Myosin-product complex in the resting state and during relaxation of smooth muscle

Previous findings suggested that in resting smooth muscle ADP is bound to myosin and that phosphorylation of the myosin, and its subsequent interaction with actin, increases the rate of ADP release. We have now extended these studies to include measurements of bound Pi as well as bound ADP in permeabilized rabbit portal vein. We report that in resting smooth muscle that has been exposed to [3H]ATP and [gamma-32P]ATP, followed by a chase in an unlabeled relaxing solution, the ratio of bound [3H]ADP to bound [32P]Pi is close to unity, and both are released at approximately the same rate. This suggests that myosin exists predominantly with both ADP and Pi bound under resting conditions and that the release of one is quickly followed by the release of the other. In contrast, there is a significant 30% excess of bound Pi over ADP in a muscle during relaxation from an isometric contraction. Under these conditions, while force output is slowly decreasing, both light chain phosphorylation and adenosinetriphosphatase (ATPase) activity have decreased to near-resting values. The time course of relaxation is similar to the time course of Pi release from both the resting and relaxing muscle. We propose that during relaxation the dephosphorylated cross bridges which are bearing force have Pi but not ADP bound and that detachment of the cross bridge (and thus force decay) is limited by Pi release from myosin which occurs at the same rate as in the resting muscle.