Histamine H3 receptors in the guinea pig ileum: evidence for a postjunctional location.

The effect of the selective histamine H3 receptor agonists (R)alpha-methylhistamine, (R)MHA and immepip (IMM) on intestinal smooth muscle contractility was investigated on isolated cells from the longitudinal muscle of the guinea pig ileum. (R)MHA (10(-13)-10(-8) M) and IMM (10(-13)-10(-8) M) did not significantly modify the basal length of intestinal cells; in contrast both agonists (10(-15)-10(-11) M) prevented the contraction produced by acetylcholine (10(-7) M). The (S)-isomer of alpha-methylhistamine, (S)MHA, was inactive both on basal contractility and on acetylcholine-induced contractions. The relaxant effect of (R)MHA was not modified by famotidine (10(-7) M), but totally prevented by the selective H3 receptor antagonist clobenpropit (10(-8) M), which per se did not modify either basal contractility or the contractile response to acetylcholine. These data indicate that inhibitory histamine H3 receptors are present on smooth muscle cells of the guinea pig ileum and can be activated by very low concentrations of selective agonists. It is not clear, however, whether they can have a functional importance in the regulation of intestinal contractility in an intact system.

Stereoselective inhibition of ethanol-induced gastric lesions in the rat by the H(3)-receptor agonist (R)-alpha-methylhistamine and its (S)-configured isomer.

The histamine H(3) receptor shows high degree of stereoselectivity for histamine analogues branched in the side chain. The hypothesis that gastroprotection by (R)-alpha-methylhistamine could be H(3) receptor-mediated was tested by comparing the effect of (R)-alpha-methylhistamine and of (S)-alpha-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)-alpha-methylhistamine (100 mg/kg i.g.). In the groups pretreated with (S)-alpha-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)-alpha-methylhistamine pretreatment resulted in an increase in number and volume of surface mucous cells, not evident after (S)-alpha-methylhistamine pretreatment. Scanning electron microscopy confirmed light microscopy evaluations. The two isomers of alpha-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 alpha-methylhistamine rests on the high degree of stereoselectivity of H(3) receptors and on the different affinities of the two isomers for these receptors.

Effects of histamine H2 receptor agonists and antagonists on the isolated guinea pig gallbladder.

Histamine H2 receptor-mediated responses were examined on cholecystokinin-octapeptide (CCK-8)-precontracted guinea pig gallbladder in vitro, testing histamine and a series of specific histamine H2 receptor agonists and antagonists. Among the antagonists tested, zolantidine and compound SKF 92857 were previously shown to antagonize H2 receptor-mediated responses in the heart, but not in the stomach. Histamine, in the presence of the H2 receptor antagonist mepyramine (10 microM), and the H2 receptor agonists dimaprit, impromidine and amthamine, inhibited CCK-8 (3 nM)-induced contractions in a concentration-dependent fashion, with the following rank orders of potency: impromidine > amthamine > histamine > dimaprit (pD2 values were 6.73 +/- 0.04, 5.44 +/- 0.07, 4.64 +/- 0.04 and 3.71 +/- 0.05, respectively). The maximal relaxation produced by dimaprit was significantly lower than that produced by histamine, as well as by impromidine and amthamine, which behaved as full agonists. All the H2 receptor antagonists examined were able to inhibit amthamine-induced relaxation. Famotidine (pA2 = 7.09 +/- 0.26), zolantidine (pA2 = 6.54 +/- 0.11), compound SKF 92857 (pA2 = 6.58 +/- 0.13) and aminopotentidine (pA2 = 6.86 +/- 0.06) competitively antagonised the amthamine-induced effect, while iodoaminopotentidine produced surmountable antagonism only at low concentrations (1 microM, pA2 = 6.83 +/- 0.21). Finally, the slowly-dissociable antagonist loxtidine caused a non-parallel displacement to the right of the concentration--response curve to amthamine (pKB = 7.55 +/- 0.24), with a significant depression of the maximal response to the agonist, even at the lowest effective concentration (0.3 microM). In conclusion, histamine H2 receptors in guinea pig gallbladder resemble those of the heart, as regards their sensitivity to zolantidine and compound SKF 92857, which, by contrast, were unable to antagonize histamine effects at gastric H2 receptors in different experimental models.

Histamine receptor-independent muscle relaxation elicited by a series of histamine H2-receptor agonists on the isolated guinea pig duodenum: a study into the mechanism of action.

1. The histamine H2 receptor agonists, dimaprit, impromidine, amthamine, and several dimaprit- and impromidine-analogues were investigated for their spasmolytic activity on the guinea pig duodenum, precontracted with acetylcholine or KCl. 2. Almost all the H2 receptor agonists exerted a histamine H2 receptor-independent muscle relaxation, which was more evident on acetylcholine- than on KCl-precontracted preparations. 3. The relaxing activity of these compounds was independent of inhibitory receptors, like beta-adrenergic, GABA-ergic, serotoninergic, etc. Similarly, modifications of cyclic nucleotide metabolism and nitric oxide production did not appear to be involved. 4. The behavior of histamine H2-receptor agonists was shared only by the Na+-blocker procaine, the intracellular Ca2+-antagonist ruthenium red and, at least in terms of efficacy, by the protein kinase C inhibitor, chelerithrine. 5. This spasmolytic effect is probably due to an impairment of receptor-mediated depolarization at the plasma membrane level and/or an inhibitory activity on the protein kinase C-dependent activation of the contractile machinery. 6. Finally, our findings suggest that the histamine H2 receptor-independent muscle relaxation is a general feature shown by H2 receptor agonists endowed with different chemical structure and the putative spasmolytic "receptor" prefers a (substituted) thiazole over a (substituted) imidazole.

The contractile effect of fedotozine on guinea pig isolated intestinal cells is not mediated by kappa opioid receptors.

The compound fedotozine, recently described as a peripheral kappa opioid receptor agonist, was tested on smooth muscle cells isolated from the longitudinal muscle layer of the guinea pig ileum, in comparison with the selective kappa receptor agonist, compound U-50488. Fedotozine (1 nmol/l-1 micromol/l) caused a concentration-dependent contraction of intestinal cells, with a maximum decrease in cell length not significantly different from that caused by acetylcholine. The kappa agonist U-50488 (0.1 pmol/l-100 nmol/l) was without effect. The contractile effect of fedotozine was not significantly modified by naloxone (0.1-1 micromol/l). These results indicate that fedotozine can have direct excitatory effects on intestinal smooth muscle cells from the guinea pig ileum not mediated by activation of kappa opioid receptors.

Rapid onset of (R)-alpha-methylhistamine protection in response to ethanol-induced histologic damage in rat gastric mucosa.

The influence of pretreatment with (R)-alpha-methylhistamine, selective agonist of histamine H3 receptors, has been investigated on gastric mucosal lesions at different time intervals, from 5 to 60 minutes, after administration of absolute ethanol in the rat. The amount and depth of lesions were quantitatively evaluated by light microscopy. In rats pretreated with (R)-alpha-methylhistamine, the integrity of the mucosa was preserved in approximately 80% of the total mucosal length measured despite ethanol challenge. Prevention of lesion formation was as great at 5 min after ethanol administration as at 60 min. When present, damage involved mainly superficial mucosa and lesions extending deeply into the gland region were evident in 1-2.5% of the total mucosa. Present findings indicate that mechanisms by which (R)-alpha-methylhistamine operates enable the mucosa to counteract damage just from the moment of exposure to ethanol and that protection is exerted both on surface and pit cells and on gastric glands.

Effects of different antisecretory drugs on gastric potential difference in the rat: comparison with sucralfate.

The proton pump inhibitors omeprazole and lansoprazole and the histamine H2 receptor antagonists ranitidine and nizatidine were investigated for their effects on gastric transmucosal potential difference (PD) in the rat, in comparison with the gastroprotective compound sucralfate. Omeprazole (1-3 mg kg-1, i.v.) and lansoprazole (1-3 mg kg-1, i.v.) did not modify basal PD, but significantly reduced (by approx. 50-60%) the drop in PD caused by intragastric administration of acetylsalicylic acid (ASA, 60 mg kg-1). Ranitidine (3-100 mg kg-1, i.v.) and nizatidine (10-30 mg kg-1, i.v.) behaved similarly to proton pump inhibitors, being ineffective on basal PD, while significantly reducing the effect of ASA. The antisecretory compounds did not change basal pH values. Sucralfate (0.5-1.5 g kg-1 intragastrically) caused a slight increase (approx. 20%) of basal PD and a dose-dependent reduction of ASA-induced fall in PD, with a maximum effect (65% reduction) comparable to that caused by the antisecretory agents. These results showed that ASA-induced disruption of the mucosal barrier can be reduced to the same extent by various antiulcer drugs, irrespective of their effects on gastric acid secretion.

SAR studies on H2 antagonists containing alkylamino substituted 1,2, 5-thiadiazole 1-oxide moieties.

A number of ranitidine analogues in which the diamino-1,2,5-thiadiazole 1-oxide substructure bearing alkyl chains of different length is present as the urea equivalent group, were synthesised and studied for their lipophilic and H2 antagonist properties. Derivatives which displayed a logP < or = 3 behaved as competitive antagonists of histamine at H2 receptors present on guinea pig right atrium. The remaining more lipophilic members of the series showed an insurmountable antagonism not completely reversible after prolonged washing. A binding study suggested that an increase in the length of alkyl chain gave rise to hydrophobic interactions with the receptor which were responsible for the apparent irreversible H2 antagonism shown by the higher homologues of the series.

The beta3-adrenoceptor agonist SR58611A inhibits gastric acid secretion in the conscious cat.

The effect of the beta3-adrenoceptor agonist [ N-[(2S)-7-ethoxycarbonyl-methoxyl-1,2,3,4-tetrahydronaphth-2-yl] (2R)-2-(3-chloro-phenyl)2-hydroxyethanamine hydrochloride] (SR58611A) on gastric acid secretion was investigated in conscious cats with a gastric fistula. Intravenous infusion of SR58611A (0.3-3 micromol/kg/h) caused a dose-dependent inhibition of the acid secretion stimulated by 2-deoxy-D-glucose (2DG), with a maximum reduction by 45%. The secretory effect of the histamine H2-receptor agonist dimaprit only tended to be reduced by SR58611A (3 micromol/kg/h). The inhibitory effect of SR5861 IA was not modified by the non selective beta1- and beta2-adrenoceptor antagonist propranolol (1.5 micromol/kg i.v.), but it was prevented by bupranolol (10 micromol/kg i.v.), a drug endowed with beta3-antagonistic properties. Both antagonists blocked the inhibitory effect of the beta2-adrenoceptor agonist clenbuterol (0.1 micromol/kg/h) on 2DG-induced acid secretion. These findings suggest that compound SR58611A inhibits gastric acid secretion in the conscious cat through activation of beta3-adrenoceptors insensitive to propranolol.

Mixed antisecretory and gastroprotective activities of a new H2-antagonist containing a nitric oxide-donor furoxan moiety.

The effect of a new histamine H2-receptor antagonist derived from the lamtidine molecule and containing a nitric oxide (NO)-donor furoxan moiety (derivative 1) was studied for its gastric antisecretory activity and for a possible gastroprotective effect, in comparison with the analog without the furoxan moiety (derivative 2). The H2-receptor antagonistic activity was also investigated in the isolated guinea pig papillary muscle. Derivative 1 was approximately 10 times less potent than derivative 2 at the H2-receptor level; conversely, it was about 10 times more effective as a gastroprotective agent against ethanol- and 0.6 N HCl-induced gastric lesions. The mechanism of the gastroprotection exerted by derivative 1 is probably connected with the release of NO, whose vasodilating action on gastric mucosa vessels is crucial. The combined antisecretory and gastroprotective activity of derivative 1 allows this compound to be considered as a prototype of a new class of antiulcer agents.

Different mechanisms are responsible for the contractile effects of histaminergic compounds on isolated intestinal smooth muscle cells.

The effects of histamine and dimaprit on intestinal smooth muscle contractility were investigated on isolated cells from longitudinal muscle of the guinea pig ileum. Both histamine (10(-14)-10(-10) M) and dimaprit (10(-13)-10(-10) M) exerted a concentration-dependent contraction of intestinal cells, causing a maximum decrease in cell length of about 20%. This effect was not significantly different from that induced by cholecystokinin-octapeptide (CCK-8) 10(-9) M. The concentration-response curves to histamine and dimaprit were shifted to the left in the presence of the histamine H2-receptor antagonist famotidine (10(-7) M) indicating the occurrence in the smooth muscle of H2 receptors mediating relaxation. Whereas the contraction produced by histamine was competitively antagonized by the H1 receptor antagonist mepyramine (10(-8) M), neither mepyramine (10(-7) M) nor temelastine (10(-7) M) did modify the contractile effect of dimaprit. In contrast, atropine (10(-8) M) significantly depressed the maximum response to dimaprit without affecting that exerted by histamine. These data indicate that histamine and dimaprit can modify intestinal contractility, by acting via different mechanisms; while the contractile action of histamine is related to H1 receptor activation, that produced by dimaprit involves cholinergic pathways.

Comparison between beta 3 and beta 2 adrenoceptor agonists as inhibitors of gastric acid secretion.

In order to investigate the role of beta 3 adrenoceptors in the regulation of gastric acid secretion we studied the effects of compound SR58611A (a selective agonist for atypical beta adrenoceptors), alone or in combination with beta-adrenoceptor antagonists, in the gastric fistula of a conscious cat. The effects of SR58611A were compared with those of clenbuterol, a selective agonist for beta 2 adrenoceptors. Intravenous infusion of SR58611A (0.3-3 mumol/kg/h) caused a dose-dependent, but partial, inhibition of the acid secretory response to 2-deoxy-D-glucose 100 mg/kg i.v., maximum effect not exceeding 40%. Clenbuterol (0.03-0.1 mumol/kg/h) caused a similar effect (maximum inhibition about 50%) at doses approximately 30 times lower. The acid secretion induced by the histamine H2-receptor agonist dimaprit (1 mumol/kg/h) was minimally affected by both beta adrenoceptor agonists. The inhibitory effect of SR58611A (3 mumol/kg/h) on 2-deoxy-D-glucose-induced acid secretion was not modified by pretreatment with the non-selective beta 1- and beta 2-adrenoceptor blocker propranolol, administered at doses (1.5 mumol/kg iv) that completely blocked the inhibitory effect of clenbuterol (0.1 mumol/kg/h). In contrast, bupranolol (10 mumol/kg i.v.) (a drug endowed with beta 3 antagonistic properties) prevented the inhibitory effects of both SR58611A and clenbuterol. The present data provide functional evidence that, besides beta 2-, also beta 3-adrenoceptors can have negative effects on gastric acid secretion, particularly when it is stimulated by indirect stimuli, like 2-deoxy-D-glucose. This gastric antisecretory activity may represent an additional mechanism for the physio-pharmacological control of gastric acid secretion.

Histological effect of (R)-alpha-methylhistamine on ethanol damage in rat gastric mucosa: influence on mucus production.

(R)-alpha-Methylhistamine, a selective agonist of histamine H3 receptors, prevents macroscopically visible gastric lesions by absolute ethanol in the rat. A further insight into its activity was the aim of our study. Rats were given saline or (R)-alpha-methylhistamine (100 mg/kg) intragastrically. After 30 min, absolute ethanol was given and gastric mucosa was sampled 60 min later. Histologic damage and intracellular and adherent mucus were quantified. Luminal surface and mucous cells were examined by scanning and transmission electron microscopy. (R)-alpha-Methylhistamine reduced the extent of lesions by ethanol from 96 to 18%. Surface mucous cells and mucous neck cells were increased in volume and number, packaging of intracellular mucus was modified, and the secretory processes were promoted by (R)-alpha-methylhistamine itself, although these modifications were mostly evident in stomachs subsequently exposed to ethanol. Adherent mucus layer thickness was increased by (R)-alpha-methylhistamine only after ethanol exposure. It is concluded that (R)-alpha-methylhistamine predisposes mucous cells to react to ethanol.

Interaction between histamine H3 receptors and other prejunctional receptor systems in the isolated guinea pig duodenum.

The hypothesis that prejunctional histamine H3 receptors and alpha-2 adrenoceptors interact with each other was assessed on the cholinergic transmission of the guinea pig duodenum. Specific agonists acting at histamine H3 receptors, alpha-2 adrenoceptors and adenosine A1 receptors, (R)-alpha-methylhistamine (1 nM-1 microM), UK 14,304 (1 nM-1 microM) and N6-cyclopentyladenosine (0.1 nM-0.1 microM), respectively, inhibited muscle contractions evoked by electrical stimulation, the effect being antagonized by specific receptor blockers, thioperamide and clobenpropit (H3 receptors), idazoxan and yohimbine alpha-2 adrenoceptors) and 8-cyclopentyl-1,3-dimethylxanthine (A1 receptors). The simultaneous activation of H3 receptors and alpha-2 adrenoceptors, using EC50 values of the specific agonists (UK 14,304: 30 nM; (R)-alpha-methylhistamine: 20 nM), produced a combined effect that did not differ from the sum of the individual effects, a result that excluded the occurrence of interactions between these receptors. Conversely, the inhibition evoked by the coadministration of N6-cyclopentyladenosine (EC50: 2.5 nm) and (R)-alpha-methylhistamine or of N6-cyclopentyladenosine and UK 14,304 was significantly lower than the sum of the individual effects, which suggests that the corresponding prejunctional receptors interact with each other. No interaction could be detected when threshold concentrations (EC(10-15)) of the different agonists were simultaneously applied. These data show a negative cooperativity between H3 and A1 receptors and between A1 and alpha-2 receptors. Conversely, no evidence of positive cooperativity emerged, even when the different agonists were applied at low or maximum concentrations. The lack of cross-reactivity between the respective agonists excludes an interaction at the recognition sites of the receptor moyeties. Therefore, these phenomena are more likely to reflect interplays between second messengers or effectors involved in modulating the ACh release.

Contractile and relaxant effects of dimaprit on guinea pig isolated intestinal cells.

The effect of the histamine H2 receptor agonist dimaprit on intestinal contractility was characterized on smooth muscle cells isolated from the longitudinal muscle of the guinea pig ileum. Dimaprit exerted two opposite effects on the contractility of isolated muscle cells: relaxation of cholecystokinin octapeptide (CCK-8)-induced contractions in the range of concentrations 10(-17)-10(-13) M and contraction at concentrations higher than 10(-13) M. The relaxant effect of dimaprit was totally prevented by the H2 blocker famotidine (10(-7) M), which, at the same time, enhanced the contractile effect of dimaprit, shifting to the left the concentration-response curve to the agonist. This contraction was not modified by the histamine H1 receptor antagonists pyrilamine and temelastine, tested both at 10(-7) M. By contrast, atropine 10(-8) M abolished the contractile effect of dimaprit, while leaving unchanged the response to CCK-8. Our results clearly indicate that longitudinal muscle cells of the guinea pig ileum possess inhibitory H2 receptors, which can be activated by very low concentrations of dimaprit; moreover, they revealed that dimaprit can have non-histaminergic effects, probably due to muscarinic receptor activation; however, concentrations about 10000 times higher than those necessary to activate H2 receptors, are required.