Cannabinoid CB 1 and CB 2 receptors antagonists AM251 and AM630 differentially modulate the chronotropic and inotropic effects of isoprenaline in isolated rat atria.

BACKGROUND: Drugs targeting CB1 and CB2 receptors have been suggested to possess therapeutic benefit in cardiovascular disorders associated with elevated sympathetic tone. Limited data suggest cannabinoid ligands interact with postsynaptic β-adrenoceptors. The aim of this study was to examine the effects of CB1 and CB2 antagonists, AM251 and AM630, respectively, at functional cardiac β-adrenoceptors.

METHODS: Experiments were carried out in isolated spontaneously beating right atria and paced left atria where inotropic and chronotropic increases were induced by isoprenaline and selective agonists of β1 and β2 -adrenergic receptors.

RESULTS: We found four different effects of AM251 and AM630 on the cardiostimulatory action of isoprenaline: (1) both CB receptor antagonists 1 μM enhanced the isoprenaline-induced increase in atrial rate, and AM630 1 μM enhanced the inotropic effect of isoprenaline; (2) AM251 1 μM decreased the efficacy of the inotropic effect of isoprenaline; (3) AM251 0.1 and 3 μM and AM630 3 μM reduced the isoprenaline-induced increases in atrial rate; (4) AM630 0.1 and 3 μM enhanced the inotropic effect of isoprenaline, which was not changed by the same concentrations of AM251.

CONCLUSIONS: Our results show that the CB1 and CB2 receptor antagonists AM251 and AM630 have bidirectional effects on the cardiostimulatory action of isoprenaline, most likely related to an interaction with β1 -adrenoceptors. Provided that the results translate to human heart, caution should be taken when using CB1 and CB2 receptor antagonists, as an enhanced sympathetic tone accompanies many cardiovascular disorders.