Role of H1 and H2 receptors in increased small airways resistance in the dog

Time course of histamine-induced bronchoconstriction and its adrenergic and H2 modulation

We characterized the complete time course of histamine-induced bronchoconstriction and its modulation via the release of endogenous catecholamines and by its actions on H2-receptors in anesthetized, tracheostomized, paralyzed, and artificially ventilated mongrel dogs. Respiratory resistance (R) and elastance (E) were estimated continuously with a recursive least squares estimator. Three protocols were followed in which multiple histamine bolus injections were given 1 h apart. We found that the time courses of E and R had consistent patterns (transient peak that returned to baseline within 1000 sec) even in cases of low mean arterial pressure (MAP). Indomethacin pre-treatments prevented tachyphylaxis to repeated i.v. challenges. beta-blockade produced a mild increase in baseline and a potentiation of the histamine-induced response in E and these effects were not altered with further alpha-or H2-blockade. Blockade of alpha-receptors increased the time to recovery in both E and MAP presumably by decreasing blood flow. Finally, we suggest that preventing the H2-receptor induced increase in bronchial blood flow may have increased the time to maximal E without affecting the recovery time.

Response localization of the pharmacological agents histamine and salbutamol along the respiratory system by forced oscillations in asthmatic subjects

The bronchodilating effect of 1 mg and 0.4 mg salbutamol on the impedance of the respiratory system was studied in 25 asthmatic subjects after histamine-induced bronchoconstriction. Histamine caused an increase of respiratory resistance (Rrs) at lower frequencies and a frequency dependence of Rrs. Respiratory reactance (Xrs) decreased at all frequencies after histamine challenge. These changes can be explained by peripheral airway obstruction. Impedance measurements performed 5 min after inhalation of 1 mg and 0.4 mg salbutamol showed a decrease of Rrs values at lower frequencies, a disappearance of the frequency dependence of Rrs, and a significant increase of Xrs values. No significant differences in absolute changes of Rrs and Xrs are observed between the salbutamol regimens. These changes after inhalation of salbutamol can be explained by supposing a predominant action on the peripheral airways.

Investigation of the effects of histamine inhalation on the tracheobronchial tree of calves by the forced oscillation technique

Effects of histamine inhalation were investigated with two different techniques in nine conscious, healthy calves. The oesophageal balloon technique was used to measure the dynamic respiratory compliance (Cdyn) and the pulmonary resistance (RL). The reactance (Xrs) and the resistance (Rrs) of the respiratory system were measured at high frequencies by the forced oscillation technique. These parameters were recorded before and after histamine inhalation. Three histamine dihydrochloride solution concentrations were used (16 mg/ml; 32 mg/ml; 64 mg/ml). Histamine inhalation induced a decrease in Cdyn and Xrs an increase in Rrs, RL and of the resonant frequency and a negative frequency dependence of Rrs. Some of these changes were satisfactorily correlated with the histamine solution concentrations. The resonant frequency was well correlated with Cdyn. It was concluded that histamine inhalation induces a decrease of the calibre of small and large airways and a non-homogeneous behaviour of the pulmonary ventilation in awake calves. The forced oscillation technique can be used to perform an inhalation provocation test in unsedated animals.

Endogenous regulation of bronchomotor tone

Airway smooth muscle responses are elicited in a complex manner through a large variety of endogenous mediators. Mediators augment or inhibit bronchomotor tone at a variety of sites through numerous different mechanisms. Interactions occur among mediators and nerves, muscle, and circulating blood elements or respiratory mast cells. Airway smooth muscle tone further is regulated by postsynaptic mediator-mediator interactions. Substances may circulate through the blood from distal sites to reach their target organ, as with epinephrine in its effects on airway smooth muscle, or may be secreted directly onto airway smooth muscle, as with the secretory products of respiratory mast cells. Recent observations have indicated that some mediators elicit airway contraction at least in part by activating efferent parasympathetic nerves and/or platelets. Direct secretion of minute quantities of mediators from adjacent epithelium or from infiltrating leukocytes may be an essential component of airway hyperreactivity. Complex interactions between the complement and kallikrein cascades have been cited as possible mechanisms of airway hyperresponsiveness. Ultimately, bronchomotor tone is mediated postsynaptically by the availability of calcium to the contractile apparatus of the smooth muscle cell. A role for the phosphoinositide system in membrane transduction and for cyclic adenosine monophosphate in regulating calcium distribution for smooth muscle contraction has been implicated. Mediator-mediator interactions distal to the synaptic cleft have been shown to augment both force and duration of airway smooth muscle contraction in a synergistic fashion. The stimuli eliciting mediator and neurotransmitter secretion, their physiologic significance, and the homeostatic infrastructure of these interactions are areas of promising investigation that require further definition.

Bronchodilatation: noncholinergic, nonadrenergic mediation demonstrated in vivo in the cat

The composite vagus nerve was stimulated during intravenous infusion of 5-hydroxytryptamine in cats subjected to pharmacologic autonomic blockade with atropine, propranolol, and phentolamine. Bronchial caliber, as assessed by changes in pulmonary resistance, demonstrated a marked dilatation, and dilatation could still be demonstrated after preliminary treatment with reserpine. By stimulating the component branches of the vagus nerve, it was determined that the parasympathetic branch is responsible for this phenomenon.