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. IP 3 (inositol 1,4,5 tris phosphate) can modulate excitability of myocytes from other regions of the heart, but it is not known whether IP 3 receptor (IP 3 -R) activation modulates AVN cell pacemaking. Consequently, this study investigated effects of IP 3 on spontaneous action potentials (APs) from AVN cells isolated from rabbit hearts. Immunohistochemistry and confocal imaging demonstrated the presence of IP 3 -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 Bt 3 -(1,4,5)IP 3 -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 IP 3 -R inhibitor xestospongin C led to a slowing in spontaneous AP rate and prevented 10 µM Bt 3 -(1,4,5)IP 3 -AM application from increasing the AP rate. UV excitation of AVN cells loaded with caged-IP 3 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 IP 3 -R activity; however, it was also found to inhibit I Ca,L and I Kr , confounding its use for studying IP 3 -R. Under AP voltage clamp, UV excitation of AVN cells loaded with caged IP 3 activated an inward current during diastolic depolarization. Collectively, these results demonstrate that IP 3 can modulate AVN cell pacemaking rate.