Temperature and histamine receptor function--what is really happening?

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

1. 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 degrees C and 16 h at 6 degrees C. The association rate constant, k1, was also temperature-dependent, but not to the same extent as k-1. 2. Plots of the observed rate constant for [3H]-QMDP-receptor complex formation, kon, versus [3H-QMDP] were linear at both 30 degrees C and 10 degrees C, consistent with the interaction of [3H]-QMDP with the H1-receptor being a simple, one-step equilibrium. 3. 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 degrees C, 30 degrees C and 25 degrees C provided some evidence that the affinity of (+)-QMDP is greater at 25 degrees C than 37 degrees C. However, the flattening of the concentration-response curves for histamine at low concentrations of (+)-QMDP at 30 degrees C and 25 degrees C is consistent with a slow dissociation of the (+)-QMDP-receptor complex and hence an incomplete equilibration with the agonist. 4. Arrhenius plots for k1 and k-1 for [3H]-QMDP were linear between 37 degrees C and 6 degrees C. The activation energies, Ea, for complex formation and dissociation were 77 +/- 4 and 129 +/- 3 kJ mol-1, respectively. 5. An Arrhenius plot for k-1 for the dissociation of [3H]-mepyramine from the H1-receptor was also linear between 37 degrees C and 6 degrees C. The activation energy was 140 +/- 2 kJ mol-1. 6. 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. 7. 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.

Receptors in the gastrointestinal tract

The receptor concept has been recently evolved and a new science was actually created, namely "receptorology". Receptors are now identified by means of different techniques (binding, agonist-antagonist interaction, autoradiography, etc.). The new techniques allowed the investigators to define new receptors and new subtypes of the "classical" ones. In the gastrointestinal (GI) tract a number of receptors have been identified and localized both on the effector organ and in the nerve terminal where they exert an important modulatory function on the neurotransmitter release. Recent biochemical studies have allowed a better understanding of the post-receptor event involving the second or third messenger regulation. Particular changes of receptors were recognized and they allow us to consider receptors not as static entities but as very dynamic components of the plasma membrane capable of different kinds of alterations, like interconversion, internalization, mobility, up- and downregulation, etc. Together with the "classical" receptors (cholinergic, adrenergic, opioid, etc.) also new receptors were identified: different subtypes of receptors for the tachykinins, for prostaglandin of the E type in the gastric parietal cell and the so-called dihydropyridine (DHP) receptor in the calcium channel of different areas of the gut. It is obvious that the precise knowledge of receptors and of their agonists and antagonists will represent the basis for a more specific and efficacious treatment of various gastrointestinal disorders.

Differential effect of temperature on histamine- and carbachol-stimulated inositol phospholipid breakdown in slices of guinea-pig cerebral cortex

Slices of guinea-pig cerebral cortex were incubated with [3H]-inositol at 37 degrees C before exposure to histamine or carbachol at 37 degrees C or 25 degrees C. Histamine-stimulated accumulation of [3H]-inositol 1-phosphate ([3H]-IP1) at 25 degrees C was only 5-7% of that at 37 degrees C, whereas for carbachol the response at 25 degrees C was 45-49% of that at 37 degrees C. The affinity of benzilylcholine, obtained from inhibition of carbachol-induced accumulation of [3H]-IP1 was similar at 25 degrees C and 37 degrees C, but the EC50 for carbachol was lower at 25 degrees C (20 +/- 2 microM) than at 37 degrees C (42 +/- 2 microM). The IC50 for histamine inhibition of [3H]-mepyramine binding to homogenates of guinea-pig cerebral cortex did not differ significantly at 25 degrees C and 37 degrees C. Histamine-induced accumulations of [3H]-IP2 and [3H]-IP3 at 25 degrees C, expressed as a percentage of the accumulation at 37 degrees C, were also much less than the corresponding value for carbachol. These observations imply that the locus or pathway(s) of agonist-induced formation of [3H]-IP1 are not the same for histamine and carbachol.

Histamine and histamine receptors: behavioral thermoregulation in the salamander Necturus maculosus

Low doses (0.01, 0.1 mg/kg, i.p.) of histamine (HA) caused selection of significantly lower temperatures, and higher doses (0.5, 1.0 mg/kg) increased temperatures by mudpuppies in linear thermal gradients. Injection of the HA precursor, L-histidine (500 mg/kg) produced an increase in the temperatures selected. Results from injections of HA H1-receptor agonist (2-pyridylethylamine) and antagonist (pyrilamine), and H2-receptor agonist (dimaprit) and antagonist (cimetidine) had significant effects on thermoregulation; H1-receptors may mediate behavioral hyperthermia and H2-receptors behavioral hypothermia. Responses to these histaminic compounds are significantly influenced by the time of day at which the responses are measured and by season and acclimation temperature. The equivalent behavioral responses in both endotherms and ectotherms to agents which produce physiological hyperthermia and hypothermia are probably behavioral hypothermia ("cold seeking") and behavioral hyperthermia ("heat seeking"), respectively.