Functional characterisation of 5-HT receptors mediating constriction of murine intrapulmonary airways

Rainu Bawa, James Moffatt. Division of Biomedical Sciences, St George’s, University of London, SW17 0RE London, United Kingdom.

5-HT is often used as a bronchomotor challenge in studies of murine airway reactivity. However, the receptor mediating the response to 5-HT in the intrapulmonary airways of the mouse lung has not been characterised. In this study, an isolated, perfused and ventilated preparation of the mouse lung was used to characterise the response to vascular 5-HT and receptor-selective 5-HT analogues and to examine the effects of receptor-selective antagonists.

Female BALB/c mice (6-8 weeks old) were pretreated with heparin (5000 U.kg-1) before being killed by a barbiturate overdose (pentobarbitone, 2g.kg-1). The trachea was cannulated and the entire heart/lung block was excised from the animal. A cannula was inserted into the stem of the pulmonary artery, which was perfused with physiological saline containing 4% bovine serum albumin at a rate of 2 ml.min-1. The airways were ventilated at a rate of 100 breaths.min-1, with a tidal volume of 100 µL. All drugs were delivered to the lung by addition to the arterial perfusate. At the end of each experiment, 100 µM methacholine was administered, to establish the maximal bronchoconstrictor response.

Methacholine (100 µM) produced a 4.5 fold increase in insufflation pressure over baseline (n = 17). The maximal concentration of 5-HT (100 µM) produced a bronchoconstrictor response representing 51.5 ± 5.2% (n = 17) of the response to methacholine. The 5-HT1A and 5-HT2B/D agonists 8-OH-DPAT (100 µM) and sumatriptan (100 µM) elicited no response, while the non-selective 5-HT2 agonist α-methyl-5-HT produced a response of 38.0 ± 4.1% of the response to methacholine (not significantly different to 5-HT). The 5-HT2A selective antagonist ketanserin (1 uM) produced an approximate 100-fold shift of the 5-HT concentration-effect curve, less than might be expected.

These results suggest that 5-HT2A receptors mediate most of the bronchoconstrictor response to 5-HT in BALB/c mice. Further work is required to determine whether other receptors contribute to the ketanserin-resistant response to 5-HT in this preparation.