Fluid absorption by proximal tubules in the absence of a colloid osmotic gradient

Proximal nephron

The kidney plays a fundamental role in maintaining body salt and fluid balance and blood pressure homeostasis through the actions of its proximal and distal tubular segments of nephrons. However, proximal tubules are well recognized to exert a more prominent role than distal counterparts. Proximal tubules are responsible for reabsorbing approximately 65% of filtered load and most, if not all, of filtered amino acids, glucose, solutes, and low molecular weight proteins. Proximal tubules also play a key role in regulating acid-base balance by reabsorbing approximately 80% of filtered bicarbonate. The purpose of this review article is to provide a comprehensive overview of new insights and perspectives into current understanding of proximal tubules of nephrons, with an emphasis on the ultrastructure, molecular biology, cellular and integrative physiology, and the underlying signaling transduction mechanisms. The review is divided into three closely related sections. The first section focuses on the classification of nephrons and recent perspectives on the potential role of nephron numbers in human health and diseases. The second section reviews recent research on the structural and biochemical basis of proximal tubular function. The final section provides a comprehensive overview of new insights and perspectives in the physiological regulation of proximal tubular transport by vasoactive hormones. In the latter section, attention is particularly paid to new insights and perspectives learnt from recent cloning of transporters, development of transgenic animals with knockout or knockin of a particular gene of interest, and mapping of signaling pathways using microarrays and/or physiological proteomic approaches.

Electron microprobe analysis of proximal tubule cellular Na, Cl and K element concentrations during acute mannitol-saline volume expansion in rats: evidence for inhibition of the Na pump

It has previously been shown that during mannitol-saline volume expansion (VE) Na transport was inhibited 50% by harvested proximal tubular fluid without a change in paracellular shunt pathway permeability to Na. To determine whether this inhibition was due to changes in cellular entry step or an effect on the pump itself, intracellular element concentrations were measured by electron microprobe X-ray ranalysis in proximal tubular cells of control (non-expanded, NE) and VE rats. Nai, Cli and phosphorusi were increased (mean +/- S.E.) from 19.3 +/- 0.8 to 23.4 +/- 0.6, 15.8 +/- 0.4 to 21.3 +/- 0.4 and 124.3 +/- 2.6 to 138.0 +/- 1.8 mmol . kg-1 wet weight (P less than 0.001) respectively while Ki remained unchanged: 122.9 +/- 2.2 and 124.2 +/- 1.3 mmol . kg-1 wet weight. The increases in Nai and Cli were in excess of cell shrinkage produced by the hyperosmolal peritubular environment while the unchanged Ki in the face of cell shrinkage indicates and actual loss. It is concluded that mannitol-saline VE inhibits the Na pump producing a rise in Nai and a fall in Ki.

The effect of peritubular protein upon fluid reabsorption in rabbit proximal convoluted tubules perfused in vitro

This paper describes an investigation of the effects of varying the peritubular protein concentration upon the rate of fluid reabsorption of proximal convoluted tubule segments of the rabbit kidney, isolated and perfused in vitro. Eleven moderately distended tubules, bathed with rabbit serum (60 g l.-1 protein) and perfused with an ultrafiltrate of serum (ca. 0 g l.-1) protein, reabsorbed fluid at a rate of 1.0 +/- 0.07 nl. mm-1 min-1. When the protein of the bathing solution was reduced by replacing the serum with ultrafiltrate there was little change in fluid reabsorption rate. A further eleven moderately distended tubules, bathed with physiological saline containing bovine serum albumin (60 g l.-1) and perfused with saline, reabsorbed fluid at 1.0 +/- 0.06 nl. mm-1 min-1. There was little change in fluid reabsorption when the protein concentration was reduced to either 20 or 0 g l.-1; these tubules responded to 10(-5) M-ouabain in the bath, or a temperature of 25 degrees C, by reducing their absorption rate to 0.13 +/- 0.05 nl. mm-1 min-1. Nine minimally distended tubules reabsorbed fluid at 0.8 nl. mm-1 min-1 when bathed with physiological saline containing bovine serum albumin (60 g l.-1) and perfused with saline. Under these circumstances reabsorption rate fell by 26% when protein was removed from the bath. A study of the pressure-diameter relationship was made for eight tubules typical of those used in this laboratory. The probable range of lumen hydrostatic pressures was discussed for the distended and undistended states. We conclude that the effect of peritubular protein concentration upon fluid reabsorption in proximal convoluted tubule segments perfused in vitro is dependent upon some property of the tubule wall that is changed when distension occurs.

Acute renal failure--an integrative discussion of morphologic and functional findings

The ultrastructural alterations at the nephron established in animal experiments, were also confirmed, by means of an electron-microscopic examination, in eight cases of human acute renal failure (ARF). Special consideration was given in this study to single cell alterations, particularly in proximal tubular cells, with emphasis being placed on alterations due to single cell damage in the region of the renal fluid compartments. The ultrastructural alterations of the tubular cells in ARF, suggest serious impairment of the cellular capacity for electrolyte transport and metabolic processes. The shunt paths between the tubular fluid compartment and the functional interstitium, arising from necrosis of the tubular cells or dissolution of the gap or tight junctions, were discussed in terms of their significance for the directional, active transport processes of the tubular cells for sodium chloride and the passive water flow. The morphologic findings were reviewed in light of recent findings on cellular membrane processes and electrolyte transport. A reinterpretation of the morphologic and functional findings in ARF is suggested. This takes into consideration single cell function and the integrity of the renal fluid compartments.

Techniques and applications of extracellular space determination in mammalian tissues

This review summarizes the ways in which the extracellular space (ECS) may be estimated in mammalian tissues, and briefly describes some of the uses to which the EC confinement of certain molecules (markers or tracers) may be put in the elucidation of physiological functions. The introductory section is followed by a description of the more commonly used marker molecules and their functional characteristics, and of factors likely to lead to the spurious over- or under-estimation of the ECS. Certain alternative methods are also described, in particular those based on morphological and electrical criteria which seek to demonstrate small, functionally important, changes in the size of specialized regions of the ECS (e.g. lateral cellular interspaces) without necessarily being required to provide a quantitatively precise estimate of their size. Section III describes the results of measurements of ECS in various mammalian tissues (muscle, gastro-intestinal tract, nervous tissue, crystalline lens, placenta, lung and kidney) and some applications of EC markers to investigation of cellular function (e.g. uptake of metabolic substrates and epithelial transport) and, in outline, characterization of capillary permeability. The available literature in this field is very extensive, and in the interests of brevity the reader is, where appropriate, referred to previous reviews covering specialized aspects of ECS determination and related topics.

Mechanisms of fluid absorption during proximal tubule development

Passive permeation characteristics and paracellular pathway ultrastructure were studied in vitro by perfusion of rabbit isolated proximal convoluted tubules during postnatal ontogenesis. The influence upon net volume flow (Jv) of transepithelial hydrostatic and protein osmotic pressure was significantly higher during early postnatal maturation than in the mature tubule. Hydraulic hydrostatic conductance (X10(-3) ml - cm-2 - min-1 - cm H2O-1) was 0.0367 +/- 0.0048 during early postnatal maturation (N = 99) and 0.0052 +/- 0.002 at maturity (N = 78). Hyperoncotic serum (12.7 +/- 0.4 g/100 ml) in the bath increased Jv by 67.5 +/- 21.1% from 0.31 +/- 0.06 to 0.52 +/- 0.08 nl-mm-1-min-1 in the neonatal proximal tubule, whereas an increase of only 25.7 +/- 20.4% from 1.08 +/- 0.15 to 1.32 +/- 0.18 nl-mm-1-min-1 was noted in the mature proximal tubule during this elevated bath protein-osmotic pressure. Electron microscopic observations showed that microperoxidase passed from tubule lumen through the basement membrane via intercellular spaces in immature tubules perfused at an increased transtubular pressure gradient. This suggests that a transepithelial shunt pathway may participate in changes of conductance during ontogenesis, although length and ultrastructural configuration of tight junctions did not vary with these variables.

CONCLUSIONS

1) Hydrostatic and oncotic water conductance of the rabbit proximal tubule changes with postnatal development. 2) Ultrastructural tracer studies suggest that the change in conductance is due to alteration of the paracellular pathway. 3) Isotonic absorption of the neonatal proximal tubule may depend more on transepithelial pressure gradients than in the mature tubule.

The measurement of nephron filtration rate and absolute reabsorption in the proximal tubule of the rabbit kidney

Micropuncture studies were performed in the rabbit to determine nephron filtration rate and absolute fluid reabsorption in the proximal tubule in order to compare the latter value with data obtained with the in vitro microperfusion technique. New Zealand white rabbits, 2-2.8 kg, were studied. Nephron filtration rate was 21 nl/min (n equal to 48) and absolute reabsorption along the length of the accesible portion of the proximal convoluted tubule was 10.3 nl/min. Correcting this value for tubular length gives a fluid reabsorption of approximately 1.9 nl/mm per min. In view of the marked difference between the in vivo and in vitro techniques and the various sources of error with each, this is reasonably similiar to the value of 1.3 nl/mm per min obtained in the isolated proximal convoluted tubule.

Influence of peritubular hydrostatic and oncotic pressures on fluid reabsorption in proximal tubules of the rat kidney

Proximal tubular reabsorption was measured with the split droplet technique under the following conditions. Group 1. Star vessel perfusions with ultrafiltrate in the presence of human serum albumin (HSA) and with ultrafiltrate alone. Group 2. Capillary perfusions, under locally high flow rates, with ultrafiltrate in the absence or presence of HSA. Group 3. Subcapsular perfusions with ultrafiltrate of high HSA concentrations but under low hydrostatic pressure and ultrafiltrate of low HSA concentrations but under elevated hydrostatic pressure. In Group 1, the presence of HSA increased the reabsorptive rate, whereas ultrafiltrate alone had no provable effect on the reabsorptive rate. In Group 2, a flow dependent decrease of the reabsorptive rate, that was not influenced by the presence of HSA, was observed. Finally, in Group 3, high concentrations of HSA did not alter the reabsorptive rate, whereas elevation of the hydrostatic pressure decreased the reabsorptive rate. The results cannot be explained on the basis of a simple passive mode of action of colloid osmotic and hydrostatic pressure on the reabsorptive rate. The possibility of a direct tubulae capillary transport route is discussed.

Diuretics: mechanism of action and clinical application

Despite the bewildering number of diuretics available to the physician, these drugs can be divided into 4 main groups, characterised by their site of action on sodium reabsorption in the kidney. Drugs acting on the ascending limb of the loop of Henle have a powerful but short acting diuretic effect; they include frusemide, ethacrynic acid and bumetanide. The benzothiadiazines and related compounds have a moderate diuretic action spread over a longer period, whilst the potassium-sparing diuretics, triamterene, amiloride and spironolactone, have only a weak diuretic effect but a marked ability to diminish urinary potassium excretion. The fourth group is made up of miscellaneous substances which function as vasodilator or osmotic agents. The pathogenesis of oedema formation in heart failure is outlined and a logical approach to treatment suggested. Duiretics are being increasingly used in the treatment of non-oedematous states, in particular hypertension, diabetes insipidus and hypercalciuria; their exact role in pregnancy and acute renal failure remains controversial. Side-effects can be related to their effect on electrolyte excretion and include hypokalaemia, hyponatraemia, hyperkalaemia and hyperuricaemia. The incidence of disturbed carbohydrate tolerance in previously normal individuals is low. Other less common side-effects are also discussed.

Pressure control of sodium reabsorption and intercellular backflux across proximal kidney tubule

The magnitude of changes in luminal hydrostatic pressure (DeltaP(L)), peritubular capillary hydrostatic pressure (DeltaP(PT)), and peritubular capillary colloid osmotic pressure (Deltapi) was determined in the Necturus kidney during volume expansion (VE). The specific effects of separate changes of each pressure parameter on proximal net sodium transport (J(Na)) were studied in isolated perfused kidneys. The combined effect of DeltaP(L), DeltaP(PT), and Deltapi, of a magnitude similar to that induced by volume expansion, decreases J(Na) by 26% in the perfused kidney. A major portion of the natriuresis in VE is due to changes in intrarenal pressures. The effect of Deltapi on the permeability characteristics of Necturus proximal tubule was studied. With increasing Deltapi, the ionic conductance of the paracellular shunt pathway decreased, since transepithelial input and specific resistance rose significantly, whereas cellular membrane resistance remained unchanged. Transepithelial permeability coefficients for sodium chloride and raffinose changed inversely proportional to transepithelial resistance, indicating an alteration of a paracellular permeation route. Net passive sodium backflux and active transport flux components were calculated. Increased net sodium transport with rising Deltapi is accompanied by a significant drop in passive back diffusion, without an increment in the active flux component. Change in passive sodium ion back diffusion thus appears to be a key physiological factor in the control of transepithelial sodium transport.