Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, PC253
Effects of anandamide on electrical activity and contraction in guinea pig cardiac ventricular myocytes
E. L. Bolton1, D. A. Terrar1
1. Department of Pharmacology, University of Oxford, Oxford, United Kingdom.
The endocannabinoid, anandamide, not only reduces left ventricular developed pressure in isolated hearts (Ford et al., 2002), but also has anti-arrhythmic effects (Krylatov et al., 2002). Although the actions of endocannabinoids are generally attributed to their association with cannabinoid CB1 or CB2 receptors, the identity of the receptor(s) or target(s) mediating the cardiac effects of anandamide are unknown. Objectives of this study were to investigate the effects of anandamide on electrical and contractile activity of isolated guinea pig ventricular myocytes, and to characterise pharmacologically any receptor(s) involved. Myocytes were stimulated to fire action potentials by a 2 ms depolarising current pulse applied via an intracellular microelectrode at a frequency of 1 Hz, and contraction measured using an edge detection system. Whole cell L-type Ca2+ currents were measured using the single electrode voltage clamp technique. Data are means ± S.E.M., compared by ANOVA. Exposure of myocytes to anandamide (1, 3, 10 μM) resulted in a concentration dependent reduction in action potential duration (APD). After 10 minutes, 10 μM anandamide reduced APD at 90 % repolarisation by 39 ± 7 % (n = 7, p < 0.001), which was accompanied by a reduction in the amplitude of myocyte contraction. Furthermore, the peak amplitude of L-type Ca2+ currents was reduced by 57 ± 11 % at 0 mV (n = 6, p < 0.01). In a separate set of experiments, 10 µM R-(+)- methanandamide (a non-hydrolysable analogue of anandamide) caused a similar reduction in APD (34 ± 6 % at 90 % repolarisation; n = 6, p < 0.001) and amplitude of contraction, showing that the actions of anandamide appear not to occur as a result of hydrolysis to arachidonic acid. The reduction in APD in response to anandamide (10 µM) was partially inhibited by the CB2 receptor antagonist AM 630 (1 µM), but the effects were smaller than predicted based on a Ki value of 31 nM for CB2 receptors. The CB1 receptor antagonist AM 281 (1 µM), the vanilloid VR1 receptor antagonist capsazepine (1 µM) and O-1918 (1 µM; an antagonist of the proposed ‘non CB1/non CB2’ receptor; Offertáler et al., 2003) failed to inhibit the response. In addition, the selective CB1 receptor agonist ACEA (10 µM) and the selective CB2 receptor agonist HU-308 (10 µM) were without significant effect on APD compared to equivalent concentrations of vehicle. Based on these data, the reduction in APD and contraction observed can, at least in part, be accounted for by the inhibition of L-type Ca2+ currents in response to anandamide. 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.
Where applicable, experiments conform with Society ethical requirements