Widening of the paracellular pathway in the kidney tubule by a transtubular osmotic gradient. Passage of graded size non-electrolytes

Pathways for water absorption and physiological role of the lateral interspaces in the kidney tubule

Possible routes for water and salt flow and the most likely theories that describe coupling between water and salt flow across leaky epithelia are presented. The osmotic theories seem the most likely ones. However, several of the theories have weaknesses that render them unsatisfactory, in particular because of the possibility of paracellular water flow in these epithelia. Puzzling are the findings that measurements of the cellular water osmotic permeability give figures that are too low for some of the exclusively transcellular theories to work. If these observations hold in the future, it may be shown that part of the water moves through paracellular pathways in these leaky epithelia. This view is supported by the observation that large extracellular markers are dragged by volume flow. Finally, experimental evidence is reviewed indicating that changes in the luminal area concentration may modulate the functional state of the nephron junctional complexes.

Sucrose fluxes and junctional water flow across Necturus gall bladder epithelium

When sucrose is present in a luminal solution bathing Necturus gall bladder epithelium, the secreted fluid is also found to contain sucrose at more than 0.9 of the luminal concentration. A first-order convection–diffusion equation has been used to calculate the emergent sucrose con­centration, by using values for the dimensions of the paracellular system and assuming that the sucrose flow is extracellular. The results indicate that most of the sucrose flow across the junctions must be convective in nature, and the size of the junctional water flow required amounts to most of the overall epithelial water flow, i. e. during secretion the fluid is generated by junctional flow and not by standing-gradient osmosis over the membrane lining the interspaces. The junctional complexes at the luminal side of the interspaces are not zonulae occludentes but parallel alignments of the cell membranes with a constant (peak to peak) spacing of 14.2 nm. As the reflexion coefficient of these junctions for salt is unknown, but could be low, it is by no means certain that the junctional water flow is driven by osmosis.

The effects of sodium chloride, urea and mannitol on the permeability in vitro fo rat papillary collecting ducts to THO, 14C-urea and 22Na

1. The diffusional permeabilities of collecting duct membranes to THO, 14C-urea and 22Na+ have been measured at different concentrations of urea, NaCl and mannitol. 2. In the absence of urea in perfusate and bath or in its presence in low concentrations, the diffusional permeability to urea was 2.0 (s.e.m. = 0.15, n = 58) micrometer s-1, compared with 0.87 (s.e.m. = 0.06, n = 29) microgram s-1 when 200 mmol/l urea was present. The permeability of the collecting ducts to THO or Na+ was not affected by the different urea concentrations. 3. High concentrations of sodium chloride increased the diffusional permeability of collecting ducts to water and urea but did not affect the diffusional permeability of the collecting duct to Na+. 4. Mannitol had effects similar to those of sodium chloride. 5. In all media tested there was an increase in THO and urea permeability when supramaximal amounts of antidiuretic hormone were added. The increases in the various media for each substance were similar, despite widely different starting permeabilities. 6. The results suggest that solutes and water move across collecting duct epithelium by several pathways that respond differently to various stimuli.

Effect of hypertonic urea and mannitol on distal nephron permeability

The paracellular pathway permeability is known to increase in perfused amphibian kidneys if the luminal fluid is made hyperosmotic with mannitol or urea. To investigate whether luminal hypertonicity increases paracellular pathway permeability in the mammalian nephron, early rat distal tubules were micropunctured and perfused through one micropipette with either isosmotic saline (IS), hyperosmotic urea (HU) or hyperosmotic mannitol (HM) solutions. A second micropipette was placed down-stream in the same tubule and test solutions of 30 nl of a mixture of 14C-inulin and 3H-mannitol or of 3H-inulin and 14C-urea were injected. Similar intratubular injections of tracers were performed in a second group of rats undergoing diuresis induced either by infusing intravenously saline alone (VS) or receiving saline plus 0.4 M urea (VU). In the latter group (VU) luminal urea concentration was increased without the tubular lumen being made hyperosmotic to its peritubular fluid. Urinary unulin recovery was essentially complete and unaffected by experimental procedures. Difference between mannitol recoveries in isosmotic saline and hyperosmotic urea perfusions IS-HU was 2.6 +/- 0.8% (P less than 0.001). Difference in urea recoveries IS-HM was 4.1 +/- 5.1% (P greater than 0.40), IS-HU was 13.9 +/- 5.3% (P equal to 0.015) and, VS-VU equal to 17.0 +/- 3.4 (P less than 0.001). Therefore, elevated luminal urea concentration increased tracer mannitol and also tracer urea permeability, both in the presence and absence of tubular hyperosmolarity. Electron microscopic observations showed changes in geometry of tubular junctional complexes compatible with the observed increase in permeability.

A new model for regulation of sodium transport in high resistance epithelia

A new three barrier, four compartment model for sodium transport in high resistance urinary epithelia is presented. This model provides a unified and simplified mechanistic explanation for sodium transport and its quantitative regulation. Sodium enters the epithelial cell by passive diffusion. Active extrusion occurs across the lateral cell membrane into the lateral intercellular space (LICS). Sodium movement from the LICS into the serosal compartment is not free and unobstructed as in the models for low resistance epithelia, but rather occurs through a regulatory channel of the LICS passing through desmosomes and the basilar slit. The exact configuration of this regulatory channel controls the rate of sodium movement from the LICS into the serosal compartment. Thus, the configuration of the regulatory channel controls the afterload on the sodium pump and thus ultimately controls the rate of transepithelial sodium transport. Antidiuretic hormone could act by increasing the effective width of this regulatory channel by contraction of intracellular microtubules or microfilaments. Present theories for regulation of transepithelial sodium transport in high resistance epithelia invoke a regulatory barrier at the apical cell membrane or at the active sodium pump located in the basolateral cell membrane. The hypothetical model presented here invokes a new alternative: regulation of the active pump rate by the sodium concentration in the LICS serving as an afterload on the pump; sodium escape from the LICS into the serosal compartment thus becomes the regulatory step for transepithelial transport.

Phosphate, calcium and magnesium fluxes into the lumen of the rat proximal convoluted tubule

In order to study fluxes of phosphate (Pi), Ca and Mg into the rat proximal tubule, a modification of the split-droplet microinjection technique was used. Injected fluids were isotonic solutions containing no Pi, Ca or Mg. The initial NaCl concentration of the injectates was either (a) 115 mM(l (which resulted in net fluid entry into the lumen), (b) 125 mM/l (no net fluid movement) or (c) 150 mM/l (net reabsorption of fluid). Injected droplets were subsequently collected from the nephron and their ionic concentrations determined using electron probe analysis. All 3 ions entered the tubular lumen. For the 115 mM and 125 mM NaCl injectates, Pi concentration increased for the first 15 s, then reached steady values of 2.07 mM/l and 2.30 mM/l respectively. Using 150 mM NaCl as injectate, Pi concentration increased for only 10 s, and then reached an average value of 2.04 mM/l. Ca and Mg concentrations in reaspirated droplets showed no correlation with time, indicating that entry into the lumen was almost immediate. The mean Ca concentration using 115 mM NaCl injectate was 1.63 mM/l, higher than with equilibrated or reabsorbed injectates (1.01 and 1.15 mM/l respectively). Mg concentration following injection of 115 mM NaCl solution (0.45 mM/l) was lower than with the other 2 injectates (0.92 and 0.85 mM/l). It is suggested that Pi and Mg enter the proximal tubular lumen from the tubular cells, while Ca entry may be transtubular and take place via intercellular pathways.

Effect of transtubular osmotic gradients on the paracellular pathway in toad kidney proximal tubule: electron microscopic observations

We have evaluated with the aid of electron microscopy changes in the width of the paracellular pathway and in the magnitude of the paracellular passage of lanthanum ions when a transtubular osmotic gradient was established by making the tubular lumen hyperosmotic with 50 mM urea or mannitol. Both transtubular gradients (with urea or mannitol) induce changes in the ultrastructure of the tight junction. The changes are characterized by widening of the tight junction and the "disappearance" of the substance forming the intermediate line. Urea has a larger effect than mannitol. The magnitude of lanthanum crossing extracellularly through the tubular epithelium also increases under the urea induced transtubular gradient.