Vanadate stimulates the pumped movements of Na in skeletal muscle

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Abstract
We have measured the effects of concentrations of vanadate ranging between 0.01 and 10 mM on the22Na efflux of frog sartorius muscles. The addition of vanadate had no effects when concentrations lower than 0.5 mM were used; higher concentrations increased Na efflux. The increase was abolished by the addition of ouabain (10−5M). In muscles pretreated with ouabain vanadate did not modify Na efflux. The stimulatory effects of vanadate on Na efflux were also observed in Na-free solutions indicating that the effux of vanadate was not caused mainly either by an increase in the exchange of Na for Na or by an increase in Na entry into the muscle. We also examined the effects of vanadate on muscles immersed in solutions containing 20 mM K+; both vanadate and increased K+ produced stimulations of Na efflux that were additive. Similarly when the effects of vanadate and insulin were measured on the Na efflux of the same muscle, additive effects were found. As the ouabain-sensitive Na efflux in frog muscle is generally agreed to be due to the activity of the Na-K ATPase, our findings suggest that the net effect of vanadate in intact muscle cells is an increase in the activity of the Na pump. Since vanadate affects many enzymes it is quite possible that the stimulatory action is not due to a direct effect on the Na-K ATPase but may be mediated through an intermediary step. Regardless of the specific mechanism, it is evident that, our results as well as other findings in the literature, strongly indicate that Na pumping by intact cells can be increased by vanadate administration. Hence it is not justified to attribute the physiological modifications caused by vanadate administration to blockade of the Na-K ATPase unless the attribution is justified by specific experimental evidence.

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