Phosphoinositide breakdown in isolated rat parotid membranes

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Abstract
Parotid gland membranes labelled with [3H]inositol were challenged with the cholinergic agonist, carbamylcholine, or with epinephrine in the presence of propranolol. Both agonists caused a significant breakdown of labelled phosphoinositides (17.5%) in membranes suspended in Krebs‐Ringer bicarbonate buffer. This effect was abolished by the respective antagonists, atropine or phentolamine. The carbamylcholine‐induced breakdown of labelled phosphoinositides did not require cytosol. The addition of cytosol alone, or the exposure of membranes to a medium of low ionic strength caused a significant breakdown of phosphoinositides (10–40%). No further breakdown due to the addition of carbamylcholine was observed under these conditions. It is suggested that neurotransmitter‐induced breakdown of phosphoinositides is effected by membrane‐associated enzyme(s) and can be observed only in a medium of high ionic strength.

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