Effects of cannabidiol on contractions and calcium signaling in rat ventricular myocytes.

Cannabidiol (CBD), a major nonpsychotropic cannabinoid found in Cannabis plant, has been shown to influence cardiovascular functions under various physiological and pathological conditions. In the present study, the effects of CBD on contractility and electrophysiological properties of rat ventricular myocytes were investigated. Video edge detection was used to measure myocyte shortening. Intracellular Ca(2+) was measured in cells loaded with the Ca(2+) sensitive fluorescent indicator fura-2 AM. Whole-cell patch clamp was used to measure action potential and Ca(2+) currents. Radioligand binding was employed to study pharmacological characteristics of CBD binding. CBD (1µM) caused a significant decrease in the amplitudes of electrically evoked myocyte shortening and Ca(2+) transients. However, the amplitudes of caffeine-evoked Ca(2+) transients and the rate of recovery of electrically evoked Ca(2+) transients following caffeine application were not altered. CBD (1µM) significantly decreased the duration of APs. Further studies on L-type Ca(2+) channels indicated that CBD inhibits these channels with IC50 of 0.1µM in a voltage-independent manner. Radioligand studies indicated that the specific binding of [(3)H]Isradipine, was not altered significantly by CBD. The results suggest that CBD depresses myocyte contractility by suppressing L-type Ca(2+) channels at a site different than dihydropyridine binding site and inhibits excitation-contraction coupling in cardiomyocytes.

Effects of endogenous cannabinoid anandamide on cardiac Na⁺/Ca²âº exchanger.

Endocannabinoid anandamide (N-arachidonoyl ethanolamide; AEA) has been shown to cause negative inotropic and antiarrhythmic effects in ventricular myocytes. In this study, using whole-cell patch clamp technique, we have investigated the effects of AEA on cardiac Na(+)/Ca(2+) exchanger (NCX1)-mediated currents. AEA suppressed NCX1 with an IC50 value of 4.7 µM. Both inward and outward components of exchanger currents were suppressed by AEA equally. AEA inhibition was mimicked by the metabolically stable analogue, methanandamide (metAEA, 10 µM) while it was not influenced by inhibition of fatty acid amide hydrolase with 1 µM URB597 incubation. The effect of AEA, was not altered in the presence of cannabinoid receptor 1 and 2 antagonists AM251 (1 µM) and AM630 (1 µM), respectively. In addition, inhibition by AEA remained unchanged after pertussis toxin (PTX, 2 µg/ml) treatment or following the inclusion of GDP-ß-S (1 mM) in pipette solution. Currents mediated by NCX1 expressed in HEK-293 cells were also inhibited by 10 µM AEA a partially reversible manner. Confocal microscopy images indicated that the intensity of YFP-NCX1 expression on cell surface was not altered by AEA. Collectively, the results indicate that AEA directly inhibits the function of NCX1 in rat ventricular myocytes and in HEK-293 cells expressing NCX1.