Peritubular uptake of lactate by Thamnophis proximal tubule.

Lactate is absorbed in the proximal tubule and also enters tubular cells at the peritubular membrane. To characterize peritubular lactate entry, lactate uptake was measured in isolated nonperfused proximal tubules. Tubules were dissected and incubated in Ringer solution with L(+)-[U-14C]lactate and 3H2O. After incubation, the tubules were extracted, and the extracts were assayed for 14C and 3H or were chromatographed to determine the percentage of tubule 14C identifiable as lactate. Maximal steady-state tubular fluid-to-bath lactate concentration ratios (TF/B lactate) occurred by 30-60 min incubation at 25 degrees C. In 30 min, one-third of the tubules established a TF/B lactate ratio greater than 1.00, and 61.4 +/- 18.6% of tubule 14C was lactate. There was no difference in TF/B lactate ratio in proximal and distal proximal segments. Uptake was depressed at 5 degrees C. Mersalyl at 10(-4) M increased the TF/B lactate ratio and tubule water content. Probenecid at 7.5-30 x 10(-4) M also increased the TF/B lactate ratio. Distal proximal tubules incubated with [3H]PAH showed a control TF/B para-aminohippurate (PAH) ratio of approximately 30, but with 10(-4) M mersalyl the TF/B PAH ratio was approximately 1.00. Lactate uptake at the peritubular membrane occurs against an electrochemical gradient, independently from the PAH transport mechanism.