Protein kinase C activation enhances the delayed rectifier potassium current in guinea-pig heart cells

The possible involvement of protein kinase C in modulating membrane currents was investigated in isolated guinea-pig ventricular cells. In a Na(+)-and K(+)-free external solution, the delayed rectifier K+ current (IK) was increased by the activator of protein kinase C (PKC), 12-O-tetradecanoylphorbol-13-acetate (TPA). The amplitude of the IK tail elicited by a return from a depolarizing pulse for 3 s at + 50 mV to a holding potential of -30 mV was increased by 32 +/- 4% (mean +/- S.E., (n = 6) after the external application of 1 nM TPA, and by 60 +/- 17% (n = 5) after 10 nM. The increase in IK produced by 1 nM TPA was abolished by the inhibitor of PKC, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7, 10 microM). In addition, the synthetic diacylglycerol 1-oleoyl-2-acetylglycerol (OAG, 125 microM) also increased IK (58 +/- 9%, n = 3). PKC purified from bovine brain remarkably increased IK (151 +/- 101%, n = 5) in the presence of 1 nM TPA when it was internally applied using the cell dialysis method. The concentration-response curve of IK for the intracellular concentration of Ca2+ was shifted to the left by 1 nM TPA, suggesting a Ca2(+)-dependent action of PKC and/or altered Ca2(+)-sensitivity of IK channels by phosphorylation. On the other hand, 1 nM TPA had no substantial influence on the Ca2+ current (decreased by 7 +/- 4%, n = 5) or the inward-rectifier K+ current (decreased by 5 +/- 5% in outward component, and 3 +/- 8% in inward component, n = 6). Therefore, the action of PKC was to specifically increase IK without affecting the other two currents.