Development of the differential response to succinylcholine in the fast and slow-twitch skeletal muscle of the kitten.

1. The response to SCh has been studied in anterior tibialis and soleus muscles of new-born and neonate cat while, in a separate group of animals of similar ages, the end-plate development in these muscles has been examined.2. At birth, when the contraction times of the anterior tibialis and soleus are not statistically different, the anterior tibialis is 1.4 times more sensitive to SCh blocking action than the soleus, but both muscles were 20-40 times less sensitive than their adult counterparts.3. With maturation, the ratio of the soleus/anterior tibialis contraction time increased to 2.7 while the ratio of the 50% blocking dose of SCh in the soleus/anterior tibialis increased to 2.8, the increases being parallel. The time to 50% recovery at 50% blocking doses of SCh was similar for both muscles at all ages although it was 3 times longer in new-born and 1-week-old animals than in animals 3 weeks of age and older.4. The areas of individual end-plates from these muscles were very similar at birth but by three weeks of age the areas of end-plates from the anterior tibialis were substantially greater than those obtained from soleus.5. We have speculated that the relationship between SCh sensitivity and end-plate area in fast and slow muscle may find its origins in the frequency of the MEPP.