R-(-)-alpha-methylhistamine, a histamine H3 receptor agonist, induces endothelium-dependent vasodilation in rat mesenteric resistance arteries

Monoaminergic Receptors as Modulators of the Perivascular Sympathetic and Sensory CGRPergic Outflows

Blood pressure is a highly controlled cardiovascular parameter that normally guarantees an adequate blood supply to all body tissues. This parameter is mainly regulated by peripheral vascular resistance and is maintained by local mediators (i.e., autacoids), and by the nervous and endocrine systems. Regarding the nervous system, blood pressure can be modulated at the central level by regulating the autonomic output. However, at peripheral level, there exists a modulation by activation of prejunctional monoaminergic receptors in autonomic- or sensory-perivascular fibers. These modulatory mechanisms on resistance blood vessels exert an effect on the release of neuroactive substances from the autonomic or sensory fibers that modify blood pressure. Certainly, resistance blood vessels are innervated by perivascular: (i) autonomic sympathetic fibers (producing vasoconstriction mainly by noradrenaline release); and (ii) peptidergic sensory fibers [producing vasodilatation mainly by calcitonin gene-related peptide (CGRP) release]. In the last years, by using pithed rats, several monoaminergic mechanisms for controlling both the sympathetic and sensory perivascular outflows have been elucidated. Additionally, several studies have shown the functions of many monoaminergic auto-receptors and hetero-receptors expressed on perivascular fibers that modulate neurotransmitter release. On this basis, the present review: (i) summarizes the modulation of the peripheral vascular tone by adrenergic, serotoninergic, dopaminergic, and histaminergic receptors on perivascular autonomic (sympathetic) and sensory fibers, and (ii) highlights that these monoaminergic receptors are potential therapeutic targets for the development of novel medications to treat cardiovascular diseases (with some of them explored in clinical trials or already in clinical use).

Cardiovascular effects of H3 histamine receptor inverse agonist/ H4 histamine receptor agonist, clobenpropit, in hemorrhage-shocked rats

Hemorrhagic shock has a potential to be life-threatening when it is not treated. The main causes of hemorrhagic shock involve: (1) forces causing injury; and (2) diseases that can cause hemorrhage., Therefore, due to the causes of hemorrhagic shock and the life-threatening potential, the search for new methods of shock treatment is extremely valuable to the modern medicine. The aim of this study was to investigate the influence of clobenpropit in the model of hemorrhagic shock. The experiments were conducted in 110 adult male Wistar rats weighing between 205 and 470g. 1, 2 and 5 μmol/kg of intravenous H₃ receptors reverse agonists, clobentropit, and/or 1, 5 and 10 μmol/kg H₃ receptor agonist, imetit, were used as general anesthetics. Irreversible hemorrhagic shock was induced by the paused bleeding until the mean arterial pressure (MAP) lowered to the level of 20–25 mmHg. It was proved that, in cases of critical hypotension, clobenpropit triggered a dose-dependent increase of: systolic blood pressure (SBP), diastolic blood pressure (DBP), MPA and heart rate (HR) of rats with critical hypotension. The most significant changes in hemodynamic parameters were achieved by administrating dosages of 2 mmol/kg. This resulted in the survival rate increase to up to 100%. However, imetit did not trigger any hemodynamic changes nor an increase in SBP, DBP, MAP or HR. Furthermore, it was found that the premedication with prazosin, yohimbine, 6-hydroxydopamine and the vasopressin V_1a receptor antagonist blocked the effects of clobenpropit. Additionally, premedication with propranolol, captopril and ZD 7155 did not cause any significant changes in the measured hemodynamic parameters. In conclusion, after an intravenous injection clobenpropit, the inverse agonist of H₃ histamine receptors/agonist of histamine receptors H₄, causes a resuscitating effect on rats in hemorrhagic shock. Moreover, such effect is based on the effector mechanisms of sympathetic nervous system and vasopressin.

Gallic acid improves endothelium-dependent vasodilatory response to histamine in the mesenteric vascular bed of diabetic rats

BACKGROUND

Endothelial dysfunction is one of the many complications caused by diabetes mellitus. The aim of the present study was to evaluate the effects of gallic acid (GA) on the mesenteric vascular bed (MVB) response to histamine in diabetic rats.

METHODS

Forty male Wistar rats were randomly assigned to a control group, an untreated alloxan-induced diabetic group and three diabetic groups treated with different doses of GA. Six weeks after induction of diabetes and GA treatment, total antioxidant capacity (TAC), malondialdehyde (MDA) concentrations, and the vasodilatory response to histamine of the MVB (measured as changes in perfusion pressure) were determined.

RESULTS

The vasodilatory response to histamine and TAC decreased, whereas MDA increased in the plasma from diabetic rats (P < 0.01). However, in the presence of 3 × 10^-5  mol/L N ^G -nitro-l-arginine methyl ester (a nitric oxide synthase inhibitor) and 1 × 10^-5  mol/L indomethacin (an inhibitor of prostaglandin production), the vasodilatory response of the MVB to histamine was reduced in all groups (P < 0.001). Treatment of diabetic rats with 20 and 40 mg/kg per day GA, but not 10 mg/kg per day GA, increased TAC and decreased MDA concentrations (P < 0.01 and P < 0.001 vs untreated diabetic rats, respectively) and significantly improved the vasodilatory response to histamine (P < 0.05 and P < 0.001, respectively).

CONCLUSION

The results show that, in diabetic rats, the endothelium-dependent vasodilatory response of the MVB to histamine is significantly decreased and depends on both nitric oxide- and prostaglandin-producing pathways and may be mediated by oxidative stress. Treatment with the antioxidant GA restored the vasodilatory response of the MVB to histamine.

Madagascine Induces Vasodilatation via Activation of AMPK

Madagascine (3-isopentenyloxyemodin) can be chemically synthesized or purified from several Rhamnus species, and it is found to have more potent biological activities than the parent compound emodin. The aim of this study is to characterize the vasodilatory effect of madagascine on vasoconstriction and sphingosylphosphorylcholine induced vasospasm in ex vivo and reveal the potential mechanisms in vitro. The effects of madagascine on vasoconstriction of rat mesenteric resistance arteries (MRAs) induced by K⁺, methoxamine, and endothelin-1 were, respectively, studied. The cholesterol-enriched porcine coronary vascular smooth muscle (VSM) strips were used to investigate the effects of madagascine on abnormal constriction induced by sphingosylphosphorylcholine (SPC) which has a pivotal role in vasospasm. The vasodilatory effect was induced by madagascine (0.3-100 μM) in isolated rat MRAs and the vasodilatory effect was blocked by NO synthase inhibitor L-NAME and AMPK inhibitor compound C. Madagascine (10 μM) also significantly relaxed the abnormal constriction in porcine VSM induced by SPC and the effect was abolished by compound C. Madagascine significantly increased the phosphorylation of endothelial nitric oxide synthase (eNOS) in endothelial cells while decreasing the phosphorylation of myosin phosphatase target subunit 1 (MYPT1) in VSM cells. Madagascine-induced vasodilatation was abrogated using small interfering RNA knockdown of AMPK. In summary, madagascine exerted vasodilatation through activating AMPK, leading to the activation of eNOS in endothelium and inhibition of ROCK/MYPT1 in VSM. This study suggests the potential value of madagascine in amelioration of vasospasm related cardiovascular diseases.

Histamine H3 and H4 receptor ligands modify vascular histamine levels in normal and arthritic large blood vessels in vivo

Growing evidence associates histamine with arthritis, but its implication in shaping vascular function in chronic inflammation remains largely elusive. This study explored the involvement of vascular histamine in the extra-articular responses in peripheral large blood vessels using a rat model of adjuvant-induced arthritis. Histamine levels were increased in the abdominal aorta and the inferior vena cava of arthritic animals. Contrary to the H1 receptor antagonist dimetindene, histamine induction was observed following administration of the H3 and H4 receptor ligands GSK334429 and JNJ7777120, respectively. In arthritis, prophylactic treatment with GSK334429 partially attenuated the clinical signs and restored basal histamine levels only in the abdominal aorta. This study is the first to implicate the H3 and H4 receptors in a concerted constitutive regulation of basal vascular histamine in the rat large blood vessels and to identify the H3 receptor as a component that may influence arterial histamine during the onset of arthritis.

Vascular responses to compound 48/80 in rat mesenteric vascular beds

A further investigation was performed on the vascular effect of endogenous histamine using the histamine releaser, compound 48/80, in rat mesenteric vascular beds with active tone. In preparations with intact endothelium, low concentrations of compound 48/80 (1.53 × 10(-5) - 3 × 1.53 × 10(-5) mg/mL) perfusion for 1 min only induced a small vasodilation. High concentrations of compound 48/80 (1.53 × 10(-4) - 3 × 1.53 × 10(-2) mg/mL) induced a biphasic vascular responses, an initial vasoconstriction followed a subsequent long-lasting vasodilation. The vasodilation induced by low concentrations of compound 48/80 and the vasoconstriction induced by high concentration of compound 48/80 was inhibited by olopatadine. However, cimetidine did not affect the responses induced by compound 48/80. Endothelium removal enlarged the compound 48/80-induced phase-2 vasoconstriction, while it attenuated the phase-3 vasodilation. Additionally, indomethacin and seratrodast significantly inhibited vasoconstriction but it did not affect the long-lasting vasodilation induced by high concentrations of compound 48/80. Ruthenium red inhibited the vasodilation induced by low concentrations and high concentrations of compound 48/80. These results suggest that the vasoconstriction induce by high concentrations of compound 48/80 is mediated by endogenous histamine released from mast cells. It is also suggested that thromboxane A2 released from mast cells is related to the vasoconstriction.

Regulation of the Cardiovascular System by Histamine

Histamine mediates a wide range of cellular responses, including allergic and inflammatory reactions, gastric acid secretion, and neurotransmission in the central nervous system. Histamine also exerts a series of actions upon the cardiovascular system but may not normally play a significant role in regulating cardiovascular function. During tissue injury, inflammation, and allergic responses, mast cells (or non-mast cells) within the tissues can release large amounts of histamine that leads to noticeable cardiovascular effects. Owing to intensive research during several decades, the distribution, function, and pathophysiological role of cardiovascular H₁- and H₂-receptors has become recognized adequately. Besides the recognized H₁- and H₂-receptor-mediated cardiovascular responses, novel roles of H₃- and H₄-receptors in cardiovascular physiology and pathophysiology have been identified over the last decade. In this review, we describe recent advances in our understanding of cardiovascular function and dysfunction mediated by histamine receptors, including H₃- and H₄-receptors, their potential mechanisms of action, and their pathological significance.

Pharmacological evidence that histamine H3 receptors inhibit the vasodepressor responses by selective stimulation of the rat perivascular sensory CGRPergic outflow

This study has investigated whether pharmacological activation of Gi/o coupled histamine H3/H4 receptors inhibits the rat vasodepressor sensory outflow. For this purpose, 100 male Wistar rats were pithed, artificially ventilated and pretreated (i.v.) with: 25mg/kg gallamine, 2mg/kg/min hexamethonium and 20μg/kg/min methoxamine, followed by i.v. continuous infusions of physiological saline (0.02ml/min) or immepip (3.1, 10 or 31μg/kg/min; a histamine H3/H4 receptor agonist). Under these conditions, electrical stimulation (0.56-5.6Hz; 50V and 2ms) of the spinal cord (T9-T12) resulted in frequency-dependent vasodepressor responses, which were: (i) unchanged during the infusions of saline or immepip (3.1μg/kg/min); and (ii) significantly but, surprisingly, not dose-dependently inhibited by 10 and 31μg/kg/min immepip. Moreover, the sensory-inhibition by 10μg/kg/min immepip (which failed to inhibit the vasodepressor responses by i.v. bolus injections of α-CGRP; 0.1-1µg/kg) was: (i) essentially unaltered after i.v. administration of saline (1ml/kg) or blocking doses of the antagonists ketotifen (100μg/kg; H1), ranitidine (1000μg/kg; H2) or JNJ7777120 (310μg/kg; H4); and (ii) abolished after i.v. thioperamide (310µg/kg; H3). In conclusion, our results suggest that immepip-induced inhibition of the vasodepressor sensory outflow is mainly mediated by prejunctional activation of histamine H3 receptors.

The expression and function of histamine H₃ receptors in pancreatic beta cells

BACKGROUND AND PURPOSE

Histamine and its receptors in the CNS play important roles in energy homeostasis. Here, we have investigated the expression and role of histamine receptors in pancreatic beta cells, which secrete insulin.

EXPERIMENTAL APPROACH

The expression of histamine receptors in pancreatic beta cells was examined by RT-PCR, Western blotting and immunostaining. Insulin secretion assay, ATP measurement and calcium imaging studies were performed to determine the function and signalling pathway of histamine H₃ receptors in glucose-induced insulin secretion (GIIS) from MIN6 cells, a mouse pancreatic beta cell line. The function and signalling pathway of H₃ receptors in MIN6 cell proliferation were examined using pharmacological assay and Western blotting.

KEY RESULTS

Histamine H₃ receptors were expressed in pancreatic beta cells. A selective H₃ receptor agonist, imetit, and a selective inverse H₃ receptor agonist, JNJ-5207852, had inhibitory and facilitatory effects, respectively, on GIIS in MIN6 cells. Neither imetit nor JNJ-5207852 altered intracellular ATP concentration, or intracellular calcium concentration stimulated by glucose and KCl, indicating that GIIS signalling was affected by H3 receptor signalling downstream of the increase in intracellular calcium concentration. Moreover, imetit attenuated bromodeoxyuridine incorporation in MIN6 cells. The phosphorylation of cAMP response element-binding protein (CREB), which facilitated beta cell proliferation, was inhibited, though not significantly, by imetit, indicating that activated H₃ receptors inhibited MIN6 cell proliferation, possibly by decreasing CREB phosphorylation.

CONCLUSIONS AND IMPLICATIONS

Histamine H₃ receptors were expressed in mouse beta cells and could play a role in insulin secretion and, possibly, beta cell proliferation.

Histamine-induced vasodilatation in the human forearm vasculature

AIM

To investigate the mechanism of action of intra-arterial histamine in the human forearm vasculature.

METHODS

Three studies were conducted to assess changes in forearm blood flow (FBF) using venous occlusion plethysmography in response to intra-brachial histamine. First, the dose-response was investigated by assessing FBF throughout a dose-escalating histamine infusion. Next, histamine was infused at a constant dose to assess acute tolerance. Finally, a four way, double-blind, randomized, placebo-controlled crossover study was conducted to assess FBF response to histamine in the presence of H1 - and H2 -receptor antagonists. Flare and itch were assessed in all studies.

RESULTS

Histamine caused a dose-dependent increase in FBF, greatest with the highest dose (30 nmol min(-1) ) infused [mean (SEM) infused arm vs. control: 26.8 (5.3) vs. 2.6 ml min(-1)  100 ml(-1) ; P < 0.0001]. Dose-dependent flare and itch were demonstrated. Acute tolerance was not observed, with an increased FBF persisting throughout the infusion period. H2 -receptor antagonism significantly reduced FBF (mean (95% CI) difference from placebo at 30 nmol min(-1) histamine: -11.9 ml min(-1)  100 ml(-1) (-4.0, -19.8), P < 0.0001) and flare (mean (95% CI) difference from placebo: -403.7 cm(2) (-231.4, 576.0), P < 0.0001). No reduction in FBF or flare was observed in response to the H1 -receptor antagonist. Itch was unaffected by the treatments. Histamine did not stimulate vascular release of tissue plasminogen activator or von Willebrand factor.

CONCLUSION

Histamine causes dose-dependent vasodilatation, flare and itch in the human forearm. H2 -receptors are important in this process. Our results support further exploration of combined H1 - and H2 -receptor antagonist therapy in acute allergic syndromes.

Adrenergic stimulation-released histamine taken-up in adrenergic nerves induces endothelium-dependent vasodilation in rat mesenteric resistance arteries

The present study investigated whether histamine was taken up by perivascular adrenergic nerves and released by periarterial nerve stimulation (PNS) to induce vascular responses. In rat mesenteric vascular beds treated with capsaicin to eliminate calcitonin gene-related peptide (CGRP)ergic vasodilation and with active tone, PNS (1 - 4 Hz) induced only adrenergic nerve-mediated vasoconstriction. Histamine treatment for 20 min induced PNS-induced vasoconstriction followed by vasodilation without affecting CGRP-induced vasodilation. Chlorpheniramine, guanethidine, combination of histamine and desipramine, and endothelium-removal abolished PNS-induced vasodilation in histamine-treated preparations. These results suggest that histamine taken up by and released from adrenergic nerves by PNS causes endothelium-dependent vasodilation in rat mesenteric arteries.

Topical application of betahistine improves eustachian tube function in an animal model

CONCLUSION

Betahistine dihydrochloride, a drug used widely in the systemic treatment of balance disorders such as Ménière's disease, was found to improve eustachian tube function when applied topically in the nasopharynx of rats.

OBJECTIVES

The study tested the effect of betahistine, a histamine receptor agonist, on eustachian tube function and tested the involvement of H1 and H3 histamine receptors.

METHODS

Eustachian tube function was measured in anaesthetized rats while middle ear pressure was increased and then monitored during induced swallowing. Betahistine and other drugs were applied topically in the nasopharynx, bulla and epipharynx, and administered intraperitoneally.

RESULTS

Systemic application of betahistine hardly changed eustachian tube function, but topical application significantly improved it. The action of topical betahistine was unaffected by the HI receptor antagonist mepyramine and was mimicked by the H3 agonist, ciproxifan.