Cholinergic activation of phospholipase D in lacrimal gland acini is independent of protein kinase C and calcium

Abstract

To determine if rat lacrimal gland acini contain phospholipase D (PLD) activity, we took advantage of PLD's unique ability, in the presence of ethanol, to catalyze a transphosphatidylation reaction to produce phosphatidylethanol (PEth). Lacrimal gland acini were labeled for 3 h with [14C]stearic acid, preincubated for 20 min in the presence of 2% ethanol, and incubated for 20 min with or without agonists. Total cellular lipids were then extracted and analyzed by thin-layer chromatography, and the radioactivity was determined by liquid scintillation counting. Carbachol (1 mM), a cholinergic agonist, stimulated the production of both [14C]PEth and [14C]phosphatidic acid ([14C]PA) twofold. This effect was completely blocked by the muscarinic antagonist atropine (10 microM). [14C]PEth accumulation was also stimulated twofold by the active phorbol esters, 4 beta-phorbol 12-myristate 13-acetate and 4 beta-phorbol 12,13-dibutyrate at 1 microM. Ionomycin (1 microM), a Ca2+ ionophore, also stimulated the production of [14C]PEth twofold. In contrast to carbachol, neither phorbol esters nor ionomycin stimulated [14C]PA production. Neither [14C]PEth nor [14C]PA production was altered by epinephrine (1 mM), a nonselective adrenergic agonist, or phenylephrine (0.1 and 1 mM), a specific alpha 1-adrenergic agonist. We concluded that PLD activity, modulated by muscarinic receptors, protein kinase C, and Ca2+, but not by adrenergic receptors, is present in rat lacrimal gland acini. We also concluded that cholinergic activation of PLD appears to be independent of PKC and Ca2+.