Recent experiments towards a model for fluid secretion in Rhodnius Upper Malpighian Tubules (UMT)

Three different methods have been used to improve a model for fluid secretion in Upper Malpighian Tubules (UMT) of the blood sucking insect Rhodnius prolixus. (I) In the first, UMT double perfusions in 5th instar Rhodnius were used to measure their fluid secretion rate. They were stimulated to secrete with 5-HT. Double perfusions allowed access separately to the basolateral and the apical cell membranes with pharmacological agents known to block different ion transport functions, namely ATPases, cotransporters and/or countertransporters and ion and water channels: ouabain, bafilomycin A1, furosemide, bumetanide, SITS, acetazolamide, amiloride, DPC, BaCl(2), pCMBS and DTT. The basic assumption is that changes in water movement reflect changes in ion transport mechanisms. (II) Intracellular Na(+) concentrations were measured with a fluorometric method in dissected R. prolixus UMT, under several experimental conditions. (III) ATPase activities were measured in R. prolixus UMT. A tentative model for the function of the UMT cell is presented. We find that (a) at the basolateral cell membrane, fundamental is a Na(+)-K(+)-2Cl(-) cotransporter; of intermediate importance are the Na(+)-K(+)-ATPase and a ouabain-insensitive Na(+)-ATPase, ion channels and Rp-MIP water channels. (b) At the apical cell membrane, most important are a V-H(+)-ATPase; and a K(+) and/or Na(+)-H(+) exchanger.

A Model for Fluid Secretion in Rhodnius Upper Malpighian Tubules (UMT)

We have measured fluid secretion rate in Rhodnius prolixus upper Malpighian tubules (UMT) stimulated to secrete with 5-OH-tryptamine. We used double perfusions in order to have access separately to the basolateral and to the apical cell membranes. Thirteen pharmacological agents were applied: ouabain, Bafilomycin A(1), furosemide, bumetanide, DIOA, Probenecid, SITS, acetazolamide, amiloride, DPC, BaCl(2), pCMBS and DTT. These agents are known to block different ion transport functions, namely ATPases, co- and/or counter-transporters and ion and water channels. The basic assumption is that water movement changes reflect changes in ion transport mechanisms, which we localize as follows: (i) At the basolateral cell membrane, fundamental are a Na(+)-K(+)-2Cl(-) cotransporter and a Cl(-)-HCO(3) (-) exchanger; of intermediate importance are the Na(+)-K(+)-ATPase, Cl(-) channels and Rp-MIP water channels; K(+) channels play a lesser role: (ii) At the apical cell membrane, most important are a K(+)-Cl(-) cotransport that is being located for the first time, a V-H(+)-ATPase; and a Na(+)-H(+) exchanger; a urate-anion exchanger and K(+) channels are less important, while Cl(-) channels are not important at all. A tentative model for the function of the UMT cell is presented.

Fluid secretion in Rhodnius upper malpighian tubules (UMT): water osmotic permeabilities and morphometric studies

We have measured the osmotic permeability of the basolateral cell membrane (Poscb) and compared it with the transepithelial permeability (Poste) to calculate the paracellular (Posp) permeability of the upper malpighian tubules (UMT) of the 5th instar of Rhodnius prolixus under several experimental conditions, namely, at rest and after stimulation to secrete with 5-HT, each under control conditions (no treatment), after treatment with pCMBS, and after addition of pCMBS and DTT. Secretion rate is negligible at rest. During stimulation mean secretion rate is 43.5 nl/cm2 sec. Secretion is severely curtailed by pCMBS and fully restored by DTT. Poscb = 9.4 (resting, control); 5.8 (control + pCMBS); 10.7 (control + pCMBS + DTT); 20.6 (stimulated, control); 14.7 (stimulated + pCMBS); 49.1 (stimulated + pCMBS + DTT) (x10?4 cm3/cm2 sec Osm). Calculated Posp are higher than the transcellular permeability, Posc, at rest and after stimulation. Electron micrograph morphometry of UMT sections show that cells significantly decrease their volume after stimulation. Lateral intercellular space (LIS) and basolateral extracellular labyrinth (BEL) are barely discernible at rest. LIS and BEL are widely dilated in stimulated UMT. Thus, ions have restricted access to the deep and narrow basolateral cell membrane indentations at rest, but they have ready access to cell membrane indentations after stimulation, because of the opening of LIS and BEL. These findings are discussed in relation to isosmotic secretion. The rate-limiting step for paracellular movement is located at the smooth septate junctions.

Identification of a new water channel (Rp-MIP) in the Malpighian tubules of the insect Rhodnius prolixus

Malpighian tubules (MT) of Rhodnius prolixus transport fluid at very high rates. To identify whether aquaporins (AQPs) are present in the MT of R. prolixus, total ribonucleic acid (RNA) was isolated from MT and used in a reverse transcription, polymerase chain reaction (RT-PCR), with two degenerate primers to highly conserved regions of the members of the AQPs family. A deoxyribonucleic acid (DNA) fragment of 370 bp was amplified; its sequence revealed a novel protein, representing a new member of the major intrinsic protein (MIP) family. The complementary DNA (cDNA) sequence of this new MIP protein was cloned by using RNA from MT and the rapid amplification of cDNA ends (RACE) technique. The cDNA had 1133 bp and the largest open reading frame coded for a protein of 286 amino acids, named R. prolixus major intrinsic protein (Rp-MIP). The hydrophobicity profile of the amino acid sequence predicts six transmembrane domains. Northern blot analysis of MT RNA showed a single transcript of about 1-1.3 kb for Rp-MIP. RT-PCR of single isolated MT and in situ hybridization analysis showed Rp-MIP transcripts in both proximal and distal segments. Expression of Rp-MIP in Xenopus laevis oocytes doubled the osmotic water permeability Pf, indicating that Rp-MIP may function as an aquaporin protein in the MT of the insect and thus may participate in urine formation in R. prolixus.

Length of the selectivity filter of aquaporin-1

We have characterized the selectivity filter of the water channel aquaporin-1 (AQP1) of proximal straight tubules (PST), as an equivalent cylindrical structure with a diameter of approximately 4.5 A, where water molecules single file. We report here efforts to evaluate its length. PST were dissected from rabbit kidneys, held with pipettes in a chamber bathed in a buffered mannitol isosmotic solution (MBS, 295 mOsm/kg). Changes in tubule cell volume with time (dV/Adt), were monitored, on line, with an inverted microscope, a TV camera and an image processor. Osmotic permeability coefficients, Pos, and reflection coefficients (sigma s) were measured with several solutes: mannitol (M), raffinose (R), sucrose (S), glycerol (G), acetamide (A) and urea (U). For this purpose PST were suddenly exposed (in approximately 80 ms and for 20 s) to a hyperosmolality step (delta Cs) achieved by adding to MBS a delta Cs of 35 mOsm/kg of either R, S, M, G, A or U. Cells shrunk within 500 ms of t = 0 to their osmometric volume and remained shrunk for the 20 s of the delta Cs. Pos was measured from the shrinking curves; Pos = 3000 +/- 25 microns/s with either R, S, M, G, A or U. This procedure also allowed to calculate sigma s; sigma s = 1.00 for R, S, M, G, A and U, indicating that these solutes do not penetrate the water channel. In contrast, the shrinking curves produced by a delta Cs = 35 mOsm/kg formamide (F) were 1/5th to 1/6th slower and smaller (subosmometric) than those produced by a delta Cs = 35 mOsm/kg of R, S, M, G, A or U. Furthermore, with F, cells did not remain shrunk. They recovered their original volume within 3 s. Pos (measured with F) is denoted as Pos*; Pos* = 480 +/- 30 microns/s. sigma s, formamide (denoted sigma sp) = 0.16 +/- 0.01. Use of sigma sp and Pos* values in Hill's equations for the bimodal theory of osmosis leads to n = 2-3, n being the number of water molecules single filing within the channel selectivity filter, whose length must lie within 6 to 9 A, a value lower than previous values calculated from the Pos/Pd* ratio.

Transport of water in proximal kidney tubules from whole tubules to single channels: length and section of the selectivity filter of aquaporin-1

Proximal straight tubule (PST) were dissected from rabbit kidneys, held with crimping pipettes in a chamber bathed in a buffered mannitol isosmotic solution (MBS, 295 mOsm/kg). Tubule cell volume changes with time (dV/Adt) after steps in MBS osmolality (delta Cs) were monitored on line with an inverted microscope, a TV camera and an image processor. Reflection coefficients sigma and osmotic permeability coefficients, Pos, for several solutes were measured using two methods. Method 1: sigma was calculated from the delta Csiso of impermeant and permeant solutes at which (dV/Adt)t-->0 = 0 (i.e., by a null point method). It is denoted as sigma 1. sigma 1 = 1.00 for mannitol (M), raffinose (R), sucrose (S), glycerol (G), acetamide (A) and urea (U). With formamide (F), sigma 1, Formamide = 0.62 +/- 0.05. These findings confirm our previous value of dp = 4.5 A for the diameter of the selectivity filter of the basolateral PST cell membrane water channel AQP1. Method 2: PST were exposed for 20 s to MBS made hyperosmotic by addition of a delta Cs of 35 mOsm/kg of R, S, M, G, A and U. Cells shrunk within 500 ms of t = 0 to their osmometric volume and remained shrunk for the 20 s of the osmotic challenge. Pos was measured from the shrinking curves. P(os) = 3000 +/- 25 microns/s with R, S, M, G, A and U. Method 2 also allowed to calculate sigma, denoted as sigma 2. sigma 2 = 1.00 for R, S, M, G, A and U. By contrast, the shrinking curve produced by a delta Cs of 35 mOsm/kg F was 1/5th to 1/6th slower and smaller (i.e., subosmometric) than that produced by a delta Cs of 35 mOsm/kg R, S, M, G, A and U. Furthermore, with F cells did not remain shrunk but recovered their original volume within 3 s. P(os) (measured with F) is denoted as P(os)*, P(os)* = 480 +/- 30 microns/s. sigma 2, Formamide = 0.16 +/- 0.01. Use of sigma 1, sigma 2 and P(os)* values in Hill's equations for the bimodal theory of osmosis leads to n = 2-9. Where n is the number of water molecules single filling within the channel selectivity filter, whose length must lie within 6 to 27 A, a value significantly lower than our previous value calculated from the P(os)/Pd* ratio.

The paracellular channel for water secretion in the upper segment of the Malpighian tubule of Rhodnius prolixus

Lumen to bath J12/C1 and bath to lumen J21/C2 fluxes per unit concentration of 19 probes with diameters (dm) ranging from 3.0-30.0 A (water, urea, erythritol, mannitol, sucrose, raffinose and 13 dextrans with dm 9.1-30.0 A) were measured during volume secretion (Jv) in the upper segment of the Malpighian Tubule of Rhodnius by perfusing lumen and bath with 14C or 3H-labeled probes. Jnet = (J12/C1-J21/C2) was studied as a function of Jv.Jv was varied by using different concentrations of 5-hydroxy tryptamine. Jnet for 3H-water was not different from Jv. We found: (i) A strong correlation between Jnet and Jv for 8 probes dm = 3.0-11.8 A (group a probes), indicating that the convective component of Jnet is more important than its diffusive component and than unstirred layers effects which are negligible. Therefore group a probes are solvent dragged as they cross the epithelium. (ii) There is no correlation between Jnet and Jv for 11 probes with dm = 11.8-30 A (group b). Therefore these probes must cross the epithelium by diffusion and not by solvent drag. (iii) In a plot of Jnet/Jv vs. dm group a probes show a steep linear relation with a slope = -0.111, while for group b probes the slope is -0.002. Thus there is a break between groups a and b in this plot. We tried to fit the data with models for restricted diffusion and convention through cylindrical or parallel slit pathways. We conclude that (i) group a probes are dragged by water through an 11.0 A-wide slit. (ii) Most of Jv must follow an extracellular noncytosolic pathway. (iii) Group b probes must diffuse through a 42 A-wide slit. (iv) A cylindrical pathway does not fit the data.

The proximal straight tubule (PST) basolateral cell membrane water channel: selectivity characteristics

Proximal straight tubules (PST) were dissected from rabbit kidneys, held by crimping pipettes in a chamber and bathed in a buffered isosmotic (295 mOsm/kg) solution containing 200 mM mannitol (MBS). Changes in tubule diameter were monitored on line with an inverted microscope, TV camera and image processor. The PST were then challenged for 20 sec with MBS made 35 mOsm/kg hyperosmotic by addition of either NaCl, KCl, mannitol (M), glycerol (G), ethylene glycol (E), glycine (g), urea (U), acetamide (A) or formamide (F). With NaCl, KCl, M, G, E, g, U, and A, tubules shrunk osmometrically within 0.5 sec and remained shrunk for as long as 20 sec without recovering their original volume (sometimes A showed some recovery). PST barely shrunk with F and quickly recovered their original volume. The permeability coefficients were 0 microns/sec (NaCl, M, g, E and U), 1 micron/sec (A), 84 microns/sec (F) and 0.02 micron/sec (G). The reflection coefficients sigma = 1.0 (NaCl, KCl, M, G, E, g and U), 0.95 (A) and 0.62 (F). Similar sigma values were obtained by substituting 200 mOsm/kg M in MBS by either NaCl, KCl, G, E, g, U, a or F. The olive oil/water partition coefficients are 5 (M), 15 (U), 85 (A) and 75 (F) (all x 10(-5)). Thus, part of F permeates the cell membrane through the lipid bilayer. The probing molecules van der Waals diameters are 7.4 x 8.2 x 12.0 (M), 3.6 x 5.2 x 5.4 (U), 3.8 x 5.2 x 5.4 (A) and (3.4 x 4.5 x 5.4 (F) A.(ABSTRACT TRUNCATED AT 250 WORDS)

Water and urea diffusive permeabilities in isolated proximal tubule cells

We measured the water (3H2O) and urea ([14C]urea) diffusive permeabilities (Pd) in intact proximal tubule cells (PTC). Isolated rabbit PTC were packed in polyethylene tubes to measure the diffusion coefficients for 3H2O and [14C]urea at 14 degrees C. Pd values for water and urea were estimated from the diffusion coefficients in intact cells, in extracellular and in intracellular media using a model of parallel and series diffusion in the packed PTC. Pd was 65 +/- 8 and 18 +/- 3 microns/s for water and urea, respectively. We examined the effect of different pharmacological agents on the Pd values for water and for urea. p-Chloromercuribenzenesulfonate at 2 mM markedly inhibited the Pd of water and urea. Phloretin (1 mM) inhibited the Pd for urea, whereas it increased Pd for water. KMnO4 (30 microM) markedly inhibited urea Pd but did not alter water Pd. The values for energy of activation of Pd for water and for urea were 2.9 +/- 2 and 7.3 +/- 4 kcal/mol, respectively. These results show that water and urea do not share the same pathway across the PTC membrane.

The fate of calcium in the diet of Rhodnius prolixus: storage in concretion bodies in the Malpighian tubules

We have investigated the fate of the large amounts of calcium ingested by Rhodnius prolixus in its meals of blood. 45Ca2+ injected into the haemolymph or fed to fifth-stage Rhodnius reared on rabbits is accumulated at high concentrations in the cells of the upper Malpighian tubules; very little is excreted from the body This 45Ca2+ accumulation goes on continuously for at least 12 days and the rate of uptake is increased several-fold within 3–4 days of a meal. The extent of calcium accumulation in tubule cells is correlated with the presence of intracellular membrane-bound concretion bodies, which are therefore likely sites of calcium deposition. X-ray diffraction showed that the calcium deposits are non-crystalline. Tubules from rabbit-fed fifth-stage Rhodnius contain 410 mmol l-1 calcium; in those from chicken-fed insects the calcium concentration is over 1 mol l-1; and in those fed in vitro on heparinised low-K+ sheep blood the calcium concentration is only 21 mmol l-1. The concentration of calcium in the haemolymph in all these insects was 8 mmol l-1 and its activity determined by an ion-selective electrode was 2.5 mmol l-1. 45Ca2+ deposited in the tubules is readily exchangeable, but the efflux preferentially passes to the haemolymph side of the tubule epithelium. The ability to sequester calcium in the Malpighian tubules may prevent calcium from interfering with reabsorptive processes in the rectum.

Solvent drag of sucrose during absorption indicates paracellular water flow in the rat kidney proximal tubule

Single convoluted proximal tubules of the rat kidney were lumen perfused in situ with isosmotic solutions containing C14-sucrose and H3-inulin as tracers, to evaluate whether the extracellular marker sucrose is entrained by water during proximal tubular reabsorption. Inulin was used as volume marker. The absorptive rate was varied by using as luminal perfusion fluids either a solution made up of (in mmole/l) 120 NaCl, 5 glucose, 25 NaHCO3 and altering the perfusion rate, or a solution containing 110 NaCl and 70 raffinose. Js, the net sucrose efflux is found to be a function of the net volume flow, Jv, such that at Jv = 0, Js is very small and at high rates of Jv, Js is over 60-fold the value observed at low Jv values. In addition, the transported to luminal sucrose concentrations decreased with Jv in a hyperbolic manner. Unstirred layers affect the diffusive component of Js, but only to a small extent. Therefore, the large remaining dependency of Js with Jv must be due to drag of sucrose by water, within the paracellular pathway. This leads to the conclusion that water flows through the paracellular pathway during absorption in the rat proximal tubule, in addition to transcellular water flow. Using equations for molecular sieving and the measured value of sigma s for sucrose of 0.76-0.91, it is calculated that the pathway where entrainment of solute by water occurs must be 1.0-1.1 nm wide. This calculation is only tentative since sigma s depends on the as yet unknown relative contribution of transcellular and paracellular pathways to transepithelial water osmotic permeability.

Diffusive water permeability in isolated kidney proximal tubular cells: nature of the cellular water pathways

The diffusive water permeability (Pd) of the plasma membrane of proximal kidney tubule cells was measured using a 1H-NMR technique. The values obtained for the exchange time (Tex) across the membrane were independent of the cytocrit and of the Mn2+ concentration (in the range 2.5 to 5 mM). At 25 degrees C the calculated Pd value was (per cm2 of outer surface area without taking into account membrane invaginations) 197 +/- 17 microns/sec. This value equals 22.3 +/- 1.9 microns/sec when the invaginations are taken into account. Cell exposure to 2.5 mM parachloromercuribenzenesulfonic acid, pCMBS, (for 20 to 35 min) reduced Pd to 45% of its control value. Five mM dithiothreitol, DTT, reverted this effect. The activation energy for the diffusive water flux was 5.2 +/- 1.0 kcal/mol under control conditions. It increased to 9.1 +/- 2.2 kcal/mol in the presence of 2.5 mM pCMBS. Using our previous values for the osmotic water permeability (Pos) in proximal straight tubular cells the Pos/Pd ratio equals 18 +/- 1, under control conditions, and 3.2 +/- 0.3 in the presence of pCMBS. These experimental results indicate the presence of pathways for water, formed by proteins, crossing these membranes, which are closed by pCMBS. Assuming laminar flow (within the pore), from Pos/Pd of 13 to 18 an unreasonably large pore radius of 12 to 15 A is calculated which would not hinder cell entry of known extracellular markers. Alternatively, for a single-file pore, 11 to 20 would be the number of water molecules which would be in tandem inside the pore.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparison of transcellular and transepithelial water osmotic permeabilities (Pos) in the isolated proximal straight tubule (PST) of the rabbit kidney

Measurements of the water osmotic permeabilities of apical and basolateral membranes of PST cells and of the transepithelial permeability have been carried out using a very fast method with high temporal and spatial resolution. At 25 degrees C the values obtained are: 80.8 +/- 11.9 x 10(-4) cm3/s osmol cm2 of apical (luminal) surface area and 90.1 +/- 13.0 x 10(-4) cm3/s osmol cm2 of basement membrane area (no membrane invaginations taken in account). These values are higher than previously published values due to the use of a faster and more accurate volume measuring and recording system. The transepithelial water osmotic permeability at 25 degrees C is 77 +/- 11 in units of 10(-4) cm3/s osmol cm2 basement membrane area. The transcellular water osmotic permeability is 32 +/- 7 (same units), leaving a paracellular contribution of 45 +/- 10 (same units). In the presence of 2.5 mM parachloromercuribenzenesulfonate (pCMBS) the apical permeability is reduced with an incubation of 10-15 min to 23% of its control value and the basolateral permeability to 8% of its control value (after 25 min) but the transepithelial permeability is only reduced to about 1/2 of the control value. This leaves a transcellular permeability of 6 x 10(-4) cm3/s osmol cm2 of basement membrane area and a paracellular contribution of 33 +/- 6 (same units). These results indicate a significant contribution of the paracellular pathway to the transepithelial water osmotic permeabilities in PST.

Renal Reabsorption of Water. Are there Pores in Proximal Tubule Cells?

Transcellular water movement occurs mainly through water channels where water molecules move in single file. These channels, which explain the large water permeability of the proximal tubule cells, are closed by mercurial sulfhydryl reagents. There are similarities between these channels and those of human red cells and those that appear in distal nephron segments and toad urinary bladder after antidiuretic hormone stimulation.

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.

Na+, K+ and Cl- transport in isolated small intestinal cells from guinea pig. Evidences for the existence of a second Na+ pump

Isolated small intestinal epithelial cells, after incubation at 4 degrees C for 30 min, reach ion concentrations (36 mM K+, 113 mM Na+ and 110 mM Cl-) very similar to those of the incubation medium. Upon rewarming to 37 degrees C, cells are able to extrude Na+, Cl- and water and to gain K+. Na+ extrusion is performed by two active mechanisms. The first mechanism, transporting Na+ by exchanging it for K+, is inhibited by ouabain and is insensitive to ethacrynic acid. It is the classical Na+ pump. The second mechanism transports Na+ with Cl- and water, is insensitive to ouabain but is inhibited by ethacrynic acid. Both mechanisms are inhibited by dinitrophenol and anoxia. The second Na+ extruding mechanism could be the Na+/K+/2Cl- cotransport system. However, this possibility can be ruled out because the force driving cotransport would work inwards, and because Na+ extrusion with water loss continues after substitution of Cl- by NO3-. We propose that enterocytes have a second Na+ pump, similar to that proposed in proximal tubular cells.

Pathways for volume flow and volume regulation in leaky epithelia

Continuous pathways must pierce the cell membrane to be used by water during osmotic equilibration between proximal straight tubular cells and the external medium, because a) the water osmotic permeability coefficient of the basolateral plasma membrane, Poscb, is high; b) its activation energy, Ea, is as that of free water movement and c) pCMBS inhibits markedly (but reversibly) Poscb and increases Ea to values similar to those observed in lipid bilayers without pores. d) Preliminary measurements of Pd the water diffusive permeability coefficient using NMR indicate that Poscb/Pd is near 4 - 5. The following two observations indicate that a significant paracellular water flow must exist in leaky epithelia. Namely, a) large extracellular solutes are dragged by water in four leaky epithelia: gall bladder, Necturus proximal tubule, rat proximal tubule and Rhodnius malpighian tubule. b) The transcellular water osmotic permeability coefficient is smaller than the transepithelial values available in the rabbit proximal straight tubule. This requires a significant paracellular permeability.

Osmotic water permeability of the apical membrane of proximal straight tubular (PST) cells

Osmotic steps, delta C, were produced across the apical cell membrane of isolated rabbit PST by perfusing their lumens with double barreled micropipettes at a rate of 0.5-0.8 nl/s. delta C = 15-46 mOsmolar were induced with mannitol. Changes in luminal diameter were recorded as a function of time with a TV camera and an integrator-processor system with space and time resolutions of 0.03 micron and 0.0167 s (3). The tubules were bathed with oil. Outer tubule diameter was time invariant. Pcaos, the apical cell osmotic permeability was therefore calculated from cell volume changes with time in units of 10(-4) cm3/cm2 X s. Osmolar. Pcaos was independent of delta C. The mean is 22.8 +/- 1.3 (n = 55). With a basolateral permeability of 50.4 (3,12), the transcellular permeability is 14 (same units) smaller than the transepithelial values available. This leads to the conclusion that a significant paracellular water osmotic permeability must exist.

Effect of para-chloromercuribenzenesulfonic acid and temperature on cell water osmotic permeability of proximal straight tubules

The apparent Arrhenius energy of activation (Ea) of the water osmotic permeability (Pcos) of the basolateral plasma cell membrane of isolated rabbit proximal straight tubules has been measured under control conditions and after addition of 2.5 mM of the sulfhydryl reagent, para-chloromercuribenzenesulfonic acid (pCMBS), of mersalyl and of dithiothreitol. Ea (kcal/mol) was 3.2 +/- 1.4 (controls) and 9.2 +/- 2.2 (pCMBS), while Pcos decreased with pCMBS to 0.26 +/- 0.17 of its control value. Mersalyl also decreased Pcos both in vitro and in vivo (using therapeutical doses). These actions of pCMBS and mersalyl were quickly reverted with 5 mM dithiothreitol and prevented by 0.1 M thiourea. Ea for free viscous flow is 4.2 and greater than 10 for non-pore-containing lipid membranes. By analogy with these membranes and with red blood cells, where similar effects of pCMBS on Pos are observed, it is concluded that cell membranes of the proximal tubule are pierced by aqueous pores which are reversibly shut by pCMBS. Part of the action of mercurial diuretics can be explained by their action on Pcos.

The continuous measurement of tubular volume changes in response to step changes in contraluminal osmolality

A new method to measure time dependent (t) volume (V) changes in proximal straight tubules (PST) is described. V is calculated from diameter (d) measurements for which a video camera and an integrating circuit are used. A tubular image of high optical contrast is recorded with the TV camera such that the scan lines run crosswise to the tubule. The video signal is analyzed by a special processor which adds 225 tubular diameters of each TV frame and feeds this analog signal to a pen recorder. The fractional error in d measurements is 10(-3). Diameter changes of less than 0.05 micron can be detected, as compared to the usual error of a single measurement of about 0.4 micron. Pcbos, the osmotic water permeability of the contraluminal cell membrane was measured by setting up osmotic steps across it in less than 0.1 s and following the ensuing delta d/delta t. The time delay between solution change and the linear part of the osmotic response was 0.51 +/- 0.05 s. Pcbos was found to be 50.4 (+/- 8.7) X 10(-4) cm3.cm-2 of basement membrane area .s-1.osmolar-1.

Cell osmotic water permeability of isolated rabbit proximal convoluted tubules

Cell osmotic water permeability, Pcos, of the peritubular aspect of the proximal convoluted tubule (PCT) was measured from the time course of cell volume changes subsequent to the sudden imposition of an osmotic gradient, delta Cio, across the cell membrane of PCT that had been dissected and mounted in a chamber. The possibilities of artifact were minimized. The bath was vigorously stirred, the solutions could be 95% changed within 0.1 s, and small osmotic gradients (10-20 mosM) were used. Thus, the osmotically induced water flow was a linear function of delta Cio and the effect of the 70-microns-thick unstirred layers was negligible. In addition, data were extrapolated to delta Cio = 0. Pcos for PCT was 41.6 (+/- 3.5) X 10(-4) cm3 X s-1 X osM-1 per cm2 of peritubular basal area. The standing gradient osmotic theory for transcellular osmosis is incompatible with this value. Published values for Pcos of PST are 25.1 X 10(-4), and for the transepithelial permeability Peos values are 64 X 10(-4) for PCT and 94 X 10(-4) for PST, in the same units. These results indicate that there is room for paracellular water flow in both nephron segments and that the magnitude of the transcellular and paracellular water flows may vary from one segment of the proximal tubule to another.

Fluid reabsorption by Necturus proximal tubule perfused with solutions of normal and reduced osmolarity

Fluid absorption in Necturus proximal tubule was studied when the kidneys were perfused with solutions of different osmolarities. The rate of fluid absorption was inversely proportional to the perfusion fluid osmolarity, while Na uptake remained constant. No difference was detected between the collected and injected luminal fluid, i.e. reabsorption was isotonic at normal and reduced osmolarities. The transtubular osmotic permeability remained fairly constant under the different perfusion osmolarities. Using our experimental results to test various models based on osmotic equilibration across the tubule wall we show that none of these provides an adequate mechanism for fluid absorption in this epithelium.

Cell osmotic water permeability of isolated rabbit proximal straight tubules

Proximal straight tubules were dissected and mounted in a chamber with their lumina occluded. The well-stirred bath could be 95% changed within 84 ms to set up osmotic gradients (delta Coi) across the peritubular cell aspect. Volume changes (less than or equal to 10 pl/mm) were estimated from continuous records of diameter changes (error less than 0.1 micrometers). delta Coi greater than or equal to 2-3 mosM could be discerned. delta Coi values from 10 to 44 mosM were used to evaluate Posc, the cell osmotic water permeability coefficient, and extrapolated to delta Coi = 0. Posc = 25.1 (+/- 2.3) X 10(-4) cm3.s-1.osM-1.cm2 tubular surface area-1. These values are lower than those reported for Pose, the transepithelial osmotic water permeability coefficient, and become lower if corrected for the real (infolded) peritubular cell surface area. Thus, for a given osmotic difference, transcellular water flow finds a higher resistance than paracellular water flow. Experiments were also performed with delta Coi greater than 100 mosM, but interpretation of these data is difficult because of the presence of volume regulatory phenomena and other undesirable effects.

Solvent drag of large solutes indicates paracellular water flow in leaky epithelia

Net fluxes ( J s n ) of sucrose, inulin, dextran (relative molecular mass 15000–17000), albumin and haemoglobin were measured across guineapig gall bladder unilateral preparations in which the absorptive flow ( J v ) was varied over a wide range (without transepithelial osmotic gradients) by perfusing their lumina either with isosmotic solutions or with solutions of reduced osmolalities (a procedure that increased J v ). The absorptive flow J v was inhibited by 10 –4 M ouabain. The net flux J s n was a linear function of J v for sucrose, inulin and dextran, the slopes being inversely related to the size of the solute. Albumin and haemoglobin barely crossed the preparation. The large increase in J s n cannot be accounted for by increased leakiness of the preparation or by unstirred layer effects. The logarithms of the unidirectional flux ratios for sucrose, inulin and dextran in bilateral preparation were also linear functions of J v . It is concluded that solvent drags these solutes via the paracellular pathway. More than 50% of the water flows paracellularly also under normal physiological conditions, since J s n for sucrose was also a linear function of J v in experiments performed only at the physiological osmolality (0.3osm). This value is calculated from the slope of the line relating J s n to J v .

The effect of external pH on osmotic permeability, ion and fluid transport across isolated frog skin

1. The rate of volume flow across frog skin induced by an osmotic gradient was measured when normal (7.4) and low pH (2.28) solutions bathed the outside. The osmotic permeabilities (Pos) were 2.4 +/- 0.4 and 4.8 +/- 1.0 micrometer/sec, respectively. The change in Pos induced by low pH was reversible. 2. Volume flow in the absence of an osmotic gradient was measured at normal and low pH. Values were 0.69 +/- 0.13 and 1.1 +/- 0.2 microliter/hr. cm2, respectively; the paired differences were significant (P less than 0.0025). This change in rate was partially reversible upon return to normal pH. 3. The potential difference (V) and short-circuit current (Is) across skins were measured under several conditions and the following equivalent parameters in a simplified electrical model were computed: total resistance (Rt); shunt resistance (Rs); electromotive force of the pump (ENa); and salt transport at open circuit (JNaCl). Representative figures were (a), at pH 7.4: Is = 14 +/- 1.6 microampere/cm2; Rt = 3.3 +/- 0.4 komega.cm2; Rs = 7.2 +/- 1.0 komega.cm2; ENA = 103 +/- 38 mV; JNaCl = 7.2 +/- 1.2 microampere/cm2; (b) at pH 2.28: Is = 8.3 +/- 2.1 microampere/cm2; Rt = 0.46 +/- 0.12 komega. cm2; Rs = 0.65 +/- 0.06 komega.cm2; ENa = 59 +/- 25 mV; JNaCl = 9.4 +/- 3.3 microampere/cm2. 4. From the electrical parameters measured concomitantly with the rate of fluid transport in given experiments, the expected salt concentration of the transported fluid was 0.30 +/- 0.08 and 0.38 +/- 0.08 mole/l. at normal and low pH, respectively, or some 3-4 times hyperosmotic with respect to the medium. 5. Treatment with low pH on the outside has been found to open the intercellular junctions in previous studies. The present results suggest that, if such an effect occurs, it is localized only to a small fraction of the cell perimeter. Making certain assumptions that fraction could be as low as 0.003. 6. Low pH on the outside reversibly changes the electrical parameters of a 'tight' epithelium like the frog skin into values more typical of 'intermediate' epithelia; both the total and shunt resistances decrease to about 0.1 of their normal values. These changes do not apparently affect the osmolarity of the transported fluid.

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.

Distal tubular tracer microinjection study of renal tubular potassium transport

Renal tubular potassium (K) transfer was studied in rats using a tracer microinjection technique in which [14C]inulin and 42K were simultaneously injected into early distal tubules during osmotic diuresis. Experiments were carried out in 1) animals on a control diet, 2) animals in which K secretion had been stimulated (high-K diet + KC1, Na2SO4, Diamox infusions), and 3) animals in which K excretion had been reduced by a low-K or low-Na diet or by amiloride. 42K excretion into the urine coincided closely in time with the excretion pattern of [14C]inulin. Efflux of 42K out of the lumen was stimulated during reduced K secretion along the distal nephron and decreased during enhanced K secretion when small tubular K loads were given. These experiments demonstrate bidirectional K movement across the distal nephron and show that changes in reabsorptive K efflux participate in the regulation of tubular net K movement.

Cell electrical potentials during enhanced sodium extrusion in guinea-pig kidney cortex slices

1. Experiments were performed on outermost slices of the guinea-pig kidney which are mainly made up of proximal tubular cells. 2. Kidney cells loaded with Na+ by chilling at 0.6 degrees C for 2.5 hr, when subsequently rewarmed to 25 degrees C in a medium containing 16 mM-K+ extrude Na+ at enhanced speed for about 10 min. This Na+ movement is accompanied by efflux of Cl and influx of K+. 3. Measurements of cell potential during enhanced Na+ extrusion show that cells hyperpolarize to values about 30 mV more negative than the K+ equilibrium potential. 4. This hyperpolarization is only partly inhibited by 1 mM ouabain or by 2 mM ethacrynic acid but both agents added together suppress it completely. 5. With 16 mM-Rb instead of 16 mM-K the hyperpolarization is smaller. 6. A diminished extracellular K+ concentration outside of the cells, within the slice, can account for only a small part of the hyperpolarization. 7. The hyperpolarization is proportional to the rate of Na+ pumping. 8. Cl- seems to shunt the hyperpolarization to a greater extent than K+. 9. It is concluded that Na+ extrusion is capable of transferring electric charge across the membrane.

Ouabain-insensitive Na+ stimulation of an Mg-2+ -dependent ATPase in kidney tissue

1. Freshly prepared microsomal fractions of the outermost cortex of guinea pig kidney show an Mg-2+-dependent ATPase activity which is partially inhibited by 100 mM NaCl, LiCl, KCl, RbCl, CsCl, NH4Cl or choline chloride. 2. If the microsomal preparation is aged by storage at 4 degrees C for 10-15 days, the Mg-2+-dependent activity shows stimulation by Na-+ and Li-+ but not by K-+, Rb-+, Cs-+, NH4-+ or choline. 3. Stimulation is similar with sodium salts of Cl-minus, HCO3-minus, CH3COO-minus, BR-minus, SO4-2-minus or methylsulphonate. 4. Stimulation is insensitive to 1 mM and 10 mM ouabain. 5. Stimulation is unaltered by the presence of 0.5 mM ethyleneglycol-bis-(beta-aminoethyl ether)N,N'-tetracetic acid. 6. Stimulation is 100% inhibited by 2 mM ethacrynic acid, a concentration which inhibits only 30% of the Mg-2+-dependent ATPase and 50% of the (Na-++K-+)-stimulated ATPase. 7. Some of these characteristics coincide with those of an ouabain-resistant, K-+-independent, ethacrynic acid-sensitive mode of Na-+ extrusion out of guinea pig kidney cortex cells.

Some aspects of proximal tubular sodium chloride reabsorption in Necturus kidney

Some aspects of proximal tubular sodium chloride reabsorption in Necturus kidney. Renal tubular reabsorption of fluid and sodium was measured by clearance methods in the doubly perfused Necturus kidney in which the bicarbonate concentration was varied between 0 and 60 mEq/liter. The effects of Damox (2.2 times 10-3M), ocubain (10-5M) and ethacrynic acid (10-4M) and of acidosis were also investigated. In addition to clearance experiments, stationary microperfusion experiments were carried out on promimal tubules to measure volume flow and steady-state sodium and chloride concentration differences across the tubular epithelium. In some experiments, the transepithelial electrical potential difference was also measured using an axial electrode system. The following results were obtained: 1) Bicarbonate is not essential to the operation of renal tubular fluid and sodium transport. 2) Total renal and proximal tubular fluid and sodium transport are partially inhibited by Diamox, ouabian and ethacrynic acid. 3) The proximal tubule maintains a significant transepithelial sodium and chloride concentration difference and a significant electrical potential difference (lumen-negative) in the presence of a poorly permeant nonelectrolyte. The direction and magnitude of the electrical polarization fully accounts for the observed chloride concentration difference. The data support the thesis that sodium chloride transport accross the proximal tubular epithelium takes place by active sodium transport and electically coupled passive chloride reabsorption. Important species differences with respect to mammalian transport mechanisms are discussed.

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.

Relationship between tubular net sodium reabsorption and peritubular potassium uptake in the perfused Necturus kidney

1. K influx from peritubular space into renal tubular cells, varphi(i) (K), was measured in doubly perfused Necturus kidneys by studying tissue uptake of (42)K added exclusively to the portal circulation. Concomitantly, net tubular Na reabsorption, varphi(n) (Na), was measured by clearance techniques. varphi(n) (Na) and varphi(i) (K) were varied widely by replacing solutions of physiological composition (controls) with solutions containing high K, low K, low Na, cyclamate instead of Cl, ouabain (10(-7)-10(-4)M) or ethacrynic acid (10(-5)-10(-4)M).2. The ratio of varphi(n) (Na) to varphi(i) (K) was found to vary with the experimental conditions, the control value of about 2 was maintained over a threefold variation in absolute Na reabsorption. This ratio increased with low K or ouabain to values near 4. With high K, ethacrynic acid, low Na or cyclamate the relationship was one or lower. Thus, net Na reabsorption can be uncoupled from peritubular K influx.3. These results can be best explained if there are two Na pumps working in parallel: pump A transporting Na (with Cl) and pump B, a Na-for-K-exchange pump. The ratio of Na efflux to K influx could approach infinity if only pump A works (if B is inhibited) and could approach one if only B works. It should vary between these limits in controls when both pumps are active, or when neither of the two pumps is completely inhibited.4. Alternatively, the experimental findings could be explained by a Na pump with a coupling ratio that varies within two extreme values, from high Na-K ratios (with Na reabsorption at, or near, control values but with very low K influx values) to low ratios (with normal K influx values but with low Na reabsorption values).

Visualization of tubular interspaces in the kidney with the aid of lanthanum

Images

Electrolyte transport in kidney tubule cells

Transtubular movement of Na and K takes place across an electrically negative cell compartment rich in K and poor in Na. Some properties of the luminal and peritubular cell boundaries with respect to ionic pump and leak characteristics are analysed. Sodium enters the tubule cell from the lumen down an electrochemical potential gradient. Peritubular Na-extrusion takes place both by an ouabain-sensitive Na-K exchange pump and by an electrogenic ouabain-insensitive Na pump. Net Na transport can be uncoupled from peritubular K uptake. It is highly likely that peritubular K uptake is pH sensitive. Once sodium has been extruded into the peritubular infoldings net Na transepithelial-transport is further critically affected by physical factors regulating capillary uptake of interstitial fluid. Several lines of evidence indicate that a large, variable intercellular transport pathway is present at the proximal tubular level. Tubular K secretion is controlled at the distal tubular level by: (1) the interplay of luminal and peritubular active K uptake into the tubule cell and (2) by a variable passive leak of K from cell into lumen across the partly depolarized luminal cell membrane. Changes in active peritubular K uptake regulate the size of a relatively small intracellular K transport pool and are critically involved in setting the rate of net tubular potassium secretion.