Neurogenic control of airway smooth muscle function

Pharmacological characterization of the interaction between the dual phosphodiesterase (PDE) 3/4 inhibitor RPL554 and glycopyrronium on human isolated bronchi and small airways

BACKGROUND

The dual PDE3/4 inhibitor RPL 554 causes bronchodilation in patients with asthma or COPD and synergistically interacts with muscarinic receptor antagonists in relaxing human isolated bronchi in acute experimental settings. In the present study we investigated the long-lasting interaction between RPL554 and glycopyrronium by testing these drugs for their ability to relax both medium and small human isolated bronchi.

METHODS

The relaxant effect and duration of action of RPL554 and glycopyrronium, alone, or in combination, were studied on the contractile tone induced by electrical field stimulation (EFS) or carbachol in medium and small human isolated bronchi. Relaxation was expressed as percentage of maximal response and synergy analyzed by Bliss Independence theory.

RESULTS

Low concentrations of RPL554 and glycopyrronium induced maximal relaxation of medium bronchi at 160 ± 20 min and 50 ± 10 min, respectively, an effect detectable for at least 4 h. Maximal synergy was observed at ≃ 2 hrs (-71.4 ± 5.1%), and the combination extended the relaxation to at least 6 hrs, when the contractile tone was -41.2 ± 8.5% of the control responses. The combination induced the greatest effectiveness for EFS at 3 Hz and low-to-middle concentrations also produced significant synergism on small airways (21.1 ± 4.0%,P < 0.05), compared with the additive response. The combination induced lumen area enhancement of 69.1 ± 2.4% (P < 0.05), compared with the additive response (51.0 ± 5.4%).

CONCLUSIONS

RPL554 and glycopyrronium demonstrated a synergistic interaction in relaxing both human medium and small isolated bronchi, in terms of peak relaxation and an extended duration of action, suggesting that this combination may have a beneficial role in the treatment of asthma or COPD.

Epithelium-dependent and -independent inhibitory effects of sivelestat, a neutrophil elastase inhibitor, on substance P-induced contraction of airway smooth muscle in lipopolysaccharide-treated guinea-pigs

The underlying mechanism involved in the interaction between neutrophil elastase inhibitors and tachykinins has not been elucidated. In this study we have examined the effects of sivelestat, a neutrophil elastase inhibitor, on the in vitro responses of airways from lipopolysaccharide (LPS)-untreated or -treated guinea-pigs to substance P. Substance P (0.01-30 micromol/l) produced concentration-dependent contractions of both tracheal and bronchial ring preparations of LPS-untreated or -treated guinea-pigs. Responsiveness to substance P in these isolated airway preparations was augmented by either epithelium removal or LPS treatment. In epithelium-intact tracheal ring preparations isolated from LPS-untreated guinea-pigs, sivelestat (100 micromol/l) significantly inhibited substance P-induced contractions. The inhibitory action was markedly attenuated by pretreatment with L-NAME (100 micromol/l) or indomethacin (2 micromol/l), and was almost undetected following removal of the epithelium. On the other hand, in bronchial ring preparations isolated from LPS-untreated guinea-pigs, sivelestat had only a very slight effect on substance P-induced contraction of the epithelium-intact preparation, whereas sivelestat greatly inhibited contraction in epithelium-removed bronchial ring preparations. In LPS-treated guinea-pigs, whether the epithelium was intact or not, sivelestat significantly inhibited the substance P-induced contraction of bronchial ring preparations. Pretreatment with L-NAME (100 micromol/l) or indomethacin (2 micromol/l) did not affect the inhibitory effect of sivelestat in bronchial ring preparations. In conclusion, epithelium removal or LPS treatment induced hyperreactivity to substance P in the guinea-pig airway. Sivelestat caused epithelium-, nitric oxide- and prostaglandin-dependent inhibition of the substance P-induced contraction of isolated guinea-pig tracheal ring preparations. In contrast, the inhibitory effect of sivelestat on substance P-induced contraction of guinea-pig bronchial ring preparations is mediated by epithelium-, nitric oxide- and prostaglandin-independent mechanisms. Sivelestat may be effective in reducing the airway hyperresponsiveness to tachykinins induced by epithelial injury as occurs in LPS-mediated inflammatory lung diseases.

Tyrosine hydroxylase- and neuropeptides-immunoreactive nerves in canine trachea

OBJECTIVE

To determine distribution of catecholaminergic and peptidergic nerve fibers in canine tracheas by use of immunohistochemistry.

SAMPLE POPULATION

10 tracheas collected from healthy adult dogs after euthanasia.

PROCEDURE

Structure of the nerve network and distribution of tyrosine hydroxylase (TH)- and 6 types of neuropeptide-containing nerves in canine tracheas were immunohistochemically studied, using neurochemical markers.

RESULTS

Intraepithelial free nerve endings with immunoreactivity for calcitonin gene-related peptide (CGRP) and substance P (SP) were observed. Tyrosine hydroxylase-, SP-, vasoactive intestinal peptide (VIP)-, and galanin (GAL)-immunoreactive nerve fibers were observed within and around the submucosal seromucous gland. In the smooth muscle layer, numerous TH- and GAL-immunoreactive nerve fibers, a moderate number of VIP- and neuropeptide Y (NPY)-immunoreactive nerve fibers, and a few SP- and methionine enkephalin (ENK)-immunoreactive nerve fibers were observed. Numerous nerve cell bodies with VIP and GAL immunoreactivity and a few with SP ENK, and NPY immunoreactivity were observed. Many TH-immunoreactive fibers were arranged in a meshwork around blood vessels. Nerves with CGRP-, SP-, VIP-, GAL-, ENK-, and NPY-immunoreactivity were also observed around blood vessels.

CONCLUSIONS

Complex innervation, including catecholamine- and neuropeptide-containing nerves, which may be related to regulation of muscle contraction and glandular secretion, are found in canine tracheas.

Capsaicin can abolish spontaneous tone in guinea-pig trachealis

The properties of spontaneous tone in isolated preparations of guinea-pig tracheal smooth muscle were examined. Experiments with control preparations revealed that 5-15 min after stretching the muscle with 0.15 mN, the spontaneous tone assumed a plateau value from which it declined gradually during the following hour. During the plateau, the force amounted to approximately 35% and 1 h later to approximately 20% of a maximum KC1 contraction. The tone was independent of tetrodotoxin, atropine and propranolol. Indomethacin quickly and completely relaxed the tone in 15 of 21 preparations. However, four preparations retained some tone even after 1 h of treatment. Exposure to the C-fibre influencing drug capsaicin resulted in a dose-dependent, reversible suppression of spontaneous tone, normally preceded by a transient increase in force. No spontaneous tone at all remained after 1 h of 10 microM capsaicin. This effect was also found in preparations pretreated with tetrodotoxin, atropine and propranolol. Preparations, deprived of spontaneous tone by capsaicin-treatment, contracted distinctly when exposed to 10 microM arachidonic acid. This contraction was almost completely abolished by indomethacin, which indicates that the prostaglandin synthesis is functioning after capsaicin treatment and, thus, that inhibition of this synthesis is not responsible for the capsaicin effect. Exposure to phosphoramidon increased the spontaneous tone almost threefold. Addition of 3 nM neurokinin A in the permanent presence of capsaicin gave weaker contractions in preparations where prostaglandin synthesis had been abolished by indomethacin, as compared to contractions in preparations with intact prostaglandin synthesis. The data indicate that a continuous release of tachykinins from sensory C-fibres is essential for the generation of spontaneous tone and that a combination of tachykinins and prostaglandins determine the size of the tone in this preparation.