Ca2+ sparks activate K+ and Cl- channels, resulting in spontaneous transient currents in guinea-pig tracheal myocytes

1. Local changes in cytosolic [Ca2+] were imaged with a wide-field, high-speed, digital imaging system while membrane currents were simultaneously recorded using whole-cell, perforated patch recording in freshly dissociated guinea-pig tracheal myocytes. 2. Depending on membrane potential, Ca2+ sparks triggered 'spontaneous' transient inward currents (STICs), 'spontaneous' transient outward currents (STOCs) and biphasic currents in which the outward phase always preceded the inward (STOICs). The outward currents resulted from the opening of large-conductance Ca2+-activated K+ (BK) channels and the inward currents from Ca2+-activated Cl- (ClCa) channels. 3. A single Ca2+ spark elicited both phases of a STOIC, and sparks originating from the same site triggered STOCs, STICs and STOICs, depending on membrane potential. 4. STOCs had a shorter time to peak (TTP) than Ca2+ sparks and a much shorter half-time of decay. In contrast, STICs had a somewhat longer TTP than sparks but the same half-time of decay. Thus, the STIC, not the STOC, more closely reflected the time course of cytosolic Ca2+ elevation during a Ca2+ spark. 5. These findings suggest that ClCa channels and BK channels may be organized spatially in quite different ways in relation to points of Ca2+ release from intracellular Ca2+ stores. The results also suggest that Ca2+ sparks may have functions in smooth muscle not previously suggested, such as a stabilizing effect on membrane potential and hence on the contractile state of the cell, or as activators of voltage-gated Ca2+ channels due to depolarization mediated by STICs.