Thermodynamic studies with acetylthiocholine on nicotinic receptors of mammalian skeletal muscle in vitro

The temperature dependency of binding of acetylthiocholine, a specific nicotinic agonist, to the nicotinic receptor of mammalian skeletal muscle was studied using isotonic contractions of the rat denervated diaphragm preparation in vitro. The dissociation constants at different temperatures (22–39 °) were determined by the Furchgott method using α-bungarotoxin as an irreversible antagonist. Both free energy of association ( ΔG ° = − 22.93 kJ/mol at 37 °) and enthalpy of binding ( ΔH ° = − 58.35 kJ/mol) calculated from K d (dissociation constant) and slope of InK d versus 1 T (van't Hoff plot) respectively were found to be negative. The negative entropy value ( ΔS ° = − 0.113 kJ/mol/deg) obtained from the intercept of this van't Hoff plot differs from the large positive value obtained earlier employing radioligand binding studies of the nicotinic receptor of Electrophorus electricus.