CONTROL OF INOSITOL 1,4,5-TRISPHOSPHATE-INDUCED CA2+ RELEASE BY CYTOSOLIC CA2+

The synergistic action of cytosolic Ca2+ and inositol 1,4,5-trisphosphate (InsP(3)) in releasing intracellular Ca2+ stores has been suggested to be responsible for the complex intracellular Ca2+ signals observed during hormonal stimulation of many cell types. However, the ability of cytosolic Ca2+ to potentiate Ca2+ release has recently been questioned because of the observed inhibitory effects of Ca2+ chelators used in previous studies. In the present study, EGTA and BAPTA [1,2-bis-(2-amino-phenoxy)ethane-NNN'N'-tetra-acetic acid] poorly inhibited InsP(3)-induced Ca2+ release from permeabilized A7r5 smooth-muscle cells. Additionally, stimulatory effects of cytosolic and luminal Ca2+ were observed either in the complete absence of Ca2+ chelator or at constant Ca2+-free chelator concentration. These data suggest that potentiation of InsP(3)-induced Ca2+ release by Ca2+ in A7r5 cells reflects an interaction between Ca2+ and InsP(3) receptors, rather than a decrease in chelator-dependent inhibition. The EC(50) for activation of InsP(3)-induced Ca2+ release by cytosolic Ca2+ was unaffected by ATP, or by changing InsP(3) concentration, although InsP(3)-induced Ca2+ release became less sensitive to the inhibitory effects of cytosolic Ca2+ as the InsP(3) concentration was elevated. Increasing H+ or Mg2+ concentration shifted the Ca2+-activation curve towards higher Ca2+ concentrations. These data suggest that, in addition to the InsP(3)-binding site, the affinity of the Ca2+-binding site(s) on InsP(3) receptors can be modulated by intracellular cations.