Isosmotic volume reabsorption in rat proximal tubule

A theoretical model incorporation both active and passive forces has been developed for fluid reabsorption from split oil droplets in rat intermediate and late proximal tubule. Of necessity, simplifying assumptions have been introduced; we have assumed that the epithelium can be treated as a single membrane and that the membrane "effective" HCO3 permeability is near zero. Based on this model with its underlying assumptions, the following conclusions are drawn. Regardless of the presence or absence of active NaCl transport, fluid reabsorption from the split oil droplet is isosmotic. The reabsorbate osmolarity can be affected by changes in tubular permeability parameters and applied forces but is not readily altered from an osmolarity essentially equal to that of plasma. In a split droplet, isosmotic flow need not be a special consequence of active Na transport, is not the result of a particular set of permeability properties, and is not merely a trivial consequence of a very high hydraulic conductivity; isosmotic flow can be obtained with hydraulic conductivity nearly an order of magnitude lower than that previously measured in the rat proximal convoluted tubule. Isosmotic reabsorption is, in part, the result of the interdependence of salt and water flows, their changing in parallel, and thus their ratio, the reabsorbate concentration being relatively invariant. Active NaCl transport can cause osmotic water flow by reducing the luminal fluid osmolarity. In the presence of passive forces the luminal fluid can be hypertonic to plasma, and active NaCl transport can still exert its osmotic effect on volume flow. There are two passive forces for volume flow: the Cl gradient and the difference in effective osmotic pressure; they have an approximately equivalent effect on volume flow. Experimentally, we have measured volume changes in a droplet made hyperosmotic by the addition of 50 mM NaCl; the experimental results are predicted reasonably well by our theoretical model.     

Non linear volume flow dependence on osmotic pressure difference in frog skin.

The volume flow dependence upon the osmotic pressure difference of both impermeant (sucrose) and permeable (NaCl) species has been investigated in leg skin bags of Rana esculenta. It is concluded: 1. The hydration-dehydration error in the flow measurement with leg skin bags is negligible. 2. The flow-force relationship is non-linear. 3. Unstirred layers and solute permeation have little, if any, influence on non linearity. 4. Structural modifications of the skin induced with hypertonic solutions have been observed and may contribute to non linearity, as well as the multiple-barrier effect.     

Salt-water coupling in leaky epithelia

Summary The theory of quasi-isotonic transport by cellular osmosis (the standing-gradient theory) has been challenged on the grounds that the osmotic permeabilities of the mucosal and interspace membranes are too low; if they were as high as the theory requires then the osmotic permeability of the whole epithelium would be 2–3 orders of magnitude higher than observed. This objection has basically been accepted for it is now claimed that these enormous permeabilities do exist, but are masked by unstirred-layer effects; I show that this is incorrect because unstirredlayer corrections are small and that the situation has not changed since 1975.The view that the route of fluid transport is junctional is replacing the cellular theory, and trans-junctional water flows seem to account for major fractions of the flow in various epithelia. I argue on grounds of general theory that these are unlikely to be osmotic flows because the junctional pores cannot satisfy both the osmotic and diffusive properties required of them, but the basic osmotic theory is also rather vague here.Non-osmotic theories, if junctional flow is accepted, must be either electro-kinetic or peristaltic.     

Methysergide reduces nonnutritive blood flow in normal and scalded skin

Methysergide is a serotonin antagonist and has been demonstrated to reduce wound blood flow and edema formation. We have determined the effect of methysergide on protein kinetics in normal and scalded skin of anesthetized rabbits. L-[ring-(13)C(6)]- or L-[ring-(2)H(5)]phenylalanine was used to reflect skin protein kinetics by use of an ear model, and L-[1-(13)C]leucine was used to reflect whole body protein kinetics. The results were that infusion of methysergide (2-3 mg. kg(-1). h(-1)) reduced the blood flow rate in normal skin by 50% without changing skin or whole body protein kinetics. After scald injury on the ear, administration of methysergide for 48 h reduced the weight of scalded ears (43 +/- 4 vs. 30 +/- 5 g, P < 0.01) and ear blood flow rate (42.6 +/- 4.9 vs. 5.8 +/- 1.0 ml. 100 g(-1). min(-1), P < 0.0001) and did not change wound protein kinetics. Methysergide reduced arteriovenous shunting and maintained inward phenylalanine transport from the blood to the skin pool. Using the microsphere technique, we found that the infusion of methysergide decreased blood perfusion by 33-36% in both normal and scalded ear skin. We conclude that methysergide administration reduces nonnutritive, as opposed to nutritive, blood flow in normal and scalded skin.     

Suitability of frequency modulated thermal wave imaging for skin cancer detection—A theoretical prediction

A theoretical study on the quantification of surface thermal response of cancerous human skin using the frequency modulated thermal wave imaging (FMTWI) technique has been presented in this article. For the first time, the use of the FMTWI technique for the detection and the differentiation of skin cancer has been demonstrated in this article. A three dimensional multilayered skin has been considered with the counter-current blood vessels in individual skin layers along with different stages of cancerous lesions based on geometrical, thermal and physical parameters available in the literature. Transient surface thermal responses of melanoma during FMTWI of skin cancer have been obtained by integrating the heat transfer model for biological tissue along with the flow model for blood vessels. It has been observed from the numerical results that, flow of blood in the subsurface region leads to a substantial alteration on the surface thermal response of the human skin. The alteration due to blood flow further causes a reduction in the performance of the thermal imaging technique during the thermal evaluation of earliest melanoma stages (small volume) compared to relatively large volume. Based on theoretical study, it has been predicted that the method is suitable for detection and differentiation of melanoma with comparatively large volume than the earliest development stages (small volume). The study has also performed phase based image analysis of the raw thermograms to resolve the different stages of melanoma volume. The phase images have been found to be clearly individuate the different development stages of melanoma compared to raw thermograms.     

Inhibition of transepithelial osmotic water flow by blockers of the glucose transporter

On the basis of evidence derived mostly from human erythrocytes, it has been suggested that water traverses cell membranes through membrane-spanning proteins such as the anion channel or the glucose transporter acting as water pores. However, specific inhibitors of such permeation processes have not been found to block water transport, and hence a precise identification of the water route has not been possible so far. We have investigated this issue by characterizing the osmotic flows across a fluid-transporting epithelium, the rabbit corneal endothelium. The rate of such flows was monitored continuously as a function of time. We confirmed prior findings of an inhibition by PCMBS on osmotic water flow, and lack of inhibition by DTNB and DIDS. On the other hand, we have found for the first time that several blockers of glucose facilitated diffusion, namely, phloretin (2 mM), phloridzin (2 mM), diallyldiethylstilbestrol (0.1 mM), cytochalasin B (20 micrograms/ml), and ethylidene-D-glucose (200 mM), all clearly inhibit osmotic flow. Our evidence is consistent with the hypothesis that both water and glucose may traverse these cell membranes through the same channel-like pathway contained in the glucose transporter membrane-spanning protein.     

Skin blood flow influences near-infrared spectroscopy-derived measurements of tissue oxygenation during heat stress.

Near-infrared (NIR) spectroscopy is a noninvasive optical technique that is increasingly used to assess muscle oxygenation during exercise with the assumption that the contribution of skin blood flow to the NIR signal is minor or nonexistent. We tested this assumption in humans by monitoring forearm tissue oxygenation during selective cutaneous vasodilation induced by locally applied heat (n = 6) or indirect whole body heating (i.e., heating subject but not area surrounding NIR probes; n = 8). Neither perturbation has been shown to cause a measurable change in muscle blood flow or metabolism. Local heating (approximately 41 degrees C) caused large increases in the NIR-derived tissue oxygenation signal [before heating = 0.82 +/- 0.89 optical density (OD), after heating = 18.21 +/- 2.44 OD; P < 0.001]. Similarly, whole body heating (increase internal temperature 0.9 degrees C) also caused large increases in the tissue oxygenation signal (before heating = -0.31 +/- 1.47 OD, after heating = 12.48 +/- 1.82 OD; P < 0.001). These increases in the tissue oxygenation signal were closely correlated with increases in skin blood flow during both local heating (mean r = 0.95 +/- 0.02) and whole body heating (mean r = 0.89 +/- 0.04). These data suggest that the contribution of skin blood flow to NIR measurements of tissue oxygenation can be significant, potentially confounding interpretation of the NIR-derived signal during conditions where both skin and muscle blood flows are elevated concomitantly (e.g., high-intensity and/or prolonged exercise).     

A New Proposal for the Action of Vasopressin, Based on Studies of a Complex Synthetic Membrane

The total osmotic flow of water across cell membranes generally exceeds diffusional flow measured with labeled water. The ratio of osmotic to diffusional flow has been widely used as a basis for the calculation of the radius of pores in the membrane, assuming Poiseuille flow of water through the pores. An important assumption underlying this calculation is that both osmotic and diffusional flow are rate-limited by the same barrier in the membrane. Studies employing a complex synthetic membrane show, however, that osmotic flow can be limited by one barrier (thin, dense barrier), and the rate of diffusion of isotopic water by a second (thick, porous) barrier in series with the first. Calculation of a pore radius is meaningless under these conditions, greatly overestimating the size of the pores determining osmotic flow. On the basis of these results, the estimation of pore radius in biological membranes is reassessed. It is proposed that vasopressin acts by greatly increasing the rate of diffusion of water across an outer barrier of the membrane, with little or no accompanying increase in pore size.     

The role of membrane in osmotic flow

Osmosis as a phenomenon caused by internal forces goes on without the necessity for the presence of any external forces. Therefore pressure gradient plays no special role in osmotic flow. Membrane as a component of solution with its molecules possessing some kind of mobility ceases to be a passive obstacle to the flow of other components, but becomes also a co-determining factor in osmotic flow. This has been shown by using the methods of irreversible thermodynamics. In the state of osmotic equilibrium osmosis does not occur. So also the mobility of water molecules which may then be found in tracer experiment does not determine the osmotic permeability coefficient. The coefficientsσ andω as defined by the parameters of the system under the condition of zero volume flow are not directly connected to L_p.     

The mechanism of water transport in the epidermis

Possible mechanisms of water transport through Stratum Corneum (SC) in the process of insensible perspiration are investigated. Electrometric methods developed for measuring water flow density through SC are used. Data showing the reverse osmotic mechanism of water transport through the SC selective membrane are obtained. It is shown that the water flow density through SC controlling the evaporation rate from the skin surface in the process of insensible perspiration depends upon the skin capillary pressure.     

Hypertonicity primes malignant melanoma cells for apoptosis

The tumor environment critically influences responsiveness of cancer cells to chemotherapies, most of which activate the mitochondria-regulated (intrinsic) apoptotic cascade to kill malignant cells. Especially skin tumors encounter an environment with remarkable biophysical properties. Cutaneous accumulation of Na⁺ locally establishes osmotic pressure gradients in vivo (hypertonicity or hyperosmotic stress), but whether cutaneous hypertonicity is a factor that modulates the responsiveness of skin cancers to therapeutic apoptosis-induction has thus far not been investigated. Here, we show that hyperosmotic stress lowers the threshold for apoptosis induction in malignant melanoma, the deadliest form of skin cancer. Hypertonic conditions enforce addiction to BCL-2-like proteins to prevent initiation of the mitochondria-regulated (intrinsic) apoptotic pathway. Essentially, hyperosmotic stress primes mitochondria for death. Our work identifies osmotic pressure in the tumor microenvironment as a cell extrinsic factor that modulates responsiveness of malignant melanoma cells to therapy.     

Osmotic flow caused by polyelectrolytes

Osmotic flow of water caused by high concentrations of anionic polyelectrolytes across semipermeable membranes, permeable only to solvent and simple electrolyte, has been measured in a newly designed flow cell. The flow cell features small solution and solvent compartments and an efficient stirring mechanism. We have demonstrated that, while the osmotic pressure of the anionic polyelectrolytes is determined primarily by micro-counterions, the osmotic flow is determined by solution-dependent properties as embodied in the hydrodynamic frictional coefficient which is determined by the polymer backbone segment of the polyelectrolyte. The variation of the osmotic permeability coefficient, L(p)(o), with concentration and osmotic pressure closely correlated with the concentration dependence of this frictional coefficient. These studies confirm previous work that the kinetics of osmotic flow across a membrane impermeable to the osmotically active solute is primarily determined by the diffusive mobility of the solute.     

Competition between split and nonmanipulated embryos in the production of identical piglets

The effect of nonmanipulated embryos on in utero survival of split porcine embryos was examined in this study. Previously, only limited success in the production of identical twin piglets has been reported. Embryos were collected from slaughtered donors (4 to 7 d post estrus) and were either split with the aid of a micromanipulator or left as whole embryos. Monozygotic pairs of split embryos were then surgically transferred to recipients with a complement of either split or nonmanipulated embryos. A total of 217 split embryos and 60 nonmanipulated embryos were transferred to 19 recipients. Nine of these recipients farrowed. In the two recipients that received only split embryos and farrowed, 31% of the split embryos survived to term, including two sets of monozygotic twins. In the remaining seven recipients, only 10% of the split embryos that were transferred along with nonmanipulated embryos survived to farrowing. This difference in split embryo survival (31 vs 10%) was significantly different (P<0.005). Sixty-nine percent of the nonmanipulated embryos survived to term in recipients that maintained pregnancy. Data presented in this study suggest that competition occurs between split and nonmanipulated embryos transferred to the same uterine environment.     

Effect of osmotic stress on photosynthesis studied with the isolated spinach chloroplast : generation and use of reducing power

The effect of increasing assay medium sorbitol concentration from 0.33 to 1.0 molar on the photosynthetic reactions of intact and broken spinach (Spinacia oleracea L. var. Long Standing Bloomsdale) chloroplasts was investigated by monitoring O₂ evolution supported by the addition of glyceric acid 3-phosphate (PGA), oxaloacetic acid (OAA), 2,5-dimethyl-p-benzoquinone, and 2,6-dichlorophenolindophenol or as O₂ uptake with methyl viologen as acceptor.

Uncoupled 2,6-dichlorophenolindophenol-supported whole chain electron transport (photosystems I and II) was inhibited from the 0.33 molar rate by 14% and 48.6% at 0.67 and 1.0 molar sorbitol in the intact chloroplast and by only 0.4% and 25.0% in the broken chloroplast preparation. Whole chain electron flow from water to other oxidants (OAA, methyl viologen) was also inhibited at increased osmoticum in intact preparations while electron flow from water to methyl viologen, ferricyanide, and NADP in broken preparations did not demonstrate the osmotic response. Electron transport to 2,5-dimethyl-p-benzoquinone (photosystem II) from H₂O and to methyl viologen (photosystem I) from 3,3′-diaminobenzidine were found to be unaffected by osmolarity in both intact and broken preparations.

The stress response was more pronounced (26-38%) with PGA as substrate in the presence of 0.67 molar sorbitol than the inhibition found with uncoupled and coupled linear electron flow. In addition, substrate availability and ATP generated by cyclic photophosphorylation evaluated by addition of Antimycin A were found not to be mediating the full osmotic inhibition of PGA-supported O₂ evolution. In a reconstituted (thylakoids plus stromal protein) chloroplast system to which a substrate level of PGA was added, O₂ evolution was only slightly (7.8%) inhibited by increased osmolarity (0.33-0.67 molar sorbitol) indicating that the level of osmotic inhibition above that contributed by adverse effects on electron flow can be attributed to the functioning of the photosynthetic carbon reduction cycle within the intact chloroplasts.

     

An analogue computer simulation of cloacal resorption of salt and water from ureteral urine in birds

The transmural flow of NaCl and water occurring during the retrograde flow of ureteral urine into the coprodeum and large intestine of birds has been simulated by analogue computation. The purpose was to estimate whether a fraction of the urine (water) which in the dehydrated state is hyperosmotic to plasma can, in spite of this, be absorbed from the narrow space between the epithelium and the central faeces core. The values of urine flow, urine osmolality, osmotic permeability, net NaCl absorption rate, and solute-linked water flow determined by in vivo perfusion studies in the domestic fowl were used in the calculation. The cloacal sojourn of ureteral urine was found to result in a net water gain but at the expense of a hyperosmotic NaCl absorption. The model was further used to evaluate the quantitative influence of the system's parameters upon the fractional water absorption. This was found very sensitive to the urine osmolality, moderately sensitive to the urine flow and NaCl absorption rate and almost unaffected by the osmotic permeability of the coprodeum and large intestine within a reasonable physiological range. The change of the epithelial transport parameters from the normally hydrated to the dehydrated state resulted in a marked increase in water absorption.     

Effect of various antimicrotubular drugs on the osmotic water flow through the wall of frog's urinary bladder]

The action of antimicrotubular drugs (colchicine, vinblastine and copper) on the osmotic water flow through the wall of the urinary bladder of Rana temporaria has been studied. The osmotic gradient was made by five- or tenfold dilution of the internal Ringer solution. The water flow was estimated gravimetrically. The water flow was induced by pituitrin (50 milliunits/ml), cyclic AMP (cAMP, 0.5-10(-3) M) and nystatine (3.5-10(-5) M). Pituitrin and cAMP and all the antimicrotubular drugs were added from the serosal surface of the bladder. Nystatine was introduced with the help of a fixed polyethylene tube. Preincubation with colchicine lasted 4 hours and that with vinblastine and copper (CuSO4), 1 hour. The drug concentrations varied between 10(-5)--10(-4) M. All the drugs studied showed a significant inhibitory effect toward pituitrin. The action of cAMP on the water flow was seen inhibited in the presence of colchicine and copper. The nystatine induced water flow was supressed by copper, colchicine being in this case inactive. A conclusion is drawn that the inhibition of cAMP formation does not cause a decreased pituitrine effect in the presence of antimicrotubular drugs. It has been assumed that the microtubules may be involved in the directed water flow within the cell.     

A new visualization chamber to study the transient volumetric response of yeast cells submitted to osmotic shifts

A visualization chamber has been developed to analyze potential correlations between osmotic step increase on yeasts and the resultant cell volume decreases. Image analysis was used to characterize the step increases in the center of the chamber and to measure the changes in the cell volume. Step increases of different intensities have been performed on the yeast Saccharomyces cerevisiae. This device has allowed the kinetics of the volumetric evolution of the cells to be observed. The water exit flow rate from the cell was found to occur in the first 10 s following the hypertonic step change. Comparison of the time constants of the chamber and of the cell volume variations allowed to conclude that the time constant of the water transfer across the membrane was short (about 1 s). (c) 1994 John Wiley & Sons, Inc.     

Measurement of skin blood flow in delayed deltopectoral flaps, using local clearance of 133Xenon.

A study was made to determine the skin blood flow at the deltoid region in 89 cases, and the regional blood flow of delayed deltopectoral flaps, using the local clearance of 133Xe. The change in the skin blood flow, before and after a delay procedure of the deltopectoral flap, was measured in 27 patients--and the following results were obtained. (1) There was a linear tendency to a decreasing flow, one found to be statistically significant, with increasing age of the patient. (2) A significant correlation was found between the skin blood flow and the blood flow of the subcutaneous tissue. (3) The blood flow after we raised one side of a deltopectoral flap and lined it with a split-skin graft was higher than that found after a U-shaped undermining and not lining a flap. (4) The rate of successful transfer of a deltopectoral flap was found to be low when the 133Xe clearance rate was less than 0.07.