Christian Leschke

Histamine receptor-dependent and/or -independent activation of guanine nucleotide-binding proteins by histamine and 2-substituted histamine derivatives in human leukemia (HL-60) and human erythroleukemia (HEL) cells.

In dibutyryl cAMP-differentiated human leukemia (HL-60) cells, the potent histamine H1-receptor agonist, 2-(3-chlorophenyl)histamine, activates pertussis toxin (PTX)-sensitive guanine nucleotide-binding proteins (G-proteins) of the Gi-subfamily by a mechanism which is independent of known histamine receptor subtypes (Seifert et al. Mol Pharmacol 45: 578-586, 1994). In order to learn more about this G-protein activation, we studied the effects of histamine and various 2-substituted histamine derivatives in various cell types and on purified G-proteins. In HL-60 cells, histamine and 2-methylhistamine increased cytosolic Ca2+ concentration ([Ca2+]i) in a clemastine-sensitive manner. Phenyl- and thienyl-substituted histamines increased [Ca2+]i as well, but their effects were not inhibited by histamine receptor antagonists. 2-Substituted histamines activated high-affinity GTPase in HL-60 cell membranes in a PTX-sensitive manner, with the lipophilicity of substances increasing their effectiveness. Although HEL cells do not possess histamine receptors mediating rises in [Ca2+]i, 2-(3-bromophenyl)histamine increased [Ca2+]i in a PTX-sensitive manner. It also increased GTP hydrolysis by Gi-proteins in HEL cell membranes. All these stimulatory effects of 2-substituted histamine derivatives were seen at concentrations higher than those required for activation of H1-receptors. In various other cell types and membrane systems, 2-substituted histamine derivatives showed no or only weak stimulatory effects on G-proteins. 2-Substituted histamine derivatives activated GTP hydrolysis by purified bovine brain Gi/Go-proteins and by pure Gi2 (the major PTX-sensitive G-protein in HL-60 and HEL cells). Our data suggest the following: (1) histamine and 2-methylhistamine act as H1-receptor agonists in HL-60 cells; (2) incorporation of bulky and lipophilic groups results in loss of H1-agonistic activity of 2-substituted histamine derivatives in HL-60 cells but causes a receptor-independent G-protein-stimulatory activity; (3) the effects of 2-substituted histamine derivatives on G-proteins are cell-type specific.

The selective histamine H1-receptor agonist 2-(3-trifluoromethylphenyl)histamine increases waking in the rat

The effects of the selective histamine H1-receptor agonist, 2-(3-trifluoromethylphenyl)histamine, were studied in rats implanted with electrodes for chronic sleep recordings. 2-(3-Trifluoromethylphenyl)histamine (80-120 micrograms) injected into the left lateral ventricle increased wakefulness, whereas slow wave sleep was reduced. Pretreatment with pyrilamine (2.0 mg/kg) prevented the effects of the H1-receptor agonist on wakefulness and slow wave sleep. Our results further support the involvement of histamine in the modulation of the waking state.

2-Alkyl-substituted histamines and hydroxyethylimidazoles with G-protein-stimulatory activity

Summary Cationic-amphiphilic 2-substituted histamines activate pertussis toxin-sensitive guanine nucleotide-binding proteins (G-proteins) by a receptor-independent mechanism. From our recent studies it became apparent that lipophilicity is an important determinant for this G-protein activation, but the influence of basicity remained unknown. We prepared seven novel 2-alkyl-substituted histamines and five novel 2-alkyl-substituted hydroxyethylimidazoles and studied their effects on high-affinity guanosine triphosphate (GTP) hydrolysis in membranes of the human leukemia cell line, HL-60. 2-Octylhistamine was found to be the most effective GTPase activator among 2-substituted histamines presently available (150% stimulation above basal), and 2-tetradecylhistamine is the most potent substance in this regard (pEC50 = 5.9). Branching of the alkyl chain and the introduction of an ether group adversely affected GTPase activation. Compared to a phenyl ring, a bulky adamantyl sphere enhanced G-protein-stimulatory activity. In the case of 2-(3-bromophenyl)histamine, 2-adamantylhistamine and 2-(3-phenylpropyl)histamine, replacement of the aminoethyl group by a hydroxyethyl group at the imidazole greatly reduced GTPase-activating properties, pointing to the importance of the basic domain in the activation process. Unexpectedly, however, in the case of a very lipophilic substituent (heptadecyl chain) the exchange of the aminoethyl group by a hydroxyethyl group had no substantial inhibitory effect, indicating that the presence of a primary amine is not a conditio sine qua non for a substance being a receptor-independent G-protein activator. Concerning histamine H1-receptors the newly prepared compounds proved to be weak antagonists.

Stereoselective Inhibition of Ethanol- Induced Gastric Lesions in the Rat by the H3-Receptor Agonist (R)-α-Methylhistamine and Its (S)-Configured Isomer

The histamine H3 receptor shows high degree of stereoselectivity for histamine analogues branched in the side chain. The hypothesis that gastroprotection by (R)-α-methylhistamine could be H3 receptor-mediated was tested by comparing the effect of (R)-α-methylhistamine and of (S)-α-methylhistamine on ethanol-induced histologic lesions in the rat gastric mucosa. Extensive damage was caused by 60 min exposure to absolute ethanol, 91% of the mucosa examined being damaged. Conversely only 23% of the mucosa was damaged after pretreatment with (R)-α-methylhistamine (100 mg/kg i.g.). In the groups pretreated with (S)-α-methylhistamine (55.44 and 166.3 mg/kg i.g.) total damage ranged from 77 to 79%, though it was confined to the upper portion of the mucosa. Morphometric analysis of stained intraepithelial mucosubstances revealed that (R)-α-methylhistamine pretreatment resulted in an increase in number and volume of surface mucous cells, not evident after (S)-α-methylhistamine pretreatment. Scanning electron microscopy confirmed light microscopy evaluations. The two isomers of α-methylhistamine differently affect the response of rat gastric mucosa to absolute ethanol and they appear to differ in their influence on surface mucous cells. A basis for interpreting the effects of the two isomers of α-methylhistamine rests on the high degree of stereoselectivity of H3 receptors and on the different affinities of the two isomers for these receptors.

Effects of the novel H1 agonists 2-(3-trifluoromethylphenyl)- and 2-(3-bromophenyl)histamine and of 2-(2-thiazolyl)ethylamine on cardiovascular paramenters in the pithed rat

The effect of the newly synthetized H1 agonists 2-(3-trifluoromethylphenyl)histamine (2-TFMPH) and 2-(3-bromophenyl)histamine (2-BPH) and of the reference compound 2-(2-thiazolyl)ethylamine (2-TEA) on diastolic blood pressure and heart rate was studied in pithed and vagotomized rats. 2-TFMPH and 2-BPH were at least equipotent with 2-TEA in producinga dimethindene-sensitive, short-lasting vasodepressor response. At the highest dose, 2-TFMPH and 2-BPH produced an additional small vasopressor response, which was followed by a long-lasting vasodepressor effect not counteracted by dimethindene and ranitidine 2-TEA, at the highest dose, induced an additional vasopressor response abolished by prazosin plus rauwolscine. Basal heart rate was increased by 2-TEA (in a desipramine-sensitive manner) but not affected by 2-TFMPH or 2-BPH. In conclusion, 2-TFMPH and 2-BPH are potent H1 agonists, devoid of an indirect sympathomimetic effect; at high doses, they produce a vasodepressor response not mediated via histamine receptors.