The Vena Cava Is Pacing The Embryonic Heart

Abstract

The mechanism and location of the pacemaker in the embryonic heart is highly controversial due to the lack of physiological in vivo recordings. Here we report fluorescent macroscopic in vivo recordings of embryonic hearts (E12.5-E14.5) from mice with cardiac expression (α-MHC or Cx40 promotor) of the Ca2+ sensor GCaMP2.Initial observations from the ventral surface showed regular uniform Ca2+ transients in the atrium, ventricle and a structure below/behind the right atrium that preceded every atrial activation. To better understand the origin of these Ca2+ transients we established a dorsal preparation leaving the heart and veins intact. Ca2+ transients activated in the region of the putative sinus node, propagated bidirectionally along the superior right and in a u-shaped pattern into the coronary sinus and left superior vena cava, and conducted faster (24±4 mm/sec) than the atria (15±2 mm/sec; n=5; p<0.02); we have termed this the “streak”. In most hearts the streak fired before every atrial activation with a delay of 79±10 ms (n=12); variations in this delay was not dependent on the heart rate (104±15 bpm, n=12). Some hearts showed 2:1 coupling and in others only the streak fired at 178±39 bpm (n=5). The streak contracted, could be electrically paced and spontaneous local field potentials were recorded as sharp spikes at the onset of Ca2+ transients. In some experiments we observed a diastolic Ca2+ increment just prior the field potential, in line with the proposed role of Ca2+ oscillations for the initiation of pacemaking in embryonic heart cells.The spontaneous activity of the streak prior to atrial activation, higher intrinsic frequency of this region, and diastolic Ca2+ release establish the pacemaker function of the streak. Thus, cardiomyocytes within/on the vena cava walls are involved in murine embryonic heart pacemaker activity. Values: mean±SEM.