Recovery of muscle from tetrodotoxin-induced disuse and the influence of daily exercise. 1. Contractile properties.

The extent of recovery in rat gastrocnemius muscles which have undergone atrophic changes due to 4 weeks of inactivity caused by sciatic nerve superfusion of tetrodotoxin, was investigated. Control and tetrodotoxin-treated female Sprague-Dawley rats were subjected to either a daily program of grid-climbing or swim-training for 4 weeks, or benefited from cage activity only. In situ contractile properties of the gastrocnemius were then measured. Twenty-eight days of recovery was insufficient to allow a complete reversal of the changes produced by 28 days of inactivity. The gastrocnemius remained atrophied (25%) and tetanically weaker than normal in the recovered animals. The maximal rate of rise of the twitch was partially recovered whereas that developed during a maximal contraction was normal. Maximal rate of rise expressed relative to the tension output was elevated, however, for both twitch (%Pt/ms) and maximal contraction (%Po/ms). Swimming did not influence the recovery of muscle size or strength. Grid-climbing, on the other hand, produced heavier and stronger muscles and a faster recovery of %Pt/ms. This study clearly illustrates the importance of muscle load in regulating muscle size and strength.