IP3-induced Ca2+ release in A7r5 vascular smooth-muscle cells represents apartial emptying of the stores and not an all-or-none Ca2+ release of separate quanta

There is still no agreement on the mechanism of the intracellular action of low concentrations of inositol 1,4,5-trisphosphate (IP3). Intracellular Ca 2+ stores may transiently release some Ca2+ before they become insensitive to IP3. Alternatively, stores with a low IP3 threshold may lose all their C a2+ and the others none. We now report that the IP3 threshold was not corre lated with the extent of Ca2+ release in permeabilized A7r5 smooth-muscle c ells. In contrast, the maximum rate of release, which was changed either by varying the level of IP3 receptor (IP3R) activation, or by changing the co ncentration of IP3R at a constant level of IP3R activation, was directly re lated to the extent of Ca2+ release. We conclude that IP3-induced Ca2+ rele ase reflects partial emptying of the stores and not all-or-none Ca2+ releas e of separate quanta.