Isotonic transport

There are two model equations that correspond to the two types of experimental observation of isotonic transport. In the first case, the mucosal bath volume is small relative to a large serosal bath, initially at the reference concentration. With time, a substantial fraction of the mucosal contents is transferred to the serosa and the mucosal bath concentration tends to a limiting value. Because of its large size, the serosal bath concentration is little changed. The mucosal bath, however, comes to transport equihbrium, with the concentration of the transported solution essentially equal to the mucosal bath concentration. This may be written... 

The condition of isotonic transport means that the mucosal deviation CJ, that satisfies Eqn. 5 is at most a few percent of the reference, Cq. The second type of experiment is that of a small serosal bath relative to a mucosal bath at the reference concentration. This includes the type of experiment in which the organ lumen is filled with the reference solution, the organ suspended, and the serosal drainage collected. (See, for example. Diamond [13] using rabbit gallbladder or Lee [14] using rat small intestine.) In this case the serosa is at transport equilibrium so that... 

Again, isotonic transport implies that the serosal deviation, Cg, that satisfies Eqn. 6 is at most a few percent of Co-... 

It is worth emphasis that the verification of isotonic transport for a particular model necessitates the solution of Eqns. 5 and 6. In general, the simple calculation of (Cr)o, the reabsorbate concentration with exactly equal bathing media, is not sufficient. Nevertheless, (Cr)q is an important theoretical aspect of a model and has been used to define the coupling coefficient of osmotic transport [ 15] by... 

The achievement of isotonic transport may be assessed by substituting Eqn. 9 into Eqns. 5 and 6 to obtain... 

In his early analysis of isotonic transport. Diamond [13] tried to use measured values of the whole epithelial water permeability, Lp, in place of the quantity Llb-The unacceptably large osmotic deviations from isotonicity that he computed caused him to reject the elementary compartment model of the lateral intercellular space. We should reconsider, therefore, the requirements imposed upon the elementary compartment model by the experimental data on rabbit gallbladder (Table 1). For N/Cq = 1.47-10 cm/s and C/Q = 0.27, Eqn. 84 requires = 5.5-10 cm/s. Eqn. 86 may be used to give a lower bound on the coupling coefficient. If mucosal equilibrium is within 2% of exact isotonicity then — C /Cq = 0.02 so that y = 0.95. Thus, if Lp = 1.7- lO"" cm/s.osm, Eqn. 85 implies L b is at least 34-10" cm/s.osm. It remains to consider these model predictions for and Llb i relation to the pertinent experimental data. 

Thus, the measured epitheUal water permeability is always less than the water permeability relevant to isotonic transport. For the case of uniform solute transport along the channel length (A = l), the right hand side of Eqn. 109 reduces to SLRTL f —y). The magnitude of the solute polarization effect is then just 1 — y, as was found for external unstirred layers Eqn. 68 and the elementary compartment model Eqn. 85. 

The phenomena usually included under the heading coupled water transport are epithelial transport of water against an adverse osmotic gradient and isotonic transport between nearly equal bathing media. We have presented above a survey of some of the mathematical models that have been used to understand these aspects of solute-solvent interaction in the transporting tissue. A major theme of this chapter has been the use of approximation wherever possible to reveal the intuitive content of a model and to address the issue of model applicability. 

V. cholerae is a gram-negative bacillus. Vibrios pass through the stomach to colonize the upper small intestine. Vibrios have filamentous protein extensions that attach to receptors on the intestinal mucosa, and their motility assists with penetration of the mucus layer.2 The cholera enterotoxin consists of two subunits, one of which (subunit A) is transported into the cells and causes an increase in cyclic AMP, which leads to a deluge of fluid into the small intestine.20 This large volume of fluid results in the watery diarrhea that is characteristic of cholera. The stools are an electrolyte-rich isotonic fluid, the loss of which results in blood volume depletion followed by low blood pressure and shock.2 Of note, the diarrheal fluid is highly infectious. 

The entry of CL into cells may be essential for the cellular entry [232] or secretion [233] of some macromolecules such as diphtheria toxin and modeccin. Sandvig and Olsnes [232] studied the entry of diphtheria toxin and modeccin into Vero cells in pH 7.2 media containing 20 mM Hepes, 1 mM Ca(OH)2, 5 mM glucose,a sufficient amount of mannitol to ensure isotonicity, and varying concentrations of NaCl. The cellular uptake of 0.1 nM diphtheria toxin at the end of 50 min was strongly dependent on CL concentration. It was 0% at 0 mM NaCl, 25% of the 140 mM NaCl control at 2 mM NaCl, and 60% of the control at 70 mM NaCl. A similar trend was observed for modeccin, i.e., no transport at 0 mM NaCl, 20% of control at 0.05 mM NaCl, 60% of control at 0.1 mM NaCl, 80% of control at 0.5 mM NaCl, and 100% of control at 2 mM NaCl. 

The coefficient of hydraulic permeability (Lp) of ozone-treated bean leaf tissue tends to decrease when measured by water loss or uptake (Fig. 6). Here, ozone-treated tissue was equilibrated with 0.2 M mannitol (approximately isotonic) immediately after exposure. The tissue was then either allowed to take up tritiated water or, after a period of tritiated water uptake, allowed to lose tritiated water into a mannitol solution. In both the influx and efflux experiments, ozone-treated tissue transported tritiated water at a lower rate than control tissue. 

Plain water crosses the walls of the intestine by osmotic action alone. However, if sodium and sugars are also present then the active transport mechanisms described earlier operate and water absorption can be enhanced. The effect is dependant on concentrations the maximum rate of water uptake occurs when the concentrations give a slightly hypotonic solution (200-250 mOsm/kg, cf. 287 mOsm/kg for isotonicity) (Wapnir and Lifshitz, 1985). Conversely, when the lumen contents are significantly hypertonic, water is secreted from plasma into the intestine by osmotic action this is a dehydrating effect. 

The colligative properties, described in Section 3.4.1 to Section 3.4.3, have been used to determine the molar mass of unknown chemical compounds. Pharmaceutical scientists and pharmacists may apply this concept in the preparation of isotonic (meaning of equal tone) solution dosage forms. These solution dosage forms can be applied to sensitive and delicate organs such as the eye, nose, or ear or directly injected into the body (i.e., blood vessels, muscles, lesions, etc.). They should have, when administered, the same osmotic pressure as body fluids. Otherwise, transport of body fluids inside and outside the cell tissues will occur, causing discomfort and damage to the tissue. Osmolarity of body fluids is approximately 0.307 osmol/L or 307 mosmol/L. 

Isotonic water resorption in the ephitelium is an example for the secondary active transport. Water and sodium ions are symported from the blood isotonically (i.e., against their concentration gradients) and there is no transport of either in the absence of the other. 

It is possible to treat moderately dehydrated horses effectively with oral replacement solutions (ORSs) (McGinness et al 1996). Oral fluids do not need to be sterUe and can be made up on the farm they are, therefore, considerably cheaper and easier to transport than i.v. fluids. It is apparently not necessary to add glucose to oral fluids for horses (Sosa Leon et al 1995) but, if feasible, electrolytes should be added. Isotonic or hypotonic fluids should be adnunistered (Sosa Leon et al 1995). A possible isotonic solution consists of 4.9 g/1 table salt and... 

The glomerular filtrate containing virtually very little protein (approximately 30 mg/dl) enters the proximal tubule. Approximately 80% of the water and solute from the glomerular filtrate is reabsorbed in the proximal tubule as an isotonic solution. Solutes such as glucose and proteins are almost totally reabsorbed. Creatinine, in contrast, is not reabsorbed. Approximately 65% of the sodium in the glomerular filtrate is reabsorbed in the proximal tubule by active transport. The rate of reabsorption of sodium in the proximal tubule, however, is under the... 

ABC superfamily 239 action potential 231 active transport 233 antiport 254 cotransport 254 depolarization 263 electrochemical gradient 232 facilitated diffusion 233 gated channel 233 hypertonic 258 hypotonic 258 isotonic 258... 

Hence, organelles are osmotically stable when suspended in isotonic KSCN. permeability, however, can be induced artificially by ionophores such as valinomycin (Figure lA). Valinomycin is considered to form a lipid-soluble complex with and functions to transport across the membranes (25). An increase in internal ld will be accompanied by the diffusion of SCN , the counter ion, across the membrane to maintain electroneutrality. 

Not all endogenous membrane transport systems are affected by the herbicides. For example, spontaneous mitochondrial swelling in isotonic solutions of ammonium phosphate or neutral amino acids, was affected only marginally by the compounds (data not shown). Swelling in these systems involves the endogenous Pi /0H anti-porter (26) and amino acid porter (27), respectively. At this... 

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