Fibroblast-myocyte electrotonic coupling: does it occur in native cardiac tissue?

Abstract

Heterocellular electrotonic coupling between cardiac myocytes and non-excitable connective tissue cells has been a long-established and well-researched fact in vitro. Whether or not such coupling exists in vivo has been a matter of considerable debate. This paper reviews the development of experimental insight and conceptual views on this topic, describes evidence in favour of and against the presence of such coupling in native myocardium, and identifies directions for further study needed to resolve the riddle, perhaps less so in terms of principal presence which has been demonstrated, but undoubtedly in terms of extent, regulation, patho-physiological context, and actual relevance of cardiac myocyte-non-myocyte coupling in vivo. This article is part of a Special Issue entitled "Myocyte-Fibroblast Signalling in Myocardium."

Keywords: Connective tissue; Electrophysiology; Fibrosis; Gap junction; Heart; Scar.

Fig. 1

Fibroblast–myocyte interrelations in native cardiac tissue. A: Confocal laser scanning microscopy images of rabbit sino-atrial node live tissue, following diffusion-loading of the CellTracker dye CMFDA. Groups of sino-atrial node myocytes (centrally located ‘large’ uniform structures) are intermingled with cardiac fibroblasts (smaller cells with numerous fine processes towards the myocytes, see white arrows). Outline view left: 158 × 158 μm; detail view right: 30 × 30 μm (corresponds to the area near the arrow at the right edge of the outline). B: Extended structures, resembling membrane nanotubes (see white arrows), in adult mouse ventricular tissue. Confocal micrographs of cardiac tissue, labelled for WGA (membranes), vimentin (fibroblasts) and α-actinin (myocytes). Scale bar: 10 μm. From in (A), and in (B); with permission.

Fig. 2

Membrane potential pairs (top and bottom), recorded simultaneously in spontaneously beating adult rat atrium using double-barrelled floating microelectrodes (tip-to-tip distance 40 μm) from putative fibroblasts (F, top) and cardiomyocytes (M, bottom). Left: F, electrically not connected to surrounding cardiomyocytes, showing a relatively depolarized membrane potential (here about - 20 mV) and the contraction-induced membrane potential change typical for these cells (label ‘a’). Middle: in some cases, presumably when weakly coupled to adjacent myocytes, F display an additional, probably electrotonically transmitted membrane depolarization (labelled ‘b’) while the myocyte membrane potential is positive to the diastolic potential in F, and a ‘capacitative’ spike (labelled ‘c’) believed to be caused by the synchronous depolarization of myocytes in the proximity of F. Right: recording of a potential waveform that would be commensurate with an F that is well-coupled to adjacent cardiomyocytes, where the electrotonically transmitted component mimics an action potential waveform of reduced amplitude and upstroke velocity (here on top of the atrial contraction-induced mechano-sensitive peak; note different potential scales for first two F potentials). Adapted from ; with permission.

Fig. 3

Connexin location relative to cardiac myocytes and connective tissue cells in native myocardium of healthy rabbit. Triple immunolabeling for myocytes (M; red: antimyomesin), fibroblasts (F; blue: antivimentin), and Cx43 or Cx40 (top and bottom rows, respectively; green: anti-Cx43/anti-Cx40) in ventricle, atrium, and atrioventricular node (AVN). Vertical arrows: homotypic myocyte contacts; horizontal arrows: homotypic non-myocyte connections; slated arrows: Cx at heterotypic cell contacts. Side-panels show 2.55 × zoomed views of the areas highlighted by dashed squares in the main images. Note significantly smaller size of Cx labels at homo- and heterotypic contact sites that involve non-myocytes. Scale bars: 20 μm. From , with permission.

Fig. 4

Propagation of electrical activation into ventricular scar tissue. A: Epicardially recorded optical maps of trans-membrane potential changes (dye used: RH237) in Langendorff-perfused rabbit isolated heart, 8 weeks after induction of transmural left-ventricular infraction (see B, left panel). Electrical pacing (at site identified in the schematic on the left by a red dot, and in the photograph (i) of the infract area by a white asterisk) in the normal zone (NZ) gives rise to a conduced wave of electrical excitation, which is delayed by about 10 ms at the peri-infarct before invading the infarct zone (PZ, IZ; respectively); see crowding and thinning of activation isochrones (ii). Normalised trans-membrane potential shapes, recorded at the locations labelled d–h, are shown in (iii). Signals from inside the scar tissue (i.e. at sites g and h) persisted even after chemical ablation of surviving endocardial muscle layers (panel B). LA: left atrium, RA: right atrium; LV: left ventricle; red vertical line in (iii): time of pacing stimulus. B: Histological substrate of scar tissue in rabbit (left, 8 weeks post-infarct, used in the experimental studies shown in A) and sheep (border zone at 1 week post infract [middle]; infarct zone 2 weeks post-infarct [right]). The panel on the left (trichrome staining) illustrates the transmural nature of infarcts used in the optical mapping studies, including the presence of islands of surviving myocytes in the infarct zone, and a surviving endocardial tissue rim (lower edge of section) that was ablated in part of the study. The images in the centre and at the right (triple immuno-labelling for Cx43 [green], myomesin to identify myocytes by their striation, and vimentin to label fibroblasts [both red]) highlights Cx43 distribution, including presence at heterocellular contact sites at the infract border (middle) and in central zone myocyte islands. Vertical arrows: homotypic myocyte contacts; horizontal arrows: homotypic non-myocyte connections; slated arrows: Cx43 at heterotypic cell contacts. From in (A) and from , respectively, in (B); with permission.