H2-receptor Blocker Cimetidine Research Articles

Histamine may directly contract urinary bladder smooth muscle

Mast cell degranulation and histamine release contribute to painful bladder syndrome, urinary tract infections, interstitial cystitis and other bladder disorders. The effects of histamine are well‐studied in other tissues, but little is known about the role of histamine and histamine receptors within the bladder. Since other smooth muscle cells express H1 (contractile) and H2 (relaxant) histamine receptors, we tested the hypothesis that histamine causes contraction of the urinary bladder smooth muscle in mice. All procedures followed institutional guidelines and were approved by the Institutional Animal Care and Use Committees of Michigan State University. Six to 12‐week‐old C57Bl/6 mice were euthanized and dissected bladders were cut into approximately 2 mm wide bladder strips, with or without the urothelium attached, for isometric contractility experiments. Both histamine H1 and H2 receptors were expressed in bladder smooth muscle cells. Increasing concentrations of histamine (100 nM – 300 μM) did not contract strips without urothelium and minimally contracted bladder strips with urothelium. However, bolus administration of 200 μM histamine caused a rapid but transient contraction in strips with and without urothelium. The H1 receptor blocker fexofenadine (5 – 10 μM) blocked these contractions (P<0.05; N=5), whereas the H2 receptor blocker cimetidine (5 – 10 μM) had no effect. In addition, transient receptor potential vanilloid 1 (TRPV1) channel blocker capsazepine significantly reduced histamine‐induced contractions only in bladder strips without urothelium, whereas the Na+ channel blocker tetrodotoxin (TTX) had no effect in any tissue (P < 0.05; N= 3–5). The mast cell activator compound 48/80 (10 μg/ml) contracted urinary bladder smooth muscle, and this contraction was also unaffected by TTX. These results indicate that H1 receptors on urinary bladder smooth cells likely modulate histamine‐induced bladder contraction, and sufficient amounts of histamine can be released from mast cells to cause a contraction.Support or Funding InformationThis research is supported by NIH K01‐DK103840 (NRT) and NIH R01‐DK119615 (NRT & GCM).

Absence of histamine-induced nitric oxide release in the human radial artery: implications for vasospasm of coronary artery bypass vessels

Radial artery (RA) bypass grafts can develop severe vasospasm. As histamine is known to induce vasospasm, its effect on RA was assessed compared with the classic bypass vessels internal mammary artery (MA) and saphenous vein (SV). The vessels were examined in organ chambers for isometric tension recording. Histamine induced contractions on baseline; the sensitivity was higher in RA and SV than MA. After precontraction with norepinephrine, histamine did not evoke relaxations of RA but induced relaxations of MA and less of SV at lower concentrations; it induced contractions at higher concentrations, reaching similar levels in all three vessels. Indomethacin did not affect the response of MA and RA but potentiated relaxations and reduced contractions of SV. Endothelium removal, N(omega)-nitro-L-arginine methyl ester (L-NAME), or the H2-receptor blocker cimetidine did not affect the response of RA, but inhibited relaxations and enhanced contractions in MA and inhibited relaxations in SV; in the latter, only L-NAME enhanced contractions. Real-time PCR detected much lower expression of endothelial H2-receptor in RA than MA or SV. Western blots revealed similar endothelial nitric oxide (NO) synthase expression in all three vessels. Relaxations to acetylcholine were identical in RA and MA. Thus histamine releases NO by activating the endothelial H2-receptor, the expression of which is much lower in RA than MA or SV. H2-receptor activation also releases prostaglandins in SV, partially antagonizing NO. The lack of histamine-induced NO production represents a possible mechanism of RA vasospasm.

Acute effects of bradykinin on cerebral microvascular permeability in the anaesthetized rat.

1. The permeability response to acutely applied bradykinin and [des-Arg9]-bradykinin on single cerebral venular capillaries has been investigated using the low molecular mass fluorescent dyes Lucifer Yellow and Sulforhodamine B with the single vessel occlusion technique. 2. When bradykinin was applied repeatedly for up to 2 h, the permeability increase was small and reversible for concentrations that ranged from 5 nM to 50 microM. 3. The logEC50 of the permeability response to bradykinin was -5.3 +/- 0.15 (logM; mean +/- s.e.m.). This was reduced to -6.37 +/- 0.24 with the angiotensin-converting enzyme inhibitor captopril, to -6.33 +/- 0.19 with the neutral endopeptidase inhibitor phosphoramidon and to -7.3 +/- 0.20 with captopril and phosphoramidon combined. 4. The permeability response to bradykinin was blocked by the bradykinin B2 receptor antagonist HOE 140, by inhibition of the Ca2+-independent phospholipase A2, by the scavenging of free radicals, or by inhibition of both cyclo-oxygenase and lipoxygenase in combination. Block of Ca2+ entry channels with SKF 96365 had no effect on the response. 5. Application of [des-Arg9]-bradykinin also increased permeability over the concentration range 5 nM to 50 microM, with a logEC50 of -5.6 +/- 0. 37. This response was not affected by free radical scavenging, but was completely blocked by the histamine H2 receptor blocker cimetidine. 6. These results imply that the acute permeability response to bradykinin is mediated via the release of arachidonic acid, which is acted on by cyclo-oxygenase and lipoxygenase resulting in the formation of free radicals, and that the response to [des-Arg9]-bradykinin is mediated via histamine.

Dirofilaria immitis:Heartworm Infection Converts Histamine-Induced Constriction to Endothelium-Dependent Relaxation in Canine Pulmonary Artery

Kaiser, L., and Williams, J. F. 1998.Dirofilaria immitis:Heartworm infection converts histamine-induced constriction to endothelium-dependent relaxation in canine pulmonary artery.Experimental Parasitology88, 146–153. Heartworm (Dirofilaria immitis) infection alters the behavior of vascular endothelial cellsin vivoandin vitro, with the potential, therefore, to influence vascular function. Histamine, an autocoid implicated in the pathogenesis of parasitic and inflammatory diseases, is vasoactive, and causes endothelium-dependent relaxation in some vascular beds. Experiments were designed to determine if histamine is an endothelium-dependent vasodilator inin vitrorings of canine pulmonary artery from heartworm and control dogs; to elucidate the mechanisms involved in histamine vasoactivity; and to measure circulating levels of histamine. Dose-response relationships to histamine were done in rings of canine pulmonary artery from heartworm and control dogs, in the presence and absence of endothelial cells, the HI receptor blocker tripelennamine, or the H2 receptor blocker cimetidine. Histamine caused a dose-dependent constriction in control, that was not influenced by endothelial cell removal. However, histamine caused an endothelium-dependent relaxation in heartworm pulmonary artery that was converted to constriction by endothelial cell removal. In heartworm, histamine relaxation was mediated by H2 receptors, but did not appear to involve nitric oxide or cyclooxygenase products. While diseases cause depression of endothelium-dependent relaxation, this is the first report of a disease that changes a constriction response to an endothelium-dependent relaxation.

Prejunctional inhibition of sympathetically evoked pupillary dilation in cats by activation of histamine H3 receptors.

Frequency-dependent pupillary dilations were evoked by electrical stimulation of the pre- or post-ganglionic cervical sympathetic nerve (sympatho-excitation) or the hypothalamus (parasympatho-inhibition) in sympathectomized anesthetized cats. Systemic administration of the selective histamine H3 receptor agonist (R)-alpha-methylhistamine (R alpha MeHA) produced a dose-dependent depression of mydriasis due to direct neural sympathetic activation but had no effect on responses elicited by parasympathetic withdrawal. The histamine H2 receptor agonist, dimaprit, was inactive. R alpha MeHA was much more effective in depressing sympathetic responses obtained at lower frequencies when compared to higher frequencies of stimulation. Responses evoked both pre- and postganglionically were inhibited by R alpha MeHA. This peripheral sympatho-inhibitory action of R alpha MeHA was antagonized by the histamine H3 receptor blocker thioperamide but not by intravenous pretreatment with the histamine H1 receptor antagonist chlorpheniramine. Histamine H2 receptor blockers cimetidine and ranitidine were also without effect. R alpha MeHA did not depress pupillary responses elicited by i.v. (-)-adrenaline. The results demonstrate that histamine H3 receptors modulate sympathetic activation of the iris at a site proximal to the iris dilator muscle. The predominant mechanism of action appears to the prejunctional inhibition of noradrenaline release from postganglionic sympathetic nerve endings. However, a concomitant ganglionic inhibitory action cannot be excluded.

Cyclic AMP generating systems in vertebrate retina: Effects of histamine and an established retinal modulator, dopamine

Histamine (HA), 1-1000 microM, significantly stimulated both basal and forskolin-activated cAMP generation in chicken and adult hen retina. The action of HA was reproduced by the selective H2-receptor agonists dimaprit and 4-methyl-histamine, but not by the selective H1-receptor agonist 2-thiazolylethylamine, and it was antagonized by the specific H2-receptor blockers cimetidine and tiotidine, but not by the H1-receptor blocker mepyramine. In parallel experiments, dopamine, an established retinal neuromodulator acting through the D1-type of receptor, also stimulated basal and forskolin-driven adenylate cyclase activity in homogenate of chicken retina. It is suggested that chicken retina contains HA H2-receptors which are positively coupled to the adenylate cyclase system.

Lack of Effect of Pinacidil on Theophylline Pharmacokinetics and Metabolism in Man

Pinacidil, a pyridyl cyanoguanidine derivative, is a new antihypertensive vasodilator drug. It shares structural similarities with the histamine H2-receptor blocker cimetidine, an imidazole cyanoguanidine derivative, which is a potent inhibitor of cytochrome P-450 and of theophylline metabolism. In the present study the pharmacokinetics and metabolism of theophylline were determined in six healthy volunteers before and on the last day of oral pinacidil administration for two weeks. The dosage of pinacidil was 12.5 mg twice a day in the first week and 25 mg in the second. There were no significant changes in theophylline plasma clearance, terminal half-life or volume of distribution during pinacidil administration. Also the renal and metabolic clearance of theophylline and the formation clearances of the major theophylline metabolites in the urine (DMU, 1MU, 3MX) did not change significantly during administration of therapeutic doses of pinacidil.

Enhanced Susceptibility to Histamine-Induced Cardiac Arrhythmias in Spontaneously Hypertensive Rats

To clarify mechanisms underlying an enhanced susceptibility to cardiac rhythm disturbances in hypertension and myocardial hypertrophy, we evaluated the vulnerability to histamine-induced arrhythmias of isolated left ventricles from spontaneously hypertensive rats (SHR) and age-matched controls (Wistar-Kyoto rats, WKY). Before drug administration, left ventricle-to-body weight ratios, spontaneous firing rates, and the incidence of arrhythmias (including delayed afterdepolarizations) were significantly increased in SHR ventricles versus WKY. In addition, action potential duration (APD) was prolonged in SHR at all levels of repolarization. In WKY but not SHR hearts, histamine (10(-7)-10(-4) M) increased spontaneous firing rates; the incidence of arrhythmias was increased in all hearts, but the response to histamine was most pronounced and occurred at lower threshold drug levels in SHR. After potentials and triggered activity were observed only in SHR. The H2-receptor blocker cimetidine (10(-5) M) and the Ca2(+)-channel antagonist verapamil (10(-6) M) each attenuated the arrhythmogenic influence of histamine in SHR and WKY preparations. Neither chlorpheniramine nor propranolol had any effect. The enhanced vulnerability to arrhythmogenesis observed in SHR myocardium may reflect elevated intracellular calcium levels, which may in turn be potentiated by local ischemia and/or intracardiac histamine release.

Response to histamine of the intact coronary system in an isolated rabbit heart preparation

In isolated Langendorff-perfused rabbit hearts concentration-response relations for histamine were analysed with and without H2-receptor blocker cimetidine (10(-5) mol l-1). The diameter of large epicardial coronary arteries was determined by video angiometry. Global coronary flow and left ventricular isovolumetric pressure were continuously registered. Histamine concentration (10(-7)-6 X 10(-6) mol l-1) reduced the diameter of large coronary arteries up to -50%, however, this vasoconstriction was maximal after 1 min and was then attenuated within the following 20 min. Coronary flow and left ventricular pressure were not significantly altered. In the presence of cimetidine the histamine-induced vasoconstriction was stabilized and not reversed with time. Moreover, the extent of vessel wall narrowing was intensified (up to -70%) when compared with histamine alone. Furthermore, severe functional myocardial failure was observed (flow reduction to 40% of control).

Voltage-dependent Vmax blockade in Na+-dependent action potentials after beta 1- and H2-receptor stimulation in mammalian ventricular myocardium.

In isolated papillary muscles of guinea pigs, the influence of isoproterenol, histamine, theophylline, and phenylephrine on the maximal rate of rise (Vmax) of Na+-dependent action potentials and on isometric contractile force was studied under rested state conditions. Isoproterenol (1 x 10(-7) mol/L), histamine (2 x 10(-5) mol/L), and theophylline (2 x 10(-3) mol/L) shifted the voltage dependence of Vmax into the hyperpolarizing direction and, consequently, led to a voltage-dependent Vmax blockade. The alpha-adrenoceptor agonist phenylephrine, on the other hand, proved to be ineffective in depressing Vmax. The beta-receptor blocker pindolol (4 x 10(-6) mol/L) or the H2-receptor blocker cimetidine (4 x 10(-5) mol/L) abolished the inhibitory effects of isoproterenol and histamine, respectively, and caused Vmax to return to the initial control value. A concentration-response relationship analysis at -65 mV revealed that isoproterenol exerted only a weak inhibitory effect on Vmax compared with its positive inotropic action. The IC50 value of the former effect amounted to approximately 5 x 10(-6) mol/L, but the EC50 value of the latter effect was 4 x 10(-8) mol/L. It is, therefore, concluded that, in physiologically relevant concentrations, beta-adrenergic agonists are unlikely to significantly modulate Na+-dependent excitability even in partially depolarized myocardium.

Characterization of the effects of histamine on the transmembrane electrical activity of guinea-pig and rabbit SA- and AV-node cells.

The effects of histamine on the transmembrane electrical activity of cells of small preparations (0.5 X 0.5 mm) of guinea-pig and rabbit sinoatrial- and atrioventricular-nodes were studied. Histamine at concentrations above 10(-7) mol/l increased the firing rate, the rate of diastolic depolarization, the maximum diastolic potential, the amplitude and the maximum rate of depolarization of the action potential of pacemaker cells of rabbit and guinea-pig sinoatrial cells and rabbit atrioventricular cells. These effects were antagonized by the H2-receptor blocker cimetidine (2.5 X 10(-6)mol/l) but they were not modified by the H1-receptor blocker chlorphenamine (2.5 and 5 X 10(-6)mol/l). Small preparations of guinea-pig atrioventricular node did not exhibit spontaneous activity, but it was induced by histamine and blocked by cimetidine. Histamine increased the maximum upstroke velocity of propagated action potential of cells of the central part of complete atrioventricular node in both species studied. These effects were blocked by cimetidine, but not by chlorphenamine. It is concluded that the increase in automaticity induced by histamine in guinea-pig and rabbit sinoatrial and atrioventricular nodes was due to stimulation of H2-receptors. Histamine did not depress electrical activity of atrioventricular node cells, but rather increased it. This effect was due to H2-receptor stimulation.

VIP and histamine H2 receptor activity in human fetal gastric glands.

Vasoactive intestinal peptide (VIP, EC50 = 6.4 X 10(-10)M) and histamine (EC50 = 3 X 10(-6)M) activated the cyclic AMP generating system in gastric glands isolated from two human fetuses at 23 weeks gestation. Histamine antagonism by the H2 receptor blockers cimetidine (Ki = 0.35 X 10(-6)M) and ranitidine (ki = 0.51 X 10(-7)M) clearly characterized the histaminic activation as being of the H2 type. It is suggested that these two vasoactive hormones may operate as neurocrine/paracrine regulators of the differentiation and/or function of the human gastric mucosa in utero.

Peptide 6A, a fibrin(ogen) degradation product, increases coronary blood flow.

The coronary hemodynamic effects of intracoronary administration of a fibrin(ogen)-derived pentapeptide, Ala-Arg-Pro-Ala-Lys (peptide 6A), were evaluated in open-chest anesthetized dogs. With administration of peptide 6A (2.5-30 mumol), coronary blood flow increased and coronary vascular resistance decreased promptly in a dose-related manner. Increase in coronary blood flow was independent of any change in indexes of myocardial O2 demand, indicating the peptide 6A exerts direct effects on coronary arterial tone. Systemic arterial and left ventricular end-diastolic pressures remained unchanged with smaller doses but decreased when higher doses of peptide 6A (greater than or equal to 20 mumol) were administered. Plasma concentrations of 6-ketoprostaglandin F1 alpha, stable hydrolysis product of prostacyclin, increased in coronary sinus blood samples in conjunction with increase in coronary blood flow. Administration of indomethacin (5 mg/kg iv) inhibited peptide 6A-induced release of prostacyclin and significantly attenuated the effects of peptide 6A on coronary hemodynamics. Pretreatment of animals with H2-receptor blocker cimetidine (500 mg iv) or with H1-and H2-receptor blocker diphenhydramine (50 mg iv) had no significant effects on peptide 6A-induced increase in coronary blood flow. This study suggests that this fibrin(ogen)-derived peptide has potent vasodilator effects on the coronary vascular bed of the dog, and these effects are in part mediated by stimulation of prostacyclin release.

Ablation of the blood-testis barrier in rats and guinea pigs by 48/80, a histamine releaser, and cadmium chloride.

When adult male guinea pigs were injected unilaterally intratesticularly with compound 48/80 (1 mg in 0.05 ml saline), their testicular peritubular capillaries were engorged with blood, intratubular edema was present on the ipsilateral side, and the blood-testicular (BT) barrier (measured by the entrance of acriflavin into the seminiferous tubule) was ablated. The contralateral testis, when injected with saline, showed no pathological changes nor a breakdown of the BT barrier. Subcutaneous injection of guinea pigs with cadmium chloride resulted in a more intense intratubular fluorescence than was observed for 48/80-treated animals. The H1 and H2 receptor blockers (diphenhydramine and cimetidine, respectively) reduced the intensity of capillary engorgement and edema. Intraperitoneal injections of 48/80 (0.5 mg) was lethal to rats, and both diphenhydramine and cimetidine induced survival even with as much as 1 mg of 48/80. Intratubular fluorescence was less severe than that observed for similarly treated guinea pigs or from cadmium chloride-treated rat testes. Neither treatment (48/80 or cadmium chloride) altered the blood-epididymal barrier of either guinea pigs or rats. Species differences were observed in the lethal effects of exogenously administered histamine.

Evidence for a direct action of acetylcholine on the gastric oxyntic cell of the amphibian.

The possibility of a direct action of acetylcholine (ACh) on the oxyntic cell not mediated by histamine release was studied in resting isolated gastric mucosae. H+ secretion and histamine release, and their relationship, were studied under ACh stimulation and other conditions. ACh released histamine from mucosal stores and stimulated H+ secretion. H2-receptor blocker cimetidine largely inhibited ACh-induced stimulation of H+ transport but not histamine release. Atropine inhibited the cholinergic response in both histamine release and H+ secretion. The dose-response curves to ACh showed nonparallel increases in H+ secretion and histamine release. When dose-response curves to exogenous and endogenous histamine (released by ACh) were compared, it was found that ACh increased Vmax for endogenous histamine. Comparison of the effects of ACh with other experimental conditions such as tetragastrin treatment, K+ depolarization, or Na+-free nutrient solution showed that the amount of histamine released by ACh was insufficient to explain the observed rates of secretion. Stimulation by ACh was faster, greater, and more transitory compared with that by histamine. In mucosae maximally stimulated by histamine, ACh induced a further increase in H+ secretion. In the presence of cimetidine, potentiation between ACh and dibutyryl cAMP or isobutyl-methylxanthine was found. The results are consistent with ACh having a dual action on oxyntic and histamine-releasing cells. The action of ACh on the oxyntic cell would potentiate the effect of released histamine. It is suggested that ACh and released histamine act on the oxyntic cell through different second messengers, resulting in the potentiated response.

Response of local blood flow in the caudate nucleus of the cat to intraventricular administration of histamine.

The effect of intraventricular histamine on blood flow in the caudate nucleus of the cat was studied by means of the hydrogen clearance technique. Bilateral ventriculo-cisternal perfusion was installed. After a control period during which both lateral ventricles were perfused with mock CSF with the same composition, the drug under study was added to one side (experimental side) while the other side was perfused further with the control mock SCF (control side). At each point in time, blood flow at the experimental side was compared to that at the control side. Histamine (10(-3) M) caused a severe vasodilatation and this effect was completely antagonised by the H2-receptor blocker cimetidine (10(-2) M). Cimetidine had no vasoactive effects of itself in the concentration used. The H2-receptor agonist Dimaprit (10(-3) M) had a vasodilator effect although less important than histamine. Indirect evidence was gained that H1-receptors are not active in the vascular bed under study.

Mechanisms of histamine stimulated secretion in rabbit ileal mucosa.

Histamine is present in high concentrations in the intestine and we investigated the possibility that it might have a role here in intestinal transport. When added to the basal side of rabbit ileal mucosa in vitro histamine (10(-4)M) induced a short-lived increase in electrical potential difference and short circuit current. It inhibited net chloride absorption but did not influence sodium transport. Alkali secretion, measured by a pH stat technique, was inhibited, suggesting that bicarbonate secretion was reduced. Both the electrical and ion flux responses to histamine were blocked by the H1 receptor blocker diphenhydramine, but not by the H2 receptor blocker cimetidine. The presence of specific H1 histamine receptors was further supported by shifts in the dose-response curve to histamine by four different concentrations of diphenhydramine. Calculation of a pA2 value from these "Schild' plots provided a figure of 7.85, which is similar to that for H1 receptors in other tissues. Aminoguanidine, a histaminase blocker, had no electrical effects alone but shifted the histamine dose response curve to the left. These studies indicate that histamine inhibits chloride absorption and alkali secretion, possibly by influencing a chloride/bicarbonate exchange process, through specific mucosal H1 receptors. Enhancement of histamine effects by a histaminase inhibitor suggests that histaminases are present in the intestinal mucosa and supports the possibility of a role for endogenous histamine in influencing ion transport. The observations indicate a mechanism by which absorption might be impaired in diseases in which histamine is liberated locally in the intestine.

Additional data on the function of hypothalamic histamine.

In a preliminary study, the stimulatory effect of histamine on an adenylate cyclase system in a solubilized cell-free preparation of the rat hypothalamus was established. The effect was dose dependent, and the histamine concentration required for half-maximal activation (Ka) was determined at 0.1 muM. At a 10-fold higher concentration, both chloropyramine, the classical histamine H1 antagonist, and metiamide, the selective H2-receptor blocker, partially blocked this action. Experiments carried out in hypothalamic slices showed a stimulatory effect of both the H1-agonist, 2-(2-pyridyl)-ethylamine, and the H2-antagonist, dimaprit, on adenylate cyclase in the range of histamine action. These effects could be reversed completely by the H1-antagonist, mepyramine, and the H2-receptor blocker cimetidine. In an additional study, histamine, histamine agonists and antagonists were tested on the spontaneous and the potassium-activated outflow of 3H-noradrenaline from rat hypothalamic slices. Histamine did not modify this outflow significantly, whereas the H1-agonist 2-(2-pyridyl)-ethylamine, produced a marked, dose-related increase in both the spontaneous and the potassium-stimulated release of noradrenaline. The H2-receptor blocker, cimetidine, also exerted a moderate but statistically significant stimulatory effect in this system. In combination studies, the noradrenaline-releasing action of these compounds could not he reversed by the selectively acting histaminic or antihistaminic agents, showing that this effect does not relate to the histaminic or antihistaminic property of the compound. It is becoming clear that histamine exerts a direct stimulatory effect on hypothalamic adenylate cyclase. The noradrenaline-releasing potency of some histaminic and antihistaminic agents showed that these compounds might modify the clear histamine effects through the release of other transmitter amines.

Effect of histamine receptor blocking on human antibody-dependent cell-mediated cytotoxicity.

The effect of H1 and H2 receptor-blocking agents on antibody-dependent cell-mediated cytotoxicity (ADCC) was studied. The H1 receptor-blocker clemastinum and the H2 receptor blocker cimetidine dose-dependently inhibited the antibody-dependent cytotoxic activity of normal human peripheral blood mononuclear cells on chicken erythrocytes. The inhibition cannot be explained either by a direct toxic effect on effector cells or by blocking of Fc receptors. The possible involvement of histamine receptor-bearing effector cells in human ADCC is suggested.

Methylation of Histamine in the Gastric Mucosa

Methylation of histamine in the gastric mucosa of various species has been studied in vitro and in vivo. When gastric mucosal homogenates of rat, guinea pig, cat, dog, and pig were incubated with 14C-histamine in the presence of S-adenosyl methionine Ntau-methylhistamine was formed as the only methylated histamine derivative. Excessive concentration of Ntau-methylhistamine inhibited the formation of Ntau-methylhistamine. Pretreatment of guinea pigs with prednisolone, aminoguanidine, or pentagastrin neither influenced in vitro the formation of Ntau-methylhistamine significantly nor shifted the methylation reaction towards the side chain. Pretreatment of guinea pigs with the histamine H2-receptor blocker cimetidine significantly enhanced the formation of Ntau-methylhistamine in vitro. In pooled samples of guinea pig gastric mucosa histamine and Ntau-methylhistamine were found. Under none of these circumstances was Nalpha-methyl-or Nalpha,Nalpha-dimethylhistamine found. It is concluded that in rat (to a small extent), guinea pig, cat, dog, and pig gastric mucosa the formation of Ntau-methylhistamine is the only methylating pathway of histamine. In cases where other methylated histamine derivates (Nalpha-methyl- and/or Nalpha,Nalpha-dimethylhistamine) are found they must origin in sources other than the gastric mucosa.