THERMODYNAMICS OF THE INTERACTION OF D-TUBOCURARINE WITH NICOTINIC RECEPTORS OF MAMMALIAN SKELETAL-MUSCLE IN-VITRO

Thermodynamic study is important for defining drug receptor interactio ns, and denervated rat hemidiaphragm is a unique preparation for such a study on nicotinic receptors. As a continuation of our earlier study with acetylthiocholine on the same preparation, Mie now report on the characteristics of temperature-dependent binding of d-tubocurarine, a reversible antagonist. The O. Arunlakshana and H.O. Schild (1959, Br. J. Pharmacol. 14, 48) equation, as improved by D.R. Woud and R.B. Par ker (1971, J. Pharmacol. Exp. Ther. 177, 13), was used to calculate th e dissociation constant of d-tubocurarine at various temperatures (10- 37 degrees C) from the parallel shift of the acetylcholine dose-respon se curve to the right by effective doses of d-tubocurarine. It was obs erved that the values of the dissociation constant increased with a de crease in temperature. Both the enthalpy (Delta H degrees) and entropy (Delta S degrees) changes as evaluated from the van't Hoff plot (InKd vs. 1/T) were found to be positive and their relative value (Delta H degrees - T Delta S degrees produced a negative free energy change whi ch characterises the binding of d-tubocurarine as an entropy-controlle d process. This finding is in agreement with the neurotoxin binding re ported earlier. The present finding and earlier observations with acet ylthiocholine reveal that agonist and antagonist binding to the nicoti nic receptor may differ depending on the experimental conditions.