Chronic low-frequency stimulation of rabbit fast-twitch muscle induced time-dependent increases in the concentration of the sarcoleminal Na+,K+-ATPase and in mitochondrial citrate synthase activity. The almost twofold increase in Na+,K+-ATPase preceded the rise in citrate synthase and was complete after 10 days of stimulation, We suggest that the increase in Na+,K+-ATPase enhances resistance to fatigue of low-frequency-stimulated muscle prior to elevations in aerobic-oxidative capacity.
