Cannabinoids (CBs) are known to alter coronary vascular tone and cardiac performance. They also exhibit cardioprotective properties, particularly in their ability to limit the damage produced by ischaemia reperfusion injury. The mechanisms underlying these effects are unknown. Here we investigate the intracellular localisation of CB receptors in the heart and examine whether they may modulate localised nuclear Ca2+ release. In isolated cardiac nuclear preparations, expression of both the inositol 1,4,5-trisphosphate receptor type 2 (IP3R) and CB receptors (CB1R and CB2R) was demonstrated by immunoblotting. Both receptors localised to the nucleus and purity of the nuclear preparations was confirmed by co-expression of the nuclear marker protein nucleolin but absence of cytoplasmic actin. To measure effects of IP3R and CBR agonists on nuclear Ca2+ release, isolated nuclei were loaded with Fluo5N-AM. This dye accumulates in the nuclear envelope. Isolated nuclei responded to IP3 with rapid and transient Ca2+ release from the nuclear envelope. Anandamide inhibited this IP3-mediated release. Preincubation of nuclear preparations with either the CB1R antagonist (AM251) or the CB2R antagonist (AM630) reversed anandamide-mediated inhibition to 80% and 60% of control values respectively. When nuclei were pre-treated with both CBR antagonists, anandamide-mediated inhibition of IP3-induced Ca2+ release was completely reversed. These results are the first to demonstrate the existence of cardiac nuclear CB receptors. They are also the first to show that anandamide can negatively modulate IP3-mediated nuclear Ca2+ release. As such, this provides evidence for a novel key mechanism underlying the action of CBs and CBRs in the heart.
- inositol trisphosphate receptor
- cannabinoid receptors
- calcium release