MUSCARINIC HYPERRESPONSIVENESS OF ANTIGEN-SENSITIZED FELINE AIRWAY SMOOTH-MUSCLE IN-VITRO

Objective-To determine the effect of in vivo antigen sensitization (As caris suum) of cats on tracheal smooth muscle (TSM) and bronchial smoo th muscle (BSM) muscarinic reactivity in vitro. Animals-Healthy domest ic shorthair cats of either sex. Procedure-Cats were sensitized and we re long-term antigen (or sham) challenge exposed for 6 weeks by aeroso lization with soluble Ascaris suum. Tracheal and BSM preparations were obtained and stimulated in vitro by electrical field stimulation (EFS ), acetylcholine (ACh, a muscarinic agonist), and physostigmine [an AC hase inhibitor). Responses were compared with responses of comparable tissues from sham antigen challenge-exposed cats. Results-Tracheal and BSM from sensitized, compared with sham-sensitized (control), cats ha d greater isometric contraction (expressed as percentage of the respon se observed for isotonic, 63 mM KCl-elicited contraction [% KCl]) in r esponse to endogenous (EFS) and exogenous muscarinic receptor activati on (ACh). Contractions in response to EFS by TSM from control cats wer e 74 % KCl vs 97 %KCI for antigen-sensitized TSM (P < 0.04). Muscarini c responses were augmented comparably by in vivo sensitization; TSM fr om control cats contracted to 190 % KCl vs 230 % KCl (P < 0.03) for TS M from immune-sensitized cats. Physostigmine augmented responses of ai l tissues to ACh so that TSM from control (290 % KCl) and antigen-sens itized (257 % KCl) cats were similar. Responses of BSM from antigen-se nsitized cats had similar augmentation of contractile response to EFS and ACh. Conclusions-long-term in vivo antigen sensitization increases numbers of muscarinic receptors on airway smooth muscle or decreases the availability or activity of AChase in cats. Clinical Relevance-Mod ulation of muscarinic receptors may be useful for treatment of asthmat ic cats with in vivo airway hyperreactivity.