Abstract
The sinoatrial node (SAN) locates in the right atrium at the junction of the crista terminals and generates the pacemaker activity. Little is known about structural correlates of complex behaviors of SAN such as pacemaker shift. We visually analyzed the spatial distribution of action potential properties in the neonatal rat SAN using a voltage sensitive dye, di-4-ANEPPS. Two parameters, upstroke time and action potential duration at 30% repolarization level (APD30) were calculated. Although these parameters showed roughly downward gradients from the leading pacemaker site, pseudocolor images of both upstroke time and APD30 exhibited patchy appearances due to the existence of multiple local maxima. The leading pacemaker activity site occurred at one of these local maxima. Further, pacemaker activity was affected by internal conditions such as gap junction conductance. A gap junction blocker, 100 μM carbenoxolone, reduced conduction velocity from 134±8 to 62±11 μm/s without affecting beating rate (n=7). Carbenoxolone changed action potential properties and shifted the leading pacemaker site in all cases. The new leading pacemaker site occurred in one of the regions where upstroke time and APD30 were at maximum levels even before the application of carbenoxolone. These results indicate that both intrinsic cellular properties and mutual interactions through gap junctions contribute to determine the action potential characteristics of SAN cells and leading pacemaker site. [J Physiol Sci. 2007;57 Suppl:S210]
