AJP - Cell Physiology, Vol 259, Issue 2 C251-C257, Copyright © 1990 by American Physiological Society
Latch-bridge model in smooth muscle: [Ca2+]i can quantitatively predict stress
C. M. Rembold and R. A. Murphy
Department of Internal Medicine, University of Virginia Health Science Center, Charlottesville 22908.
Ca2+ concentration ([Ca2+])-dependent cross-bridge phosphorylation by
myosin light chain kinase is postulated to be the primary regulator of
stress development in smooth muscle. A four-state model of cross-bridge
function, regulated only by [Ca2+]-dependent changes in myosin kinase
activity, has been proposed to explain contraction and the latch state of
smooth muscle (high force with reduced cross-bridge cycling and ATP
consumption). A key test of this model is to determine whether changes in
myoplasmic [Ca2+], per se, can quantitatively predict changes in myosin
kinase activity, cross-bridge phosphorylation, and therefore force
production. We find that changes in aequorin-estimated myoplasmic [Ca2+]
can quantitatively predict the time course of phosphorylation and isometric
stress production in response to stimulation with histamine and angiotensin
II and during adenosine 3',5'-cyclic monophosphate-mediated relaxation when
[Ca2+] is not changing rapidly. These results suggest that changes in
myoplasmic [Ca2+] and activation of myosin light chain kinase may be
sufficient to explain both contraction and relaxation of agonist stimulated
swine carotid arterial smooth muscle.
