Aqueous fraction from Costus spiralis (Jacq.) Roscoe leaf reduces contractility by impairing the calcium inward current in the mammalian myocardium

AqF from Costus spiralis reduces the calcium entry in myocardial cells as showed by its effect on the Bowditch phenomenon and on the positive inotropic effect produced by both BAY K8644 and isoproterenol. Direct measurements of the L-type calcium current (ICa, L) as well as of the intracellular free calcium determined by the fluorescence of FLUO 4AM (F/F0) confirmed such hypothesis. Brazilian folk medicine uses infusion of Costus spiralis leaf to help people to treat arterial hypertension and syndromes of cardiac hyperexcitability. Evaluate the aqueous fraction (AqF) effect on atrial contractility and investigate its mechanism of action. The AqF effect on the cardiac contractility was studied on isolated electrically driven guinea pig left atria. Atropine and tetraethylammonium (TEA) were employed to investigate whether potassium contributes for the inotropic mechanism of the AqF. The role of calcium in this effect was also studied. This was done by analysing the AqF effect on the Bowditch's phenomenon, as well as by studying whether it could interfere with the concentration–effect curve for CaCl2, isoproterenol, and BAY K8644. Mice isolated cardiomyocytes were submitted to a whole-cell patch-clamp technique in order to evaluate whether the L-type calcium current participates on the AqF effect. Furthermore, the intracellular calcium transient was studied by confocal fluorescence microscopy. AqF depressed the atrial contractile force. It was the most potent fraction from C. spiralis leaf (EC50=305±41mg/l) (crude extract: EC50=712±41; ethyl acetate: EC50=788±121; chloroform: EC50=8948±1346mg/l). Sodium and potassium content in the AqF was 0.15mM and 1.91mM, respectively. Phytochemical analysis revealed phenols, tannins, flavones, xanthones, flavonoids, flavonols, flavononols, flavonones, and saponins. Experiments with atropine and TEA showed that potassium does not participate of the inotropic mechanism of AqF. However, this fraction decreased the force overshoot characteristic of the Bowditch's phenomenon, and shifted the concentration–response curve for CaCl2 (EC50 from 1.12±0.07 to 7.23±0.47mM) indicating that calcium currents participate on its mechanism of action. Results obtained with isoproterenol (1–1000pM) and BAY K8644 (5–2000nM) showed that AqF abolished the inotropic effect of these substances. On cardiomyocytes, 48mg/l AqF reduced (∼23%) the L-type calcium current density from −6.3±0.3 to −4.9±0.2 A/F (n=5 cells, p