Diacylglycerol metabolism in isolated aortic smooth muscle cells

The metabolism of the cell-permeable diacylglycerol (DG) analogue, 1,2-dioctanoyl-sn-glycerol (diC8), by isolated intact smooth muscle cells from rabbit aorta was determined. Radiolabeled diC8 was rapidly converted to monoacylglycerol, with smaller amounts of radioactivity recovered as free glycerol and in the total phospholipid fraction that included phosphatidic acid. The greater metabolism of diC8 by the lipase pathway (formation of monoacylglycerol and glycerol) as compared with the kinase pathway (formation of phosphatidic acid and other phospholipids) was observed consistently when experimental conditions were changed by varying the time of incubation (5-30 min), diC8 concentration (0.5-50 microM), and the content of smooth muscle cells in the incubation (0.5-5 X 10(5) cells). These results with the metabolism of exogenous diC8 by intact smooth muscle cells are consistent with in vitro determinations of DG kinase and lipase activities, where lipase activity (especially hydrolysis of the sn-1 position to yield the monoacylglycerol intermediate) was much greater than kinase activity measured with both soluble and particulate subcellular fractions from aortic smooth muscle cells. Both DG kinase and lipase activities were inhibited when 1-monoolein was added to the in vitro assay. Treatment of intact smooth muscle cells with 1-monoolein (200 microM), however, did not reduce the formation of monoacylglycerol from diC8.