Endothelin-Induced Sarcoplasmic Reticulum Calcium Depletion Waves in Vascular Smooth Muscle Cells

Agonist-stimulated waves of elevated cytoplasmic Ca concentration ([Ca]i) regulate blood vessel tone [1] and vasomotion [2] in vascular smooth muscle. Previous studies employing cytoplasmic Ca indicators revealed that these Ca waves were generated by a combination of inositol 1,4,5-trisphosphate (IP3) and Ca induced Ca release (CICR) from the sarcoplasmic reticulum (SR) [3, 4]; although, some of the mechanistic details remain uncertain. However, these findings were derived indirectly from observing agonist-induced [Ca]i fluctuations in the cytoplasm. Here, for the first time, we have recorded Endothelin-1 (ET-1) induced waves of Ca depletion from the SR lumen in vascular smooth muscle cells (VSMCs) using a calsequestrin-targeted Ca indicator. Our findings show that these waves: (1) are due to regenerative CICR by the receptors for IP3 (IP3R), (2) have a marked latency period, (3) are characterized by a transient increase in SR Ca ([Ca]SR) both at the point of origin and at the wave front, (4) proceed with diminishing velocity, and (5) are arrested by the nuclear envelope. Our quantitative model indicates that the gradual decrease in the velocity of the SR depletion wave, in the absence of external Ca, results from continuity of the SR luminal network. In VSMCs, Ca controls multiple functions, including contraction-relaxation, proliferation, migration and apoptosis, in health and disease [5–7]. This could be explained by the widely held view that functional selectivity of the Ca signal is encoded in its spatial and temporal characteristics [8–10]. The first agonist-stimulated waves of elevated [Ca]i in SMCs were reported in 1990 by Neylon et al [11], who proposed a mechanism of wave propagation based on interaction between IP3-sensitive receptors (IP3Rs) and ryanodinesensitive receptors (RyRs) located in separate endo/sarcoplasmic reticulum (ER/SR) com-