Osmoregulation and Excretion

The Journal of General Physiology, 1965

Equations expressing the effect of the diffusion potential on the trace ion transfer across a porous charged membrane have been derived. These equations have been tested with experiments with human cadaver skin. The transfer of sotalol and salicylate was measured varying the salt (NaCl) concentration in the donor and receiver compartments. It appears that osmotic pressure and ion-exchange make a significant contribution to the flux enhancement by the diffusion potential.

Wiley Interdisciplinary Reviews: Membrane Transport and Signaling, 2012

Water transport across cell membranes is central to most physiological functions. About 200 L of water move across epithelial cells each day in humans in order to maintain whole-body homeostasis; water transport in and out of organs such as the brain and eye are of major clinical importance. It is well established that the water transport is driven by ion transport, but how? Osmosis is not always the answer: water can be transported against considerable osmotic gradients, apparently without any external osmotic or hydrostatic driving forces. It is generally accepted that cotransporters of the symport type play a key role for the coupling between ion and water fluxes. Models of coupling are either molecular or based on unstirred layer effects, and can be distinguished by their response time: for molecular models, water transport follows changes of substrate transport instantaneously; in unstirred layer models there is a delay while the osmolarity changes in the solutions surrounding the cotransport protein. For cotransporters expressed heterologously in Xenopus oocytes, influx of water can be detected about 1 second after initiation of cotransport of ions and other substrates. This is 20 times faster than expected (and observed) for unstirred layer effects. Water transport in cotransporters is best explained by a molecular model in which ion and water fluxes are coupled by a mechanism within the protein. This would also clarify how cotransporters exploit the free energy in the ion fluxes for the uphill transport of water.

Bulletin of the Australian Mathematical Society, 1997

The Danish physiologist Ussing was the first to present the use of the flux-ratio as an investigative tool in membrane transport studies . In a set of two complementary experiments, steady unidirectional outfluxes of a tracer from opposite faces of a membrane slab are measured. In the first experiment, face A of the slab is kept at constant tracer concentration, while face B is washed at zero concentration, and the steady outflux j^ at face B is measured. In the second experiment, the roles of faces A and B are interchanged, and the steady outflux j * u ( at face A is measured.

Biochemistry and Cell Biology, 2002

Historically, water transport across biological membranes has always been considered a passive process, i.e., the net water transport is proportional to the gradients of hydrostatic and osmotic pressure. More recently, this dogma was challenged by the suggestion that secondary active transporters such as the Na/glucose cotransporter (SGLT1) could perform secondary active water transport with a fixed stoichiometry. In the case of SGLT1, the stoichiometry would consist of one glucose molecule to two Na + ions to 220-400 water molecules. In the present minireview, we summarize and criticize the evidence supporting and opposing this water cotransport hypothesis. Published and unpublished observations from our own laboratory are also presented in support of the idea that transport-dependent osmotic gradients begin to build up immediately after cotransport commences and are fully responsible for the cell swelling observed.

Biochimica et Biophysica Acta (BBA) - Biomembranes, 2004

The electrical conductance of ions across the peritoneal membrane of young buffalo (approximately 18-24 months old) has been recorded. were used. The conductance values have been found to increase with increase in concentration as well as with temperature (15 to 35 8C) in these cases. The slope of plots of specific conductance, j, versus concentration exhibits a decrease in its values at relatively higher concentrations compared to those in extremely dilute solutions. Also, such slopes keep on increasing with increase in temperature. In addition, the conductance also attains a maximum limiting value at higher concentrations in the said cases. This may be attributed to a progressive accumulation of ionic species within the membrane. The j values of electrolytes follow the sequence for the anions:

Acta Physiologica, 2009

Solute-coupled water transport and isotonic transport are basic functions of low-and high-resistance epithelia. These functions are studied with the epithelium bathed on the two sides with physiological saline of similar composition. Hence, at transepithelial equilibrium water enters the epithelial cells from both sides, and with the reflection coefficient of tight junction being larger than that of the interspace basement membrane, all of the water leaves the epithelium through the interspace basement membrane. The common design of transporting epithelia leads to the theory that an osmotic coupling of water absorption to ion flow is energized by lateral Na + /K + pumps. We show that the theory accounts quantitatively for steady-and time dependent states of solute-coupled fluid uptake by toad skin epithelium. Our experimental results exclude definitively three alternative theories of epithelial solute-water coupling: stoichiometric coupling at the molecular level by transport proteins like SGLT1, electro-osmosis and a 'junctional fluid transfer mechanism'. Convection-diffusion out of the lateral space constitutes the fundamental problem of isotonic transport by making the emerging fluid hypertonic relative to the fluid in the lateral intercellular space. In the Na + recirculation theory the 'surplus of solutes' is returned to the lateral space via the cells energized by the lateral Na + /K + pumps. We show that this theory accounts quantitatively for isotonic and hypotonic transport at transepithelial osmotic equilibrium as observed in toad skin epithelium in vitro. Our conclusions are further developed for discussing their application to solute-solvent coupling in other vertebrate epithelia such as small intestine, proximal tubule of glomerular kidney and gallbladder. Evidence is discussed that the Na + recirculation theory is not irreconcilable with the wide range of metabolic cost of Na + transport observed in fluid-transporting epithelia. Keywords isotonic transport, Na + recirculation theory, physical-mathematical modelling of epithelial transport, metabolic cost of active sodium transport, solute-coupled fluid transport, toad skin epithelium.

Biophysical Journal, 1968

Flux and flux-ratio equations are derived on the basis of the phenomenological equations of irreversible thermodynamics. Deviations of flux-ratios from that given by the often quoted Ussing (1949) relation are predicted, even in the absence of active transport, by considering the dependence of coupled fluxes on the membrane potential. The treatment is extended to include the interpretation of fluxes measured with tracers. Estimation of the numerical values of the resistance coefficients show that the voltage dependence of the entrainment terms can adequately account for the departures from the Ussing relation and the discrepancies between isotopically and electrically measured membrane conductances.

2021

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