Regulation of phosphatidate synthesis by secretagogues in parotid acinar cells

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Abstract
The metabolism of phosphatidate in rat parotid acinar cells was investigated, particularly with regard to the actions of agonists known to act by mobilizing Ca2+. When cells were incubated in medium containing 10 .mu.M-[32P]Pi, phosphatidate was rapidly labeled, approaching an apparent steady-state with a half-time of .apprx. 20 min. Methacholine provoked a more than doubling of phosphatidate radioactivity, which was reversed by the muscarinic antagonist atropine. Phosphatidate apparently labels to near steady-state rapidly and in cells prelabeled for 60 min, the increase in radioactivity, induced by agonists, probably reflects net synthesis rather than an increase in specific radioactivity. Phosphatidate synthesis in response to methacholine was rapid and occurred, within the resolution of a few seconds, with no measurable latency. Adrenaline and substance P also stimulated phosphatidate synthesis, but both agonists were less efficacious than methacholine. A Ca2+ ionophore, ionomycin, did not provoke phosphatidate synthesis. By using a protocol that eliminates the receptor-regulated Ca2+ pool, it was demonstrated that methacholine-induced phosphatidate formation does not come about as a consequence of Ca2+ influx nor of Ca2+ release. The phosphatidate synthesis response has characteristics compatible with its previously suggested role as a primary mediator of membrane Ca2+-gating.

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