Evidence of a Novel Site Mediating Anandamide-induced Negative Inotropic and Coronary Vasodilatator Responses in Rat Isolated Hearts

1. Cannabinoids are known to cause coronary vasodilatation and reduce left ventricular developed pressure (LVDP) in isolated hearts although the identity of the receptor(s) mediating these responses is unknown. Our objective was to pharmacologically characterize cannabinoid receptors mediating cardiac responses to the endocannabinoid, anandamide. 2. Dose-response curves for coronary perfusion pressure (CPP) and LVDP were constructed to anandamide, R-(+)-methanandamide, palmitoylethanolamide (PEA) and JWH015 in isolated Langendorff-perfused rat hearts. Anandamide dose-response curves were also constructed in the presence of antagonists selective for CB(1), CB(2) or VR(1) receptors. 3. Anandamide and methanadamide significantly reduced CPP and LVDP but the selective CB(2) receptor agonists, PEA and JWH015 had no significant effect, compared with equivalent vehicle doses. 4. Single bolus additions of the selective CB(1)-receptor agonist, ACEA (5 nmol), decreased LVDP and CPP. When combined with JWH015 (5 nmol) these responses were not augmented. 5. Anandamide-mediated reductions in CPP were significantly blocked by the selective CB(1) receptor antagonists SR 141716A (1 microM) and AM251 (1 microM) and the selective CB(2) receptor antagonist SR 144528 (1 microM) but not by another selective CB(2) receptor antagonist AM630 (10 microM) nor the vanilloid VR(1) receptor antagonist capsazepine (10 microM). 6. SR 141716A, AM281 and SR 144528 significantly blocked negative inotropic responses to anandamide that were not significantly affected by AM251, AM630 and capsazepine. 7. One or more novel sites mediate negative inotropic and coronary vasodilatatory responses to anandamide. These sites can be distinguished from classical CB(1) and CB(2) receptors, as responses are sensitive to both SR 141716A and SR 144528.