Renal glutamine metabolism: studies with the perfused kidney and with isolated cortical tubules

The isolated perfused kidney system was developed in our lab and functionally defined. With this technique kidneys from rats which had been subjected to an NH₄CL induced metabolic acidosis for 7-days and 24-hour recovery from this condition were perfused with glutamine. Rates of ammmoniagenesis and gluconeogenesis were measured. Ammoniagenesis was back to normal after 24 hours of recovery which is what occurs with in vivo NH₃ excretion. This implies an intrarenal adaptation in the 24-hour recovered situation. Acidotic rats were subjected to a metabolic alkalosis following a 7-day acidotic challenge. Isolated kidneys perfused 8-hours after this accelerated recovery was initiated maintained acidotic rates of ammoniagenesis. This is contrary to in vivo NH₃ excretion in rats treated in a similar manner in which a return of NH₃ excretion to normal is observed. Thus, an acute, reversible, extrarenal influence is responsible for the supervision of ammoniagenesis under these conditions. -- With the isolated cortical tubule preparation, rates of ammoniagenesis from glutamine were found to be stimulated by acute acidosis (pH 7.06) and with L-methionine-s-sulfoximime (M.S.O.) an inhibitor of glutamine synthetase (G.S.). In tubules incubated with glutamate NH₄CL and lactate acute acidosis was found to inhibit NH₃ uptake and glutamine synthesis. This reaction to acute acidosis was also demonstrated in cortical tubules from acidotic and 24-hour recovery animals. From these results it is clear that in some way the acute change in H⁺ concentration effectively decreases the activity of G.S. resulting in an apparent stimulation of ammoniagenesis. -- A model is presented which correlates this observation with anatomical heterogenicity and in vivo responses to acute acidosis