Protein kinase C enhances the rapidly activating delayed rectifier potassium current, IKr, through a reduction in C-type inactivation in guinea-pig ventricular myocytes

1. The rapidly activating delayed rectifier potassium current, I-Kr, was studied in guinea-pig ventricular myocytes in the presence of thiopentone, which blocks the more slowly activating component of the delayed rectifier potassium current, I-Ks, and using whole cell perforated patch damp or switched voltage clamp with sharp electrodes to minimise intracellular dialysis. 2. Activation of protein kinase A (PKA) by isoprenaline or forskolin caused an increase in I-Kr tail currents. Following a 300 ms depolarising step to +20 mV, mean tail current amplitude was increased 47 +/- 12 % by isoprenaline, and 73 +/- 13 % by forskolin. No increase in I-Kr was observed when I-Kr was studied using whole cell ruptured patch clamp and there was no change in the reversal potential of I-Kr in the presence of isoprenaline. 3. The rectification of the current sensitive to E4031, a selective I-Kr blocker, was markedly reduced in the presence of isoprenaline and the region of negative slope was absent. This is consistent with a reduction in the inactivation of I-Kr and was supported by the finding that I-Kr, in the presence of isoprenaline, was somewhat less sensitive to block. E4031 (5 mu M) blocked only 81 +/- 5% of I-Kr in the presence of isoprenaline compared to 100 +/- 0 % in control. 4. The forskolin- and isoprenaline-induced increases in I-Kr were inhibited by staurosporine and by the selective protein kinase C (PKC) inhibitor bisindolylmaleimide I. Direct activation of PKC by phorbol dibutyrate increased I-Kr tail currents by 24 +/- 5%. Both the isoprenaline-and forskolin-induced increases in I-Kr were inhibited when calcium entry was reduced by block of I-Ca with nifedipine or when myocytes were pre-incubated in BAPTA-AM. 5. The selective PKA inhibitor KT5720 prevented the isoprenaline-induced increase in I-Kr only when the increase in I-Ca was also suppressed. 6. These data show a novel mechanism of regulation of I-Kr by PKC and this kinase was activated by beta-adrenoceptor stimulation. I-Kr seems to be enhanced through a reduction in the C-type inactivation which underlies the rectification of the channel and such a mechanism may occur in other channels with this type of inactivation.