Inhibition of force and shortening in smooth muscle by vanadate

We have investigated the effects of vanadate (Vi) on force generation by, and shortening of, chemically skinned smooth muscle preparations from guinea-pig taenia coli at 22 degrees C. A method, using phosphatase inhibitors, was introduced to obtain stable, long-lasting contractions in thiophosphorylated preparations. Vi (10-1000 microM) dose-dependently inhibited active force, to about 20% of its maximum level. At a higher temperature (30 degrees C), the rate of inhibition was faster but the extent of inhibition was less. The rate of contraction following photolytic release of ATP to fibres in rigor was not affected by Vi (30 microM). The maximal shortening velocity (Vmax) was inhibited in a similar manner as active force by Vi (30 microM). In conclusion, the results suggest that Vi interacts with a force-generating actomyosin-ADP (AMADP) state reached after phosphate release. The rate of inhibition of smooth muscle contraction was markedly lower than in skeletal muscle, suggesting differences either in properties of the Vi-bound states or, more likely, in the concentration of AMADP states capable of binding Vi. This suggests that the long duty cycle in smooth muscle is not associated with a higher relative population of AMADP states reached immediately after Pi release, but rather by an increase in the population of subsequent force-generating cross-bridge states. The Vi-bound cross-bridges introduce an internal load to shortening, possibly acting in a similar manner as cross-bridge states introduced at low levels of activation.