Adaptation to Taurine Deprivation in the Phosphate-Depleted Rat

Abstract

ABSTRACT. 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 V,,, of uptake ( p < 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 < 0.001).This was independent of the taurine status of the animals. Although there was no effect of diet on the urinary excretion of 8-alanine, P depletion, irrespective of taurine status, resulted in a 2-fold increase in the Km of 8-alanine uptake. We conclude that the taurinuria of P depletion is reversed in taurine deprivation. The adaptive response involves an increase in the V,,, of uptake. However, the increase in Km of taurine uptake observed in P depletion does not reverse with taurine depletion. (Pediatr Res 30: 146-149, 1991) Abbreviations P, phosphorus BBMV, brush border membrane vesicle HEPES, N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid P+T-, phosphate replete, taurine-depleted P-T-, phosphate-and taurine-depleted P-T+, phosphate-depleted, taurine replete The renal tubular epithelium is able to conserve amino acids during periods of decreased dietary intake. Rats or mice fed diets low in sulfur-containing amino acids adapt by increasing the tubular reabsorption of these amino acids, particularly taurine (1, 2). The adaptive response manifests at the luminal surface by enhanced taurine uptake by BBMV prepared from rats fed these diets (3). These changes are associated with an increase in the initial rate of uptake (V,,,) rather than a decrease in the affinity of the symport (Km) (3). The physiologic signal(s) for the observed changes relate, in part, to alterations in the renal cortical content of taurine (4, 5). On the other hand, it has been shown that vitamin D-and/or phosphate-depleted rats develop taurinuria in the absence of secondary hyperparathyroidism (6). The taurinuria manifests at the renal brush border membrane by a decrease in the affinity of the taurine symport, resulting in decreased accumulation of taurine by BBMV (7). No changes could be elicited for the V,,, of uptake. In addition, the perturbation in uptake was not associated with alterations in plasma or renal cortex taurine concentrations, suggesting that overflow is not a regulatory signal in this model.Although the adaptive response has been shown to exist in terms of urinary excretion at the apical surface in adult rats, little is known about the response of these animals to taurine deprivation in the presence of phosphate depletion. Herein, we ...

Early Renunciants as Hadīth Transmitters

“…He is then told yes by Muḥammad ibn Yaḥyá al-Taymī, after which he spends lavishly on the mosque in gratitude. 141…”