In primary bovine adrenal glomerulosa cells, the signaling enzyme phospholipase D (PLD) is suggested to mediate priming, the enhancement of aldosterone secretion after pretreatment with and removal of angiotensin II (AngII), via the formation of persistently elevated diacylglycerol (DAG). To further explore PLD's role in priming, glomerulosa cells were pretreated with an exogenous bacterial PLD. Using this approach, phosphatidic acid (PA) is generated on the outer, rather than the inner, leaflet of the plasma membrane. Although PA is not readily internalized, the PA is nonetheless rapidly hydrolyzed by cell-surface PA phosphatases to DAG, which efficiently flips to the inner leaflet and accesses the cell interior. Pretreatment with bacterial PLD resulted in priming upon subsequent AngII exposure, supporting a role of DAG in this process, because the increase in DAG persisted after exogenous PLD removal. To determine the PLD isoform mediating aldosterone secretion, and presumably priming, primary glomerulosa cells were infected with adenoviruses expressing GFP, PLD1, PLD2, or lipase-inactive mutants. Overexpressed PLD2 increased aldosterone secretion by approximately 3-fold over the GFP-infected control under basal conditions, with a significant enhancement to about 16-fold over the basal value upon AngII stimulation. PLD activity was also increased basally and upon stimulation with AngII. In contrast, PLD1 overexpression had little effect on aldosterone secretion, despite the fact that PLD activity was enhanced. In both cases, the lipase-inactive PLD mutants showed essentially no effect on PLD activity or aldosterone secretion. Our results suggest that PLD2 is the isoform that mediates aldosterone secretion and likely priming.
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