Effects of Sodium Vanadate on the Smooth Muscle of the Rat Portal Vein

The effects of vanadate on the mechanical and electrical activity of the rat portal vein were examined. Vanadate increased the height of the spontaneous contractions of the portal vein without affecting basal tension. This effect was not blocked by alpha-blockers or atropine. Vanadate did not exert any effect when the spontaneous contractions were abolished by high concentrations of verapamil or in the absence of Ca. Vanadate did not change the resting membrane potential of the smooth muscle cells of the vein, but it increased the rate of rise and the amplitude of the action potentials and the number of action potentials per burst. The intracellular Na content was increased slightly by vanadate. However, the effects of Na,K-pump inhibition by ouabain or K-removal were quite different from those of vanadate, and these procedures did not block the effect of vanadate, although they attenuated it. Vanadate also potentiated the K-contracture of the portal vein. These results indicate that vanadate increases the amplitude of the spontaneous contractions through an increase in the number of action potentials per burst which underly the contraction. These effects are not mediated by intrinsic nerve excitation or inhibition of Na,K pump activity. The facilitation of Ca entry through the voltage dependent Ca channel may be the main cause of the effect of vanadate on mechanical activity.