Name: Norah Verbout

Physiology and Pharmacology

Norah G. Verbout, Jesse K. Lorton, David B. Jacoby, Allison D. Fryer

Atropine Pretreatment Enhances Airway Hyperreactivity in Antigen Challenged Guinea Pigs

Abstract: Airway hyperreactivity in antigen challenged guinea pigs is mediated

by eosinophil major basic protein, which blocks inhibitory M2

muscarinic receptors on parasympathetic nerves, increasing ACh release

and bronchoconstriction. Anticholinergics are effective

bronchodilators during asthma attacks, but are not when administered

prophylactically. Here we tested whether the nonselective muscarinic

receptor antagonist atropine affects eosinophil activation and airway

hyperreactivity if administered prophylactically, before antigen

challenge.

Guinea pigs were sensitized to ovalbumin, and 21d later

challenged with inhaled ovalbumin. Animals received either atropine

(1mg/kg, ip) or saline 1h before and 6h after antigen challenge.

Airway reactivity to electrical stimulation of the vagi was measured

in anesthetized animals 24h after challenge, by which time atropine

had worn off. Vagally induced bronchoconstriction was significantly

increased by challenge. Paradoxically, atropine pretreatment

significantly potentiated vagally induced bronchoconstriction in

antigen challenged animals but had no effect in controls.

Bronchoconstriction induced by iv ACh was not changed by antigen

challenge or by atropine pretreatment.

Eosinophil number in airways

was significantly increased by antigen challenge. In contrast, there

were fewer identifiable eosinophils in lungs of challenged animals

pretreated with atropine. Deposition of eosinophil major basic protein

in lungs (assessed blindly) was increased by challenge and was

significantly potentiated by atropine pretreatment, suggesting that

atropine increased eosinophil activation and degranulation. Depleting

eosinophils (240µg Ab IL-5/kg, ip) before sensitization prevented

hyperreactivity in atropine pretreated antigen challenged animals,

supporting a role of eosinophils in atropine induced potentiation of

airway hyperreactivity.

Atropine induced potentiation of airway

hyperreactivity suggests there may be other muscarinic receptors

contributing to airway pathology in antigen challenged animals. We

showed that M3 and M4, but not M1, M2 or M5, muscarinic receptors are

expressed by eosinophils, isolated from guinea pig peritoneum, as

assessed by RT-PCR and immunocytochemistry.  These data demonstrate

that muscarinic receptor blockade before antigen challenge potentiates

vagally induced airway hyperreactivity, an effect that is eosinophil

mediated, possibly by muscarinic receptors that are on eosinophils

themselves.