The expression of functional postsynaptic α2-adrenoreceptors in the corpus cavernosum smooth muscle

Abstract

1. The purpose of this study was to determine if corpus cavernosum smooth muscle expresses functional postsynaptic α2-adrenoceptors (AR). 2. The α2-adrenoceptor agonist UK 14,304 elicited concentration-dependent contractions in rabbit corpus cavernosum smooth muscle (CCSM). The half-maximal response occurred at 0.32 ± 0.03 μM and the maximum contraction at 10 μM UK 14,304. 3. Pretreatment of CCSM strips with selective α2-adrenoceptor antagonists, rauwolscine and RS-15385, produced rightward shifts in the dose-response curves to UK 14,304 (pA2 values 7.1 and 8.5, respectively). In contrast, these antagonists did not alter contraction induced by the α1-adrenoceptor agonist phenylephrine (PE) or oxymetazoline. UK 14,304-induced contractions were also inhibited by prazosin (pA2 = 9.08). 4. UK 14,304-induced contractions, unlike those to PE, were highly dependent on the presence of extracellular Ca2+. 5. [3H]-rauwolscine bound to CCSM membranes with high affinity (K(d) = 1.5 nM). [3H]-rauwolscine binding was displaced by unlabelled rauwolscine, RS-15385, UK 14,304 and prazosin, but not by PE. 6. UK 14,304 inhibited forskolin and prostaglandin E1 (PGE1)-induced increases in intracellular cyclic AMP concentration in primary cultures of rabbit CCSM cells. 7. These results demonstrate that CCSM expresses G(i)-coupled postsynaptic α2-adrenoceptors, and activation of these receptors causes contraction of trabecular smooth muscle.