IP₃ induced Ca²⁺ release in A7r5 vascular smooth-muscle cells represents a partial 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 Ca²⁺ stores may transiently release some Ca²⁺ before they become insensitive to IP₃. Alternatively, stores with a low IP₃ threshold may lose all their Ca²⁺ and the others none. We now report that the IP₃ threshold was not correlated with the extent of Ca²⁺ release in permeabilized A7r5 smooth-muscle cells. In contrast, the maximum rate of release, which was changed either by varying the level of IP₃ receptor (IP₃R) activation, or by changing the concentration of IP₃R at a constant level of IP₃R activation, was directly related to the extent of Ca²⁺ release. We conclude that IP₃-induced Ca²⁺ release reflects partial emptying of the stores and not all-or-none Ca²⁺ release of separate quanta.