Protein Kinase C Activates ATP-Sensitive K + Current in Human and Rabbit Ventricular Myocytes

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Abstract
Mediators involved in ischemic preconditioning, such as adenosine and norepinephrine, can activate protein kinase C (PKC), and a variety of observations suggest that both PKC and ATP-sensitive K+ current (IKATP) play essential roles in ischemic preconditioning. PKC is therefore a candidate to link receptor binding to IKATP activation, but it has not been shown whether and how PKC can activate IKATP in the heart. The present study was designed to determine whether PKC can activate IKATP in rabbit and human ventricular myocytes. Under conditions designed to minimize Na+ and Ca2+ currents, dialysis of rabbit ventricular myocytes with pipette solutions containing reduced [ATP] elicited IKATP, with a 50% effective concentration (EC50) of 260 μmol/L. In cells that failed to show IKATP under control conditions, superfusion with 1 μmol/L phorbol 12,13-didecanoate (PDD) elicited IKATP in a fashion that depended on pipette [ATP], with an [ATP] EC50 of 601 μmol/L. PDD-induced IKATP activation was concentration dependent, with an EC50 of 7.1 nmol/L. The highly selective PKC inhibitor bisindolylmaleimide totally prevented IKATP activation by PDD, and in blinded experiments, 1 μmol/L PDD elicited IKATP in eight of nine cells, whereas its non–PKC-stimulating analogue 4α-PDD failed to elicit IKATP in any of the five cells tested (P=.003). Similar experiments were conducted in human ventricular myocytes and showed that 0.1 μmol/L PDD elicited IKATP at pipette [ATP] of 100 and 400 μmol/L (five of five cells at each concentration) but not at 1 mmol/L [ATP] (none of five cells). We conclude that PKC activates IKATP in rabbit and human ventricular myocytes by reducing channel sensitivity to intracellular ATP. This finding has potentially important implications for understanding the mechanisms of ischemic preconditioning.