Sensitivity of rodent skeletal muscles to dicholines: dependence on innervation and age

Depolarizations provoked by acetylcholine (ACh) and three dicholines, succinylcholine (SCh), glutarylcholine (GCh) and azelainylcholine (AzCh) were measured in normal and 3-7 days denervated muscles of adult, and in normal muscles of 0-27 days old mice and rats. Soleus and flexor digitorum superficialis muscles were mainly used. To prevent movement during measurement of the membrane potential by microelectrodes, the muscles were bathed in solutions containing 30 mM K+. At the adult endplate region of muscles of the mouse, the -log concentrations required for 7 mV depolarization were 6.5 for AzCh and approximately 5.9 for SCh, GCh and ACh. In extrajunctional areas of denervated muscles the values were 7.1, 6.4, 5.9 and 5.6 for AzCh, ACh, GCh and SCh, respectively. In non-denervated soleus muscles of 1-3 days-old mice, the average difference in sensitivity (estimated as mentioned above) to AzCh and SCh was approximately 2.0 log units, not significantly different from that found in denervated adult muscles. Between days 3 and 24 this difference decreased to about 0.8 at the endplate region but it remained greater in the periphery of soleus muscles. The AzCh-SCh sensitivity differences were smaller in muscles from juvenile rats than in juvenile mice. Sensitivity to ACh and AzCh, but not that to SCh, was by 0.6-0.8 log units greater in the presence than in the absence of anticholinesterases in endplates at any age of the animals. The blocking effectiveness of d-tubocurarine was low in newborn animals (similar to that observed in denervated adult muscles) and it increased with age.