K+ transport in mitoplasts

Permeability of the mitochondrial outer membrane to organic cations

Inhibition of mitochondrial respiration by hydrophobic fluorescent dyes (Rhodamine 6G, Safranine O, Pyronine B) is much less potentiated by digitonin-lysis of the outer membrane than that by polyamines or adriamycin. This situation may be explained by impermeability of the anion-selective channels in the outer mitochondrial membrane to large cations and by the ability of hydrophobic (but not polar or amphipathic) ions to directly permeate lipid bilayers.

The regulation of the matrix volume of mammalian mitochondria in vivo and in vitro and its role in the control of mitochondrial metabolism

The purpose of this article is to describe briefly the methods by which the intra-mitochondrial volume may be measured both in vitro and in situ, to summarise the mechanisms thought to regulate the mitochondrial volume and then to review in more detail the evidence that changes in the intra-mitochondrial volume play an important part in the regulation of liver mitochondrial metabolism by glucogenic hormones such as glucagon, adrenaline and vasopressin. It will be shown that these hormones cause an increase in matrix volume sufficient to produce significant activation of fatty acid oxidation, respiration and ATP production, pyruvate carboxylation, citrulline synthesis and glutamine hydrolysis. These are all processes activated by such hormones in vivo. I will go on to demonstrate that the increase in matrix volume is brought about by an increase in mitochondrial [PPi]. This is able to stimulate K+ entry into the matrix, perhaps through an interaction with the adenine nucleotide translocase. The rise in matrix [PPi] is a consequence of an increase in cytosolic and hence mitochondrial [Ca2+] which inhibits mitochondrial pyrophosphatase. In the final section of the review I provide evidence that changes in mitochondrial volume may be important in the responses of a variety of tissues to hormones and other stimuli. I write as a metabolist with a working knowledge of bioenergetics rather than the converse, and this will certainly be reflected in the approach taken. If I cause offence to any dedicated experts in the field of bioenergetic by my ignorance or lack of understanding of their studies I can only offer my apologies and ask to be corrected.

Reconstitution of transmembrane K+ transport with a 53 kilodalton mitochondrial protein

A 53 kDa protein has been purified from a Triton X-100 extract of liver mitochondrial membranes, by affinity chromatography on immobilized quinine, a K+ transport inhibitor. KCl-containing lipid vesicles reconstituted with this protein lose K+ to a medium low in K+ faster than vesicles lacking protein. With bacteriorhodopsin reconstituted in vesicles containing K+, light induces faster development of a pH gradient if the 53 kDa protein is included during vesicle preparation. This effect is like that of valinomycin, which catalyzes K+ efflux, dissipating the membrane potential arising from H+ entry. Evidence that vesicles containing the 53 kDa protein are permeable to K+, but exhibit low permeability to H+, indicates that this protein acts as a K+ uniporter.

Enhanced uptake of spermidine and methylglyoxal-bis(guanylhydrazone) by rat liver mitochondria following outer membrane lysis

Isolated rat liver mitochondria rapidly bound the 14C-labeled organic cations spermidine, a physiologically important polyamine, and methylglyoxal-bis(guanylhydrazone) (MGBG), an anticancer drug. This rapid, Mg2+-sensitive, respiration-independent binding is assumed to involve adsorption to anionic surface groups. A slower progressive uptake of the organic cations exhibited respiration dependence, indicating that it involves transport across the inner mitochondrial membrane into the matrix compartment. Addition of digitonin, to lyse the outer mitochondrial membrane, caused an increase in the mitochondrial content of the organic cations and enhanced the rate of progressive, respiration-dependent cation uptake. The data are consistent with the interpretation that the outer mitochondrial membrane limits access of the organic cations, spermidine and MGBG, to the inner mitochondrial membrane. This conclusion is supported also by published data indicating that outer membrane lysis enhances inhibitory effects of the organic cations on mitochondrial respiration. The uptake of spermidine by mitochondria was inhibited by MGBG.

Regulation of the mitochondrial matrix volume in vivo and in vitro. The role of calcium

The ability of alpha-adrenergic agonists and vasopressin to increase the mitochondrial volume in hepatocytes is dependent on the presence of extracellular Ca2+. Addition of Ca2+ to hormone-treated cells incubated in the absence of Ca2+ initiates mitochondrial swelling. In the presence of extracellular Ca2+, A23187 (7.5 microM) induces mitochondrial swelling and stimulates gluconeogenesis from L-lactate. Isolated liver mitochondria incubated in KCl medium in the presence of 2.5 mM-phosphate undergo energy-dependent swelling, which is associated with electrogenic K+ uptake and reaches an equilibrium when the volume has increased to about 1.3-1.5 microliter/mg of protein. This K+-dependent swelling is stimulated by the presence of 0.3-1.0 microM-Ca2+, leading to an increase in matrix volume at equilibrium that is dependent on [Ca2+]. Ca2+-activated K+-dependent swelling requires phosphate and shows a strong preference for K+ over Na+, Li+ or choline. It is not associated with either uncoupling of mitochondria or any non-specific permeability changes and cannot be produced by Ba2+, Mn2+ or Sr2+. Ca2+-activated K+-dependent swelling is not prevented by any known inhibitors of plasma-membrane ion-transport systems, nor by inhibitors of mitochondrial phospholipase A2. Swelling is inhibited by 65% and 35% by 1 mM-ATP and 100 microM-quinine respectively. The effect of Ca2+ is blocked by Ruthenium Red (5 micrograms/ml) at low [Ca2+]. Spermine (0.25 mM) enhanced the swelling seen on addition of Ca2+, correlating with its ability to increase Ca2+ uptake into the mitochondria as measured by using Arsenazo-III. Mitochondria derived from rats treated with glucagon showed less swelling than did control mitochondria. In the presence of Ruthenium Red and higher [Ca2+], the mitochondria from hormone-treated animals showed greater swelling than did control mitochondria. These data imply that an increase in intramitochondrial [Ca2+] can increase the electrogenic flux of K+ into mitochondria by an unknown mechanism and thereby cause swelling. It is proposed that this is the mechanism by which alpha-agonists and vasopressin cause an increase in mitochondrial volume in situ.

Stimulation of K+ flux into mitochondria by phenylarsine oxide

The dithiol-reactive reagent phenylarsine oxide causes a pH-dependent stimulation of unidirectional K+ flux into respiring rat liver mitochondria. This stimulation is diminished by subsequent addition of either the dithiol 2,3-dimercaptopropanol or the monothiol 2-mercaptoethanol. In contrast, uncoupling by phenylarsine oxide is reversed by 2,3-dimercaptopropanol but not by 2-mercaptoethanol. The data suggest separate sites of interaction of phenylarsine oxide with mechanisms of K+ entry and ATP synthesis. Stimulatory effects of mersalyl and phenylarsine oxide on K+ influx are not additive. Thus PheASO and mersalyl may affect K+ influx at a common site. Pretreatment of the mitochondria with DCCD, which inhibits K+ influx, fails to alter sensitivity to PheAsO or mersalyl. Thus the DCCD binding site associated with the K+ influx mechanism appears to be separate from and independent of the sulfhydryl group(s) which mediate stimulation of K+ influx by PheAsO and mersalyl. PheAsO, like mersalyl, also increases the rate of unidirectional K+ efflux from respiring mitochondria. The combined presence of PheAsO plus mersalyl causes a greater stimulation of K+ efflux than is observed with either reagent alone.