Differential effects of sex hormones and phytoestrogens on peak and steady state contractions in isolated rabbit detrusor.

Abstract

Recent evidence suggests that sex steroids may produce rapid inhibition of voltage operated Ca2+ channels (VOCCs). Detrusor smooth muscle is highly dependent upon Ca2+ influx for receptor-activated contractions. Thus, we examined the relative effectiveness of a select group of sex steroids and dietary phytoestrogens to relax detrusor contracted with the muscarinic receptor agonist, bethanechol (BE) and the purinergic P2X receptor agonist, alpha,beta-methylene ATP (alpha,beta-MeATP). Isolated strips of rabbit detrusor were secured to isometric force transducers in a tissue bath and length-adjusted until maximum contractions were achieved. Peak (P) contractile responses were recorded for alpha,beta-MeATP (P(ATP)) and BE (P(BE)) and steady-state (SS) responses were recorded for BE (SS(BE)) in the presence and absence of selected sex steroids and phytoestrogens (10 microM, unless indicated). The L-type VOCC inhibitor, nifedipine (1 to 10 microM), completely inhibited P(ATP) but reduced SS(BE) by approximately 50%, whereas the VOCC and non-VOCC inhibitor, SKF 96365, inhibited SS(BE) by approximately 95%, suggesting that P(ATP) was entirely dependent on L-type VOCCs, but (BE)-induced contractions depended also on activation of non-VOCCs. 17Beta-estradiol (estradiol) and progesterone inhibited P(ATP) by approximately 60% and 20%, respectively, and 32 microM estradiol and ethinyl estradiol inhibited SS(BE) by approximately 80 and 95%, respectively. Inhibition by estradiol was potentiated, rather than blocked, by the nuclear estrogen receptor antagonist, tamoxifen. Moreover, tamoxifen alone nearly completely relaxed SS(BE). The inactive metabolite of estradiol, 17alpha-estradiol, inhibited both P(ATP) and P(BE) by approximately 40%. Testosterone had no effect on P(ATP) and P(BE). The phytoestrogen and tyrosine kinase inhibitor, genistein, inhibited SS(BE) by 44%, whereas daidzein, a phytoestrogen without tyrosine kinase inhibitory activity, produced only a 7% inhibition. None of the phytoestrogens examined inhibited P(BE), whereas all inhibited P(ATP) by approximately 20 to 35%. A comparison of inhibition of (BE) and alpha,beta-MeATP-induced contractions by selected estrogen isomers showed some distinct differences. For example, estrone did not inhibit P(BE) or SS(BE), but inhibited P(ATP) by approximately 20%, whereas DES inhibited SS(BE) by nearly 90%, but P(ATP) by a lesser degree (approximately 70%). Our data support the hypothesis that 17beta-estradiol, ethinyl estradiol, DES, tamoxifen and genistein may relax detrusor contractions by inhibition of both VOCCs and non-VOCCs. Moreover, our data show that genistein, a dietary phytoestrogen with tyrosine kinase inhibitory activity, selectively reduced alpha,beta-MeATP-induced peak and BE-induced steady-state contractions, sparing the maximum response to BE. Lastly, the inactive isomer, 17alpha-estradiol, inhibited both BE- and alpha,beta-MeATP-induced contractions. These data suggest that certain dietary phytoestrogens (for example, genistein) or sex steroids, especially those with weak activity at the nuclear steroid site (for example, 17alpha-estradiol), or tamoxifen may prove therapeutically useful in treating overactive bladder caused by elevated muscarinic and purinergic receptor activation.