Ethanol and inositol 1,4,5-trisphosphate release calcium from separate stores of brain microsomes

The effects of ethanol and inositol 1,4,5-trisphosphate (IP3) on releasable Ca stores were examined in microsomes isolated from mouse whole brain. Ca release was monitored by determination of changes in the extra-microsomal Ca concentration using Indo-1, a fluorescent Ca indicator. In the absence of ATP, ethanol released Ca from microsomes in a concentration-dependent manner, with a threshold for Ca release between 25 and 50 mM. Ethanol-induced release of microsomal Ca was reduced by approximately 50% after ATP-stimulated uptake of Ca, indicating that the ethanol-releasable pool was diminished by ATP-dependent uptake of Ca into an ethanol-insensitive microsomal pool. Release of Ca produced by ethanol was linear with concentration (up to 400 mM). By contrast, IP3-induced Ca release was saturable and was dependent on prior ATP-stimulated Ca uptake. Simultaneous addition of ethanol and IP3 produced additive responses. These results show that pharmacologically relevant concentrations of ethanol release Ca from an IP3-insensitive intracellular Ca store. Furthermore, our results demonstrate the existence of at least two releasable stores of Ca in brain microsomes.