Functional depression of H2 histamine receptors in sheep with experimental allergic asthma

Histamine: metabolism, physiology, and pathophysiology with applications in veterinary medicine

OBJECTIVE

To review the human and veterinary literature on histamine physiology and pathophysiology and potential applications for clinical use in veterinary critical care.

DATA SOURCES

Human and veterinary clinical studies, reviews, texts, and recent research in histamine receptor and antagonist therapy.

HUMAN DATA SYNTHESIS

Recent progress in molecular biology has led to a more complete understanding of the enzymes involved in histamine metabolism and histamine receptor physiology. The past decade of research has confirmed the role of histamine in the classical functions (contraction of smooth muscle, increase in vascular permeability, and stimulation of gastric acid secretion) and has also elucidated newer ones that are now under investigation. Data on the roles of histamine in angiogenesis, circadian rhythm, bone marrow regeneration, bacterial eradication, and cancer are emerging in the literature. Newer histamine antagonists are currently in drug trials and are expected to advance the clinical field in treatment of allergic, gastrointestinal, and cognitive disorders.

VETERINARY DATA SYNTHESIS

Veterinary histamine research is directed at identifying the effects of certain pharmacological agents on blood histamine concentrations and establishing the relevance in clinical disease states. Research demonstrates important species differences in regards to histamine receptor physiology and tissue response. Studies in the area of trauma, sepsis, anaphylaxis, allergy, and gastrointestinal disorders have direct applications to clinical veterinary medicine.

CONCLUSIONS

Histamine plays a key role in the morbidity and mortality associated with allergy, asthma, gastric ulcers, anaphylaxis, sepsis, hemorrhagic shock, anesthesia, surgery, cardiovascular disease, cancer, CNS disorders, and immune-mediated disease. Histamine antagonism has been in common use to block its adverse effects. With recent advances in the understanding of histamine receptor physiology, pharmaceutical agents targeting these receptors have increased the therapeutic options.

Increased airway reactivity in human RSV bronchiolitis in the guinea pig is not due to increased wall thickness

Bronchiolitis due to the respiratory syncytial virus (RSV) is the most common cause of lower respiratory tract infection in the first year of life. It has been suggested that RSV infection may cause subsequent asthma, but a mechanism for this relationship has not been demonstrated. Studies examining the presence of airway reactivity in infants with RSV bronchiolitis are limited by our inability to administer provocative agents such as histamine to such ill infants. This makes a small animal model of this condition a useful tool in which to investigate the pathophysiology of RSV bronchiolitis. We, therefore, evaluated airway responsiveness in vivo and airway morphometric changes in 20 guinea pigs infected by instilling 4,000 plaque-forming units of human RSV virus onto the nasal mucosa under halothane anaesthesia, while 20 control animals received an equivalent volume of sterile cell culture medium. Six days following instillation, 10 infected animals and 10 controls underwent measurements of pulmonary resistance (RL) following increasing doses of inhaled acetylcholine (Ach). These guinea pigs were then sacrificed and the lung and heart removed en bloc for morphometric studies. There were no differences in baseline RL between infected and control groups. At Ach concentrations of 15 and 50 mg/mL, RSV-infected animals had higher RL values than controls (P < 0.05). Fourteen days following RSV instillation no differences in Ach responses were detected in the 10 infected and 10 control animals studied. To determine whether the increase in airway reactivity 6 days after RSV instillation was associated with changes in airway wall morphometry, 125 airways (69 infected, 56 control) were studied. Analysis of wall area, wall area internal to the smooth muscle, or smooth muscle area standardized by the internal perimeter of the airway showed no significant differences between the infected and control airways. These results demonstrated that airway hyperresponsiveness correlated with previously reported histologic changes of acute bronchiolitis 6 days after guinea pigs were infected with human RSV, but neither hyperresponsiveness nor histological changes persisted following resolution of the primary infection. The increased airway reactivity and the previously observed histological changes seen at day 6 following infection was not due to increased airway wall thickness.

Histamine receptors in the lung

Histamine, which has long been implicated as a mediator of allergic airway disease, has multiple effects on airways which are mediated by at least three histamine receptors. H1-receptors mediate bronchoconstriction, vasoconstriction and dilatation, microvascular leak and activation of sensory nerves. H2-receptors mediate vasodilatation in some species and mucus secretion. H3-receptors mediate vasodilatation in some species and mucus secretion. H3-receptors modulate cholinergic neurotransmission, the release of neuropeptides from sensory nerves and allergen-induced bronchoconstriction.

The pharmacology of airway hyperresponsiveness and inflammation

Hyperresponsiveness is a key abnormality in asthma and deserves recognition both in the clinical presentation of asthma and its pathogenesis. Available evidence strongly suggests that inflammatory processes are important in producing this aspect of disordered airway physiology. Pharmacologic studies have greatly increased our knowledge of mechanisms involved in hyperresponsiveness but fall short of a complete explanation of the phenomenon. It is most likely that hyperresponsiveness, as measured in the laboratory, has many different components and that any single mechanism will be an insufficient explanation.

Suppression of gastric H2-receptor mediated function in patients with bronchial asthma and ragweed allergy

We have previously demonstrated a depression of airway, vascular, and cutaneous H2-histamine receptor function in sheep with experimental allergic asthma. In the present investigation, we wished to determine if there is a depression of gastric H2-receptor function in subjects with allergic bronchial asthma. In eight normal subjects and seven subjects with allergic bronchial asthma and bronchial reactivity to ragweed antigen, gastric H2-receptor function was assessed by measuring basal and maximal stimulated acid output following pretreatment with a placebo or the H2-antagonist, cimetidine. Maximal stimulated acid output was defined as the peak acid output (PAW mEq/hr) of hydrochloric acid following a subcutaneous injection of histalog (1.5 mg/kg), and selective H2-stimulation as delta PAO = PAOplacebo-PAOcimetidine. While basal acid output was not different between the two groups, mean (+/- SD) PAO was significantly lower in the asthmatic group (14.0 +/- 8.2 mEq/hr) than the normal group (27.9 +/- 9.4 mEq/hr) (p less than 0.01). Mean PAO expressed as percent of predicted maximum was 112 +/- 36 percent in the normal group and 61 +/- 34 percent in the asthmatic group (p less than 0.01). Mean delta PAO was significantly higher in the normal group (17.1 +/- 4.8 mEq/hr) than in the asthmatic group (7.0 +/- 5.3 mEq/hr) (p less than 0.005) indicating suppressed selective H2-receptor stimulation in the latter. We conclude that in subjects with bronchial asthma and marked bronchial hyperreactivity to ragweed antigen, there is a depression of gastric H2-histamine receptor function.