Carbamoylcholine stimulated phosphatidylinositol breakdown in rat parotid-gland fragments incubated in either a Krebs--Ringer medium or an ion-depleted iso-osmotic sucrose medium. This suggest that phosphatidylinositol breakdown is not initiated by a change in the intracellular concentration of Na+ or of K+, and previous experiments have already indicated that it is independent of cyclic nucleotides and Ca2+. Thus it seems that this reaction may be initiated by a direct interaction at the plasma membrane between the activated muscarinic cholinergic receptor system and the enzyme that catalyses phosphatidylinositol breakdown.
