Relaxation of arterial smooth muscle by calcium sparks

Local increases in intracellular calcium ion concentration ([Ca²⁺](i)) resulting from activation of the ryanodine-sensitive calcium-release channel in the sarcoplasmic reticulum (SR) of smooth muscle cause arterial dilation. Ryanodine-sensitive, spontaneous local increases in [Ca²⁺](i) (Ca²⁺ sparks) from the SR were observed just under the surface membrane of single smooth muscle cells from myogenic cerebral arteries. Ryanodine and thapsigargin inhibited Ca²⁺ sparks and Ca²⁺-dependent potassium (K(Ca)) currents, suggesting that Ca²⁺ sparks activate K(Ca) channels. Furthermore, K(Ca) channels activated by Ca²⁺ sparks appeared to hyperpolarize and dilate pressurized myogenic arteries because ryanodine and thapsigargin depolarized and constricted these arteries to an extent similar to that produced by blockers of K(Ca) channels. Ca²⁺ sparks indirectly cause vasodilation through activation of K(Ca) channels, but have little direct effect on spatially averaged [Ca²⁺](i), which regulates contraction.