THE DOSE-RELATED RESPONSE OF RABBIT FAST MUSCLE TO LONG-TERM LOW-FREQUENCY STIMULATION

Rabbit tibialis anterior muscles were stimulated continuously at 2.5 H z, 5 Hz, or 10 Hz for 10 months. The resulting adaptive transformation was dose-related for contractile speed, myosin isoform composition, a nd enzyme activities. The ''fast-oxidative'' state produced by stimula tion at 2.5 Hz was stable: even after 10 months, 84% of the fibers wer e of type 2A. Absence of a secondary decline in oxidative activity in these muscles provided strong evidence of a causal link between myosin transitions and metabolic adaptation. Significant fiber loss occurred only after prolonged stimulation at 10 Hz. The myosin isoform composi tion of individual muscles stimulated at 5 Hz resembled that of muscle s stimulated at either the lower or the higher frequency, behavior con sistent with a threshold for fiber type change. In clinical applicatio ns such as cardiomyoplasty, muscles could be used more effectively by engineering their properties to combine speed and power of contraction with the necessary resistance to fatigue. (C) 1998 John Wiley & Sons, Inc.