Effects of Chronic Tenotomy on the Post-Tetanic Responses of Fast and Slow Muscles in Rats

Abstract

The effects of tenotomy present for two weeks on the post-tetanic responses of isolated fast (extensor digitorum longus) and slow (soleus) muscles were studied by isometric recording of muscle contraction in rats. After tetanic stimulation with bursts of 100 impulses at a frequency of 20 impulses/sec, the force of single fast muscle contractions exceeded the pretetanic level by 20‐25% (p < 0.05) in both intact and tenotomized muscles. In similar conditions, the slow muscle soleus showed a decrease in the amplitudes of single contractions, i.e., post-tetanic depression. Post-tetanic depression was not seen after tenotomy. Caffeine (4 mM) increased the force of single and tetanic contractions in control and tenotomized fast muscles, though no corresponding increase in post-tetanic responses were seen. In slow muscles, caffeine had no effect on the nature of post-tetanic responses. Dantrolene (10 μM) decreased the force of single and tetanic contractions but did not prevent the appearance of post-tetanic potentiation in fast muscles and depression in intact slow muscles. The fact that the phenomenon of post-tetanic potentiation disappeared after denervation of fast muscles within two weeks while it was present after tenotomy for as long as 40 days after surgery provides grounds for suggesting the existence of a potentiating factor which reaches muscles by axonal transport and increases single contractions.