Inhibition of force and shortening in smooth muscle by vanadate

We have investigated the effects of vanadate (V-i) on force generation by, and shortening of, chemically skinned smooth muscle preparations from guine a-pig taenia coli at 22 degrees C. A method, using phosphatase inhibitors, was introduced to obtain stable, long-lasting contractions in thiophosphory lated preparations. V-i (10-1000 mu M) dose-dependently inhibited active fo rce, 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 les s. The rate of contraction following photolytic release of ATP to fibres in rigor was not affected by V-i (30 mu M). The maximal shortening velocity ( V-max) was inhibited in a similar manner as active force by V-i (30 mu M). In conclusion, the results suggest that V-i interacts with a force-generati ng actomyosin-ADP (AMADP) state reached after phosphate release. The rate o f inhibition of smooth muscle contraction was markedly lower than in skelet al muscle, suggesting differences either in properties of the V-i-bound sta tes or, more likely, in the concentration of AMADP states capable of bindin g V-i. This suggests that the long duty cycle in smooth muscle is not assoc iated with a higher relative population of AMADP states reached immediately after P-i release, but rather by an increase in the population of subseque nt force-generating cross-bridge states. The V-i-bound cross-bridges introd uce an internal load to shortening, possibly acting in a similar manner as crossbridge states introduced at low levels of activation.