Atypical presynaptic alpha-adrenoceptor modulation of neurally-mediated cholinergic responses in guinea-pig tracheal smooth muscle

Moxonidine acting centrally inhibits airway reflex responses

We examined the role of I1-imidazoline (I1-IR) receptors in control of airway function, by testing the effects of systemic administration of the I1-IR agonist moxonidine on reflex responses of tracheal smooth muscle (TSM) tone to either lung deflation or mechanical stimulation of intrapulmonary rapidly adapting receptors. Experiments were performed in either alpha-chloralose anesthetized or decorticate, paralyzed, and mechanically ventilated beagle dogs. Moxonidine (10-100 micrograms/kg) administered via three different routes (femoral vein, muscular branch of superior thyroid artery, and vertebral artery) attenuated TSM responses to stimulation of airway sensory nerve fibers by two different ways and caused a decrease in arterial pressure and heart rate. These effects were dose dependent and were significantly reversed by efaroxan (an I1-IR and alpha 2-adrenergic blocker) administered via the vertebral artery. Intravertebral efaroxan abolished the hemodynamic effects of moxonidine. Intravenous moxonidine (10-100 micrograms/kg) did not alter airway smooth muscle responses to electrical stimulation of the peripheral vagus nerve. In addition, in vitro moxonidine (1-100 micrograms/ml) had no effect on contractile responses to increasing doses of acetylcholine. These findings indicate that moxonidine may act at a central site to suppress reflex airway constriction, even when given into the systemic circulation. Given the presence of I1-IR sites and alpha 2-adrenergic receptors in brain regions participating in airway reflexes, these receptor classes may be involved in brainstem control of the cholinergic outflow to the airways.

I1-imidazoline receptors and cholinergic outflow to the airways

We examined the role of I1-imidazoline receptors in the control of airway function, by testing the effects of systemic administration of the I1-imidazoline agonist moxonidine on reflex responses of tracheal smooth muscle (TSM) tone to either lung deflation or mechanical stimulation of intrapulmonary rapidly adapting receptors. Experiments were performed in either alpha-chloralose anaesthetized or decorticate, paralyzed and mechanically ventilated beagle dogs. Moxonidine (10-100 microg/kg) administered via three different routes (the femoral vein, muscular branch of superior thyroid artery, and vertebral artery) attenuated TSM responses to stimulation of airway sensory nerve fibers by two different ways, and caused a decrease in arterial pressure and heart rate. These effects were dose-dependent, and were significantly reversed by efaroxan (an I1-imidazoline and alpha2-adrenergic blocker) administered via the vertebral artery. Intravertebral efaroxan abolished the hemodynamic effects of moxonidine. Intravenous moxonidine (10-100 microg/kg) did not alter airway smooth muscle responses to electrical stimulation of the peripheral vagus nerve. In addition, in vitro moxonidine (1-100 microg/ml) had no effect on contractile responses to increasing doses of acetylcholine. These findings indicate that moxonidine may act at a central site to suppress reflex airway constriction, even when given into the systemic circulation. Given the presence of I1-imidazoline sites and alpha2-adrenergic receptors in brain regions participating in airway reflexes, these receptor classes may be involved in brainstem control of the cholinergic outflow to the airways.

Interaction of the renin-angiotensin system, bradykinin and sympathetic nerves with cholinergic transmission in the rat isolated trachea

1. The present study was undertaken to investigate the interaction of the renin-angiotensin system (RAS), bradykinin and the sympathetic nervous system with cholinergic transmission in the rat airways. Experiments were performed on epithelium-intact and epithelium-denuded preparations of rat isolated trachea which had been incubated with [3H]-choline to incorporate [3H]-acetylcholine into the cholinergic transmitter stores. Tracheal preparations were subjected to electrical field stimulation (trains of 1 ms pulses, 5 Hz, 15 V) and the stimulation-induced (S-I) efflux taken as an index of transmitter acetylcholine release. 2. In both epithelium-intact and epithelium-denuded tracheal preparations, the alpha 2-adrenoceptor agonist UK14304 (0.1 and 1 microM) inhibited the S-I efflux, in a concentration-dependent manner. The inhibition of S-I efflux produced by UK14304 (1 microM) was antagonized by the selective alpha 2-adrenoceptor antagonist idazoxan (0.3 microM). Idazoxan (0.3 microM) alone had no effect on the S-I efflux. 3. Angiotensin II (0.1 and 1 microM) was without effect on the S-I efflux in either epithelium-intact or epithelium-denuded tracheal preparations. When angiotensin-converting enzyme was inhibited by perindoprilat (10 microM), angiotensin II (1 microM) was also without effect on the S-I efflux. Similarly, in the presence of idazoxan (0.3 microM), to block prejunctional alpha 2-adrenoceptors, angiotensin II (0.1 and 1 microM) did not alter the S-I efflux. When added alone, perindoprilat (10 microM) did not alter the S-I efflux. 4. In epithelium-denuded preparations, bradykinin (0.01-1 microM) inhibited the S-I efflux. In epithelium-intact preparations, there was also a tendency for bradykinin (0.1 and 1 microM) to inhibit the S-I efflux but this was not statistically significant. However, when angiotensin-converting enzyme and neutral endopeptidase were inhibited by perindoprilat (10 microM) and phosphoramidon (1 microM), respectively, bradykinin (1 microM) significantly inhibited the S-I efflux in epithelium-intact preparations as well as in epithelium-denuded preparations. The inhibition of the S-I efflux produced by bradykinin, in the combined presence of perindoprilat (10 microM) and phosphoramidon (1 microM), was unaffected by the additional presence of the cyclo-oxygenase inhibitor indomethacin (10 microM) and/or the nitric oxide synthase inhibitor NG-nitro-L-arginine (100 microM), in either epithelium-intact or epithelium-denuded preparations. 5. In conclusion, the findings of the present study suggest that airway parasympathetic nerves are endowed with alpha 2-adrenoceptors which subserve inhibition of transmitter acetylcholine release. Under the present conditions, however, transmitter acetylcholine release is not subject to transneuronal modulation by noradrenaline released from adjacent sympathetic nerves in the airways. Moreover, angiotensin II and perindoprilat do not appear to modulate acetylcholine release from parasympathetic nerves of the airways. In contrast, bradykinin inhibits acetylcholine release from airway parasympathetic nerves but this action of bradykinin is limited by the activity of epithelial angiotensin-converting enzyme and/or neutral endopeptidase. The inhibitory action of bradykinin on cholinergic transmission in the airways does not appear to involve the liberation of prostaglandins or nitric oxide.

Effects of three alpha 2-adrenoceptor agonists, rilmenidine, UK 14304 and clonidine on bradykinin- and substance P-induced airway microvascular leakage in guinea-pigs

The effects of three alpha 2-adrenoceptor agonists, clonidine, rilmenidine and UK 14304 on the increase of microvascular permeability induced by bradykinin or substance P in guinea-pigs airways have been studied in vivo. Extravasation of intravenously (i.v.) injected Evans blue dye was used as index of permeability. The effects of the three alpha 2-adrenoceptor agonists on the contraction induced by bradykinin (0.3 micrograms.kg-1, i.v.) and substance P (0.3 micrograms.kg-1, i.v.) were also studied on the isolated guinea-pig trachea. The increase of plasma exudation induced by bradykinin (0.3 micrograms.kg-1, i.v.) was inhibited partially by rilmenidine and UK 14304 (20 micrograms and 100 micrograms, intratracheally) respectively. These two substances had no action on the effects of substance P. The effects of rilmenidine and UK 14304 were abolished by alpha 2-blockers (idazoxan 1mg.kg-1 i.v. and RX 821001 100 micrograms.kg-1, i.v.), but they were not altered by the alpha 1-blocker prazosin (30 micrograms.kg-1, i.v.). Under similar conditions, clonidine or the alpha 1-adrenoceptor agonist methoxamine were without significant effects. In vitro, rilmenidine and UK 14304 inhibited partially the contractile effects of bradykinin but not those of substance P. To conclude, both rilmenidine and UK 14304 inhibit the bradykinin-induced increase of vascular permeability in the airways, and they probably do so on peptidergic nerve endings at the prejunctional level since these substances are without effect on substance P. The absence of activity of clonidine in our study might be due to a difference in spectrum of action on the several types of alpha 2-adrenergic or imidazole receptors.