Mitochondrial potassium transport: the role of the MitoK ATP

1. Mitochondrial potassium transport: the role of the MitoKATP WeiGuo 2005.1.14

2. Mitochondrial potassium cycle • Mitochondria are structurally complex. The inner membrane contains the essential components of the electron transport proteins and all of the exchange carriers

3. Mitochondrial potassium cycle • The mitochondrial K+ cycle consists of influx and efflux pathways for K+, H+, and anions • These ions are exchanged between the matrix and the intermembrane space ( IMS ); however, the outer membrane (OM) does not present a barrier to further exchange of small ions with the cytosol

4. K+ K+ matrix Pi- Matrix alkalinization K+ leak ETS MitoKATP ∆Ψ Pi-H+ symporter IMS H+ OH- Influx pathway for potassium • Electron transport system (ETS) generates membrane potential (∆Ψ). • ∆Ψ can drive K+ influx by diffusion (‘‘K+ leak’’) and via the mitoKATP. • This K+ for H+ exchange will alkalinize the matrix, causing phosphate to enter via the Pi-H+ symporter.

5. K+-H+ antiporter Efflux pathway for potassium • Net uptake of K+ salts will be accompanied by osmotically obligated water, resulting in matrix swelling. Excess matrix K+ is then ejected by the K+/H+ antiporter

6. Early work on the potassium cycle • Diffusive K+ influx would be sufficient to cause matrix water content to increase, with eventual lysis. This would be avoided by the K+/H+ antiporter

7. Maintain matrix volume mito-KATP synthesizing ATP at very high rates ∆Ψ decreases matrixcontraction MitoKATP meets a different need in volume regulation

8. DE matrix volume return to original state 10–15% contraction in matrix volume 5-HD Addition of antimycin A to simulate ischemia depolarization and decrease in diffusive K+ influx addition of ADP to trigger state 3 respiration MitoKATP on matrix and IMS volumes • MitoKATP opening was shown to regulate matrix volume during ischemia and state 3 respiration

9. MitoKATP on matrix and IMS volumes • Changes in IMS could be estimated by means of membrane surface areas (SA) • Studies shown that mitoKATP opening decreases IMS volume • Physiological changes in matrix volume may have important effects on IMS structure–function

10. Two distinct consequences of opening mitoKATP • When ∆Ψ is high → opening mitoKATP → matrix alkalinization → production of reactive oxygen species (ROS) ↑ • When ∆Ψ is depressed → opening mitoKATP → prevent contraction of the matrix and expansion of the IMS

11. Is mitoKATP involved in all modes of cardioprotection ? • Ischemic preconditioning √ • Calcium preconditioning √ • KCO preconditioning √ • Delayed preconditioning √ • Adaptive preconditioning √ • Na+/H+ exchange inhibition √ • Ischemic post-conditioning ?

12. Preconditioning phase Ischemic phase Reperfusion phase As a triggerof cardioprotection As a end-effector of cardioprotection As a end-effector of cardioprotection During which phase is mitoKATP opening crucial for cardioprotection? • MitoKATP is proposed to play distinct roles in each phase of ischemia– reperfusion

13. During the preconditioning phase • The role of mitoKATP opening is to increase production of ROS • Moderate increases in ROS play an important second messenger role in a variety of signaling pathways

14. IMS OH- Pi-H+ symporter K+ K+ K+leak MitoKATP Matrix Pi- Pi- uptake will be less than K+ uptake ROS↑ Matrix alkalinization A proposed mechanism for increased ROS • K+ uptake creating a gradient for uptake of Pi on the Pi–H+ symporter, Pi uptake will be less than K+ uptake, because Pi is present in much lower concentrations than K+. For this reason, matrix pH always increases when matrix volume increases due to uptake of K+ and Pi.

15. During the ischemic phase • mitochondrial permeability transition (MPT) • The primary role of matrix Ca2+ is to stimulate ROS production upon reperfusion • Ca2+ cannot open MPT unless ROS are present • Cytosolic Ca2+ may play an additional role in promoting ROS oxidation of adenine nucleotide translocase (ANT)

16. The mechanism by which mitoKATP protects the heart during ischemia phase • The opening of mitoKATP preserves the structure–function of the IMS and maintains the low permeability of the outer membrane to adenine nucleotides, thereby preserving ADP for phosphorylation upon reperfusion

17. Outer Mem VDAC IMS CK Cr / PCr ANT Inner Mem matrix ATP ADP Outer mitochondrial membrane permeability to ADP and ATP was controlled by voltage-dependent anion channel (VDAC) • In heart, VDAC is normally in a low-conductance state that is poorly permeable to nucleotides, and energy transfers are mediated instead by creatine and creatine phosphate.

18. MitoKATP regulation of VDAC permeability to nucleotides duringischemia • During ischemia, ∆Ψ will decrease,resulting in reduced uptake of K+, • contraction of the matrix, and expansion of the IMS • IMS expansion will cause Mi-CK to dissociate from VDAC, leading to a high outer membrane conductance to ATP and ADP • This means that all of cellular ATP is available for hydrolysis, and, ultimately, unavailability of ADP for rephosphorylation upon reperfusion

19. During the reperfusion phase • The opening of mitoKATP facilitates rapid energy conversion to phosphocreatine (PCr) . Under these conditions, mitochondria will not produce a burst of ROS upon reperfusion, and the irreversible opening of the MPT will not occur

20. Energy transfer from mitochondria to myofibrils is mediated by two parallel pathways—creatine/creatine phosphate (Cr/CrP) and ATP/ADP Outer Mem VDAC • Cr/CrP is more efficient • About 67% of the energy production in heart has been found to arise from the CK system CK Cr / PCr ANT Inner Mem ATP / ADP • In the Cr/CrP system, myofibrillar creatine kinase converts ADP to creatine. Mi-CK bridge the IMS between outer membrane VDAC and inner membrane ANT.

21. MitoKATP facilitates rapid energy conversion to phosphocreatine (PCr) during the reperfusion phase • During reperfusion, expansion of the IMS will cause Mi-CK to dissociate from VDAC, leading to a high outer membrane conductance to ATP and ADP • If mitoKATP is open, the outer membrane will retain its low permeability to nucleotides, and the mitochondria can restore energy levels using the more efficient metabolic channeling via Mi-CK

22. Summary • Mitochondria potassium cycle • Two distinct consequences of Opening mitoKATP • mitoKATP plays cardio-protective effect during all three phases of the ischemia–reperfusion injury

23. Thank you !