The quantal nature of calcium release to caffeine in single smooth muscle cells results from activation of the sarcoplasmic reticulum Ca2+-ATPase

Calcium release from intracellular stores occurs in a graded manner in response to increasing concentrations of either inositol 1,4,5-trisphosphate or caffeine, To investigate the mechanism responsible for this quantal release phenomenon, [Ca2+] changes inside intracellular stores in isolated single smooth muscle cells were monitored with mag-fura 2. Following permeabilization with saponin or alpha-toxin the dye, loaded via its acetoxymethyl ester, was predominantly trapped in the sarcoplasmic reticulum (SR), Low caffeine concentrations in the absence of ATP induced only partial Ca2+ release; however, after inhibiting the calcium pump with thapsigargin the same stimulus released twice as much Ca2+. When the SR Ca2+-ATPase was rendered non-functional by depleting its ''ATP pool,'' submaximal caffeine doses almost fully emptied the stores of Ca2+. We conclude that quantal release of Ca2+ in response to caffeine in these smooth muscle cells is largely due to the activity of the SR Ca2+-ATPase, which appears to return a portion of the released Ca2+ back to the SR, even in the absence of ATP, Apparently the SR Ca2+-ATPase is fueled by ATP, which is either compartmentalized or bound to the SR.