Following myocardial infarction, fibroblasts are recruited to the wounded area¹ where they transform into myofibroblasts². Myofibroblasts are proliferative, migratory, and contractile².The mechanisms underlying membrane potential, and whether modulation of membrane potential can influence proliferation, migration, and contraction are not completely understood. Animal protocols complied with Canadian Council on Animal Care regulations and were approved by the University of Calgary Animal Resources Centre and the Animal Care Committee of the University of Manitoba. Hearts were removed from adult Sprague Dawley rats under isoflurane anaesthesia and Langendorffperfused with collagenase II. Dispersed fibroblasts were separated from other cell types and cultured in Dulbecco′s Modified Eagle′s Medium/10%Fetal Calf Serum. Within 6 days, fibroblasts had transformed into myofibroblasts, as evidenced by expression of α-smooth muscle actin and embryonic smooth muscle myosin. As previously reported³,inwardly rectifying and outward time- and voltage-dependent K⁺ currents were expressed in ventricular myofibroblasts. The K⁺ currents appear to be the primary determinants of membrane potential. To determine if modulating membrane potential can modulate myofibroblast function, [K⁺]_o was varied from 1.5 mM to 20 mM (by isotonic substitution with Na⁺) and myofibroblast membrane potential, cell number, migration, and contraction were monitored. Bis-(1,3-dibutylbarbituric acid)trimethine oxonol (DiBAC₄(3)) is a bis-barbituric acid oxolol compound which partitions into the membrane as a function of membrane potential⁴. Hyperpolarization is associated with extrusion of the dye and decreased fluorescence while depolarization is associated with enhanced fluorescence⁴.In 1.5 mM [K⁺]_o Tyrode′s solution, DiBAC₄(3) emission decreased ~35%, indicative of hyperpolarization. Elevating [K⁺]_o (15 mM) increased emission ~30%, indicative of depolarization. Myofibroblasts were previously shown to produce significantly greater contraction in 20 mM [K⁺]_o, suggesting that depolarization enhanced contraction³.Migration, measured by the Boyden chamber assay⁵, was decreased by 1 mM tetraethylammonium (TEA): 17±2 (mean±se) cells per test chamber with TEA present, as compared to 98±9 cells (control).

Myofibroblast number following 24 hours incubation with 1.7 mM [K⁺]_o Tyrode′s solution was elevated (13315±205 vs. 8484±867 (control) cells/well, n = 3), suggesting that hyperpolarization can increase cell number. In combination, these results indicate that membrane potential in myofibroblasts is regulated by K⁺ conductance, and that changes in membrane potential can modulate myofibroblast function.