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MEMBRANE TRANSPORTERS, ION CHANNELS, AND PUMPS

The vitamin D receptor agonist elocalcitol upregulates L-type calcium channel activity in human and rat bladder

Department of ¹Clinical Physiopathology, ²Physiological Sciences, ³Pharmacology, ⁴Urology, University of Florence, Florence; ⁵Bioxell, Milan; and ⁶National Institute of Biostructures and Biosystems, Rome, Italy

Human bladder contraction mainly depends on Ca²⁺ influx via L-type voltage-gated Ca²⁺ channels and on RhoA/Rho kinase contractile signaling, which is upregulated in overactive bladder (OAB). Elocalcitol is a vitamin D receptor agonist inhibiting RhoA/Rho kinase signaling in rat and human bladder. Since in the normal bladder from Sprague-Dawley rats elocalcitol treatment delayed the carbachol-induced contraction without changing maximal responsiveness and increased sensitivity to the L-type Ca²⁺ channel antagonist isradipine, we investigated whether elocalcitol upregulated L-type Ca²⁺ channels in human bladder smooth muscle cells (hBCs). In hBCs, elocalcitol induced a rapid increase in intracellular [Ca²⁺], which was abrogated by the L-type Ca²⁺ channel antagonist verapamil. Moreover, hBCs exhibited L-type voltage-activated Ca²⁺ currents (I_Ca), which were selectively blocked by isradipine and verapamil and enhanced by the selective L-type agonist BAY K 8644. Addition of elocalcitol (10^–7 M) increased L-type I_Ca size and specific conductance by inducing faster activation and inactivation kinetics than control and BAY K 8644, while determining a significant negative shift of the activation and inactivation curves, comparable to BAY K 8644. These effects were strengthened in long-term treated hBCs with elocalcitol (10^–8 M, 48 h), which also showed increased mRNA and protein expression of pore-forming L-type _1C-subunit. In the bladder from Sprague-Dawley rats, BAY K 8644 induced a dose-dependent increase in tension, which was significantly enhanced by elocalcitol treatment (30 µg·kg^–1·day^–1, 2 wk). In conclusion, elocalcitol upregulated Ca²⁺ entry through L-type Ca²⁺ channels in hBCs, thus balancing its inhibitory effect on RhoA/Rho kinase signaling and suggesting its possible efficacy for the modulation of bladder contractile mechanisms.

vitamin D analogue; human bladder smooth muscle cells; voltage-gated calcium channel; overactive bladder

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