AJP - Cell Physiology, Vol 258, Issue 2 C369-C375, Copyright © 1990 by American Physiological Society
Smooth muscle energetics and theories of cross-bridge regulation
R. J. Paul
Department of Physiology, Monash University, Clayton, Victoria, Australia.
The energetics of smooth muscle is characterized by low tension cost (rate
of ATP utilization per isometric force/cross-section area), ranging from
100- to 500-fold less than skeletal muscle. The efficiency (ATP usage per
work) of smooth muscle, although less well documented, is also somewhat
(4-fold) less than skeletal muscle. Another well-known characteristic of
smooth muscle is the linear relation between the steady-state of ATP
utilization (JATP) and isometric force. Recently, Murphy and colleagues
[C.-M. Hai and R. A. Murphy. Am. J. Physiol. 254 (Cell Physiol. 23)
C99-C106, 1988] have put forth a kinetic model of cross-bridge regulation
that predicts the time course of stress and myosin light chain
phosphorylation (MLC-Pi). The energetics consequences of this model, in
brief, are that the low tension cost is partly attributed to a slow
detachment rate of the myosin cross bridge when dephosphorylated when
attached to actin ("latch state"), whereas the lower efficiency is ascribed
to a high rate of myosin phosphorylation-dephosphorylation inherent to a
fit of data to this kinetic scheme. This latter corollary is somewhat
controversial in light of current interpretations of smooth muscle
energetics data. Using SCoP software (National Biomedical Simulation
Resource, Duke University), we tested this model in terms of fitting
existing data with respect to 1) is a high myosin-dephosphorylation
adenosine triphosphatase (ATPase) necessary to fit the available data on
the time course of stress and MLC-Pi?; and 2) can this model predict the
observed linear relation between the steady-state rate of ATP hydrolysis
(JATP) and isometric force?(ABSTRACT TRUNCATED AT 250 WORDS)
