CONTROL OF CORONARY VASCULAR TONE BY NITRIC-OXIDE

A specific difference-spectrophotometric method was used to measure nitric oxide (NO) release into the coronary effluent perfusate of isolated, constant-flow-perfused guinea pig hearts. Authentic NO applied into the coronary release of cyclic GMP (cGMP) fivefold. Increasing oxygen tension in aqueous solutions from 150 to 700 mm Hg decreased NO half-life (5.6 seconds) by 32%. During single passage through the intact coronary system, 86% of the infused NO was converted to nitric ions. Oxidation of NO was more than 30 times faster within the heart than in aqueous solution. Endogenously formed NO was constantly released into the coronary effluent perfusate at a rate of 161 ± 11 pmol/min. The NO scavenger oxyhemoglobin and methylene blue increased coronary resistance and decreased cGMP release (basal release, 342 ± 4 fmol/min), whereas superoxide dismutase reduced coronary resistance. L-Arginine (10-5 M) slightly decreased coronary perfusion pressure and enhanced release of cGMP. N(G)-Monomethyl L-arginine (10-4 M) reduced basal release of NO and cGMP by 26% and 31%, respectively, paralleled by a coronary vasoconstriction. Bradykinin in the physiological range from 5 x 10-11 M to 10-7 M dilated coronary resistance vessels, which was paralleled by the release of NO and cGMP. Onset of NO release preceded onset of coronary vasodilation in all cases. Upon stimulation with bradykinin, amounts of endogenously formed NO were within the same range as the dose-response curves for exogenously applied NO both for changes in coronary resistance and cGMP release. Acetylcholine (10-5 M), ATP (10-5 M), and serotonin (10-8 M) increased the rate of NO and cGMP release, resulting in coronary vasodilation. Our data suggest the following: 1) NO, the most rapidly acting vasodilator presently known, is metabolized within the heart mainly to nitrite and exhibits a half-life of only 0.1 second; 2) in the unstimulated heart, basal formation of NO may play an important role in setting the resting tone of coronary resistance vessels; 3) the kinetics and quantities of NO formation suggest that NO is causally involved in the bradykinin-induced coronary vasodilation; and 4) amounts of NO formed within the heart stimulated with ATP, acetylcholine, and serotonin are effective for vasodilation.