Differential effects of sex hormones and phytoestrogens on peak and steadystate contractions in isolated rabbit detrusor

Purpose: Recent evidence suggests that sex steroids may produce rapid inhib ition 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). Materials and Methods: Isolated strips of rabbit detrusor were secured to i sometric force transducers in a tissue bath and length-adjusted until maxim um 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 (SSBE) in the presence and absence of selected sex st eroids and phytoestrogens (10 mu M, unless indicated). Results: The L-type VOCC inhibitor, nifedipine (1 to 10 mu M), completely i nhibited P-ATP but reduced SSBE by similar to 50%, whereas the VOCC and non -VOCC inhibitor, SKF 96365, inhibited SSBE by similar to 95%, suggesting th at P-ATP was entirely dependent on L-type VOCCs, but BE-induced contraction s depended also on activation of non-VOCCs. 17 beta-estradiol (estradiol) a nd progesterone inhibited P-ATP by similar to 60% and 20%, respectively, an d 32 mu M estradiol and ethinyl estradiol inhibited SSB, by similar to 80 a nd 95%, respectively. Inhibition by estradiol was potentiated, rather than blocked, by the nuclear estrogen receptor antagonist, tamoxifen. Moreover, tamoxifen alone nearly completely relaxed SSB,. The inactive metabolite of estradiol, 17 alpha-estradiol, inhibited both P-ATP and P-BE by similar to 40%. Testosterone had no effect on P-ATP and P-BE. The phytoestrogen and ty rosine kinase inhibitor, genistein, inhibited SSBE by 44%, whereas daidzein , a phytoestrogen without tyrosine kinase inhibitory activity, produced onl y a 7% inhibition. None of the phytoestrogens examined inhibited P-BE, wher eas all inhibited P-ATP by similar to 20 to 35%. A comparison of inhibition of BE and alpha,beta-MeATP-induced contractions by selected estrogen isome rs showed some distinct differences. For example, estrone did not inhibit P -BE Or SSBE, but inhibited P-ATP by similar to 20%, whereas DES inhibited S SBE by nearly 90%, but P-ATP by a lesser degree (similar to 70%). Conclusions: Our data support the hypothesis that 17 beta-estradiol, ethiny l estradiol, DES, tamoxifen and genistein may relax detrusor contractions b y inhibition of both VOCCs and non-VOCCs. Moreover, our data show that geni stein, a dietary phytoestrogen with tyrosine kinase inhibitory activity, se lectively reduced alpha,beta-MeATP-induced peak and BE-induced steady-state contractions, sparing the maximum response to BE. Lastly, the inactive iso mer, 17 alpha-estradiol, inhibited both BE- and alpha,beta-MeATP-induced co ntractions. These data suggest that certain dietary phytoestrogens (for exa mple, genistein) or sex steroids, especially those with weak activity at th e nuclear steroid site (for example, 17 alpha-estradiol), or tamoxifen may prove therapeutically useful in treating overactive bladder caused by eleva ted muscarinic and purinergic receptor activation.