CALCIUM MOBILIZATION FROM INTRACELLULAR STORES IN SMOOTH-MUSCLE CELLS

Ca2+ mobilization from intracellular stores in smooth muscle cells was investigated. Two kinds of Ca2+ release channels in the membrane of Ca2+ stores, the Ca2+-induced Ca2+ release (CICR) channel and the inositol 1,4,5-trisphosphate (IP3)-induced Ca2+ release (IICR) channel, are demonstrated and characterized. These two kinds of channels distribute heterogeneously in the store membrane; some store (S(alpha)) has both CICR and IICR channels while the other (S(beta)) has only IICR channels. S(alpha) and S(beta) are present not separately in different cells but coexist in all the cells so far tested. IICR is accelerated by submicromolar level of Ca2+ and this effect of Ca2+ is rapid enough to constitute a positive feedback loop during a single agonist-induced Ca2+ release from the store so that Ca2+ release occurs in an all-or-none like manner.