Effect of Direct Tetanization on Twitch Tension in Developing Cat Leg Muscles

The effect of tetanization, using both indirect and direct stimulation, on the twitch response to post-tetanic single shocks and on the tension response to a second tetanization has been studied in the gastrocnemius, soleus and extensor carpi radialis longus (ECRL) muscles of kittens ranging in age from 3–140 days, and in adult cats. Direct stimulation was applied in completely curarized animals. Nembutal® anaesthesia was used. Equal tension was found to be produced by a first tetanization and a second one, the latter elicited during the period of twitch potentiation following the first. The second tetanization was, however, found to give a steeper rise in tension than the first. In each animal, the maximal post-tetanic potentiation (PTP) of twitch tension obtained was of about equal magnitude following tetanization by indirect and direct stimulation. In newborn kittens, practically no PTP occurred in the gastrocnemius, whereas in the soleus there was a PTP of about 50 %. With age, the PTP produced by direct stimulation was found to increase in the gastrocnemius and decrease in the soleus, as after stimulation through the motor nerve. The time course of the effects produced by the two types of stimulation was also similar. After tetanization by both types of stimulation, potentiated twitches showed a steeper rise in tension than pre-tetanic control twitches, and in the gastrocnemius they also showed a marked lengthening of half relaxation time. In the kitten soleus, little change took place in the latter. Non-potentiated twitches of adult cat soleus muscle showed shortening of contraction and half relaxation time. It is concluded that the main mechanism of twitch potentiation in the neuromuscular system of kittens and cats is located within the muscle fibres.