AJP - Cell Physiology, Vol 256, Issue 1 C59-C66, Copyright © 1989 by American Physiological Society
ADP release from myosin in permeabilized smooth muscle
T. M. Butler, D. S. Pacifico and M. J. Siegman
Department of Physiology, Jefferson Medical College, Philadelphia, Pennsylvania 19107.
The purpose of this study was to determine the nucleotide bound to myosin
and its rate of release under relaxed and activated conditions in
permeabilized rabbit portal veins. Incubation of the muscles in a relaxing
solution containing [3H]-ATP resulted in the formation of 60-70 microM
radiolabeled ADP in the muscle whether or not the myosin light chains had
been thiophosphorylated. This value was similar to the estimate of the
concentration of myosin subfragment 1. Upon transfer of the muscles to a
chase solution containing no labeled ATP, there was a very slow loss of
labeled ADP when the light chains were unphosphorylated, but a much faster
release occurred when the light chains were thiophosphorylated. The results
suggest that smooth muscle myosin exists primarily in a complex with ADP
under both relaxed and phosphorylated conditions and that phosphorylation
of all of the light chains results in a large increase in the rate of
release of the products of ATP splitting from all of the myosin.
Interestingly, the exponential release of ADP in relaxed muscle shows two
components, one of which contains about two-thirds of the total ADP and is
5- to 10-fold faster than the other. If the difference in rates of ADP
release observed in relaxed muscle persists when the myosin is
phosphorylated, then it is possible that there is a 5- to 10-fold
difference in rates of cycling for different phosphorylated cross bridges
in smooth muscle.
