Waves of Calcium Depletion in the Sarcoplasmic Reticulum of Vascular Smooth Muscle Cells: An Inside View of Spatiotemporal Ca(2+) Regulation

Agonist-stimulated smooth muscle Ca(2+) waves regulate blood vessel tone and vasomotion. Previous studies employing cytoplasmic Ca(2+) indicators revealed that these Ca(2+) waves were stimulated by a combination of inositol 1,4,5-trisphosphate- and Ca(2+)-induced Ca(2+) release from the endo/sarcoplasmic reticulum. Herein, we present the first report of endothelin-1 stimulated waves of Ca(2+) depletion from the sarcoplasmic reticulum of vascular smooth muscle cells using a calsequestrin-targeted Ca(2+) indicator. Our findings confirm that these waves are due to regenerative Ca(2+)-induced Ca(2+) release by the receptors for inositol 1,4,5-trisphosphate. Our main new finding is a transient elevation in SR luminal Ca(2+) concentration ([Ca(2+)](SR)) both at the site of wave initiation, just before regenerative Ca(2+) release commences, and at the advancing wave front, during propagation. This strongly suggests a role for [Ca(2+)](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 Ca(2+), 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 Ca(2+) signalling.