Organisation of the mouse sinoatrial node: structure and expression of HCN channels.
OBJECTIVE
To reveal the structural characteristics of the sinoatrial node (SAN) and the distribution of hyperpolarization-activated cyclic nucleotide-gated cation channels (HCN) in the SAN in the mouse.


METHODS
The structure of the SAN and the distribution of HCN channels in the SAN in the mouse were studied by histology and immunolabelling of ANP, Cx43 and HCN channels.


RESULTS
The mouse SAN is a comma-shaped structure with a length of approximately 1.5 mm parallel to the crista terminalis and is separated from atrial muscle by connective tissue at the border both with the crista terminalis and the atrial septum. A unique compact nodal structure with densely-packed nodal cells was identified at the head of the comma-shaped SAN. Cell size and fibre orientation vary regionally in the SAN: the cells in the compact node are small and are orientated perpendicular to the crista terminalis, whereas the cells in the more inferior part are larger and more loosely-packed and are orientated parallel to the crista terminalis. All SAN cells exhibited labelling of HCN4, but no cell exhibited detectable labelling of HCN1, HCN2, ANP and Cx43, while surrounding atrial cells exhibited labelling of ANP and Cx43, but not HCN1, HCN2 and HCN4. A specialised interface between the SAN and surrounding atrial muscle was also identified: strands of HCN4-positive nodal cells protrude into the atrial muscle and strands of Cx43-positive atrial cells protrude into the SAN; thus, there are interdigitations between the SAN and atrial muscle.


CONCLUSIONS
In the mouse, (i) the SAN is structurally complex with a densely-packed head and loosely-packed tail; (ii) HCN4 is the only HCN isoform detectable and is present throughout the SAN; and (iii) there is a specialised interface between the SAN and surrounding atrium that may be necessary for the SAN to drive the more hyperpolarized atrial muscle.
Liu
Jie
J
University Laboratory of Physiology, University of Oxford, Oxford OX1 3PT, UK.
Dobrzynski
Halina
H
Yanni
Joseph
J
Boyett
Mark R
MR
Lei
Ming
M
D000200:Action Potentials; D000818:Animals; D009320:Atrial Natriuretic Factor / Q000032:analysis; D015415:Biomarkers / Q000032:analysis; D018031:Connexin 43 / Q000032:analysis; D054815:Cyclic Nucleotide-Gated Cation Channels; D006325:Heart Atria / Q000737:chemistry / Q000378:metabolism; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D007150:Immunohistochemistry; D015640:Ion Channel Gating / Q000502:physiology*; D007473:Ion Channels / Q000032:analysis; D008297:Male; D008564:Membrane Potentials; D051379:Mice; D008810:Mice, Inbred C57BL; D018408:Patch-Clamp Techniques; D015221:Potassium Channels / Q000032:analysis*; D012849:Sinoatrial Node / Q000033:anatomy & histology* / Q000378:metabolism*
D015415:Biomarkers; D018031:Connexin 43; D054815:Cyclic Nucleotide-Gated Cation Channels; C578666:Hcn1 protein, mouse; C578711:Hcn2 protein, mouse; D064428:Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels; D007473:Ion Channels; D015221:Potassium Channels; D009320:Atrial Natriuretic Factor