Cardiac and Electrophysiological Effects of Primary and Refined Extracts from Leonurus cardiaca L. (Ph.Eur.)

Abstract

Although several antiarrhythmic drugs of chemical origin are in clinical use since decades, their application is often limited by their adverse effects and especially by their inherited proarrhythmic risk, which can lead to a significantly increased mortality in patients receiving these compounds. On the other hand, aqueous extracts from the aerial parts of the European Lamiaceae Leonurus cardiaca (Ph.Eur.) have been used for centuries as a remedy against tachyarrhythmia and other cardiac disorders. Nevertheless, a scientific basis for the claim of direct cardiac electrophysiological, antiarrhythmic, or functional effects of Leonurus cardiacae herba (LCH) preparations has not been established until now. In order to enrich the active constituents from the primary extract which was tested as the most cardioactive, namely the aqueous Soxhlet extract, and to eliminate undesired substances such as the dichloromethanic fraction or potassium, a bioassay guided fractionation procedure was applied, resulting in the development of a Leonurus cardiaca refined extract (LCRE) which was characterised together with Leonurus crude extracts by a newly developed gradient elution HPLC fingerprint analysis for separation and quantification of six major phenolics as well as by qNMR for determining the stachydrine content. This refined extract was applied intracoronarily in isolated rabbit hearts perfused according to the Langendorff technique. Mapping experiments with 256 electrodes on the heart surface showed a reduction of left ventricular pressure and an increase of relative coronary flow at concentrations of 1.0 and 2.0 mg/mL LCRE. Furthermore, the PQ-interval was prolonged and both the basic cycle length and the activation recovery interval increased. In addition, voltage-clamp measurements were performed on the following cell models in order to characterise the electrophysiological profile of LCRE: neonatal rat ventricular cardiomyocytes to investigate the effect on I_Na and I_Ca.L, sinoatrial node cells and ventricular myocytes isolated from adult guinea pigs to test effects on I_f and action potential (AP) duration, as well as HERG-transfected HEK 293 cells to analyse the influence on the I_K.r. In these voltage clamp experiments LCRE exerted a calcium-antagonistic activity by I_Ca.L blockade, reduced the repolarising current I_K.r, and prolonged the AP-duration, while I_Na was not affected. Although LCRE displayed only weak effects on the I_f amplitude and voltage dependence, it significantly prolonged the activation time constant of I_f. Thus, LCRE acts on multiple electrophysiological targets, specifically I_Ca.L, I_K.r, and I_f, observed both at whole organ and single cell level.

Key words

Leonurus cardiaca - Lamiaceae - refined extract - plant analysis - antianginal and antiarrhythmic effects - voltage clamp - epicardial mapping

References

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1 These two authors contributed equally.