The modulatory effects of prostaglandins on both excitatory and inhibitory non-adrenergic non-cholinergic neurotransmission in guinea-pig airways

Influence of maturation on constrictive response to stimulation of C-fiber afferents in isolated guinea pig airways

We investigated whether the airway constrictive response to stimulation of bronchopulmonary C-fiber afferents is altered during the maturation process. Isometric tension was measured in airway rings isolated from three tracheobronchial locations (intrathoracic trachea and main and hilar bronchi) and compared in mature [M, 407 ± 10 (SE) g body wt, n = 36] and immature (IM, 161 ± 5 g body wt, n = 35) guinea pigs. Our results showed no difference in the ACh (10−5 M)- or KCl (40 mM)-induced contraction between M and IM groups, regardless of the airway location. In sharp contrast, the concentration-response curves of 10−8–10−6 M capsaicin were distinctly lower in IM hilar bronchi; for example, response to the same concentration of capsaicin (10−6 M) was 89.2 ± 15.3% of the response to 10−5 M ACh in IM and 284.7 ± 43.2% in M animals. Similar, but smaller, differences in the bronchoconstrictive response to capsaicin between IM and M groups were also observed in the trachea and main bronchus. Electrical field stimulation induced airway constriction in all three locations in M and IM groups. However, after administration of 10−6 M atropine and 10−6 M propranolol, electrical field stimulation-induced contraction was significantly smaller in the hilar bronchus of IM than M animals, and this difference was not prevented by pretreatment with 5 × 10−5 M indomethacin. Although radioimmunoassay showed no difference in the tissue content of substance P between M and IM airways, the constrictive responses to exogenous substance P and neurokinin A were markedly greater in M airways at all three locations. In conclusion, the constriction of isolated airways evoked by C-fiber stimulation was significantly weaker in the IM guinea pigs, probably because of a less potent effect of tachykinins on the airway smooth muscle.

Modulation of sensory nerve function in the airways

Several clinical studies document a greater discrimination between asthmatic and healthy subjects in bronchial responsiveness to a range of stimuli such as cold air, distilled water and sodium metabisulphite, than to conventional bronchoconstrictor agonists including histamine and methacholine. One of the mechanisms thought to account for the bronchoconstriction induced by these agents is via reflex activation of the cholinergic pathway. An increase in sensory nerve (afferent) activity in asthma might account for the increased responsiveness to these agents. If so, a number of strategies are available to inhibit the function of afferent nerves which could lead to a suppression of bronchial hyperresponsiveness, including (1) inhibition of afferent activity, (2) inhibition of neuropeptide release and (3) antagonism of tachykinin receptors. As there are numerous reviews dealing with the latter, in this review Domenico Spina, Saloni Shah and Selena Harrison focus on the first two strategies.

The role of prostanoids in ozone-induced changes in airway responsiveness: receptor activation-specific prostanoid release

We studied the effect of in vivo ozone exposure (3 ppm, 2 h) on methacholine- and histamine-induced guinea pig tracheal smooth muscle contractions in vitro and the role of cyclooxygenase products in this process. After exposure to ozone, methacholine stimulation showed a functional hyperreactivity, whereas after stimulation with histamine a hyporeactivity was observed. These effects could be explained by the release of prostanoids. In a control situation an increase in PGF(2α), PGE(2) and PGD(2) release is observed after stimulation of the histaminergic receptor system. After ozone exposure the release of prostanoids was also enhanced (unstimulated, PGF(2α) and TxB(2); histamine, PGF(2α), PGE(2); methacholine, PGF(2α), TxB(2), 6-kPGF(1α), PGE(2)). This study shows that the prostanoid release is strongly dependent on the receptor system stimulated to induce smooth muscle contraction and the importance of prostanoids in ozone-induced changes in guinea pig tracheal smooth muscle reactivity.

Thromboxane A2 and related prostaglandins in airways

Asthma is now thought to be a chronic inflammatory disease of the airways. The roles of prostanoids, thromboxane A2 (TXA2) and the prostaglandins (PGs) in the pathogenesis and pathophysiology of asthma have fostered a wealth of studies but remain controversial. TXA2 and the bronchoconstrictor PGs, PGD2 and PGF2 alpha, are generated in greater amounts in asthmatic than in normal subjects. TXA2 is a potent constrictor of airway smooth muscle, an inducer of acetylcholine release and of airway microvascular leakage. It may participate in the thickening and the remodeling of the airway wall which may contribute to the airway hyperresponsiveness, a typical feature of asthma. Strategies for inhibition of TXA2 effects include antagonism of the TXA2 receptor (TP receptor) and inhibition of the thromboxane synthase. TP receptor antagonists could block the effects of all the bronchoconstrictor prostanoids because TXA2 as well as the bronchoconstrictor PGs act through activation of lung TP receptor. The recent development of specific and potent TP receptor antagonists and inhibitors of thromboxane synthase has provided tools to assess the role of TXA2 and broncho-constrictor PGs in the pathophysiology of asthma.

Regulation by phosphodiesterase isoenzymes of non-adrenergic non-cholinergic contraction in guinea-pig isolated main bronchus

1. We have investigated the role of phosphodiesterase isoenzymes in modulating electric field stimulation (EFS), substance P and capsaicin-induced contraction of the guinea-pig isolated main bronchus. 2. Non-adrenergic non-cholinergic contractile responses were elicited by EFS (3 Hz, 20 s) in the guinea-pig isolated main bronchus in the presence of the non-selective muscarinic antagonist, atropine (0.1 microM), the non-selective beta-adrenoceptor antagonist, propranolol (1 microM), the neutral endopeptidase inhibitor, thiorphan (10 microM) and the cyclo-oxygenase inhibitor, indomethacin (5 microM). The type III, type III/IV, type IV and type V phosphodiesterase isoenzyme inhibitor, SKF 94836, benzafentrine, Ro-20-1724 and zaprinast respectively, significantly attenuated the contractile response to EFS. The IC50 (95% confidence limits) value for SKF 94836, benzafentrine, Ro-20-1724 and zaprinast was 8.3 microM (0.89-78); 0.7 microM (0.1-4.5); 0.5 microM (0.2-1.2) and 13 microM (2-87) respectively. 3. The phosphodiesterase isoenzyme inhibitors, SKF 94836, Ro-20-1724 and zaprinast, partially attenuated the contractile response to substance P (10 nM). Benzafentrine significantly inhibited the contractile response to substance P, yielding an IC50 value of 1.9 microM (0.9-3.8). 4. The phosphodiesterase isoenzyme inhibitor, Ro-20-1724 (0.1-100 microM) failed to reduce significantly the contractile potency of capsaicin (P > 0.05). In contrast, SKF 94836 (1 microM), benzafentrine (10 microM) and zaprinast (100 microM) significantly reduced the contractile potency of capsaicin (P < 0.05). 5 The selective phosphodiesterase isoenzyme inhibitors, SKF 94836, benzafentrine, Ro-20-1724 andzaprinast (0.01-100 microM) reversed in a concentration-dependent manner the contractile response toexogenously administered capsaicin (EC50) yielding ICm values of 3.91 microM (0.68-22); 3.37 microM (1.86-6.11); 0.366 microM (0.201-0.564) and 50.1 microM (18.6- 135) respectively.6 In conclusion, phosphodiesterase isoenzymes appear to regulate the contractile response to electricalfield stimulation and our results provide circumstantial evidence for a regulatory role ofphosphodiesterase type IV isoenzyme on sensory nerve function in vitro.

Influence of tolfenamic acid on airway contractility in guinea pigs

The effect of the NSAIDs tolfenamic acid and indomethacin was tested on acetylcholine-induced and antigen-induced contractions in guinea pig airways. Indomethacin potentiated antigen-induced contractions while tolfenamic acid showed dilatory properties. The effects of the drugs on acetylcholine-induced contractions showed no significant differences; indomethacin had a slightly increasing tendency while tolfenamic acid reduced the contractile response. These results indicate that tolfenamic acid does not have bronchoconstrictive properties, which is a common side effect of other NSAIDs in asthmatics.