Effects of histamine H1- and H2-receptor antagonists on cardiovascular function during systemic anaphylaxis in guinea pigs

The Roles of Cardiovascular H2-Histamine Receptors Under Normal and Pathophysiological Conditions

This review addresses pharmacological, structural and functional relationships among H₂-histamine receptors and H₁-histamine receptors in the mammalian heart. The role of both receptors in the regulation of force and rhythm, including their electrophysiological effects on the mammalian heart, will then be discussed in context. The potential clinical role of cardiac H₂-histamine-receptors in cardiac diseases will be examined. The use of H₂-histamine receptor agonists to acutely increase the force of contraction will be discussed. Special attention will be paid to the potential role of cardiac H₂-histamine receptors in the genesis of cardiac arrhythmias. Moreover, novel findings on the putative role of H₂-histamine receptor antagonists in treating chronic heart failure in animal models and patients will be reviewed. Some limitations in our biochemical understanding of the cardiac role of H₂-histamine receptors will be discussed. Recommendations for further basic and translational research on cardiac H₂-histamine receptors will be offered. We will speculate whether new knowledge might lead to novel roles of H₂-histamine receptors in cardiac disease and whether cardiomyocyte specific H₂-histamine receptor agonists and antagonists should be developed.

Mouse anaphylactic shock is caused by reduced cardiac output, but not by systemic vasodilatation or pulmonary vasoconstriction, via PAF and histamine

AIMS

Systemic anaphylaxis is life-threatening, and its pathophysiology is not fully clarified. Mice are frequently used for experimental study on anaphylaxis. However, the hemodynamic features and mechanisms of mouse anaphylactic hypotension remain unknown. Therefore, we determined mechanisms of systemic and pulmonary vascular response to anaphylactic hypotension in anesthetized BALB/c mice by using receptor antagonists of chemical mediators.

MAIN METHODS

Anaphylaxis was actively induced by an intravenous injection of the ovalbumin antigen into open-chest artificially ventilated sensitized mice. Mean arterial pressure (MAP), pulmonary arterial pressure (PAP), left atrial pressure, central venous pressure, and aortic blood flow (ABF) were continuously measured.

KEY FINDINGS

In sensitized control mice, MAP and ABF showed initial, transient increases, followed by progressive decreases after the antigen injection. Total peripheral resistance (TPR) did not decrease, while PAP initially and transiently increased to 18.5±0.5mmHg and pulmonary vascular resistance (PVR) also significantly increased. The antigen-induced decreases in MAP and ABF were attenuated by pretreatment with either a platelet-activating factor (PAF) receptor antagonist, CV6209, or a histamine H1 receptor antagonist, diphenhydramine, and were abolished by their combination. Diphenhydramine augmented the initial increases in PAP and PVR, but did not affect the decrease of the corresponding MAP fall. The antagonists of either leukotriene C4 or serotonin, alone or in combination with CV6209, exerted no significant effects.

SIGNIFICANCE

Mouse anaphylactic hypotension is caused by a decrease in cardiac output but not vasodilatation, via actions of PAF and histamine. The slight increase in PAP is not involved in mouse anaphylactic hypotension.

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.

Anaphylaxis in dogs and cats

OBJECTIVE

To review and summarize current information regarding the pathophysiology and clinical manifestations associated with anaphylaxis in dogs and cats. The etiology, diagnosis, treatment, and prognosis is discussed.

ETIOLOGY

Anaphylaxis is a systemic, type I hypersensitivity reaction that often has fatal consequences. Many of the principal clinical manifestations involve organs where mast cell concentrations are highest: the skin, the lungs, and the gastrointestinal tract. Histamine and other deleterious inflammatory mediators promote vascular permeability and smooth muscle contraction; they are readily released from sensitized mast cells and basophils challenged with antigen. Anaphylaxis may be triggered by a variety of antigens including insect and reptile venom, a variety of drugs, vaccines, and food.

DIAGNOSIS

Anaphylaxis is a clinical diagnosis made from a collection of signs and symptoms. It is most commonly based on pattern recognition. Differential diagnoses include severe asthma, pheocromocytoma, and mastocytosis.

THERAPY

Epinephrine is considered the drug of choice for the treatment of anaphylaxis. It acts primarily as a vasopressor in improving hemodynamic recovery. Adjunctive treatments include fluid therapy, H1 and H2 antihistamines, corticosteroids, and bronchodilators; however, these do not substitute for epinephrine.

PROGNOSIS

Prognosis depends on the severity of the clinical signs. The clinical signs will vary among species and route of exposure. The most severe clinical reactions are associated when the antigen is administered parenterally.

Food anaphylaxis

Anaphylaxis is a life-threatening allergic reaction, and food is one of the most common responsible allergens in the outpatient setting. The prevalence of food-induced anaphylaxis has been steadily rising. Education regarding food allergen avoidance is crucial as most of the fatal reactions occurred in those with known food allergies. The lack of a consensus definition for anaphylaxis has made its diagnosis difficult. Symptoms affect multiple organ systems and include pruritus, urticaria, angioedema, vomiting, diarrhoea, abdominal cramps, respiratory difficulty, wheezing, hypotension, and shock. Prompt recognition of anaphylaxis is essential as delayed treatment has been associated with fatalities. Although epinephrine is accepted as the treatment of choice, timely administration does not always occur, partly due to a lack of awareness of the diagnostic criteria. Several novel tools are currently being investigated, which will potentially aid in the diagnosis and treatment of food-induced anaphylaxis.

Cardiovascular aspects of anaphylaxis: implications for treatment and diagnosis

PURPOSE OF REVIEW

Anaphylactic cardiovascular collapse can be resistant to treatment with epinephrine (adrenaline) and, in some cases, diagnostic uncertainty compromises follow-up care. The purpose of this review is to examine recent studies relevant to the management and diagnosis of this condition.

RECENT FINDINGS

Nausea, vomiting, incontinence, diaphoresis, dyspnoea, hypoxia, dizziness and collapse are associated with hypotension. Relative bradycardia (falling heart rate despite hypotension) is a consistent feature of hypotensive insect sting anaphylaxis and may represent a non-specific physiological response to severe hypovolaemia in conscious individuals. Upright posture has been found to be associated with death from anaphylaxis. Animal studies have found the intramuscular route for epinephrine is ineffective, intravenous boluses temporarily effective, but intravenous infusions of epinephrine are able to reverse anaphylactic shock. In one animal model, antihistamines were found to be harmful. A prospective human study provides evidence for the efficacy of treatment with intravenous epinephrine infusion and fluid (volume) resuscitation. Case reports support the use of the vasoconstrictors metaraminol, methoxamine and vasopressin if adrenaline is ineffective. Repeated measurements of mast cell tryptase are more sensitive and specific than a single measurement for the diagnosis of anaphylaxis.

SUMMARY

Current evidence supports use of the supine/Trendelenburg position, epinephrine by intravenous infusion and aggressive volume resuscitation. If these fail, atropine should be considered for severe bradycardia and potent vasoconstrictors may be useful. To confirm the diagnosis of anaphylaxis, serial measurements of mast cell tryptase may be preferable to a single measurement.

Minor role of the C3a receptor in systemic anaphylaxis in the guinea pig

Previously, Regal et al. [Regal, J.F., Fraser, D.G., Toth, C.A., 1993. Role of the complement system in antigen-induced bronchoconstriction and changes in blood pressure in the guinea pig. J. Pharmacol. Exp. Ther. 267, 979-988] demonstrated that preventing complement system activation resulted in inhibition of anaphylaxis in the guinea pig, and that the C-terminal 21 amino acids of guinea pig C3a (C3a-peptide) mimic the symptoms of anaphylactic shock in the guinea pig [Regal, J.F., 1997. Role of the complement system in pulmonary disorders. Immunopharmacology 38, 17-25]. To determine if C3a is an essential mediator of systemic anaphylaxis, the anaphylactic response to ovalbumin (OA) was assessed in guinea pigs genetically deficient in the C3a receptor (C3aR-) compared to their control strain of animals which were C3a receptor positive (C3aR+). In addition, the response to another control strain of animals, Hartley guinea pigs, was determined. Sensitized guinea pigs were anesthetized, and bronchoconstriction and changes in blood pressure were monitored in response to intravenous (i.v.) injection of either C3a-peptide, recombinant human C5a (rHuC5a) or OA. Both Hartley guinea pigs and C3aR+ animals responded similarly to C3a-peptide and rHuC5a. C3aR- animals, however, were unresponsive to C3a-peptide and responded normally to rHuC5a, confirming their functional deficiency of the C3a receptor. In response to OA, C3aR+ animals and Hartley guinea pigs responded with a severe bronchoconstriction, an initial transient hypotension, followed by an increase in blood pressure and a delayed prolonged hypotensive response. In contrast, in C3aR- animals, the increased blood pressure response to OA was significantly prolonged, the delayed hypotensive response was blunted, and the bronchoconstriction was delayed compared to the C3aR+ animals. The difference in the anaphylactic response could not be explained by differing amounts of OA-specific IgG1 antibody or C3a generated during the anaphylactic response. Thus, these data suggest that C3a plays a minor role in the hypotension of systemic anaphylaxis and investigation of a role for other products of complement system activation, either alone or in combination with C3a, is clearly warranted.

Latex anaphylaxis causing heart block: role of ranitidine

PURPOSE

Treatment with H2 receptor antagonists may cause the heart to be more susceptible to atrioventricular conduction delay when exposed to an overwhelming insult by histamine released during an anaphylactic reaction. We present the case of a woman, pretreated with ranitidine, who developed 3:1 heart block secondary to latex anaphylaxis. We propose that H2 antagonist premedication alone in patients susceptible to anaphylaxis increases their risk of heart block.

CLINICAL FEATURES

A 38 yr old obese woman with cervical cancer presented for a radical hysterectomy. Systems review yielded a history of sleep apnea, orthopnea, gastroesophageal reflux, and sciatica. Medications included preoperative ranitidine, 150 mg. There was no history of atopy or allergy. Following general anesthesia induction, at the onset of the surgical procedure the patient developed a severe anaphylactic reaction which was heralded by the onset of 3:1 heart block, with decreases in SpO2, P(ET)CO2 and a decrease in systolic blood pressure to 45 mmHg. This was diagnosed as a possible latex reaction and treated using epinephrine boluses and infusion, fluids, 50 mg diphenhydramine, 50 mg ranitidine and 100 mg hydrocortisone. Following a 48 hr stay in the ICU the patient made an uneventful recovery. Allergy testing with intradermal latex injection and increased plasma tryptase levels confirmed a latex anaphylaxis.

CONCLUSION

The use of H2 antagonists alone as a prophylaxis for gastroesophageal reflux may increase the risk of heart block in patients who develop anaphylaxis.

Anaphylactic shock: mechanisms and treatment

This paper reviews the mechanisms of anaphylactic shock in terms of the immunoglobulin and non-immunoglobulin triggering events, and the cellular events based on the rise in intracellular cyclic AMP and calcium that release preformed granule-associated mediators and the rapidly formed, newly synthesized mediators predominantly based on arachidonic acid metabolism. These primary mediators recruit other cells with the release of secondary mediators that either potentiate or ultimately curtail the anaphylactic reaction. The roles of these mediators in the various causes of cardiovascular collapse are examined. The treatment of anaphylactic shock involves oxygen, adrenaline and fluids. The importance and safety of intravenous adrenaline are discussed. Combined H1 and H2 blocking antihistamines and steroids have a limited role. Glucagon and other adrenergic drugs are occasionally used, and several new experimental drugs are being developed.