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

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°C and 16 h at 6°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°C and 10°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°C, 30°C and 25°C provided some evidence that the affinity of (+)‐QMDP is greater at 25°C than 37°C. However, the flattening of the concentration‐response curves for histamine at low concentrations of (+)‐QMDP at 30°C and 25°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°C and 6°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°C and 6°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 favourable for [3H]‐mepyramine, a tertiary amine, than for [3H]‐QMDP, a quaternary amine. 1988 British Pharmacological Society