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

摘要

class="enumerated" style="list-style-type:decimal">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.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.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.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.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.