KCa channel insensitivity to Ca2+ sparks underlies fractional uncoupling in newborn cerebral artery smooth muscle cells

摘要

In smooth muscle cells, localized intracellular Ca²⁺ transients, termed “Ca²⁺ sparks,” activate several large-conductance Ca²⁺-activated K⁺ (KCa) channels, resulting in a transient KCa current. In some smooth muscle cell types, a significant proportion of Ca²⁺ sparks do not activate KCa channels. The goal of this study was to explore mechanisms that underlie fractional Ca²⁺ spark-KCa channel coupling. We investigated whether membrane depolarization or ryanodine-sensitive Ca²⁺ release (RyR) channel activation modulates coupling in newborn (1- to 3-day-old) porcine cerebral artery myocytes. At steady membrane potentials of −40, 0, and +40 mV, mean transient KCa current frequency was ∼0.18, 0.43, and 0.26 Hz and KCa channel activity [number of KCa channels activated by Ca²⁺ sparks × open probability of KCa channels at peak of Ca²⁺ sparks (NPo)] at the transient KCa current peak was ∼4, 12, and 24, respectively. Depolarization between −40 and +40 mV increased KCa channel sensitivity to Ca²⁺ sparks and elevated the percentage of Ca²⁺ sparks that activated a transient KCa current from 59 to 86%. In a Ca²⁺-free bath solution or in diltiazem, a voltage-dependent Ca²⁺ channel blocker, steady membrane depolarization between −40 and +40 mV increased transient KCa current frequency up to ∼1.6-fold. In contrast, caffeine (10 μM), an RyR channel activator, increased mean transient KCa current frequency but did not alter Ca²⁺ spark-KCa channel coupling. These data indicate that coupling is increased by mechanisms that elevate KCa channel sensitivity to Ca²⁺ sparks, but not by RyR channel activation. Overall, K_Ca channel insensitivity to Ca²⁺ sparks is a prominent factor underlying fractional Ca²⁺ spark uncoupling in newborn cerebral artery myocytes.