Graded dose effects of angiotensin II on aldosterone production in man during various levels of potassium intake

Effect of Increased Potassium Intake on Adrenal Cortical and Cardiovascular Responses to Angiotensin II: A Randomized Crossover Study

Background Increased potassium intake lowers blood pressure in patients with hypertension, but increased potassium intake also elevates plasma concentrations of the blood pressure-raising hormone aldosterone. Besides its well-described renal effects, aldosterone is also believed to have vascular effects, acting through mineralocorticoid receptors present in endothelial and vascular smooth muscle cells, although mineralocorticoid receptors-independent actions are also thought to be involved. Methods and Results To gain further insight into the effect of increased potassium intake and potassium-stimulated hyperaldosteronism on the human cardiovascular system, we conducted a randomized placebo-controlled double-blind crossover study in 25 healthy normotensive men, where 4 weeks treatment with a potassium supplement (90 mmol/day) was compared with 4 weeks on placebo. At the end of each treatment period, we measured potassium and aldosterone in plasma and performed an angiotensin II (AngII) infusion experiment, during which we assessed the aldosterone response in plasma. Hemodynamics were also monitored during the AngII infusion using ECG, impedance cardiography, finger plethysmography (blood pressure-monitoring), and Doppler ultrasound. The study showed that higher potassium intake increased plasma potassium (mean±SD, 4.3±0.2 versus 4.0±0.2 mmol/L; P=0.0002) and aldosterone (median [interquartile range], 440 [336-521] versus 237 [173-386] pmol/L; P<0.0001), and based on a linear mixed model for repeated measurements, increased potassium intake potentiated AngII-stimulated aldosterone secretion (P=0.0020). In contrast, the hemodynamic responses (blood pressure, total peripheral resistance, cardiac output, and renal artery blood flow) to AngII were similar after potassium and placebo. Conclusions Increased potassium intake potentiates AngII-stimulated aldosterone secretion without affecting systemic cardiovascular hemodynamics in healthy normotensive men. Registration EudraCT Number: 2013-004460-66; URL: https://www.ClinicalTrials.gov; Unique identifier: NCT02380157.

Minireview: aldosterone biosynthesis: electrically gated for our protection

Aldosterone produced by adrenal zona glomerulosa (ZG) cells plays an important role in maintaining salt/water balance and, hence, blood pressure homeostasis. However, when dysregulated, aldosterone advances renal and cardiovascular disease states. Multiple steps in the steroidogenic pathway require Ca(2+), and the sustained production of aldosterone depends on maintained Ca(2+) entry into the ZG cell. Nevertheless, the recorded membrane potential of isolated ZG cells is extremely hyperpolarized, allowing the opening of only a small fraction of low-voltage-activated Ca(2+) channels of the Ca(v)3.x family, the major Ca(2+) conductance on the ZG cell membrane. As a consequence, to activate sufficient Ca(2+) channels to sustain the production of aldosterone, aldosterone secretagogs would be required to affect large decreases in membrane voltage, a requirement that is inconsistent with the exquisite sensitivity of aldosterone production in vivo to small changes (0.1 mm) in extracellular K(+). In this review, we evaluate the contribution of membrane voltage and voltage-dependent Ca(2+) channels to the control of aldosterone production and consider data highlighting the electrical excitability of the ZG cell. This intrinsic capacity of ZG cells to behave as electrical oscillators provides a platform from which to generate a recurring Ca(2+) signal that is compatible with the lengthy time course of steroidogenesis and provides an alternative model for the physiological regulation of aldosterone production that permits both amplitude and temporal modulation of the Ca(2+) signal.

Angiotensin II and aldosterone regulation

The circulating renin-angiotensin system is a major regulator of the secretion of the adrenocortical hormone, aldosterone. This renin-angiotensin aldosterone system is important in the control of salt and water balance and blood pressure. This review describes the historical background leading to the discovery of aldosterone in the 1950s and the recognition in the 1960s that angiotensin II was involved in its control. Although angiotensin II is important in the regulation of aldosterone secretion, its action is influenced by multiple other factors, especially potassium and atrial natriuretic peptide. In addition to the circulating renin-angiotensin system, a local renin-angiotensin system is present in the zona glomerulosa cell. This local system also appears to be involved in the regulation of aldosterone production. The mechanism by which angiotensin II stimulates the adrenal zona glomerulosa cell is described in some detail. Angiotensin II interacts with the angiotensin receptor (AT1) membrane receptor that is coupled to cellular second messengers. Specific AT1 receptor antagonists are now clinically used to block angiotensin II's action on various target organs, including the adrenal gland.

Changes in adrenal responsiveness and potassium balance with shifts in sodium intake

Dietary sodium modulates the aldosterone response to angiotensin, but available evidence does not indicate whether there is a gradual change in adrenal responsiveness with intermediate sodium intakes or a sharp shift from a low to a high responsive level at some threshold sodium intake. Nine normal subjects received angiotensin II infusions while in balance on five levels of sodium intake over two orders of magnitude, 3 to 300 mEq sodium per day. Basal plasma renin activity, plasma aldosterone and plasma angiotensin II concentrations gradually fell as dietary sodium intake increased. The adrenal was quite sensitive to the state of sodium balance since a shift in sodium intake of as little as 20 mEq (e.g. 10 to 30 mEq/day) induced a significant change in basal plasma aldosterone. Adrenal responsiveness to infused angiotensin varied inversely with the log of urinary sodium excretion over the entire range of sodium intake, indicating that there is a gradual modulation of adrenal responsiveness with changes in dietary sodium. In addition, despite a constant dietary potassium intake, serum potassium gradually fell from 4.55 +/- .06 to 3.98 +/- .07 mEq/l, (P less than 0.02), as sodium intake increased, partly due to increased urinary potassium excretion (mean cumulative potassium loss of 129 mEq). Thus, changes in potassium balance may be important in modulating the sodium-associated variation in aldosterone secretion.

Endogenous angiotensin-aldosterone-pressure relationships during sodium restriction

The effects of moderate restriction of dietary sodium and potassium supplementation on plasma levels of renin, angiotensin II, aldosterone, and cortisol and on arterial pressure were studied in 12 patients with mild essential hypertension. To define hormone-blood pressure relationships, venous hormone levels were measured hourly and intra-arterial pressure continuously for 24 hours after 4 to 6 weeks of sodium restriction, 4 to 6 weeks of potassium supplementation, and a similar period of control diet. Our results show that compared with the control diet, moderate sodium restriction was associated with increased levels of aldosterone but no overall change in renin, angiotensin II, or cortisol levels. Further, slopes of regression lines relating log renin and log angiotensin II to aldosterone were increased, as were log cortisol/aldosterone regression lines. On the contrary, regression lines of log renin and log angiotensin II versus arterial pressure were unaltered by sodium restriction. Hormone and blood pressure relationships were not changed by the potassium supplemented diet. Although confirmatory data are needed, our findings suggest that moderate sodium restriction enhances aldosterone responsiveness to endogenous angiotensin II and adrenocorticotropic hormone without diminishing the pressor activity of endogenous angiotensin II. These results may explain in part the disappointingly small hypotensive effect of modest sodium restriction in mild essential hypertension.

Effects of angiotensin II on arterial pressure, renin and aldosterone during exercise

To evaluate the effect of isotonic exercise on the response to angiotensin II, angiotensin II in saline solution was infused intravenously (7.5 ng X kg-1 X min-1) in seven normal sodium replete male volunteers before, during and after a graded uninterrupted exercise test on the bicycle ergometer until exhaustion. The subjects performed a similar exercise test on another day under randomized conditions when saline solution only was infused. At rest in recumbency angiotensin II infusion increased plasma angiotensin II from 17 to 162 pg X ml-1 (P less than 0.001). When the tests with and without angiotensin II are compared, the difference in plasma angiotensin II throughout the experiment ranged from 86 to 145 pg X ml-1. The difference in mean intra-arterial pressure averaged 17 mmHg at recumbent rest, 12 mmHg in the sitting position, 9 mmHg at 10% of peak work rate and declined progressively throughout the exercise test to become non-significant at the higher levels of activity. Plasma renin activity rose with increasing levels of activity but angiotensin II significantly reduced the increase. Plasma aldosterone, only measured at rest and at peak exercise, was higher during angiotensin II infusion; the difference in plasma aldosterone was significant at rest, but not at peak exercise. In conclusion, the exercise-induced elevation of angiotensin II does not appear to be an important factor in the increase of blood pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Primary aldosteronism: effects of inhibition of ACTH and potassium administration on plasma aldosterone concentration

The relative roles of ACTH, angiotensin and potassium in influencing aldosterone secretion in primary aldosteronism were assessed by direct or indirect means. In untreated patients with primary aldosteronism caused either by adrenal adenoma or hyperplasia plasma aldosterone and cortisol concentrations fluctuated in unison and dexamethasone reduced both hormones markedly. Only when renin-angiotensin system was greatly activated and plasma potassium normalized by medical treatment was dexamethasone less successful in lowering plasma aldosterone concentration. Potassium infusion of 10,20 and 30 mEq/hr in patients with adenoma failed to elicit any increase in plasma aldosterone concentration despite significant increases in plasma potassium levels. These results suggest that patients with primary aldosteronism due to adrenal adenoma are relatively more sensitive to small changes in plasma ACTH level than those in plasma angiotensin or potassium levels. In recumbent patients with adrenal hyperplasia ACTH also modulates plasma aldosterone concentration.