REGIONAL DIFFERENCES IN ENDOTHELIAL FUNCTION IN HORSE LUNGS - POSSIBLE ROLE IN BLOOD-FLOW DISTRIBUTION

We investigated regional differences of in vitro responses of pulmonar y arteries (6-mm OD) from the dorsocaudal (top) and cranioventral (bot tom) lung regions to endothelium-dependent vasodilators (methacholine, bradykinin, and calcium ionophore A-23187). Methacholine relaxed endo thelium-intact top vessels; however, in bottom vessels, a small relaxa tion preceded a profound contraction. In top vessels, removal of endot helial cells converted relaxation to contraction, and in bottom vessel s it abolished relaxation and enhanced contraction. Bradykinin and A-2 3187 were more potent and caused greater endothelium-mediated relaxati on in top than in bottom arteries. The endothelium-independent vasodil ator sodium nitroprusside caused similar relaxations in all rings. N-G -nitro-L-arginine and N-G-monomethyl-Larginine and methylene blue abol ished relaxation of top and bottom arteries to methacholine; meclofena mate had little effect. We conclude that regional differences in endot helium-mediated relaxation are caused by differences in the magnitude of the endothelial release of nitric oxide. Similar differences in end othelium-dependent flow-mediated vasodilation and endothelial nitric o xide release may result in preferential perfusion of caudodorsal lung regions.