Ph dependence of myosin binding-induced activation of the thin
filament in cardiac myocytes and skeletal muscle fibers.
Metzger, Joseph M.
Department of Physiology, University of Michigan, School of
Medicine, Ann Arbor, MI 48109
APStracts 2:0393H, 1995.
The pH dependence of myosin binding-induced thin filament activation
was determined in permeabilized cardiac myocytes and slow and fast
single skeletal muscle fibers by experimentally lowering the
concentration of MgATP in the Ca2+-free solutions bathing the
permeabilized preparations. As the pS (where S is MgATP and pS is
-log[MgATP]) was increased from 3.0 to 8.0, isometric tension
increased to a peak value in the range of pS from 4.9 to 5.3. At pH
7.00, the transition from the relaxed to the activated rigor state
was steep in cardiac myocytes (Hill value, n, = 21.2 +/- 3.1; x +/-
SEM) and due to the apparent effect of strongly bound cross-bridges
to cooperatively activate the thin filament in the absence of added
Ca2+. At pH 6.20, the steepness of the tension-pS relationship was
markedly reduced (n = 6.1 +/- 1.0) and the mid-point of the
relationship (pS50) was shifted to higher pSs in cardiac myocytes. In
comparison, reduced pH had no effect on the steepness or position of
the tension-pS relationship in single slow or fast skeletal muscle
fibers. These findings suggest myosin binding-induced activation of
the thin filament is pH dependent in cardiac myocytes but not
skeletal muscle fibers under these experimental conditions in which
Ca2+ is absent.
Received 25 January 1995; accepted in final form 10 July 1995.
APS Manuscript Number H70-5.
Article publication pending Am. J. Physiol. (Heart Circ. Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 23 September 1995.