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Cardiac fibroblasts account for about 75% of all cardiac cells, but because of their small size contribute only ∼10-15% of total cardiac cell volume. They play a crucial role in cardiac pathophysiology. For a long time, it has been recognized that fibroblasts and related cell types are the principal sources of extracellular matrix (ECM) proteins, which organize cardiac cellular architecture. In disease states, fibroblast production of increased quantities of ECM proteins leads to tissue fibrosis, which can impair both mechanical and electrical function of the heart, contributing to heart failure and arrhythmogenesis. Atrial fibrosis is known to play a particularly important role in atrial fibrillation (AF). This review article focuses on recent advances in understanding the molecular electrophysiology of cardiac fibroblasts. Cardiac fibroblasts express a variety of ion channels, in particular voltage-gated K(+) channels and non-selective cation channels of the transient receptor potential (TRP) family. Both K(+) and TRP channels are important determinants of fibroblast function, with TRP channels acting as Ca(2+)-entry pathways that stimulate fibroblast differentiation into secretory myofibroblast phenotypes producing ECM proteins. Fibroblasts can couple to cardiomyocytes and substantially affect their cellular electrical properties, including conduction, resting potential, repolarization, and excitability. Co-cultured preparations of cardiomyocytes and fibroblasts generate arrhythmias by a variety of mechanisms, including spontaneous impulse formation and rotor-driven reentry. In addition, the excess ECM proteins produced by fibroblasts can interrupt cardiomyocyte-bundle continuity, leading to local conduction disturbances and reentrant arrhythmias. A better understanding of the electrical properties of fibroblasts should lead to an improved comprehension of AF pathophysiology and a variety of novel targets for antiarrhythmic intervention.

Rationale: Cardiac fibrosis contributes to pathogenesis of atrial fibrillation (AF), which is the most commonly sustained arrhythmia and a major cause of morbidity and mortality. Although it has been suggested that Ca(2+) signals are involved in fibrosis promotion, the molecular basis of Ca(2+) signaling mechanisms and how Ca(2+) signals contribute to fibrogenesis remain unknown., Objective: To determine the molecular mechanisms of Ca(2+)-permeable channel(s) in human atrial fibroblasts, and to investigate how Ca(2+) signals contribute to fibrogenesis in human AF., Methods and Results: We demonstrate that the transient receptor potential (TRP) melastatin related 7 (TRPM7) is the molecular basis of the major Ca(2+)-permeable channel in human atrial fibroblasts. Endogenous TRPM7 currents in atrial fibroblasts resemble the biophysical and pharmacological properties of heterologous expressed TRPM7. Knocking down TRPM7 by small hairpin RNA largely eliminates TRPM7 current and Ca(2+) influx in atrial fibroblasts. More importantly, atrial fibroblasts from AF patients show a striking upregulation of both TRPM7 currents and Ca(2+) influx and are more prone to myofibroblast differentiation, presumably attributable to the enhanced expression of TRPM7. TRPM7 small hairpin RNA markedly reduced basal AF fibroblast differentiation. Transforming growth factor (TGF)-beta1, the major stimulator of atrial fibrosis, requires TRPM7-mediated Ca(2+) signal for its effect on fibroblast proliferation and differentiation. Furthermore, TGF-beta1-induced differentiation of cultured human atrial fibroblasts is well correlated with an increase of TRPM7 expression induced by TGF-beta1., Conclusions: Our results establish that TRPM7 is the major Ca(2+)-permeable channel in human atrial fibroblasts and likely plays an essential role in TGF-beta1-elicited fibrogenesis in human AF.

Background: We have shown that rapid atrial activation, as occurs during atrial fibrillation (AF), reduces L-type Ca2+ current (ICa) and that this is the principal mechanism of the action potential duration and refractoriness changes that characterize tachycardia-induced atrial remodeling. The present study was designed to determine whether atrial tachycardia alters biochemical indices of the number of L-type Ca2+ channels and/or of the number and binding affinity of β-adrenergic receptors. Methods: In canine atrial sarcolemmal preparations, the number and binding affinity of dihydropyridine receptors were determined with the use of 3H-nitrendipine and that of β-adrenergic receptors with 125I-iodocyanopindolol. Results were obtained with preparations from dogs paced at 400/min for 1 (P1, n=20), 7 (P7, n=9), and 42 (P42, n=9) days, and compared with observations in sham-operated controls (P0, n=14). Results: Pacing reduced the Bmax of dihydropyridine receptors, from 157±18 fmol/mg (P0) to 116±9 fmol/mg (P1, P <0.05), 100±14 fmol/mg (P7, P <0.05) and 94±9 fmol/mg (P42, P <0 .01). The affinity of dihydropyridine receptors was unchanged, with the Kd averaging 711±102 pM, 656±74 pM, 633±155 pM and 585±92 pM in P0, P1, P7 and P42 dogs. Neither Bmax nor Kd of β-adrenergic receptors was altered by rapid pacing. Values of Bmax of dihydropyridine receptors correlated with atrial ICa current density (r2=0.95) and ERP (r2=0.99). Conclusions: Rapid atrial activation results in downregulation in the number of dihydropyridine receptors without altering the number or affinity of β-adrenergic receptors. The reductions in ICa that play an important role in the atrial electrical remodeling by which ‘AF begets AF’ appear to be due at least in part to a decrease in the number of L-type Ca2+ channels in cardiac cell membranes. [ABSTRACT FROM PUBLISHER]