Cholinergic stimulation of phosphatidic acid formation by rat cornea in vitro

The cornea has one of the highest acetylcholine (ACh) concentrations of any tissue but the function of the ACh has remained enigmatic. During studies on corneal arachidonic acid metabolism, we observed that ACh stimulates formation of labeled phosphatidic acid in rat corneas whose phospholipids were prelabeled with [14C]arachidonate. ACh did not affect the metabolism of free [14C]arachidonate. [14C]Arachidonyl-phosphatidic acid formation was doubled after 10 min of incubation in the presence of ACh concentrations of 10(-4) M or greater. The stimulation by ACh could be completely blocked by atropine and scopolamine and partially blocked by d-tubocurarine. These studies suggest that intact rat cornea has muscarinic cholinergic receptors and that the enzymes of the inositol phospholipids pathway are present since phosphatidic acid is an obligatory intermediate in that cycle of reactions.