Hollywood et al. (2001) described the presence of a nickel-sensitive inward current in human proximal urethral smooth muscle, which had properties typical of T-currents found in other tissues. Recently, Koh et al. (2001) described a novel, voltage-sensitive inward current in murine colon that had kinetics and nickel sensitivity similar to T-currents but was blocked by external Ba²⁺ and almost abolished in 5 mM Na⁺. The aim of the present study was to determine whether the currents in human bladder neck smooth muscle cells possessed characteristics more typical of T-type calcium current or the voltage-dependent non-specific cation current described by Koh et al. (2001).

Smooth muscle cells were isolated from biopsy samples taken from the proximal 1 cm of the urethra in seven male and two female patients (age range 20-68 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. 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 currentÐvoltage (IÐV) relationship consisted of two peaks: one at ~-40 mV and the other at 0 mV. In the presence of 300 nM nifedipine, the current at 0 mV was reduced from -156 ± 37 to -38 ± 13 pA (mean ± S.E.M., n = 6, P < 0.05, paired t test) whereas the current at -40 mV was little affected (-47 ± 13 to -41 ± 11 pA, n = 6, P = 0.2). The nifedipine-insensitive current at -40 mV peaked within 15 ± 2 ms (n = 4) and inactivated at -50, -40 and -30 mV with time constants of 31 ± 4, 22 ± 3 and 16 ± 2 ms, respectively (n = 7).

The current at -30 mV was unaffected by TTX and not significantly reduced in 5 mM Na⁺ (-85 ± 4 to -66 ± 10 pA, P = 0.14, n = 5), suggesting that Na⁺ ions made very little contribution. When Ca²⁺ was replaced with Ba²⁺, the current at -30 mV was increased from -80 ± 16 to -91 ± 15 pA (P = 0.34, n = 5). Replacement of Ca²⁺-containing Hanks’ solution with 5 mM EGTA, Mg²⁺-substituted Hanks’ solution, reduced the peak current at -40 mV from -67 ± 12 to -33 ± 9 pA (P < 0.01, n = 6). When IÐV relationships were constructed before and after Ca²⁺ removal, currents activated at potentials positive to -70 mV, peaked at ~-10 mV and inactivated at negative potentials. When Mg²⁺ was removed from the calcium-free solution, the current was dramatically enhanced, suggesting that it represented the flow of monovalent cations through T-type calcium channels.

In conclusion, these results suggest that the negatively activating inward current in human bladder neck myocytes resembles T-type calcium current rather than a voltage-sensitive non-selective cation conductance.

This work was supported by The Wellcome Trust and Action Research.

All procedures accord with current UK legislation