HORMONAL-REGULATION OF GLUTAMINE-METABOLISM BY OK CELLS

The precise mechanism(s) of action of PTH, insulin or glucagon in the regulation of renal glutamine and ammonia metabolism is unknown. Our a im was to delineate the effects and the site(s) of action of these hor mones on renal glutamine metabolism. Experiments were carried out usin g OK cells as a model system. Cell cultures were incubated for three h ours in a bicarbonate buffer of pH 7.4 supplemented with either 1 mM [ 2-N-15] or [5-N-15] glutamine and 10(-7) M PTH, insulin or glucagon. C omparative studies were performed at pH 6.8, 7.4 or 7.6 without hormon e. PTH and acute acidosis significantly stimulated glutamine metabolis m via both the phosphate-dependent glutaminase (PDG) and glutamate deh ydrogenase (GLDH) pathways. The opposite was observed at pH 7.6. Insul in augmented fluxvia PDG with little effect on the GLDH pathway. Gluca gon had insignificant effects on either PDG or GLDH pathways. Intracel lular [N-15] glutamate formed from [2-N-15] glutamine was removed part ially by transamination to alanine, aspartate and serine and partially by translocation to an extracellular compartment. Acidosis, PTH and i nsulin enhanced the formation of [N-15] alanine with little effect on [N-15] aspartate. PTH, insulin and glucagon significantly stimulated t he production of [N-15]serine, whereas acidosis had little effect. The translocation of intracellular glutamate was significantly increased by acidosis, PTH and insulin and decreased by acute alkalosis. The dat a indicate that: (a) PTH mimicks the effect of acute acidosis on renal glutamine metabolism, that is, augmented glutamine metabolism through both PDG and GLDH pathways and stimulated the output of intracellular glutamate. This effect might be mediated via decreased activity of th e Na+-H+ exchanger associated with cellular acidification and/or throu gh a second messenger; (b) insulin, but not glucagon, increased glutam ine uptake and metabolism, and simultaneously enhanced output of intra cellular glutamate sufficiently to stimulate the PDG pathway; and (c) overall, glucagon had little effect on glutamine metabolism by OK cell s compared with either PTH or insulin.