The mechanisms of α2-adrenoceptor agonist-induced contraction in longitudinal muscle of the porcine uterus

The aim of the present study was to clarify the cellular mechanisms underlying the alpha(2)-adrenoceptor-mediated contraction of porcine myometrium (nonvascular smooth muscle). Acetylcholine (3 nM-1 mu M), clonidine (1 nM-10 mu M) and 5-bromo-N-[2-imidazolin-2-yl]-6-quinoxalinamine (UK14304) (1 nM-10 mu M) in Krebs solution caused a concentration-dependent contraction in the longitudinal muscles of the porcine uterus with similar EC50 values and maximum responses. A lowered external Ca2+ concentration and verapamil (10 nM-10 mu M) decreased the contractile response to clonidine and UK14304 more markedly than the response to acetylcholine. However, in Kumagai solution, neither clonidine nor UK14304 caused contractile responses, but acetylcholine remained effective. The effects of alpha(2)-adrenoceptor agonists on intracellular Ca2+ concentration ([Ca2+](i)) and smooth muscle force were measured simultaneously using fura-PE3-loaded muscle preparations. Clonidine and UK14304 caused increases in [Ca2+](i) and force of the longitudinal muscle. The increases in [Ca2+](i) and muscle force were markedly inhibited by verapamil and in Ca2+-free solution (EGTA, 1 mM). In the absence of external Ca2+, clonidine caused only a small increase in [Ca-2+](i) in Ca2+-loaded preparations compared with those increases caused by carbachol, histamine, and oxytocin. Ca2+ (2.5 mM) caused increases in [Ca2+](i) and force of the longitudinal muscles in a Ca2+-free high K+ solution. Clonidine concentration dependently potentiated the Ca2+-induced contraction without significantly changing the increase in [Ca2+](i), and this potentiation was inhibited by yohimbine. These results suggested that clonidine increases the Ca2+ sensitivity of the contractile elements through activation of alpha(2)-adrenoceptors. During the development of the contractile response to clonidine(1 mu M, 0-5 min), tissue cyclic AMP levels did not change significantly. In vitro treatment with pertussis toxin(1 mu g/ml for 2 h) significantly decreased the contraction induced by clonidine without affecting the responses to carbachol and high K+. The present results indicate that in porcine myometrium, alpha(2)-adrenoceptor stimulation caused contraction of the longitudinal muscles by mechanisms largely dependent on the influx of extracellular Ca2+, probably through voltage-dependent Ca2+ channels (VDCCs), and that the potentiation of the Ca2+ sensitivity of the contractile elements is another mechanism of the contractile responses. These actions involve a pertussis-toxin-sensitive G protein (probably G(i) type) in the signal transduction pathway. (C) 2000 Elsevier Science B.V. All rights reserved.