Recovery of sensitivity to acetylcholine following desensitization in muscles of different vertebrate species.

The recovery after desensitization (DS) of the frog sartorius muscle postsynaptic membrane has a constant rate with a half-time of about 6 sec at 22 degrees C. Several other avian and mammalian species were also examined to answer the question, whether this parameter differs in muscles of different function and acetylcholine receptor properties. The following muscles were studied using iontophoretic microapplication of ACh: frog m. cutaneus pectoris, chick anterior and posterior latissimus dorsi, innervated and denervated rat diaphragm, and diaphragm of awake and hibernating golden hamsters. In all cases, desensitization developed when series of 6-20 short ACh doses were applied to the end-plate region of the muscle fibre membrane without any DS potentiating drugs. Rate of DS onset was dependent on the dose and/or frequency of ACh application. The rate of recovery, however, differs very little in all the muscles studied, the half-times being in the range of 5-6 sec at 22 degrees C. A similar rate of DS recovery was thus found in muscles with different pharmacological properties of cholinoreceptors (e.g. different sensitivity of frog and rat innervated and denervated muscles to d-tubocurarine), or different ionic pathways during activation (hibernating golden hamster). These results support the assumption that desensitization is not of receptor origin and indicate that the system operating during DS does not undergo substantial changes during the functional differentiation of vertebrate muscle membrane.