Multiple serotonin receptors on large arterioles in striated muscle

In rat striated muscle, serotonin (5-hydroxytryptamine; 5-HT) constricts large arterioles (first-order; A1) via 5-HT2 receptor activation but dilates smaller arterioles via a 5-HT1-like receptor. In this preparation, A1 arterioles possess little basal tone. The purpose of this study was to determine if 5-HT would elicit A1 dilation if arteriolar tone was first induced. In anesthetized rats, A1 diameters of the cremaster muscle were measured via in vivo videomicroscopy. Topical application of angiotensin II caused a 26 +/- 3% constriction of the vessels. 5-HT dilated the preconstricted A1s by 36 +/- 7% while constricting normal tone A1s by 33 +/- 5%. This dilation was enhanced by blocking 5-HT2 receptors with LY53857, but abolished with methysergide, a 5-HT1 and 5-HT2 receptor antagonist indicating that the dilation was mediated by a 5-HT1-like receptor. Thus, A1 arterioles possess both 5-HT2 and 5-HT1-like receptors. The net result of 5-HT application in striated muscle, dilation or constriction, will depend on the initial tone of the vessels.