Possible Tracheal Relaxant and Antimicrobial Effects of the Essential Oil of Ethiopian Thyme Species (Thymus serrulatus Hochst. ex Benth.): A Multiple Mechanistic Approach

Pharmacologic effects approach of essential oils and their components on respiratory diseases

ETHNOPHARMACOLOGICAL RELEVANCE

Essential oils (EOs) are concentrated hydrophobic liquids with volatility and a unique aroma. Formed by aromatic plants as secondary metabolites, EOs have been used as traditional medicines to treat various health problems worldwide. Historical records show that herbs rich in EOs have been widely used to treat respiratory diseases in China, Europe, and many other regions.

AIM OF THE REVIEW

This review summarizes the traditional applications and modern pharmacological mechanisms of EOs derived from aromatic herbs and their active ingredients in respiratory diseases in preclinical and clinical trials through multitarget synergy.

MATERIALS AND METHODS

Information about EOs and respiratory diseases was collected from electronic databases, such as ScienceDirect, Web of Science, PubMed, Google Scholar, Baidu Scholar, and the China National Knowledge Infrastructure (CNKI).

RESULTS

This review presents the preventive and therapeutic effects of EOs on respiratory diseases, including chronic obstructive pulmonary disease, bronchial asthma, acute lung injury, pulmonary infection, and pulmonary fibrosis. The molecular mechanisms of EOs in treating different lung diseases are summarized, including anti-inflammation, anti-oxidation, mucolytic, and immune regulatory mechanisms.

CONCLUSIONS

EOs show potential as supplements or substitutes for treating lung diseases.

Comparative GC Analysis, Bronchodilator Effect and the Detailed Mechanism of Their Main Component; Cinnamaldehyde of Three Cinnamon Species

Cinnamon is one of the most commonly used spices worldwide. In some Arab countries, cinnamon is used with other ingredients to relieve bronchospasm and treatment of airways-related disorders. In the current study, GC, GC-MS and tracheal relaxant effect comparison were performed using the three available types in Saudi Arabia, Cinnamomum verum (Ceylon cinnamon), C. cassia (Chinese cinnamon) and C. loureiroi (Vietnamese cinnamon). The essential oil of C. verum was the most potent in the relaxation of guinea pig isolated tracheal muscles against carbachol (CCh, 1 uM)-evoked bronchospasm at the concentration range from 0.03 to 3 mg/mL followed by C. bureiroi at 0.03 to 5 mg/mL; whereas, C. cassia was the least potent oil. Cinnamaldehyde (1), isolated as the main component of the three oils induced complete relaxation of low K+ (25 mM)-evoked contractions, with mild effect on the contractions evoked by high K+ (80 mM). Pre-incubation of the tracheal tissues with glibenclamide (10 mM) significantly opposed the relaxation of low K+ by cinnamaldehyde. The standard drug, cromakalim also inserted glibenclamide-sensitive inhibition of low K+ without relaxing high K+. These results indicate that cinnamaldehyde acts predominantly by ATP-specific K+ channel opening followed by weak Ca++ antagonistic effects. The obtained results justify the medicinal value of cinnamon oil in respiratory disorders.

Bronchodilator Secondary Metabolites from Rhazya stricta Decne Aerial Parts

The plant kingdom comprises medicinally useful plants that have provided many new drugs used to treat various diseases. In our search for bronchodilator secondary metabolites from plants growing in Saudi Arabia, the total extract of Rhazya stricta showed activity against carbamylcholine- (CCh) induced bronchoconstriction in guinea pig tracheal muscles used as an ex vivo model. The fractions obtained from liquid–liquid extraction process were tested for bronchodilator effects. The most active ethyl acetate fraction (RS-E) and aqueous fraction (RS-H) were subjected to biologically guided phytochemical study using different stationary phases and chromatographic techniques to isolate the pure secondary metabolites. Five known compounds were isolated from the active fractions. Three alkaloids namely; (-)-quebrachamine (1), (+)-eburenine (2), (+)-stemmadenine (3) as well as the two iridoid glycosides loganic acid (4) and loganine (5) were identified by various spectroscopic methods. Among the isolated compounds 1 and 5 were the only active as bronchodilators in the plant. It is worth to mention that iridoid glycosides are isolated for the first time from R. stricta.

Achillea fragrantissima Essential Oil: Composition and Detailed Pharmacodynamics Study of the Bronchodilator Activity

The bronchodilator effect of the Achillea fragrantissima essential oil (AFO) was studied in guinea pigs’ tracheas and the influence of drying on the quantity and composition of AFO was studied using GC-MS and GC analyses. AFO produced a complete and potent relaxation against carbachol (CCh), while lower potency and partial efficacy were observed against high K+ (80 mM), thus producing dual inhibitory effects similar to dicyclomine. The anticholinergic-like action was further confirmed when pre-incubation tracheal tissues were used at lower concentrations with AFO displacing the CCh concentration‒response curves (CRCs) to the right in a competitive manner similar to atropine. However, non-parallel shifts in CCh CRCs were observed with higher doses, similar to dicyclomine. Further confirmation of the CCB-like effect was obtained from the non-specific deflection of Ca++ CRCs toward the right using the pre-incubated tissues with AFO in Ca++ free medium, similar to verapamil. When AFO was tested against low K+-mediated contractions to explore the possible involvement of additional antispasmodic mechanism(s), AFO interestingly showed a complete inhibition with a higher potency. This inhibition was found to be sensitive to tetraethylammonium (TEA) and 4-aminopyridine (4-AP), whereas glibenclamide (Gb) remained inactive. These results show that AFO possesses bronchodilator effects predominantly from its anticholinergic and K+ channel activation followed by weak Ca++ channels inhibition.

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 EC₅₀ 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 Ca²⁺ 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 Ca²⁺ channel blocker, showed selectively higher potency against high K⁺ compared to CCh-mediated contractions with respective EC₅₀ 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 Ca²⁺ inhibitory-like action of fenchone pretreated tracheal tissues were authenticated by the rightward shift of Ca²⁺ CRCs with suppression of maximum response, similar to verapamil, a standard Ca²⁺ channel blocker. Fenchone showed a spasmolytic effect in isolated trachea mediated predominantly by K⁺ channel activation followed by dual inhibition of PDE and Ca²⁺ channels. Further in silico molecular docking studies provided the insight for binding of fenchone with Ca²⁺ channel (−5.3 kcal/mol) and K⁺ channel (−5.7), which also endorsed the idea of dual inhibition.