In Silico and Ex Vivo Studies on the Spasmolytic Activities of Fenchone Using Isolated Guinea Pig Trachea

Fenchone is a bicyclic monoterpene found in a variety of aromatic plants, including Foeniculum vulgare and Peumus boldus, and is used in the management of airways disorders. This study aimed to explore the bronchodilator effect of fenchone using guinea pig tracheal muscles as an ex vivo model and in silico studies. A concentration-mediated tracheal relaxant effect of fenchone was evaluated using isolated guinea pig trachea mounted in an organ bath provided with physiological conditions. Sustained contractions were achieved using low K(+) (25 mM), high K(+) (80 mM), and carbamylcholine (CCh; 1 µM), and fenchone inhibitory concentration–response curves (CRCs) were obtained against these contractions. Fenchone selectively inhibited with higher potency contractions evoked by low K(+) compared to high K(+) with resultant EC(50) values of 0.62 mg/mL (0.58–0.72; n = 5) and 6.44 mg/mL (5.86–7.32; n = 5), respectively. Verapamil (VRP) inhibited both low and high K(+) contractions at similar concentrations. Pre-incubation of the tracheal tissues with K(+) channel blockers such as glibenclamide (Gb), 4-aminopyridine (4-AP), and tetraethylammonium (TEA) significantly shifted the inhibitory CRCs of fenchone to the right towards higher doses. Fenchone also inhibited CCh-mediated contractions at comparable potency to its effect against high K(+) [6.28 mg/mL (5.88–6.42, n = 4); CCh] and [6.44 mg/mL (5.86–7.32; n = 5); high K(+)]. A similar pattern was obtained with papaverine (PPV), a phosphodiesterase (PDE), and Ca(2+) inhibitor which inhibited both CCh and high K(+) at similar concentrations [10.46 µM (9.82–11.22, n = 4); CCh] and [10.28 µM (9.18–11.36; n = 5); high K(+)]. However, verapamil, a standard Ca(2+) channel blocker, showed selectively higher potency against high K(+) compared to CCh-mediated contractions with respective EC(50) values of 0.84 mg/mL (0.82–0.96; n = 5) 14.46 mg/mL (12.24–16.38, n = 4). The PDE-inhibitory action of fenchone was further confirmed when its pre-incubation at 3 and 5 mg/mL potentiated and shifted the isoprenaline inhibitory CRCs towards the left, similar to papaverine, whereas the Ca(2+) inhibitory-like action of fenchone pretreated tracheal tissues were authenticated by the rightward shift of Ca(2+) CRCs with suppression of maximum response, similar to verapamil, a standard Ca(2+) channel blocker. Fenchone showed a spasmolytic effect in isolated trachea mediated predominantly by K(+) channel activation followed by dual inhibition of PDE and Ca(2+) channels. Further in silico molecular docking studies provided the insight for binding of fenchone with Ca(2+) channel (−5.3 kcal/mol) and K(+) channel (−5.7), which also endorsed the idea of dual inhibition.