Ca2+ transport by the synaptosomal plasma membrane Ca2+-ATPase and the effect of thioridazine

Thioridazine inhibits the activity of the synaptic plasma membrane Ca2+-ATPase from pig brain and slightly decreases the rate of Ca2+ accumulation by synaptic plasma membrane vesicles in the absence of phosphate. However, in the presence of phosphate, thioridazine increases the rate of Ca2+ accumulation into synaptic plasma membrane vesicles. Phosphate anions diffuse through the membrane and form calcium phosphate crystals, reducing the free Ca2+ concentration inside the vesicles and the rate of Ca2+ leak. The higher levels of Ca2+ accumulation obtained in the presence of thioridazine could be explained by a reduction of the rate of slippage on the plasma membrane ATPase.