T‐ and L‐type Ca 2+ Currents in Freshly Dispersed Smooth Muscle Cells from the Human Proximal Urethra

The purpose of the present study was to characterise Ca 2+ 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 Ca 2+ currents, were present in these cells. When steady‐state inactivation curves for the L current were fitted with a Boltzmann equation, this yielded a V 1/2 of −45 ± 5 mV. In contrast, the T current inactivated with a V 1/2 of −80 ± 3 mV. The L currents were reduced in a concentration‐dependent manner by nifedipine (ED 50 = 159 ± 54 n m ) and Ni 2+ (ED 50 = 65 ± 16 μ m ) but were enhanced when external Ca 2+ was substituted with Ba 2+ . The T current was little affected by TTX, reduction in external Na + , application of nifedipine at concentrations below 300 n m or substitution of external Ca 2+ with Ba 2+ , but was reduced by Ni 2+ with an ED 50 of 6 ± 1 μ m . When cells were stepped from −100 to −30 mV in Ca 2+ ‐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 Mg 2+ with an ED 50 of 32 ± 16 μ m . These data support the idea that human urethral myocytes possess currents with electrophysiological and pharmacological properties typical of T‐ and L‐type Ca 2+ currents.