ATP-sensitive
potassium (
KATP) channels are abundantly expressed in the myocardium. Although a definitive role for the channel remains elusive they have been implicated in the phenomenon of cardioprotection, but the precise mechanism is unclear. We set out to test the hypothesis that the channel protects by opening early during
ischemia to shorten action potential duration and reduce electrical excitability thus sparing intracellular
ATP. This could reduce
reperfusion injury by improving
calcium homeostasis. Using a combination of contractile function analysis,
calcium fluorescence imaging and patch clamp electrophysiology in cardiomyocytes isolated from adult male Wistar rats, we demonstrated that the opening of sarcolemmal
KATP channels was markedly delayed after cardioprotective treatments: ischemic preconditioning,
adenosine and PMA. This was due to the preservation of intracellular
ATP for longer during simulated
ischemia therefore maintaining sarcolemmal
KATP channels in the closed state for longer. As the simulated
ischemia progressed,
KATP channels opened to cause contractile,
calcium transient and action potential failure; however there was no indication of any channel activity early during simulated
ischemia to impart an energy sparing hyperpolarization or action potential shortening. We present compelling evidence to demonstrate that an early opening of sarcolemmal
KATP channels during simulated
ischemia is not part of the protective mechanism imparted by ischemic preconditioning or other PKC-dependent cardioprotective stimuli. On the contrary, channel opening was actually delayed. We conclude that sarcolemmal
KATP channel opening is a consequence of
ATP depletion, not a primary mechanism of
ATP preservation in these cells.