Time-dependent interaction of a new H2-receptor antagonist, IT-066, with the receptor in the atria of guinea pig.

3-Amino-4-[4-[4-(1-piperidinomethyl)-2-pyridyloxy]-cis-2- butenylamino]-3-cyclobutene-1,2-dione hydrochloride (IT-066) showed a highly potent and long lasting H2-receptor blocking action in guinea pig atria. The inhibitory effect of IT-066 on the histamine-induced positive chronotropic response increased concentration- and time-dependently. A short period of treatment with IT-066 shifted the concentration-response curve of histamine to the right in parallel, without decreasing the maximal response to histamine. With prolongation of the treatment, the concentration-response curve shifted further to the right with time-dependent suppression of the maximal response to histamine. The inhibitory effect of IT-066 was irreversible. The dissociation constant for histamine (KA) was not changed by prolongation of the time of incubation with IT-066. The dissociation constant for IT-066 (KB) was decreased with the prolongation of the treatment. Kinetic analysis of the time-dependent inhibition showed a two-step reaction: the first was reversible and the second was irreversible. Preincubation of the atria with ranitidine, however, protected the H2-receptor from the apparently irreversible antagonism of IT-066. These results suggest that IT-066 has a time-dependent and irreversible interaction with the H2-receptor and that the interaction may be responsible for the potent and long lasting H2-receptor blocking action of IT-066.     

H2-receptor antagonism by a novel compound IT-066 assessed from [14C]aminopyrine accumulation in rabbit parietal cells

The H2-receptor antagonistic activities and properties of IT-066 were investigated in rabbit isolated gastric mucosal cell and were compared with those of famotidine and cimetidine. IT-066 inhibited dose dependently the histamine-stimulated [14C]aminopyrine ([14C]AP) accumulation in parietal cells. The antagonism was unsurmountable. The inhibitory action of IT-066 was enhanced by lengthening of its incubation time with the cells. IT-066 following 30 min preincubation with the parietal cells showed 32 and 1560 times the activity of famotidine and cimetidine, respectively, as an inhibitor of [14C]AP accumulation. The inhibitory activity of IT-066 on the histamine effect remained after repeated washing of the cells while with famotidine and cimetidine the response to histamine was restored after washout of the drugs from the cells. These data indicate that the antagonism of IT-066 on histamine H2-receptors is high and is unsurmountable in the isolated parietal cell model.     

Effects of the new H2-receptor antagonist 3-amino-4-[4- [4- (1-piperidinomethyl)-2-pyridyloxy]-cis-2-butenylamino]-3-cyclobutene-1, 2- dione hydrochloride on gastric acid secretion and ulceration

The effect of 3-amino-4-[4-[4-(1-piperidinomethyl)-2-pyridyloxy]-cis- 2-butenylamino]-3-cyclobutene-1,2-dione hydrochloride (IT-066), a new H2-receptor antagonist, on gastric acid secretion and on various experimental ulcers was investigated. IT-066 showed very potent and long lasting antisecretory action in pylorus ligated rats. The inhibitory potency of IT-066 given subcutaneously for gastric acid secretion was 1285 and 44 times higher than for cimetidine and famotidine, respectively. The duration of the inhibitory action of IT-066 was significantly longer than that of famotidine and cimetidine. In pylorus ligated rats, IT-066 showed almost 20 times higher potency than omeprazole with intraduodenal administration, and almost the same duration of action as omeprazole with one tenth the dose in oral administration. IT-066 showed a powerful protective effect on various experimental ulcer models. The potency of IT-066 administered subcutaneously was significantly higher compared with that of cimetidine, famotidine and omeprazole. IT-066 given orally also showed a more powerful antiulcer effect than cimetidine and omeprazole, and was comparable with that of famotidine in restraint and water immersion stress and cold-stress plus indometacin induced ulcer models in rats. These results suggest that IT-066 has powerful and long lasting antisecretory and antiulcer effects and is a useful antisecretory drug for treatment of peptic ulcer diseases.     

Is [3H]-tiotidine a specific ligand for the H2-receptor?

The H2-antagonist tiotidine inhibited the H2-receptor-mediated, histamine-induced increase in cyclic AMP in dispersed mucosal cells from guinea pig stomach (Ki, 4 X 10(-8) M). The radiolabeled [3H]-tiotidine bound specifically and reversibly to the same cells with a half-maximal binding occurring at 5 X 10(-7) M tiotidine. The dissociation of bound [3H]-tiotidine from gastric cells and the Scatchard analysis of the binding binding data suggest the existence of additional binding sites for tiotidine. Eight other antagonists which inhibited the H2-receptor-mediated increase in cyclic AMP also inhibited [3H]-tiotidine binding. However, the potencies of these agents for binding did not agree with their effects on cyclic AMP. The selective H2-agonists impromidine and dimaprit which increased cyclic AMP caused only partial inhibition of [3H]-tiotidine binding. These results demonstrate that [3H]-tiotidine has limited binding to the H2-receptors and as such [3H]-tiotidine is not a suitable ligand for labelling the H2-receptor on gastric mucosal cells.     

Effects of IT-066, a new histamine H2-receptor antagonist, on gastric acid secretion and experimental gastric ulcers in rats and dogs

We examined the effects of a new histamine H2-receptor antagonist, 3-amino-4-(4-[4-(1-piperidinomethyl)-2-pyridyloxy]-cis-2- butenylamino)-3- cyclobutene-1,2-dione hydrochloride (IT-066), on gastric acid secretion and the healing process of experimental ulcers in rats and dogs. Famotidine, a well-established H2-receptor antagonist, was used as the reference drug. Male Donryu rats (240-260 g) and Beagle dogs of both sexes (8-10 kg), having Heidenhain pouches, were used. IT-066 dose-dependently inhibited the basal gastric acid secretion of rats, and this inhibition significantly persisted for 12 hr. In addition, the agent significantly inhibited histamine-stimulated acid secretion in both normal rats and rats with acetic acid ulcers. IT-066, given p.o. twice daily for 2 and 3 weeks after ulceration, significantly accelerated both spontaneous and delayed healing (with indomethacin) of acetic acid-induced gastric ulcers in rats. The effects of IT-066 on acid secretion and ulcer healing were almost the same or slightly more potent than those observed with famotidine. IT-066, when given i.v. or p.o., dose-dependently inhibited the gastric acid secretion stimulated by histamine, pentagastrin, or carbachol in dogs. The antisecretory effects of the agent on histamine-stimulated acid secretion significantly persisted for more than 6 hr. These results indicate that IT-066 appears to be a promising antisecretory and anti-ulcer agent.     

Interaction of histamine H2-receptor antagonists with GABA and benzodiazepine binding sites in the CNS

The histamine H2-receptor antagonist cimetidine potently inhibited [3H]muscimol and enhanced [3H]flunitrazepam binding in membranes prepared from several brain regions in the rat, including the dorsal raphe nucleus. As further examined in cortical membranes, this effect on both GABA and benzodiazepine binding sites was specific for imidazole-derived H2-receptor antagonists (potency: cimetidine greater than metiamide greater than tiotidine) and not observed with either several H1-receptor antagonists or histamine. These data indicate a striking similarity between the actions of cimetidine (and other imidazole-derived H2-receptor antagonists) and GABA on binding parameters at the GABA receptor complex.     

Cardiac effects of the new H2-receptor antagonists

A series of new H2-receptor antagonists were tested for their effects on different isolated heart preparations. In the guinea-pig atria and papillary muscle the inhibitory effect on histamine H2-receptors was evaluated. In the perfused rabbit heart and in strips of human atria the effect of the H2-antagonists on the spontaneous or electrically-stimulated contractions was evaluated. In the first two preparations some main quantitative differences were pointed out, tiotidine and compound SKF 93479 being the most potent antagonists, cimetidine, metiamide and ranitidine the less effective. In the rabbit heart and in human atria results were quite different: cimetidine and ranitidine were virtually ineffective up to the maximum concentration tested (3 x 10(-3) M), oxmetidine and compound SKF 93479 had a negative inotropic and chronotropic effect starting from concentrations of 3 x 10(-6)-10(-5) M. On the basis of the behaviour of other compounds endowed with negative cardiac effects (propranolol, anaesthetic-like compounds, verapamil) and of that of compounds capable of counteracting the effect of oxmetidine (increased concentration of calcium ions and isoproterenol) it was hypothesized that oxmetidine may interfere in the transport of calcium ions. Our data emphasize the importance of the different structure of the H2-antagonists in determining non-specific effects absolutely independent of the primary action that is the H2-receptor blockade.     

Effect of some new H2-receptor antagonists on gastrointestinal motility

Some new histamine H2-receptor antagonists were tested for their effects on gastrointestinal motility. Ranitidine was found to possess definite stimulatory effects which appeared to be connected with an interference with the cholinergic system and occurred, though in different degree, from the lower esophageal sphincter (LES) to the colon. Etintidine, on the contrary, showed a remarkable antimuscarinic effect on the LES of the rat and the guinea-pig. Cimetidine, SK&F 93479 and tiotidine were virtually ineffective whereas oxmetidine exerted a consistent inhibitory activity on both basal motility and on the contractions induced by a variety of stimulatory agents. This effect, which was completely independent of the autonomic nervous system appeared to be connected with an inhibition of the transport of calcium ions. All the above results suggest that the H2-antagonists so far available may not be absolutely selective for the H2-receptor but may be endowed with non-specific effects which could have an interest at least from a pharmacological viewpoint.     

Selective disappearance of histamine H2-receptor activity in the human gastric cancer cell line HGT-1 after short-term or chronic treatment by histamine or its H2-antagonists

Homologous loss of histamine H2-receptor activity (cAMP generation) was observed after short-term (10-20 min) or chronic treatment (6 days) of cultured HGT-1 cells with histamine (desensitization) or the H2-receptor antagonist SKF 93479. This inactivation process was not observed when HGT-1 cells were exposed to the classical H2-antihistamine cimetidine. The data show: (1) that the compound SKF 93479 has a very prolonged inhibitory action on histamine receptor activity, suggesting an irreversible interaction between the antagonist and the receptor; (2) that cimetidine is a reversible H2-receptor antagonist which can be removed without changing the the efficacy and the potency of histamine on gastric cells; (3) that the H2-receptor antagonists cimetidine and SKF 93479 specifically block histamine H2-receptor activity in HGT-1 cells since cAMP generation induced by other hormones such as vasoactive intestinal peptide (VIP), glucagon or gastric inhibitory peptide (GIP) was unchanged after treatment; (4) the first evidence for time-dependent (half-life: 20 min) desensitization of gastric H2-receptors.     

Asp125 and Thr130 in transmembrane domain 3 are major sites of alpha1b-adrenergic receptor antagonist binding

Site-directed mutagenesis was used to investigate the molecular interactions involved in prazosin binding to the human alpha(1b)-adrenergic receptor (alpha(1b)-AR) receptor. Based on molecular modeling studies, Thr130 and Asp125 in transmembrane region III of the alpha(1b)-AR receptor were found to interact with prazosin. Thr130 and Asp125 were mutated to alanine (Ala) and expressed in HEK293 cells. The radioligand [(3)H]prazosin did not show any binding to Asp125Ala mutant of alpha(1b)-AR. Therefore, it was not possible to find any prazosin affinity to the mutant using the radioligand [(3)H]prazosin. The mutation also abolished phenylephrine-stimulated inositol phosphate (IP) formation of [(3)H]myo-inositol. On the other hand, the Thr130Ala mutant showed reduced binding affinity for [(3)H]prazosin (dissociation constant, K(d) 674.27 pM versus 90.27 pM for the wild-type receptor) and had reduced affinity for both tamsulosin and prazosin (11-fold and 9-fold, respectively). However, the Thr130Ala mutant receptor retained the ability to stimulate the formation of [(3)H]myo-inositol. The results provide direct evidence that Asp125 and Thr130 are responsible for the interactions between alpha(1b)-AR receptor and radioligand [(3)H]prazosin as well as tamsulosin.     

Novel histamine H(3)-receptor antagonists and partial agonists with a non-aminergic structure

We determined the affinities of eight novel histamine H(3)-receptor ligands (ethers and carbamates) for H(3)-receptor binding sites and their agonistic/antagonistic effects in two functional H(3)-receptor models. The compounds differ from histamine in that the ethylamine chain is replaced by a propyloxy chain; in the three ethers mentioned below (FUB 335, 373 and 407), R is n-pentyl, 3-methylbutyl and 3,3-dimethylbutyl, respectively. The compounds monophasically inhibited [(3)H]-N(alpha)-methylhistamine binding to mouse cerebral cortex membranes (pK(i) 7.51 - 9.53). The concentration-response curve of histamine for its inhibitory effect on the electrically evoked [(3)H]-noradrenaline overflow from mouse cortex slices was shifted to the right by these compounds (apparent pA(2) 6.61 - 8.00). Only FUB 373 and 407 inhibited the evoked overflow by themselves (intrinsic activities 0.3 and 0.4); these effects were counteracted by the H(3)-receptor antagonist clobenpropit. [(35)S]-GTPgammaS binding to mouse cortex membranes was stimulated by the H(3)-receptor agonist (R)-alpha-methylhistamine in a manner sensitive to clobenpropit. Among the novel compounds only FUB 373 and 407 stimulated [(35)S]-GTPgammaS binding (intrinsic activities 0.6 and 0.4). In conclusion, the novel compounds are partial H(3)-receptor agonists (FUB 373 and 407) or H(3)-receptor antagonists; comparison with FUB 335 shows that the transition from antagonist to agonist is caused by a slight structural change. A protonated N atom in the side chain is not necessary for agonism at H(3) receptors, proposing a receptor-ligand interaction different from that of classical agonists.     

Inhibitory histamine H2-receptor in the guinea-pig urinary bladder

The histamine H2-receptor in the guinea-pig urinary bladder was characterized by determining the effects of histamine and impromidine on contractions induced by electrical transmural stimulation (ETS). The contractile responses to ETS (0.5 ms, 15 V, for 15 s) at frequencies of 1 to 30 Hz were abolished by treatment with tetrodotoxin, and were partly inhibited by scopolamine, indicating that the ETS-induced contraction has scopolamine-sensitive and -resistant components. Histamine and impromidine inhibited the scopolamine-resistant contraction induced by ETS but not the ETS-induced scopolamine-sensitive contraction and nicotine- and acetylcholine (ACh)-induced contractions. The inhibitory effects of histamine and impromidine were antagonized by cimetidine, but not by diphenhydramine and mepyramine. Thus, the inhibitory effect of histamine may be mediated through H2-receptors. As impromidine did not affect the tetrodotoxin-sensitive and Ca2+-dependent ETS-evoked release of ACh and noradrenaline (NA) from the isolated urinary bladder preloaded with [3H]choline and [3H]NA, respectively, the H2-receptor may not be involved in the cholinergic and adrenergic mechanisms. These results indicate that histamine H2-receptors are present in the guinea-pig urinary bladder. The H2-receptor located on non-cholinergic excitatory neurons may be involved in the inhibitory action produced by histamine.     

Effects of impromidine, a specific H2-receptor agonist and 2(2-pyridyl)-ethylamine, an H1-receptor agonist, on stimulation-induced release of [3H]-noradrenaline in guinea-pig isolated atria

1 The specific histamine H2-receptor agonist, impromidine (3-100 nmol/l), increased the rate and force of beating of guinea-pig isolated atria. These effects were blocked by the H2-receptor antagonist, cimetidine (30 mumol/l), but not by the H1-receptor antagonist, mepyramine (0.1 mumol/l). 2 In atria that had previously been incubated in [3H]-noradrenaline, impromidine (3-100 nmol/l) had no effect on the resting efflux of radioactivity, but concentrations of 50 and 100 nmol/l significantly increased the efflux induced by electrical stimulation (2 Hz for 10 s) of the intramural sympathetic nerves by approximately 38%; lower concentrations (3, 10 and 25 nmol/l) had no effect. 3 The effect of impromidine in enhancing stimulation-induced efflux of radioactivity was abolished by cimetidine (30 mumol/l) and by mepyramine (0.1 mumol/l). It was unaffected by the alpha-adrenoceptor antagonist, phentolamine (30 mumol/l). 4 Impromidine produced some inhibition of the uptake of [3H]-noradrenaline, but this did not account for the enhancement of the stimulation-induced efflux of radioactivity, since impromidine (50 mumol/l) still increased release in the presence of cocaine (30 mumol/l). 5 The specific H1-receptor agonist, 2-(2-pyridyl)-ethylamine (10-100 mumol/l), increased both the resting and stimulation-induced efflux of radioactivity. These effects were not blocked by mepyramine (0.1 mumol/l) or the beta-adrenoceptor antagonist, metoprolol (0.1 mumol/l). 6 The prejunctional inhibitory histamine receptors in guniea-pig atria are not classifiable into H1- or H2-type by the use of relatively specific postjunctional histamine H1- or H2-receptor agonists and antagonists.     

Histamine2 (H2)-receptor antagonists in the treatment of urticaria

Urticaria may develop in response to a number of stimuli such as allergic reactions, drugs, cold, pressure, stings and, most interestingly, neuropsychological upheavals. Classical treatment has utilised H1-receptor antagonists, in view of the fact that histamine released from local mast cells acts on H1-receptors on the vasculature and participates in the pathophysiology of this syndrome. More recently, H2-receptor antagonists have also been tried, alone or in combination, with encouraging results. The question still remains why H2-receptor antagonists should have any beneficial effect since H2-receptors are mostly present on exocrine cells and on T-suppressor lymphocytes, where they are stimulatory, or mast cells, where they are auto-inhibitory. Possible explanations may include the ratio of H1- to H2-receptors on local vasculature and the effect of H2-antagonists on responses elicited through nervous system activity via cholinergic or neuropeptidergic neurons. Finally, evidence is presented that certain tricyclic H-receptor antagonists may have powerful inhibitory effects on secretion from both peripheral and central nervous system mast cells, as well as from neurons. The possible role of H3-receptors in this process is also discussed. At present, the available evidence does not justify the routine use of H2-antagonists in the treatment of urticaria.     

Pharmacological interactions between ranitidine, cimetidine, metiamide and tiotidine at histamine H2-receptor sites in guinea-pig atria

Cumulative concentration-response curves to histamine or dimaprit were constructed on guinea-pig isolated right atria and agonist dose-ratios were determined following addition of ranitidine, cimetidine, metiamide or tiotidine alone or a combination of any two of these H2-receptor blocking drugs. The observed histamine or dimaprit dose-ratios for combinations of any two of the H2-antagonists tested were consistent with results predicted from the equation, DR1+2 = DR1 + DR2 - 1, for two antagonists competing for the same receptor sites. Therefore we conclude that all four H2-antagonists compete for the same histamine H2-receptor.     

Evidence for H2-receptor-mediated inhibition of histamine release from isolated rat mast cells

Impromidine, an H2-receptor agonist, inhibited the release of histamine from isolated purified rat mast cells evoked by compound 48/80 and acetylcholine. Pyridilethyl-amine (PEA), and H1-receptor agonist, on the other hand, was only slightly effective at very high concentrations. The inhibitory effect of impromidine was blocked by preincubating the cells with cimetidine, but not by chlorpheniramine. The existence of an H2-mediated inhibitory feed-back regulation of histamine release was also suggested by the demonstration of specific binding sites for [3H]-cimetidine in rat mast cell membranes.     

[3H]-(+)-N-methyl-4-methyldiphenhydramine, a quaternary radioligand for the histamine H1-receptor.

1. A series of derivatives of 4-methyldiphenhydramine have been examined as potential quaternary radioligands for the histamine H1-receptor. 2. [3H]-(+)-N-methyl-4-methyldiphenhydramine ([3H]-QMDP), 83 Ci mmol-1, was synthesized by methylation of the tertiary analogue and purified by high-voltage electrophoresis. 3. [3H]-QMDP bound to H1-receptors in a washed homogenate from guinea-pig cerebellum with an affinity constant, Ka, of 1.14 +/- 0.03 x 10(9) M-1. The proportion of non-specific binding of 0.3-0.6 nM [3H]-QMDP, defined by 0.4 microM mepyramine, was usually in the range 15-45%, depending on the method of measurement of binding. The affinity of [3H]-QMDP was similar in guinea-pig cerebellum, cerebral cortex and hippocampus, but was lower, 1.4 x 10(8) M-1, in rat cerebral cortex. 4. Evidence was obtained for the presence of a secondary, non-muscarinic, binding site for [3H]-QMDP in guinea-pig cerebellum, approximate Ka 1.5 x 10(7) M-1, accounting for circa 4% of the total binding of 1 nM [3H]-QMDP. 5. There was a very good correlation between the affinities of 15 compounds for the H1-receptor determined from inhibition of [3H]-QMDP binding and from inhibition of [3H]-mepyramine binding. 6. The potential utility of [3H]-QMDP for studies of H1-receptors in the plasma membrane of cells in culture is discussed.     

Quantification of H2-agonism by clonidine and dimaprit in an adenylate cyclase assay

In homogenates of guinea-pig ventricle clonidine and dimaprit both stimulate adenylate cyclase and exhibit "bell-shaped' E/[A] curves. The two properties (stimulatory and inhibitory) could be resolved using a theoretical model assuming a "down line' auto-inhibitory mechanism. In the case of clonidine a further depressive property could be seen in the presence of high concentrations of the selective H2-receptor antagonist tiotidine which is not explicable in terms of this model. The results suggest that clonidine has a direct agonistic effect on H2-receptors in guinea-pig heart. However, like dimaprit, clonidine appears to be a partial agonist because its expression is confounded by a secondary inhibitory property(ies).     

Clathrin-independent internalization of the human histamine H1-receptor in CHO-K1 cells

The aim of the present study was to investigate the cellular pathway involved in histamine-stimulated internalization of the human H1-receptor in CHO-K1 cells expressing N-terminal myc-tagged H1-receptor (Myc-H1) or N-terminal myc-C-terminal green fluorescent protein (Myc-GFP H1) versions of the receptor. Studies of 3H-mepyramine binding and histamine-stimulated 3H-inositol phosphate accumulation in these cells showed that the Myc-H1 and Myc-GFP H1-receptors had identical pharmacology to the wild-type H1-receptor. The Myc-H1-receptor was rapidly internalized in CHO-K1 cells following stimulation with histamine (0.1 mM). This response occurred within 15 min, and could be prevented by the quaternary H1-receptor antagonist alpha-pirdonium. Similar data were obtained with the Myc-GFP H1-receptors. Internalization of the Myc-GFP H1-receptor was maintained in the absence of extracellular calcium and was not inhibited by the CAM kinase II inhibitor KN-62 (10 microM). Phorbol dibutyrate, an activator of protein kinase C, was also able to stimulate internalization of the H1-receptor. However, inhibition or downregulation of protein kinase C (which significantly modified histamine-stimulated inositol phosphate responses) was without effect on the internalization of the H1-receptor stimulated by histamine. Hypertonic sucrose did not prevent histamine-induced internalization of the Myc-GFP H1-receptor, but was able to attenuate internalization of transferrin via clathrin-mediated endocytosis in the same cells. In contrast, preincubation of cells with filipin or nystatin, which disrupts caveolae and lipid rafts, completely inhibited the histamine-induced internalization of the Myc-GFP H1-receptor, but was without effect on the sequestration of transferrin. The H1-receptor and cholera toxin subunit B were colocalized under resting conditions at the cell surface. Immunohistochemical studies with an antibody to caveolin-1 confirmed that this protein was also localized predominantly to the plasma membrane. However, following stimulation of CHO-Myc-GFP H1 cells with histamine, there was no evidence for internalization of caveolin-1 in parallel with the H1-receptor.These data provide strong evidence that the H1-receptor is internalized via a clathrin-independent mechanism and most likely involves lipid rafts.     

Effects of the H2-receptor agonist dimaprit on lymphocyte responsiveness in vitro

The histamine H2-agonist dimaprit was found to increase the response of rat spleen cells to the T-cell mitogen Concanavalin A, when present at concentrations of 10(-5) and 10(-4)M. Higher concentrations of dimaprit were cytotoxic. The enhanced response seemed to be associated with an inhibitory effect of dimaprit on T-suppressor cell activity rather than with a direct mitogen-like stimulation of lymphocyte proliferation or with an interference with monocyte/macrophage functions. The stimulatory effects of dimaprit were not reversed by the H2-receptor antagonist, cimetidine, nor by the beta-receptor antagonists metoprolol and H 35/25. Addition of the H1-receptor antagonist, mepyramine, further increased the stimulatory effect of dimaprit on lymphocyte responsiveness.