Effects of vanadate on the functional properties of the isolated toad bladder.

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Abstract
Vanadate, considered a candidate for physiological modulation of Na pumping activity, was studied in the isolated toad urinary bladder. It produced a decrease in Na transport activity that was reversible and could be partially antagonized by pretreatment with SITS [4-acetamino-4''-isothiocyano-2-2''-disulfonic stilbene]. This suggest an intracytoplasmic site of action for vanadate. Other transition metal salts, prepared from Ta and Nb, produced instead a transient rise in Na transport and this was unaffected by SITS. Comparison between ouabain and vanadate showed clear-cut differences: inhibition of Na transport by the former, not the latter, was partially overcome by increasing cell Na or serosal K. Unexpectedly intracellular K did not decrease appreciably following vanadate treatment, in contrast to what occurs when ouabain was used instead. Vasopressin-induced hydro-osmotic flow was irreversibly inhibited by vanadate but not by ouabain; pretreatment with SITS attenuated this effect. Vanadate blocked urinary acidification in this epithelium, thus supporting the hypothesis that proton flow occurs through an enzyme distinct from the mitochondrial ATPase, insensitive to vanadate.

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