Lactate transport by Thamnophis proximal tubule: sodium dependence

We examined the effects of experimental conditions on unidirectional lactate fluxes in isolated perfused Thamnophis proximal tubule. Fluxes were determined by adding L(+)-[U-14C]lactate to perfusate or bath as appropriate (lactate concentration = 1 mM). The lumen-to-bath lactate flux (Jlb lact) was not affected by the manipulations required to exchange perfusate or bath or by substitution of phosphate for bicarbonate buffer. During measurement of Jlb lact, lactate was ordinarily added to the perfusate only. However, addition of 1 mM lactate to the bath had no effect on Jlb lact, indicating no role for exchange diffusion in renal lactate absorption. In contrast, substitution of tetramethylammonium (TMA+) or choline for Na+ decreased Jlb lact by about 75%. The bath-to-lumen flux (Jbl lact) was also decreased by TMA+ substitution for Na+, although by only about 25%. From these and previous results we suggest that in Thamnophis proximal tubule, lactate is absorbed by an active Na+-dependent transport process that probably derives its energy from the lumen-to-cell electrochemical gradient for Na+.