QUANTAL CA2-OR-NONE RELEASE FROM FUNCTIONALLY DISCRETE INTRACELLULAR STORES( MOBILIZATION BY RYANODINE RECEPTORS IS DUE TO ALL)

Low caffeine concentrations were unable to completely release the caff eine- ryanodine-sensitive intracellular Ca2+ pool in intact adrenal ch romaffin cells. This 'quantal' Ca2+ release is the same as that previo usly observed with inositol Ins(1,4,5)P-3-induced Ca2+ release. The mo lecular mechanism underlying quantal Ca2+ release from the ryanodine r eceptor was investigated using fura-2 imaging of single chromaffin cel ls. Our data indicate that the intracellular caffeine-sensitive Ca2+ p ool is composed of functionally discrete stores, that possess heteroge neous sensitivities to caffeine. These stores are mobilized by caffein e in a concentration-dependent fashion, and, when stimulated, individu al stores release their Ca2+ in an 'all-or-none' manner. Such quantal Ca2+ release may be responsible for graded Ca2+ responses in single ce lls.