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At least 343 records · Page 19Linked to original sources
PubMed2000

Involvement of protein tyrosine kinase in osmoregulation of Na(+) transport and membrane capacitance in renal A6 cells.

Renal A6 cells have been reported in which hyposmolality stimulates Na(+) transport by increasing the number of conducting amiloride-sensitive 4-pS Na(+) channels at the apical membrane. To study a possible role of protein tyrosine kinase (PTK) in the hyposmolality-induced signaling, we investigated effects of PTK inhibitors on the hyposmolality-induced Na(+) transport in A6 cells. Tyrphostin A23 (a PTK inhibitor) blocked the stimulatory action of hyposmolality on a number of the conducting Na(+) channels. Tyrphostin A23 also abolished macroscopic Na(+) currents (amiloride-sensitive short-circuit current, I(Na)) by decreasing the elevating rate of the hyposmolality-increased I(Na). Genistein (another type of PTK inhibitor) also showed an effect similar to tyrphostin A23. Brefeldin A (BFA), which is an inhibitor of intracellular translocation of protein, blocked the action of hyposmolality on I(Na) by diminishing the elevating rate of the hyposmolality-increased I(Na), mimicking the inhibitory action of PTK inhibitor. Further, hyposmolality increased the activity of PTK. These observations suggest that hyposmolality would stimulate Na(+) transport by translocating the Na(+) channel protein (or regulatory protein) to the apical membrane via a PTK-dependent pathway. Further, hyposmolality also caused an increase in the plasma (apical) membrane capacitance, which was remarkably blocked by treatment with tyrphostin A23 or BFA. These observations also suggest that a PTK-dependent pathway would be involved in the hyposmolality-stimulated membrane fusion in A6 cells.

Animals↗
PubMed2002

Role of taurine in osmoregulation during endurance exercise.

Taurine is released by contracting muscles, but its actual role remains unspecified. In this study, we investigated whether the exercise-stimulated release of taurine from muscle into the plasma regulates the modification of osmolality induced by intramuscular osmolyte production. Six subjects performed 90 min of cycling exercise (at 70% maximum power output) on two occasions, with (HC) or without (DC) fluid intake. Taurine content was determined in plasma, blood cells and urine before and after the endurance events, together with plasma osmolality. Plasma osmolality increased by 4% in the DC experiment ( P<0.01), but remained stable in the HC condition. The exercise also induced changes in the mean (SD) plasma taurine content to a greater degree in HC [+63 (26)%] than in DC [+33 (18)%; P<0.05], supporting the hypothesis that taurine is released into the plasma via an osmoregulatory process. However, the higher plasma taurine content in HC was not related to changes in renal taurine. In addition, the increase of taurine in plasma was not related to its release from blood cells since their taurine concentration increased by 70% both in HC [429 (77) to 680 (82) microM; P=0.003] and in DC [451 (57) to 731 (34) microM; P<0.001]. The lack of correlation between plasma volume modification and the mass ratio of taurine would exclude a major role for taurine exchange in plasma volume regulation. Sodium ( R=0.967, P<0.001), chloride ( R=0.917, P<0.001) and osmolality ( R=0.924, P<0.001) seem to be the main regulators of plasma volume changes during exercise. In conclusion, changes in the plasma taurine content during endurance exercise is related to an osmoregulatory process, but this alone does not control plasma volume changes.

Adult↗
PubMed2000

Role of osmoregulation in the actions of taurine.

Taurine regulates an unusual number of biological phenomena, including heart rhythm, contractile function, blood pressure, platelet aggregation, neuronal excitability, body temperature, learning, motor behavior, food consumption, eye sight, sperm motility, cell proliferation and viability, energy metabolism and bile acid synthesis. Many of these actions are associated with alterations in either ion transport or protein phosphorylation. Although the effects on ion transport have been attributed to changes in membrane structure, they could be equally affected by a change in the activity of the affected transporters. Three common ways of altering transporter activity is enhanced expression, changes in the phosphorylation status of the protein and cytoskeletal changes. Interestingly, all three events are altered by osmotic stress. Since taurine is a key organic osmolyte in most cells, the possibility that the effects of taurine on ion transport could be related to its osmoregulatory activity was considered. This was accomplished by comparing the effects of taurine, cell swelling and cell shrinkage on the activities of key ion channels and ion transporters. The review also compares the phosphorylation cascades initiated by osmotic stress with some of the phosphorylation events triggered by taurine depletion or treatment. The data reveal that certain actions of taurine are probably caused by the activation of osmotic-linked signaling pathways. Nonetheless, some of the actions of taurine are unique and appear to be correlated with its membrane modulating and phosphorylation regulating activities.

Animals↗
PubMed1988

Glycinebetaine as an osmoregulant and compatible solute in the marine cyanobacterium Spirulina subsalsa.

Glycinebetaine was found to be the major organic substrate accumulating under hypersaline growth conditions in the halotolerant cyanobacterium Spirulina subsalsa. In addition to its proposed role as osmolite, glycinebetaine is shown to specifically protect enzymatic activity. Glucose-6-phosphate dehydrogenase from S. subsalsa retained full activity in the presence of NaCl at concentrations as high as 1.5 M, provided that comparable concentrations of glycinebetaine were also present in the reaction mixture. A kinetic analysis indicated that glycinebetaine protected the enzyme against both NaCl-induced decrease in Vmax and reduction in affinity to glucose 6-phosphate. The alternative osmolites, glycerol and proline, protected the enzyme against the reduction in Vmax but not against the reduction in affinity to glucose 6-phosphate.

Betaine↗
PubMed1984

Osmoregulation of amino acid transport activity in cultured fibroblasts.

The effect of exposure of chick embryo cells to increasing concentrations of Na+ in the culture medium on the subsequent amino acid transport as determined at physiological osmolarity was investigated in detail. It was found that the hyperosmolar treatment stimulated amino acid transport in a dose-dependent manner up to 200 mM Na+. Changes were measurable as early as 1 h after altering Na+ and reached a maximum after 4 h, remaining constant thereafter. The maintenance of this effect required continuous exposure of the cell to high Na+ in the culture medium. Hyperosmolarity-mediated increases in amino acid transport activity by system A have been detected with L-proline and L-alanine. Transport activities of systems ASC and L did not change appreciably after exposure of the cells to high Na+. Inhibition of protein synthesis by cycloheximide or RNA synthesis by actinomycin D (actD) prevented these uptake changes. Kinetic analysis indicated that the stimulation of the activity of transport system A by high Na+ treatment occurred through a mechanism affecting Vmax rather than Km.

Amino Acids↗
PubMed1991

Osmoregulation in Saccharomyces cerevisiae. Studies on the osmotic induction of glycerol production and glycerol-3-phosphate dehydrogenase (NAD+)

Production of glycerol and a key enzyme in glycerol production, glycerol 3-phosphate dehydrogenase (NAD+) (GPD), was studied in Saccharomyces cerevisiae cultured in basal media or media of high salinity, with glucose, raffinose or ethanol as the sole carbon source. At high salinity, glycerol production was stimulated with all carbon sources and glycerol was accumulated to high intracellular concentration in cells grown on glucose and raffinose. Cells grown on ethanol accumulated glycerol to a lower level but showed an increased content of trehalose at high salinity. However, the trehalose concentration corresponded only to about 20% of the glycerol level, and did not compensate for the shortfall in intracellular osmolyte content. Immunoblot analysis demonstrated an increased production of GPD at high salinity. This increase was osmotically mediated but was lower when glycerol was substituted for NaCl or sorbitol as the stress-solute. The enzyme also appeared to be subject to glucose repression; the specific activity of GPD was significantly lower in cells grown on glucose, than on raffinose or ethanol.

Ethanol↗