Calcium loading properties of carbachol-sensitive calcium store in calcium-depleted stomach smooth muscle of the newt

The amount of carbachol (CCh)-sensitive Ca store, which was loaded during Ca contracture in Ca-depleted stomach smooth muscle of the newt, was estimated from the amplitude of the CCh contracture induced in Ca-free high K solution. This store was loaded to nearly maximum level by simultaneous application of 2.8 X 10(-6)M Ca and 113.5 mM K, while Ca contracture induced at this time was less than 10% in amplitude of control (1.8 mM Ca). When the loading was interrupted after 5 sec of 1.8 mM Ca application, the amplitude of Ca contracture reached nearly its maximum, while the loading of the store was only about 30% of that after 90 sec. Contracture induced by Ca-free high K solution immediately after the brief (10 sec) exposure of the muscle to 1.8 mM Ca was about 50% that of K contracture, while little Ca was observed to be loaded. The [K]0-Ca contracture tension relation and the [K]0-Ca loading relation showed sigmoid and linear characteristics, respectively.