Interstitial cells of Cajal (ICC) have recently been reported to exist in the guinea-pig bladder (McCloskey & Gurney, 2002). To date, little is known of their physiological properties, although they are likely to be involved in controlling the activity of bladder smooth muscle. In the present study, the patch clamp technique was used to investigate outward currents in ICC from the guinea-pig detrusor.

Bladders were removed from guinea-pigs humanely killed by cervical dislocation. A heterogeneous population of cells was isolated as described previously (McCloskey & Gurney, 2002), the majority of which had a smooth muscle-like morphology. A subpopulation of branched cells were typical of the kit-positive ICC reported previously. These cells were selected for patch-clamp studies using K⁺-based pipette solution.

Application of depolarising pulses to cells voltage clamped at -80 mV resulted in the generation of outward currents. Penitrem A (100 nM) significantly inhibited the outward currents across the range of voltages tested; mean control current at +50 mV was reduced from 1922 ± 535 pA (mean ± S.E.M.) to 427 ± 70 pA (P = 0.038, n = 5, Student’s paired t test). The penitrem A-sensitive current was large and noisy and activated positive to -40 mV typical of large-conductance Ca²⁺-activated K⁺ (BK) currents. This current was also reduced by iberiotoxin (100 nM, n = 3). Nifedipine (1 µM) inhibited the current at +40 mV, from 1666 ± 463 pA to 1040 ± 274 pA (n = 5, P = 0.034) thereby demonstrating its Ca²⁺ dependence. When the BK current was blocked by penitrem A and nifedipine, a voltage-dependent current was unmasked which activated at potentials positive to -50 mV and displayed voltage-dependent inactivation (half-maximal inactivation, -71 ± 4 mV, n = 8). Examination of the shift in reversal potential of tail currents from -65 ± 2 mV (n = 6, actual E_K -83 mV) in 5.8 mM [K⁺]_o, to -27 ± 3 mV in 60 mM [K⁺]_o, (n = 3, actual E_K -21 mV) suggested that this current was largely carried by K⁺ ions. Tetraethylammonium ions inhibited the current in a concentration-dependent fashion (EC₅₀ 4.1 ± 0.5 mM, n = 5) but 4-AP (10 mM), charybdotoxin (100 nM) and apamin (50 nM) had no effect suggesting that SK, I_K, Kv1.2 and Kv1.3 channels are unlikely to be components.

In conclusion, bladder ICC have at least two components of outward K⁺ current. A major portion of the total outward K⁺ current was BK and a delayed rectifier current made up a significant component of the remainder. These may be involved in repolarization of the action potential and might contribute to the resting membrane potential or hyperpolarization of the cells.

This work was funded by the BBSRC.