Relationship between Ca²⁺ sparklets and sarcoplasmic reticulum Ca²⁺ load and release in rat cerebral arterial smooth muscle

Ca⁺ sparklets are subcellular Ca²⁺ signals produced by the opening of sarcolemmal L-type Ca²⁺ channels. Ca²⁺ sparklet activity varies within the sarcolemma of arterial myocytes. In this study, we examined the relationship between Ca²⁺ sparklet activity and sarcoplasmic reticulum (SR) Ca²⁺ accumulation and release in cerebral arterial myocytes. Our data indicate that the SR is a vast organelle with multiple regions near the sarcolemma of these cells. Ca²⁺ sparklet sites were located at or <0.2 μm from SR-sarcolemmal junctions. We found that while Ca²⁺ sparklets increase the rate of SR Ca²⁺ refilling in arterial myocytes, their activity did not induce regional variations in SR Ca²⁺ content or Ca²⁺ spark activity. In arterial myocytes, L-type Ca²⁺ channel activity was independent of SR Ca²⁺ load. This ruled out a potential feedback mechanism whereby SR Ca²⁺ load regulates the activity of these channels. Together, our data suggest a model in which Ca²⁺ sparklets contribute Ca²⁺ influx into a cytosolic Ca²⁺ pool from which sarco(endo)plasmic reticulum Ca²⁺-ATPase pumps Ca²⁺ into the SR, indirectly regulating SR function.