Modulation of Spontaneous Action Potential Rate by Inositol Trisphosphate in Myocytes from the Rabbit Atrioventricular Node

The atrioventricular node (AVN) is a key component of the cardiac conduction system and takes over pacemaking of the ventricles if the sinoatrial node fails. IP₃ (inositol 1,4,5 <i>tris</i>phosphate) can modulate excitability of myocytes from other regions of the heart, but it is not known whether IP₃ receptor (IP₃-R) activation modulates AVN cell pacemaking. Consequently, this study investigated effects of IP₃ on spontaneous action potentials (APs) from AVN cells isolated from rabbit hearts. Immunohistochemistry and confocal imaging demonstrated the presence of IP₃-R2 in isolated AVN cells, with partial overlap with RyR2 ryanodine receptors seen in co-labelling experiments. In whole-cell recordings at physiological temperature, application of 10 µM membrane-permeant Bt₃-(1,4,5)IP₃-AM accelerated spontaneous AP rate and increased diastolic depolarization rate, without direct effects on I_Ca,L, I_Kr, I_f or I_NCX. By contrast, application via the patch pipette of 5 µM of the IP₃-R inhibitor xestospongin C led to a slowing in spontaneous AP rate and prevented 10 µM Bt₃-(1,4,5)IP₃-AM application from increasing the AP rate. UV excitation of AVN cells loaded with caged-IP₃ led to an acceleration in AP rate, the magnitude of which increased with the extent of UV excitation. 2-APB slowed spontaneous AP rate, consistent with a role for constitutive IP₃-R activity; however, it was also found to inhibit I_Ca,L and I_Kr, confounding its use for studying IP₃-R. Under AP voltage clamp, UV excitation of AVN cells loaded with caged IP₃ activated an inward current during diastolic depolarization. Collectively, these results demonstrate that IP₃ can modulate AVN cell pacemaking rate.