Einflu� von Adrenalektomie und Glucocorticoiden auf die Wasserpermeabilit�t corticaler Nephronabschnitte der Rattenniere

Regulation of collecting duct AQP3 expression: response to mineralocorticoid

Adrenocortical steroid hormones are importantly involved in the regulation of extracellular fluid volume. The present study was aimed at examining whether aldosterone and/or glucocorticoid regulates the abundance of aquaporin-3 (AQP3), -2, and -1 in rat kidney. In protocol 1, rats were adrenalectomized, followed by aldosterone replacement, dexamethasone replacement, or combined aldosterone and dexamethasone replacement (rats had free access to water but received a fixed amount of food). Protocol 2 was identical to protocol 1, except that all groups received fixed daily food and water intake. In both protocols 1 and 2, aldosterone deficiency was associated with increased fractional Na excretion and severe hyperkalemia. Semiquantitative immunoblotting revealed that aldosterone deficiency was associated with a dramatic downregulation of AQP3 abundance. Consistent with this, immunocytochemistry and immunoelectron microscopy revealed a marked decrease in AQP3 labeling in the basolateral plasma membranes of collecting duct principal cells. In contrast, AQP1 and AQP2 abundance and distribution were unchanged. Glucocorticoid deficiency revealed no changes in AQP3, -2, or -1 abundance. In protocol 3, Na restriction (to increase endogenous aldosterone levels) or exogenous aldosterone infusion in either normal rats or vasopressin-deficient Brattleboro rats was associated with a major increase in AQP3 abundance. In protocol 4, aldosterone levels were clamped by infusion of aldosterone, while Na intake was altered from a low to a high level. Under these circumstances, there were no changes in AQP3 or AQP2 abundance, although the level of the thiazide-sensitive Na-Cl cotransporter was decreased. In conclusion, the results uniformly demonstrate that aldosterone regulates AQP3 abundance independently of Na intake. In contrast, changes in glucocorticoid levels in these models do not influence AQP3 or AQP2 abundance. Therefore, in the collecting duct aldosterone may regulate, at least in part, AQP3 expression in addition to regulating Na and K transport.

A numerical model of the renal distal tubule

A numerical model of the rat distal tubule was developed to simulate water and solute transport in this nephron segment. This model incorporates the following: 1) Na-Cl cotransporter, K-Cl cotransporter, Na channel, K channel, and Cl channel in the luminal membrane; 2) Na-K-ATPase, K channel, and Cl channel in the basolateral membrane; and 3) conductances for Na, K, and Cl in the paracellular pathway. Transport rates were calculated using kinetic equations. Axial heterogeneity was represented by partitioning the model into two subsegments with different sets of model parameters. Model equations derived from the principles of mass conservation and electrical neutrality were solved numerically. Values of the model parameters were adjusted to minimize a penalty function that was devised to quantify the difference between model predictions and experimental results. The developed model could simulate the water and solute transport of the distal tubule in the normal state, as well as in conditions including thiazide or amiloride application and various levels of sodium load and tubular flow rate.

Aldosterone metabolism in rat renal tissue in vitro. Formation of lipid soluble metabolites

In the present study the formation of lipid soluble metabolites from 3H-aldosterone was investigated in vitro in isolated kidneys and kidney and liver slices of Sprague Dawley rats. The steroids were separated by HPLC (forward and reversed phase systems) and detected on-line as UV- or 3H-chromatograms. Apart from an unenzymatically formed substance, isoaldosterone, three less polar metabolites were traced (A1, A2, A3). The structure of the quantitatively most important metabolite (A1), was identified as 5 alpha-dihydroaldosterone using a combination of techniques such as chromatographic comparison with reference steroids, antibody binding and mass spectrometry. Evidence for further conversion of DHaldo to 3 alpha, 5 alpha-tetrahydroaldosterone was obtained in chromatographic and antibody binding studies. The formation of metabolites was not dependent on glomerular filtration. Furthermore it displayed regional heterogeneity with highest activity in the outer medulla. Finally it was observed that the in vitro metabolism of aldosterone was not saturable over a range of initial aldo concentration of 10(-9) to 10(-5) M.

Changes in sodium and potassium excretion and urinary volume in rats submitted to hypervitaminosis A

Hypervitaminosis A induces the following changes in rat kidney: decrease in sodium and potassium excretion and an increase of urinary volume. In order to determine the possible reversibility of these alterations, the authors allowed the hypervitaminotic animal to recover for 30 and 60 days. After a 30-day recovery period for urinary volume, a 20-day recovery period for sodium excretion, and a 10-day recovery period for potassium excretion, the alterations were reverted to normal.

The effects of dexamethasone administration and withdrawal on water permeability across the blood-brain barrier

We examined the effects of dexamethasone administration and withdrawal on regional cerebral blood flow (CBF) and the permeability surface area (PS) product for water in the central nervous system of conscious rats. There were no significant changes in CBF. Dexamethasone treatment produced a significant decrease in the PS product for water in the cerebral cortex, while withdrawal of dexamethasone resulted in a significant increase. Water content of the cerebral cortex was also increased in rats from which dexamethasone had been withdrawn. These findings may help in understanding the pathogenesis of brain swelling in some patients.

Detection and comparative evaluation of aldosterone antagonists in glucocorticoid-treated, adrenalectomized rats

An improved method for the evaluation of aldosterone antagonists in adrenalectomized, glucocorticoid-treated rats is described. The method involved assessing the pharmacological effects of spironolactone and potassium canrenoate and comparing them with the action of prorenone and potassium prorenoate, respectively. Adrenalectomized rats were pretreated with fluocortolone caproate (10 mg/kg s.c.), a long-acting glucocorticoid, immediately after surgery. Fluocortolone (1.25 mg/kg s.c.), a short acting preparation, was administered 4 days after this treatment. On the 5th day after adrenalectomy, the actual diuresis experiment was performed. The rats received a continuous i.v. infusion of aldosterone [1 microgram/(kg x h)] for 10, 15 or 20 h. Spirnolactone or prorenone (6.7, 13.4 or 26.8 mg/kg of each steroid) were administered in single oral doses 1 h before or 4 h after the start of the i.v. infusion. Potassium canrenoate and potassium prorenoate (1.9, 3.8, or 6.7 mg/(kg x h) of each compound) were infused intravenously over 10 or 15 h. Urine was collected in 1 h fractions and the anti-aldosterone activity was assessed by the ability of the compounds to reverse the aldosterone effect on the Na/K ratio. The anti-aldosterone activity of the steroids studies was clearly detectable with the method described. Prorenone was as potent as spironolactone and potassium prorenoate was on the average 3.9 times as potent as potassium canrenoate. The method appears suitable for the characterization of the time course and duration of anti-aldosterone activity and for the calculation of relative potencies in comparison to standard compounds.

Phosphaturic response of hydrocortisone in the presence and the absence of parathyroid hormone

The effect of hydrocortisone (HC) on the renal Pi transport in the presence and in the absence of parathyroid hormone (PTH) in the rat was examined using clearance techniques. During constant infusion of PTH in parathyroidectomized (PTX) rats, HC decreased the reabsorption of Pi from 2.84 to 2.33 mumol/min (P less than 0.001), the TmPi from 4.55 to 3.83 mumol/min (P less than 0.001), the reabs. Pi/GFR values from 0.93 to 0.74 (P less than 0.001) and the TmPi/GFR from 1.73 to 1.42 mumol/ml (P less than 0.001). In the absence of PTH, HC diminished the TmPi from 8.36 to 6.58 mumol/min (P less than 0.005) and the TmPi/GFR from 3.36 to 2.51 mumol/ml (P less than 0.001). It is concluded that the phosphaturic response of hydrocortisone may be important in the homeostasis of inorganic phosphate in the body.

Influence of aldosterone on epithelial leakiness of rat proximal tubules. Microconductivity Measurements*

Oil blocked superficial segments of late proximal tubules were perfused with salt free isosmolal mannitol solution. Time course of total net ion influx into the lumen after sudden stop of perfusion was monitored by continuous measurement of the conductivity of tubular fluid inside the lumen. Double barreled microelectrodes with high coupling resistance (Rc = 350-450 kOhm) were used as microconductivity probes. The observed time course was interpreted in terms of a two compartment system. Time constants for equilibration were 5.9 plus or minus 2.8 (SD) s in control rats, 14.4 plus or minus 8.0 s in adrenalectomized rats and 5.5 plus or minus 2.3 s in adrenalectomized rats substituted with 100 microgram . kg-1 . d-1 aldosterone for 3 days. The observed effect of aldosterone on the equilibration time constant indicates that this hormone increases the leakiness of proximal tabular epithelium for passive ion influx into the lumen.

Acute effects of hydrocortisone on the reabsorption of inorganic phosphate in normal, adrenalectomized and parathyroidectomized rats

Clearance studies have been performed in order to examine the acute effects of hydrocortisone on the reabsorption of inorganic phosphage (Pi) in normal, acutely adrenalectomized (Adr. X) and acutely parathyroidectomized (PTX) rats. Hydrocortisone increased the glomerular filtration rates in all groups, normalized the capacity to excrete water in the Adr. X animals, did not change consistently the fractional reabsorption of sodium in all series and diminished the fraction reabsorption of inorganic phosphate in Adr. X rats from 91.4 +/- 6.5 (mean +/- S.D.) to 78.2 +/- 8.5% (P less than 0.001). It is concluded that the decrease of the fractional reabsorption of Pi in the Adr. X animals was rather the result of the increased filtered load than a direct effect of hydrocortisone on the Pi transport.

A functional model of the rat kidney

A mathematical model of the nephron was developed by writing a set of material balance equations for the flow of urea, salt and water along the foregoing study and are taken here as a basis, in particular the model configuration of the collecting duct system. The stimulation of the model equatentration profiles which at the ends of the several tubular sections were consistent with the values observed in experimental investigations.e medullary interstitial solute concentration profiles are taken to increase linearly in outer and inner zone. The several transeptithelial fluxes are driven by diffusion, osmosis, solvent drag and active transport. The development of osmotic gradient in the inner medulla is taken here to be caused by active secretion of salt into the descending LImb of Henle's loop. The parameters in the flux equations for all parts of the nephron and the concentration values at the end of each tubular section are determined by collecting and averaging the values given in literature and by extrapolating the measurement data. The simulation of the model equations with these averaged parameters resulted in concentration profiles which at the ends of the several tubular sections were consistent with the values observed in experimental investigations.

Effects of dietary protein restriction and glucocorticoid administration on urea excretion in rats

Rats were sampled for clearance studies under anesthesia after 0, 2, 4, 7 and 14 days of dietary protein restriction. Mean fractional urea excretion decreased between day 2 and day 7 of protein restriction after a two-day lag in response. Seven-day administration of dexamethasone in protein-restricted rats caused a significant increase in mean fractional urea excretion. Adrenalectomized rats fed a normal diet had fractional urea excretion values resembling those in protein-restricted rats. Chronic administration of dexamethasone in adrenalectomized rats caused a consistent increase in fractional urea excretion. Fractional urea excretion values were no lower in protein-restricted adrenalectomized rats fed a normal diet. The mean plasma corticosterone concentration (measured 6 to 8 PM) was decreased in rats fed a low protein diet relative to rats fed a high protein diet. The results suggest that glucocorticoids may play a role in the tubular regulation of urea excretion either by a direct effect on the renal tubule or through some intermediate factor. A mediating role of glomerular filtration rate in glucocorticoid-induced changes of fractional urea excretion could not be ruled out, however.

The hydraulic conductivity of the rat proximal tubular wall determined with colloidal solutions

The hydraulic conductivity of the rat proximal tubular wall was determined using colloidal solutions perfused in short (50--200 mum) (SMP) or long (90--200 mum) (LMP) proximal tubular segments. In SMP human serum albumin (HSA) or polyvinylpyrrolidone (PVP) was added to raffinose solutions. A Lp of 0.019 nl-min-1-mm-1-mm Hg-1 was found when high colloid concentrations were used while values of 0.055--0.092 were found when low colloid concentrations were used. In other experiments, the Lp was determined by perfusing short tubular segments with pure raffinose solutions. A value of 0.015 nl-min-1-mm-1-mm Hg-1 was found. This is twice the value found when raffinose solutions were perfused through long tubular segments and it is concluded that the short microperfusion technique overestimates Lp with a factor of two. When microperfusions of long tubular segments were conducted, PVP was added to an equilibrium solution consisting of NaCl (110 mM) and raffinose (80 mM). Lp was found to be 0.018--0.021 when high colloid concentrations were used, while a value of 0.029 was found when a low colloid concentration was used. As found in both SMP and LMP a decrease in Lp's with increasing colloid concentrations indicates that a significant influence of radial concentration differences is highly probable. It is therefore suggested that the highest Lp derived when using the lowest colloid concentrations represents the best estimate. With this Lp value (0.03--0.05 nl-min-1-mm-1-mm Hg-1) and the existing transtubular hydrostatic and oncotic pressure difference it can be calculated that these passive forces might constitute the driving force for 1/3 of the fluid reabsorbed in the proximal tubule.

Evaluation of peritubular capillary microperfusion method by morphological and functional studies

The present study was undertaken in anesthetized rats to evaluate the technique of efferent arteriolar and branch capillary microperfusion in the kidney. In contrary to previous suggestions, subsurface efferent arteriolar branches were demonstrated to occur prior to the surface site of insertion of the microperfusion pipette into an efferent arteriole. This finding, in addition to the presence of continued blood flow around the microperfusion pipette, provided an explanation for continued filtration and tubular reabsorption in nephrons under study. Moreover, additional microdissection studies demonstrated that a mean of 3.8 nephrons are surrounded by artificially perfused capillaries from a micropipette inserted into a single efferent arteriole. Thus, any obstructive effect of the microperfusion technique on post-glomerular blood flow might be expected to alter nephron function in only a minority of nephrons which are surrounded by artificial perfusate. The results also demonstrated the importance of avioding osmotic gradients between the animal's plasma and the artificial perfusate. A method is also described in which the correlation between in vivo diagrams and in vitro microdissections allows assessment of the completeness of the artificial perfusion of capillaries surrounding the surface loops of nephrons under study.