Nordimaprit,homodimaprit, clobenpropit and imetit: affinities for H3 binding sites and potencies in a functional H3 receptor model

Summary We determined the affinities of nordimaprit, homodimaprit, clobenpropit and imetit for H₃ binding sites (labelled by ³H-N-methylhistamine) in rat brain cortex homogenates and their potencies at presynaptic H_3A receptors on noradrenergic nerve endings in mouse brain cortex slices. ³H-N-Methylhistamine bound saturably to rat brain cortex homogenates with a K_d of 0.70 nmol/l and a B_max of 98 fmol/mg protein. Binding of ³H-N-methylhistamine was displaced monophasically by dimaprit (pK_i 6.55), nordimaprit (5.94), homodimaprit (6.44), clobenpropit (9.16), imetit (9.83), R-(–)--methylhistamine (8.87) and histamine (8.20), and biphasically by burimamide (pK_{i high} 7.73, pK_{i low} 5.97). In superfused mouse brain cortex slices preincubated with ³H-noradrenaline, the electrically (0.3 Hz) evoked tritium overflow was inhibited by imetit (pIC₃₅ 8.93),R-(–)--methylhistamine (7.87) and histamine (7.03). The effect of histamine was attenuated by nordimaprit, homodimaprit, clobenpropit and N-ethoxycarbonyl-2- ethoxy-1,2-dihydroquinoline (EEDQ); EEDQ (but not nordimaprit, homodimaprit and clobenpropit) attenuated the effect of histamine also in slices pre-exposed to the drug 60–30 min prior to superfusion. The concentration-response curve of histamine was shifted to the right by homodimaprit and clobenpropit; Schild plots yielded straight lines with a slope of unity for both drugs (pA₂ 5.94 and 9.55, respectively). Nordimaprit depressed the maximum effect of histamine (pD₂ 5.55) and also slightly increased the concentration of histamine producing the half-maximum effect.In conclusion, nordimaprit and homodimaprit possess similar affinities for H₃ binding sites like dimaprit; nordimaprit and homodimaprit as well as clobenpropit and imetit do not differentiate between H_3A and H_3B binding sites. Nordimaprit is a reversible noncompetitive H₃ receptor antagonist, homodimaprit and clobenpropit are reversible competitive H₃ receptor antagonists and imetit is an H₃ receptor agonist. Correspondence to: E. Schlicker at the above address     

Role of histamine receptors in the regulation of edema and circulation postburn

Despite histamine being a potent endogenous vasoactive agent released in increasing amounts postburn, its role in postburn oedema formation has been controversial and its effect on burn circulation poorly investigated. The present study investigated the involvement of H₁, H₂ and H₃ receptors in postburn edema in rats exposed to skin and muscle burns and their influence on skin circulation postburn. We used the selective antagonists clemastine (H₁), ranitidine (H₂), thioperamide (H₃) and the selective H₃ receptor agonist, imetit. Results showed that none of the antagonists or the H₃ agonist had significant effect on postburn edema measured by quantitative spectrophotometric analysis of extravasated Evans blue-albumin in the full-thickness burned skin or muscle. Clemastine and thioperamide failed to induce significant effect on blood flow in the partial- or full-thickness skin burn injury as measured by laser Doppler flowmetry, while ranitidine significantly (P<0.01) reduced blood flow in the full-thickness burn. In contrast, the H₃ receptor agonist, imetit, significantly increased blood flow, both in the partial-thickness burn injury (P<0.05) and in the full-thickness burn (P<0.01). Moreover, imetit significantly (P<0.01) increased mean arterial pressure while thioperamide significantly (P<0.01) reduced systemic pressure. In conclusion, H₁, H₂ and H₃ receptors are not important actors in the regulation of vascular patency permeability, whereas H₃ receptors play an important role by increasing skin circulation postburn, presumably by relaxation of vascular smooth muscle and/or by interacting with other inflammatory neurotransmitters. Data also suggest that H₂ receptor blockers may not be best choice for stress ulcer prophylaxis in burn patients.     

Characterization of histamine H3 receptors inhibiting 5-HT release from porcine enterochromaffin cells: Further evidence for H3 receptor heterogeneity

The nature of the histamine receptor mediating inhibition of 5-HT release was investigated in strips of the porcine small intestine by investigating the effects of histamine ligands on the overflow of endogenous 5-HT and its metabolite 5-hydroxyindoleacetic acid (5-HIAA). The overflow was measured by HPLC, combined with electrochemical detection and represents calcium-sensitive 5-HT release from enterochromaffin cells, as reported previously. The histamine H₃ receptor selective agonists (R)--methyl-histamine and imetit inhibited the overflow of 5-HT maximally by 50–60%, with EC₅₀ values of 48 and 3.2 nmol/l, respectively. Effects on 5-HT overflow were always accompanied by similar effects on the overflow of 5-HIAA. Thioperamide (100 nmol/l) shifted the concentration response curve of (R)--methyl-histamine to the right (pK_B value 8.38). The inhibitory effect of 1 mol/l (R)--methyl-histamine was antagonized in a concentration-dependent manner by thioperamide (IC₅₀: 65 nmol/l) and dimaprit (IC₅₀: 8.6 mol/l); however, the effect of (R)--methyl-histamine was weakly antagonized by burimamide (by 38% at 100 mol/l) and not significantly affected by other H₃ receptor antagonists, such as impromidine, betahistine and phenylbutanoyl-histamine (each up to 100 mol/l). In conclusion, H₃ receptors mediating inhibition of 5-HT release from porcine enterochromaffin cells have a particular pharmacological profile indicating that heterogeneity of H₃ receptors may exist. The data suggest that histamine H₃ receptors modulating 5-HT release in pig small intestine do not belong to either H_3A or H_3B receptors as defined in rat tissue. Correspondence to: K. Racke at the above address     

Inhibitory H3 receptors on sympathetic nerves of the pithed rat: activation by endogenous histamine and operation in spontaneously hypertensive rats

Our previous results demonstrate the occurrence of presynaptic inhibitory histamine H₃ receptors on sympathetic neurons innervating resistance vessels of the pithed rat. The present study, in which new H₃ receptor ligands with increased potency and selectivity (imetit, clobenpropit) were used, was designed to further explore the role of H₃ receptors in the regulation of the rat cardiovascular system. In particular we were interested whether these receptors may be activated by endogenous histamine and whether they are detectable in an experimental model of hypertension. All experiments were performed on pithed and vagotomized rats treated with rauwolscine 1 μmol/kg. In normotensive Wistar rats the electrical (1 Hz, 1 ms, 50 V for 20 s) stimulation of the preganglionic sympathetic nerve fibres increased diastolic blood pressure by about 35 mmHg. Two H₃ receptor agonists, R-(–)-α-methylhistamine and imetit, inhibited the electrically induced increase in diastolic blood pressure in a dose-dependent manner. The maximal effect (about 25%) was obtained for R-(–)-α-methylhistamine at about 10 μmol/kg and for imetit at about 1 μmol/kg. Two H₃ receptor antagonists, thioperamide 1 μmol/kg and clobenpropit 0.1 μmol/kg, attenuated the inhibitory effect of imetit. The neurogenic vasopressor response was increased by about 15% by thioperamide 1 μmol/kg and clobenpropit 0.1 μmol/kg and decreased by 25% by the histamine methyltransferase inhibitor metoprine 37 μmol/kg. R-(-)-α-Methylhistamine, imetit, thioperamide, clobenpropit and metoprine did not affect the vasopressor response to exogenously added noradrenaline 0.01 μmol/kg (which increased diastolic blood pressure by about 40 mmHg). Metoprine had only a very low affinity for H₃ binding sites (labelled by ³H-N^α-methylhistamine; pK_i 4.46). In pithed Wistar Kyoto (WKY) and spontaneously hypertensive (SHR) rats, electrical (1 Hz, 1 ms, 50 V for 10 s) stimulation increased diastolic blood pressure by 28 and 37 mmHg, respectively. Imetit inhibited the neurogenic vasopressor response to about the same extent in WKY and SHR rats (maximal effect of about 30%). The inhibitory influence of imetit was diminished by thioperamide 1 μmol/kg to about the same degree in rats of either strain. The present study confirms the occurrence of presynaptic H₃ receptors on sympathetic nerve fibres involved in the inhibition of the neurogenic vasopressor response. Moreover, it demonstrates that these H₃ receptors are probably activated by endogenous histamine and appear to be operative in hypertension. Received: 25 July 1996 / Accepted: 24 October 1996