Renal ammoniagenesis in vitro at medium pH 7.0 and 7.8 is of similar magnitude with glutamine as substrate (1, 2), whereas ammoniagenesis from glutamate is greater at the lower pH (3). Our purpose was to determine why such differences exist. To do this, we followed certain aspects of glutamine and glutamate metabolism by rat kidney slices at medium pH 7.0 and 7.8 under various conditions. We also followed the effects of pH on phosphate dependent glutaminase (PDG) activity in homogenates. We conclude that metabolism of glutamine by rat kidney slices at pH 7.0 and pH 7.8 is different despite a similar magnitude of ammonia production.Materials and Methods. We used male Spague-Dawley rats weighing 250-300 g. These rats were fed Purina Rat Chow and H,O ad lib. In some studies, the rats were given 2 intraperitoneal injections of methionine sulfoximine (20 mg/100 g) 17 and 4 hr prior to study. Two slices from each rat's kidney were cut on a Stadie-Riggs microtome and then bisected-each half approximated 30-40 mg. The medium was bicarbonate buffered so that the initial pH and pH following incubation in all studies approximated 7.0 and 7.8 (12 and 48 mM bicarbonate, respectively). One slice half was incubated at pH 7.0 and the other half at 7.8 for 90 min. Some media contained ISupported by NIH Grant 15458 and a Grant 2 Fellow of the Washington Heart Assoc. 3Fellow of the Bureau of Medicine, USN. from Washington Heart Assoc. 803no substrate glutamine 2 mM, NH,Cl 1 mM, glutamate 1 mM or 10 mM, and/or a-ketoglutarate 1 mM; and the gas phase was either 95% 0,-5% CO, or 95% N,-5% CO,. Glutamate was measured by the method of Meiss, Peyser and Miller (4). Phosphate dependent glutaminase I (PDG) activity was estimated using a procedure detailed by Janicki ( 5 ) . The method of deproteinization of medium and estimations of ammonia and glucose have been described previously (6-8). pH was determined anaeroblcally using the expanded scale of a Copenhagen Radiometer pH meter, model 27. Results from both slice pairs were averaged to give one result, and the data compared statistically by paired analysis using Student's t test. Statistical significance was set at P < .05.Results. Efiects of p H on aerobic glutamine and glutamate metabolism. Slice studies performed in the bicarbonatebuffered media gassed with 95% 0, and 5% COz are shown in Table I. Without substrate, ammonia production from endogenous sources was not different at the pHs studied although gluconeogenesis was greater at the lower pH (P < .Ol). In agreement with others (1, 2), rat kidney slices incubated aerobically in the presence of glutamine showed no significant difference in renal ammoniagenesis but did show a significant increase in gluconeogenesis at the lower pH. Although ammoniagenesis with glutamine present was similar at either pH, glutamate accumulation was greater at pH 7.8. In contrast to glutamine and in agreement with previous findings (3), total ammonia production from glutamate was increased at pH 7.0 in the presence of

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Deficiences in acid excretion during hypercalcemia have been reported, and this defect has been ascribed to a deficiency in ammonia excretion. Because no changes in urine pH and urine flow occurred to explain decreased ammonia excretion, this suggested to us that decreased excretion was secondary to decreased renal ammonia production. To test this hypothesis, we investigated the ability of kidney slices from rats to produce ammonia and glucose and to consume oxygen when incubated in varying concentrations of calcium. High medium concentrations of calcium (3 and 4 mM) decreased kidney slice ammoniagenesis from glutamine and glutamate and kidney slice oxygen consumption while not affecting gluconeogenesis. Based upon our findings, we propose that hypercalcemia decreases urine ammonia excretion by depressing renal ammonia production.