Role of contractile protein activation in the length-dependent
modulation of tracheal smooth muscle force.
Mehta, Dolly, Ming-Fang Wu, and Susan J. Gunst.
Dept. of Physiology and Biophysics, Indiana Univ. School of
Medicine, Indianapolis, IN, 46202
APStracts 2:0250C, 1995.
The active isometric force a muscle develops decreases at muscle
lengths below an optimal length, Lo. However, when the length of an
actively contracting muscle is abruptly decreased, a lower level of
isometric force is reached during force redevelopment than when the
contraction is initiated at the shorter length. This has been
attributed to a deactivation of contractile proteins caused by
shortening. In this study, intracellular Ca2+ ([Ca2+]i) and myosin
light chain (MLC) phosphorylation were measured to assess the
mechanisms for the modulation of isometric force caused by changing
smooth muscle length before or during isometric contraction. The
decline in isometric force between Lo and 0.5Lo was associated with
decreases in both MLC phosphorylation and [Ca2+]i during contractions
elicited by ACh or 60 mM KCl. Quick release of the muscle during
contraction depressed force redevelopment at the shorter length but
not MLC phosphorylation. We conclude that decreases in Ca2+
-calmodulin dependent MLC phosphorylation contribute significantly to
the decline in isometric force at lengths below Lo, but the
depression of contractility associated with the quick release of
actively contracted smooth muscle is not caused by a shortening
-induced deactivation of contractile proteins.
Received 30 March 1995; accepted in final form 28 June 1995.
APS Manuscript Number C178-5.
Article publication pending Am. J. Physiol. (Cell Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 18 July 1995.