The proposed experiments will continue an ongoing investigation of the mechanism of lactate trasport by the proximal tubule of the Garter snake, Thamnophis species. In previous work using isolated perfused proximal tubule dissected from the kidney of Thamnophis, active, Na ion dependent lactate absorption was demonstrated. Next, the effect on the unidirectional lactate absorptive flux of varying lactate concentration in perfusate and bath will be explored. By raising lactate concentration, it should be possible to determine if the absorptive flux saturates at some attainable lactate concentration. If evidence for saturation is obtained, the data will be subjected to Michaelis-Menton analysis, to estimate the maximal flux rate (Jmax) and the affinity constant (Km) for this transport process. The effect on the backflux of lactate of raising bath lactate concentration will also be considered. Further, since high lactate concentrations have been shown to damage renal tissue in other systems, experiments will be carried out to determine if the lactate concentrations used affect the passive permeability characteristics Themnophis proximal tubules perfused in vitro. Other experiments involving selection of a better volume marker for determination of volume absorption by Thamnophis proximal tubules may also be carried out if time permits.