Effects of Anandamide on Electrical Activity, Calcium Transients and Contraction in Guinea Pig Cardiac Ventricular Myocytes

Abstract

Anandamide is an endocannabinoid which reduces cardiac contractility. Although the effects of endocannabinoids are generally attributed to their association with G-protein-coupled cannabinoid CB1 and CB2 receptors, the identity of the receptor(s) or target(s) responsible for the effects of anandamide in the heart are unknown.Anandamide (10 μM) reduced action potential (AP) duration and contraction in guinea pig ventricular myocytes stimulated to fire APs using an intracellular microelectrode. In addition, anandamide reduced peak amplitude of L-type Ca2+ currents over the range −30 to + 60 mV (switched voltage clamp, step depolarisations from −40 mV), with a reduction in peak current at 0 mV of 57 ± 11 % (n = 6, p < 0.01). Ca2+ transients measured in field-stimulated cells using Fluo-5F were also reduced. The reduction in AP duration in response to anandamide was partially inhibited by the CB2 receptor antagonist AM 630 (1 μM), but effects appeared non-competitive, and were smaller than predicted based on a Ki value of 31 nM for CB2 receptors. The CB1 receptor antagonist AM 281 (1 μM), the TRPV1 receptor antagonist capsazepine (1 μM), and O-1918 (an antagonist of a reported non CB1/non CB2 cannabinoid receptor; 1 μM) failed to inhibit responses to 10 μM anandamide. Furthermore, the selective CB1 receptor agonist ACEA, and the selective CB2 receptor agonist HU-308 (both 10 μM) were without significant effect on AP duration.The effects of anandamide can, at least in part, be accounted for by an inhibition of L-type Ca2+ currents. However, additional effects on other ion channels or intracellular targets cannot be ruled out. It appears that if a cannabinoid receptor mediates these effects, the characteristics of this receptor are not those expected of conventional CB1 or CB2 receptors.