Diabetes-Induced Cardiac Autonomic Neuropathy
Posted by Scott December 18th, 2008 | 5,253 words | No Comments »
Diabetes-Induced Cardiac Autonomic Neuropathy
Scott W. Harden
12/15/08
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Abstract
Diabetes mellitus is a common disease (affecting approximately 3% of the global
population) caused by disordered metabolism which has the potential to damage every
organ system, resulting in a plethora of pathological conditions. One of the most serious
complications of diabetes is damage to the autonomic nervous system. Diabetes-induced
autonomic neuropathy is responsible for many of the gastrointestinal, visual, and
cardiovascular problems commonly associated with diabetes. Few mammalian organ
systems are as critical to immediate survival as the cardiovascular system; temporary
malfunction of which provokes life-threading pathological states and can even induce
sudden death. The high prevalence of diabetes combined with its association with
cardiac autonomic neuropathy and the current lack of effective treatment options
makes diabetic cardiac autonomic neuropathy a serious threat to a large portion of the
human population. This review outlines the relationship between diabetes mellitus and
autonomic neuropathy, with special emphasis on the neurophysiological pathology of
the cardiovascular system caused by this deadly epidemic.
Contents
0.1 Introduction
The autonomic nervous system is the portion of the nervous system responsible for
maintaining
homeostasis by regulating individual organ functions not under voluntary control. Few organs are
as critical to immediate survival as the heart, which must undergo constant adaptive changes to
regulate proper function in response to a continuously changing environment. Cardiac indices such
has inotropy (contractility), chronotropy (heart rate), and dromotropy (conduction velocity) must
be constantly adjusted, working in concert with other organ systems to maintain normal
body function. Diabetic autonomic neuropathy is a condition that can damage the
entire autonomic nervous system. Furthermore, selective damage to specific components
of the autonomic nervous (such as the stimulatory sympathetic nervous system, or
the inhibitory parasympathetic nervous system) system can result in the dangerous
condition of imbalanced autonomic control of organ function. Since every organ of the
body is heavily innervated by autonomic fibers, diabetic autonomic neuropathy has the
potential to cause devastating multi-organ dysfunction and even sudden death. This is
especially true in the case of the cardiovascular system, where even momentary loss of
proper function can result in permanent damage do the body and possibly even sudden
death.
1 Diabetes Mellitus
Diabetes mellitus (often simply referred to as diabetes) refers to the group of diseases
which cause
disordered metabolism resulting in prolonged hyperglycemia (blood sugar concentration sustained
at pathologically high levels). The phrase “diabetes mellitus” is derived from the Greek words for
“siphon” and “honey”, reflecting the excessive passage of urine (like a siphon) and the sweetness of
its taste (like honey), which are symptoms of diabetes. Regarded as a prevalent and growing
epidemic (Fang etal.,2004), approximately 171 million people (2.8% of the global human
population) were estimated to have diabetes in 2000, with a projected 4.4% of the population to
be affected by 2030 (Wild etal.,2004). Although there is no cure for diabetes, multiple treatment
options are often available to help manage it. Diabetes is known as a systemic disease because it
impacts multiple organ systems and has the potential to affect the body as a whole.
Hyperglycemia itself has the potential to directly cause death, but a majority of the deaths
of patients afflicted with diabetes are due to failing organ systems which had been
negatively affected by chronic exposure to hyperglycemic conditions as a result of their
disease.
1.1 Role of Insulin in Diabetes
Insulin is the primary hormone which regulates the uptake of glucose from blood plasma into
most
cells. After meals, carbohydrates are broken down into monosaccharide glucose, the main source of
fuel in the body. Insulin is produced by beta (beta) cells in the pancreas and released
into the blood stream in response to increased blood glucose concentration. Diabetes
can be caused autoimmune conditions which attack and kill beta-cells (type 1 diabetes),
or by the impaired production, secretion, reception, or adsorption of insulin (type 2
diabetes).
1.1.1 Insulin Secretion Pathway
Insulin is a metabolic hormone produced (in pancreatic beta-cells in responce to rising blood glucose
concentrations) whose general function is to cause many body cells (especially liver, muscle,
and fat cells) to absorb and store circulating glucose from the blood stream. This is
accomplished through a complicated cascade or protein interactions [Figure 1A]. As
circulating glucose concentrations rise, beta-cells detect the increase and uptake glucose
(through the membrane-bound GLUT2 channels) and metabolize it, producing ATP.
Increasing intracellular ATP concentration results in the closure of KATP-channels
(which normally function to maintain membrane polarization). Subsequent depolarization
of the plasma membrane potential results in an influx of Ca2+, triggering release of
Ca2+stored in the endoplasmic reticulum. This rapid increase in Ca2+concentration
allows for the release of insulin (stored in intracellular vesicles) into the blood stream,
similar to the way neurons release vesicle-enveloped neurotransmitters when they are
depolarized. After circulating throughout body, insulin in the blood is then detected by
receptors on the surface of many body cells which (upon activation by insulin) initiates a
protein activation cascade to allow for the absorption and glycosylation of circulating
glucose, allowing it to be converted into glycogen (polysaccharide glucose), a short
term energy storage molecule. In short, insulin the chemical messenger that beta-cells use
to tell other cells throughout the body to use absorb the glucose circulating in the
bloodstream.
1.1.2 Type 1 Diabetes
Type 1 diabetes is an autoimmune disease in which pancreatic beta-cells (normally responsible for
producing insulin in response to rising blood glucose concentration) are attacked and damage by
the T-cells. Approximately 10% North American and European diabetic cases are of the type 1
classification. Most of the people affected by type 1 diabetes are healthy prior to the onset
of diabetes. Because it is the predominant type of diabetes afflicting children, type 1
diabetes is often referred to as “juvenile diabetes”. It is also commonly referred to a
“insulin-dependent diabetes”, because the careful administration of insulin into the blood stream
to maintain physiological circulating glucose concentrations is the primary method of
treatment.
1.1.3 Type 2 Diabetes
Unlike type 1 diabetes, type 2 diabetes is a condition where pancreatic beta-cells are present in
proper number. Type 2 diabetes is a disease where a problem occurs somewhere in the
glucose/insulin pathway. It is associated with insulin resistance, reduced insulin sensitivity, or
reduced insulin secretion by pancreatic beta-cells. Often occurring in overweight individuals
with a family history of the disease, type 2 diabetes is the primary type of diabetes
afflicting adults. This form of diabetes occurs in two stages. In the early stage of type 2
diabetes, insulin sensitivity is greatly reduced and the body attempts to compensate
for the lack of sensitivity by producing copious amounts of insulin and releasing it
into the blood stream. As type 2 diabetes progresses into later stages, insulin secretion
becomes impaired. This type of diabetes is commonly referred to as “non-insulin-dependent
diabetes”, because the administration of insulin into the blood stream usually does
not treat the hyperglycemia. The best forms of treatment for type 2 diabetes include
adjusting glucose intake and by encouraging weight loss by carefully monitoring diet and
exercising regularly. In many cases, these lifestyle changes substantially restore insulin
sensitivity.
1.2 Direct Symptoms of Diabetes
The three major symptoms of diabetes are polyuria (frequent urination), polydipsia
(increased thirst) and polyphagia (increased appetite). When diabetes causes blood
glucose concentration rises above a certain level (known as the renal threshold), the
kidneys begin to remove it from the blood and pass it with the urine (a condition called
glycosuria). The increased osmotic pressure of the glucose-rich urine inhibits reabsorption of
water by the kidney, resulting in excess fluid loss and glucose release through the urine
(polyuria). Consequently, the decrease in blood volume (caused by water leaving the
bloodstream) is osmotically replaced by water held in the cells of the body, resulting in
dehydration and increased thirst (polydipsia). These symptoms develop rapidly as a result of
type 1 diabetes, but can develop slowly (or sometimes not at all) as a result of type 2
diabetes.
1.3 Chronic Complications
Although extreme hyperglycemia can directly cause complications, chronic hyperglycemia has
been known to cause a plethora of detrimental affects on virtually every major body process. The
principle cause of these complications can be attributed to damage of blood vessels sustained as a
result of chronic hyperglycemia, a condition known as angiopathy. Angiopathy occurs when cells
lining the inside of blood vessels (endothelial cells) uptake too much glucose. Unlike most cells in
the body, endothelial cells do not have the ability to internalize their glucose transporters to
regulate the amount of glucose that gets absorbed, so their intracellular glucose concentration is
dependent entirely upon blood glucose concentration. As a result of hyperglycemia, these
cells form more glycoproteins (membrane proteins covalently attached to extracellular
oligosaccharide chains called glycans) than normal and the membrane grows thick (Carlson
etal.,2003)and weak. Angiopathy can be broken down into two subclassifications,
microangiopathy and macroangiopathy, referring to damage to small and large blood vessels
respectively. Each of these types of angiopathy can cause major problems, but in different
ways.
1.3.1 Microangiopathy
Vision problems are commonly associated with advanced diabetes, affecting 80% of patients
who
have had diabetes for at least 10 years. Chronic diabetes can cause new blood vessels in the retina
to become extremely week and prone to rupture (hemmoraging). Diabetic retinopathy (the main
cause of diabetic vision impairments) is caused by damage to the eyes due to hemmoraged
microvessels in the retina. Initially, small granules of coagulated blood collect in the eye resulting
in small dots in the visual field. Eventually, large and repeated hemmorages can occur
(often during sleep) which leak enough blood into the eye to cloud the entire visual
field, resulting in extremely poor vision, sensitivity only to light/dark changes, or total
blindness.
In addition to visual complications, microangiopathy as a result of chronic diabetes
commonly
causes renal, cardiac, and neurological complications. Diabetes mellitus is the most common cause
of adult kidney failure. In many cases, kidney dialysis is eventually required for many diabetic
patients. Diabetic cardiomyopathy is a common condition in diabetic patients, causing myocyte
hypertrophy (a pathological increase in cell size) resulting in diastolic dysfunction and eventual
heart failure. Another affect of microangiopathy is diabetic neuropathy, a condition in which nerve
fibers are damaged as a result of the deterioration of the small vessels that supply them
with oxygen. This can result in mononeuritis (peripheral nerve damage), amyotrophy
(muscle weakness due to damage fo the nerve fibers responsible for their stimulation),
or autonomic neuropathy (damage to the neurons comprising the autonomic nervous
system).
1.3.2 Macroangiopathy
Macroangiopathy (damage to large blood vessels) due to chronic diabetes can affect a
variety of
organ systems. Both heart attack (as a result of coronary artery disease) and stroke (of the
ischemic type) can be caused by diabetic macroangiopathy. Diabetic myonecrosis (the
deterioration of muscle cells) can also be caused by diabetic macroangiopathy. “Diabetic foot” is a
term given to a collection of problems that can affect the feet following chronic diabetes,
generally caused by a combination of sensory neuropathy and macroangiopatical damage.
Exertion-related leg and foot pain (intermittent claudication) is commonly caused by
peripheral vascular disease in diabetic patients. Increased rates of infection, skin ulcers,
necrosis and gangrene of the feet have also been associated with diabetes. “Diabetic
foot” is the most common cause of non-traumatic adult amputation in the developed
world.
2 Diabetic Autonomic Neuropathy
Diabetes is a systemic disease which causes altered function of many organ systems.
Similarly,
autonomic neuropathy has the potential to negatively impact the entire body, since every organ is
innervated by the autonomic nervous system. Statistical evidence demonstrates that autonomic
neuropathies (and their respective symptoms) are more prevalent in diabetic patients,
suggesting that diabetic autonomic neuropathy exists as a unique clinical disease. Clinical
symptoms of diabetic autonomic neuropathy usually do not occur until after 1 year of type
2 diabetes or 2 years of type 1 diabetes (Pfeifer etal.,1984). Although diabetes is
known to increase the likelihood of autonomic neuropathies, some are still reluctant to
consider diabetic autonomic neuropathy as a unique clinical entity because many of its
symptoms (i.e., cardiac disease) could occur in the absence of diabetes, and it is often it is
impossible to determine the specific cause of neuropathies and their symptoms (Fang
etal.,2004).
2.1 Suspected Mechanisms of Diabetes-Induced Autonomic Neuropathy
2.1.1 Microvascular Disease
The primary theory as to the mechanism by which diabetes promotes autonomic neuropathy
relates to microvascular disease. Diseases impacting microvessels often affect nerve fibers as well,
and small nerve fibers (a major component of the autonomic nervous system) depend on properly
functioning microvasculature for oxygen. Damage to microvessels as a result of chronic
hyperglycemia (vasoconstriction and vascular membrane thickening (Carlson etal.,2003)) can
result in hypoxic conditions which eventually disable or destroy the neural fibers that rely on them
for oxygen. Neuronal ischemia (damage due to lack of oxygen) is a common symptom
of diabetes. Microvascular dysfunction caused by diabetes often serves as the cause
for the majority of structural, functional, and clinical changes observed in diabetic
neuropathy.
2.1.2 Advanced Glycation Endproduct
Pathologically high concentrations of circulating glucose increases the likelihood of
random
(non-enzymatic) covalent binding of glucose to proteins, altering their structure (likely inhibiting
their function). These inhibited glucose-bound proteins, often referred to as AGEs (Advanced
Glycation End products), effect nearly every type of cell in the body and are believed to be a
major factor in aging and age-related effects of chronic diabetes. AGEs also exert oxidative stress
on the body, which can lead to numerous complications, especially those associated with aging.
Additionally, AGEs have the potential to active the immune system resulting in inflammatory
diseases such as atherosclerosis, asthma, arthritis, myocardial infarction, nephropathy, retinopathy
and neuropathy.
2.1.3 Protein Kinase C
Increased levels of circulating glucose results in the activation of a signaling cascade
which leads to
the increase absorption of extracellular fatty acids. One of these fatty acids (diacylglycerol) is
implicated in the pathology of diabetic neuropathy. Increased intracellular diacylglycerol leads to
the activation of protein kinase (PK) C. PKC is believed to modulate acetylcholine release
(Allgaier etal.,1988), a principle neurotransmitter for the autonomic nervous system (especially
in the parasympathetic branch), which is assumed to result in a decrease in nerve conduction
velocity.
2.1.4 Polyol Pathway
While most body cells require insulin to regulate glucose update, cells of the retina,
kidney, and
nervous system are insulin-independent, meaning that their intracellular glucose concentration is
completely dependent upon blood plasma glucose concentration. Glucose absorbed and not
immediately used in energy metabolism may enter the polyol pathway and eventually be converted
into sorbitol by the enzyme aldose reductase (a process which requires NADPH). Under
physiological conditions, aldose reductase has low affinity for glucose, and there is plenty
of available NADPH to fuel this reaction. However, in hyperglycemic conditions the
affinity of aldose reductase for glucose increases. This increased affinity results in a
higher production of sorbitol, the depletion of NADPH, and the release of reactive
oxygen species. Since sorbitol does not have the ability to be exported from the cell, the
rate of production of sorbitol becomes greater than the rate of its metabolism. This
results in the accumulation of sorbitol in the cell, drawing water into the cell by osmotic
pressure. Reactive oxygen species released by the breakdown of NADPH place the cell
under increased oxidative stress. Additionally, as NADPH decreases, glutathione (which
requires NADPH for its production) concentrations decrease, resulting in hemolysis (the
rupture of red blood cells) as a result of oxidative stress. Furthermore, high sorbitol
levels reduce the cellular uptake of myoinositol, decreasing the activity of the plasma
membrane Na+/K+ ATPase pump required for nerve function, further contributing to the
neuropathy.
2.2 Diabetic Cardiac Autonomic Neuropathy
Approximately 20% of diabetic patients are afflicted with some form cardiac autonomic
neuropathy, increasing their risk of mortality by 500% (Ziegler etal.,1998). The estimated 8-year
survival rate for diabetic patients with cardiac autonomic neuropathy is 77%, compared to the
97% survival rate estimate of diabetic patitens without cardiac autonomic neuropathy (Rathmann
etal.,1993). Affecting one fourth of type 1 diabetics and one third of type 2 diabetics, cardiac
autonomic neuropathy is one of the most well-studied forms of diabetic autonomic neuropathy
because it is one of the most life threatening, and it is also one of the easiest forms of
autonomic neuropathy to assess. The autonomic nervous system modulates the electrical
and physiological activity of the myocardium via the sympathetic (stimulatory) and
parasympathetic (inhibitory) branches of the autonomic nervous system, which in concert
with each other to promote proper cardiac function. Damage to autonomic nerve fibers
innervating the heart and blood vessels caused by diabetes can result in impaired control of
heart rate and vascular dynamics. An imbalance of the sympathetic / parasympathetic
control of cardiac function can lead to the development of deadly heart arrhythmias
(Sztajzel,2004).
One of the greatest difficulties in the study of autonomic neuropathy is the lack of
accurate,
noninvasive, and quantitative methods to measure autonomic function. Because imaging small
nerve fibers in the target organs of living creatures is limited, and noninvasive methods to measure
neuron stimulation threshold and neurotransmitter release are essentially nonexistent, most
in vivo studies which measure autonomic neuropathy rely on secondary indicators of
neurological efficiency. Because the cardiovascular system constantly and rapidly changes
cardiac indices in response to environmental stimuli, most of the assessment methods for
measuring autonomic neuropathy are centered around the measurement of cardiovascular
function.
Two major hindrances to the study of diabetic cardiac autonomic neuropathy is the lack of
a
standard for quantitative assessment of neuropathy, and the interplay of various mechanisms and
symptoms caused by this condition. Studies designed to assess the prevalence of diabetic cardiac
autonomic neuropathy are often difficult to compare due to the lack of a standard criteria to
measure and classify the degree of neuropathy. Additionally, many diabetic patients exhibiting
only a few, seemingly random symptoms of neuropathy. Therefore, one test alone is usually
not a conclusive assessment as to whether a patient possesses autonomic neuropathy,
or the degree to which it impacts cardiovascular control. As a result, the symptoms
of diabetic autonomic neuropathy are also the fundamental tools for assessing this
disease.
2.2.1 Sympathetic / Parasympathetic Control
In most cases, diabetes mellitus is associated with an imbalance of the autonomic nervous
system,
with the sympathetic (accelatory) branch overpowering the parasympathetic (suppressive) branch.
This contributes to many of the symptoms associated with diabetes-induced autonomic
neuropathy. Metoprolol is a drug which blocks cardiac sympathetic nervous system activity by
selectively inhibiting beta1 receptors. In a mouse model of diabetes, administration of metoprolol
decreased heart rate by 59 bpm, compared to a mere 5 bpm in nondiabetic control mice
(daCostaGoncalves etal.,2008a). The effect of disabling sympathetic input to the heart was
more dramatic for diabetic animals, suggesting that diabetes increases sympathetic
activity.
Conversely, parasympathetic activity is decreased by diabetes mellitus. In the same study
(daCostaGoncalves etal.,2008a), atropine (which decreases parasympathetic activity by
competitively inhibiting acetylcholine receptors) was administered to diabetic and nondiabetic
control mice. The resulting change in heart rate in the diabetic mice was less than half of that of
nondiabetic controls (59 bpm vs. 144 bpm). The effect of disabling parasympathetic input to the
heart was less dramatic in the case of diabetes, suggesting that diabetes attenuates
parasympathetic activity.
Does diabetes increase sympathetic activity and decrease parasympathetic activity by two
different mechanisms? This question is difficult to address, because the mechanisms by which
diabetes-induced autonomic neuropathy are not completely understood, and the methods by
which these mechanisms degrade different portions of the autonomic nervous system are
even more speculatory. As mentioned previously, sympathetic and parasympathetic
nervous systems accelerate and suppress heart function respectively. It is important to
note that, rather than simply being considered as opposite forces, the sympathetic
and parasympathetic branches of the autonomic nervous system should be regarded as
complimentary forces which continuously work together to facilitate proper cardiac function.
Parasympathetic activity suppresses sympathetic activity and visa versa. Tonic (continuous)
parasympathetic activity is normally exerted over the heart primarily through the vagus
nerve. It is possible that a single mechanism is responsible for the diabetes-associated
activity changes detected in both branches of the autonomic nervous system. This single
mechanism may selectively hinder function of the vagus nerve, decreasing parasympathetic
activity, intern decreasing sympathetic suppression, resulting in the increased sympathetic
activity.
2.2.2 Silent Myocardial Ischemia
Silent myocardial ischemia (cardiac denervation syndrome) is a dangerous form of cardiac
autonomic neuropathy. In this syndrome, damage to the nerve fibers innervating the heart reaches
an extent that extreme trauma (such as a severe heart attack) is never recognized due to the
absence of chest pain (angina), often resulting in delayed treatment (if any is sought at all).
Although the exact mechanism of silent myocardial ischemia is not fully understood (Maser and
Lenhard,2005), altered pain thresholds, subthreshold ischemia (not strong enough to produce
pain), and dysfunction of the afferent (sensory) cardiac autonomic nerve fibers have all
been suggested as possible mechanisms contributing to this condition (Shakespeare
etal.,1994).
2.2.3 Sudden Cardiac Death
Diabetic cardiac autonomic neuropathy greatly increases the risk of sudden unexpected death
due
to severe but asymptotic myocardial ischemia which can produce lethal arrhythmias. As a result,
there are many unexpected deaths in diabetic patients in the absence of ischemia. This sudden
cardiac death is sometimes called “dead in bead syndrome”, because the victim is often relatively
healthy and dies unexpectedly in their sleep. This major unresolved health problem
claims over 300,000 lives per year in the United States alone, accounting for 29% of all
diabetic deaths. In most cases of sudden cardiac death, the direct case is ventricular
fibrillation. Sudden cardiac death has been associated with sympathetic autonomic
hyperinnervation, associated with nerve sprouting in response to myocardial infarction in dogs
(Cao etal.,2000).
2.2.4 Orthostatic Hypotension
When moving from a lying position to a standing position, blood pools in the legs and blood
pressure in the upper part of the body rapidly decreases. Under normal conditions, this
drop in blood pressure (termed orthostasis hypotension) is detected by baroreceptors
(afferent, sensory fibers which detect decreased blood pressure) and corrected by peripheral
vasoconstriction (Ewing,1978). In those suffering from diabetic autonomic neuropathy, damage to
the sympathetic efferent branch of the autonomic nervous system (responsible for the
baroreceptor-induced vasoconstriction and cardiac acceleration to compensate for the drop in
blood pressure) results in sustained orthostatic hypotension. This may result in dizziness,
weakness, fatigue, and visual blurring, or could be asymptotic depending on the degree of
sympathetic neuropathy. It should be noted that neuropathy affecting any portion of the
baroreflex arc (efferent, afferent, or central components) would produce impairment in the
baroreflex.
2.2.5 Resting Tachycardia
Resting tachycardia (rapid heart rate) is a characteristic of diabetic patients with severe
autonomic neuropathy affecting the vagus nerve (Ewing and Clarke,1986). The vagus nerve is the
primary source of parasympathetic input to the heart, and parasympathetic tone through the
vagus nerve serves a cardioinhibitory function, suppressing heart rate below its intrinsic level.
Impairment of the vagus nerve removes this inhibitory function, resulting in rapid heart rate
(often in excess of 100 bpm) in the absence of exercise. Because early stages of severe diabetic
cardiac autonomic neuropathy primarily impact the parasympathetic nervous system, resting
tachycardia increases, then decreases as the disease progresses, impacting the sympathetic nervous
system. Once the parasympathetic and sympathetic nervous systems are both sufficiently
damaged, resting tachycardia diminishes. Because severe parasympathetic and sympathetic
autonomic neuropathy decreases resting tachycardia, it can not be used as the sole
measurement of cardiac autonomic neuropathy. By point in the progression of the disease
where both branches of the autonomic nervous system are severely impacted, heart rate
variability decreases to a point where heart rate is essentially fixed, regardless of exercise or
sleep.
2.2.6 Hypertension
Most patients with type 2 diabetes are afflicted by hypertension as a result of sympathetic
overactivation (Huggett etal.,2003), likely the result of reduced parasympathetic tone associated
with diabetes (Lin etal.,2008). Similar conditions are observed in experimental animal models of
diabetes. In Leptin receptor deficient db/db diabetic mice, the average daytime (sleeping) blood
pressure is significantly higher than in db/+ control mice (daCostaGoncalves etal.,2008a).
[Figure 2] Chronic hypertension significantly increases the risk for strokes, heart attacks, heart
failure, arterial aneurysm, and renal failure.
2.2.7 Baroreflex Sensitivity
Decreased baroreflex sensitivity is commonly associated with diabetes mellitus. Power
spectral
analysis is an established method capable of assessing baroreflex sensitivity (daCostaGoncalves
etal.,2008b). In db/db diabetic mice, baroreflex sensitivity was found to be significantly reduced
compared to db/+ nondiabetic controls (daCostaGoncalves etal.,2008a). Baroreflex sensitivity
was found to be decreased in OVE26 diabetic mice as well, assessed by measuring responses to the
chemical modification of blood pressure via sodium nitroprusside (a vasodilator which reduces
blood pressure) or phenylephrine (a vasopressor which increases blood pressure) administration as
well as responses to vagal nerve stimulation (?). This study demonstrated that the magnitude of
changes of heart rate in responses to blood pressure changes was decreased by diabetes.
Surprisingly, this study found that aortic depressor node activity was similar in response to
blood pressure changes between diabetic and control mice, suggesting the function
of the afferent (sensory) component of the baroreflex is not significantly impacted by
diabetes. However, stimulation of the aortic depressor node produced a lesser magnitude of
change of heart rate in diabetic mice than nondiabetic controls. Finally, direct vagal
stimulation produced a dramatically attenuated heart rate response in diabetic mice
compared to nondiabetic controls, suggesting that the central component is damaged by
diabetes, likely being the most significant contribution to diabetic impairment of baroreflex
function.
2.2.8 Exercise Intolerance
Cardiac autonomic dysfunction impairs the ability of the body to respond to environmental
stresses by sensing that stress and adapting to it by altering heart rate, blood pressure, and other
cardiac indices. In the case of exercise, cardiac output normally directs peripheral blood flow to
skeletal muscles to meet the demands placed on them. However, as a result of cardiac autonomic
neuropathy, the sympathetic and parasympathetic branches of the autonomic nervous system are
impaired, resulting in diminished blood flow to the skeletal muscles. Some of the cardiac functions
which are impaired include reduced ejection fraction, systolic dysfunction, and decreased diastolic
filling (Vinik etal.,2003). Due to the decreased ability to react to the stress of exercise, patients
with diabetes are encouraged to undergo cardiac stress tests before beginning any exercise
program.
2.2.9 Diastolic Dysfunction
Left ventricle diastolic dysfunction is reportedly common in patients with type 2 diabetes
mellitus
in the absence of clinically detectable heart disease (Fang etal.,2004). Most diabetic patients
with left ventricular diastolic dysfunction display evidence of cardiac autonomic neuropathy, but
the severity of their dysfunction is unrelated to microvascular complications (Rajan and
Gokhale,2002). This suggests that diabetes-induced diastolic dysfunction is largely due to cardiac
autonomic neuropathy. Noninvasive assessment of diastolic dysfunction can be obtained utilizing
Doppler echocardiography. Diminished diastolic function has been reported reported in
transgenic GLUT4 diabetic mice compared to control mice. Diabetes-induced changes
included fractional shortening and reduction in circumferential fiber shortening velocity
(Semeniuk etal.,2002). Analogous to diabetic patients, diastolic deficiencies in animal
models also include reduced left ventricle ejection time and an increased preinjection
period.
2.2.10 Sympathetic Autonomic Fiber Distribution
One study utilizes immunohistochemical analysis of sectioned ventricular tissue to measure
and
compare sympathetic nerve density between diabetic and nondiabetic humans (Park etal.,2002).
Surprisingly, the results of this study indicate that there is no difference in cardiac nerve
density or heterogenecity as a result of diabetes. However, the same study reported that
noroepinephrine concentrations were markedly reduced in apical sections of postmortem hearts
taken from diabetic patients, which is indicative of sympathetic denervation. Recently,
positron emission tomography (PET) imaging techniques have been used to monitor the
uptake of radiolabeled catecholamine analogues, such as carbon-11 hydroxyephedrine
(HED), allowing the assessment of sympathetic autonomic distribution (Hartmann
etal.,1999). In nondiabetic patients with heart failure, it was found that there was a global
reduction (and an increase in regional abnormalities) of the sympathetic uptake of
HED.
2.2.11 Circulating Catecholamine Levels
Catecholamines are neurotransmitters released by the sympathetic autonomic nervous system
which serve to stimulate cardiac function and promote vasoconstriction to raise blood pressure.
Studies conducted in streptozotocin-induced diabetic rats demonstrated that diabetes alters the
normal concentration of circulating catecholamines, resulting in different concentrations depending
on the progression of the disease (Fushimi etal.,1988). It was reported that after 6 weeks of
streptozotocin treatment, blood plasma catecholamine concentration was significantly increased
above control rats. After 13 weeks of streptozotocin-induced diabetes, circulating catecholamine
concentrations returned to normal levels. [Figure 3] It was hypothesized that the progressively
impaired secretion of catecholamines throughout the progression of diabetes may be
caused by sympathetic diabetic autonomic neuropathy, whereas the initial spike in
concentrations was due to the diminished sympatho-inhibitory action as a result of rapid
parasympathetic neuropathy. Similar results were reported in spontaneously diabetic Chinese
hamsters (Uekita etal.,1997), where 3 weeks following the onset of diabetes cardiac
catecholamine content (as assessed by HPLC) in diabetic hamsters was above normal
(177% of the controls), and progressively decreased approaching the control levels by 35
weeks.
These studies support the theory that diabetes alters the balance of the sympathetic and
parasympathetic autonomic nervous system. The parasympathetic branch of the autonomic
nervous system is known to experience neuropathy in response to diabetes before the sympathetic
nervous system. Since parasympathetic tone suppresses parasympathetic function, the degradation
of parasympathetic tone would likely lead to sympathetic overactivity while the sympathetic
nervous system remains largely unaffected by neuropathy. As the disease progresses, this
sympathetic dominance would likely return to normal levels as the sympathetic nervous
system too succumbs to diabetes-induced neuropathy. This theory is supported by data
indicating increased catecholamine levels at the onset of diabetes, and catecholamine levels
near those of controls as the disease becomes severe, involving dramatic autonomic
neuropathy for both the sympathetic and parasympathetic branches of the autonomic nervous
system.
2.2.12 Heart Rate / Blood Pressure Variability
Throughout the course of a normal day, individuals perform many different types of tasks
which
require that the body compensate for a changing environment (such as a drop in blood pressure
caused by standing rapidly) by modifying cardiac indices, such as heart rate and blood pressure.
The ability of the body to rapidly sense these environmental changes and compensate for them
relies on the sensory and efferent components of the autonomic nervous system. Cardiac
autonomic neuropathy impairs these reflexes and dampens the response of cardiac indices to
changes in the internal environment. Analysis of electrocardiogram (ECG) recordings over long
periods of time allows the assessment of heart rate variability by comparing the times
between heart beats (R-R) using a technique called power spectral analysis (where R-R
interval data is analyzed via the fast Fourier transformation algorithm). The magnitude
of the decrease in heart rate variability indicates the severity of cardiac autonomic
neuropathy. Measurements of heart rate variability provide one of the earliest indicators of
cardiac autonomic neuropathy (Maser and Lenhard,2005;Lin etal.,2008;Vinik
etal.,2003).
The resulting data, when graphed with respect to frequency power, usually produces three
distinct bands. The very low frequency (VLF) band is caused by the sympathetic branch of the
autonomic nervous system increasing cardiac output by decreasing the amount of time between
heart beats (usually a product of thermoregulation). Conversely, a high frequency (HF) band is
caused by the parasympathetic branch of the autonomic nervous system decreasing cardiac output
by increasing the amount of time between heart beats (generally in response to respiratory
activity). In between these two bands is a low frequency (LF) band, which represents minor
changes in heart beat rate (likely due to baroreceptor activity). In humans, the LF
band is presumed to be caused by both the parasympathetic and sympathetic branches
of the autonomic nervous system, whereas in mice the LF band is attributed only to
the parasympathetic branch of the autonomic nervous system (daCostaGoncalves
etal.,2008a).
Heart rate variability can be quantitatively assessed by obtaining a trace of blood
pressure
throughout the sleep cycle of diabetic and nondiabetic animal models via a telemetric data
acquisition system. This trace can then detrended by subtracting a moving window average from
the trace. Peaks in the trace (representing the R stroke of each heartbeat) can be converted into
data points, and each R-R interval can be determined and added to a data set. This data can then
be autoregressively processed by the fast Fourier transformation formula to yield a
power spectral density trace. Unlike the power spectral density traces from humans,
rodents generally produce two strong peaks, dubbed LF and HF, for parasympathetic and
sympathetic activity respectively. This trace (with overlapping peaks) can be mathematically
deconvoluted, producing overlapping traces of the two distinct peaks. The integrated
value of each peak produces a quantitative value of its power in (ms2) units. [Figure
4]
A significant contribution to the study of heart rate variability in animal models of
diabetes
involved the study of heart rate and blood pressure variability in streptozotocin-induced diabetic
rats (Lin etal.,2008). This study found that diabetes not only decreases heart rate variability,
but also increases systolic blood pressure variability. An interesting result of this study was the
finding that diabetes decreased the magnitude of both the parasympathetic (LF) and sympathetic
(HF) powers. However, parasympathetic power was decreased significantly more than sympathetic
power, altering the parasympathetic/sympathetic ratio. This ratio (an indicator of the
sympatho-vagal balance in heart control (Zaza and Lombardi,2001)) decreased from ~2 to
~.45 for 4-week streptozotocin-induced diabetic mice, and increased to ~.48 for 8-week
streptozotocin-induced diabetic mice. This proves that parasympathetic function is attenuated
more than sympathetic function, but also suggests that parasympathetic function is
attenuated faster since the ratio begins to approach the normal physiological value with
increasing time (likely due to increasing damage sustained by the sympathetic nervous
system).
2.2.13 Myocardial Fibrosis
Myocardial fibrosis (the accumulation of fibrils in the muscle tissue) and myocyte
hypertrophy
(the pathological swelling of cardiomyocytes) are assumed to be the most significant contributors
to cardiac changes in non-neuropathological diabetic cardiomyopathy, eliciting functional
alterations in cardiac output not directly related to changes in autonomic function. As diabetes
progresses, structural alterations have been reported in the heart in absence of coronary disease.
The most significant of these changes is cardiomyocyte loss and replacement fibrosis. A
study analyzing the hearts of of 15 week old OLETF diabetic rats demonstrates these
significant diabetes-induced structural changes which impact the left ventricle (Mizushige
etal.,2000). The collagen area / visual field ratio was greater in diabetic rats than
nondiabetic rats, and the collagen content / dry heart weight ratio was significantly
increased, suggesting that left ventricle fibrosis occurs in the early stages of type 2
diabetes mellitus. A study of streptozotocin-induced diabetic rats demonstrated that
the heart weight / body weight ratio was increased and the volume of extracellular
components was increased 3-fold due to diabetes (Warley etal.,1995). The same study
found that, although the structure of the myocardium was significantly changed due
to diabetes, the actual left ventricular volume remained unchanged. Studies of right
ventricular endomyocardial biopsies of diabetic and nondiabetic patients confirmed
that interstitial fibrosis is increased by diabetes in humans (Nunoda etal.,1985). The
same study reported an increase in the mean diameter of myocardial cells of diabetic
patients.
2.3 Gastrointestinal Diabetic Autonomic Neuropathy
Gastrointestinal disturbances often accompany diabetes mellitus (20% of diabetic patients
report
experiencing diarrhea regularly, and over 60% report constipation), and these disturbances usually
become more severe with time. Many of these issues are the result of diabetes-induced
gastrointestinal autonomic neuropathy. The gastrointestinal tract represents a large and complex
organ system, and the location of autonomic neuropathy dictates the symptoms that are
produced. The major complications of the gastrointestinal system due to diabetic autonomic
neuropathy are as follows (Vinik etal.,2003):
- Esophangeal enteropathy (disordered peristalsis, abnormal lower esophageal sphincter
function)
- Gastroparesis diabeticorum (nonobstructive impairment of gastric propulsive activity)
– usually clinically silent, but has the potential to be one of the most debilitating of
all diabetic GI complications. It interferes with nutrient delivery to the small intestine
and disrupts glucose absorption and exogenous insulin administration, resulting in
sporadic variations of blood glucose concentration.
- Diarrhea (impaired or increased motility of the small bowel)
- Constipation (dysfunction of intrinsic and extrinsic intestinal neurons, decreased or
absent gastrocolic reflex)
- Fecal incontinence (abnormal internal and/or external anal sphincter tone, impaired
rectal sensation)
Diabetic gastrointestinal autonomic neuropathy is much more difficult and takes a far
greater amount
of time to assess than diabetic cardiac autonomic neuropathy due to the slow movements and
reflexes of the gastrointestinal system compared to the cardiovascular system. Surprisingly,
there does not appear to be a strong correlation between gastrointestinal problems
(disturbed gastric emptying, impaired gall bladder contraction, or prolonged colonic
transit time) and cardiovascular autonomic neuropathy in diabetic patients (Werth
etal.,1992). This curious finding suggests that diabetes-induced autonomic neuropathy
may differentially (and possibly randomly) impact specific regions of the body, and
that disturbed motor function in the gut could be one of the many affects of diabetic
neuropathy.
3 Developing Research
3.1 Diabetic Autonomic Structural Changes
A majority of the research dedicated to the study of diabetes-induced autonomic neuropathy
has been conducted from a neurophysiological standpoint (daCostaGoncalves
etal.,2008b,a;Ewing,1978;Fushimi etal.,1988;Huggett etal.,2003;Pfeifer
etal.,1984;Rajan and Gokhale,2002;Shakespeare etal.,1994;Sztajzel,2004;Uekita
etal.,1997;Zaza and Lombardi,2001). Far fewer studies have sought to study the morphological
alterations of the autonomic nervous system as a result of diabetes mellitus. Of the few studies
that attempt to quantify the distribution of the autonomic fibers in the myocardium, virtually all
methods utilized involve performing immunohistochemical analysis on sectioned tissue (Park
etal.,2002). To date, no studies have been conducted to quantitatively assess changes in the
coverage and distribution of the sympathetic and parasympathetic fibers in the heart due to
diabetes mellitus. Since clear functional changes occur as a result of diabetes-induced cardiac
autonomic neuropathy, one would expect anatomical changes to occur as well. However, the
specific structural changes caused by diabetes-induced cardiac autonomic neuropathy
are difficult to predict. On one hand, decreased sympathetic influence of heart rate
(daCostaGoncalves etal.,2008a) suggests decreased sympathetic coverage compared to
parasympathetic coverage. On the other hand, sympathetic hyperactivity (Huggett
etal.,2003) and increased myocardial catecholamine levels (Park etal.,2002) suggest
increased sympathetic coverage compared to parasympathetic coverage. Quantitative
functional assessments measuring sympathetic coverage, parasympathetic coverage,
and the ratio of these two measurements should be conducted to further elucidate the
under-studied structural changes of the autonomic nervous system that may coincide with the
established functional changes associated with diabetes-induced cardiac autonomic
neuropathy.
3.2 Animal Models of Type 1 Diabetes
3.2.1 Streptozotocin (STZ) Injection
Streptozotocin (STZ) is a chemical produced by Streptomyces achromogenes, a soil-dwelling
gram-positive bacteria. Initially researched for its antibiotic properties, STZ is now commonly
used to study type 1 diabetes mellitus in animal models. Although STZ may exhibit cell toxicity
through multiple mechanisms, its primary method of action is as an alkylating agent, possessing
the potential to cause massive DNA damage (by cross-linking adjacent DNA bases) if it reaches
the nucleus of a cell. However, it is impermeable to cell membranes. Therefore, in order for it to
be toxic to a cell, it must be actively transported inside. Structurally, STZ is similar
to glucose, and has the potential to be uptaken by the GLUT2 glucose transporter
(Schnedl etal.,1994). Since pancreatic beta-cells contain high levels of GLUT2 (Wang and
Gleichmann,1998), they are most affected by the toxic effects of STZ-mediated DNA
alkylation.
3.2.2 OVE26 Transgenic Mice
The OVE26 diabetic mouse is a transgenic model used to study severe early-onset type 1
diabetes.
These mice develop severe diabetes within hours of birth (Epstein etal.,1989)and experience
high concentrations of circulating glucose by 35 days (450 mg/dl) with a 42% reduction in insulin
secretion (Carlson etal.,2003). These mice can survive for more than a year with no insulin
administration, likely due to the small level of residual insulin secretion. OVE26 transgenic mice
contain a DNA insertion of the calmodulin gene regulated by an insulin promoter. This results in
the expression of large amounts of calmodulin (a protein which can bind up to four
calcium ions) in any cell which normally expresses large amounts of insulin, namely
pancreatic beta-cells. [Figure 1B] The excess intracellular calmodulin binds much of the
available intracellular calcium, preventing intracellular calcium concentrations from
reaching levels high enough to induce insulin secretion. This theory is supported by
electron microscopic data which demonstrates that OVE26 transgenic pancreatic beta-cells
contain copious amounts of insulin granules which failed to be properly secreted (Bhatt
etal.,2000). These observations suggested that excess calmodulin may specifically impair
the insulin secretory process. However, decreased insulin secretion alone may not be
the sole cause of diabetes in OVE26 diabetic mice. By 3 days of age, the number of
pancreatic beta-cells is less than half of the normal value, and remaining cells appear
markedly abnormal with greatly diminished cytoplasmic space (Epstein etal.,1989).
Although the specific mechanism (or combination of mechanisms) whereby the OVE26
transgene induces type 1 diabetes in mice is not entirely understood, the ability to
selectively inhibit the production of insulin by pancreatic beta-cells without toxic effects
to other body cells is greatly beneficial to the study of diabetes-induced pathological
states.
3.3 Animal Models of Type 2 Diabetes
3.3.1 Otsuka Long-Evans Tokushima Fatty (OLETF) Rats
A common type 2 diabetic animal model is the OLETF (Otsuka Long-Evans Tokushima Fatty)
rat. Males of this transgenic animal line develop late onset hyperglycemia. OLETF rats have lower
rates of glucose infusion (60% of the control at 12 weeks of age and 20-30% of the control at 18,
24, 30 and 39 weeks of age) than age-matched controls, indicating the development of insulin
resistance with age, accompanied by a 45% reduction in the amount of insulin-stimulated glucose
uptake (Sato etal.,1995). Characteristic features of OLETF rats is the presence of hyperplastic
(continuously growing) foci of pancreatic islets (regions where hormone producing cells are
found, including beta-cells) which increases in severity continuously through 40 weeks
of age (Kawano etal.,1992). Although this model is a generally accepted model of
type 2 diabetes, it is not without its imperfections. Acellular capillaries and pericyte
ghosts (characteristic morphological changes in early human type 2 diabetic retinopathy)
are not found in large quantities in OLETF rats, suggesting that OLETF rat is not a
suitable animal model for the study of angiopathic diabetic retinopathy (Matsuura
etal.,2005).
3.3.2 NOD Mice
Non-obese diabetic (NOD) mice are susceptible to the spontaneous development of type 2
diabetes
mellitus. Spontaneous contraction of diabetes is known to be the result of insulitis (inflammation
and beta-cell death as the result of an autoimmune attack from white blood cells infiltrating the
pancreatic islets where hormone producing cells are located) which results in non-fasting
hyperglycemia. Because NOD mice spontaneously contract autoimmune-induced type 1 diabetes,
they also highly susceptible to developing other autoimmune syndromes. The mechanism by which
NOD mice spontaneously contract type 2 diabetes is very complex and not completely
understood, however it is believed to be related to genetic polymorphisisms in the Idd3
locus which is associated with interkeukin-2 production, the loss which contributes to
the development of autoimmunity in NOD mice.(Delovitch and Singh,1997;Tang
etal.,2008)
3.3.3 Leptin Receptor Deficient db/db Mice
Another mouse model that offers the ability to study type 2 diabetes mellitus is the leptin
receptor deficient db/db mouse. Leptin is a protein which circulates in the body proportional to
the amount of body fat, enters the central nervous system in proportion to its circulating
concentration, and is believed to bind to receptors in the brain, playing a role in the regulation of
hunger. In this transgenic mouse model, Leprdb (a leptin receptor) is mutated into a
nonfunctional form, therefore the central nervous system views the body as if it were continuously
starved for food, leading to hyperphagia (persistent hunger), obesity, insulin resistance, and
pancreatic beta-cell insulin secretory defects, resulting in hyperglycemia by 12-14 weeks of age
(Carley and Severson,2008;Gibbs etal.,1995). This animal model closely mimics the forms of
autonomic neuropathy observed in human type 2 diabetic patients (daCostaGoncalves
etal.,2008a).
4 Conclusion
Diabetic autonomic neuropathy is a severe condition often associated with chronic diabetes.
It has
the potential to cause a large range of health problems, from diarrhea to sudden death, and is very
difficult to assess and treat. By the time symptoms of diabetic autonomic neuropathy begin to
manifest themselves, nerve damage is likely irreversible and carries poor prognosis. Therefore, the
best treatment options are usually aimed at preventing the progression of diabetes, therefore
limiting the development of the autonomic neuropathies that are associated with it.
This usually involves precise maintenance of diabetes by carefully monitoring diet,
blood sugar, and insulin needs. However, the best way to ensure that one does not
experience the debilitating effects of diabetic autonomic neuropathy is to maintain a
healthy diet and exercise regimen to prevent the development of diabetes in the first
place.
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