The majority of Na + channels in the heart are composed of the tetrodotoxin (TTX)-resistant (K D , 2-6 µM) Na v 1.5 isoform; however, recently it has been shown that TTX-sensitive (K D , 1-10 nM) neuronal Na + channel isoforms (Na v 1.1, Na v 1.3 and Na v 1.6) are also present and functionally important in the myocytes of the ventricles and the sinoatrial (SA) node. In the present study, in mouse SA node pacemaker cells, we investigated Na + currents under physiological conditions and the expression of cardiac and neuronal Na + channel isoforms. We identified two distinct Na + current components, TTX resistant and TTX sensitive. At 37• C, TTX-resistant i Na and TTX-sensitive i Na started to activate at ∼ −70 and ∼ −60 mV, and peaked at −30 and −10 mV, with a current density of 22 ± 3 and 18 ± 1 pA pF −1 , respectively. TTX-sensitive i Na inactivated at more positive potentials as compared to TTX-resistant i Na . Using action potential clamp, TTX-sensitive i Na was observed to activate late during the pacemaker potential. Using immunocytochemistry and confocal microscopy, different distributions of the TTX-resistant cardiac isoform, Na v 1.5, and the TTX-sensitive neuronal isoform, Na v 1.1, were observed: Na v 1.5 was absent from the centre of the SA node, but present in the periphery of the SA node, whereas Na v 1.1 was present throughout the SA node. Nanomolar concentrations (10 or 100 nM) of TTX, which block TTX-sensitive i Na , slowed pacemaking in both intact SA node preparations and isolated SA node cells without a significant effect on SA node conduction. In contrast, micromolar concentrations (1-30 µM) of TTX, which block TTX-resistant i Na as well as TTX-sensitive i Na , slowed both pacemaking and SA node conduction. It is concluded that two Na + channel isoforms are important for the functioning of the SA node: neuronal (putative Na v 1.1) and cardiac Na v 1.5 isoforms are involved in pacemaking, although the cardiac Na v 1.5 isoform alone is involved in the propagation of the action potential from the SA node to the surrounding atrial muscle.

Background-There is an effort to build an anatomically and biophysically detailed virtual heart, and, although there are models for the atria and ventricles, there is no model for the sinoatrial node (SAN

Clinical studies have shown that sinoatrial node dysfunction occurs at the highest incidence in the elderly population. Guinea-pigs were studied throughout their lifespan (i.e. birth to 38 months) to investigate the possible mechanism leading to nodal dysfunction. Using immunofluorescence with confocal microscopy, Cx43 protein expression was shown at birth to be present throughout the sinoatrial node and atrial muscle, however, at one month Cx43 protein was not expressed in the centre of the sinoatrial node. Throughout the remainder of the animal's lifespan the area of tissue lacking Cx43 protein progressively increased. Western blot provided verification by quantitative analysis that Cx43 protein expression within the sinoatrial node decreased with age; however, the expression of other cardiac connexins, Cx40 and Cx45, did not differ with age. Analysis of conduction maps showing propagation of the action potential across the sinoatrial node, from the initiation point to the crista terminalis, found that the action potential conduction time taken and conduction distance increased proportionally with age; conversely the conduction velocity decreased with age. We have shown ageing induces degenerative changes in action potential conduction, contributed to by the observed loss of Cx43 protein. Our data identify Cx43 as a potential therapeutic target for quashing the age-related deterioration of the cardiac pacemaker.

We investigated the densities of the L-type Ca(2+) current, i(Ca,L), and various Ca(2+) handling proteins in rabbit sinoatrial (SA) node. The density of i(Ca,L), recorded with the whole-cell patch-clamp technique, varied widely in sinoatrial node cells. The density of i(Ca,L) was significantly (p<0.001) correlated with cell capacitance (measure of cell size) and the density was greater in larger cells (likely to be from the periphery of the SA node) than in smaller cells (likely to be from the center of the SA node). Immunocytochemical labeling of the L-type Ca(2+) channel, Na(+)-Ca(2+) exchanger, sarcoplasmic reticulum Ca(2+) release channel (RYR2), and sarcoplasmic reticulum Ca(2+) pump (SERCA2) also varied widely in SA node cells. In all cases there was significantly (p<0.05) denser labeling of cells from the periphery of the SA node than of cells from the center. In contrast, immunocytochemical labeling of the Na(+)-K(+) pump was similar in peripheral and central cells. We conclude that Ca(2+) handling proteins are sparse and poorly organized in the center of the SA node (normally the leading pacemaker site), whereas they are more abundant in the periphery (at the border of the SA node with the surrounding atrial muscle).

Fluorescent imaging has revealed that posterior nodal extensions provide the anatomical substrate for the dual-pathway electrophysiology of the atrioventricular (AV) node during normal conduction and reentry. The reentry can be intranodal, or as well as the posterior nodal extensions, it can involve an endocardial layer of atrial/atrial-nodal (A/AN) cells as part of the AV nodal reentry (AVNR) circuit. Using fluorescent imaging with a voltage-sensitive dye and immunolabeling of Cx43, we mapped the electrical activity and structural substrate in 3 types of AVNR induced by premature atrial stimulation in 8 rabbit hearts. In 6 cases, the AVNR pathway involved (1) a fast pathway (FP), (2) the A/AN layer, and (3) a slow pathway (SP). In 4 cases, reentry took the path (1) SP, (2) A/AN layer, and (3) FP. In 2 cases, reentry was intranodal, propagating between the 2 posterior nodal extensions. Immunolabeling revealed that the FP and SP are formed by Cx43-expressing bundles surrounded by tissue without Cx43. Cx43-expressing posterior nodal extensions are the substrate of AVNR during both intranodal and extranodal reentry.

Abstract-Recent work on isolated sinoatrial node cells from rabbit has suggested that sarcoplasmic reticulum Ca 2ϩ release plays a dominant role in the pacemaker potential, and ryanodine at a high concentration (30 mol/L blocks sarcoplasmic reticulum Ca 2ϩ release) abolishes pacemaking and at a lower concentration abolishes the chronotropic effect of ␤-adrenergic stimulation. The aim of the present study was to test this hypothesis in the intact sinoatrial node of the rabbit. Spontaneous activity and the pattern of activation were recorded using a grid of 120 pairs of extracellular electrodes. Ryanodine 30 mol/L did not abolish spontaneous activity or shift the position of the leading pacemaker site, although it slowed the spontaneous rate by 18.9Ϯ2.5% (nϭ6). After ryanodine treatment, ␤-adrenergic stimulation still resulted in a substantial chronotropic effect (0.3 mol/L isoproterenol increased spontaneous rate by 52.6Ϯ10.5%, nϭ5). In isolated sinoatrial node cells from rabbit, 30 mol/L ryanodine slowed spontaneous rate by 21.5Ϯ2.6% (nϭ13). It is concluded that sarcoplasmic reticulum Ca 2ϩ release does not play a dominating role in pacemaking in the sinoatrial node. The full text of this article is available at http://www.circresaha.org. release activates inward Na ϩ -Ca 2ϩ exchange current, and this helps to generate the pacemaker depolarization. Two studies by Lakatta and coinvestigators 1,2 published recently in Circulation Research have placed the spotlight on this mechanism: Bogdanov et al 1 suggested that SR Ca 2ϩ release may be obligatory for pacemaking, because they observed that a high concentration (30 mol/L) of ryanodine, which blocks the SR Ca 2ϩ release channel, abolishes the spontaneous activity of isolated SA node cells from rabbit. Vinogradova et al 2 boldly suggested that the positive chronotropic effect of ␤-adrenergic stimulation is the result of the increase in the Ca 2ϩ transient caused by ␤-adrenergic stimulation, because they observed that the chronotropic effect in isolated SA node cells from rabbit is abolished or greatly reduced after the suppression of the Ca 2ϩ transient by a submaximal concentration of ryanodine (3 mol/L). These recent reports are surprising. Previously, the role of SR Ca 2ϩ release was thought to be more minor, because in isolated SA node cells from rabbit and guinea pig, suppression of the Ca 2ϩ transient by a variety of interventions (including up to 10 mol/L ryanodine) did not abolish pacemaking and just decreased spontaneous rate by 21% to 37%. [3][4][5] In this scenario, it is assumed that multiple ionic currents (I Na , I Ca,L , I Ca,T , I K,r , I b,Na , and I f as well as I NaCa ) are involved in the generation of the pacemaker potential. 6 As highlighted by DiFrancesco and Robinson, 7 the conclusion that the chronotropic effect of ␤-adrenergic stimulation is the result of an increase in the Ca 2ϩ transient is also surprising, because the chronotropic effect of ␤-adrenergic stimulation has been previously attributed to actions on ionic currents such as I Ca,...

No abstract

The heart's pacemaker, the sinoatrial node, does not consist of a group of uniform sinoatrial node cells embedded in atrial muscle. Instead, it is a heterogeneous tissue with multiple cell types and a complex structure. Evidence suggests that from the periphery to the center of the sinoatrial node, there is a gradient in action potential shape, pacemaking, ionic current densities, connexin expression, Ca2+ handling, myofilament density, and cell size. This complexity may be necessary for the sinoatrial node to pacemake under diverse conditions, drive the more hyperpolarized atrial muscle, and resist proarrhythmic perturbations.