Adaptation to taurine deprivation in the phosphate-depleted rat

The nonessential amino acid taurine, which is inert in renal tissue, was used to study renal adaptation in the presence of phosphate (P) depletion in the rat. Weanling rats were placed on the control diet (0.7% P, normal taurine) for 4 wk, then fed one of the experimental diets or continued on the control diet for 1 wk. The diets used were 1) P+T- (0.7% P, low taurine), 2) P-T+ (0.1% P, normal taurine); and 3) P-T- (0.1% P, low taurine). Taurine deficiency was associated with avid tubular reabsorption of taurine, irrespective of P status. This was associated with a 4- to 5-fold increase in the Vmax of uptake (p less than 0.001). On the other hand, P depletion increased the Km of taurine uptake by 6.6- to 9.5-fold, suggesting a decrease in the affinity of the taurine symport (p less than 0.001). This was independent of the taurine status of the animals. Although there was no effect of diet on the urinary excretion of beta-alanine, P depletion, irrespective of taurine status, resulted in a 2-fold increase in the Km of beta-alanine uptake. We conclude that the taurinuria of P depletion is reversed in taurine deprivation. The adaptive response involves an increase in the Vmax of uptake. However, the increase in Km of taurine uptake observed in P depletion does not reverse with taurine depletion.