P381The right atrioventricular pacemaker ring in the avian heart, myth or reality?

Abstract

Background: In the avian heart right atrioventricular (AV) valve there is a large, unicuspid, muscular flap, in remarkable contrast to the fibrous structure characteristic of the mammalian tricuspid valve. Histological studies showed that in addition to a sinus node, AV node, His bundle, left and right bundle branches, the birds also have the right AV Purkinje ring and a special middle bundle branch. The atrioventricular Purkinje ring is located in the atrial sheet of the AV muscular valve along all its base length. Purpose: To perform electrophysiological identification of cells on the atrial side of the chicken right muscular AV valve. Methods: Experiments were conducted on spontaneously contracting right muscular AV valve and contiguous atrial muscle preparations isolated from nine adult chicken hearts using the standard microelectrode technique. Results: The rate of spontaneous contractions of isolated preparations of muscular valve was 121 ± 7 beats/min. Three types of action potentials (APs) were registered from the atrial surface of the valve: APs of working myocardium cells, APs of pacemaker cells and APs of conducting cells. A working myocardium AP with a constant level of membrane potential during phase 4 was the most frequently recorded voltage waveform in the atrial surface of the muscular valve. Pacemaker APs have much lesser values of a maximum upstroke rate (4.3 ± 0.6 vs 193.9 ± 4.9 V/s), an amplitude (73.4 ± 1.7 vs 108.7 ± 1.0 mV ) but higher an amplitude of diastolic depolarization (7.7 ± 0.8 vs 1.4 ± 0.1 mV) than conducting APs (P<0.05). Conducting APs are usually registered from the part of the free edge of the valve, attached to the right ventricle free wall and also from the areas adjacent to dorsal and ventral sides of the interventricular septum. Detailed microelectrode mapping of APs in an isolated muscular valve preparation revealed the presence of the pacemaker zone on the valve atrial side which is permanently localized along the whole length of its base from ventral to dorsal attachment of the valve to the interventricular septum where the fibrous tissue is present. Conclusion: Our study provides evidence of the regularity of the course and the electrophysiological organization of the pacemaker zone in isolated chicken muscular valve. Location of found out pacemaker zone in the muscular valve coincide with histological description of topography of valve component of the right AV Purkinje ring. We believe that we have offered the proof that the avian right AV Purkinje ring is in reality the right AV pacemaker ring. This study was supported by the RFBR.