OKADAIC ACID UNCOUPLES CALCIUM ENTRY FROM DEPLETION OF INTRACELLULAR STORES

The mechanism by which the depletion of intracellular Ca2+ stores stim ulates Ca2+ influx is poorly understood. However, the coupling of depl etion to influx is broken during mitosis [Preston, S.F. et. al., (1991 ) Cell Regul., 2, 915-925]. Thus, in interphase HeLa cells, activation of the histamine H1 receptor, or incubation with thapsigargin, which inhibits the Ca2+-ATPase of storage vesicles and depletes Ca2+ stores, strongly stimulate Ca2+ influx. In mitotic cells, however, neither hi stamine nor thapsigargin stimulate Ca2+ influx. Since it has been foun d that okadaic acid treatment of interphase cells induces a mitotic-li ke state with respect to a number of other membrane processes, we have asked if okadaic acid might also uncouple Ca2+ depletion from stimula ted influx. We show that okadaic acid specifically does suppress this coupling: thapsigargin and histamine deplete stores in control and oka daic-acid-treated HeLa cells, but after treatment with okadaic acid, s timulation of Ca2+ influx is barely detectable. This suggests that a p rotein phosphorylation/dephosphorylation event controls the coupling o f Ca2+ stores to influx, and that there may be a physiological mechani sm for control of the Ca2+ response to hormonal signals at the level o f coupling.