The voltage-dependent effect of Agauria salifolia leaf extract on the sodium current of isolated frog skeletal and cardiac muscle cells

Abstract

An ethanolic extract of leaves of Agauria salicifolia was tested on voltage-clamped isolated skeletal and cardiac frog muscle cells using the double sucrose-gap (skeletal) and the whole-cell patch-clamp (cardiac) methods, at concentrations ranging from 10 −8 to 5 × 10 −5 g/l. At 10 −6 g/l a progressive and limited decrease was observed in resting membrane potential (≈ 5 mV). On normally polarized cells (−90 mV) the extract induced a dose- and use-dependent increase in the transient inward current which is accounted for by an increase in the sodium membrane conductance and an opposite shift of the activation and inactivation curves vs. potential. This agonistic effect on I Na is fully reversed when the holding potential is maintained depolarized above −80 mV as a result of an actual decrease in the sodium conductance and a reversal of the shifts of activation and activation curves. The inhibitory effect observed on depolarized fibres can be partially relieved by a subsequent repolarization. It is concluded that the extract acts on the sodium channel regardless of its state (resting, activated and inactivated) on at least two different sites.