In vivo estimation of changes in distal tubule flow and their role in dexamethasone-induced kaliuresis in control and potassium-adapted rats

Progesterone: not just a sex hormone anymore?

The adrenal cortex synthesizes many steroids, but the mineralocorticoid aldosterone and the glucocorticoids cortisol and corticosteroid have been viewed as the principal steroids responsible for regulation of renal electrolyte excretion. A study by Elabida et al. challenges that paradigm and suggests that progesterone, classically viewed as a sex steroid, also participates in renal electrolyte balance.

Pomc knockout mice have secondary hyperaldosteronism despite an absence of adrenocorticotropin

Aldosterone production is controlled by angiotensin II, potassium, and ACTH. Mice lacking Pomc and its pituitary product ACTH have been reported to have absent or low aldosterone levels, suggesting that ACTH is required for normal aldosterone production. However, this is at odds with the clinical finding that human aldosterone deficiency is not a component of secondary adrenal insufficiency. To resolve this, we measured plasma and urine electrolytes, together with plasma aldosterone and renin activity, in Pomc(-/-) mice. We found that these mice have secondary hyperaldosteronism (elevated aldosterone without suppression of renin activity), indicating that ACTH is not required for aldosterone production or release in vivo. Exogenous ACTH stimulates a further increase in aldosterone in Pomc(-/-) mice, whereas angiotensin II has no effect, and the combination of angiotensin II and ACTH is no more potent than ACTH alone. These data suggest that aldosterone production and release in vivo do not require the action of ACTH during development or postnatal life and that secondary hyperaldosteronism in Pomc(-/-) mice is a consequence of glucocorticoid deficiency.

Role of glucocorticoid in excretion of an acute potassium load in patients with Addison's disease and panhypopituitarism

Glucocorticoid (GC) has been shown to stimulate potassium (K) excretion in various conditions, but it is still incompletely resolved whether its presence is essential for the normal K homeostasis. We addressed this question in patients with selective GC deficiency (panhypopituitarism) and with combined GC and mineralocorticoid deficiency (Addison's disease), studied 24 hours after withdrawal of their regular substitution therapy. Compared to data in healthy subjects, both basal K excretion and the kaliuresis after a KCl load (1 mmol/kg body wt orally) were impaired in either patient group (P < 0.05). Physiological cortisol supplementation (20 mg 3 hr prior to test, and 1 mg/hr during test) increased basal K excretion (from 10.6 +/- 1.8 to 19.2 +/- 1.9 mmol/5 hr, P < 0.01) and KCl stimulated kaliuresis (from 47.9 +/- 6.1 to 54.8 +/- 4.7 mmol/5 hr, P = 0.06) to normal levels in panhypopituitarism. Cortisol also improved basal K excretion (from 10.2 +/- 1.5 to 16.9 +/- 3.5 mmol/5 hr, P < 0.05) and KCl-stimulated K excretion (from 31.6 +/- 2.5 to 45.2 +/- 3.8 mmol/5 hr, P < 0.05) in Addison's disease, although KCl-stimulated K excretion remained below normal (P < 0.01). The effects of cortisol on sodium excretion differed between the two patient groups (P < 0.05) in that only in Addison's disease the improved K excretion was associated with sodium retention. Additional experiments with the purely GC compound dexamethasone (0.5 mg 3 hr prior to test, and 0.03 mg/hr during test) in the patients with Addison's disease also improved K excretion (P < 0.05), but without the concomitant sodium retention observed after cortisol.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Potassium excretion in renal failure in the rat: the role of distal tubule flow and aldosterone

1. This study examines the contribution of an increased distal tubule flow and of aldosterone to the handling of a potassium load in conscious rats with renal failure induced by subtotal nephrectomy or by gentamicin on a control of high K+ diet. 2. Glomerular filtration rate was reduced by subtotal nephrectomy to 40% and by gentamicin treatment to 60% of control. Subtotal nephrectomy induced significant hypertrophy of glomeruli and proximal and distal tubules, but gentamicin did not. Both experimental groups had a normal iothalamate space and plasma potassium after a 20 h fast. 3. Two hours after an acute KCl load rats with renal failure excreted less potassium than control rats. There was also a lesser natriuretic effect of KCl in the renal failure groups. 4. A high K+ diet, given for 5-7 days, increased excretion of an acute KCl load in control rats and rats with renal failure. 5. (UNaV + UKV) was used as an estimate of distal tubule flow. Potassium excretion, related to distal tubule flow, was similar in the renal failure and control rats when on the same diet. This is consistent with potassium excretion being strongly, but not necessarily solely, dependent on distal flow. 6. Adrenalectomy reduced, and aldosterone restored, potassium excretion in the renal failure and control groups. This suggests a role for aldosterone in excretion of an acute potassium load with this degree of renal failure.