Effects of indomethacin on muscarinic inhibition of endogenous noradrenaline release from rat isolated trachea

The release of endogenous noradrenaline from rat isolated tracheae was evoked by electrical field stimulation (3 Hz, 540 pulses) in the presence of yohimbine, desipramine and tyrosine. The muscarine receptor agonist oxotremorine concentration-dependently inhibited the evoked release of noradrenaline by 95% at 1 μmol/l, EC50 values in two series of experiments 41 and 57 nmol/l, respectively. The effect of oxotremorine was antagonized by the non-selective muscarine receptor antagonist scopolamine (10–1000 nmol/l) in a manner suggesting a simple competitive interaction (slope of Schild plot −0.94; pA2 value 8.88). However, the M2 selective muscarine receptor antagonist methoctramine (0.1–10 μmol/l) affected the action of oxotremorine in a manner suggesting a complex interaction (slope of Schild plot −0.47). Addition of indomethacin (3 μmol/l) caused an increase of the evoked release of noradrenaline by 45% and low concentrations of oxotremorine (0.01 and 0.1 μmol/l, but not 1 μmol/l) became less effective resulting in a slight shift to the right of the concentration response curve (EC50 169 nmol/l). Moreover, in the presence of indomethacin methoctramine (0.1–10 μmol/l) antagonized the effects of oxotremorine in a manner suggesting a simple competitive interaction (slope of Schild plot −0.93, pA2 value 7.61). In the presence of indomethacin, the concentration response curve of oxotremorine was only slightly shifted to the right in the presence of the M1 receptor selective antagonist pirenzepine (1 μmol/l, −log KB 6.1) and not significantly affected by the M3 receptor selective antagonist pfluoro-hexahydrosiladifenidol (1 μmol/l). In conclusion, the release of noradrenaline in the rat trachea is inhibited via presynaptic muscarine heteroreceptors of the M2 subtype. In addition, activation of muscarine receptors affects the release of noradrenaline also indirectly, probably via the release of inhibitory prostanoids, and this involves muscarine receptors which are pharmacologically different from those at the sympathetic nerve terminals.