Underlying mechanisms involved in progesterone-induced relaxation to the pig bladder neck

Progesterone increases bladder capacity and improves the bladder compliance by its relaxant action on the detrusor. A poor information, however, exists concerning to the role of this steroid hormone on the bladder outflow region contractility. This study investigates the progesterone-induced action on the smooth muscle tension of the pig bladder neck. To this aim, urothelium-denuded bladder neck strips were mounted in myographs for isometric force recordings and for simultaneous measurements of intracellular Ca2+ concentration ([Ca2+]i) and tension. On phenylephrine (PhE)-precontracted strips, progesterone produced concentration-dependent relaxations only at high pharmacological concentrations. The blockade of progesterone receptors, nitric oxide (NO) synthase, guanylyl cyclase, large conductance Ca2+-activated K+ (BKCa) or ATP-dependent K+ (KATP) channels reduced the progesterone relaxations. The presence of the urothelium and the inhibition of cyclooxygenase (COX), intermediate- and small-conductance Ca2+-activated K+ channels failed to modify these responses. In Ca2+-free potassium rich physiological saline solution, progesterone inhibited the contraction to CaCl2 and to the L-type voltage-operated Ca2+ (VOC) channel activator BAY-K 8644. Relaxation induced by progesterone was accompanied by simultaneous decreases in smooth muscle [Ca2+]i. These results suggest that progesterone promotes relaxation of pig bladder neck through smooth muscle progesterone receptors via cGMP/NO pathway and involving the activation of BKCa and KATP channels and inhibition of the extracellular Ca2+ entry through L-type VOC channels.