Inhibition of the effects of angiotensin II on adrenal steroid production by dietary sodium

Changing dietary sodium alters the chronic cardiovascular effects of losartan in rats

Introduction. We have previously demonstrated a profound hypotensive response to the angiotensin II type 1 (AT1)-receptor antagonist losartan in rats consuming a normal salt diet that is not seen in salt-loaded rats, presumably due to a suppression of the renin-angiotensin system (RAS) by high sodium levels.The purpose of the present study was to examine the cardiovascular effects of changing dietary sodium intake during chronic treatment with losartan.We hypothesised that during blockade of AT1-receptors by chronic losartan infusion, when renin levels would be elevated regardless of dietary sodium, changing diets from high to normal or normal to high salt would have no effect on mean arterial pressure (MAP).

Materials and methods. To test this hypothesis, groups of rats instrumented with radiotelemetry transducers for MAP monitoring and venous catheters for infusion were initially placed on either a 0.4% salt content diet, referred to as Losartan Normal diet — High salt diet (LosN-HI, n = 7), or a 4.0% salt content diet, referred to as U Losartan High salt diet — Normal diet (LosHI-N, n = 9). After a thee-day control period, infusion of losartan was begun in all rats (10 mg/kg/day in 7 ml/day isotonic saline i.v.).After 10 days, diets were switched between groups and data were collected for another 10 days, after which losartan infusion was terminated for a 10-day recovery period.

Results. At the start of losartan infusion MAP was observed to be similar between LosN-HI rats (101±2 mmHg) and LosHI-N rats (101±2 mmHg). By day seven of the first 10 day protocol, MAP in LosN-HI rats had fallen to 71±4 mmHg while decreasing to 90±2 mmHg in LosHI-N rats. Five days after switching diets, MAP in LosN-HI rats had risen back to 85±3 mmHg, while MAP in LosHI-N rats had fallen to 75±2 mmHg. Conclusions. These results do not support our hypothesis, suggesting that changing dietary sodium can alter the chronic hypotensive response to losartan regardless of the initial state of the RAS.

Effects of a 1 day fast on biohumoral variables associated with human circadian rhythmicity

1. The aim of the present study was to investigate the effects of a short (1 day) fast by testing biohumoral variables associated with the human circadian rhythm. 2. Fifteen clinically healthy male volunteers (32 +/- 8 years old) participated in the study. Subjects were fed a control diet for 7 days. The last day was a control day and the following 8th day was the fasting day. Each subject was asked to collect urine seven times over a 24 h period. Chemical and hormonal variables were measured in each fractionated urine specimen. The time- qualified urinary excretion rates were biometrically analysed using conventional and chronobiological methods. 3. During fasting, significant incremental changes were detected in the urinary excretion rates of potassium, aldosterone, 17-hydroxycorticosteroids and adrenaline and significant decremental changes were detected in the excretion rates of sodium, chloride, creatinine, urea nitrogen, uric acid, 17-ketosteroids, noradrenaline and dopamine. The circadian rhythmicity of the variables was well preserved and remained almost stable throughout the fasting phase. 4. Fasting affected the mean oscillatory levels and oscillatory amplitudes of variables, suggesting that nutrients may have played roles as tonic and phasic modulators on the mechanisms that physiologically regulate ircadian rhythmicity.

Aldosterone responses to angiotensin II in anorexia nervosa

Patients with anorexia nervosa (AN) tend to have renin-angiotensin-aldosterone (RAA) abnormalities caused by abnormal behaviors such as strict dieting, fasting, vigorous exercise, self-induced vomiting and abuse of laxatives and/or diuretics. Adrenal responsiveness to angiotensin II (A II) was studied in 13 AN patients before and after therapy and in 6 normal sex- and age-matched controls: adrenal responses to postural change (1 h of walking following 1 h in a supine position) and to exogenous A II injection (A II: 10 ng/kg/min intravenous infusion for 30 min). The 24-h urine sodium concentration was significantly lower in AN patients before therapy than after therapy. Plasma aldosterone secretory response to A II was significantly higher in AN patients before therapy in both postural change and exogenous A II injection tests compared with after therapy response and that of controls. On the other hand, there was no significant difference in adrenal response to postural change or to exogenous A II between AN patients after therapy and controls. In conclusion, increased A II sensitivity caused by chronic sodium deficiency in AN patients normalized over time as the patients recovered.

Dissociation in plasma renin and adrenal ANG II and aldosterone responses to sodium restriction in rats

In rats, plasma renin activity (PRA) increases sharply, reaching a plateau within hours of sodium restriction. Plasma aldosterone increases gradually, not reaching a plateau for 1-2 days. To determine whether this dissociation is secondary to the time needed to modify adrenal sensitivity to angiotensin II (ANG II) and to assess the role of locally produced ANG II in this process, rats were salt restricted for 0-120 h. Plasma hormone levels were assessed, adrenal ANG II was measured, and basal and ANG II (1 x 10(-8) M)-stimulated steroidogenesis were determined in vitro. Although PRA attained an elevated plateau within 8 h, plasma aldosterone did not peak until after 48 h of sodium depletion. The in vitro aldosterone sensitivity to exogenous ANG II was not apparent until rats had been salt restricted for 16 h. A plateau (4-fold increase above the ANG II response on high salt) was achieved between 24 and 48 h. Adrenal ANG II also exhibited a similar delayed response that correlates significantly with changes in aldosterone biosynthesis and late pathway activity. Thus the dissociation between PRA and plasma aldosterone may be secondary to a lag in the zona glomerulosa's (ZG) steroidogenic response to ANG II as well as a parallel lag in tissue ANG II production, suggesting that changes in tissue ANG II may mediate ZG sensitivity to ANG II during sodium deprivation.

Time course of enhanced adrenal responsiveness to angiotensin on a low salt diet

To assess the rate of activation of the renin-angiotensin-aldosterone axis and enhancement of adrenal responsiveness to angiotensin II (Ang II) with restriction of sodium intake, 16 healthy male subjects were placed initially on a 200 meq daily sodium intake; adrenal responsiveness to Ang II was assessed, and then daily sodium intake was reduced abruptly to 10 meq. Adrenal responses to Ang II were assessed again during the non-steady state interval 24 and 48 hours later, and after balance was achieved in 5-7 days. Renin-angiotensin system activation was evident within 24 hours after sodium intake was restricted. The increase in basal plasma aldosterone concentration and enhancement of the adrenal response to Ang II, on the other hand, tended to lag. Within 24 hours of restricting sodium intake, despite a significant increase in both plasma renin activity (1.0 +/- 0.2 vs. 2.4 +/- 0.7 ng/ml/hr, p less than 0.01) and Ang II concentration (22.0 +/- 1.9 vs. 29.5 +/- 1.3 pg/ml, p less than 0.05), there was no increase in basal plasma aldosterone concentration (10.4 +/- 1.3 vs. 11.7 +/- 1.2 ng/dl). At 48 hours, despite little further change in plasma renin activity or plasma Ang II concentration, there was a sharp increase in basal plasma aldosterone concentration (22.5 +/- 3.6 ng/dl, p less than 0.01). The adrenal response to Ang II was increased significantly at 24 hours, evident at only a 10 ng/kg/min dose, but showed progressive further enhancement with time.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of a mild and prolonged restriction in sodium or food intake on the circadian rhythm of aldosterone and related variables

The effect of a mild reduction in dietary sodium intake (-30 mEq/24 hr) and body weight (-2 kg/2 months) on circadian rhythms of urinary aldosterone (UA), sodium (UNa), potassium (UK), creatinine (UC) and volume (UV) have been investigated in nine clinically healthy subjects. The mild reduction in dietary sodium is associated with: (1) a decrease in the 24-hr excretion rate of UNa, UK and UV, and an increased mesor of UA and UC; (2) a lowered extent of the circadian variation for UNa, UK, UV and a greater amplitude for UA and UC (3) a later crest in the temporal phase for UK, UA, UC, an earlier phasic wave for UNa. The mild reduction in calorie intake resulting in a body weight loss is associated with a more pronounced decrease in the 24-hr excretion rate of UNa and UK, and in the extent of circadian fluctuation for UNa. Peculiar events are: (1) the decreased 24-hr excretion rate for UA, and the increased mesor for UV; (2) the extent variability increased for UV, decreased for UC. Such effect may have a practical resonance for heuristic physiology since the role of dietary sodium and food intake has been better clarified. Dietary sodium and food can be regarded as 'chronomodulatory agents' for the adrenal cortex since their adrenotropic influence is extended to the tonic as well as phasic secretion of aldosterone.

Mechanism of adrenal angiotensin II receptor changes after nephrectomy in rats

At 48 h after bilateral nephrectomy in rats there is a two- to threefold increase in the number of adrenal angiotensin II receptors and a decrease in Kd of smooth muscle angiotensin II receptors. These changes have been attributed to the absence of circulating angiotensin II. Serum K+, which increases after nephrectomy may be an important and overlooked modulator. Therefore, the present experiments were designed to assess the role of K+ as a regulator of angiotensin II receptors after nephrectomy. Serum K+ was controlled with Na polystyrene sulfonate (Kayexalate), a resin designed to exchange Na+ for K+ in the gastrointestinal tract. Acutely nephrectomized rats were divided into two groups: experimental animals received Kayexalate resin every 12 h for four doses, and controls received Kayexalate exchanged with KCl in vitro before gavage. There was a significant positive correlation serum K+ and aldosterone (r = 0.78, P less than 0.001). Kayexalate maintained a normal serum K+ of 5.9 +/- 0.2 meq/liter (n = 27), aldosterone 25 +/- 3 ng/dl (n = 27) and adrenal receptor concentration of 934 +/- 156 fmol/mg protein (n = 4). Control animals had significantly higher serum K+ of 10.5 +/- 0.4 meq/liter (n = 23), aldosterone 435 +/- 32 (n = 23), and adrenal receptors of 2726 +/- 235 fmol/mg protein (n = 4). There was a linear relationship between serum K+ and number of adrenal receptors (r = 0.87). No such relationship was present in uterine smooth muscle. Therefore, these studies demonstrate that K+ modulates the number of adrenal but not smooth muscle angiotensin II receptors after nephrectomy. This is the first evidence that potassium modulates angiotensin II receptors independently of changes in angiotensin II blood levels.

Hormonal regulation of membrane receptors and cell responsiveness: a review

Hormone receptors are those components of target-cells that specifically bind hormones and convey the hormonal message to the intracellular machinery. Such receptors can be localized inside the cell, such as the nuclear receptors of thyroid hormones and the nuclear and cytoplasmic receptors of steroid hormones, or on the outer surface of the plasma membrane, such as the membrane-bound receptors of polypeptide hormones and neurotransmitters. Extensive studies during recent years have shown that the interaction between hormone and membrane-bound receptor can affect the receptor characteristics in at least two ways. Firstly, receptor occupancy can modify, by way of cooperativity, the affinity of homologue receptors for the given hormone. Secondly, the binding capacity of a target cell appears to vary as a function of the preexposure of the cell to the hormone. The latter phenomenon has been related to the so-called states of subsensitivity, desensitization, or refractoriness, and might be responsible for the physiologic regulation of the target cell sensitivity and for the hormone resistance which accompanies various metabolic disorders. In this review we attempt to describe the major findings related to hormone desensitization or resistance of these hormones that have plasma-membrane-bound receptors. Data from the literature are presented independently for each hormone and when applicable, conflicting results are discussed in each section. The various theories which might explain hormone desensitization are outlined in the last section of this paper.

Metabolic effects of sodium restriction and thiazides in tetraplegic patients

The metabolic effects of sodium restriction, alone or with thiazide, were studied in 12 healthy subjects, in 24 tetraplegics during the intial 8 months of paralysis (early) and in 16 others during the subsequent period (late). The diuresis caused by both treatments led to more haemoconcentration in early than in late patients. In contrast with the healthy subjects on low sodium, the tetraplegics had a delayed urinary sodium retention and no fall in calciuria. During thiazide, urinary sodium depletion occurred early and the urine calcium fell after 3 days in all tetraplegics. During both treatments, aldosterone and renin increased more in early patients than in the other groups. The clinical implications of inducing dehydration and a sustained stimulation of the renin-angiotensin-aldosterone axis in recently injured tetraplegics with severe orthostatic hypotension are discussed.

Regulation of angiotensin II receptors in the rat adrenal cortex by dietary electrolytes

The binding affinity and concentration of specific angiotensin II receptor sites of rat adrenal cortical cells and homogenates were determined after 1 and 6 wk of altered sodium and potassium intake. Sodium deprivation caused marked increases in plasma renin, blood angiotensin II, and plasma aldosterone, and was accompanied by a significant increase (+74%) in the number of specific angiotensin II receptor sites per adrenal cortical cell. High potassium intake was followed by increased serum potassium and markedly elevated plasma aldosterone, with subnormal levels of renin and angiotensin II and a 170% increase in the number of angiotensin II receptors per cell after 1 wk. Sodium loading and potassium deprivation were followed by the opposite effect upon adrenal receptors, with reduction of the angiotensin II-binding capacity. None of the dietary electrolyte changes were accompanied by an ancrease in receptor affinity above the control value of 2 nM-1. A decrease in receptor affinity was noted after 6 wk of either low sodium or low potassium intake, when the renin and angiotensin II levels were increased by 104-129%. The adrenals of normal rats infused acutely with synthetic angiotensin II, or anesthetized with ether or sodium pentobarbital, which markedly increased plasma renin activity, contained fewer angiotensin receptors. These reductions in binding site concentration were not accompanied by changes in affinity and were attributed to occupancy by angiotensin II. These studies have demonstrated that chronic changes in sodium or potassium balance and acute changes in blood angiotensin II levels can exert modulating effects upon the adrenal content and/or affinity of specific receptor sites for angiotensin II.

Reciprocal influence of salt intake on adrenal glomerulosa and renal vascular responses to angiotensin II in normal man

The adrenal glomerulosa cell and the renal vasculature respond to similar arterial angiotensin II (A II) levels. We have assessed the effect of decreased sodium intake on their responses to A II in man. Studies were performed in 42 normal subjects in balance on a daily intake of 100 meq potassium and either 200 or 10 meq sodium/day. Renal blood flow was measured with (133)Xe and arterial A II, renin and aldosterone concentrations by radioimmunoassay. A II was infused intravenously (1, 3, or 10 ng/kg/min) for 40-60 min; 14 subjects received graded doses. The A II level increased linearly with dose and plateaued within 3 min; blood pressure and renal vascular resistance showed a similar time-course. Aldosterone rose within 10 and plateaued within 20 min. Dose-response relationships were established between the rate of A II infusion and the adrenal, the renal vascular, and pressor responses. Sodium restriction reduced the pressor (P < 0.01) and the renal vascular response (P < 0.01), but potentiated the adrenal response to A II (P < 0.01). An excellent correlation was found between the plasma A II and aldosterone levels, but the slope of their regression relationship on a high (y = 0.13x + 6) and low salt intake (y = 0.32x + 14) differed significantly (P < 0.0005). Thus, sodium intake reciprocally influences vascular and adrenal responses to A II: salt restriction blunts the vascular response and potentiates the adrenal's, a physiologically important influence in view of aldosterone's role in sodium conservation.

Angiotensin II vascular receptors: their avidity in relationship to sodium balance, the autonomic nervous system, and hypertension

During intravenous administration of varying doses of angiotensin II antibody to anesthetized rats, apparently specific vascular receptors were characterized. These receptors compete with administered antibody to bind circulating angiotensin. This competitive phenomenon was used to evaluate the affinity of these receptors for angiotensin. Apparent vascular receptor affinity was defined by the amount of antibody required to block the blood pressure response to exogenous angiotensin. It was found that this receptor affinity varies directly with sodium intake so that the amount of antibody required to block was eightfold greater in normal animals on a high sodium intake, as compared with those on a low sodium intake. Sodium dependence of receptors was also demonstrated in nephrectomized animals, in desoxycorticosterone (DOC)-treated rats, and in chronic renal hypertension. Thus the observed changes in receptor affinity were usually inversely related to measured endogenous angiotensin II levels. Ganglionic blockade increased antibody requirement eightfold. All of these changes were consistent, with no overlap observed in response of individual animals from different groups. These results may explain the variation in pressor activity of angiotensin associated with changes in salt balance and ganglionic blockade. In general, when sufficient antibody was injected to block the effect of exogenous angiotensin a blood pressure lowering effect was also observed. Two exceptions were the nephrectomized and the one-kidney renal hypertensive animals, in both of which antibody administration had no effect on blood pressure. Additional results suggest that changes in receptor affinity are involved in the pathogenesis of various types of experimental hypertensions because the amount of antibody required to block angiotensin was enhanced in renal (twofold), DOC (fourfold), and genetic (fourfold) hypertension. Accordingly, changes in the affinity of these receptors could be critically involved in normal blood pressure control and in various forms of experimental and clinical hypertension, even when circulating angiotensin II levels are normal.