Human bladder contraction mainly depends on Ca2+ influx, via L-type voltage-gated Ca2+ channels, and on RhoA/Rho-kinase contractile signalling, a pathway up-regulated in overactive bladder (OAB) syndrome. Elocalcitol (Elo) is a vitamin D receptor agonist inhibiting RhoA/Rho-kinase signalling in rat and human bladder, shown to ameliorate OAB symptoms in a clinical study. Since in normal bladder from SpragueDawley (SD) rats Elo treatment delayed the carbachol-induced contraction without changing maximal effect, we hypothesized, based on increased sensitivity to the selective L-type Ca2+ channel antagonist isradipine in bladder strips from Elo-treated rats, an up-regulation of L-type Ca2+ channels. Thus, this hypothesis was further investigated. In human bladder smooth muscle cells (hBCs), Elo induced a rapid increase in intracellular [Ca2+], which was abrogated by the specific L-type Ca2+ channel antagonist verapamil and was undetectable in the absence of extracellular Ca2+. Moreover, hBCs exhibited voltage-activated Ca2+ currents (ICa), T-type and L-type (ICa,L). Accordingly, both isradipine and verapamil only blocked the slow ICa,L, which was enhanced by the selective L-type agonist Bay K8644 (Bay). Addition of Elo (10−7 M) increased ICa,L size and specific conductance (Gm/Cm), by inducing faster activation and inactivation kinetics than control and Bay, and determined a significant negative shift of the activation (Va) and inactivation curves (Vh), as Bay. These effects resulted potentiated in long-term treated hBCs with Elo (10−8 M, 48 h), which also showed increased mRNA and protein expression of pore forming L-Type α1C subunit. In bladder strips from SD rats, Bay induced a dose-dependent increase in tension and its maximal contractile effect was significantly enhanced by Elo-treatment (30 μg/kg per day, 2 weeks). In conclusion, Elo upregulated Ca2+ entry through L-type Ca2+ channels in human bladder smooth muscle cells, thus balancing its inhibitory effect on RhoA/Rho-kinase signalling, and providing a mechanistic basis for using this drug in the treatment of OAB.
03 - 07 May 2008
European Society of Endocrinology