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

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 testedthe hypothesis that mechanical loading, not stimulation frequency perse, plays a key role in determining the plasticity of myosin heavychain (MHC) protein isoform expression in muscle undergoing resistancetraining. Female Sprague-Dawley rats were randomly assigned toresistance-training programs that employed active1) shortening(n = 7) or2) lengthening contractions(n = 8). The medial gastrocnemius (MG)muscles in each group trained under loading conditions thatapproximated 90-95% of maximum isometric tetanictension but were stimulated at frequencies of 100 and~25 Hz, respectively. Lengthening and shortening contractions wereproduced by using a Cambridge ergometer system. The MG muscles trainedevery other day, performing a total of 16 training sessions. Bothtraining programs produced significant (P < 0.01) and similar reductions inthe fast type IIB MHC protein isoform in the white MG muscle, reducingits relative content to ~50% of the total MHC protein isoform pool.These changes were accompanied by increases in the relative content ofthe fast type IIX MHC protein isoform that were of similar magnitudefor both groups. The results of this study clearly demonstrate thatstimulation frequency does not play a key role in modulating MHCisoform alterations that result from high-resistance training.