Ca2+ signalling and Ca2+-activated K+ channels in smooth muscle.

In smooth muscle, transient subsarcolemma increases in Ca2+ of approximately 200 nM from the sarcoplasmic reticulum activate Ca2+-activated K+ channels (KCa) in the sarcolemma giving rise to spontaneous transient outward currents (STOCs). In the present study we have examined whether (1) STOCs are spatially restricted membrane currents, (2) single KCa channel activity is regulated by changes in bulk average cytosolic Ca2+ concentrations ([Ca2+]c) without concomitant local subsarcolemma Ca2+ changes, and (3) a relationship exists between the voltage-dependent Ca2+ current (ICa) and KCa channel activity. Guinea-pig single colonic myocytes were voltage clamped in the whole cell configuration (to measure macroscopic currents) and bulk average [Ca2+]c measured simultaneously using the dye Fura-2. Single channel activity was also recorded with a second electrode, on the same cell, in the cell-attached mode. KCa activity was identified by reversal potential and conductance measurements. If STOCs are not spatially restricted events but reflect increased KCa channel activity throughout the sarcolemma, the voltage-dependence of single KCa channels should be similar to that of STOCs. Prolonged depolarization (-60 mV to +50 mV) increased [Ca2+]c, the amplitude and frequency of STOCs, and single KCa channel activity. [Ca2+]c peaked around -20 mV. Between -50 and -20 mV, the increase in STOC frequency was markedly voltage-dependent (e-fold for 5 mV depolarization); beyond -20 mV less so. Single KCa channel activity increased about e-fold for a 20 mV depolarization and thus was demonstrably different in this respect from that of STOC activity, evidence consistent with the proposed spatially restricted nature of STOCs. Simultaneous depolarization (3 s) of both the whole cell and the membrane patch, from -70 to 0 mV, elevated [Ca2+]c to about 800 nM and evoked single KCa channel activity, the latter began after about 10 ms, peaked around 100 ms, then declined. On repolarization to -70 mV KCa channel activity ceased abruptly. Depolarization (to 0 mV) of the whole cell only, with the patch transmembrane potential maintained at -70 mV, increased [Ca2+]o to about 800 nM but, importantly, did not increase KCa channel activity. Conversely depolarization (to 0 mV) of the patch alone, the whole cell being maintained at -70 mV, did not alter the bulk [Ca2+]c but evoked single KCa channel activity. The time course of KCa channel activity was remarkably similar to that of ICa suggesting that Ca2+ influx through voltage-dependent Ca2+ channels may serve as a trigger for KCa channel activation. Together these results suggest that STOCs are spatially restricted membrane currents and that KCa channels are sensitive to both depolarization and local subsarcolemma Ca2+ increases but not to alterations in [Ca2+]c.