Thermodynamics of the interaction of d-tubocurarine with nicotinic receptors of mammalian skeletal muscle in vitro.
Thermodynamic study is important for defining drug receptor interactions, 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, we 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. Parker (1971, J. Pharmacol. Exp. Ther. 177, 13), was used to calculate the dissociation constant of d-tubocurarine at various temperatures (10-37 degrees C) from the parallel shift of the acetylcholine dose-response curve to the right by effective doses of d-tubocurarine. It was observed that the values of the dissociation constant increased with a decrease in temperature. Both the enthalpy (delta H degree) and entropy (delta S degree) changes as evaluated from the van't Hoff plot (In Kd vs. 1/T) were found to be positive and their relative value (delta H degree - T delta S degree) produced a negative free energy change which characterises the binding of d-tubocurarine as an entropy-controlled process. This finding is in agreement with the neurotoxin binding reported earlier. The present finding and earlier observations with acetylthiocholine reveal that agonist and antagonist binding to the nicotinic receptor may differ depending on the experimental conditions.
Banerjee
B
B
Division of Pharmacology and Experimental Therapeutics, Indian Institute of Chemical Biology, Calcutta, India.
Ganguly
D K
DK
D000818:Animals; D003964:Diaphragm / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D005260:Female; D066298:In Vitro Techniques; D008297:Male; D018482:Muscle, Skeletal / Q000187:drug effects; D018733:Nicotinic Antagonists / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D011978:Receptors, Nicotinic / Q000502:physiology; D013816:Thermodynamics; D014403:Tubocurarine / Q000378:metabolism*
D018733:Nicotinic Antagonists; D011978:Receptors, Nicotinic; D014403:Tubocurarine