RENAL TUBULAR REABSORPTION, METABOLIC UTILIZATION AND ISOMERIC FRACTIONATION OF LACTIC ACID IN THE DOG

Abstract

Na dl lactate given to the normal dog by mouth was almost completely retained. When the plasma lactate concn. was in- creased by intravenous infusion, urinary excretion was quite small until the plasma concn. approached 1 mg./ml. At greater plasma concns. up to 4 mg./ml., the rate of excretion was proportional to the rate of glomerular filtration of lactate. The mechanism for renal tubular reabsorption of lactic acid exhibited a reproducible maximum capacity. The excretion of lactate at rates of filtration insufficient for complete saturation of the reabsorptive mechanism was discussed in relation to existing theories. Simultaneous saturation of the reabsorptive mechanisms for both glucose and lactic acid had no conspicuous effect on the reabsorption of either substance. In 9 expts. dl mixtures containing from 35-71% of the 1(+) isomer were infused. The isomeric composition of lactate from samples of blood and urine collected over the same period was detd.; the mean 1( + ) fraction in the infusion, blood and urine lactate was 52, 43, and 38% respectively. The 1 (+) and d( [long dash]) components of the dl mixtures were compared according to the fraction of the filtered lactic acid that was reabsorbed and the fraction of infused lactic acid that was utilized. The ratio of the d ([long dash]) fraction to the 1 (+) fraction was substantially the same for both reabsorption and utilization; the means were 0.68 and 0.65, respectively. This suggests a wide-spread preference in the dog for the "natural" over the d( [long dash]) isomer in the ratio of 3 to 2.