Cardiac fibroblasts are involved in the maintenance of myocardial tissue structure. However, little is known about ion currents in human cardiac fibroblasts. It has been recently reported that cardiac fibroblasts can interact electrically with cardiomyocytes through gap junctions. Ca(2+)-activated K(+) currents (I (K[Ca])) of cultured human cardiac fibroblasts were characterized in this study. In whole-cell configuration, depolarizing pulses evoked I (K(Ca)) in an outward rectification in these cells, the amplitude of which was suppressed by paxilline (1 microM: ) or iberiotoxin (200 nM: ). A large-conductance, Ca(2+)-activated K(+) (BK(Ca)) channel with single-channel conductance of 162 +/- 8 pS was also observed in human cardiac fibroblasts. Western blot analysis revealed the presence of alpha-subunit of BK(Ca) channels. The dynamic Luo-Rudy model was applied to predict cell behavior during direct electrical coupling of cardiomyocytes and cardiac fibroblasts. In the simulation, electrically coupled cardiac fibroblasts also exhibited action potential; however, they were electrically inert with no gap-junctional coupling. The simulation predicts that changes in gap junction coupling conductance can influence the configuration of cardiac action potential and cardiomyocyte excitability. I (k(Ca)) can be elicited by simulated action potential waveforms of cardiac fibroblasts when they are electrically coupled to cardiomyocytes. This study demonstrates that a BK(Ca) channel is functionally expressed in human cardiac fibroblasts. The activity of these BK(Ca) channels present in human cardiac fibroblasts may contribute to the functional activities of heart cells through transfer of electrical signals between these two cell types.

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Aconitine (ACO), a highly toxic diterpenoid alkaloid, is recognized to have effects on cardiac voltage-gated Na(+) channels. However, it remains unknown whether it has any effects on K(+) currents. The effects of ACO on ion currents in differentiated clonal cardiac (H9c2) cells and in cultured neonatal rat ventricular myocytes were investigated in this study. In H9c2 cells, ACO suppressed ultrarapid-delayed rectifier K(+) current (I(Kur)) in a time- and concentration-dependent fashion. The IC(50) value for ACO-induced inhibition of I(Kur) was 1.4 microM. ACO could accelerate the inactivation of I(Kur) with no change in the activation time constant of this current. Steady-state inactivation curve of I(Kur) during exposure to ACO could be demonstrated. Recovery from block by ACO was fitted by a single-exponential function. The inhibition of I(Kur) by ACO could still be observed in H9c2 cells preincubated with ruthenium red (30 microM). Intracellular dialysis with ACO (30 microM) had no effects on I(Kur). I(Kur) elicited by simulated action potential (AP) waveforms was sensitive to block by ACO. Single-cell Ca(2+) imaging revealed that ACO (10 microM) alone did not affect intracellular Ca(2+) in H9c2 cells. In cultured neonatal rat ventricular myocytes, ACO also blocked I(Kur) and prolonged AP along with appearance of early afterdepolarizations. Multielectrode recordings on neonatal rat ventricular tissues also suggested that ACO-induced electrocardiographic changes could be associated with inhibition of I(Kur). This study provides the evidence that ACO can produce a depressant action on I(Kur) in cardiac myocytes.

The effects of rottlerin, a known inhibitor of protein kinase C-delta activation, on ion currents were investigated in pituitary tumor (GH3) cells. Rottlerin (0.3-100 microM) increased the amplitude of Ca2+-activated K+ current (I K(Ca)) in a concentration-dependent manner with an EC50 value of 1.7 microM. In intracellular perfusion with rottlerin (1 microM) or staurosporine (10 microM), phorbol 12-myristate 13-acetate-induced inhibition of I K(Ca) in these cells was abolished. In cell-attached mode, rottlerin applied on the extracellular side of the membrane caused activation of large-conductance Ca2+-activated K+ (BK(Ca)) channels, and a further application of BAPTA-AM (10 microM) to the bath had no effect on rottlerin-stimulated channel activity. When cells were exposed to rottlerin, the activation curve of these channels was shifted to less positive potential with no change in the slope factor. Rottlerin increased BK(Ca)-channel activity in outside-out patches. Its change in kinetic behavior of BK(Ca) channels is primarily due to an increase in mean open time. With the aid of minimal kinetic scheme, a quantitative description of rottlerin stimulation on BK(Ca) channels in GH3 cells was also provided. Under current-clamp configuration, rottlerin (1 microM) decreased the firing of action potentials. I K(Ca) elicited by simulated action potential waveforms was enhanced by this compound. In human cortical HCN-1A cells, rottlerin (1 microM) could also interact with the BK(Ca) channel to stimulate I K(Ca). Therefore, rottlerin may directly activate BK(Ca) channels in neurons or endocrine cells.