DIFFERENTIAL EFFECTS OF SEX HORMONES AND PHYTOESTROGENS ON PEAK AND STEADY STATE CONTRACTIONS IN ISOLATED RABBIT DETRUSOR

Purpose: Recent evidence suggests that sex steroids may produce rapid inhibition of voltage operated Ca 2+ channels (VOCCs). Detrusor smooth muscle is highly dependent upon Ca 2+ 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, α,β-methylene ATP (α,β-MeATP). Materials and Methods: 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 α,β-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 μM, unless indicated). Results: The L-type VOCC inhibitor, nifedipine (1 to 10 μM), completely inhibited P ATP but reduced SS BE by ∼50%, whereas the VOCC and non-VOCC inhibitor, SKF 96365, inhibited SS BE by ∼95%, suggesting that P ATP was entirely dependent on L-type VOCCs, but BE-induced contractions depended also on activation of non-VOCCs. 17β-estradiol (estradiol) and progesterone inhibited P ATP by ∼60% and 20%, respectively, and 32 μM estradiol and ethinyl estradiol inhibited SS BE by ∼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, 17α-estradiol, inhibited both P ATP and P BE by ∼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 ∼20 to 35%. A comparison of inhibition of BE and α,β-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 ∼20%, whereas DES inhibited SS BE by nearly 90%, but P ATP by a lesser degree (∼70%). Conclusions: Our data support the hypothesis that 17β-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 α,β-MeATP-induced peak and BE-induced steady-state contractions, sparing the maximum response to BE. Lastly, the inactive isomer, 17α-estradiol, inhibited both BE- and α,β-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, 17α-estradiol), or tamoxifen may prove therapeutically useful in treating overactive bladder caused by elevated muscarinic and purinergic receptor activation.