Cotransport Of Water Research Articles

Uncoupled active transport mechanisms accounting for low selectivity in multidrug carriers: P-glycoprotein and SMR antiporters.

The extraordinarily low substrate specificity of P-glycoprotein conflicts with the notion that specific substrate interactions are required in the control of the reaction path in an active transport system. The difficulty is shown to be overcome by a half-coupled mechanism in which the ATP reaction is linked to carrier transformations, as in a fully coupled system, but in which the transported substrate plays a passive role. The mechanism, which requires no specific interaction with the substrate, brings about uphill transport. A half-coupled mechanism is directly supported by two observations: (i) almost completely uncoupled ATPase activity in purified P-glycoprotein, and (ii) a pattern of substrate specificity like that of passive systems, where maximum rates for different substrates vary little (unlike active systems, where maximum rates vary greatly). The mechanism accommodates other findings: partial inhibition of ATPase activity by an actively transported substrate; simultaneous binding and translocation of more than one substrate molecule; and stimulation or inhibition of the transport of one substrate molecule by another. A half-coupled system associated with an internal competitive inhibitor should behave as if tightly coupled, in agreement with the effects of the synthetic peptide, polytryptophan. The degree of coupling in the intact system is yet to be determined, however. A half-coupled ATPase mechanism could originally have evolved in a flippase, where immersion of the carrier in its substrate, the membrane lipid, precludes uncoupled ATP hydrolysis. These concepts may have wider application. An uncoupled antiport mechanism, driven by a proton gradient rather than ATP, can explain low selectivity in the SMR multidrug carriers of bacteria, and a half-coupled mechanism for the ion-driven cotransport of water (the substrate in which the carrier site is immersed) can explain a recently proposed uphill flow of water.

The human Na+-glucose cotransporter is a molecular water pump.

1. The human Na+-glucose cotransporter (hSGLT1) was expressed in Xenopus laevis oocytes. The transport activity, given by the Na+ current, was monitored as a clamp current and the concomitant flux of water followed optically as the change in oocyte volume. 2. When glucose was added to the bathing solution there was an abrupt increase in clamp current and an immediate swelling of the oocyte. The transmembrane transport of two Na+ ions and one sugar molecule was coupled, within the protein itself, to the influx of 210 water molecules. 3. This stoichiometry was constant and independent of the external parameters: Na+ concentrations, sugar concentrations, transmembrane voltages, temperature and osmotic gradients. 4. The cotransport of water occurred in the presence of adverse osmotic gradients. In accordance with the Gibbs equation, energy was transferred within the protein from the downhill fluxes of Na+ and sugar to the uphill transport of water, indicative of secondary active transport of water. 5. Unstirred layer effects were ruled out on the basis of experiments on oocytes treated with gramicidin or other ionophores. Na+ currents maintained by ionophores did not lead to any initial water movements. 6. The finding of a molecular water pump allows for new models of cellular water transport which include coupling between ion and water fluxes at the protein level; the hSGLT1 could account for almost half the daily reuptake of water from the small intestine.

Effects of extracellular environment on the osmotic signal transduction involved in activation of motility of carp spermatozoa.

The mechanism by which a hypo-osmotic shock activates motility of carp spermatozoa was studied. The direct role of osmolality at the axoneme was investigated after demembranation of spermatozoa with Triton X-100 and reactivation in various ionic or anionic solutions containing Mg-ATP: demembranated spermatozoa remain motile in solutions of osmolality up to 550 mOsm kg-1 while non-demembranated spermatozoa are immotile when osmolality rises above 250 mOsm kg-1 with the same salt solutions as well as in non-ionic solutions. Suspension in hypo-osmotic saline solutions triggered the swelling of native carp spermatozoa. No motility or swelling occurred above 200-300 mOsm kg-1 and this osmolality is probably that of the cytosol. The swelling of carp spermatozoa is the result of an entrance of water but this was not affected by pCMBS, an inhibitor of the aquaporin CHIP28, or by various inhibitors of the co-transport of water with ions. Various pharmacological agents that affect the motility of different sperm species had no effect on carp sperm motility when used under similar conditions. However, prolonged exposure to a solution devoid of K+ or Cl- affects the activation of motility in a reversible manner, suggesting that these ions have a role in the perception or transduction of the osmotic signal. Altering the concentration of intracellular second messengers such as Ca2+ and cAMP, and the pH did not affect the motility of carp spermatozoa. However, DMSO at 1-20% (400-3200 mOsm kg-1) affects the motility of carp spermatozoa 3-4 min after mixing. These results show that the activation signal of carp sperm motility differs from that known for spermatozoa of other species of fish such as trout. Our results indicate that the activation mechanism may involve a co-transport of ions or specific 'stretch-activated channels' that are sensitive to osmotic pressure.

Cotransport of water by the Na+/glucose cotransporter.

Water is transported across epithelial membranes in the absence of any hydrostatic or osmotic gradients. A prime example is the small intestine, where 10 liters of water are absorbed each day. Although water absorption is secondary to active solute transport, the coupling mechanism between solute and water flow is not understood. We have tested the hypothesis that water transport is directly linked to solute transport by cotransport proteins such as the brush border Na+/glucose cotransporter. The Na+/glucose cotransporter was expressed in Xenopus oocytes, and the changes in cell volume were measured under sugar-transporting and nontransporting conditions. We demonstrate that 260 water molecules are directly coupled to each sugar molecule transported and estimate that in the human intestine this accounts for 5 liters of water absorption per day. Other animal and plant cotransporters such as the Na+/CI-/gamma-aminobutyric acid, Na+/iodide and H+/amino acid transporters are also able to transport water and this suggests that cotransporters play an important role in water homeostasis.

Effects of carbonic anhydrase II (CAII) deficiency on CNS structure and function in the myelin‐deficient CAII‐deficient double mutant mouse

In the choroid plexus carbonic anhydrase II (CAII) supports the transport of bicarbonate ions, sodium ions, and water from blood to the CSF, and in the myelin sheath CAII supports compaction of myelin by stimulating cotransport of ions and water out from between the myelin membranes. In view of the latter, it is surprising that mutant mice deficient in CAII (Car-2n) have compact myelin. Since myelin basic protein also takes part in myelin compaction, we bred double CAII-deficient, myelin-deficient (Mld) mutant mice, in which the adults would have some compact myelin sheaths and a partial deficiency in myelin basic protein, with a view to examining oligodendrocytes and myelin sheaths in the double mutant. Like the parent Mld strain, the double mutants displayed tremors and seizures; however, the onset of seizures was delayed significantly in the double mutants, and the lifespan increased by several months. Like the brains of Car-2n mutants, those of double mutants (MldCar-2n) were deficient in mRNA and protein for CAII and showed upregulation of a different isozyme, CAIV. In the double mutants, oligodendrocytes were reduced in number, and the myelin sheaths and oligodendrocytes were swollen. The partial protection against seizures, which CAII deficiency conferred, suggests that acidosis in the central nervous system (CNS) of the Car-2n and MldCar-2n mice, due to absence of CAII from the choroid plexus, may downregulate the activity of NMDA receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Cotransport of salt and water in membrane proteins: Membrane proteins as osmotic engines

Cotransport of salt and water in membrane proteins: Membrane proteins as osmotic engines

Comparison of the separation of lactic acid and l-leucine by liquid emulsion membranes

A mass transfer model originally proposed for the permeation of zinc and l-phenylalanine was slightly modified and used for calculating the mass transfer resistances in lactic acid permeation (using the secondary amine Amberlite LA-2 as a carrier) and l-leucine permeation (using the quaternary ammoniumchloride Aliquat 336 as a carrier). The results show that under optimized conditions lactic acid permeation is controlled by the rate of reaction and that l-leucine permeation is controlled by the rate of reaction and the diffusion through the emulsion globule. The model results were compared with batch experiments separating lactic acid and l-leucine from real fermentation broths. The comparison shows that the separation rate is affected by co-transport of pH regulating agents and to a smaller extent by the co-transport of water.

Adrenergic Swelling of Nucleated Erythrocytes: Cellular Mechanisms in A Bird, Domestic Goose, and Two Teleosts, Striped Bass and Rainbow Trout

ABSTRACT The mechanism of adrenergic swelling and associated pH changes was investigated in avian (goose) and teleost (striped bass and rainbow trout) erythrocytes. The swelling of goose red cells was probably caused by Na+/K+/Cl− cotransport and consecutive osmotic flow of water into the cell. Goose red cells swelled when exposed to isoproterenol in the presence of elevated extracellular K+, but not at physiological K+ concentrations. The swelling was quantitatively inhibited by furosemide, and by removing Cl− from the incubation medium, but was not affected by DIDS. The isoproterenol-induced swelling of fish erythrocytes may be due to loosely coupled Na+/H+ and Cl/HCO3− exchanges. Furosemide did not completely inhibit the swelling of striped bass red cells. The cell volume increased even if K+ was completely removed from the incubation medium. In contrast, both DIDS and amiloride treatment, and the removal of Na+ from the incubation medium, inhibited the volume changes. In fish red cells the swelling is associated with a clear acidification of the medium and alk-alinization of the red cell contents. This phenomenon was most pronounced when the cells were treated simultaneously with DIDS and isoproterenol; the intracellular pH became higher than the extracellular one. Both amiloride and removal of Na+ from the incubation medium prevented the reversal of the transmembrane pH gradient in cells treated simultaneously with DIDS and isoproterenol.