Waves of Calcium Depletion in the Sarcoplasmic Reticulum of Vascular Smooth Muscle Cells: An Inside View of Spatiotemporal Ca2+ Regulation

Agonist-stimulated smooth muscle Ca2+ waves regulate blood vessel tone and vasomotion. Previous studies employing cytoplasmic Ca2+ indicators revealed that these Ca2+ waves were stimulated by a combination of inositol 1,4,5-trisphosphate- and Ca2+-induced Ca2+ release from the endo/sarcoplasmic reticulum. Herein, we present the first report of endothelin-1 stimulated waves of Ca2+ depletion from the sarcoplasmic reticulum of vascular smooth muscle cells using a calsequestrin-targeted Ca2+ indicator. Our findings confirm that these waves are due to regenerative Ca2+-induced Ca2+ release by the receptors for inositol 1,4,5-trisphosphate. Our main new finding is a transient elevation in SR luminal Ca2+ concentration ([Ca2+]SR) both at the site of wave initiation, just before regenerative Ca2+ release commences, and at the advancing wave front, during propagation. This strongly suggests a role for [Ca2+]SR in the activation of inositol 1,4,5-trisphosphate receptors during agonist-induced calcium waves. In addition, quantitative analysis of the gradual decrease in the velocity of the depletion wave, observed in the absence of external Ca2+, indicates continuity of the lumen of the sarcoplasmic reticulum network. Finally, our observation that the depletion wave was arrested by the nuclear envelope may have implications for selective Ca2+ signalling.