Chronic Stimulation Modifies the Isotonic Shortening Velocity of Denervated Rat Slow-Twitch Muscle

Rat soleus muscles were denervated and stimulated in vivo for periods of up to 104 days. Stimuli used were trains of 1 ms pulses at 100 Hz delivered for periods of 1 s; trains were repeated every 10-100 s. In a majority of animals the tension of the muscles was maintained at about 10% of normal, equivalent to muscles denervated but unstimulated for 20 days. At the longest periods the stimulated muscles developed ten times more tension than ones that were denervated but not stimulated. In denervated and denervated-stimulated muscles twitch contraction and relaxation times were prolonged, compared with controls, for up to 3 weeks. Thereafter both sets showed a speeding of the isometric twitch that was greater in the stimulated muscles. At the longest periods the twitch was as short as that of a denervated fast muscle. Stimulation did not affect contralateral denervated muscles. Twitch: tetanus ratios remained high despite stimulation, and muscles showed little post-tetanic potentiation. Tension developed more rapidly in the tetani of the stimulated muscles, even allowing for larger final values. Maximum velocity of shortening was increased in many of the stimulated muscles, and there was a proportional flattening of the force-velocity curve, i.e. a/P₀ increased. Maximum velocity and a/P₀ increased reciprocally with twitch time to peak, so that those muscles that had twitches most changed by stimulation also had their isotonic properties modified to the greatest extent. Even at the longest period of stimulation, twitch time course and tetanic tension were not converted to those of normal fast muscle.