Possible role of Ca2+ channels in the vasodilating effect of 5β-dihydrotestosterone in rat aorta

It has previously been shown that the androgen, 5 beta-dihydrotestosterone (17 beta-hydroxy-5 beta P-androstan-3-one, 5 beta-DHT), is able to produce an endothelium-independent vasodilating effect in rat aorta. The present study analyzed the mechanisms underlying the above vasodilator effect of 5 beta-dihydrotestosterone, with particular emphasis on verifying a possible interaction with GABA(A) receptors, beta-adrenoceptors and Ca2+ channels. Rat aortic rings without endothelium were isometrically recorded. 5 beta-Dihydrotestosterone produced a concentration-dependent relaxation on the contractions induced by noradrenaline (NA; 0.3 mu M) or K+ (KCI; 60 mM), with the latter being more sensitive to 5 beta-dihydrotestosterone-induced relaxation than the former; the concentration-response curves showed that 5 beta-dihydrotestosterone is significantly more potent than 17 beta-estradiol (1,3,5(10)-estratrien-5,17 beta-diol) to induce vasodilatation. The vasodilating effect of 5 beta-dihydrotestosterone on noradrenaline-induced contraction was resistant to blockade by the GABA(A) receptor antagonists, picrotoxin or bicuculline, and the beta-adrenoceptor antagonist, propranolol, a finding that excludes an interaction of the steroid with GABA(A) receptors and beta-adrenoceptors. Interestingly, the contractions evoked by calcium in depolarized tissues were substantially inhibited by 5 beta-dihydrotestosterone, implying that this steroid could be an endogenous calcium channel blocker; consistent with this finding, 5 beta-dihydrotestosterone was able to relax tissues precontracted with the calcium channel opener, Bay K 8644. Moreover, although the rings precontracted with noradrenaline and potassium were almost equipotently relaxed by 5 beta-dihydrotestosterone. Nifedipine was more potent than 5 beta-dihydrotestosterone to block the potassium-induced contraction, but the steroid was more effective than nifedipine to prevent noradrenaline-induced contraction. The above results suggest that 5 beta-dihydrotestosterone causes relaxation of rat aorta by acting directly on the membrane of smooth muscle cells; this non-genomic action may be explained in terms of a blockade of voltage- and receptor-dependent calcium channels, a mechanism that restricts the availability of extracellular calcium in the contractile machinery. (C) 1999 Elsevier Science B.V. All rights reserved.