Evans, Christopher M.

M2muscarinic receptors limit acetylcholine release from the pulmonary parasympathetic nerves. M2receptors are dysfunctional in antigen-challenged guinea pigs, causing increased vagally mediated bronchoconstriction. Dysfunction of these M2receptors is due to eosinophil major basic protein, which is an antagonist for M2receptors. Histamine-induced bronchoconstriction is composed of a vagal reflex in addition to its direct effect on airway smooth muscle. Because hyperreactivity to histamine is seen in antigen-challenged animals, we hypothesized that hyperreactivity to histamine may be due to increased vagally mediated bronchoconstriction caused by dysfunction of M2receptors. In anesthetized, antigen-challenged guinea pigs, histamine-induced bronchoconstriction was greater than that in control guinea pigs. After vagotomy or atropine treatment, the response to histamine in antigen-challenged animals was the same as that in control animals. In antigen-challenged animals, blockade of eosinophil influx into the airways or neutralization of eosinophil major basic protein prevented the development of hyperreactivity to histamine. Thus hyperreactivity to histamine in antigen-challenged guinea pigs is vagally mediated and dependent on eosinophil major basic protein.

Neuronal muscarinic (M2) receptors inhibit release of acetylcholine from the vagus nerves. Hyperreactivity in antigen-challenged guinea pigs is due to blockade of these M2autoreceptors by eosinophil major basic protein (MBP) increasing the release of acetylcholine. In vivo, substance P-induced hyperactivity is vagally mediated. Because substance P induces eosinophil degranulation, we tested whether substance P-induced hyperreactivity is mediated by release of MBP and neuronal M2receptor dysfunction. Pathogen-free guinea pigs were anesthetized and ventilated. Thirty minutes after intravenous administration of [Sar9,Met(O2)11]- substance P, guinea pigs were hyperreactive to vagal stimulation and M2receptors were dysfunctional. The depletion of inflammatory cells with cyclophosphamide or the administration of an MBP antibody or a neurokinin-1 (NK1) receptor antagonist (SR-140333) all prevented substance P-induced M2dysfunction and hyperreactivity. Intravenous heparin acutely reversed M2receptor dysfunction and hyperreactivity. Thus substance P releases MBP from eosinophils resident in the lungs by stimulating NK1receptors. Substance P-induced hyperreactivity is mediated by blockade of inhibitory neuronal M2receptors by MBP, resulting in increased release of acetylcholine.