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We tested the hypothesis that serotonin activates phospholipase D (PLD) in cultured rat mesangial cells. The formation of phosphatidylethanol (PET) in ethanol was used as a measure of PLD activity. Serotonin [5-hydroxytryptamine (5-HT)] stimulated PET production, with an initial 10-fold increase in PET content within 15-30 s, followed by a decrease in PET to values only sixfold above baseline at 45-60 s. Thereafter, the values increased again at 5 min to a plateau 10-fold above baseline. The decrease in PET values, following the initial increase, was due to metabolism of PET, possibly by a phosphatidic acid phosphohydrolase, which led to a 76% decrease in PET within 1 h. Inhibition of phosphohydrolase with propranolol increased the phosphatidic acid and decreased the 1,2-diacylglycerol (DAG) content of 5-HT-stimulated cells. Finally, exogenous PLD induced mesangial cell proliferation as well as increases in phosphatidic acid, PET, and DAG. We conclude that phosphatidic acid contributes to DAG formation in mesangial cells. Furthermore, we suggest that PLD is involved in 5-HT-mediated mesangial cell proliferation.

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