Although a number of studies have characterised the ionic currents in urethral smooth muscle of animals (Cotton et al. 1997; Hollywood et al. 2000), little is known about the electrophysiology of human lower urinary tract smooth muscle. In the present study, we have used the patch-clamp technique to characterise the currents in freshly dispersed human smooth muscle cells.

Smooth muscle cells were isolated from biopsy samples taken from the proximal 1 cm of the bladder neck in seven male and two female patients (age 51 ± 7 years, mean ± S.E.M., range 19-79 years) who gave written, informed consent. This study was approved by the Queen’s University Ethical Committee. Cells were perfused with Hanks’ solution containing 1.8 mM Ca²⁺ at 37 °C and studied using the amphotericin B perforated-patch technique with Cs⁺-rich pipette solution as described previously (Cotton et al. 1997). When cells were held at -100 mV and depolarised in 10 mV steps for 500 ms, inward currents were evoked at potentials positive to -70 mV. The current-voltage (I-V) relationship consisted of two peaks: one at ~-40 mV and the other at 0 mV. Separation of these was achieved by stepping from holding potentials of -100 or -60 mV. The I-V curve obtained at a holding potential of -60 mV showed little inward current at potentials negative to -40 mV. Subtraction of the currents obtained at -60 from those at -100 mV revealed a current that activated at potentials positive to -70 mV, peaked at -30 mV and reversed at potentials near +20 mV. A series of 2 s preconditioning potentials from -110 to 0 mV was applied before stepping to either -30 or 0 mV to examine the voltage dependence of inactivation of both currents. The current evoked by a step to -30 mV half-maximally inactivated at -80 ± 3 mV (n = 8) compared with -45 ± 5 mV (n = 6) for the current evoked by a step to 0 mV. Ni²⁺-reduced currents evoked at potentials negative to -20 mV in a concentration-dependent manner (EC₅₀ = 14 ± 5 µM, n = 6) but had little effect on currents at 0 mV. In four cells, 100 µM Ni²⁺ reduced the current evoked by a step to -40 mV from -62 ± 15 to -9 ± 5 pA. In contrast the current evoked by a step to 0 mV was only reduced from -253 ± 68 to -221 ± 73 pA in the presence of 100 µM Ni²⁺.

These results suggest that human bladder neck myocytes possess both low- and high-voltage-activated currents similar to T- and L-type calcium currents found in other smooth muscle cells.This work was supported by Action Research.

Cotton, K.D., Hollywood, M.A., McHale, N.G. & Thornbury, K.D. (1997). J. Physiol. 505, 121-131. abstract

Hollywood, M.A., McCloskey, K.D., McHale, N.G. & Thornbury, K.D. (2000). Am. J. Physiol. 279, C420-428.