Activation of phospholipase D in a rat mast (RBL 2H3) cell line. A possible unifying mechanism for IgE-dependent degranulation and arachidonic acid metabolite release.

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Abstract
RBL 2H3 cells (a model of mast cell function) were sensitized with anti-TNP IgE (0.5 micrograms/ml) and triggered to secrete both histamine and arachidonic acid (AA) metabolites by the addition of TNP-OVA (0 to 100 ng/ml). After a 3-min delay, the release of both groups of mediators proceeded in a parallel manner. In cells labeled with [14C]-AA, TNP-OVA produced a rapid increase in phosphatidic acid (PA), and subsequently, 1,2-diacylglycerol (DAG) and intracellular AA levels. Concurrently, there was a decrease in [14C]-AA labeled phosphatidylcholine. The release of labeled AA from phosphatidylcholine in response to TNP-OVA was paralleled by a liberation of free choline but no evidence of liberation of phosphorylcholine. When ethanol (0.05 to 2% v/v) was included in the culture medium, phosphatidylethanol was synthesized at the expense of PA and DAG, with a resulting inhibition of secretion. D,1 propranolol, an inhibitor of PA phosphohydrolase, inhibited the IgE-dependent production of [14C]-DAG, and [14C]-free fatty acid but not [14C]-PA. The IgE-dependent release of both histamine and AA metabolites was completely inhibited by pretreatment with propranolol. Taken together, the above results suggest that phospholipase D is activated upon cross-bridging of IgE receptors on the surface of RBL 2H3 cells and that this may be a pivotal step in the signal transduction cascade leading to the release of both presynthesized and de novo synthesized mediators.

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