Control of sodium reabsorption

Cortisol and 11 beta-hydroxysteroid dehydrogenase type 2 as potential determinants of renal citrate excretion in healthy children

BACKGROUND

In patients with Cushing disease, renal citrate excretion is reduced. A low urinary citrate concentration is a risk factor for nephrolithiasis. Since higher acid loading is one major determinant of reduced citrate excretion, we aimed to examine whether glucocorticoids still within the physiological range may already impact on urinary citrate excretion independently of acid-base status.

METHODS

Overall, 132 healthy prepubertal participants of the DOrtmund Nutritional and Anthropometric Longitudinally Designed (DONALD) Study who had collected two successive 24-h urine samples (at 1 and 2 years) before the start of their pubertal growth spurt were included in the study. Net acid excretion capacity (NAEC), urinary potential renal acid load (PRAL), creatinine, calcium, and various cortisol metabolites were measured in all samples. Glucocorticoid quantification was done by GC-MS and radioimmunoassay.

RESULTS

In regression models multivariable-adjusted for 24-h urinary PRAL, NAEC, creatinine and calcium, urinary free cortisol (UFF), 6β-hydroxycortisol, and 20α-dihydrocortisol showed significant inverse relationships (P ≤ 0.02) with 24-h renal citrate output. By contrast, the estimate of renal 11β-hydroxysteroid dehydrogenase type 2 (11β-HSD2), i.e., the ratio of urinary free cortisone/UFF, associated positively with urinary citrate (P = 0.04).

CONCLUSIONS

In line with studies in hypercortisolic state, even moderately high cortisol levels in healthy children, still within the physiological range, may negatively impact on the kidney's citrate excretion. Besides, a higher 11β-HSD2 activity, favoring cortisol inactivation, is paralleled by an increased citrate excretion.

Search for a natriuretic mechanism sensitive to sodium in the brain of the monkey

1. The effects of hypertonic saline infusion into the third ventricle were investigated in ten monkeys which were pre-operated, trained, and used in the conscious state under controlled conditions. 2. In non-hydrated monkeys, intraventricular infusion of NaCl 1.0 M, 0.01 ml./min for 30 min did not affect urine volume or Na output but produced a small increase in urine osmolality. Comparable infusion of NaCl 0.15 M had no effect on any parameter. 3. In monkeys undergoing water diuresis (with i.v. infusion of 5% dextrose), intraventricular hypertonic saline produced large reciprocal changes in urine volume and osmolality while urine Na showed no significant change. The effects on urine volume and osmolality were greater than those of lysine-vasopressin 30 m-u./kg i.v. 4. The absence of natriuresis after intraventricular hypertonic saline infusion in the monkey was in notable contrast to the results reported in lower species. However, the data suggested that the infusion probably released ADH as in other species.

Hypertension associated with early stage kidney disease. Complementary roles of circulating renin, the body sodium/volume state and duration of hypertension

Interrelations among blood pressure, exchangeable sodium, blood volume and plasma renin activity were studied in 40 normal subjects and in 40 patients with early stage kidney disease (mean plasma creatinine, 2 mg/100 ml). Findings in eight normotensive patients did not differ significantly from those in normal subjects. However, 32 hypertensive patients showed increases (p less than 0.05) in mean exchangeable sodium and in the products of the logarithm of plasma renin activity and exchangeable sodium or blood volume. In normal subjects, blood pressure did not correlate with any of the parameters measured. In the patients, it correlated significantly (p less than 0.05) with duration of hypertension (r = 0.70), exchangeable sodium (r = 0.34) and with sodium-renin (r = 0.38) or volume-renin (r = 0.30) products, but not with blood volume or circulating renin individually. Multiple regression analysis with blood pressure as a dependent variable, and duration of hypertension and the sodium-renin or volume-renin products as independent variables, revealed correlation coefficients of 0.77 and 0.76, respectively. These findings suggest that hypertension accompanying early stage kidney disease may depend at least partly on subtle abnormalities in the sodium volume-renin feedback mechanism as well as on a factor related to the duration of preexisting hypertension.

Fluid and electrolyte metabolism in the newborn infant

This chapter has sought to gather together the background information on systems controlling homoeostasis of salt and water and their clinical derangement. The temptation to adopt an all-embracing approach to the management of these problems is strong but such an approach is difficult to achieve and indeed dangerous. The circumstances of each sick infant are unique and plans for treatment must be individually tailored and flexible, dependent upon clinical and biochemical progress. Future developments in this field are likely to involve further understanding of renal and hormonal control of fluid and electrolyte and it might be expected that as new methods of management emerge they will be accompanied by their own peculiar risks of inducing secondary homoeostatic disturbances. With regard to infant feeding each advance appears to underline the desirability of breast feeding and support current moves toward provision of low solute feeds for those who are artificially fed. Paediatricians should be vocal in their advocation of breast feeding and ensure that the major principles of salt and water handling are understood by all who work with small infants. An extra scoop of powdered milk can be more of a poison than a food.

Intestinal transport of water and electrolytes during extracellular volume expansion in dogs

The effects of extracellular fluid volume expansion on intestinal transport of salts and water were studied in dogs by perfusing loops of bowel in vivo. Saline loading caused depression of duodenal and jejunal absorption with net secretion of salt and water into the lumen. Studies of unidirectional transport of (22)Na(+) revealed that the negative net sodium flux was due primarily, and perhaps exclusively, to increased serosal to mucosal transport, for mucosal to serosal sodium transport was not changed during volume expansion. Net transport of water and potassium paralleled net sodium flux. Administration of deoxycorticosterone did not affect the intestinal response to saline loading. Hemodilution, accomplished by equilibrating the dogs' blood with a reservoir of saline, did not affect intestinal absorption, but isotonic, iso-oncotic expansion of the extracellular fluid produced by reinfusing the saline-blood mixture from the reservoir resulted in negative net transport of water, sodium, and potassium by the duodenum. It is suggested that the small bowel is capable of secreting salts and water through intercellular spaces, and that this process is stimulated by extracellular fluid volume expansion.

On the mechanism of inhibition in fluid reabsorption by the renal proximal tubule of the volume-expanded rat

We undertook to determine the extent to which the inhibition in absolute proximal fluid reabsorption in response to expansion of extracellular volume with noncolloid-containing solutions is the result of concomitant reductions in postglomerular (efferent arteriolar) protein concentration. Selective elevation of efferent arteriolar oncotic pressure in volume-expanded rats (Ringer's 10% body weight) to levels slightly in excess of normal by microperfusion with 9-10% albumin-Ringer's solution nearly completely reversed the inhibition in absolute and fractional reabsorption in adjacent proximal tubules. In contrast, during similar microperfusion with a 6-7% albumin solution, no increase in proximal reabsorption was measured. We interpret these findings to indicate that the bulk of the inhibition in absolute proximal reabsorption in response to volume expansion with colloid-free solutions is causally mediated by the accompanying parallel decline in postglomerular vascular protein concentration.

Inhibition of transepithelial sodium transport in the frog skin by a low molecular weight fraction of uremic serum

An inhibitor of transepithelial sodium transport was found in a low molecular weight fraction obtained from serum of patients with far advanced chronic renal disease. In 18 nondialyzed patients, the mean inhibition of short circuit current (SCC) was 24.9 +/-2.2% (SE). With a comparable fraction from 11 normal subjects. SCC decreased by only 5.3 +/-1.5%. There was significantly greater inhibition with the serum fractions of patients with end stage renal disease being maintained on chronic hemodialysis than in the normal control group; but the degree of inhibition in the dialyzed population was significantly less than that observed in the nondialyzed chronically uremic patients. The inhibition of SCC produced by the serum fractions of a group of seven patients with acute renal failure was not significantly different from the control group despite the presence of high grade uremia in the former. The inhibitory fraction has characteristics identical with the uremic serum fraction which previously has been shown to inhibit p-aminohippurate (PAH) uptake by rabbit kidney cortical slices. With gel filtration through Sephadex G-25, the active fraction appears after the major peaks of substances as small as urea and sodium; hence it may have been retarded on the column. But its ultrafiltration characteristics suggest that its molecular weight may be less than 1000. The inhibitory capability was not destroyed by boiling, freezing, or digestion with chymotrypsin or pronase. Neither methylguanidine nor guanidinosuccinic acid in concentrations well above those present in the serum of uremic patients inhibited sodium transport in the frog skin. The data suggest that there is an inhibitor of sodium transport in the serum of patients with chronic uremia. The role of this material in the regulation of sodium excretion in uremia as well as its possible role as a uremic toxin are subjects of both theoretical and practical interest.