Introduction Sinus node (SN) dysfunction is a common clinical condition. When the sinus node fails, there are subsidiary atrial pacemakers (SAP) that can take over as the leading pacemaker. In the goat heart, we compared the structures of the SAP and SN.

Methods Adult female goats (n = 8) underwent either epicardial SN ablation (n = 4) sufficient to cause a ~50% fall in the heart rate or had a sham operation (n = 4). The site of earliest activation in these hearts was localised after 4 weeks using in vivo epicardial mapping technique. Preparations of SN (n = 4) and SAP (n = 4) were harvested and frozen, serially sectioned and stained for histology (Masson’s trichrome) and immunolabelled for connexin43 (Cx43; major connexin in the working myocardium; negative marker for nodal cells) HCN4 (main pacemaker channel; positive marker of nodal cells) and Sodium-Calcium Exchanger (NCX1; major ion-channel of the "calcium-clock"). Images were taken with light and confocal microscopy. The extent and morphology of the SN and SAP were determined. Comparative analysis of Cx43, HCN4 and NCX1 protein expression in the SN, SAP and atrial myocardium (AM) was performed.

Results In control goats, earliest activation was in the SN, whereas after SN ablation earliest activation was in the SAP. The SN was located in the intercaval region parallel to the crista terminalis (CT) and extended half of its length in the long axis towards the inferior vena cava (IVC). The centre of the SN occupied the full thickness of the intercaval region. The SN, as in other species, consisted of cells, which were small, lightly stained (compared to atrial myocytes) and embedded in a network of connective tissue. The SAP was located along the caudal part of the CT near the IVC. The morphology of the SAP was intermediate to nodal and atrial tissue and it contained a mixture of atrial and nodal cells. HCN4 and NCX1 expression was SN=SAP >AM (p < 0.05) whereas Cx43 expression was AM = SAP >SAN (p < 0.05).

Conclusions The SAP in the goat is situated caudal to the SN and expresses in abundance both HCN4, and NCX1 the principal ion-channels of the "membrane" and "calcium-clock" respectively; allowing pacemaking function. Furthermore, its anatomical location and its mixture of atrial and nodal cells suggest it is similar to the ‘paranodal area’ recently described in the human. Because of the subsidiary pacemaker function of this region and its sinus node-like ion channel expression, it may provide a useful target for novel methods of ‘biological pacemaking’.