Calcium and the Muscarinic Synaptosomal Phospholipid Labeling Effect

Abstract

Abstract: The role of calcium in the muscarinic phospholipid labeling effect in synaptosomes has been investigated. In the absence of added calcium, acetylcholine doubled phosphatidate labeling and increased phosphatidy linositol labeling 40% in synaptosomes when incubated in a medium that contained [ 32 P]orthophosphate. Inclusion of calcium or the omission of magnesium resulted in a marked elevation of acetylcholine‐stimulated phosphatidylinositol labeling (70–80%) while phosphatidate stimulation was unaltered. Calciumchelating agents, EGTA and EDTA, reduced the stimulated labeling of both phosphatidate and phosphatidylinositol, but this inhibition could be reversed by calcium addition. The calcium ionophore A23187, which promotes entry of calcium into cells, selectively increased labeling of both phosphatidate and phosphatidyl‐inositol. This effect, unlike acetylcholine‐stimulated labeling, was not blocked by the addition of atropine. The calcium dependency of the acetylcholine stimulation, on the one hand, and the insensitivity of the ionophore to a muscarinic antagonist, on the other, argue strongly that the acetylcholine‐receptor interaction regulates calcium mobilization and that the latter is linked to the stimulated labeling of phosphatidate and phosphatidylinositol.