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A Szewczyk

Publications and source records attributed to A Szewczyk.

At least 37 records · Page 2Linked to original sources
PubMed1997

An antagonist of ATP-regulated potassium channels, the guanidine derivative U-37883A, stimulates the synthesis of phosphatidylserine in rat liver endoplasmic reticulum membranes.

The guanidine derivative U-37883A has been found to stimulate in vitro synthesis of phosphatidylserine in endoplasmic reticulum membranes, catalyzed exclusively by a serine-specific base exchange enzyme. The stimulation of the enzyme activity by the drug was concentration-dependent, with EC50 of 54 microM, while the biologically inactive analog of U-37883A, U-42069, was without effect. The stimulation caused by U-37883A was enhanced under the conditions when active transport of Ca2+ into the lumen of microsomal vesicles was induced, whereas it was inhibited by a calcium ionophore, A23187, and by a specific inhibitor of Ca2+-ATPase, thapsigargin. On the other hand, a potassium ionophore, valinomycin, had no effect on phosphatidylserine synthesis. U-37883A did not affect the Km of the base exchange enzyme for serine, but greatly reduced the EC50 value of the enzyme for calcium. Furthermore, Ca2+ uptake by endoplasmic reticulum vesicles has been found to increase in the presence of U-37883A. These observations suggest that U-37883A enhances phosphatidylserine synthesis indirectly by acting on calcium transport, thus affecting calcium concentration within the lumen of endoplasmic reticulum membranes. Alternatively, the effect of the drug could be propagated via the mechanism by which phospholipid flip-flop movement, known to regulate the serine-specific base exchange reaction, is modulated.

Adamantane↗
PubMed1997

The mitochondrial sulfonylurea receptor: identification and characterization.

Biochemical identification of mitochondrial sulfonylurea receptors has been carried out through binding studies performed with [3H]glibenclamide. The presence of a single class of low affinity binding sites for glibenclamide in the inner mitochondrial membrane has been found, with a KD of 360 +/- 48 nM and BMAX of 48 +/- 7 pmoles/mg in beef heart mitochondria. Glibenclamide binding was affected by other sulfonylureas (glipizide, glisoxepide) but not by potassium channel openers (diazoxide, pinacidil, RP66471). In both rat liver and beef heart mitochondria adenine nucleotides (ATP, ADP, AMP) and nucleotide analogs (triazine dyes) produced large inhibition (from 60 to 80%) of [3H]glibenclamide binding. Photoaffinity labeling of submitochondrial particles with [125I]-glibenclamide revealed a single specifically labeled polypeptide band of 28 kDa by SDS-PAGE that is postulated to be (or to form a part of) the mitochondrial sulfonylurea receptor.

ATP-Binding Cassette Transporters↗
PubMed1997

Effects of K+ channel inhibitors on potassium transport in bovine adrenal chromaffin granules.

The Ca(2+)-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.

Adrenal Medulla↗
PubMed1997

Interaction of sulfhydryl reagents with K+ transport in adrenal chromaffin granules.

In the present study the functional role of SH groups in the Ca(2+)-independent K(+)-selective channel activity in the membrane of bovine adrenal gland chromaffin granules has been studied. Ionic channel activity has been estimated using 86Rb+, a K+ analogue, flux measurements. The inhibition of chromaffin granules K+ channel by SH modifying agents, such as N-ethylmaleimide, mersalyl and phenylarsenoxide, is described.

Adrenal Medulla↗
PubMed1996

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

Mitochondria from several tissues contain a potassium-specific channel similar to the ATP-regulated K+ (K ATP) channel of the plasma membrane. The mitochondrial channel shares with the plasma membrane K ATP 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 K ATP channel reconstituted into liposomes requires the ATP-Mg complex for inhibition. The mitochondrial K ATP 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.

Adenine Nucleotides↗
PubMed1996

Glibenclamide inhibits mitochondrial K+ and Na+ uniports induced by magnesium depletion.

Magnesium depletion induces K+ and Na+ uniports in rat liver mitochondria. The purpose of the present study was to investigate the effects exerted by the antidiabetic sulfonylurea, glibenclamide, a well known blocker of ATP-sensitive potassium channels, on mitochondrial K+ and Na+ uniports. The K+ and Na+ uniport activities were monitored indirectly, in energized mitochondria, by following K+ and Na+ influxes as measured by light scattering. The membrane potential of the mitochondria was determined using a TPP+ selective electrode. Equilibrium binding measurements of glibenclamide to the inner mitochondrial membrane was performed with [3H]glibenclamide. Mitochondrial K+ and Na+ uniports were found to be inhibited by glibenclamide in a concentration-dependent manner, with IC50 of 20 +/- 7 and 15 +/- 8 microM, respectively. On lowering of the pH value, the potency of glibenclamide to inhibit the uniports activity was increased. Binding studies revealed the presence of a single class of low affinity binding sites for glibenclamide in the inner mitochondrial membrane, with a Kd of 4 +/- 2 microM and a BMAX of 148 +/- 50 pmoles/mg of protein. The present study provides evidence that both mitochondrial K+ and Na+ uniport activities are sensitive to the antidiabetic sulfonylurea, glibenclamide.

Animals↗
PubMed1996

Effects of inhibitors and activators of ATP-regulated K+ channel on mitochondrial potassium uniport.

The effect of inhibitors and activators (channel openers) of ATP-regulated potassium channel on potassium ion transport was studied in isolated rat liver mitochondria. In order to follow the potassium transport into mitochondrial matrix light scattering measurements have been employed. The membrane potential of rat liver mitochondria, upon addition of potassium channel openers, was measured with the use of TPP-ion selective electrode. The results show that some of the potassium channel openers like HOE 234 and Y27152 are able to activate potassium transport into mitochondria. On the other hand, mitochondrial potassium uniport found was inhibited by the antidiabetic sulfonylureas and was not affected by other potassium channel inhibitors like 4-amino pyridine.

Adenosine Triphosphate↗
PubMed1996

The ATP-regulated K+ channel in mitochondria: five years after its discovery.

Mitochondria contain a potassium specific channel (mitoKATP channel) sensitive to ATP and antidiabetic sulfonylureas. The mitochondrial KATP channel plays an important role in the mitochondrial volume control and in regulation of the components of protonmotive force. This minireview describes the properties and current hypotheses concerning the function of mitoKATP channel.

ATP-Binding Cassette Transporters↗
PubMed1995

The role of mitochondrial potassium fluxes in controlling the protonmotive force in energized mitochondria.

The two components of the protonmotive force, the pH gradient (delta pH) and the transmembrane electric potential (delta psi), were measured in rat liver mitochondria as a function of K+ concentration in the suspending medium. It was found that both the rate of formation and the final level of delta pH upon energization of mitochondria with succinate increased with increasing [K+]. Concomitantly, delta psi decreased so that the level of the protonmotive force remained practically unchanged. Potassium channel opener RP66471 further potentiated both the formation rate and the level of delta pH. These results are interpreted as showing that the electrophoretic K+ influx enables the formation of delta pH by partly compensating charge transfer due to the proton pumping.

Animals↗
PubMed1995

Potassium channel opener, RP 66471, induces membrane depolarization of rat liver mitochondria.

Effect of potassium channel openers on membrane potential of rat liver mitochondria was studied. It has been found that potassium channel opener RP 66471 induces depolarization of the mitochondrial membrane. Since neither the inhibition of mitochondrial respiration nor the uncoupling of mitochondria was observed concomitantly, the specific effect on the mitochondrial potential is postulated. Most likely the effect is caused by the increase of permeability of the inner mitochondrial membrane to potassium ions. Interestingly, however, it was found that no other potassium channel openers tested but RP 66471 was able to induce depolarization of mitochondrial membrane.

Animals↗
PubMed1993

Demonstration of glibenclamide-sensitive K+ fluxes in rat liver mitochondria.

Influx of K+ into rat liver mitochondria driven by the transmembrane potential was investigated. Under non-energized conditions (non-respiring mitochondria suspended in isotonic KSCN) K+ influx (manifested by mitochondrial swelling) was enhanced by pinacidil and its derivative P1060, known as K+ channel openers in the plasma membrane; this stimulation being inhibited by potassium channel blocker glibenclamide. Under energized conditions (respiring mitochondria) the rate of K+ uptake (measured with K(+)-sensitive electrode) was increased by P1060 and slowed down by glibenclamide. These results indicate functioning of a specific potassium channel in intact liver mitochondria.

Animals↗
PubMed1992

ATP-sensitive K+ channels in insulinoma cells are activated by nonesterified fatty acids.

Both 86Rb+ efflux experiments and electrophysiological studies have shown that arachidonic acid and other nonesterified fatty acids activate ATP-sensitive K+ channels in insulinoma cells (HIT-T15). Activation was observed with arachidonic, oleic, linoleic, and docosahexaenoic acid but not with myristic, stearic, and elaidic acids. Fatty acid activation of ATP-sensitive K+ channels was blocked by antidiabetic sulfonylureas such as glibenclamide. The activating effect of arachidonic acid was unaltered by indomethacin and by nordihydroguaiaretic acid, indicating that it is not due to metabolites of arachidonic acid via cyclooxygenase or lipoxygenase pathways. Moreover, the nonmetabolizable analogue of arachidonic acid, eicosatetraynoic acid, was an equally potent activator. Activation of ATP-sensitive K+ channels by fatty acids was potentiated by diacylglycerol and was inhibited by calphostin C, an inhibitor of protein kinase C. These findings indicate that fatty acid activation of ATP-sensitive K+ channels is most likely due to the participation of arachidonic acid (and other fatty acid)-activated protein kinase C isoenzymes. Activation of ATP-sensitive K+ channels by nonesterified fatty acids is not involved in the control of insulin secretion since arachidonic acid stimulates insulin secretion from insulinoma cells instead of inhibiting it.

5,8,11,14-Eicosatetraynoic Acid↗
PubMed1991

Inhibition of cell proliferation by alpha-tocopherol. Role of protein kinase C.

The effect of alpha-tocopherol (vitamin E) on the proliferation of vascular smooth muscle cells (A7r5), human osteosarcoma cells (Saos-2), fibroblasts (Balb/3T3), and neuroblastoma cells (NB2A) has been studied. The proliferation of vascular smooth muscle cells was inhibited by physiologically relevant concentrations of alpha-tocopherol, neuroblastoma cells were only sensitive to higher alpha-tocopherol concentrations, and proliferation of the other cell lines was not inhibited. The inhibition of smooth muscle cell proliferation was specific for alpha-tocopherol. Trolox, phytol, and alpha-tocopherol esters had no effect. Proliferation of smooth muscle cells stimulated by platelet-derived growth factor or endothelin was completely sensitive to alpha-tocopherol. If smooth muscle cells were stimulated by fetal calf serum, proliferation was 50% inhibited by alpha-tocopherol. No effect of alpha-tocopherol was observed when proliferation of smooth muscle cells was stimulated by bombesin and lysophosphatidic acid. The possibility of an involvement of protein kinase C in the cell response to alpha-tocopherol was suggested by experiments with the isolated enzyme and supported by the 2- to 3-fold stimulation of phorbol ester binding induced by alpha-tocopherol in sensitive cells. Moreover, alpha-tocopherol also caused inhibition of protein kinase C translocation induced by phorbol esters and inhibition of the phosphorylation of its 80-kDa protein substrate in smooth muscle cells. A model is discussed by which alpha-tocopherol inhibits cell proliferation by interacting with the cytosolic protein kinase C, thus preventing its membrane translocation and activation.

Animals↗
PubMed1991

Azido derivative of tricarboxylic acid for photoaffinity labeling.

A new photoaffinity probe, 5-(1-hydroxy-4-azidophenylazo)-1,2,3-benzenetricarboxylic acid, was synthesized and characterized. This reagent can be potentially used in photoaffinity labeling of the mitochondrial tricarboxylate carrier, as well as of enzymes interacting with tricarboxylic acids. Inhibition and labeling of the mitochondrial tricarboxylate carrier is presented.

Affinity Labels↗
PubMed1991

Alpha-tocopherol (vitamin E) regulates vascular smooth muscle cell proliferation and protein kinase C activity.

Alpha-Tocopherol (vitamin E) protects against free radical damage, which has been implicated in aging, cancer initiation, and atherosclerosis. We have found that physiological concentrations of alpha-tocopherol specifically inhibited aorta smooth muscle cell (VSMC, line A7r5) proliferation and protein kinase C (PKC) activity. Other water and lipid soluble antioxidants were inactive. alpha-Tocopherol inhibition of PKC and of VSMC proliferation may represent a physiological mechanism, relevant to the onset of diseased states such as atherosclerosis.

Adenosine Triphosphate↗