RESUMEN
INTRODUCTION: Fatigue disrupts muscle force summation and is associated with a decrease in cytoplasmic Ca(2+) concentration. The purpose of this study was to compare summation during fatigue and recovery with summation during dantrolene-induced inhibition of Ca(2+) release. METHODS: Rat medial gastrocnemius muscles were evaluated before and after fatigue, or during exposure to dantrolene. Summation was quantified by the ratio of the force transient associated with the final activation in a train of stimuli (Twf), obtained by subtraction of the force with one less stimulus, and the force of the twitch (Tw). RESULTS: This ratio (Twf/Tw) decreased from 2.46 ± 0.11 (mean ± SEM) to 0.8 ± 0.1 during intermittent contractions, but was still significantly different from non-fatigued muscle after 10 min of recovery. Dantrolene altered summation, as Twf/Tw was 1.7 ± 0.2 and 1.27 ± 0.15 at a low dose and a high dose, respectively. CONCLUSIONS: Inhibition of Ca(2+) release alters summation, but repetitive stimulation leading to fatigue changes it more substantially.