Spontaneous Ca2+ activated Cl- currents in isolated urethral smooth muscle cells

PURPOSE: We identified and characterized the membrane currents underlying spontaneous transient depolarization in the urethra. MATERIALS AND METHODS: Myocytes were isolated from sheep urethra by enzymatic digestion and studied by the amphotericin B patch clamp method. RESULTS: Just more than 10% of cells had spontaneous transient inward currents when maintained at -60 mV. Mean amplitude plus or minus standard error of mean of the spontaneous transient inward currents was 102 +/- 35 pA. and mean frequency was 17 +/- 3 minutes-1 in 18 preparations. Within each cell currents sometimes consisted of up to 3 phases but in 16 of 18 cells monophasic spontaneous transient inward currents were also identified. These currents decayed relatively slowly with a mean time constant of 570 +/- 97 ms. Spontaneous transient inward currents were identified as Ca2+ activated Cl- currents because they reversed near the calculated Nernst potential for chloride ions. They were blocked by the Cl- channel blockers 100 microM. niflumic acid and 1 mM. anthracene-9-carboxylic acid as well as in Ca2+-free solution, 10 mM. caffeine and 30 microM. ryanodine. The latter results suggest that spontaneous transient inward currents require intact intracellular Ca2+ stores. Amplitude and frequency were unaffected by 10 microM. nifedipine but were reduced by the nonspecific Ca2+ entry blockers 10 microM. SKF 96365 and 1 mM. La3+. We interpret these results as indicating that the Ca2+ stores underlying the spontaneous transient inward currents may refill by plasmalemmal Ca2+ channels that differ from L-type channels. CONCLUSIONS: Urethral cells fire large spontaneous transient inward currents, mediated by Ca2+ activated Cl- channels, which are adequate to account for the spontaneous transient depolarizations seen in whole urethral tissue.