Muscarinic control of airway function

Muscarinic M(1), M(2), and M(3) receptor subtypes have been shown to be involved in the pre- and postjunctional control of airway diameter of various species, including man. In a guinea pig model of allergic asthma, the prejunctional M(2) receptor was shown to become dysfunctional already during the early allergic reaction, thereby contributing to exaggerated vagal reflex activity and airway hyperreactivity. Moreover, a deficiency of endogenous nitric oxide was observed after allergen provocation, which may also contribute to an enhanced postjunctional M(3) receptor-mediated cholinergic response. Both in human and in animal airway preparations it was shown that enhanced cholinergic contractions are relatively resistent to beta-adrenoceptor-mediated relaxation. The reduced beta-adrenoceptor function may primarily be due to transductional cross-talk between PI metabolism and adenylyl cyclase, including protein kinase C-induced uncoupling of the beta-adrenoceptor from the effector system. Cross-talk between postjunctional M(2) receptor-mediated inhibition and beta-adrenoceptor-induced activation of adenylyl cyclase appears to be of minor functional importance, but could be enhanced in allergic asthma due to increased expression of the inhibitory G protein as induced by cytokines

Muscarinic M2 receptors do not participate in the functional antagonism between methacholine and isoprenaline in guinea pig tracheal smooth muscle

We investigated whether muscarinic M(2) receptors, known to inhibit adenylyl cyclase activity in airway smooth muscle, also inhibit isoprenaline-induced relaxation of guinea pig tracheal smooth muscle, as has recently been described for the dog (Fernandes et al., 1992, J. Pharmacol. Exp. Ther. 262, 119). Smooth muscle strips were contracted with various concentrations of methacholine or histamine (which served as a control) in the absence or presence of the M(2)-selective muscarinic receptor antagonist, gallamine (30 mu M), and cumulative isoprenaline-relaxation curves were obtained. It was found that muscarinic M(2) receptor blockade had no significant effect on isoprenaline pD(2) and E(max) values, neither with histamine nor with methacholine. The results show that, in guinea pig trachea, muscarinic M(2) receptors do not significantly influence the functional antagonism of cholinergic smooth muscle contraction by isoprenaline

Muscarinic inhibitory autoreceptors in different generations of human airways

The present study was undertaken to investigate the functional presence of inhibitory muscarinic M(2) autoreceptors on postganglionic cholinergic nerve endings in different generations of human airways. To this end, the effects of the M(2)-selective muscarinic receptor antagonists AQ-RA 741 and gallamine were studied on electrical field stimulation-induced twitch contractions of preparations from trachea and from bronchial airways of varying diameter. Furthermore, electrically evoked release of endogenous acetylcholine from human bronchial preparations, and the effect of the muscarinic receptor antagonist atropine thereon, was measured by high-performance liquid chromatography. On average, twitch contractions were significantly but only slightly (11 to 15%) potentiated by M(2)-selective concentrations of AQ-RA 741 and gallamine, despite approximately half of the preparations showing no potentiation at all. A subdivision into airway generations showed that M(2) autoreceptor function was not readily detectable in bronchioles and subsegmental bronchi. By contrast both with AQ-RA 741 and gallamine a clear potentiation (26 to 36%) of the twitch contractions was observed in approximately half of the terminal bronchi and in all central airway preparations. Moreover, the evoked release of endogenous acetylcholine in terminal and subsegmental bronchi was significantly facilitated by atropine, to 162 to 189% of controls. These results provide strong and partly direct evidence for the existence of inhibitory muscarinic M(2) receptors on postganglionic cholinergic nerve endings in human central airways and subsegmental and terminal bronchi, but not in bronchioli. It remains to be established, however, why these M(2) receptors exhibit a rather variable functionality in regulating cholinergic nerve-mediated contraction in different airway generations

Differences in the prejunctional effects of methacholine and pilocarpine on the release of endogenous acetylcholine from guinea-pig trachea

We investigated the effects of the full muscarinic acetylcholine receptor agonist methacholine and the partial and putatively M(2)-selective agonist pilocarpine on endogenous acetylcholine release from guinea-pig trachea by use of high-performance liquid chromatography with electrochemical detection. Atropine-induced increases in acetylcholine release were used to monitor the system. Electrical field stimulation (8 V, 30 Hz, 0.5 ms for 5 min)-induced acetylcholine release in the presence of neostigmine, with or without preincubation with choline to maximally enhance acetylcholine output, was increased to about 225% by 0.3 mu M atropine, indicating functional autoinhibition. However, methacholine (10 mu M) did not affect the acetylcholine release, whereas it was enhanced to 166% by 30 mu M pilocarpine. When electrical field stimulation was applied at lower intensity (8 V, 16 Hz, 0.1 ms for 5 min) and in the absence of neostigmine, an increase by 0.3 mu M atropine (to 177%) but a decrease of the acetylcholine release by 10 mu M methacholine (to 65%) and 30 mu M pilocarpine (to 63%) were observed. These results clearly demonstrate (i) that inhibition of evoked endogenous acetylcholine release from prejunctional nerve terminals in guinea-pig trachea can only be demonstrated under conditions of low junctional concentrations of acetylcholine, and (ii) that pilocarpine, as a partial muscarinic agonist, behaves as an antagonist under high junctional concentrations of the neurotransmitter