Nephrology forum. Mechanisms of urine concentration

3D simulation of solutes concentration in urinary concentration mechanism in rat renal medulla

In this work, a mathematical model was developed to simulate the urinary concentration mechanism. A 3-D geometry was derived based on the detail physiological pictures of rat kidney. The approximate region of each tubule was obtained from the volume distribution of structures based on Walter Pfaller's monograph and Layton's region-based model. Mass and momentum balances were applied to solve for the change in solutes concentration and osmolality. The osmolality of short and long descending nephrons at the end of the outer medulla was obtained to be 530 mOsmol/kgH2O and 802 mOsmol/kgH2O, respectively, which were in acceptable agreement with experimental data. The fluid osmolality of the short and long ascending nephrons was also compatible with experimental data. The osmolality of CD fluid at the end of the inner medulla was determined to be 1198 mOsmol/kgH2O which was close the experimental data (1216 ± 118). Finally, the impact of the position of each tubule on the fluid osmolality and solutes concentration were obvious in the results; for example, short descending limb a1, which is the closest tubule to the collecting duct, had the highest urea concentration in all tubules. This reflects the important effect of the 3D modeling on the precise analysis of urinary concentration mechanism.

Compensatory changes in enzymes of arginine metabolism during renal hypertrophy in mice

The present study investigates enzyme activities of the urea cycle, transamidinase and ornithine-proline inter-conversion in the hypertrophied kidney after unilateral nephrectomy in mice. Surgical removal of the left kidney in mice led to compensatory enlargement of the right kidney after 1 and 14 days. This renal growth was associated with an increase in glomerular volume (but not number) and enlargement of the proximal convoluted tubules. The total renal protein content increased in proportion to the increase in kidney weight, but the protein per gram weight of kidney did not change. The specific activity of only ornithine aminotransferase (OAT), the rate-limiting enzyme in the conversion of ornithine to proline, increased in 2 weeks of hypertrophy. The specific activity of all other enzymes was unchanged. However, the total enzyme activity per kidney of all the enzymes, without exception, was elevated in the hypertrophied kidney. While the increase in total OAT activity was much more than the increase in kidney weight, all other enzymes increased more or less in proportion to the increase in renal mass. The results suggest that compensation in OAT activity to chronic reduction in renal mass was complete, but only partial in the case of other enzymes.

Effects of prenatal ethanol exposure on postnatal renal function and structure in the rat

Effects of prenatal ethanol exposure on postnatal renal function and structure in the rat. Renal function and morphology were studied in 90-day-old offspring of ethanol-fed (E) rats and were compared to pair-fed control (C) animals. Compared to C rats, E rats were smaller at birth, had higher fractional sodium excretion (p less than 0.01) and lower fractional potassium excretion (p less than 0.01). In E rats, sodium (Na) restriction resulted in a significant increase in urine flow and Na wastage, whereas C rats remained in Na balance. E rats developed hyperkalemia, when potassium (K) intake was increased from 2.8 to 14 mEq/day. Baseline creatinine clearance, urine and blood osmolalities and pH, plasma electrolytes and aldosterone concentrations were similar in both groups. There was no significant difference in wet or dry kidney weight, renal water content, or renal tissue concentrations of Na or K between the two groups. No difference was found in gross morphology or light microscopic appearances of the kidneys between E and C rats. Thus rats exposed to ethanol during fetal life have a defect in urine concentration and Na conservation when fed a low Na diet and a defect in K excretion when given a K load without evidence of any gross or light microscopic renal structural abnormalities at 90 days of age.

Renal tubular dysfunction in fetal alcohol syndrome

Renal function was evaluated in six patients with fetal alcohol syndrome (FAS) and eight control subjects before and after fluid restriction and acute acid loading. Baseline serum electrolytes, creatinine clearance, fractional sodium excretion, tubular reabsorption of phosphate, urine and blood pH and osmolalities, plasma renin activity, and plasma aldosterone level were normal in all subjects, but fractional potassium excretion (FEK) was lower in FAS patients than in control subjects (P less than 0.001). Despite equivalent plasma osmolalities (295 +/- 3 vs 293 +/- 2 mosmol/kg, P = 0.2), the maximum urinary osmolality after 12 h of water deprivation in patients with FAS was significantly lower compared with controls (560 +/- 107 vs 965 +/- 77 mosmol/kg; P less than 0.001) and increased to only 578 +/- 101 mosmol/kg after vasopressin administration. After ammonium chloride loading, minimum urine pH was significantly higher in patients than in controls (5.7 +/- 0.17 vs 4.81 +/- 0.19; P less than 0.001). Net acid excretion and FEK were also lower in patients than in controls (102 +/- 11 vs 139.6 +/- 11.3 microEq/min per 1.73 m2 and 23.5 +/- 1.3 vs 29 +/- 1.6%, respectively; P less than 0.001). The data indicate a subclinical renal tubular defect in urine concentration and acidification in patients with FAS.

The role of the kidney in the maintenance of water balance

This chapter shows how the mammalian kidney is able to regulate the excretion of water independently from that of solutes. For this function, which derives from several evolutionary steps among vertebrates, it takes advantage of the diluting ability of the thick ascending limb to produce osmotic energy which is then used to concentrate solutes in the urine. This concentration is permitted by a highly sophisticated architecture of nephrons and vessels in the renal medulla, combined with special permeability characteristics of the different nephron segments and specific hormonal regulation. Two different types of loops of Henle and several well-insulated vascular compartments contribute to this process. The major nitrogenous waste product, urea, is concentrated by an indirect process involving a transfer of osmotic energy from the outer to the inner medulla. As known for several decades, concentrating function is primarily regulated by the effect of antidiuretic hormone (ADH) on water permeability of the collecting duct. However, as discovered more recently, it is also largely dependent upon the effect of the same hormone on urea permeability in the terminal collecting duct. In addition, recent investigations have revealed a much more complex hormonal regulation of the concentrating process than previously thought. ADH itself acts on many other structures in the kidney, and many other hormones and mediators, the secretion of which is not thought to be influenced by the water status, do affect urine concentration either directly or by their interaction with ADH. Rodents display a wide spectrum of morphological and functional renal adaptations improving water conservation. Their study has brought a better understanding of the significant steps and anatomical structures that contribute to the concentrating process. Finally, it is also apparent that the capacity to concentrate urine is influenced in individual animals of a given species by the availability of water, by specific feeding patterns, and by the protein content of the diet.

Arginine vasopressin and body fluid homeostasis in the fetal alcohol exposed rat

Studies involving fluid homeostasis were carried out in adult Long-Evans rats born to mothers given liquid diets containing 35% of the calories derived from ethanol and compared to offspring of dams given isocaloric liquid diets containing no ethanol. Plasma levels of arginine vasopressin (AVP), plasma and urine osmolality, and urine production were determined in water-sated and water-deprived offspring. In the water-sated condition, the group exposed to alcohol prenatally had plasma levels of AVP seven-fold above control levels. This increase was associated with a large increase in within-group variability. Water consumption was also significantly elevated in the group of fetal alcohol exposed (FAE) rats. Plasma and urine osmolality and urine production were similar to control levels. In the control animals, 24-hr of water deprivation produced the expected increase in AVP, in plasma and urine osmolality, and decrease in urine production. The FAE animals, however, showed parallel changes in plasma and urine osmolality and urine production with no significant change in AVP. Examination of basal glucose metabolic rates in the cerebral structures involved in fluid homeostasis revealed that despite the large increase in AVP levels in the FAE rats, only the neurohypophysis and supraoptic nuclei showed significant increases in activity. These data suggest that fetal alcohol exposure causes a long-term disruption in the central mechanisms regulating vasopressin release and fluid homeostatic responses.

Hyponatremia

The serum sodium concentration reflects the osmolality of the extracellular fluid and provides no direct information about total body sodium content. Patients with hyponatremia may have decreased, normal, or increased total body sodium content. The first step in the approach to the patient with hyponatremia is measurement of plasma osmolality. Hyponatremia with normal plasma osmolality results from hyperlipemia or hyperproteinemia whereas hyponatremia with increased plasma osmolality results from hyperglycemia or mannitol infusion. Patients with hyponatremia and decreased plasma osmolality may be hypovolemic, hypervolemic, or normovolemic. The volume status of the patient is best determined by history, physical examination, and a few ancillary tests (e.g., total plasma protein concentration, hematocrit, blood pressure, central venous pressure). The clinical signs of hyponatremia are related more to the rapidity of onset than to the severity of the associated plasma hypoosmolality and reflect influx of water into the central nervous system. The main goals of treatment in hyponatremia are to manage the underlying disease and, if necessary, to increase serum sodium concentration and plasma osmolality.

Correlates of diuretic renography in experimental hydronephrosis

We studied the correlations between diuretic renographs and kidney function in experimental hydronephrosis in rabbits. Features of furosemide-stimulated 99mTc-diethylenetriamine-pentaacetic acid renographs were compared to the growth rate, thirst test and endogenous creatinine clearance rate in a chronic solitary-kidney animal model. Intravenous pyelograms, done four weeks after laparotomy, left nephrectomy, bladder resection and constriction of the right pyeloureteric junction, showed signs of obstruction in all the 12 animals of the experimental group. An absent tracer washout after intravenous furosemide, found in five animals, was associated with retarded growth, isosthenuria and an abnormal creatinine clearance. In all of the other seven animals, a distinct tracer washout after intravenous furosemide was accompanied with a normal growth rate and creatinine clearance. However, no one of these seven animals had a normal ability to retain water and concentrate urine in the thirst test. We conclude that, in this experimental model, a furosemide-induced tracer washout from the kidney pelvis cannot be taken as a proof of the absence of any upper urinary tract obstruction.

Fractional excretion of magnesium and renal concentrating capacity in refluxing renal units

To evaluate renal function in kidney units exposed to vesicoureteral reflux we quantified the fractional renal excretion of magnesium and the maximal renal concentrating capacity in 22 renal units of infants and children who presented with grade III to V vesicoureteral reflux on standard voiding cystourethrography. The lowest normal value for maximal concentrating capacity and the highest normal value for fractional excretion of magnesium were set at 800 mOsm. per kg. and 8 per cent, respectively. Concentrating capacity was abnormal in all 22 renal units and the excretion of magnesium was abnormal in 5. Failure of a renal unit to retain magnesium normally was always associated with an inability to concentrate urine. We conclude that in cases of vesicoureteral reflux function of the distal part of the nephron is the first to become abnormal and concentrating capacity is a more sensitive indication of dysfunction than fractional excretion of magnesium.

Role of the urinary concentrating process in the renal effects of high protein intake

High protein diet is known to increase glomerular filtration rate (GFR) and induce kidney hypertrophy. The mechanisms underlying these changes are not understood. Since the mammalian kidney comprises different nephron segments located in well-delineated zones, it is conceivable that the hypertrophy does not affect all kidney zones and all nephron segments uniformly. The present experiments were designed to study the chronic effects of high or low isocaloric protein diets (HP = 32% or LP = 10% casein, respectively) on kidney function and morphology in Sprague-Dawley rats. HP diet induced significant increases in kidney mass, GFR, free water clearance, and maximum urine concentrating ability. Kidney hypertrophy was characterized by: 1. a preferential increase in thickness of the inner stripe of the outer medulla (IS) (+54%, P less than 0.001, while total kidney height, from cortex to papillary tip, increased only by 18%); 2. a marked hypertrophy of the thick ascending limbs (TAL) in the inner stripe (+40% epithelium volume/unit tubular length, P less than 0.05) but not in the outer stripe nor in the cortex; 3. an increase in heterogeneity of glomeruli between superficial (S) and deep (D) nephrons (D/S = 1.47 in HP vs. 1.17 in LP, P less than 0.05). In contrast, normal kidney growth with age and kidney hypertrophy induced by uninephrectomy were not accompanied by preferential enlargement of IS structures. The morphologic changes induced by high protein intake parallel those we previously reported in rats fed a normal diet (25% protein) but in which the operation of the urine concentrating mechanism was chronically stimulated by ADH infusion or by reduction in water intake.(ABSTRACT TRUNCATED AT 250 WORDS)