Aldosterone and in vivo Mineralocorticoid Activity in Normotensive and Hypertensive Man

Role of 18-hydroxylated cortisols in hypertension

The isolation of 18-hydroxycortisol and 18-oxocortisol was recently described. These steroids have been shown to be excreted in exaggerated quantities in patients with primary aldosteronism, with adrenal adenomas and in glucocorticoid suppressible aldosteronism. We report the measurement of both steroids in the urine of patients with essential hypertension. 18-Oxocortisol excretion did not differ in patients with normal renin essential hypertension (0.7 +/- 0.7 micrograms/24 h), low renin essential hypertension (0.7 +/- 0.5 micrograms/24 h) and normal individuals (1.2 +/- 0.9 micrograms/24 h). Patients with normal renin hypertension excreted 54 +/- 43 micrograms/24 h of 18-hydroxycortisol, those with low renin essential hypertension excreted 58 +/- 54 micrograms/24 h, and normal individuals excreted 63 +/- 36 micrograms/24 h. Three of the low renin and one of the normal renin hypertensive subjects excreted greater quantities of 18-hydroxycortisol than the upper limit of normal, but all excreted normal quantities of 18-oxocortisol. As 18-hydroxycortisol is inactive, the meaning of this elevated excretion is unclear, but it may be a marker of an adrenal enzymatic abnormality which may be playing a more direct role in hypertension.

Sodium transport inhibition, cell calcium, and hypertension. The natriuretic hormone/Na+-Ca2+ exchange/hypertension hypothesis

Sodium plays a critical role in the etiology of essential hypertension, but the mechanism by which excess dietary sodium actually leads to the elevation of blood pressure is not understood. The hypothesis described shows how an excessive sodium load can lead to the development of hypertension. The underlying factor must be a genetic or acquired deficiency or limitation in renal sodium excretion that may be undetectable by standard renal function tests. The resultant tendency towards sodium, water, and extracellular fluid volume expansion is compensated by the secretion of a natriuretic hormone that promotes sodium excretion by inhibiting sodium pumps in the kidney tubule cells. The hormone also inhibits sodium pumps in other cells, including vascular smooth muscle cells, causing intracellular sodium to increase. Then, because the vascular smooth muscle cells contain a Na+-Ca2+ exchange transport system in their plasma membranes, more calcium than normal is delivered to these cells. This causes the increased contractility and reactivity that underlies the increased vascular tone and peripheral vascular resistance that elevates the blood pressure.

Salt sensitivity in humans is linked to enhanced sympathetic responsiveness and to enhanced proximal tubular reabsorption

If high sodium intake is involved in the pathogenesis of essential hypertension, the effects of changing the sodium intake should be demonstrable in the susceptible part of the normotensive population. Therefore, we have investigated the effects of moderate salt restriction in 52 young normotensive subjects with and without a family history of hypertension; 22 (42%) responded to moderate salt restriction (200 to 50 mmol/day) over 2 weeks, with a significant fall in blood pressure shown by continuous automatic blood pressure recordings. Accordingly, these subjects were classified as salt-sensitive, and the remainder as salt-resistant. Compared to salt-resistant subjects, salt-sensitive subjects showed a 2.5-fold higher incidence of a positive family history of hypertension (p less than 0.01), and a significantly higher blood pressure and lower salivary sodium concentration during the usual high sodium diet. Although there were no differences in Na,K-ATPase activity and in Na-K cotransport of erythrocytes, the pressor response to infused norepinephrine in salt-sensitive subjects was double that of salt-resistant subjects independent of the diet and this was linked to indirect evidence for enhanced proximal tubular sodium reabsorption. On the usual high sodium diet, 40% of the normal population may be salt-sensitive and prone to develop hypertension. Hypersensitivity to catecholamines (genetically determined?) may be the cause of salt sensitivity. A low sodium concentration in saliva deserves further study as a simple screening test to identify salt-sensitive subjects.

Low sodium/high potassium diet for prevention of hypertension: probable mechanisms of action

20 normotensive subjects (10 with a family history of hypertension) were investigated as to whether moderate salt restriction and/or a high potassium intake had a beneficial effect on blood pressure regulation and prevention of hypertension. In all subjects a moderate reduction of salt intake from 200 to 50 mmol/day over 2 weeks reduced the rise in blood pressure induced by various doses of noradrenaline (0.1, 0.2, and 0.4 microgram/kg/min). Furthermore, of 20 subjects 12 (8 with a family history of hypertension) responded to salt restriction with a fall in systolic or diastolic blood pressure of at least 5 mm Hg. There were no significant differences in plasma renin, aldosterone, vasopressin, and catecholamine levels between responders (salt-sensitive subjects) and non-responders, but salt-sensitive subjects had a mean baseline diastolic blood pressure which was higher than that of salt-insensitive subjects by 13 mm Hg (77.3+/-3.26 vs. 64.6+/-2.06, p less than 0.001). A high potassium intake reduced diastolic blood pressure by at least 5 mm Hg in 10 out of 20 subjects, of the 10 7 had a family history of hypertension and 9 responded to salt restriction. A high potassium intake also improved compliance with a low salt regimen, promoted sodium loss, prevented the rise in plasma catecholamines induced by a low salt diet, and increased the sensitivity of the baroreceptor reflex. These four effects occurred in the group as a whole and were probably the means by which a high potassium intake reduced blood pressure. In all subjects 2 weeks of a combined low sodium/high potassium intake reduced blood pressure rises induced by mental stress or noradrenaline infusion by 10 mm Hg. The results of this study suggest that moderate salt restriction combined with a high potassium intake helps to prevent hypertension, that salt-sensitive subjects exist, and that these individuals would profit most.

Influence of exposure to moderate altitude on the plasma concentraton of cortisol, aldosterone, renin, testosterone, and gonadotropins

The influence of 11 days at moderate altitude (2,000 m) combined with exercise on plasma concentration of testosterone, FSH (follicle-stimulating hormone), LH (luteinizing hormone), cortisol, aldosterone, and renin activity was studied in ten healthy subjects. Within 48 h of arrival at moderate altitude a significant increase in testosterone was found whereas FSH had decreased significantly and LH showed a tendency to decrease. Cortisol increased significantly at the beginning and reached a maximum at the end of altitude exposure. The plasma aldosterone level rose continuously and on the last day of altitude was significantly elevated. Plasma renin activity showed a tendency to decrease. On return to low land all measured parameters returned to base line values within 2 days. The findings of increases in plasma levels of aldosterone and testosterone (and serum T3 and T4, as reported by others) are in contrast to the previously found decrease of urinary excretion of all these hormones. This appears to be a distinct dissociation of serum levels of adrenal (and thyroid) hormones from their urinary excretion. The observed increase in plasma aldosterone is probably mediated through ACTH and the rise in plasma potassium, since plasma renin activity showed an opposite trend. The rise in plasma testosterone is probably of adrenal origin since plasma gonadotropins declined simultaneously. The increase of plasma levels of glucocorticoids, mineralocorticoids, and androgens after an ascent from 600 m to 2,000 m above sea level is compatible with an ACTH-mediated stimulation of the entire adrenal cortex and/or a diminished elimination of adrenal steroids: The concomitant fall of FSH, LH, and plasma renin would then be a consequence of a direct negative feedback inhibition of these hormones.