Abstract
Effects of vanadate (NH4VO3: VAN) on tension development, membrane potential and cellular Na content were investigated in guineapig taenia coli depolarized by 62.7 mM KCI solution. VAN (10-4-10-3 M) caused a transient increase in the K developed tension followed by a relaxation. The VAN-induced contraction was observed even in a low Ca (0.13 mM) solution but was inhibited by the removal of external Ca. After the addition of verapamil (5×10-8, 10-7 M), VAN still produced a contraction. Further, the VAN-induced contraction was observed in 142.2 mM KCl (Na 11.9 mM) solution containing ouabain (10-4 M). On the other hand, the effect of VAN to relax the K-induced contraction was dependent on the concentration of VAN. In low Na (choline-substituted) solution, the VAN-induced relaxation was decreased. VAN increased cellular Na content of the depolarized muscle, and a correlation was obtained between the cellular Na accumulation and the relaxation. These results suggest that the relaxation is mainly attributable to the accumulation of Na following the inhibition of the Na pump, while the contraction is independent of the inhibition of the Na pump and less sensitive to the external Ca than the K-induced process.
