Muscarinic inhibition of endogenous noradrenaline release from rabbit isolated trachea: receptor subtype and receptor reserve.

The aim of the present study was to characterize putative muscarine receptors on sympathetic nerve terminals in the rabbit trachea. Release of endogenous noradrenaline from in vitro incubated rabbit trachea was evoked by electrical field stimulation (3 Hz, 540 pulses) and quantified by high performance liquid chromatography with electrochemical detection. The muscarine receptor agonist oxotremorine inhibited the evoked release of noradrenaline completely at 1 mumol/l (EC50: 64 nmol/l). The concentration response curve was very steep (Hill coefficient of 2.3). Scopolamine shifted the concentration response curve of oxotremorine to the right (-log KB 8.48) demonstrating specific, inhibitory muscarine receptors. Several subtype-preferring muscarine receptor antagonists also shifted the concentration response curve of oxotremorine to the right. The rank order of potency was (-log KB or pA2*): scopolamine (8.48) > AF-DX 384 (7.88*; slope of Schild plot 1.1) > (R)-trihexyphenidyl (7.87) > 4-DAMP (7.85) > AQ-RA 741 (7.77) >> methoctramine 6.18) > pirenzepine (6.0) > p-fluoro-hexahydrosiladifenidol (p-FHHSiD, 5.68). When these affinity constants were plotted against reported -log Ki values determined in binding studies on human cloned muscarine receptor subtypes (m1-m5), the best correlation was obtained for m2. Indomethacin (3 mumol/l), which on its own increased the evoked noradrenaline release by about 45%, affected neither the inhibitory effect of oxotremorine nor the antagonistic potency of methoctramine or p-FHHSiD.(ABSTRACT TRUNCATED AT 250 WORDS)