Mitragynine and its potential blocking effects on specific cardiac potassium channels

Area of Science:

• Cardiovascular Pharmacology
• Molecular Toxicology
• Pharmacology

Background:

• Mitragyna speciosa (kratom) is used recreationally, and mitragynine, its active compound, has been linked to poisoning and fatalities.
• Cardiac ion channels, including hERG, Kir2.1, and GIRK, are crucial for maintaining normal heart function and electrical stability.
• The precise mechanisms underlying mitragynine's cardiotoxicity remain incompletely understood.

Purpose of the Study:

• To investigate the effects of mitragynine on the function and expression of cardiac ion channels, specifically hERG, Kir2.1, and GIRK.
• To elucidate the molecular interactions between mitragynine and the hERG channel.
• To assess the potential additive cardiotoxicity risks associated with mitragynine's effects on multiple cardiac ion channels.

Main Methods:

• Electrophysiological recordings (whole-cell patch-clamp) of hERG, Kir2.1, and GIRK channel currents in hERG-transfected HEK293 cells and Xenopus oocytes.
• Site-directed mutagenesis of the hERG channel (Y652A, F656A) to probe drug-binding sites.
• Western blot analysis to assess hERG protein expression.
• Quantitative PCR (qPCR) to evaluate hERG mRNA expression.
• Molecular docking simulations to predict binding interactions.

Main Results:

• Mitragynine significantly inhibited hERG channel currents with IC50 values of 1.62µM (HEK293 cells) and 1.15µM (oocytes).
• Mutations at Y652A and F656 in the hERG channel attenuated mitragynine's inhibitory effect, suggesting interaction within the pore cavity.
• Mitragynine inhibited IKACh (GIRK channel) currents (IC50 = 3.32µM) but did not significantly affect IK1 (Kir2.1 channel) currents.
• Mitragynine reduced hERG protein expression but not mRNA levels, indicating post-transcriptional regulation.
• Molecular docking supported mitragynine's interaction with high-affinity binding sites in the hERG channel.

Conclusions:

• Mitragynine directly inhibits cardiac hERG and GIRK channels, contributing to potential cardiotoxicity.
• The interaction with hERG occurs within the channel pore, specifically at the Y652 and F656 residues.
• Combined blockade of hERG and GIRK channels by mitragynine may pose additive risks for cardiac arrhythmias and adverse events.
• Mitragynine's effect on protein expression warrants further investigation into its post-transcriptional regulatory mechanisms.