The effects of essential oil ofMentha x villosa on skeletal muscle of the toad

The effects of the essential oil of Mentha x villosa (EOMV) was investigated in saponin-permeabilized fibres and in intact sartorius muscle of the toad. In intact muscle EOMV at concentrations from 1 to 100 μg/mL blocked the contraction induced by high (80 mM) potassium and induced potentiation of caffeine-induced muscle contraction. From 1 to 6 mg/mL EOMV elicited contraction in both depolarized (by 110 mM K+) and non-depolarized (4 mM K+) muscle fibres. This contraction was blocked by procaine. At 3 mg/mL EOMV depolarized the normal resting potential from −80.77 to −71.21 mV, but did not alter the transmembrane potential in the presence of 60 mM K+. EOMV (1 mg/mL) induced contraction of saponin-permeabilized muscle fibres with an intact sarcoplasmic reticulum (SR). These data suggest that EOMV: (i) induces muscle contraction by releasing Ca2+ from SR; (ii) impairs the excitation–contraction coupling at a step before Ca2+ release from the SR.