Lactate absorption in Thamnophis proximal tubule: transport versus metabolism

Abstract

Proximal tubules from the kidney of Thamnophis (garter snake) were perfused in vitro and unidirectional fluxes of lactate measured using L(+)-[U-14C]lactate, (lactate concentration, 1 mM). The lumen-to-bath (absorptive) flux (Jlb lact) significantly exceeded the bath-to-lumen flux (backflux) (Jbl lact) in each of 12 tubules (seven distal proximal and five proximal proximal). The flux ratio (Jlb lact/Jbl lact) was approximately 3.00. At flow rates of 13-16 nl/min and lactate concentration of 1 mM the net flux was about 1.60 pmol . min-1 . mm-1 in both proximal proximal and distal proximal segments. Both fluxes were decreased by perfusion at 5 degrees C. To determin e the contribution of metabolism of lactate to its absorption, Jlb lact was measured at 25 degrees C in 10 distal proximal tubules during perfusion with [14C]lactate, lactate concentration, 1 mM, and with [methoxy-3H]inulin. In these experiments, the amount of 14C found in the bath was 93% of the amount of 14C absorbed from the lumen. Chromatography showed that all of the 14C found in the bath was [14C]lactate. These data establish that in Thamnophis proximal tubule lactate absorption occurs against an electro chemical gradient by transport of the intact lactate molecule without significant metabolism.

Comparative Physiology of the Kidney

“…These data suggest, although they do not demonstrate conclusively, that absorbed and metabolized lactate are taken up by the tubule ceUs at opposite membranes and that the pools of absorbed and metab-olized lactate are separate. They support the idea that renal absorptive transport conserves substrate for the entire organism, whereas peritubular accumulation supplies nutrients for the individual tubule cells (33).…”

“…However, among the nonmammalian vertebrates, the tubular absorptive process has been studied only with isolated tubules from garter snakes (Thamnophis spp.) [ Table 5; (33)(34)(35)]. Net absorption occurs throughout the length of the proximal tubule via an energy-requiring, sodium-dependent process (33,35).…”

“…[ Table 5; (33)(34)(35)]. Net absorption occurs throughout the length of the proximal tubule via an energy-requiring, sodium-dependent process (33,35). The lactate transported by perfused tubules from the lumen to the bath is not metabolized, whereas the lactate transported into the cells of nonperfused tubules from the peritubular side is significantly metabolized (33,34).…”

“…Net absorption occurs throughout the length of the proximal tubule via an energy-requiring, sodium-dependent process (33,35). The lactate transported by perfused tubules from the lumen to the bath is not metabolized, whereas the lactate transported into the cells of nonperfused tubules from the peritubular side is significantly metabolized (33,34). These data suggest, although they do not demonstrate conclusively, that absorbed and metabolized lactate are taken up by the tubule ceUs at opposite membranes and that the pools of absorbed and metab-olized lactate are separate.…”

Lactate Oxidation by Three Segments of the Rabbit Proximal Tubule

“…In order to prevent diffusion of PEG out of the cut ends of the tubules, PEG was added to the bath at 10 mA4 throughout the remainder of this experiment. The method of tubule perfusion is based on that originally reported by Burg et al (7) and used previously by us (8). Opening of the tubular lumen during perfusion is readily observed with the stereomicroscope.…”