VASODILATION INDUCED BY ACETYLCHOLINE AND BY GLYCERYL TRINITRATE IN RAT AORTIC AND MESENTERIC VASCULATURE

In endothelium-containing rings of rat aorta which had been precontracted with phenylephrine, addition of acetylcholine (ACh) (0.010-10-mu-M) resulted in concentration-dependent, graded relaxation through the release of endothelium-derived relaxing factor (EDRF). Hemoglobin (3 and 10-mu-M) and methylene blue (10-mu-M) both produced marked inhibition of this EDRF-mediated relaxation. In the perfused mesenteric arterial vasculature of the rat, ACh-induced vasodilation was also inhibited by hemoglobin and by methylene blue, although to a lesser extent than was ACh-induced relaxation of aortic rings by these two agents. These findings indicate that EDRF mediates in large part ACh-induced relaxation of resistance vessels in the mesenteric vascular bed as well as large arteries. The nitrovasodilator glyceryl trinitrate (GTN) caused endothelium-independent relaxation of aortic rings as well as vasodilation of mesenteric arterial vasculature. GTN-induced relaxation of aortic rings was antagonized by hemoglobin as well as methylene blue, but to a lesser extent than was ACh-induced relaxation. However, hemoglobin did not inhibit and methylene blue actually potentiated GTN-induced vasodilation in the perfused mesenteric vasculature. Possible explanations of these paradoxical results are discussed.