CFTR Modulation by Plantago asiatica Flavonoids: A Potential Antisecretory Mechanism Against Diarrhoea

Plantago asiatica L. (P. asiatica) is traditionally used to treat diarrhoea in veterinary medicine, but its molecular basis remains unclear. In rodent models, the water extract and active fractions reduced intestinal fluid accumulation without significant transit inhibition by the crude extract, supporting a substantial antisecretory component. A phenotype-guided analysis of 14 compounds identified from the active fractions prioritised cystic fibrosis transmembrane conductance regulator (CFTR), the principal apical chloride channel driving intestinal fluid secretion. Apigenin, luteolin and scutellarein were then evaluated at the lipid-exposed ivacaftor site of porcine CFTR in a POPC membrane. Although this regulatory site can support both CFTR potentiation and inhibition, the ligand-specific binding arrangements underlying these opposite effects remain unclear. Luteolin formed a persistent Glu874-centred network and produced the largest reproducible shift in local TM8 hinge geometry, providing a structural hypothesis for inhibitory modulation. Apigenin sampled a TM4-shifted State A and a TM8-facing State B that overlapped the ivacaftor-associated pocket. In light of its reported concentration-dependent block and potentiation, these ensembles are proposed as inhibition-associated and potentiation-associated modes, respectively. Scutellarein remained associated with the shared pocket and exhibited a distinct interaction pattern, further supporting the ligand-dependent conformational plasticity of this regulatory site. These findings support CFTR modulation by P. asiatica flavonoids as a plausible mechanism for reducing chloride-driven intestinal fluid secretion.