Effects of pressure on glucose transport in human erythrocytes

Functional characterisation of glucose transport in bovine articular chondrocytes

The adequate provision of glucose to articular chondrocytes is essential to sustain their predominantly anaerobic metabolism; glucose is also a precursor for the extracellular matrix macromolecules which these cells synthesise. Impaired glucose uptake would compromise cell function and potentially result in an imbalance of matrix synthesis and degradation, leading to osteoarthritis. We studied the glucose influx pathway into bovine articular chondrocytes using 2-deoxy- d-[(3)H]-glucose (DOG). Uptake occurs via an extracellular pH (pH(o))-insensitive, phloretin- and cytochalasin B-sensitive pathway, hallmarks of the GLUT family of facilitative glucose transporters, with a K(m) of 0.35+/-0.11 mM. Uptake was affected by a number of physiologically relevant factors: (1) raised hydrostatic pressure (1-30 MPa) inhibited DOG uptake by up to 30%; (2) interleukin-1 (IL-1beta) reduced uptake via an increase in transporter affinity; (3) glucosamine inhibited glucose uptake in a manner consistent with the actions of a competitive inhibitor. Given the involvement of IL-1beta in osteoarthritis and the protective role assigned to glucosamine, these findings implicate an important role for glucose delivery in chondrocyte energy production and matrix metabolism, which, therefore, may potentially affect the maintenance of cartilage integrity.

Tryptophan permease gene TAT2 confers high-pressure growth in Saccharomyces cerevisiae

Hydrostatic pressure in the range of 15 to 25 MPa was found to cause arrest of the cell cycle in G(1) phase in an exponentially growing culture of Saccharomyces cerevisiae, whereas a pressure of 50 MPa did not. We found that a plasmid carrying the TAT2 gene, which encodes a high-affinity tryptophan permease, enabled the cells to grow under conditions of pressure in the range of 15 to 25 MPa. Additionally, cells expressing the Tat2 protein at high levels became endowed with the ability to grow under low-temperature conditions at 10 or 15 degrees C as well as at high pressure. Hydrostatic pressure significantly inhibited tryptophan uptake into the cells, and the Tat2 protein level was down-regulated by high pressure. The activation volume associated with tryptophan uptake was found to be a large positive value, 46.2 +/- 3.85 ml/mol, indicating that there was a net volume increase in a rate-limiting step in tryptophan import. The results showing cell cycle arrest in G(1) phase and down-regulation of the Tat2 protein seem to be similar to those observed upon treatment of cells with the immunosuppressive drug rapamycin. Although rapamycin treatment elicited the rapid dephosphorylation of Npr1 and induction of Gap1 expression, hydrostatic pressure did not affect the phosphorylation state of Npr1 and it decreased the level of Gap1 protein, suggesting that the pressure-sensing pathway may be independent of Npr1 function. Here we describe high-pressure sensing in yeast in comparison with the TOR-signaling pathway and discuss an important factor involved in adaptation of organisms to high-pressure environments.

Glucose affinity for the glucose transporter Glut1 in native or reconstituted lipid bilayers. Temperature-dependence study by biomembrane affinity chromatography

The affinity of D-glucose and the transport inhibitor cytochalasin B (CB) for the glucose transporter Glut1 was studied at 5-42 degrees C by quantitative frontal affinity chromatography on sterically immobilized human red cell membrane vesicles, and on proteoliposomes containing reconstituted red cell membrane proteins. Glut1 in the vesicles showed the highest glucose affinity; the dissociation constant Kd(glc) was nearly constant (16 +/- 3 mM) from 15 degrees C to 37 degrees C. For Glut1 in proteoliposomes Kd(glc) decreased from 56 mM at 5 degrees C to 26 mM at 42 degrees C. The CB-Glut1 affinity was strongest around 20 degrees C and was mostly higher with the vesicles, Kd (CB) being 49 nM at 19 degrees C. The entropy and entropy and enthalpy changes for the interactions were calculated.

Ionic fluxes through myelin membrane vesicles

Technical difficulties and a lack of reproducibility in procedures aimed at the production of myelin vesicles have delayed functional studies on membrane transport through myelin. Myelin vesicles could provide an excellent model for the study of the transport of ions and water, etc., across this type of membrane. They could also help improve our understanding of the molecular functions of the myelin sheath. In this investigation, a novel, nonaggressive method of producing central nervous system myelin vesicles is presented. Purified bovine myelin was incubated with iminodiacetic acid (an insoluble chelating agent that is easy to remove and does not interfere with further functional assays), and rendered insoluble on 1% crosslinked polystyrene beads (Chelex-100). Myelin vesicles obtained were impermeable to sugars (sucrose, glucose, and galactose), but showed a degree of permeability towards potassium salts as determined by light-scattering. Further experiments with fluorescent probes revealed an electrogenic K+ influx, as measured by oxonol V fluorescence quenching, and a significant H+ permeability measured using the pH-sensitive probe acridine orange. H+ permeability was not detected in control liposomes made from the same endogenous myelin lipids without protein. The results are discussed with reference to previous studies performed using purified myelin proteins in reconstituted systems. The relevance of these results with respect to ionic transport across myelin membrane is discussed.