Endogenous prostaglandins selectively mask large arteriole constriction to angiotensin II.
Television microscopy was used to observe the responses of in vivo arterioles and venules of the rat cremaster muscle to the topical application of angiotensin II (10(-8) and 10(-6) M). Neither the first- (A1) or second-order arterioles (A2) nor the first- (V1) or second-order venules (V2) constricted significantly to angiotensin II. However, after the inhibition of local prostaglandin synthesis with either mefenamic acid or indomethacin, both A1 and A2, but not the venules, gave a significant constrictor response to angiotensin II (10(-6) M). Arterioles and venules, which were preconstricted with norepinephrine, dilated to their initial baseline diameters after angiotensin II (10(-6) M), a response not observed when the microvessels were pretreated with either an angiotensin antagonist or a prostaglandin synthesis inhibitor. These observations indicate that endogenous prostaglandins exert a significant dilator influence on the larger arterioles, that this dilator influence appears to oppose the constrictor effect of angiotensin II, and that angiotensin II acts on specific receptors to induce synthesis and/or release of dilator prostaglandins in large arterioles. However, prostaglandins cannot account for the absence of a venular constriction to angiotensin.