Osmotic regulation of synthesis of glycerophosphocholine from phosphatidylcholine in MDCK cells

Glycerophosphocholine (GPC) is osmotically regulated in renal medullary cells and in cultured Madin-Darby canine kidney (MDCK) cells. Previously, it was shown that a high extracellular concentration of urea or NaCl causes these cells to accumulate large amounts of GPC. GPC is known 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 accumulation. When 1-palmitoyl-2- lysophosphatidyl-[methyl-³H]choline ([³H]LPC is added to the medium, it is taken up by the cells and most of it is rapidly converted to PC. During a chase, ³H lost from PC appears almost exclusively in GPC and sphingomyelin. The rate of catabolism of PC is twofold greater in cells exposed to high NaCl (200 mosmol/kgH₂O, 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 increase in GPC synthesis from PC; sphingomyelin synthesis decreases, and total cell PC does not change. Also, neither total mass nor specific radioactivity of lysophosphatidylcholine changes. PC catabolism is unaffected by short (2 h) exposure to high NaCl or urea. To investigate the enzymatic basis for the increased PC catabolism in response to high NaCl, phospholipase activity was measured in cell homogenates with 1-palmitoyl-2-[1- ¹⁴C]palmitoyl-PC as a substrate. Exposure of cells to high NaCl for 2 days (but not 2 h) increases activity 2.8-fold compared with control or high-urea medium. Lysophospholipase activity (measured with [³H]LPC as the substrate) is unchanged. The increased phospholipase activity occurs with dipalmitoyl PC, but not sn-2-arachidonyl PC, as a substrate. Collectively, these data suggest a role for a phospholipase, unrelated to the arachidonyl-selective enzyme, in the regulation of PC catabolism during accumulation of GPC induced by prolonged exposure to high extracellular NaCl.