Modulation of ion channel function by anandamide in rat atrioventricular nodal myocytes

Kathryn Yuill. University of Bath, Pharmacy and Pharmacology, Bath BA2 7AY, United Kingdom.

The atrioventricular node (AVN) is the primary conduction pathway from the atria to the ventricles and is essential to normal cardiac function. This region displays unique properties, acting to slow conduction from atria to ventricles, thus timing ventricular filling. Modulation of AVN function can therefore result in observable bradycardias and AV block. Cannabinoids (CB) can modulate neuronal, vascular, and ventricular ion channels through CB receptor-dependent and independent actions (Pertwee, 2005; Hiley, 2009). At present, their effects on the cellular electrophysiological characteristics of the cardiac conduction system are unknown. The present study has examined the effect of the endogenous CB receptor agonist, anandamide, on isolated AVN myocytes.

Adult male Wistar rats (200-250g) were killed humanely by cervical dislocation. The heart was rapidly removed and retrogradely perfused with an enzymatic solution (containing protease, collagenase and bovine serum albumin), using a modified langendorff apparatus. The AVN region was excised and cells isolated using a combination of mechanical and enzymatic dispersion (Hancox et al., 1993; Yuill et al., 2010). Electrophysiological recordings were only conducted on healthy AVN myocytes exhibiting regular spontaneous activity. All recordings were carried out at 37°C, using the perforated patch mode of the voltage clamp technique, using standard physiological potassium-based solutions. Statistical analysis was performed using a paired t-test, with Bonferroni post-hoc correction.

Bath application of the 1μM anandamide altered spontaneous action potential activity. An increase in action potential duration was observed at 90% repolarisation (APD90) from 102.6 ± 2.3ms to 146.3 ± 3.5ms (P<0.05, n = 5). Anandamide exposure also increased the duration but reduced the amplitude of the maximum diastolic potential by 8.1 ± 0.3mV (n = 5, P<0.05), resulting in a reduction of the overall spontaneous action potential firing rate. Recordings of membrane current revealed that the external application of 1μM anandamide, induced a significant reduction in the amplitude of both the L-type calcium current (ICaL; n = 5), and the hyperpolarisation activated ‘funny’ current (If; n = 5).

These findings show that the cannabinoid receptor agonist, anandamide, attenuates spontaneous action potentials in the AVN, via actions on ICaL, and If. These findings may help to further elucidate the actions of cannabinoids on the cardiac conduction system, but further work is required to elucidate the mechanisms by which these effects are mediated.

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Pertwee, R.G (2005). Handb Exp Pharmacol. 168: 1-51.

Hancox, J.C. et al., (1993). Am. J. Physiol. 265: H755-766

Yuill, K.H et al., (2010). Exp. Physiol. 95 (4): 508-517.