OSMOTIC REGULATION OF SYNTHESIS OF GLYCEROPHOSPHOCHOLINE FROM PHOSPHATIDYLCHOLINE IN MDCK CELLS

Glycerophosphocholine (GPC) is osmotically regulated in renal medullar y cells and in cultured Madin-Darby canine kidney (MDCK) cells. Previo usly, it was shown that a high extracellular concentration of urea or NaCl causes these cells to accumulate large amounts of GPC. GPC is kno wn to be a product of phosphatidylcholine (PC) catabolism. The purpose of the present experiments was to examine the role of changes in the rate of GPC synthesis from PC in hyperosmotically induced GPC accumula tion. When 1-palmitoyl-2-lysophosphatidyl-[methyl-H-3]choline ([H-3]LP C) is added to the medium, it is taken up by the cells and most of it is rapidly converted to PC. During a chase, H-3 lost from PC appears a lmost exclusively in GPC and sphingomyelin. The rate of catabolism of PC is twofold greater in cells exposed to high NaCl (200 mosmol/kgH(2) O, added for 2 days) than in control or high-urea medium. Increased PC catabolism in NaCl-treated cells is associated with a 2.6-fold increa se in GPC synthesis from PC; sphingomyelin synthesis decreases, and to tal cell PC does not change. Also, neither total mass nor specific rad ioactivity of lysophosphatidylcholine changes. PC catabolism is unaffe cted by short (2 h) exposure to high NaCl or urea. To investigate the enzymatic basis for the increased PC catabolism in response to high Na Cl, phospholipase activity was measured in cell homogenates with 1-pal mitoyl-2-[1-C-14]palmitoyl-PC as a substrate. Exposure of cells to hig h NaCl for 2 days (but not 2 h) increases activity 2.8-fold compared w ith control or high-urea medium. Lysophospholipase activity (measured with [H-3]LPC as the substrate) is unchanged. The increased phospholip ase activity occurs with dipalmitoyl PC, but not sn-2-arachidonyl PC, as a substrate. Collectively, these data suggest a role for a phosphol ipase, unrelated to the arachidonyl-selective enzyme, in the regulatio n of PC catabolism during accumulation of GPC induced by prolonged exp osure to high extracellular NaCl.