Cardiac fibroblasts comprise the largest group of cells within the mammalian heart. They reside primarily in the interstitium of the ventricular and atrial myocardium (Zak 1973). In health, fibroblasts are responsible for maintaining the extracellular matrix and synthesizing a number of autocrine and paracrine factors such as transforming growth factor β (TGFβ) (Eghbali 1989). Following myocardial infarction, fibroblasts are activated by the presence of necrotic muscle (Weber & Brilla 1992) and by TGFβ which is released by immune cells (de Almeida et al. 2002). Fibroblasts convert into myofibroblasts which lay down scar tissue (Hao et al. 1999). Despite these important roles of fibroblasts and myofibroblasts, little is known about how the membrane potential is controlled in cardiac fibroblasts and myofibroblasts, or how modulation of membrane potential may affect fibroblast or myofibroblast function.

We have recorded membrane currents in both freshly dissociated and primary cultures of adult rat cardiac fibroblasts and measured collagen I gel deformation due to myofibroblast contraction in the presence of different [K⁺]_o concentrations. Hearts were removed from adult Sprague-Dawley rats under isoflurane anaesthesia and were Langendorff perfused, then digested with collagenase II. Dispersed fibroblasts were kept at 4 °C and studied within 6 h or were separated from myocytes and other cell types and cultured in DMEM-10 % FCS.

Using whole-cell patch-clamp methods, K⁺ currents in freshly dissociated and cultured cardiac fibroblasts were recorded. Inward currents in 10 mM [K⁺]_o Tyrode solution were blocked by 300 µM Ba²⁺. At -120 mV, in freshly dissociated fibroblasts, the Ba²⁺-sensitive current measured -13.8 ± 6.7 pA pF^-1 (means ± S.E.M.), (n = 3). In cultured fibroblasts, inward currents were recorded at -120 mV in 5.4 mM [K⁺]_o Tyrodes solution, and these were blocked by 1 mM Ba²⁺ (Ba²⁺-sensitive current = -7.4 ± 2.2 pA pF^-1, n = 20). Time- and voltage-dependent outward currents (51.2 ± 14.3 pA pF^-1, +40 mV, n = 47) were recorded from the majority of freshly dissociated fibroblasts. A component of this outward current was blocked by 1 mM TEA (TEA-sensitive current = 7.6 ± 3.1 pA pF^-1 at +40 mV, n = 3). Similar time- and voltage-dependent outward currents (11.9 ± 3.0 pA pF^-1, +40 mV, n = 20) were recorded in cultured fibroblasts. The outward currents were also sensitive to 100 µM 4-aminopyridine (4-AP) in both freshly dissociated and cultured cardiac fibroblasts. The 4-AP-sensitive current measured 12.9 ± 6.6 pA pF^-1 at +40 mV in freshly dissociated fibroblasts (n = 3) and 6.0 ± 1.8 pA pF^-1 at +40 mV in cultured fibroblasts (n = 5).

In cultured fibroblasts, anti-α smooth muscle actin antibody staining was positive, indicating that when cultured, cardiac fibroblasts became activated and transformed into contractile myofibroblasts. Populations of myofibroblasts were seeded onto collagen I gels following first passage, and gel deformation due to myofibroblast contraction was compared when myofibroblasts were depolarised by exposure to 20 mM [K⁺]_o Tyrode solution. At 8 h, gel surface area was significantly reduced in 20 mM [K⁺]_o (90 ± 2 mm² (n = 8) vs. 113 ± 4 mm² (n = 15) under control conditions, P = 0.034, Student’s unpaired t test). This suggests that modulation of membrane potential by modulating K⁺ currents can affect myofibroblast function.