Cellular and Molecular Pathobiology of the Cardiac Conduction System

Sinoatrial Node Anatomy

Originally described by the pioneering work of Keith, Mackenzie, Leipzig, Aschoff, Tawara, and Flack over a century ago, the human sinoatrial node ( Figure 7.2 ) is called a ‘spindle’- or ‘cresent’-shaped collection of excitable cells located in a subepicardial location juxtaposed with the crista terminalis (terminal crest). The cells are surrounded by a dense and fibrous tissue environment usually 10–20 mm in length and 2–3 mm in width. The position of the node within the right atria is most often located 1 mm from the surface of the epicardium, laterally in the sulcus terminalis of the right atria (near the junction of the right atrium and the superior vena cava; Figure 7.2 ). The precise position of the SAN may vary by individual, but is usually localized in an environment that includes a nodal artery.

Sinoatrial Node Cell Morphology

The pacemaker is a collection of weakly coupled, heterogeneous cells, including pacemaker cells as well as non-pacemaker cells such as atrial myocytes, adipocytes and fibroblasts. Within the node, pacemaker cells vary by size and electrophysiological properties with smaller cells showing slower intrinsic firing concentrated in the SAN center and larger, faster-firing cells in the periphery. Pacemaker cells have been successfully isolated from a wide range of species and can be separated into three main categories based on gross morphology. Spindle-shaped cells are relatively small with total membrane capacitance less than 30 pF and sparse apparent myofilaments ( Figure 7.3 ). Elongated cells are larger (membrane capacitance between 35 pF and 50 pF) especially in the longitudinal axis and have a higher myofilament density. Finally, spider cells are distinguished by their branched appearance, although the functional consequences are unclear. It is commonly believed that spindle-shaped cells are concentrated in the central SAN and serve as the leading pacemakers, while elongated cells are more likely found in the periphery. As discussed in the next section, these morphologically distinct cells possess unique electrophysiological properties that are critical for synchronized pacemaking.

Sinoatrial Node Action Potential and Electrical Components

The SAN action potential is distinct from that measured in atrial or ventricular cells ( Figure 7.4 ). Most importantly, unlike ventricular or atrial cells, the SAN action potential shows a phase 4 spontaneous depolarization phase with a maximum diastolic value around –60 mV (compared to rest potential close to –90 mV for ventricular myocytes) that eventually reaches the threshold of action potential generation without an external depolarizing stimulus. Control of this spontaneous depolarization phase is critical for pacemaking and involves the coordinated effort of multiple ion channels/transporters/exchangers.

Pacemaking in automatic cells from the SAN depends on a unique electrophysiological profile that allows for generation of spontaneous action potentials. One key channel highly expressed in pacemaker cells is the f- or ‘funny’ channel (primarily HCN4 in the SAN), a hyperpolarization-activated channel that is permeable to both Na ⁺ and K ⁺ . F-channel current ( I _f ) is unique in that it was the first voltage- and time-dependent current that was discovered to be activated upon hyperpolarization of the membrane (rather than depolarization like most other channels). The mixed conductance of Na ⁺ and K ⁺ provides a reversal potential for the channel around –10 mV so that during diastole, the current is depolarizing. I _f is believed to be a key determinant of intrinsic excitability of SAN pacemaker cells. Importantly, these channels are heavily regulated by β-adrenergic and muscarinic receptor activation mediated in large part by direct activation by cAMP.

Expression of Na ⁺ channels is highly heterogeneous in the pacemaker with variability across species. In general, I _Na is higher in mouse than in larger animals (e.g. rabbit). Both tetrodotoxin (TTX)-sensitive neuronal-type channels and TTX-resistant cardiac isoforms have been found in adult mouse SAN cells. In rabbit, I _Na is found in larger cells from the SAN periphery but not in smaller, central SAN. Furthermore, I _Na measured in larger rabbit cells shows increased TTX sensitivity compared to that measured in ventricular myocytes, indicating a greater contribution from neuronal isoforms (e.g. Na _v 1.1). These findings are significant for the design of Na+ channel-based therapies for cardiac conduction disease.

L-type voltage-gated Ca ²⁺ channels are essential for generating the SAN action potential upstroke, which as a consequence is much slower than the upstroke recorded in ventricular or atrial myocytes. In SAN cells, Ca ²⁺ current is contributed to by a variety of channels including multiple isoforms of L- and T-type channels. L-type Ca ²⁺ current in SAN cells is likely the result of both Ca _v 1.2 and Ca _v 1.3 channels, unlike ventricular I _Ca , which is predominantly Ca _v 1.2. Ca _v 1.3 channels activate earlier (at more negative potentials) and inactivate more slowly compared to Ca _v 1.2.

An important electrophysiological feature of SAN cells is their low expression of inward rectifier K ⁺ channels that regulate I _K1 , a current responsible for maintaining the stable resting potential in atrial and ventricular myocytes ( Figure 7.4 ). The lack of this major repolarizing current facilitates the spontaneous diastolic depolarization phase in these cells and increases the importance of other K ⁺ channel currents (e.g. I _Kr , I _Ks ) in controlling the maximum diastolic potential. In mouse and rabbit SAN, I _Kr block using E-4031 depolarizes the maximum diastolic potential, decreases the action potential amplitude, and delays repolarization leading to a decrease in SAN cell spontaneous firing rate, although to varying degrees depending on species. In contrast, larger animals such as the pig seem to depend more heavily on I _Ks for control of maximum diastolic potential. The transient outward K ⁺ current ( I _to ) is also expressed in SAN and displays a heterogeneous distribution with increasing density as cells become larger (i.e. from SAN center to periphery). I _to likely regulates SAN repolarization but not maximum diastolic potential or firing rate. In addition to I _Kr , I _Ks , and I _to, multiple additional K ⁺ channel currents play important regulatory roles in the SAN. Notably, SAN cells express acetylcholine-activated K ⁺ channels (Kir3.1/Kir3.4) that give rise to an inward rectifying current ( I _KACh ), that is activated by muscarinic and adenosine receptor agonists. Also found in SAN cells are K _ATP channels ( I _KATP ) that activate under conditions of low ATP (e.g. myocardial ischemia) to help decrease pacemaker activity and slow heart rate.

Normal homeostasis of intracellular ions (Na ⁺ , Ca ²⁺ , K ⁺ , Cl ^– ) is tightly controlled in SAN cells through similar mechanisms as found in atrial and ventricular cells. In particular, just as in other cell types, the sarcolemmal Na ⁺ /Ca ²⁺ exchanger (NCX) exploits the Na ⁺ gradient to extrude Ca ²⁺ from inside the cell (forward mode) and help maintain normal homeostasis of intracellular Ca ²⁺ . Importantly, in the forward mode, NCX generates a depolarizing current, providing a potential link between intracellular Ca ²⁺ /Na ⁺ levels and membrane excitability. In particular, I _NCX has been proposed to be an important determinant of the diastolic depolarization rate. In contrast, the Na ⁺ /K ⁺ ATPase extrudes three Na ⁺ from the cell for two K ⁺ generating a net repolarizing current that is thought to help set the maximum diastolic potential. Finally, while diastolic depolarization rate (DDR) is heavily influenced by activity of I _f , recent studies demonstrate that intracellular Ca ²⁺ cycling is yet an additional important pathway for controlling DDR and thus heart rate. Specifically, local Ca ²⁺ release from sarcoplasmic reticulum (SR) ryanodine receptor Ca ²⁺ channels has been measured during diastole and is believed to facilitate spontaneous depolarization through forward-mode Na/Ca ²⁺ exchange, which generates a depolarizing current that can affect the rate of spontaneous diastolic depolarization.

Catecholamine-dependent increases in heart rate (positive chronotropy) depend on activation of beta-adrenergic receptors and subsequent activation of a myriad of sarcolemmal and SR ion channels, exchangers and transporters. Increases in cAMP associated with beta-adrenergic receptor activation directly modulate the activities of several channels, including I _f , while downstream activation of protein kinase A leads to enhanced phosphorylation and activity of L-type Ca ²⁺ channels, SR uptake, and release.