Net Volume Flux Research Articles

Ionic requirements of proximal tubular sodium transport. I. Bicarbonate and chloride.

Simultaneous perfusion of peritubular capillaries and proximal convoluted tubules was used to study the effect of varying transepithelial ionic gradients on ionic fluxes. Results show that net sodium influx and volume flux was one-third of normal when bicarbonate was absent, no chloride gradient existed, and glucose and amino acids were absent. Addition of bicarbonate to the luminal fluid did not restore the flux to normal, but peritubular bicarbonate did restore it. A chloride gradient imposed when no bicarbonate was present could only increase the fluxes slightly, but his flux was significant even after cyanide had poisoned transport. Reversing the chloride concentration gradient decreased the net sodium and volume fluxes whether bicarbonate was present or not. Glucose had no effect on fluxes, but substitution of Na by choline abolished them entirely. It is concluded that sodium is actively transported, that a chloride concentration gradient from lumen to plasma could account for up to 20% of net transport, and that peritubular bicarbonate is necessary for normal rates of sodium and fluid absorption.

Bilayer lipid membrane as a model for vasopressin, prostaglandin and ca 2+ effects on water permeability

1. 1. Bilayer lipid membranes were generated in an aqueous medium from egg phosphatidylcholine and the water permeability of the black membranes was determined by measuring the net volume flux produced by a NaCl gradient. 2. 2. In agreement with previous reports, vasopressin increased the water permeability coefficient of the lipid membrane. Cyclic AMP, over a wide concentration range, had little or no effect on the water permeability, substantiating a direct effect of the antidiuretic hormone on the membrane. 3. 3. Similar to observations in biological systems, Ca 2+ and prostaglandin E 1 inhibited, apparently competitively, the action of vasopressin on the water flow of the model membrane. The inhibitory effects were observed if Ca 2+ and prostaglandin E 1 were added prior to, but not after, vasopressin. 4. 4. The formation of a vasopressin-lipid membrane complex stabilized by electrostatic and hydrophobic interactions is proposed. 5. 5. Unlike valinomycin, neither vasopressin nor cyclic AMP changed significantly the electrical conductance of the bilayer membranes in a medium containing NaCl and KCl. 6. 6. It is concluded that bilayer lipid membranes apparently possess the characteristics of the proposed site of action of vasopressin in biological membranes related to water transport, while the second site, related to Na + transport, is lacking.

Water permeability of bilayer lipid membranes: Sterol-lipid interaction

Bilayer lipid membranes were generated in an aqueous medium from synthetic, egg or plant phosphatidyl choline (PC) or from plant monogalactosyl diglyceride (MG). The water permeability of the black membranes was determined by measuring the net volume flux produced by a NaCl gradient. The osmotic permeability coefficient,P os, was markedly affected by the number of double bonds in the fatty acid conjugates of the lipids: the greater the degree of unsaturation, the higher the value ofP os. The temperature dependence ofP os of the lipid membranes was studied over a range of 29 to 40°C. The experimental activation energy,E a , estimated from the linear plots of log (P os)versus 1/T, was significantly higher for MG membranes (17 kcal/mole) than for the various PC membranes (11 to 13 kcal/mole), probably owing to hydrogen bonding between MG and water molecules. In comparison with PC membranes, the membranes generated from PC and cholesterol (1∶1 molar ratio) had lowerP os but similarE a values. Likewise, either stigmasterol or β-sitosterol decreasedP os of MG membranes, whileE a was not affected by the sterols. MG-cholesterol membranes were specifically characterized by a unique value ofE a (-36 kcal/mole) thus indicating temperature dependent structural changes.

Effect of peritubular protein concentration on reabsorption of sodium and water in isolated perfused proxmal tubules.

Micropuncture studies have indicated that variation in peritubular oncotic pressure influences net transport of fluid out of the proximal tubule. The present in vitro studies on isolated perfused rabbit proximal convoluted tubules were designed to examine whether protein concentration gradient must act across the peritubular capillary membrane to influence reabsorption, or whether it can exert a direct effect across the tubular basement membrane 71 proximal tubules were perfused with ultrafiltrate made isosmolal to bathing fluids, the latter having identical electrolyte composition as the perfusing ultrafiltrate, but adjusted to three oncotic pressures: hypooncotic, protein 0.0 g/100 ml; control isooncotic serum, protein 6.4 g/100 ml; and hyperoncotic, protein 12.5 g/100 ml. Net volume flux (nl/mm per min), net Na flux (nEq/mm per min), unidirectional Na flux from bath to lumen (nEq/mm per min), and passive permeability coefficient (x 10(-5) cm/sec) for Na (P(Na)), urea (P(urea)), and sucrose (P(sucrose)) were determined using isotopic techniques. When the bath was hypooncotic, there was (as compared with isooncotic serum) a significant decrease in net volume (38%) and net sodium (40%) flux, but no change in P(Na), P(urea), or transtubular potential; however, P(sucrose) increased significantly (78%). In experiments in which hyperoncotic bath was used, there was (compared with isooncotic serum) an increase in net volume (28%) and net sodium (30%) flux, but transtubular potential difference did not change significantly. These data demonstrated that changes in the ambient protein concentration gradient exert direct effects upon proximal tubular reabsorption. Because penetration of sucrose (an index of intercellular movement) but not urea (an index of transcellular movement) varied with changes in tubular reabsorption, it is suggested that oncotic pressure acts by altering the rate of back-leak of reabsorbate through extracellular pathways between tubular cells. It is concluded that an effect of protein concentration on reabsorption can be exerted directly across the basement membrane, without necessary interposition of the capillary bed.

Properties of liquid bilayer membranes separating two aqueous phases: Temperature dependence of water permeability

The temperature dependence of the water permeability of lipid bilayer membranes has been determined over the interval 7 to 49 °C, by measuring the net volume flux produced by a NaCl gradient. The membranes were generated from solutions of lecithin and cholesterol in decane, either 0.5% (w/v) lecithin and 0.25% cholesterol (solution A) or 1% lecithin and 1% cholesterol (solution B). At 25 °C, P os, the osmotic permeability coefficient, is 31.7 ± 4.3 μ/sec and 24.4 ± 3.7 μ/sec for membranes formed from solutions A and B, respectively. Plots of In ( P os) versus 1 T are linear and yield values of 12.7 ± 0.3 and 13.1 ± 0.7 kcal./ mole for the experimental activation energies E a. These results, together with literature data for the solubility and diffusion of water in hydrocarbons, are consistent with a simple solubility-diffusion mechanism for the water permeation process. The similarity between the values of both P os and E a for cellular membranes and the lipid bilayer suggests that a similar mechanism may operate in the biological systems.