Solute Transport Rate Research Articles

The use of transmembrane differences in saturation transfer for measuring fast membrane transport; application to H 13C0 3− exchange across the human erythrocyte

We consider the NMR signal intensity of a solute B, that is both in exchange across the membrane of cells in suspension and also in chemical exchange with another solute C, when the nuclei of C are saturated with selective radiofrequency radiation. This extension of the classical saturation transfer experiment potentially enables the measurement of fast unidirectional membrane transport rates of solutes that are in chemical equilibrium, when the chemical shifts of the intra- and extracellular solute are identical. The application of the theory and experimental methods to a biological system is illustrated by using 13C NMR with the H 13 CO 3 − 13 CO 2 exchange system in human erythrocytes. In the buffer conditions employed the estimated value of the exchange rate of H 13C0 3 − was comparable to values estimated with NMR and other non-NMR methods.

Does Availability of Inorganic Phosphate Regulate Cellular Oxidative Metabolism?

Cells require inorganic phosphate for formation of ATP and a variety of other metabolic and transport functions. Studies of renal tubules indicate that availability of phosphate does regulate rates of oxidative metabolism and solute transport, that intracellular metabolic processes compete for inorganic phosphate, and that transepithelial phosphate transport provides inorganic phosphate for use within the cell.

Transport of solute in proximal tubules is modified by changes in medium osmolality.

To examine the hypothesis that fluid absorption is driven by transepithelial osmotic differences, we measured such differences in proximal tubules perfused under oil and attempted to increase the rate of fluid absorption (Jv) and hence the absorbate-perfusate osmotic differences by lowering the osmolality of the perfusate. We were able to consistently increase Jv in this manner only in proximal straight tubules perfused with a simple perfusate that contained no bicarbonate or amino acids. With the simple perfusate, a small but significant increase in the absorbate-perfusate osmolality difference was seen with increased Jv, which is expected if the volume absorption is driven by a transepithelial osmotic difference. In addition, lowering the perfusate osmolality from 290 to 160 mosmol/kg H2O increased the rate of solute absorption from 79 +/- 7 to 91 +/- 8 posmol . min-1 . mm-1; this increase was partly accounted for by an increase in the rate of absorption of glucose from 6.6 +/- 0.9 to 9.5 +/- 1.1 pmol . min-1 . mm-1. In contrast, with the complete perfusate in proximal straight tubules there was little or no increment in Jv, no change in transepithelial osmolality differences, and a decrease in the rate of solute transport with hypoosmolality from 136 +/- 21 to 87 +/- 22 posmol . min-1 . mm-1. In proximal convoluted tubules, similar results were obtained, but a time-dependent decline of Jv complicated the interpretation of the results in the convoluted tubules. It is hypothesized that the observed changes in solute transport with hypotonic perfusate may be the result of changes in membrane permeability that are subsequent to cell swelling.

Relationship between pump and leak: Part III. Electrical coupling of Na +-solute uptake to the Na,K-atpase

The relationship between metabolism and Na +-linked secondary active transport is discussed. It is suggested that Na + symports are electrically coupled to other ion fluxes, including pumped fluxes so that inhibition of the pump results in an immediate decrease in the rate of solute transport. Similarly ouabain decreases Na +-Ca + exchange across the plasma membrane of cardiac cells partly by stopping that fraction of exchange that was coupled to the Na,K-ATPase.

Epithelial water transport in a balanced gradient system

The relationship between epithelial fluid transport, standing osmotic gradients, and standing hydrostatic pressure gradients has been investigated using a perturbation expansion of the governing equations. The assumptions used in the expansion are: (a) the volume of lateral intercellular space per unit volume of epithelium is small; (b) the membrane osmotic permeability is much larger than the solute permeability. We find that the rate of fluid reabsorption is set by the rate of active solute transport across lateral membranes. The fluid that crosses the lateral membranes and enters the intercellular cleft is driven longitudinally by small gradients in hydrostatic pressure. The small hydrostatic pressure in the intercellular space is capable of causing significant transmembrane fluid movement, however, the transmembrane effect is countered by the presence of a small standing osmotic gradient. Longitudinal hydrostatic and osmotic gradients balance such that their combined effect on transmembrane fluid flow is zero, whereas longitudinal flow is driven by the hydrostatic gradient. Because of this balance, standing gradients within intercellular clefts are effectively uncoupled from the rate of fluid reabsorption, which is driven by small, localized osmotic gradients within the cells. Water enters the cells across apical membranes and leaves across the lateral intercellular membranes. Fluid that enters the intercellular clefts can, in principle, exit either the basal end or be secreted from the apical end through tight junctions. Fluid flow through tight junctions is shown to depend on a dimensionless parameter, which scales the resistance to solute flow of the entire cleft relative to that of the junction. Estimates of the value of this parameter suggest that an electrically leaky epithelium may be effectively a tight epithelium in regard to fluid flow.

Epithelial cell volume modulation and regulation.

Epithelial cell volume is a sensitive indicator of the balance between solute entry into the cell and solute exit. Solute accumulation in the cell leads to cell swelling because the water permeability of the cell membranes is high. Similarly, solute depletion leads to cell shrinkage. The rate of volume change under a variety of experimental conditions may be utilized to study the rate and direction of solute transport by an epithelial cell. The pathways of water movement across an epithelium may also be deduced from the changes in cellular volume. A technique for the measurement of the volume of living epithelial cells is described, and a number of experiments are discussed in which cell volume determination provided significant new information about the dynamic behavior of epithelia. The mechanism of volume regulation of epithelial cells exposed to anisotonic bathing solution is discussed and shown to involve the transient stimulation of normally dormant ion exchangers in the cell membrane.

Salt and water transport by epithelial cells

Epithelial cell volume is a sensitive indicator of the balance between solute entry into the cell and solute exit. Solute accumulation in the cell leads to cellular swelling because the water permeability of the cell membranes is high. Similarly, solute depletion leads to cellular shrinkage. The rate of volume change under a variety of experimental conditions may be utilized to study the rate and direction of solute transport by an epithelial cell. The pathways of water movement across an epithelium may also be deduced from the changes in cellular volume. A technique for the measurement of the volume of living epithelial cells is described and a number of experiments are discussed in which cellular volume determinations provided significant new information about the dynamic behavior of epithelia.

Sediment and solute transport in a representative basin

Suspended sediment and total dissolved material concentrations in stream flow were studied in Deep Creek Representative Basin near Cessnock, N.S.W. between April, 1976 and June, 1977. Sediment and solute transport rates were calculated at two gauging stations for individual flood events and low flow periods using the hourly hydrograph record, stage/season concentration‐discharge relationships and calculated rating curves. Limitations in the use of the rating curve suggest that during flood hydrographs of long return period, calculated transport rates may be in error. A continuous record of concentration variation was sampled during one runoff event enabling comparisons to be made between methods of load calculation. For the satisfactory calculation of transport rates during extreme flood hydrographs, it is essential that concentration variation be accurately defined.

Comparison of the Responses of Corn Root Tissue to Fusicoccin and Washing

A comparison has been made of the effects of fusicoccin with those of washing on segments of corn (Zea mays L.) root tissue. Both fusicoccin and washing caused increases in K(+)((86)Rb) influx, net H(+) efflux, and electrogenic cell membrane potential, but with no effect on respiration rate. The similarity was most evident with fresh tissue during the initial phases of washing, prior to the developmental changes which augment the anion and general solute transport rates of the tissue. After the development of enhanced transport capacity the proportional response to fusicoccin was much diminished. It is suggested that the fusicoccin-like response to washing may be a manifestation of recovery from injury.

Computation of the osmotic water permeability of perfused tubule segments

The development of in vitro perfusion of isolated segments of the nephron [1] has made the evaluation of tubular functions more precise than was previously possible. The measurement of fluid and solute transport rates is more accurate as the perfusion rate and the composition of the bath and perfusion solutions can be better controlled. These techniques have been used to measure the water permeabilities of different nephron segments [2–5].

Effects of ultrafiltration on dialysance in commercially available coils

A decrease in urea dialysance in Travenol 1.9m 2 cellophane (190) hemodialysis coils was demonstrated as outlet pressure was increased by partial clamping of outlow tubing [11. Suggested explanations include impingement of membrane against mesh with loss of area and increased resistance of fluid films as intracoil hydrostatic pressure increases. It has been generally asumed that increasing ultrafiltration by raising transmembrane hydrostatic pressure with outlet clamping decreases dialysis efficiency in all coil types. In these studies the relationships of dialysance rates and mean hydrostatic intracoil pressure (MCP) are compared in 190 and newer type coils. The studies demonstrate that in newer coils solute transport rates do not decrease with intracoil pressures as previously assumed.

Influence of Carbon Transport Kinetics on Solution Growth of β‐Silicon Carbide Crystals

Twinned β‐silicon carbide platelets with a (111) habit were grown from the walls of graphite crucibles in liquid silicon solutions with carbon concentrations below 0.1 a/o (atomic per cent). The observed crystal growth effects resulting from increasing the carbon solute transport rate by stirring the solution are described and correlated with boundary layer theory. Both forward growth of platelets into the solution and lateral growth of platelets were accelerated by stirring. However, crystal growth in the forward direction was eventually limited by a boundary layer region, beyond which the solution was no longer supersaturated. Because of the decrease in boundary layer thickness with increasing stirring speed, the terminal lengths of silicon carbide platelets decrease with increasing stirring speed.

Standing-gradient osmotic flow. A mechanism for coupling of water and solute transport in epithelia.

At the ultrastructural level, epithelia performing solute-linked water transport possess long, narrow channels open at one end and closed at the other, which may constitute the fluid transport route (e.g., lateral intercellular spaces, basal infoldings, intracellular canaliculi, and brush-border microvilli). Active solute transport into such folded structures would establish standing osmotic gradients, causing a progressive approach to osmotic equilibrium along the channel's length. The behavior of a simple standing-gradient flow system has therefore been analyzed mathematically because of its potential physiological significance. The osmolarity of the fluid emerging from the channel's open end depends upon five parameters: channel length, radius, and water permeability, and solute transport rate and diffusion coefficient. For ranges of values of these parameters encountered experimentally in epithelia, the emergent osmolarity is found by calculation to range from isotonic to a few times isotonic; i.e., the range encountered in epithelial absorbates and secretions. The transported fluid becomes more isotonic as channel radius or solute diffusion coefficient is decreased, or as channel length or water permeability is increased. Given appropriate parameters, a standing-gradient system can yield hypertonic fluids whose osmolarities are virtually independent of transport rate over a wide range, as in distal tubule and avian salt gland. The results suggest that water-to-solute coupling in epithelia is due to the ultrastructural geometry of the transport route.

INFLUENCE OF LIGHT ON GROWTH, GEOTROPISM AND GUTTATION OF AVENA SEEDLINGS GROWN IN TOTAL DARKNESS

Investigations were conducted with etiolated seedlings of Avena on the influence of red, far red and blue light on: a. the growth of sections of coleoptile and mesocotyl b. the geotropic reactions of coleoptile and mesocotyl c. guttation of the seedlings. The experiments were carried out in darkness, the experimental irradiations excepted. Two systems of reactions to light appear to be present in the seedlings. One of them shows a very low saturation value of about 1 erg cm−2 for red and far red light, the sensitivity to blue light being much lower than for the former wavelengths. This system was found in section growth and the geotropic reaction. The other system shows a dependency on the amount of light energy applied up to at least 105 erg cm−2 for red, far red and blue light. In this system the sensitivity to far red light is much lower than to red light and about as high as to blue light. This system was found to affect guttation and the shape of the geotropic curves of the seedlings. The latter system appears to exert its influence through changes in transport rates of water and solutes, as is indicated by its influence on guttation. From the results is concluded that this change in the rate of water transport influences the transport rates of auxins and, consequently, the relative growth rates of different parts of the seedlings. Connections of the conclusions drawn with some data from the literature on phototropism are discussed.

Rate of Solute Transport out of Emulsion Droplets in Micron Size Range

An experimental verification of the theory for rate of external-phase-diffusion-controlled solute transport from emulsion droplets in the micron size range has been carried out. The Coulter counter was employed to follow the change in size distribution in aqueous sodium chloride solutions of dibutyl phthalate droplets with time. The rate was proportional to particle radius according to theory, and the diffusion coefficient of dibutyl phthalate calculated from data and theory agreed well with the Stokes-Einstein law predictions. The effect of the presence of a water immiscible component in the dibutyl phthalate droplet was also studied.