Cryptotanshinone as a Multi-Target Natural Terpenoid with Bronchodilator Potential: Insights from Integrated In Vitro and In Silico Studies

Asthma is a chronic airway disease characterized by inflammation, bronchial hyperresponsiveness, and airflow limitation, highlighting the need for novel bronchodilator agents. Cryptotanshinone (CT), a bioactive diterpenoid derived from Salvia miltiorrhiza, exhibits anti-inflammatory and vasodilatory properties; however, its direct effects on airway smooth muscle remain poorly characterized. This study investigated the bronchodilatory activity of CT and its pharmacological mechanisms. Molecular docking was performed to evaluate potential interactions with M3 muscarinic receptors and L-type calcium channels. Functional experiments were conducted using isolated guinea pig tracheal smooth muscle preparations. The relaxant effects of CT (10−7–3 × 10−4 M) were evaluated against carbachol (1 µM)- and high-K+ (80 mM)-induced contractions. Docking predicted favorable binding of CT to the M3 receptor and L-type Ca2+ channel, with binding energies of −9.854 and −9.951 kcal/mol, respectively. In vitro, CT produced concentration-dependent relaxation of CCh-induced contractions, reaching a maximal effect of 41.9 ± 2.58% at 3 × 10−4 M (pEC50 = 4.60). CT produced minimal relaxation in high-K+-induced contractions, suggesting receptor-mediated rather than non-selective smooth muscle inhibition. CT also produced a parallel rightward shift of the CCh concentration–response curve at 10−5 M, whereas a higher concentration (10−4 M) altered the maximal contractile response, suggesting concentration-dependent pharmacological effects. Pharmacological inhibition studies indicated the involvement of muscarinic receptor-mediated mechanisms, with additional contributions from calcium channel-related mechanisms and partial involvement of the NO/cGMP pathway, while β2-adrenergic signaling and potassium channels were not significantly involved. These findings suggest that CT exerts bronchodilatory effects through the involvement of multiple pharmacological pathways relevant to airway smooth muscle regulation and provide preliminary mechanistic evidence supporting further investigation.