H+-dependent sulfate secretion in the marine teleost renal tubule

Renal sulfate secretion was studied in the southern flounder Paralichthys lethostigma. In the intact animal sulfate clearance averaged 12 times the glomerular filtration rate and about 93% of the excreted sulfate was secreted. The secretory maximum was 150 mumol . kg body wt-1 . h-1 at 5 mM plasma sulfate. DIDS inhibited sulfate secretion in vivo. In vitro sulfate uptake by renal tubule fragments was saturable (Km = 0.92 mM) and could be inhibited by DIDS, SITS, and antimycin A. Na-free medium and ouabain also inhibited sulfate uptake, suggesting involvement of the bath-to-cell Na gradient in sulfate secretion. However, no Na effect on sulfate uptake could be demonstrated in brush border membrane (BBM) or basolateral membrane (BLM) vesicles. Instead, concentrative sulfate uptake (i.e., overshoot) by BLM but not BBM vesicles could be produced by an out greater than in proton gradient. Generation of an inside-positive membrane potential (valinomycin in the presence of an out greater than in potassium gradient) slightly stimulated sulfate uptake by BLM and had no effect on BBM, indicating that the rate-limiting step for sulfate uptake at both membranes was an electroneutral process. H+ gradient-dependent sulfate uptake by BLM was inhibited by the proton ionophore FCCP, by the anion transport inhibitor DIDS, and by HgCl2. A tentative model of sulfate secretion is proposed.