The smooth muscle of the urethra is capable of playing an important role in continence by generating sufficient tone to prevent voiding of urine from the bladder (Bridgewater et al. 1993). This tone appears to be myogenic, at least in part, but the mechanism of its generation and maintenance are poorly understood. We have learned a lot in recent years, using the patch-clamp technique, about the conductances underlying excitation in isolated smooth muscle cells dispersed from sheep and rabbit urethra (Cotton et al. 1997; Hollywood et al. 2000) but this technique does not allow us to assess the relevance of any given conductance to the function of the syncitium. In the present study we have used intracellular microelectrodes to record the effects of some ion channel blockers on spontaneous action potentials in segments of rabbit urethra.
The bladder and urethra were removed from both male and female rabbits immediately after they had been killed by lethal injection of pentobarbitone. The most proximal 3 cm of the urethra was removed and placed in Krebs solution. This was then opened up, the urothelium removed and the preparation pinned out on a silicon rubber base and superfused with Krebs solution at 35-37 °C. Smooth muscle cells were impaled with glass microelectrodes filled with 3 M KCl (80-120 MV). Resting membrane potential varied from -40 to -65 mV (mean ± S.E.M., 49.3 ± 2.28, n = 29) Approximately 90 % of the preparations showed spontaneous electrical activity. Three different types of spontaneous activity were observed: (1) slow waves consisting of a small initial spike followed by a large plateau (of duration at half-amplitude 0.99 ± 0.16 s-1, n = 4); (2) spike complexes consisting of between 1 and 12 (mean 4.48 ± 0.68, n = 19) rapid spikes superimposed on a small plateau (15-20 mV, mean 12.5 ± 1.54 mV); and (3) continuous firing of single rapid spikes.
The effects of Penitrem A (a specific blocker of the BKCa channel) on the above activity were examined. Penitrem increased the frequency of firing of spike complexes (from 4.7 ± 3.47 to 9.9 ± 5.9 min-1), increased the maximum spike amplitude (from 34.2 ± 3.8 to 55.9 ± 6.15 mV) and decreased the duration at half amplitude of the plateau (from 1.5 ± 0.34 to 0.48 ± 0.087 s-1). The other obvious effect of Penitrem A was the blockade of a rapid hyperpolarisation following each individual spike.
We conclude that calcium-activated potassium current in rabbit urethra plays an important role in repolarisation of the action potential and that its activation is sufficiently rapid to limit the spike amplitude under normal conditions.
This work was supported by the Northern Ireland Department of Education and Learning.