Calcium channel blocking activity of Mentha longifolia L. explains its medicinal use in diarrhoea and gut spasm

Mentha longifolia has a reputation in traditional medicine in the indications of diarrhoea and gut spasm. This study was carried out to provide a possible pharmacological basis for its medicinal use in hyperactive gut disorders. In a castor oil induced diarrhoeal model, the crude extract of Mentha longifolia (Ml.Cr), at doses of 100-1000 mg/kg, provided 31-80% protection, similar to loperamide. In isolated rabbit jejunum preparations, Ml.Cr caused inhibition of spontaneous and high K⁺-induced contractions, with respective EC₅₀ values of 1.80 (1.34-2.24; n = 6-8) and 0.60 mg/mL (0.37-0.85; n = 6-8), which suggests spasmolytic activity, mediated possibly through calcium channel blockade (CCB). The CCB activity was further confirmed when pretreatment of the tissue with Ml.Cr (0.3-1 mg/mL) caused a rightward shift in the Ca⁺⁺ concentration-response curves (CRCs), similar to verapamil. Loperamide also inhibited spontaneous and high K⁺-induced contractions and shifted the Ca⁺⁺ CRCs to the right. Activity-directed fractionation revealed that the petroleum spirit fraction was more potent than the parent crude extract and aqueous fraction. These data indicate that the antidiarrhoeal and spasmolytic effects of the crude extract of Mentha longifolia are mediated through the presence of CCB-like constituent(s), concentrated in the petroleum spirit fraction and this study provides indirect evidence for its medicinal use in diarrhoea and spasm.