Calcium release by ryanodine receptors in smooth muscle.

Recent experiments have revealed an unanticipated complexity in Ca2+ release processes in smooth muscle. While Ca2+ release via stimulation of phospholipase C activity and the gating of inositol-1,4,5-trisphosphate (InsP3) receptor Ca2+ channels has been well characterized, the role of the homologous family of sarcoplasmic reticulum (SR) Ca2+ channels, ryanodine receptors (RyRs), in excitation-contraction coupling in smooth muscle is less clear. These Ca2+-gated SR Ca2+ channels produce unitary Ca2+ sparks, which in turn open Ca2+-activated membrane ion channels such as Ca2+-activated Cl- channels (ClCa) and Ca2+-activated K+ channels. In this manner local Ca2+ sparks trigger spontaneous transient inward and outward currents, thereby driving physiological electrical activity in smooth muscle. Here we summarize some of our recent findings on Ca2+-induced Ca2+ release (CICR) and stretch-induced Ca2+ release (SICR) in smooth muscle.