Effects of vanadate on mechanical responses and Na-K pump in vascular smooth muscle

The effects of vanadate on tension and on the Na-K pump in isolated guinea-pig aorta were investigated. Vanadates (NH₄VO₃ or Na₃VO₄ · nH₂O) (10^?3 M) produced a sustained contraction (about 0.5 g) which was not influenced by phentolamine (10^?6 M). In the absence of external Ca, vanadate and norepinephrine (2 × 10^?6 M) induced a small contraction, although high K (45.4 mM) did not. In a Ca-depleted, high K (142.2 mM) solution, vanadate and norepinephrine still caused muscle contraction. D600 (10^?6 M) slightly inhibited the contractions induced by vanadate and norepinephrine, while this agent completely inhibited the contraction induced by high K. Sodium nitroprusside (10^?5 M) strongly inhibited the contractions induced by vanadate and norepinephrine but not the contraction induced by high K. Vanadate produced a contraction in K-free solution with ouabain (10^?4 M). The tissue K content did not change during a 2 h treatment of the muscle with vanadate. Reaccumulation of K following a 3 h treatment of the muscle with K-free solution was inhibited by ouabain but not by vanadate. These results indicate a similarity between the contractions induced by vanadate and by norepinephrine and suggest that the vanadate-induced contraction is not due to an inhibition of the Na-K pump.