AJP - Cell Physiology, Vol 258, Issue 6 C1092-C1099, Copyright © 1990 by American Physiological Society
Myosin-product complex in the resting state and during relaxation of smooth muscle
T. M. Butler, M. J. Siegman, S. U. Mooers and S. R. Narayan
Department of Physiology, Jefferson Medical College, Thomas Jefferson University, Philadelphia, Pennsylvania 19107.
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.
