T‐ and L‐type Ca2+ Currents in Freshly Dispersed Smooth Muscle Cells from the Human Proximal Urethra

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Abstract
The purpose of the present study was to characterise Ca2+ currents in smooth muscle cells isolated from biopsy samples taken from the proximal urethra of patients undergoing surgery for bladder or prostate cancer. Cells were studied at 37 °C using the amphotericin B perforated-patch configuration of the patch-clamp technique. Currents were recorded using Cs+-rich pipette solutions to block K+ currents. Two components of current, with electrophysiological and pharmacological properties typical of T- and L-type Ca2+ currents, were present in these cells. When steady-state inactivation curves for the L current were fitted with a Boltzmann equation, this yielded a V1/2 of −45 ± 5 mV. In contrast, the T current inactivated with a V1/2 of −80 ± 3 mV. The L currents were reduced in a concentration-dependent manner by nifedipine (ED50 = 159 ± 54 nm) and Ni2+ (ED50 = 65 ± 16 μm) but were enhanced when external Ca2+ was substituted with Ba2+. The T current was little affected by TTX, reduction in external Na+, application of nifedipine at concentrations below 300 nm or substitution of external Ca2+ with Ba2+, but was reduced by Ni2+ with an ED50 of 6 ± 1 μm. When cells were stepped from −100 to −30 mV in Ca2+-free conditions, small inward currents could be detected. These were enhanced 40-fold in divalent-cation-free solution and blocked in a concentration-dependent manner by Mg2+ with an ED50 of 32 ± 16 μm. These data support the idea that human urethral myocytes possess currents with electrophysiological and pharmacological properties typical of T- and L-type Ca2+ currents.