The ferret SA node is located at the junction of the superior vena cava and right atrium near the sulcus terminalis region (Fig. 1, A and B). Cross sections of the right atrium revealed an ovoid shaped SA node positioned epicardially (Fig. 1C). A light microscopic view of the SA node region shows two nodal arteries, centrally located, surrounded by a ring of dark cells (Fig. 1D). Larger, pale cells form a concentric ring around the dark cells (Fig. 1D). Thus, while being smaller, many features of ferret SA node displayed striking similarities to classic descriptions of human SA node (29; 69; 70).
To characterize specific anatomical and functional regions of ferret SA node, we next ...view middle of the document...
2E). Our TnIS and TnIC data demonstrate that these two transcripts are useful markers for distinguishing central nodal (TnIS) versus outer (TnIC) regions in the ferret.
In several species, the SA node is well innervated by the autonomic nervous system, which plays an important role in SA nodal function (29; 30; 43; 68; 70). Because the presence of neuronally expressed ion channels could confound our analysis, we sought to indentify autonomic nerve fibers within the ferret SA node. A well-characterized marker of autonomic nerves is GAP-43, a protein associated with axonal growth and regeneration, which has been identified in cultured sympathetic neurons, cholinergic neurons, and sympathetic and parasympathetic autonomic ganglia of normal adult rats (1; 13; 25; 73; 77; 78). We observed GAP-43 fluorescence as filamentous tracts within both the central and surrounding SA nodal regions (Fig. 2F). There was less GAP-43 surrounding the SA nodal arteries. Further, our data showed marked differences between the widespread filamentous pattern of antibody to GAP-43 and the antibody to NF. While both NF and TnIS were localized within the central position of the SA node, the distribution of NF was more confined than that of TnIS.
II) Distribution and Expression Levels of Cation Ion Channel Transcripts
To understand the contributions of different cation channels to cardiac pacemaker activity, we examined the distribution of channel transcripts using FISH. This analysis included the four hyperpolarization-activated cation channels (HCN1-4), six voltage-gated Na+ channels (Nav1.1, Nav1.3-5, Nav1.8 and 1.9), three voltage-gated Ca2+ channels (Cav1.2, Cav3.1 and 3.2) and 20 voltage-gated K+ channel subunits from multiple families (Kv1.1-1.6, Kv2.1 and 2.2, Kv3.1-4, Kv4.1-3, Kv7.1, Kv11.1-3). We also probed for three K+ channel ancillary subunits (minK, Kv5.1, Kv6.1).
We detected transcript for all 33 ion channels and subunits in the ferret SA node. It was perhaps surprising that the same transcripts were detected in both the SA node and right atrial appendage despite the marked differences in their electrophysiological properties. While it is was clear that a transcript is more abundant in some areas, caution should be exercised in comparing the relative abundance of different transcripts, because there are several well known factors that confound this type of comparison. For example, the efficiency of hybridization and labeling may vary between probes. With these caveats in mind, we quantified the fluorescence emitted by each transcript probe relative to an internal standard. Signals from the central nodal region, and the right atrial appendage are shown in Table 2.
The strongest fluorescent signals were obtained from Kv1.1, Kv1.4, Kv1.5, and the T-type calcium channel transcripts, Cav3.1 and Cav3.2; all had signals greater than 62 relative fluorescence units (RFU). Several channels were apparently expressed at somewhat lower...