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The ATP-sensitive potassium channel from the inner mitochondrial membrane (mitoK ATP ) is a highly selective conductor of K ؉ ions. When isolated in the presence of nonionic detergent and reconstituted in liposomes, mitoK ATP is inhibited with high affinity by ATP (K 1/2 ‫؍‬ 20 -30 M). We have suggested that holo-mitoK ATP is a heteromultimer consisting of an inwardly rectifying K The importance of the mitochondrial K ϩ cycle for volume regulation, reviewed by Garlid and Paucek (1) was recognized by Mitchell (2) long before any of the components of this cycle were discovered. K ϩ is driven into the matrix by the high membrane potential (⌬⌿) 1 generated by the proton-pumping electron transport system, and excess K ϩ is removed by the regulated K ϩ /H ϩ antiporter. Electrophoretic K ϩ influx occurs by diffusion and by means of ATP-sensitive potassium channel from the inner mitochondrial membrane (mitoK ATP ). At the high values of ⌬⌿ maintained by mitochondria, both of these processes increase exponentially with ⌬⌿ (3, 4) and are consequently very sensitive to fluctuations in ⌬⌿. These fluctuations, in turn, are high in tissues such as heart, which undergo large variations in energy demand and ATP synthesis rates (5). Thus, regulation of K ϩ influx and efflux pathways can be seen as a means of regulating volume in the face of the changing energy requirements of the cell. MitoK ATP plays more than a housekeeping role in cell physiology. There is now general agreement that mitoK ATP plays a key role in cardioprotection against ischemia-reperfusion injury (6, 7). The proposed mechanisms of this protective effect of mitoK ATP opening (5, 8, 9) are plausible; however, it is evident that more needs to be known about the functional properties of mitoK ATP before its role in vivo can be ascertained.By using a novel ethanol extraction technique, Mironova et al. (10) were the first to report reconstitution in lipid bilayer membranes of a 55-kDa K ϩ channel from mitochondria. Paucek et al. (3) used a detergent extraction technique and were the first to report reconstitution of mitoK ATP in liposomes. The latter channel was associated with two proteins of molecular mass 55 and 63 kDa, and we hypothesized that mitoK ATP is a heteromultimeric complex consisting of a 55-kDa inwardly rectifying K ϩ channel (mitoKIR) and a 63-kDa sulfonylurea receptor (mitoSUR), analogous to the plasma membrane ATPdependent K ϩ channel (cellK ATP ) (11,12). In this report, we focus on three interactions that address the key question of whether the 55-kDa K ϩ channel protein observed in the ethanol purification is the same as the 55-kDa protein purified with detergents. First, we show that UDP reverses ATP-inhibition of K ϩ flux mediated by both mitoKIR and mitoK ATP reconstituted in liposomes. Moreover, UDP exerts the same action in isolated mitochondria, and the affinities for the opening effect of UDP are about the same in each preparation. Second, we show that the mitoKIR opener p-diethylaminoethylbenzoate (DEB) also activates K ϩ flux via ...

Formation of palmitic acid/Ca(2+) (PA/Ca(2+)) complexes was suggested to play a key role in the non-classical permeability transition in mitochondria (NCPT), which seems to be involved in the PA-induced apoptosis of cardiomyocytes. Our previous studies of complexation of free fatty acids (FFA) with Ca(2+) showed that long-chain (C:16-C:22) saturated FFA had an affinity to Ca(2+), which was much higher than that of other FFA and lipids. The formation of FFA/Ca(2+) complexes in the black-lipid membrane (BLM) was demonstrated to induce a nonspecific ion permeability of the membrane. In the present work, we have found that binding of Ca(2+) to PA incorporated into the membrane of sulforhodamine B (SRB)-loaded liposomes results in an instant release of a part of SRB, with the quantity of SRB released depending on the concentration of PA and Ca(2+). The pH-optimum of this phenomenon, similar to that of PA/Ca(2+) complexation, is in the alkaline range. The same picture of SRB release has been revealed for stearic, but not for linoleic acid. Along with Ca(2+), some other bivalent cations (Ba(2+), Sr(2+), Mn(2+), Ni(2+), Co(2+)) also induce SRB release upon binding to PA-containing liposomes, while Mg(2+) turns out to be relatively ineffective. As revealed by fluorescence correlation spectroscopy, the apparent size of liposomes does not alter after the addition of PA, Ca(2+) or their combination. So it has been supposed that the cause of SRB release from liposomes is the formation of lipid pores. The effect of FFA/Ca(2+)-induced permeabilization of liposomal membranes has several analogies with NCPT, suggesting that both these phenomena are of similar nature.

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