There is an emerging consensus that pharmacological opening of the mitochondrial ATP-sensitive K(+) (K(ATP)) channel protects the heart against ischemia-reperfusion damage; however, there are widely divergent views on the effects of openers on isolated heart mitochondria. We have examined the effects of diazoxide and pinacidil on the bioenergetic properties of rat heart mitochondria. As expected of hydrophobic compounds, these drugs have toxic, as well as pharmacological, effects on mitochondria. Both drugs inhibit respiration and increase membrane proton permeability as a function of concentration, causing a decrease in mitochondrial membrane potential and a consequent decrease in Ca(2+) uptake, but these effects are not caused by opening mitochondrial K(ATP) channels. In pharmacological doses (<50 microM), both drugs open mitochondrial K(ATP) channels, and resulting changes in membrane potential and respiration are minimal. The increased K(+) influx associated with mitochondrial K(ATP) channel opening is approximately 30 nmol. min(-1). mg(-1), a very low rate that will depolarize by only 1-2 mV. However, this increase in K(+) influx causes a significant increase in matrix volume. The volume increase is sufficient to reverse matrix contraction caused by oxidative phosphorylation and can be observed even when respiration is inhibited and the membrane potential is supported by ATP hydrolysis, conditions expected during ischemia. Thus opening mitochondrial K(ATP) channels has little direct effect on respiration, membrane potential, or Ca(2+) uptake but has important effects on matrix and intermembrane space volumes.

Protection of heart against ischemia-reperfusion injury by ischemic preconditioning and K ATP channel openers is known to involve the mitochondrial ATPsensitive K ؉ channel (mitoK ATP ). Brain is also protected by ischemic preconditioning and K ATP channel openers, and it has been suggested that mitoK ATP may also play a key role in brain protection. However, it is not known whether mitoK ATP exists in brain mitochondria, and, if so, whether its properties are similar to or different from those of heart mitoK ATP . We report partial purification and reconstitution of a new mitoK ATP from rat brain mitochondria. We measured K ؉ flux in proteoliposomes and found that brain mitoK ATP is regulated by the same ligands as those that regulate mitoK ATP from heart and liver. We also examined the effects of opening and closing mitoK ATP on brain mitochondrial respiration, and we estimated the amount of mitoK ATP by means of green fluorescence probe BODIPY-FL-glyburide labeling of the sulfonylurea receptor of mitoK ATP from brain and liver. Three independent methods indicate that brain mitochondria contain six to seven times more mitoK ATP per milligram of mitochondrial protein than liver or heart.

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