Renal handling of taurine, L-alanine, L-glutamate and D-glucose in Opsanus tau: studies on isolated brush border membrane vesicles.

Renal brush border membrane vesicles (bbmv) from the aglomerular toadfish (Opsanus tau), isolated by differential precipitation, were tested for their ability to actively translocate (i) taurine, known to be secreted by the kidney of several marine teleosts, and (ii) L-alanine, L-glutamic acid, and D-glucose, solutes that are normally reabsorbed in the filtering nephron. Vesicular taurine uptake displayed a Na+ dependence. Transport was greatest under conditions of an inward-directed Na+ gradient, but a significant stimulation by Na+ over K+ could also be observed in the absence of a salt gradient. At high extravesicular K+, the addition of valinomycin reduced taurine uptake. Na+-dependent 3H-taurine flux was almost completely inhibited by non-labeled taurine (tracer replacement) or beta-alanine, but was unaffected by L-alanine. Replacement of medium chloride by SCN- or NO3- in the presence of Na+ resulted in significantly lower uptake rates under both anion gradient and anion equilibrium conditions, whereas Br- could almost fully substitute for the stimulatory Cl- action. These results indicate the presence of an electrogenic Na+-cotransport mechanism with specificity for beta-amino acids in the toadfish renal brush border. Whether the system under physiological conditions mediates reabsorption or secretion of taurine remains to be determined. Toadfish bbmv also translocated L-alanine and L-glutamic acid in a Na+-dependent manner. Possible roles for these most likely reabsorptive transport systems in a non-filtering kidney are discussed. D-glucose uptake, however, appeared to occur via Na+-independent pathways, since it was not affected by phlorizin in the presence of Na+, or by Na+ replacement.