Figure 7. Sparks activate Ca²⁺-sensitive membrane currents in smooth muscle.

A, activation of Ca²⁺-activated K⁺ channels by Ca²⁺ sparks results in spontaneous transient outward currents (STOCs), hyperpolarizing the plasma membrane (increased membrane potential; V_m). This reduces activation of voltage-operated Ca²⁺ channels (VOCCs), decreasing Ca²⁺ influx and global [Ca²⁺]. This can act as a negative feedback mechanism, whereby localized Ca²⁺ release from the sarcoplasmic reticulum (SR) lowers mean cytosolic [Ca²⁺], restricting further store loading. B, spark-dependent activation of Ca²⁺-activated Cl⁻ channels causes spontaneous transient inward currents (STICs), with membrane depolarization and Ca²⁺ channel activation. This results in positive feedback, with Ca²⁺ release from the SR promoting further Ca²⁺ influx.