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

1. 1. The release of Ca²⁺ from the intracellular store site, as induced by inositol 1,4,5-trisphosphate, was studied in relation to free Ca²⁺ concentrations or amounts of stored Ca²⁺ in smooth muscle cells. 2. 2. The maximal Ca²⁺ release induced by inositol 1,4,5-trisphosphate was observed when the amount of Ca²⁺ in the store site was about 50% of the maximal capacity of the Ca²⁺ storage, and when the extra-vesicular free Ca²⁺ concentration was less than 1.5 × 10^-6 M. 3. 3. The Ca²⁺ 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 Ca²⁺ release appeared to be specific for intracellular Ca²⁺ store sites (mainly sarcoplasmic reticulum), and this Ca²⁺ release was not apparent in the sarcolemmal fraction.