Roles of Ca2+ on the inositol 1,4,5-trisphosphate-induced release of Ca2+ from saponin-permeabilized single cells of the porcine coronary artery.

Abstract 1. 1. The release of Ca 2+ from the intracellular store site, as induced by inositol 1,4,5-trisphosphate, was studied in relation to free Ca 2+ concentrations or amounts of stored Ca 2+ in smooth muscle cells. 2. 2. The maximal Ca 2+ release induced by inositol 1,4,5-trisphosphate was observed when the amount of Ca 2+ in the store site was about 50% of the maximal capacity of the Ca 2+ storage, and when the extra-vesicular free Ca 2+ concentration was less than 1.5 × 10 −6 M. 3. 3. The Ca 2+ release induced by inositol 1,4,5-trisphosphate was accelerated by ATP and 5'-adenylylimidodiphosphate (AMPPNP), but not by ADP and AMP. 4. 4. This inositol 1,4,5-trisphosphate-induced Ca 2+ release appeared to be specific for intracellular Ca 2+ store sites (mainly sarcoplasmic reticulum), and this Ca 2+ release was not apparent in the sarcolemmal fraction.