NA⁺‐ and K⁺‐channels as molecular targets of the alkaloid ajmaline in skeletal muscle fibres

Abstract

Background and purpose: Ajmaline is a widely used antiarrhythmic drug. Its action on voltage-gated ion channels in skeletal muscle is not well documented and we have here elucidated its effects on Na þ and K þ channels. Experimental approach: Sodium (I Na ) and potassium (I K ) currents in amphibian skeletal muscle fibres were recorded using 'loose-patch' and two-microelectrode voltage clamp techniques (2-MVC). Action potentials were generated using current clamp. Key results: Under 'loose patch' clamp conditions, the IC 50 for I Na was 23.2 mM with Hill-coefficient h ¼ 1.21. For I K , IC 50 was 9.2 mM, h ¼ 0.87. Clinically relevant ajmaline concentrations (1-3 mM) reduced peak I Na by B5% but outward I K values were reduced by B20%. Na þ channel steady-state activation and fast inactivation were concentration-dependently shifted towards hyperpolarized potentials (B10 mV at 25 mM). Inactivation curves were markedly flattened by ajmaline. Peak-I K under maintained depolarisation was reduced to B30% of control values by 100 mM ajmaline. I K activation time constants were increased at least two-fold. Lower concentrations (10 or 25 mM) reduced steady-state-I K slightly but peak-I K significantly. Action potential generation threshold was increased by 10 mM ajmaline and repolarisation prolonged. Conclusions and implications: Ajmaline acts differentially on Na þ and K þ channels in skeletal muscle. This suggests at least multiple sites of action including the S4 subunit. Our data may provide a first insight into specific mechanisms of ajmaline-ion channel interaction in tissues other than cardiac muscle and could suggest possible side-effects that need to be further evaluated.

Molecular Modes of Action of Defensive Secondary Metabolites

“…ACh activates the nAChR, resulting in a rapid influx of Na + in the postsynapse, which open voltage-gated Na + -channels. This event generates an action potential, which spreads along the membrane via voltage-gated Na + -channels (Friedrich et al, 2007).…”

The Sick and the Weak: Neuropathies/Myopathies in the Critically Ill

“…This potential threshold is mainly set by the steady-state inac-tivation of the channels (253,270). Both changes in steady-state activation and inactivation modulate excitability and action potential properties in skeletal muscle (219) and are thus of special interest in many diseases associated with channelopathies (e.g., Refs. 104, 380).…”

Indole alkaloids from the Marquesan plant Rauvolfia nukuhivensis and their effects on ion channels

“…The ionisation states of compounds at physiological pH were deduced with the help of Chemaxon software (Chemaxon, XXXX). However, as reported in the literature, the interaction site of Ajmaline is located in the membrane (Friedrich et al, 2007). In this lipidic medium, ionisation states and, more generally, pKa values are difficult to evaluate.…”

“…Missing amino-acid sidechains were added with the help of the xleap module of Amber according to the protein force-field ff99SB (Wickstrom et al, 2009). The interacting site was located around the S4 fragment of the channel, in the transmembrane domain, as elucidated for the ajmaline compound (Friedrich et al, 2007) (See Supporting Information). Molecular docking calculations were performed with the Autodock 4.2 software (Morris et al, 2009).…”

“…Because some recent studies underlined the activity of ajmaline derivatives on ion channels, we decided to assess the potential of the isolated http://dx.doi.org/10.1016/j.phytochem.2014.10.026 0031-9422/Ó 2014 Elsevier Ltd. All rights reserved. alkaloids on these biological targets (Bebarova et al, 2005;Friedrich et al, 2007).…”