Calcium transport across peritubular surface of the marine teleost renal tubule.

Isolated renal tubules of seawater-acclimated flounder (Pseudopleuronectes americanus) readily accumulated Ca and Mg when exposed to increased concentrations of these ions. Evidence based on the ability of the tubules to maintain high K levels indicated that the Ca was neither free in the cytosol nor taken up by mitochondria. Two phases of Ca uptake were seen. The fast phase was kinetically complex and not affected by Mg, dinitrophenol, cytochalasin B, or Na-free medium, but was stimulated by ouabain. The slow phase showed Michaelis-Menten kinetics and was inhibited by Mg and dinitrophenol, but was stimulated by Na-free medium, cytochalasin B, and ouabain. The slow phase was also more responsive to changes in bath Ca activity and was thought to be involved with the powerful divalent cation secretory system of these tubules. No correlation between divalent cation uptake and fluid content of the tubules was seen.