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

We identified and characterized the membrane currents underlying spontaneous transient depolarization in the urethra.

Myocytes were isolated from sheep urethra by enzymatic digestion and studied by the amphotericin B patch clamp method.

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.

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.