KCa channel insensitivity to Ca2+ sparks underlies fractional uncoupling in newborn cerebral artery smooth muscle cells.
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 approximately 0.18, 0.43, and 0.26 Hz and KCa channel activity [number of KCa channels activated by Ca2+ sparksxopen probability of KCa channels at peak of Ca2+ sparks (NPo)] at the transient KCa current peak was approximately 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 approximately 1.6-fold. In contrast, caffeine (10 microM), 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, KCa channel insensitivity to Ca2+ sparks is a prominent factor underlying fractional Ca2+ spark uncoupling in newborn cerebral artery myocytes.
Li
Anlong
A
Department of Physiology, University of Tennessee Health Science Center, Memphis, TN 38163, USA.
Adebiyi
Adebowale
A
Leffler
Charles W
CW
Jaggar
Jonathan H
JH
D000818:Animals; D000831:Animals, Newborn; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D002450:Cell Communication / Q000187:drug effects* / Q000502:physiology; D002462:Cell Membrane / Q000502:physiology; D002536:Cerebral Arteries / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D004110:Diltiazem / Q000494:pharmacology; D004594:Electrophysiology; D008564:Membrane Potentials / Q000502:physiology; D009131:Muscle, Smooth, Vascular / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D024681:Potassium Channels, Calcium-Activated / Q000187:drug effects* / Q000502:physiology; D019837:Ryanodine Receptor Calcium Release Channel / Q000502:physiology; D013552:Swine
D002121:Calcium Channel Blockers; D024681:Potassium Channels, Calcium-Activated; D019837:Ryanodine Receptor Calcium Release Channel; D002110:Caffeine; D004110:Diltiazem; D002118:Calcium