Nałecz Mj

ATP-regulated K+ channel in mitochondria: pharmacology and function.

Mitochondria from several tissues contain a potassium-specific channel similar to the ATP-regulated K+ (KATP) channel of the plasma membrane. The mitochondrial channel shares with the plasma membrane KATP channel the sensitivity to sulfonylurea derivatives and some other blockers as well as to channel openers of diverse chemical character. In contrast to the plasma membrane channel, which is blocked by free ATP, the mitochondrial KATP channel reconstituted into liposomes requires the ATP-Mg complex for inhibition. The mitochondrial KATP channel, possibly in a concerted action with other K+ permeability pathways, plays an important role in mitochondrial volume control. Its function in the regulation of the components of the protonmotive force is also suggested.

Effect of externally added carnitine on the synthesis of acetylcholine in rat cerebral cortex cells

Acetylcholine synthesis from radiolabelled glucose was monitored in cerebral cortex cells isolated from brains of suckling and adult rats. Acetylcholine synthesis was found much higher in suckling animals, both in the absence and presence of acetylcholinesterase (acetylcholine hydrolase, EC 3.1.1.7) inhibitor, paraoxon. Together with choline (20 microM), carnitine was found to stimulate acetylcholine synthesis in a synergistic way in cortex cells from adult rats (18%). Choline, however, was incapable of reversing an inhibitory effect exerted by carnitine on acetylcholine synthesis in cortex cells from suckling animals. Distribution of carnitine derivatives was found significantly different in the cells from young and old animals, the content of acetylcarnitine decreased with age with a corresponding increase of free carnitine. The observed differences in carnitine effect on acetylcholine synthesis suggested that high acetylcarnitine in cells capable of beta-oxidation might be correlated with the lower level of acetylcholine synthesis.

The activity of pyruvate carrier in a reconstituted system: substrate specificity and inhibitor sensitivity.

The pyruvate carrier, of molecular mass 34 kDa, was purified from mitochondria isolated from rat liver, rat brain, and bovine heart, by affinity chromatography on immobilized 2-cyano-4-hydroxycinnamate. Its activity after reconstitution in phosphatidylcholine vesicles was measured either as uptake of [1-14C]pyruvate or as exchange with different 2-oxoacids. All preparations exhibited similar apparent Km values for pyruvate, but somewhat different V(max) values. The ability to exchange different anions of physiological significance, including branched-chain 2-oxoacids, confirmed the known substrate specificity described for the pyruvate carrier in mitochondria. The sensitivity of pyruvate transport toward phenylglyoxal suggested an important role of arginyl residues in the transport activity, while a role of lysyl and histidyl residues was not confirmed.

Transport of carnitine in RBE4 cells ‐ an in vitro model of blood‐brain barrier

Accumulation of carnitine was studied in immortalized rat brain capillary endothelial cells RBE4, as an in vitro model of blood-brain barrier. Both uptake and efflux phenomena were found independent on [Na+] gradient. A quick acylation of carnitine in RBE4 cells was observed, especially leading to formation of long-chain acylcarnitines (46%), while 38% of the accumulated compound was found in the form of free carnitine. Exposure of basolateral membrane to carnitine-free medium induced a quick efflux, leaving, however, about 50% of accumulated carnitine inside the cells. The process of accumulation was found to be sensitive to butyrobetaine and cysteine, but not to choline or Hemicholinium-3, pointing to sterical demands of a transporter responsible for carnitine uptake.

EFFECTS OF K* CHANNEL INHIBITORS ON POTASSIUM TRANSPORT IN BOVINE ADRENAL CHROMAFFIN GRANULES

The Ca2+‐independent K+ selective channel in the membrane of adrenal gland chromaffin granules has earlier been identified. In the present report we describe new properties of this potassium channel using 86Rb+, a K+ analogue, flux measurements. The studies are performed in membrane vesicles prepared from chromaffin granules. The electrogenic 86Rb+ transport is inhibited by quinacrine, barium, zinc and magnesium. The effects of other potassium channel blockers on 86Rb+ transport into chromaffin granules are also reported.

Is there sufficient experimental evidence to consider the mitochondrial cytochromebc1 complex a proton pump? Probably no.

The electron flow through the cytochromebc1 complex of the mitochondrial respiratory chain is accompanied by vectorial proton translocation, though the mechanism of the latter phenomenon has not yet been clarified. Several proposed hypotheses are briefly presented and discussed here. Recently, a number of papers have appeared claiming the existence of a proton pump in the enzyme mainly on the basis of the interaction of the complex with N,N′-dicyclohexylcarbodiimide. These data are reviewed here with the aim of showing their ability to fit multiple interpretations. This together with some other arguments leads to the conclusion that a proton pump in the mitochondrialbc1 complex has not yet been demonstrated.