| pA2
online © Copyright 2003 The British Pharmacological Society |
011P
University of Surrey Summer Meeting June 2003 |
Non-neuronal
acetylcholine mediates the contractile response of the mouse isolated
trachea to 5-Hydroxytryptamine (5-HT)
|
 Print abstract Search PubMed for: |
The contractile effect of
5-HT on the mouse isolated trachea is assumed to be due to the activation
of parasympathetic cholinergic nerves (Eum et al., 1999). In the
present study the 5-HT receptor subtype that mediates this effect and
the involvement of neuronal acetylcholine was assessed.
Female BALB/c mice, 6-8 weeks, were killed with an overdose of urethane.
Each trachea was then cut in half to generate two separate ring preparations
which where mounted in tissue baths. Indomethacin (3 µM) was present
in all experiments to prevent the actions of endogenous protanoids that
are generated by airway preparations in vitro. All values are expressed
as the mean ± the standard error of the mean.
Atropine (1-10 nM) caused a rightward shift of the methacholine concentration-effect
curves with a pA2
of 8.9. Over the same concentration range atropine collapsed the 5-HT
concentration-effect curve, suggesting that 5-HT acts as an indirect agonist
(Black et al., 1985). The rank order of potency of 5-HT receptor
agonists (5-HT=
-methyl 5-HT>5-carboxamidotryptamine)
suggests that the receptor resembles the murine 5-HT2A
receptor (McKune & Watts, 2001). Furthermore, the 5-HT concentration-effect
curve was right-shifted by ketanserin (0.3-3.0 nM) with a pA2
of 9.4.While cholinergic twitch responses to electrical field stimulation
were abolished by combined blockade of N-, P- and Q-type voltage-operated
calcium channels, responses to 5-HT were unaffected. Responses to 5-HT
were also unaffected by the fast sodium channel blocker tetrodotoxin at
a concentration (300 nM) that abolished cholinergic twitch responses.
Conversely, the inositol triphosphate receptor antagonist 2-aminoethoxy-diphenylborate
(50 µM) reduced the maximal response to 5-HT (control 53.94 ±
7.78 % vs. 39.14 ± 4.95 %; P<0.05) without affecting responses
to either methacholine or electrical field stimulation. These findings
suggest that the source of acetylcholine is non-neuronal and prompted
a structural examination of other sources of acetylcholine in this preparation.
Choline acetyltransferase immunoreactivity was localised to ganglia and
nerve fibers as well as approximately half the epithelial cells in the
preparation. Enzymatic removal of the epithelial layer with bacterial
pronase completely prevented the contractile response to 5-HT.
These findings suggest that 5-HT, acting at 5-HT2A
receptors on epithelial cells, initiates the release of acetylcholine
and subsequent contraction of the mouse isolated trachea. This phenomenon
should be borne in mind in asthma studies since choline acetyltransferase
is present in human airway epithelium (Klapproth et al., 1997)
and 5-HT is known to mediate atropine-sensitive anaphylactic bronchoconstriction
in mice (Eum et al., 1999).
Black JW et al. (1985) Br. J. Pharmacol., 86,:589-599.
Eum SY et al. (1999) Br. J. Pharmacol., 126,312-316.
Klapproth H et al. (1997) Naunyn Schmiedebergs Arch. Pharmacol.,
355, 515-523.
McKune CM & Watts SW. (2001) J. Pharmacol. Exp. Ther., 297,
88-95.