Temperature-dependence of the kinetics of the binding of [3H]-(+)-N-methyl-4-methyldiphenhydramine to the histamine H1-receptor: comparison with the kinetics of [3H]-mepyramine

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Abstract
The dissociation of [3H]-(+)-N-methyl-4-methyldiphenhydramine ([3H]-QMDP) from the histamine H1-receptor was markedly temperature-dependent. The t1/2 was 4 min at 37.degree. C and 16 h at 6.degree.C. The association rate constant, k1, was also temperature-dependent, but not to the same extent as k-1. Plots of the observed rate constant for [3H]-QMDP-receptor complex formation, kon, versus [3H-QMDP] were linear at both 30.degree. C and 10.degree. C, consistent with the interaction of [3H]-QMDP with the H1-receptor being a simple, one-step equilibrium. The ratio of the kinetic constants, k1/k-1, indicated that the affinity constant of [3H]-QMDP for the H1-receptor should increase with decreasing temperature. Measurement of (+)-QMDP antagonism of the contraction of longitudinal muscle strips from guinea-pig small intestine induced by histamine at 37.degree. C, 30.degree. C and 25.degree. C provided some evidence that the affinity of (+)-QMDP is greater at 25.degree. C than 37.degree. C. However, the flattening of the concentration-response curves for histamine at low concentrations of (+)-QMDP at 30.degree. C and 25.degree. C is consistent with a slow dissociation of the (+)-QMDP-receptor complex and hence an incomplete equilibration with the agonist. Arrhenius plots for k1 and k-1 for [3H]-QMDP were linear between 37.degree. C and 6.degree. C. The activation energies, Ea, for complex formation and dissociation were 77 .+-. 4 and 129 .+-. 3 kJ mol-1, respectively. An Arrhenius plot for k-1 for the dissociation of [3H]-mepyramine from the H1-receptor was also linear between 37.degree. C and 6.degree. C. The activation energy was 140 .+-. 2 kJ mol-1. Activation energies for complex formation with the H1-receptor, Eaf, and complex dissociation, Ead, were similar for [3H]-QMDP and [3H]-mepyramine. The energy difference, Eaf-Ead, equivalent to the enthalpy change, did not differ significantly for the two ligands (-52 and -48 kJ mol-1, respectively). The larger values of k1 and k-1 for [3H]-mepyramine compared to [3H]-QMDP imply the presence of an entropic component in the interaction. The simplest explanation for these observations is that transfer from the aqueous phase into a hydrophobic region is a significant factor in antagonist-H1-receptor interaction. This would be entropically more favorable for [3H]-mepyramine, a tertiary amine, than for [3H]-QMDP, a quaternary amine.