The effect of Agauria salicifolia leaf extract on the sodium current of tetrodotoxin-treated frog skeletal muscle fibres.
An ethanolic extract of Agauria salicifolia leaf (AS) was tested on voltage-clamped single muscle fibres from frogs, using the double sucrose-gap technique, in the presence of tetrodotoxin (TTX) to partially block the transient inward sodium current. Below 5 x 10(-10) g/ml TTX, 10(-6) g/l AS reversed the blocking effect of TTX on INa and increased the current amplitude. For TTX concentrations ranging from 5 x 10(-10) to 5 x 10(-8) g/ml, which partially blocked INa and lengthened its time to peak, 10(-6) g/l AS reinforced the blocking effect on the current. The inhibitory effect of AS was then studied in the presence of 10(-9) g/ml TTX which was assumed to block all the sarcolemmal sodium channels and not the tubular ones. It appeared to be strongly voltage-dependent since it was fully relieved when the cell was hyperpolarized by 20 mV and was maximum at holding potential (h.p.) -70 mV (100% blockage of INa instead of 50% at h.p. -90 mV). Contrary to observations in depolarised fibres (in which the inhibitory effect of AS decreased as concentration rose) the INa blockage increased with concentration in TTX-treated fibres. It is concluded that AS extract exerts an antagonistic effect on tubular sodium channels by acting on a single site which is different from the high affinity inhibitory site demasked in normal fibres by depolarization of the resting membrane potential.
Ecole Normale Supérieure, University of Yaoundé, Cameroon.
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Ewane-Nyambi
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Raymond
D000818:Animals; D066298:In Vitro Techniques; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D009132:Muscles / Q000187:drug effects* / Q000502:physiology; D010936:Plant Extracts / Q000494:pharmacology*; D010946:Plants, Medicinal; D011895:Rana ridibunda; D012508:Sarcolemma / Q000187:drug effects / Q000502:physiology; D015222:Sodium Channels / Q000187:drug effects* / Q000502:physiology; D013779:Tetrodotoxin / Q000494:pharmacology*
D010936:Plant Extracts; D015222:Sodium Channels; D013779:Tetrodotoxin