Initial plasma disappearance and distribution volume of [131l]albumin and [125l]fibrinogen in man

The simultaneous plasma disappearance curves of albumin and fibrinogen were recorded in eight normal subjects from 10 to 60 min following intravenous injection. Additional samples were taken at 3, 4, 5, 6 and 7 min. The initial distribution volume (IDV) of albumin calculated by semilogarithmic extrapolation to zero time was 5.56% (range 3.73–8.53) larger than that of fibrinogen, denoting an initial high-rate function of albumin efflux extending from zero to about 10 min after tracer injection. The following slower phase of the albumin curve from 10 to 60 min was found to be similar to the so-called transcapillary escape rate (TER) of single-tracer experiments. By introducing the value C_p(o) (i.e. the estimated curve height at t= 0, from the injected amount of albumin tracer divided by the IDV_f), the entire initial part of the albumin curves was analysed. From this analysis the mean value of 0.0135+ 0.0038 min¹ was determined for initial slope, corresponding to a whole-body unidirectional albumin efflux [j(0)] of 0.0572 + 0.0160 ml 100 g^-1 min^-1. The result is about 16 times higher than normal estimates of total lymphatic albumin return, indicating a huge backflux of interstitial albumin at the whole-body capillary level. Both phases of efflux seem to reflect uptake in a variety of peripheral tissues, and the hypothesis that the second phase (TER) expresses the initial slope of albumin escape into non-liver tissues is not substantiated. Based on the difference in IDV of the tracers demonstrated, the uncritical use of albumin as a plasma volume marker is not justified.     

Hydrostatic pressure in the subcapsular interstitial space of rat and dog kidneys

Summary Experiments were performed in rats and dogs in order to reevaluate the concept of a high renal interstitial pressure. Assuming that the renal subcapsular pressure represents the pressure 2 of the superficial interstitium, catheters were implanted in the subcapsular space and the pressure was continuously recorded with a transducer of a very low volume displacement. In 17 rats a mean subcapsular pressure of 3.8 cm H₂O±2.0 was measured, while 6 dogs had an average subcapsular pressure of 10.8 cm H₂O±3.0. The subcapsular pressure was found to increase during renal venous constriction and ureteral pressure elevation, procedures which are very likely to lead to a rising renal interstitial pressure.To demonstrate a functional communication between the subcapsular and the deep renal interstitium I¹³¹-labelled albumin was injected into the subcapsular space of 5 rats, while the hilar lymph was collected through a cannulated lymph vessel. It was found that already in the first collection period of 20 min duration a considerable I¹³¹ activity was present in the lymph which consisted mainly of albumin-bound iodide. It is concluded that the low subcapsular pressure is probably valid for the entire renal interstitial compartment.     

Interdigitating reticulum cells in human renal grafts

Summary Seventeen human renal graft biopsies taken 1 h to 50 days after transplantation and 3 human renal non-graft biopsies (2 minimal change and 1 non-tumour portion of angiomyolipoma) were investigated with immunoelectron microscopy in order to identify interdigitating reticulum cells (IDC) or dendritic cells (DC) in renal tissues. The antibodies used consisted of a rabbit polyclonal antibody of antihuman S100 protein, mouse monoclonal antibodies of antihuman HLA-DR, anti-CD3, and anti-CD1a. IDC or DC were identified in 11 renal grafts. They were found both in the glomerular and interstitial (peritubular) capillary lumens but not in the interstitium of 1 case; both were present in the interstitial capillary lumens and interstitium of another case, and in the interstitium only of 9 cases. In the remaining 6 grafts and 3 non-grafts they were not detected. These 6 grafts and 3 non-grafts did not show any pathological change except for foot process fusion of the glomerular epithelia in 2 cases of minimal change. These findings suggest that IDC or DC are not normally present in human renal tissues. The presence of the cell in the glomerular and peritubular capillary lumens of a biopsy taken after 1 h and their presence in the interstitial capillary lumens of another graft biopsy, suggest that the IDC or DC in human renal grafts are derived from recipients, not donors, and that they migrate from the circulating blood toward the interstitium.     

Potassium kinetics in human muscle interstitium during repeated intense exercise in relation to fatigue

Accumulation of K⁺ in skeletal muscle interstitium during intense exercise has been suggested to cause fatigue in humans. The present study examined interstitial K⁺ kinetics and fatigue during repeated, intense, exhaustive exercise in human skeletal muscle. Ten subjects performed three repeated, intense (61.6±4.1 W; mean±SEM), one-legged knee extension exercise bouts (EX1, EX2 and EX3) to exhaustion separated by 10-min recovery periods. Interstitial [K⁺] ([K⁺]_interst) in the vastus lateralis muscle were determined using microdialysis. Time-to-fatigue decreased progressively (P<0.05) during the protocol (5.1±0.4, 4.2±0.3 and 3.2±0.2 min for EX1, EX2 and EX3 respectively). Prior to these bouts, [K⁺]_interst was 4.1±0.2, 4.8±0.2 and 5.2±0.2 mM, respectively. During the initial 1.5 min of exercise the accumulation rate of interstitial K⁺ was 85% greater (P<0.05) in EX1 than in EX3. At exhaustion [K⁺]_interst was 11.4±0.8 mM in EX1, which was not different from that in EX2 (10.4±0.8 mM), but higher (P<0.05) than in EX3 (9.1±0.3 mM). The study demonstrated that the rate of accumulation of K⁺ in the muscle interstitium declines during intense repetitive exercise. Furthermore, whilst [K⁺]_interst at exhaustion reached levels high enough to impair performance, the concentration decreased with repeated exercise, suggesting that accumulation of interstitial K⁺ per se does not cause fatigue when intense exercise is repeated.     

The polymerisation kinetics of lower dialkyl itaconates

The free radical polymerisation kinetics of diethyl- (DEI), dipropyl- (DnPI), dibutyl- (DnBI), and dihexyl itaconate (DHI) in the bulk were studied in the temperature range from 50 to 70°C. The concentration of the initiator, 2,2′-azoisobutyronitrile (AIBN), was varied between 0.02 and 0.085 mol/dm³. The rate of polymerisation (R_p), degree of polymerisation (DP), overall polymerization rate constant (K), the ratio of the propagation and termination rate constants (k_p/k_t^1/2), as well as the chain transfer constant to monomer (C_M) were determined. The values of R_p, K, and k_p/k_t^1/2 of the investigated monomers all increase with increasing size of the alkyl group in the ester substituent, whereas C_M decreases when going from the dimethyl to the dihexyl ester. The values of C_M are larger than the corresponding values for the alkyl esters of methacrylic acid.     

Effect of thickness and PEG addition on the hydrolytic degradation of PLLA

We previously demonstrated that cylindrical, biodegradable reservoirs fabricated with polylactide-polyethylene glycol (PLLA : PEG) films maintain constant permeability and enable zeroorder drug delivery for up to 6 weeks in vitro. This research proposes that PEG not only enhances permeability but also extends of life of the device by allowing the escape of soluble degraded monomers thereby minimizing autocatalysis of PLLA. To test this hypothesis, cylindrical PLLA films with varying PEG concentrations (0–30%, w/w) and film-thickness (0.05–0.18 mm) were fabricated, and their degradation rate and thermal properties monitored for 23 weeks in vitro. The decrease in PLLA molecular weight for all films followed bi-exponential kinetics that fit the equation:y_t = M(e^–K1t + e^{– K2t} ), as was determined by a Pearson's coefficient > 0.95 for all films. The constant M was empirically determined to be equal to have the initial molecular weight of the degrading polymer. The value of K ₁ was 5–60 orders of magnitude greater than K ₂ and was attributed to the autocatalytic degradation based on its dependence on PEG concentration, film thickness, and correlation with the enthalpy change associated with the glass transition (ΔC _p). K ₂ was attributed to simple hydrolytic cleavage of PLLA. The decrease in the value of K ₁ with PEG concentration and thickness, and the correlation of K ₁ with ΔC _p, confirmed that the PLLA degradation can be controlled by incorporating PEG, as well as by modifying thickness.     

Targeting mitochondrial mRNA translation to tackle obesity‐induced insulin resistance: thumbs up for exercise

The control of renal vascular tone is important for the regulation of salt and water balance, blood pressure and the protection against damaging elevated glomerular pressure. The K⁺ conductance is a major factor in the regulation of the membrane potential (V_m) in vascular smooth muscle (VSMC) and endothelial cells (EC). The vascular tone is controlled by V_m via its effect on the opening probability of voltage‐operated Ca²⁺ channels (VOCC) in VSMC. When K⁺ conductance increases V_m becomes more negative and vasodilation follows, while deactivation of K⁺ channels leads to depolarization and vasoconstriction. K⁺ channels in EC indirectly participate in the control of vascular tone by endothelium‐derived vasodilation. Therefore, by regulating the tone of renal resistance vessels, K⁺ channels have a potential role in the control of fluid homoeostasis and blood pressure as well as in the protection of the renal parenchyma. The main classes of K⁺ channels (calcium activated (K_Ca), inward rectifier (K_ir), voltage activated (K_v) and ATP sensitive (K_ATP)) have been found in the renal vessels. In this review, we summarize results available in the literature and our own studies in the field. We compare the ambiguous in vitro and in vivo results. We discuss the role of single types of K⁺ channels and the integrated function of several classes. We also deal with the possible role of renal vascular K⁺ channels in the pathophysiology of hypertension, diabetes mellitus and sepsis.     

Aristolochic acid nephropathy revisited: a place for innate and adaptive immunity?

Pozdzik A A, Berton A, Schmeiser H H, Missoum W, Decaestecker C, Salmon I J, Vanherweghem J‐L & Nortier J L
(2010) Histopathology 56 , 449–463 Aristolochic acid nephropathy revisited: a place for innate and adaptive immunity? Aims: The histological features of aristolochic acid nephropathy (AAN) consist of paucicellular interstitial fibrosis, severe tubular atrophy, and almost intact glomeruli with media lesions of interlobular arteries. As an early phase of interstitial inflammation preceded peritubular fibrosis in the rat model of AAN, the aim was to investigate the presence of inflammatory cells in human AAN. Methods and results: Reports of confirmed cases and case series of AAN were reviewed in terms of interstitial inflammation and found to have very conflicting results. This prompted us to search for and characterize inflammatory cells within the native kidneys provided from four end‐stage AAN patients. Prior aristolochic acid exposure was attested by the intrarenal presence of the typical aristolactam I‐derived DNA adduct. Besides the tubulointerstitial lesions usually seen in the cortex, a massive infiltration of macrophages, T and B lymphocytes was detected by immunohistochemistry in the medullary rays and in the outer medullae with some extension to the upper cortical labyrinth. Conclusions: In parallel with histological findings reported in the rat model, inflammatory cells are present preferentially in the interstitium of the medullary rays and of the outer medulllae in renal interstitium from human AAN cases, even in the terminal stages. Further studies must be undertaken to determine the respective roles of innate and adaptive immunity in the progression of AAN.     

Glomerular filtration and tubular absorption of the basic polypeptide aprotinin

The basic polypeptide aprotinin (Ap), mol. wt 6500, pi 10.5, is filtered in the glomeruli, virtually completely taken up by the proximal tubular cells and retained there for many hours. This process was studied in rats by determining the renal plasma clearance (C_Ap) as the amount of [¹²⁶I]Ap accumulated in the kidney plus that excreted in the urine per unit of time divided by the integrated plasma concentration. In periods lasting 4–20 min after i.v. bolus injection or infusion to constant plasma concentration, C_Ap was 65% of glomerular filtration rate (GFR) estimated as kidney plus urinary clearance of [⁵¹Cr]EDTA (C_Cr._EDTA). Less than 0.8% of the filtered Ap appeared in the urine. C_Ap varied inversely with plasma protein concentration in mgml“¹: C_Ap/C_CrEDTA= 0.98-0.0058 × P_Pr, corresponding to a glomerular Gibbs-Donnan distribution for a net molecular charge of + 6, in agreement with the amino acid composition of Ap. C_Ap (kidney + urinary) was not altered by inhibiting tubular uptake of [¹²⁵I]Ap by maleate or by exceeding the uptake capacity with large doses of unlabelled Ap. Neutralized Ap (malonylated) did not accumulate in the kidney, but showed a urinary clearance indistinguishable from that of [⁵¹Cr]EDTA. Both C_Ap and C₍._rEDTA were reduced to 0.04 ml min^-1 when glomerular filtration pressure was lowered by ureteral stasis and increased P_Pr (80–90 mg ml”¹). These findings indicate: (1) no steric or charge restriction to filtration of Ap in the glomerular membrane, (2) the Gibbs-Donnan equilibrium should be considered when estimating glomerular sieving of charged polypeptides in intact animals (3) charge dependent tubular uptake, (4) little or no transtubular transport of intact Ap, (5) no appreciable tubular uptake of Ap from the peritubular side and (6) local renal accumulation of Ap in a period of up to 20 min may be used to estimate local glomerular filtration and/or local proximal tubular reabsorption rates. Model analysis based on the appearance of ¹²⁵I in plasma, the time course of renal Ap content, and literature data on subcellular Ap distribution are consistent with two populations of endosomes, transporting Ap at widely different rates from the proximal tubular brush border to the lysosomes where breakdown occurs at a high rate.     

Potassium-transporting proteins in skeletal muscle: cellular location and fibre-type differences

Potassium (K⁺) displacement in skeletal muscle may be an important factor in the development of muscle fatigue during intense exercise. It has been shown in vitro that an increase in the extracellular K⁺ concentration ([K⁺]_e) to values higher than approx. 10 mm significantly reduce force development in unfatigued skeletal muscle. Several in vivo studies have shown that [K⁺]_e increases progressively with increasing work intensity, reaching values higher than 10 mm . This increase in [K⁺]_e is expected to be even higher in the transverse (T)-tubules than the concentration reached in the interstitium. Besides the voltage-sensitive K⁺ (K_v) channels that generate the action potential (AP) it is suggested that the big-conductance Ca²⁺-dependent K⁺ (K_Ca1.1) channel contributes significantly to the K⁺ release into the T-tubules. Also the ATP-dependent K⁺ (K_ATP) channel participates, but is suggested primarily to participate in K⁺ release to the interstitium. Because there is restricted diffusion of K⁺ to the interstitium, K⁺ released to the T-tubules during AP propagation will be removed primarily by reuptake mediated by transport proteins located in the T-tubule membrane. The most important protein that mediates K⁺ reuptake in the T-tubules is the Na⁺,K⁺-ATPase α₂ dimers, but a significant contribution of the strong inward rectifier K⁺ (Kir2.1) channel is also suggested. The Na⁺, K⁺, 2Cl⁻ 1 (NKCC1) cotransporter also participates in K⁺ reuptake but probably mainly from the interstitium. The relative content of the different K⁺-transporting proteins differs in oxidative and glycolytic muscles, and might explain the different [K⁺]_e tolerance observed.     

Influence of potassium depletion on potassium conductance in proximal tubules of frog kidney

In order to test for the contribution of intracellular potassium activity to the link of sodium/potassium-ATPase activity and potassium conductance, studies with conventional and potassium selective microelectrodes were performed on proximal tubules of the isolated perfused frog kidney. The peritubular transference number for potassium (t _k), i.e., the contribution of peritubular slope potassium conductance to the slope conductance of the cell membranes (luminal and peritubular), was estimated from the influence of peritubular potassium concentration on the potential difference across the peritubular cell membrane (PD _pt). During control conditions,PD _pt is –65±1 mV, intracellular potassium activity (K _i) 57±2 mmol/l andt _k 0.41±0.05. The resistance in parallel of the luminal and peritubular cell membranes (R _m) is 44±4 kcm, the resistance of the cellular cable (R _c) 137±13 M/cm. When the cells are exposed 10 min to potassium free perfusates (series I),PD _pt increases by –28±3 mV within 2 min and then decreases gradually to approach the control value within 10 min.K _i decreases by 22±3 mmol/l andR _c increases by 35±10%. After a transient decrease,R _m increases by 36±9%. Readdition of peritubular potassium leads to a transient increase ofPD _pt, a gradual decrease ofR _m andR _c as well as a gradual increase ofK _i t _k recovers only slowly to approach 65±8% of control value within 3 and 79±10% within 6 min. When the cells are exposed 10 min to potassium free perfusates containing 1 mmol/l barium (series II),PD _pt depolarizes by +28±4 mV andK _i decreases by 7±1 mmol/l within 10 min. Within 2 min of reexposure to control perfusatesPD _pt approaches the control value.t _k recovers significantly faster than in series I and approaches 92±8% of control value within 3 min and 107±8% within 6 min reexposure to control perfusates. In conclusion, the effect of potassium free perfusates on peritubular potassium conductance depends on the degree of potassium depletion of the cell.     

Role of K+ channels in the regulation of electrical spontaneous activity of the mouse small intestine

The roles of K⁺ channels in the regulation of slow waves and pacemaker potentials recorded from mouse small intestine were investigated using intracellular recording techniques in the presence of nifedipine. Iberiotoxin (0.1 μM) and charybdotoxin (0.1 μM) had no effect on the generation of slow waves recorded from circular smooth muscle cells. Apamin (0.3 μM) depolarized the membrane and decreased the amplitude of early, rapid repolarization of slow waves, without altering the amplitude, frequency, duration, or maximum rate of rise of the initial upstroke phase (dV/dt _max). The early, rapid repolarization was enhanced by phenylephrine (15 μM). 4-Aminopyridine (4-AP, 5 mM) depolarized the membrane and increased the amplitude and dV/dt _max of slow waves. Both apamin and 4-AP depolarized the membrane and decreased the amplitude and dV/dt _max of pacemaker potentials recorded from interstitial cells of Cajal distributed in the myenteric region (ICC-MY). Membrane depolarization with a high-K⁺ solution decreased the amplitude and dV/dt _max of slow waves. These results suggest that apamin-sensitive K⁺ conductance and 4-AP-sensitive K⁺ conductance may contribute to the resting membrane potential of circular smooth muscle cells. The early, rapid repolarization of slow waves appears to result from the opening of apamin-sensitive K⁺ conductance. 4-AP-sensitive K⁺ conductance is likely to be activated in the initial upstroke component (primary component) of slow waves. In ICC-MY, membrane depolarization induced by apamin or 4-AP may result from electrotonic spread from smooth muscle cells.     

Influence of cation markers on sulfate distribution in mouse gastrocnemius muscle

Further studies are reported which examines the influence of fixed negative sites on anion distribution in the interstitium of mouse gastrocnemius muscle. For this purpose, the³⁵S-sulfate distribution in in vitro muscles was examined in the absence and presence of a number of cations which are thought to bind to the interstitial fixed negative sites. Each of the cation treated muscles gave sulfate space measurements which were statistically greater than the control muscles. Ferritin, spermidine and ruthenium red treatment resulted in muscle sulfate spaces similar to the true morphometric ECV. This study suggests that the fixed negative sites in the interstitium of mouse gastrocnemius muscle exists in sufficient density to influence the distribution of anions in these tissue.     

On the kinetics of the polymerization of some ammonium methacrylates with different alkyl chain lengths in aqueous solution

Methacryloyloxyalkyltrimethylammonium chlorides ( 1a–c ) with different alkyl chain lengths were synthesized and polymerized radically with 4,4′-azobis(4-cyanovalerianic acid) and K₂S₂O₈ as initiators. With K₂S₂O₈ as initiator, reaction orders of 0,5 and 1 with respect to initiator and monomer, were found. For the ionic monomers with longer alkyl chains the ratio of rate constants k_p(2fk_d/k_t)^0,5 was determined over a wide concentration range. It was found that with decreasing monomer concentration the ratio of rate constants increases, which is caused by a diminished termination rate because of an increased electrostatic repulsion of the ionic polymer radicals in dilute solution (increase of dissociation). Methacryloyloxyethyltrimethylammonium chloride ( 1a ) doesn't show this effect. This can be explained by the diminished effective charge of the ammonium group in the preferred gauche conformation of the molecule.     

Axial heterogeneity of sodium-bicarbonate cotransport in proximal straight tubule of rabbit kidney

Intracellular microelectrodes were used to investigate rheogenic Na⁺(HCO ₃ ^– ) _n cotransport in different segments of isolated proximal straight tubule (PST) of rabbit kidney. In the first portion (S2 segment) the peritubular cell membrane potentialV _b averaged –46.0, SE±1.3 mV (n=20), while in the terminal portion (S3 segment) it averaged –68.3, SE±2.5 mV (n=10). This difference may reflect different modes of anion permeation across the peritubular cell membrane. In S2 segments, sudden 101 reduction of bath HCO ₃ ^– concentration caused a fast transient cell depolarization, V _b=–45.8, SE±1.2 mV (n=33) as expected from the presence of Na⁺(HCO ₃ ^– ) _n contransport. As the puncture site moved further distally, V _b declined and gradually changed its time course by superposition of a slower secondary depolarization. In this region the transient cell depolarization could be recuperated by inhibiting the peritubular K⁺ conductance with Ba²⁺ (1 mmol/l). In S3 segments, however, the HCO ₃ ^– -dependent transient cell depolarization was completely lost both in the absence and presence of Ba²⁺. In addition, sudden reduction of bath Na⁺ concentration did not acidify the cell, as it did in the S2 segment. The data indicate that the expression of Na⁺(HCO ₃ ^– ) _n cotransport in the peritubular cell membrane gradually diminishes towards the end of the S2 segment and is lost in the S3 segment.     

Computational Model of Interstitial Transport in the Spinal Cord using Diffusion Tensor Imaging

Local drug delivery methods, including convection-enhanced delivery (CED), are being used to increase distribution in selected regions of nervous tissue. There is a need for 3D models that predict spatial drug distribution within these tissues. A methodology was developed to process magnetic resonance microscopy (MRM) and diffusion tensor imaging (DTI) scans, segment gray and white matter regions, assign tissue transport properties, and model the interstitial transport of macromolecules. Fiber tract orientation was derived from DTI data and used to assign directional dependence of hydraulic conductivity, K, and tracer diffusivity, D _t , transport tensors. Porous media solutions for interstitial fluid pressure, velocity, and albumin distribution were solved using a finite volume method. To test this DTI-based methodology, a rat spinal cord transport model was developed to simulate CED into the dorsal white matter column. Predicted distribution results correspond well with small volume (∼1 μl) trends found experimentally, although albumin loss was greater at larger infusion volumes (>2 μl). Simulations were similar to those using fixed transport properties due to the bulk alignment of white matter fibers along the cord axis. These findings help to validate the DTI-based methodology which can be applied to modeling regions where fiber tract organization is more complex, e.g., the brain.     

Interstitial distribution of charged macromolecules in the dog lung: A kinetic model

A mathematic model was constructed to investigate conflicting physiologic data concerning the charge effect of continuous capillaries to macromolecules in the lung. We simulated the equilibration kinetics of lactate dehydrogenase (MR 4.2 nM) isozymes LDH 1 (pI=5.0) and LDH 5 (pI=7.9) between plasma and lymph using previously measured permeability coefficients, lung tissue distribution volumes (V_A) and plasma concentrations (C_P) in lung tissue. Our hypothesis is that the fixed anionic charges in interstitium, basement membrane, and cell surfaces determine equilibration rather than charged membrane effects at the capillary barrier, so the same capillary permeability coefficients were used for both isozymes. Capillary filtration rates and protein fluxes were calculated using conventional flux equations. Initial conditions at baseline and increased left atrial pressures (P_LA) were those measured in animal studies. Simulated equilibration of isozymes over 30 h in the model at baseline capillary pressures accurately predicted the observed differences in lymph/plasma concentration ratios (C_L/C_P) between isotopes at 4 h and eequilibration of these ratios at 24 h. Quantitative prediction of isozyme C_L/C_P ratios was also obtained at increased P_LA. However, an additional cation selective compartment representing the surface glycocalyx was required to accurately simulate the initial higher transcapillary clearances of cationic LDH 5. Thus experimental data supporting the negative barrier, positive barrier, and no charge barrier hypotheses were accurately reproduced by the model using only the observed differences in interstitial partitioning of isozymes without differences in capillary selectivity.     

Base induced hyperpolarization of the cell potential in HCO 3 − free perfused necturus renal proximal tubules

Short-term peritubular alkalinization from 7.5 to 8.5 hyperpolarized (–8.8 mV) the basolateral membrane potential (V ₁) in HCO ₃ ^– free Hepes buffered Necturus renal proximal tubule cells. This sustained base induced hyperpolarization (BIH) was associated with an increase in the peritubular apparent transference number for potassium ( ). The apparent transference number for potassium ( ) was estimated at pH 7.5 and 8.5 by raising peritubular K⁺ from 2.5 to 10 mmol/l. increased linearly asV ₁ hyperpolarized, whereas measured in the presence of peritubular Ba²⁺ at pH 7.5 and 8.5 was nearly zero. However, the BIH persisted in the presence of barium at the peritubular, luminal or both sides of the epithelium. Moreover this BIH was also accompanied by a small hyperpolarization (–0.4 mV) of the transepithelial membrane potential (V ₃) in the absence or presence of peritubular and/or luminal Ba²⁺. Therefore we conclude that BIH must originate from additional mechanisms other than an increase in peritubular or luminal potassium conductance.     

Substrate specificity of the luminal Na+-dependent sulphate transport system in the proximal renal tubule as compared to the contraluminal sulphate exchange system

The efflux of [³⁵S]sulphate from the lumen of the proximal renal tubule into tubular cells of rats was measured by the stop-flow tubular-lumen microperfusion technique. The transport parameters obtained and the apparent K _i values of competing substrates were compared with those of the contraluminal influx of [³⁵-S]sulphate from the interstitium into tubular cells. For the luminal sulphate efflux a K _m(l, SO ₄ ^2– ) of 0.8 mmol/l and a J _max(l, SO ₄ ^2– ) of 0.2 pmol s^–1 cm^–1 were found. The corresponding contraluminal values were K _m(cl,SO ₄ ^2– ) 1.4 mmol/l and J _max(cl, SO ₄ ^2– ) 1.2 pmol s^–1 cm^–1. Omission of Na⁺ from the perfusates reduced the luminal efflux of sulphate by 83%, while the contraluminal influx of sulphate was not changed. Increase in HCO ₃ ^– concentration inhibited both luminal efflux and contraluminal influx of sulphate, while a change of pH from 6.0 to 8.0 was without effect. Comparing the apparent K _i(SO ₄ ^2– ) values for luminal and contraluminal sulphate transport, a relationship close to 11 was seen for some inorganic substrates with tetrahedral molecular structure (thio-sulphate, sulphate, molybdate and selenate). The same holds for phosphate, while for oxalate the contraluminal K _i(SO ₄ ^2– ) value was lower than the luminal one (1.2 and 4.5 mmol/l). Some of the dicarboxylates and disulphonates tested show the same affinity to the luminal Na⁺-dependent sulphate transporter and the contraluminal sulphate exchange system, whereas most of the benzene carboxylate and benzenesulphonate derivatives tested exhibit higher luminal than contraluminal k _i values. The inhibitory potency increased with rising numbers of substituents on the benzene ring. This effect was more pronounced for the contraluminal sulphate transporter. In general, only disulphonates and analogues as well as similarly structured compounds (5-sulphosalicylate, 2-hydroxy-5-nitrobenzenesulphonate, eosine-5-isothiocyanate) have a good inhibitory potency toward the luminal sulphate transporter [apparent K _i 0.9–3.1 mmol/l]. All the tested sulphamoyl and phenoxy diuretics, and fluorescein and phenolphthalein dyes showed no or a smaller inhibitory potency to the luminal sulphate transport system than to the contraluminal. The most effective inhibitors of both sulphate transport systems are 8-anilino-1-naphthalenesulphonate, orange G, and H₂-DIDS. The data indicate that the Na⁺-dependent luminal and the Na⁺-independent contraluminal sulphate transport systems accommodate a similar spectrum of anionic substrates, whereby the inhibitory potency against the luminal Na⁺-dependent sulphate transport system is identical or smaller than against the contraluminal transporter.     

Electrochemical potentials of potassium in proximal renal tubule of rat

Summary By means of double-barreled K⁺ selective liquid ion-exchange microelectrodes, the electrical potential differences across individual cell membranes were determined simultaneously with the K⁺ concentration in single cellular elements of the proximal tubular epithelium of the rat. Proximal tubular fluid [K⁺] and plasma [K⁺] were also determined electrometrically. Thin cortical slices of the rat kidney analyzed by flame photometry yielded a mean [K⁺] of 136.3±4.2 mM per kg cell water. This electrometric study yielded a mean intracellular [K⁺] of 54.4±2.5 mM, a value which is about 1/3 of the total K⁺ content of proximal tubule cells. The electrometric mean proximal tubule fluid (second half) [K⁺] was 3.7±0.1 mM while plasma [K⁺] was 4.3±0.1 mM, yielding a fluid/plasma concentration ratio of 0.85±0.02. The calculated K⁺ equilibrium potentials (E _K )across the two individual from their respective measured membrane electrical PDs. This signifies that K⁺ exhibits an electrochemical equilibrium distribution across the luminal and peritubular cell boundaries of the proximal tubular epithelium. Thus it is no longer necessary to postulate the presence of an active K⁺ pump in either the luminal or peritubular cell membranes.