Role of aldosterone in the control of sodium excretion in patients with advanced chronic renal failure

Potassium tolerance

The maintenance of potassium homeostasis with an increased potassium intake or decreased renal function is dependent in part on the renal adaptation observed in 'potassium tolerance'. However other factors, including control of ingestion, and increased distal delivery of fluid, also play a role.

Contribution of the distal tubule to potassium excretion in experimental glomerulonephritis

The renal adaptations that maintain potassium homeostasis in diffuse forms of glomerular disease are not well defined. Thus, handling of potassium by superficial nephron segments was examined in a rat model of antiglomerular basement membrane nephritis. Sampling the same nephron successively from the end and beginning of the distal tubule and the end of the proximal tubule allowed a segmental analysis. Despite a 40% reduction in GFR, potassium excretion in the glomerulonephritis animals was normal due to an increase in FEK. The proximal tubule and loop segment did not contribute to the enhanced FEK seen in these animals. In contrast, potassium entry along the distal tubule was significantly greater in the experimental group averaging 13.7 +/- 4.3 pmol/min compared to 1.2 +/- 1.7 pmol/min in controls (P less than 0.01). Multiple linear regression analysis showed that distal tubule potassium entry at any level of flow was enhanced in glomerulonephritis compared to controls (P less than 0.0001). Plasma aldosterone levels were similar in both groups of animals. Thus, the adaptation to potassium excretion seen in glomerulonephritis is partly achieved by the distal tubule through flow-rate independent mechanisms and appears to be independent of plasma aldosterone levels.

Increased secretion of potassium in the rectum of humans with chronic renal failure

Previous studies have shown that the intestinal excretion of potassium increases in patients with renal failure and serves to guard against severe potassium retention. It is not known, however, whether the rise in intestinal potassium excretion occurs because of an increase in intestinal potassium secretion or a reduction in potassium absorption. Therefore, studies were performed to evaluate the rate of potassium secretion in the human rectum of controls and subjects with renal insufficiency, using a dialysis bag technique. The results demonstrate that net potassium secretion was increased in subjects with renal failure (-5.2 +/- 0.9 microEq X min-1) compared with the control value of -2.0 +/- 0.4 microEq (P less than .05). This change in intestinal secretion of potassium was shown to be independent of the passive effects of plasma potassium. The rise in potassium secretion, however, correlated directly with an increase in transepithelial potential difference (lumen-negative). Although plasma aldosterone levels were higher in patients than in controls, the scatter of data precludes an assessment of the role of aldosterone in the mechanism of the rise in potassium secretion. These data suggest that augmented intestinal potassium excretion in patients with chronic renal insufficiency is caused by increased net potassium secretion in the large intestine, and highlight the role of the intestine in maintaining potassium balance.

Disorders of body fluids, sodium and potassium in chronic renal failure

A stable volume and composition of extracellular fluid are essential for normal functioning of the body. Since the kidney is primarily responsible for regulating extracellular fluid, loss of kidney function should have catastrophic consequences. Fortunately, even with loss of more than 90 percent of renal function, a remarkable capacity to regulate body fluid volumes and sodium and potassium persists. Nevertheless, this capacity is limited to chronic renal disease and this has important consequences for clinical management of these patients. How can sodium and potassium homeostasis be assessed? Methods for evaluating the steady-state regulation of sodium include measurement of body fluids and their distribution in different compartments and measurement of exchangeable and intracellular sodium. Short-term regulation of body sodium can be assessed from measurement of sodium balance during changes in dietary salt. Potassium is predominantly contained within cells and thus the assessment of its regulation requires special emphasis on measurement of steady-state body stores and potassium distribution across cell membranes. However, the methods used to make all of these measurements require assumptions that may not hold in the altered state of uremia. This raises problems in interpretation requiring critical analysis before conclusions can be made regarding sodium and potassium homeostasis in patients with chronic renal failure. This review focuses on abnormalities of body fluids, sodium and potassium in patients with creatinine clearances of less than 20 ml/min due to chronic renal failure and the impact of conservative therapy, dialysis and renal transplantation on these patients.

Plasma aldosterone concentrations in chronic renal disease

The disagreement in the literature concerning the role of aldosterone in the maintenance of potassium homeostasis in chronic renal disease might be partially explained by differences in plasma renin activity (PRA) among individual patients. Therefore, a study was done in 28 selected patients with varying degrees of renal insufficiency whose serum potassium and PRA concentrations were within the normal range. The results indicate that at comparable serum potassium and PRA concentrations, plasma aldosterone is in most instances elevated when creatinine clearance is lower than 50% of normal.

Renal handling of potassium in dogs with chronic renal insufficiency

The dynamics of potassium excretion were examined in normal dogs and dogs with chronic renal insufficiency of at least 4 weeks' duration (remnant model). All animals, in balance on diets providing 15, 50, or 100 mEq of potassium and 100 mEq of sodium, were challenged with 50 mEq of potassium chloride. Immediately thereafter, hourly clearances were obtained for 5 hours. Irrespective of dietary potassium, mean fasting serum potassium and urinary potassium excretion (UKV) were similar in normal and remnant dogs with mean GFR's of 57 +/- 3 and 16 +/- 3 ml/min, respectively. After orogastric administration of 50 mEq potassium, serum potassium rose significantly more in remnant (2.2 to 2.5 mEq/liter) than in normal (0.9 to 1.2 mEq/liter) groups (P less than 0.001). Conversely, UKV increased significantly less, 70 to 96 vs. 151 to 194 micro Eq/min, respectively (P less than 0.001). In 5 hours, normal animals excreted 61 to 67% of the load, but remnant dogs only 30 to 37% (P less than 0.001). In all groups, UKV correlated directly with serum potassium concentration. But this relationship was markedly attenuated in the remnant groups (P less than 0.001) and independent of dietary potassium. In contrast, the same slope describes the relationship between UKV/GRF and serum potassium for all, normal and remnant, animals. The blunted kaliuresis occurred despite the more severe hyperkalemia in remnant than in normal dogs; it was not associated with significant changes in acid-base, diuresis, natriuresis, serum glucose, insulin, and glucagon concentrations and occurred despite prolonged hyperaldosteronism. The results demonstrate a severe limitation of the remnant kidney's ability to rapidly excrete a potassium load. Changes in serum potassium, or a consequence thereof, are important for the urinary excretion of potassium following its acute administration.

Prevalence, pathogenesis, and functional significance of aldosterone deficiency in hyperkalemic patients with chronic renal insufficiency

Our findings indicate that hypoaldosteronism occurs commonly (23/31 patients) in hyperkalemic patients with chronic renal insufficiency and that the deficiency of aldosterone contributes to the pathogenesis of the hyperkalemia. In most patients (83%), hypoaldosteronism could be accounted for by deficient renal secretion of renin, but in some patients (17%) overt renin deficiency did not appear to be present, and therefore other (unidentified) causes of aldosterone deficiency must be invoked. The results also indicate that the urinary excretion rate of aldosterone secretion rate in this group of patients.