Frequency modulation of Ca2+sparks is involved in regulation of arterial diameter by cyclic nucleotides

Abstract

Forskolin, which elevates cAMP levels, and sodium nitroprusside (SNP) and nicorandil, which elevate cGMP levels, increased, by two- to threefold, the frequency of subcellular Ca²⁺release (“Ca²⁺sparks”) through ryanodine-sensitive Ca²⁺release (RyR) channels in the sarcoplasmic reticulum (SR) of myocytes isolated from cerebral and coronary arteries of rats. Forskolin, SNP, nicorandil, dibutyryl-cAMP, and adenosine increased the frequency of Ca²⁺-sensitive K⁺(K_Ca) currents [“spontaneous transient outward currents” (STOCs)] by two- to threefold, consistent with Ca²⁺sparks activating STOCs. These agents also increased the mean amplitude of STOCs by 1.3-fold, an effect that could be explained by activation of K_Cachannels, independent of effects on Ca²⁺sparks. To test the hypothesis that cAMP could act to dilate arteries through activation of the Ca²⁺spark→K_Cachannel pathway, the effects of blockers of K_Cachannels (iberiotoxin) and of Ca²⁺sparks (ryanodine) on forskolin-induced dilations of pressurized cerebral arteries were examined. Forskolin-induced dilations were partially inhibited by iberiotoxin and ryanodine (with no additive effects) and were entirely prevented by elevating external K⁺. Forskolin lowered average Ca²⁺in pressurized arteries while increasing ryanodine-sensitive, caffeine-induced Ca²⁺transients. These experiments suggest a new mechanism for cyclic nucleotide-mediated dilations through an increase in Ca²⁺spark frequency, caused by effects on SR Ca²⁺load and possibly on the RyR channel, which leads to increased STOC frequency, membrane potential hyperpolarization, closure of voltage-dependent Ca²⁺channels, decrease in arterial wall Ca²⁺, and, ultimately, vasodilation.