An electrophysiological study of the effects of D-tubocurarine, atropine, and alpha-bungarotoxin on the cholinergic receptor in innervated and chronically denervated mammalian skeletal muscles

An electrophysiological study of the action of d-tubocurarine (d-TC), atropine, and α-bungarotoxin (BuTX) was made on the innervated and chronically denervated diaphragm and soleus muscles of the rat and mouse. All three drugs were able to block endplate potentials of innervated muscles as well as acetylcholine (ACh) sensitivity of chronically denervated muscles. The effects of atropine and d-TC were fully reversed upon washing with Ringer's solution, white the effects of BuTX were only partially reversed. The reversibility of BuTX was more evident at extrajunctional areas in chronically denervated muscles than at the innervated endplate region, but in both cases only a fraction of the normal response could be detected after intensive washing for at least 2 hr. The blockade of ACh sensitivity in chronically denervated muscles required a concentration of d-TC 10-fold higher than that necessary to block the endplate potentials of innervated muscles. BuTX and d-TC did not affect the ionic permeabilities of the muscle fiber during an action potential while atropine decreased both Na⁺ and K⁺ conductances, the magnitude of the effect being dependent on the frequency of stimulation. At the endplate region, d-TC was much more effective than atropine in protecting against the irreversible effect of BuTX. In the chronically denervated preparation, however, neither of the two drugs effectively protected against BuTX. It is concluded that in terms of their reactivities to cholinolytic agents, the extrajunctional receptors induced by chronic denervation of skeletal muscles are qualitatively similar to those found at endplate regions of normal muscles, but that they exhibit differences in their quantitative interaction with different cholinolytic agents. The data further indicate that atropine interacts with the ionic conductance modulator unit associated with the cholinergic receptor, rather than with the ACh receptor itself.