Effects of carbon dioxide and tetanus duration on relaxation of frog skeletal muscle

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Abstract
Sartorius and semitendinosus muscles of the frogRana temporaria (0° C) were stimulated under isometric (fixed-end) conditions and force during stimulation and relaxation was recorded. Relaxation time was assessed by measuring the time from the last stimulus until force declined to 90% or 50% of its value at that time. The relaxation time increased linearly with concentration of CO2 in the Ringer's solution (pH 7.2), whereas, at constant CO2, relaxation was hardly affected by changes in extracellular HCO3 − and pH. This is consistent with the view that CO2 enters the cells, but H+ and HCO3 −do not, and that entry of CO2 causes intracellular acidification and slows relaxation. For tetani lasting between 0.5 and 5 s, relaxation is slower the longer the tetanus; further increase in tetanus duration to 10 s has little additional effect. The presence of 3.3 mmoll− CO2 in the Ringer's solution has a small effect on relaxation after a brief tetanus, but greatly slows relaxation as tetanus duration increases. Experiments with paired tetani (conditioning and test) showed that relaxation from a test tetanus was slower if a conditioning tetanus was given. However, the size of this effect was independent of muscle length during the conditioning tetanus. After a conditioning tetanus, an interval of many minutes is required for the muscle to recover its ability to relax quickly from a test tetanus. The results indicate that changes in metabolite levels caused by contraction itself are not responsible for the slowing of relaxation. Possible mechanisms of the effects of CO2 on relaxation are discussed.