THE EXPRESSION OF FUNCTIONAL POSTSYNAPTIC ALPHA(2)-ADRENOCEPTORS IN THE CORPUS CAVERNOSUM SMOOTH-MUSCLE

1 The purpose of this study was to determine if corpus cavernosum smoo th muscle expresses functional postsynaptic alpha(2)-adrenoceptors (AR ). 2 The alpha(2)-adrenoceptor agonist UK 14,304 elicited concentratio n-dependent contractions in rabbit corpus cavernosum smooth muscle (CC SM). The half-maximal response occurred at 0.32 +/- 0.03 mu M and the maximum contraction at 10 mu M UK 14,304. 3 Pretreatment of CCSM strip s with selective alpha(2)-adrenoceptor antagonists, rauwolscine and RS -15385, produced rightward shifts in the dose-response curves to UK 14 ,304 (pA(2) values 7.1 and 8.5, respectively). In contrast, these anta gonists did not alter contraction induced by the alpha(1)-adrenoceptor agonist phenylephrine (PE) or oxymetazoline. UK 14,304-induced contra ctions were also inhibited by prazosin (pA(2) = 9.08). 4 UK 14,304-ind uced contractions, unlike those to PE, were highly dependent on the pr esence of extracellular Ca2+. 5 [H-3]-rauwolscine bound to CCSM membra nes with high affinity (K-d = 1.5 nM). [H-3]-rauwolscine binding was d isplaced by unlabelled rauwolscine, RS-15385, UK 14,304 and prazosin, but not by PE. 6 UK 14,304 inhibited forskolin and prostaglandin E-1 ( PGE(1))-induced increases in intracellular cyclic AMP concentration in primary cultures of rabbit CCSM cells. 7 These results demonstrate th at CCSM expresses G(i)-coupled postsynaptic alpha(2)-adrenoceptors, an d activation of these receptors causes contraction of trabecular smoot h muscle.