Role of Nitric Oxide in Cardiovascular Disorders - Applications of Biotechnology in Cardiovascular Therapeutics - page 64

production following pharmacological stimulation of the endothelium utilizing
acetylcholine (Ach). In a study on healthy subjects, reactive hyperemia of the
forearm, a physiological endothelial stimulus, results in a 52.5% increase in mean
plasma nitrite concentrations (Allen et al. 2005 ). The subjects who could exercise
hardest also produced the most NO. In contrast, subjects with cardiovascular dis-
ease showed no significant increases. However, plasma nitrite is readily oxidized
to nitrate within plasma, and thus its utility as a marker of NO production within
the clinical setting may be limited. Alternatively, NOx (predominantly nitrate) is
relatively stable in plasma (microM concentrations), but is produced by sources
other than the vasculature and has been shown to be unsuitable as a measure of
localized NO production.
Hemoglobin, Oxygen, and Nitric Oxide
Hb consists of four subunits that bind oxygen on heme-iron groups. It captures
oxygen in the lungs and releases it in peripheral tissues. Hb also serves the function
of discharging carbon dioxide, the cellular waste product of respiration. With the
advent of NO research there is now abundant literature about the interaction of Hb
with NO. The confinement of Hb within red blood cell (RBC) reduces the rate of
reaction of Hb with NO by a factor of 1,000 because of two reasons:
1. The red cell membrane creates a barrier to the diffusion of NO.
2. A thin RBC-free zone, created by blood flow at the perimeter in vessels greater than
20 mm in diameter, separates NO from RBC and reduces the NO uptake rate.
It is generally accepted that Hb is crucial for oxidative inactivation of NO by
reaction to nitrate and methemoglobin. The rapid destruction of NO by Hb raises
the question of whether it is paracrine agent with only local effects or whether, like
a hormone, it disseminates throughout the body. Various studies of reactions of
NO with erythrocytes, plasma, and Hb, indicate that under normal physiological
conditions, the reactions of NO with metals, heme groups, and free radicals
predominate.
It is now recognized that NO senses oxygen levels and is an innate part of the
body's oxygen delivery system. NO in blood cells is an active regulatory molecule
that senses the oxygen level in tissue and causes Hb to undergo subtle shape changes
to release its oxygen in tissues when levels are low but holds on to it when oxygen
levels in tissue are high. Interactions of NO with Hb could regulate the uptake and
delivery of oxygen by subserving the classical physiological responses of hypoxic
vasodilation and hyperoxic vasoconstriction in the human respiratory cycle. In healthy
adults alternately exposed to hypoxia or hyperoxia (to dilate or constrict arteries),
binding of NO to hemes (FeNO) and thiols (SNO) of Hb varies as a function of
HbO 2 saturation (FeO 2 ). RBC/SNO-mediated vasodilator activity is inversely propor-
tional to FeO 2 over a wide range, whereas RBC-induced vasoconstriction correlates