Kir6.2 limits Ca²⁺ overload and mitochondrial oscillations of ventricular myocytes in response to metabolic stress

Abstract

ATP-sensitive K(+) (KATP) channels are abundant membrane proteins in cardiac myocytes that are directly gated by intracellular ATP and form a signaling complex with metabolic enzymes, such as creatine kinase. KATP channels are known to be essential for adaption to cardiac stress, such as ischemia; however, how all the molecular components of the stress response interact is not fully understood. We examined the effects of decreasing the KATP current density on Ca(2+) and mitochondrial homeostasis and ischemic preconditioning. Acute knockdown of the pore-forming subunit, Kir6.2, was achieved using adenoviral delivery of short hairpin RNA targeted to Kir6.2. The acute nature of the knockdown of Kir6.2 accurately shows the effects of Kir6.2 depletion without any compensatory effects that may arise in transgenic studies. We also investigated the effect of reducing the KATP current while maintaining KATP channel protein in the sarcolemmal membrane using a nonconducting Kir6.2 construct. Only 50% KATP current remained after Kir6.2 knockdown, yet there were profound effects on myocyte responses to metabolic stress. Kir6.2 was essential for cardiac myocyte Ca(2+) homeostasis under both baseline conditions before any metabolic stress and after metabolic stress. Expression of nonconducting Kir6.2 also resulted in increased Ca(2+) overload, showing the importance of K(+) conductance in the protective response. Both ischemic preconditioning and protection during ischemia were lost when Kir6.2 was knocked down. KATP current density was also important for the mitochondrial membrane potential at rest and prevented mitochondrial membrane potential oscillations during oxidative stress. KATP channel density is important for adaption to metabolic stress.

KATPChannels in the Cardiovascular System

“…In addition to the effects on intracellular Ca 2ϩ , K ATP channels may additionally protect against stress by preserving mitochondrial function. In rat ventricular myocytes, for example, shRNA knockdown of Kir6.2 (by 50%) not only leads to disrupted intracellular Ca 2ϩ homeostasis, but also to oscillations of mitochondrial membrane potential (764). Mitochondrial K ATP channels may of course also directly participate in the protection against stress (259).…”

“…The effects of these compounds, however, may be complex since K ATP channel openers such as diazoxide and pinacidil have opposite effects on mitochondrial respiration under different energetic conditions (669). Mitochondrial integrity may also potentially be indirect in that sarcolemmal K ATP channel opening diminishes intracellular Ca 2ϩ overload, which in turn may protect mitochondrial function (764). K ATP channel opening in the vasculature may also contribute to increase blood flow to the ischemic tissue and thus to lessen the damaging effects of ischemia.…”

Early opening of sarcolemmal ATP-sensitive potassium channels is not a key step in PKC-mediated cardioprotection

“…as the predominant subunit combination [2][3][4]. Other K ATP subunits are expressed in cardiomyocytes and may form components of the postulated mitoK ATP [4].…”

“…Evidence for a key role for the cardiac K ATP channel comes from the observation that pharmacological blockade [12,18], disruption of channel expression with interacting fragments of SUR [19], shRNA knockdown [3] and knockout of Kir6.2 subunits [20,21] all have deleterious effects on the ability of IPC and other cardioprotective stimuli to impart protection.…”

A syndrome of congenital hyperinsulinism and rhabdomyolysis is caused byKCNJ11mutation

“…However, very recent work on the cardiac isoform of Kir6.2 provides tantalising evidence for what might be happening in skeletal muscles in patients with severe Kir6.2 loss of function. Storey and colleagues have elegantly shown that disruption of cardiac Kir6.2 has major adverse consequences on cardiomyocyte adaptation to ischaemia 19. This should not be surprising because ischaemia leads to a toxic increase in intracellular Ca, which not only impairs contractility but can also initiate apoptosis through triggering the mitochondrial permeability transition pore 20.…”