Modulation of myosin isoform expression by mechanical loading: role of stimulation frequency

Abstract

Caiozzo, Vincent J., Michael J. Baker, and Kenneth M. Baldwin. Modulation of myosin isoform expression by mechanical loading: role of stimulation frequency. J. Appl. Physiol. 82(1): 211–218, 1997.—This study tested the hypothesis that mechanical loading, not stimulation frequency per se, plays a key role in determining the plasticity of myosin heavy chain (MHC) protein isoform expression in muscle undergoing resistance training. Female Sprague-Dawley rats were randomly assigned to resistance-training programs that employed active 1) shortening ( n = 7) or 2) lengthening contractions ( n = 8). The medial gastrocnemius (MG) muscles in each group trained under loading conditions that approximated 90–95% of maximum isometric tetanic tension but were stimulated at frequencies of 100 and ∼25 Hz, respectively. Lengthening and shortening contractions were produced by using a Cambridge ergometer system. The MG muscles trained every other day, performing a total of 16 training sessions. Both training programs produced significant ( P < 0.01) and similar reductions in the fast type IIB MHC protein isoform in the white MG muscle, reducing its relative content to ∼50% of the total MHC protein isoform pool. These changes were accompanied by increases in the relative content of the fast type IIX MHC protein isoform that were of similar magnitude for both groups. The results of this study clearly demonstrate that stimulation frequency does not play a key role in modulating MHC isoform alterations that result from high-resistance training.