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

The airway cholinergic system: physiology and pharmacology

The present review summarizes the current knowledge of the cholinergic systems in the airways with special emphasis on the role of acetylcholine both as neurotransmitter in ganglia and postganglionic parasympathetic nerves and as non-neuronal paracrine mediator. The different cholinoceptors, various nicotinic and muscarinic receptors, as well as their signalling mechanisms are presented. The complex ganglionic and prejunctional mechanisms controlling the release of acetylcholine are explained, and it is discussed whether changes in transmitter release could be involved in airway dysfunctions. The effects of acetylcholine on different target cells, smooth muscles, nerves, surface epithelial and secretory cells as well as mast cells are described in detail, including the receptor subtypes involved in signal transmission.

Prejunctional muscarinic receptors regulating neurotransmitter release in airways

Prejunctional pA2 values of five muscarinic antagonists were determined in the guinea-pig trachea under stimulation conditions in which the antagonists alone did not enhance acetylcholine release. The antagonists were partly selective at M1 (pirenzepine), M2 (AQ-RA 741, himbacine) and M3 receptors (hexahydrosiladifenidol, dicyclomine). The profile of the antagonist affinities was different from that obtained at cardiac M2 receptors but resembled the profile reported in the literature for the cloned m4 receptor. This suggests that autoinhibition of acetylcholine release in the trachea is mediated via M4 receptors.

Prostanoid receptors of the EP3 subtype mediate inhibition of evoked [3H]acetylcholine release from isolated human bronchi

1. The release of neuronal [3H]acetylcholine (ACh) from isolated human bronchi after labelling with [3H]choline was measured to investigate the effects of prostanoids. 2. A first period of electrical field stimulation (S1) caused a [3H]ACh release of 320+/-70 and 200+/-40 Becquerel (Bq) g(-1) in epithelium-denuded and epithelium-containing bronchi respectively (P>0.05). Subsequent periods of electrical stimulation (Sn, n=2, 3, and 4) released less [3H]ACh, i.e. decreasing Sn/ S1 values were obtained (0.76+/-0.09, 0.68+/-0.07 and 0.40+/-0.04, respectively). 3. Cumulative concentrations (1-1000 nM) of EP-receptor agonists like prostaglandin E2, nocloprost, and sulprostone (EP1 and EP3 selective) inhibited evoked [3H]ACh release in a concentration dependent manner with IC50 values between 4- 14 nM and maximal inhibition of about 70%. 4. The inhibition of evoked [3H]ACh release by prostaglandin E2, nocloprost and sulprostone was not affected by the DP-, EP1- and EP2-receptor antagonist AH6809 at a concentration of 3 microM, i.e. a 3-30 times greater concentration than its affinity (pA2 values) at the respective receptors. 5. Circaprost (IP-receptor agonist; 1-100 nM), iloprost (IP- and EP1-receptor agonist; 10-1000 nM) and U-46619 (TP-receptor agonist; 100-1000 nM) did not significantly affect [3H]ACh release. 6. Blockade of cyclooxygenase by 3 microM indomethacin did not significantly modulate evoked [3H]ACh release in epithelium-containing and epithelium-denuded bronchi. Likewise, the combined cyclo- and lipoxygenase inhibitor BW-755C (20 microM) did not affect evoked [3H]ACh release. 7. In conclusion, applied prostanoids appear to inhibit [3H]ACh release in epithelium-denuded human bronchi under the present in vitro conditions, most likely via prejunctional prostanoid receptors of the EP3 subtype.

Modulation of noradrenaline release in rat isolated stomach by prostanoids, but not by histaminergic mechanisms

Several gastric functions are modulated by the sympathetic nervous system, but local mechanisms involved in the control of noradrenaline release are largely unknown. Overflow of endogenous noradrenaline was studied from isolated rat stomach incubated in Ussing chambers allowing the separate determination of mucosal and serosal overflow. Spontaneous noradrenaline overflow was similar at the mucosal and serosal side, but electrical field stimulation caused a frequency-dependent increase in noradrenaline overflow selectively at the serosal side. Evoked noradrenaline overflow was blocked by tetrodotoxin, not affected by indometacin and markedly enhanced (by about 250%) by yohimbine. In the presence of indometacin and yohimbine, sulprostone (an agonist at EP1/EP3 receptors) and misoprostol (an agonist at EP2/EP3 receptors) reduced the noradrenaline overflow evoked by stimulation at 3 Hz maximally by about 80% (EC50: 6 nmol/l and 11 nmol/l, respectively). The EP1 receptor selective antagonist AH 6809 (6-isopropoxy-9-oxoxanthene-2-carboxylic acid) did not antagonize the inhibition by sulprostone. Noradrenaline overflow evoked by stimulation at 1 Hz and 3 Hz was increased by scopolamine by about 50% and almost completely inhibited by oxotremorine. Neither, histamine nor the H3 receptor selective agonist (R)-alpha-methyl-histamine, nor the H1, H2 and H3 selective receptor antagonists mepyramine, cimetidine and thioperamide significantly affected noradrenaline overflow evoked by stimulation at 1 Hz or 3 Hz. In conclusion, impulse-induced noradrenaline release in the rat stomach is controlled by multiple presynaptic mechanisms involving alpha 2-adrenergic autoreceptors, EP3 prostanoid and muscarine heteroreceptors, whereas histaminergic mechanisms do not appear to be significant.

Mucosa-dependent muscarinic liberation of prostaglandins from rat isolated trachea

1. The present study examined whether cholinoceptor stimulation modulates the release of arachidonic acid-derived mediators from rat isolate tracheae. 2. Tracheae were preincubated with [3H]-arachidonic acid and the outflow of 3H-compounds was determined. Acetylcholine and the muscarinic agonist, carbachol but not nicotine, increased the rate of tritium outflow maximally by about 30%. The M3 receptor-preferring antagonist rho-fluoro-hexahydrosiladiphenidol was more effective than pirenzepine and methoctramine in antagonizing the effect of acetylcholine. 3. High performance liquid chromatography analysis (methanol gradient) of the released 3H-compounds showed that one peak, co-eluting with [14C]-prostaglandin E2([14C]-PGE2) and [3H]-PGD2, was enhanced almost 10 fold following muscarinic receptor activation, whereas the outflow of [3H]-arachidonic acid remained unaffected. 4. Using an acetonitril gradient separation it was shown that [3H]-PGE2, [3H]-PGD2 and [3H]-PGF2alpha are released spontaneously, but [3H]-PGE2 represented the major fraction of 3H-prostaglandins. Acetylcholine enhanced the release of all three 3H-prostaglandins, but the effect on PGE2 was most pronounced and most consistent. 5. After removal of the mucosa the muscarinic effect of acetylcholine on total tritium and on that of the 3H-prostaglandins ([3H]-PGE2/PGD2 peak) was abolished. 6. Acetylcholine also enhanced the outflow of radioimmunologically determined PGE2 in a mucosa-dependent manner. 7. After inhibition of cyclo-oxygenase by 3 microM indomethacin, the outflow of 3H-prostaglandins was reduced to almost undetectable levels and acetylcholine evoked a marked release [3H]-arachidonic acid. The phospholipase A2 inhibitor, quinacrine (up to 100 microM) also blocked the effect of acetylcholine on the outflow of 3H-prostaglandins, but this was not followed by a compensatory increase in the outflow of [3H]-arachidonic acid. 8. In conclusion, activation of muscarinic receptors which have characteristics of the M3 subtype can evoke release of prostaglandins from the airway mucosa.