BETAINE AND INOSITOL REDUCE MDCK CELL GLYCEROPHOSPHOCHOLINE BY STIMULATING ITS DEGRADATION

The amount of glycerophosphocholine (GPC) in renal medullary cells in vivo and in cultured renal [Madin-Darby canine kidney (MDCK)] cells va ries with extracellular NaCl and urea. We previously showed that this is largely due to modulation of GPC degradation catalyzed by GPC:choli ne phosphodiesterase (GPC: PDE). GPC also varies inversely with the le vels of other compatible osmolytes, the accumulation of which is induc ed by high tonicity. We tested whether GPC:PDE activity and GPC degrad ation are affected by accumulation of compatible osmolytes other than GPC. We find that MDCK cell GPC content decreases when the cells take up betaine and/or inositol from the medium. The effect is considerably greater for cells in isosmotic or high-NaCl medium than in high-urea medium. This difference is associated with suppression of betaine and inositol accumulation with high urea. We then measured GPC:PDE activit y with a novel chemiluminescent assay. Addition of inositol and/or bet aine to the medium greatly increases GPC:PDE activity in cells in isos motic or high-NaCl media, but the increase is much less in high-urea m edium. The increases in GPC:PDE activity, associated with the presence of betaine, are accompanied by commensurate increases in absolute rat es of endogenous GPC degradation by cells in isosmotic or high-NaCl me dium. We found previously that, in MDCK cells incubated for 2 days in high-NaCl medium, the rate of GPC synthesis from phosphatidylcholine i s increased, correlated with an increase in phospholipase activity. Ho wever, in the present experiments, betaine accumulation has no effect on phospholipase activity under those conditions and, thus, presumably does not affect GPC synthesis. Collectively, these data support the c onclusion that betaine and/or inositol reduces GPC by increasing GPC d egradation catalyzed by GPC:PDE. This mechanism enables GPC to be reci procally regulated relative to other compatible osmolytes, thus mainta ining an appropriate total osmolyte content.