Lactate metabolism in the isolated perfused rat kidney: relations to renal function and gluconeogenesis.

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Abstract
In the intact dog decreases in both glomerular filtration rate and net renal Na+ reabsorption due to raised ureteral pressure were not associated with a decrease in renal lactate oxidation rate, although total renal CO2 production decreased in proportion to the changes in net renal reabsorption of Na+ and glomerular filtration rate. In order to determine whether in the absence of other added substrates the metabolism of lactate supports only the basal renal metabolism or can enhance renal function as well, the rate of lactate utilization and decarboxylation by the isolated perfused rat kidney were quantified in relation to renal function and 1 measure of renal basal metabolism, glucose production. The perfusate was Krebs-Ringer bicarbonate (pH 7.35-7.48) with Fraction V bovine serum albumin, 6g/100 ml. L-(+)-lactate was added to raise the lactate concentration from endogenous levels to 2.5, 5.0 or 10 mM. The following were determined: net lactate utilization rate, lactate decarboxylation rate (14CO2 produced from L-(+)-[U-14C]lactate), net glucose production rate, and net re-absorptive rate of Na+. The apparent Km and Vmax for lactate oxidation were 2.1 mM and 1.29 .mu.mole .cntdot. g-1 .cntdot. min-1, respectively. There was no apparent maximum for total lactate utilization rate due to continuing increases in glucose production rate as lactate concentration was raised. At approximately 10 mM lactate, glucose production accounted for about half of the total lactate utilized. Therefore the basal energy requirements of the kidney need not be constant, since glucose production increases as lactate concentration is raised. Both lactate oxidation rate and lactate utilization rate were significantly correlated with the net reabsorption of Na+ by the renal tubules, with the percentage of filtered Na+ reabsorbed and with the glomerular filtration rate. The major fraction of the net renal reabsorption of Na+ was probably supported by the metabolism of substrates either bound to albumin or derived from renal tissue since the percentage of filtered Na+ reabsorbed increased from approximately 78%, when no lactate was added, to 97% when initial lactate concentration was 10 mM. Addition of lactate increased both the basal metabolism and tubular function. Whether it was the presence of lactate, or its utilization by oxidative or by other pathways which enhanced net renal reabsorption of Na+ and the glomerular filtration rate was not determined.

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