Ca²⁺Release through Ryanodine Receptors in the Sarcoplasmic Reticulum and Ca²⁺Sequestration by the Mitochondria in Smooth Muscle Cells

The contribution of the Ca2+-induced Ca2+ release (CICR) mechanism in excitation-contraction (E-C) coupling and the tightness of the coupling between Ca2+ influx and Ca2+ release are still controversial in smooth muscle cells (SMC). In SMC isolated from the guinea-pig vas deferens or urinary bladder, a depolarizing stimulus initially induced spot-like increases in the intracellular Ca2+ concentration ([Ca2+] i ), called “Ca2+ hot spots,” at several superficial areas in the cell. When a weak stimulus (a small or a short depolarizing step) was applied, only a few Ca2+ hot spots appeared transiently in the superficial area but did not spread into other regions, to trigger global [Ca2+] i rise. Such depolarization-evoked local Ca2+ transients were distinctive from spontaneous Ca2+ sparks, since the former were susceptible to Ca2+ blockers, ryanodine, and inhibitors of the Ca2+ pump in the sarcoplasmic reticulum (SR), suggesting pivotal roles of Ca2+ influx through voltage-dependent Ca2+ channels (VDCC) and Ca2+ release from the SR through ryanodine receptors (RyR) for the activation of Ca2+ spots. Frequently discharging Ca2+ spark sites (FDS) under resting conditions were located exactly in the same areas as Ca2+ hot spots evoked by depolarization, indicating the existence of distinct local junction sites for tight coupling between VDCC in the plasmalemma and RyR in the SR. Co-localization of clusters of RyR and large-conductance Ca2+-activated K+ (BK) channels was also suggested. The fast and tight coupling for CICR in these junctional sites was triggered also by an action potential, whereas a slower spread of Ca2+ wave to the whole-cell areas suggests the loose coupling in propagating CICR to other cell areas. It can therefore be postulated that CICR may occur in two steps upon depolarization; the initial CICR in distinct junctional sites shows tight coupling between Ca2+ influx and release, and the following CICR may propagate slow Ca2+ waves to other areas. Ryanodine receptors form a multiprotein complex with molecules such as calsequestrin, junctin, triadin, junctophilins, and FK506-binding proteins, which directly or indirectly regulate the RyR activity and the tight coupling. Moreover, an evoked Ca2+ spot may enhance Ca2+ uptake by neighboring mitochondria and their ATP production to increase energy supply to the Ca2+ pump of the SR in the microdomain.