pH dependence of myosin binding-induced activation of the thin filament in cardiac myocytes and skeletal fibers
Metzger, J.M.
American Journal of Physiology 270(3 Pt 2): H1008-H1014
1996
ISSN/ISBN: 0002-9513 PMID: 8780197 Document Number: 458304
The pH dependence of myosin binding-induced thin filament activation was determined in permeabilized cardiac myocytes and slow- and fast-twitch single skeletal muscle fibers by experimental lowering of (MgATP) in the Ca-2+-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 pS range of 4.9-5.3. At pH 7.00, the transition from the relaxed to the activated rigor state was steep in cardiac myocytes (Hill value (n-H) = 21.2+- 3.1 (SE)) and due to the apparent effect of strongly bound cross bridges to cooperatively activate the thin filament in the absence of added Ca-2+. At pH 6.20, the steepness of the tension-pS relationship was markedly reduced (n-H = 6.1 +- 1.0) and the midpoint of the relationship (pS-50) was shifted to higher pS values 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-twitch skeletal muscle fibers. These findings suggest that myosin binding-induced activation of the thin filament is pH dependent in cardiac myocytes but not in skeletal muscle fibers under these experimental conditions in which Ca-2+ is absent.