Ryanodine receptors regulate arterial diameter and wall [Ca2+] in cerebral arteries of rat via Ca2+-dependent K+ channels

1. The effects of inhibitors of ryanodine-sensitive calcium release (RyR) channels in the sarcoplasmic reticulum (SR) and Ca2+-dependent potassium (K(Ca)) channels on the membrane potential, intracellular [Ca2+], and diameters of small pressurized (60 mmHg) cerebral arteries (100-200 μm) were studied using digital fluorescence video imaging of arterial diameter and wall [Ca2+], combined with microelectrode measurements of arterial membrane potential. 2. Ryanodine (10 μM), an inhibitor of RyR channels, depolarized by 9 mV, increased intracellular [Ca2+] by 46 nM and constricted pressurized (to 60 mmHg) arteries with myogenic tone by 44 μm (~ 22%). Iberiotoxin (100 nM), a blocker of K(Ca) channels, under the same conditions, depolarized the arteries by 10 mV, increased arterial wall calcium by 51 nM, and constricted by 37 μm (~ 19%). The effects of ryanodine and iberiotoxin were not additive and were blocked by inhibitors of voltage-dependent Ca2+ channels. 3. Caffeine (10 mM), an activator of RyR channels, transiently increased arterial wall [Ca2+] by 136 ± 9 nM in control arteries and by 158 ± 12 nM in the presence of iberiotoxin. Caffeine was relatively ineffective in the presence of ryanodine, increasing [calcium] by 18 ± 5 nM. 4. In the presence of blockers of voltage-dependent Ca2+ channels (nimodipine, diltiazem), ryanodine and inhibitors of the SR calcium ATPase (thapsigargin, cyclopiazonic acid) were without effect on arterial wall [Ca2+] and diameter. 5. These results suggest that local Ca2+ release originating from RyR channels (Ca2+ sparks) in the SR of arterial smooth muscle regulates myogenic tone in cerebral arteries solely through activation of K(Ca) channels, which regulate membrane potential through tonic hyrperpolarization, thus limiting Ca2+ entry through L-type voltage-dependent Ca2+ channels. K(Ca) channels therefore act as a negative feedback control element regulating arterial diameter through a reduction in global intracellular free [Ca2+].