Abstract
The reaction kinetics for drug blockade of postsynaptic channels at the mouse neuromuscular junction were studied. The activation energies associated with the onward and off-rate constants, from a sequential model, were measured for procaine, scopolamine, octanol, and heptanol, and from these the transition state thermodynamic parameters of free energy (delta G++), enthalpy (delta H++), and entropy (delta S++) of activation were determined. All agents showed positive entropies of activation for the channel blocking rate constant kappa 2, with the values for the alcohols significantly greater than those for the two positively charged local anesthetics. No significant differences in the activation energies for the off-rate constant kappa-2 were observed between any of the agents. The magnitude of kappa-2 decreased as the hydrophobicity of the agent increased. The large activation energies measured for kappa 2 and kappa-2 (in excess of 10 kcal/mol) are higher than expected for simple rate-limiting diffusion, and the possibility exists that channel blockade involves conformational changes in a protein segment, induced by a hydrophobic interaction between agent and intrachannel site or sites. The large entropy increase observed in the blocking step is indicative of the agent replacing structured waters of hydration near a hydrophobic region of a protein.