RECEPTOR-MEDIATED AND PHORBOL ESTER-MEDIATED PHOSPHOLIPASE-D ACTIVATION IN RAT PAROTID INVOLVES 2 DIFFERENT PATHWAYS

We have investigated phospholipase D (PLD) activation in rat parotid acini prelabeled with [C-14]stearic acid. In the presence of 2% ethanol, muscarinic and a-adrenergic agonists stimulated the formation of [C-14]phosphatidylethanol as a result of a PLD activity. The calcium ionophore, ionomycin, and the phorbol esters, 4 beta-phorbol 12-myristate 13-acetate (PMA) and phorbol 12,13-dibutyrate (PDBu), also stimulated phosphatidylethanol accumulation, but 1-oleyl-2-acetyl-sn-glycerol (GAG), a permeant analogue of diacylglycerol did not. Chelerythrine and staurosporine, two inhibitors of protein kinase C, failed to affect any response. These results suggest that protein kinase C was not involved in the regulation of PLD activity. A difference between PLD regulation by PMA and receptor-mediated agonists was observed with regard to the extracellular calcium requirement. Our results strongly suggest that PLD activation in parotid acini involved different pathways: a calcium-dependent pathway activated by receptor-mediated agonists and a calcium-independent pathway activated by phorbol esters. Moreover, we observed that PLD activation did not result in any change in phosphatidic acid level. We propose that the phosphatidyl transferase activity of PLD reflected a metabolic pathway which may allow a base-exchange reaction in parotid gland.