A kinetic approach for an interpretation of the acetylcholine — d-tubocurarine interaction on chronically denervated skeletal muscle

1. In the chronically denervated rat diaphragm acetylcholine and related agonists produced a biphasic response under isotonic conditions. 2. The initial fast phasic shortening reached its maximum within 3–12 sec. The amplitude of the effects depended both on the concentration and the rate of application of the agonist. An equilibrium of diffusion for the agonist was not attained at the maximum of the effect. No direct proportionality existed between receptor occupation and effect, due to the influence of the rate of receptor occupation on the height of the mechanical response. The Michaelis-Menten kinetics are therefore not applicable to this particular problem. 3. The rate of onset and washout of the inhibitory action of d-tubocurarine obeyed the diffusion of the antagonist into and out of a superficial layer, consisting only of one or two fiber's thickness, since already the excitation of a superficial layer will be sufficient to induce a maximum shortening under strictly isotonic conditions. The thickness of the muscle layer, which is responsible for this effect, was increased after inhibition by DFP of the acetylcholinesterase. Comparing identical concentrations of ACh, the period required to reach the maximum response after application of the agonist was prolonged, whereas the rate of shortening remained unaltered. 4. d-tubocurarine diminished the rate of shortening of the phasic response. Evaluations of the effects yielded dose-response curves, which apparently demonstrated a non-competitive mechanism. Both the decreased rate of shortening and the flattening of the dose-response curves in the presence of d-tubocurarine are considered to be induced by the low dissociation rate of the d-tubocurarine—receptor complex, compared with the rate of drug access to the superficial muscle layer.