Nitric
oxide and its role in health and diabetes.
Thomas
Burke Ph.D. Part
4. How Nitric Oxide (NO) causes vasodilation. NO
initiates and maintains vasodilation through a cascade of biological
events that culminate in the relaxation of smooth muscle cells that
line arteries, veins, lymphatics.
While somewhat complex, the sequence of biological events that
are triggered by NO is described below: Step
1. NO gas released from nitrosothiols in hemoglobin or from
endothelial cells, diffuses into smooth muscle cells that line small
blood vessels. Step
2.Once inside the smooth muscle cell, NO binds to an enzyme, called
guanylate cyclase (GC) and this binding results in GC activation. Step
3. Activated GC is able to cleave two phosphate groups from another
compound called guanosine triphosphate (GTP). This results in the
formation of cyclic guanosine monophosphate (cGMP) that is used to
phosphorylate (Phosphorylation is the addition of a phosphate group)
proteins, including the smooth muscle contractile protein called
myosin. Step
4. Once phosphorylated,
smooth muscle cell myosin relaxes, resulting in dilation of the vessel
that was originally exposed to NO. As
one can imagine, only a limited number of GC enzymes are present in
any one smooth muscle cell and once all the GC enzymes have been
activated, additional NO will not initiate any further vasodilation.
Any “extra” NO is simply sequestered as a nitrosothiols bound to
hemoglobin in RBC for future use. Eventually
the phosphate groups bound to myosin in smooth muscle cells must be
removed to return the blood vessels to their normal diameter. This
removal, or de-phosphorylation, is accomplished by another enzyme, a
phosphatase. If the phosphatase enzyme is inhibited, then NO/GC/cGMP
mediated vasodilation will be sustained for a longer period of time.
This, in fact, is the basis of the erectile dysfunction drug Viagra™;
which inactivates the phosphatase. What
does this mean for people with diabetes and for their physicians?
Clearly, normal vasodilation cannot occur in patients whose NO production or
release is depressed, as we pointed out in previous articles.
Without vasodilation, healing of ulcers will be slow, development of
nerve damage will accelerate, and circulation to organs such as eyes,
kidney, heart, and intestine will remain below normal. Some
may ask whether it isn’t “too much” vasoconstriction rather than
“too little” vasodilation that characterizes poor perfusion in
people with diabetes? In normal subjects, the control of perfusion
involves several vasoconstrictor hormones and activation of
sympathetic nerves, which together cause vasoconstriction. To induce
vasodilation, the body must reduce these biologic responses or counter
them with vasodilators such as NO (or a prostaglandin called
prostacyclin). Therefore, in the absence of normal concentrations of
NO, even normal levels of vasoconstrictive hormones or nervous
activity results in abnormally low blood flow (vasoconstriction and
its effect to reduce tissue perfusion). One does not need to implicate
“too much” vasoconstrictive activity (via hormones or nerves) as a
cause of perfusion problems in people with diabetes, although this
certainly can be a contributing factor in some instances. In
summary, NO causes vasodilation by initiating a cascade of biological
events that relax smooth muscle cells lining blood vessels. This vasodilation continues until a phosphatase enzyme
dissociates the phosphate from myosin (which may be delayed by
Viagra). Since
vasodilation through NO only occurs when there is GC able to bind NO,
additional NO, is sequestered for future use as a nitrosothiol,
including those found in hemoglobin. NO is the most important of the
body’s countermeasures against normal vasoconstriction and, if
production or release of NO is impaired, as in the case of people with
diabetes, poor circulation, and all the consequences thereof, ensues. Many
people with diabetes exhibit loss of sensation, a phenomena that is
linked directly to an abnormality in nerve function. The loss of nerve
structure and function has been attributed to a decreased circulation
induced, in part, by decreased production of NO. The relationship
between NO, vasodilation, blood flow, and nerve function will be
discussed in the next article. Dr.
Tom Burke received his PhD in Physiology from University of Houston,
Post Doctoral Training at Duke Medical School, He was an Associate
Professor of Medicine and Physiology at the University of Colorado
Medical School. He has authored more than 70 published scientific
clinical articles and has been a visiting scientist at the Mayo
Clinic, Yale University, University of Alabama, and University of
Florida. He is a recognized international lecturer on cell injury and
nephrology.
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