Sensitization of the Adrenal Cortex to Angiotensin II in Sodium-Deplete Man

Cardiovascular and adrenal sensitivity to angiotensin II in essential hypertension

Regulation of aldosterone secretion by sodium chloride is impaired in a group of essential hypertensives: high-salt diet fails to suppress aldosterone in these patients despite low renin values. The mechanism of this impaired regulation of aldosterone has not been clarified so far. We tested the sensitivity of aldosterone secretion and blood pressure to A II in 20 normotensive controls (aged 20-60, MAP 92 +/- 3 mm Hg), in ten normotensives with one or two parents with hypertension, and in 21 patients with essential hypertension (aged 17-65, MAP 119 +/- 4 mm Hg). After a period of 6 days on high-salt intake (300-320 mEq Na+/day), A II (0.1, 0.5, 1.0 and 2.0 ng/kg/min) was infused, each concentration for 30 min. According to aldosterone excretion during sodium loading, patients were divided into group A with complete suppression (n = 12, aldosterone excretion 3.6 +/- 0.4 microgram/day) and in group B with insufficient suppression (n = 9, aldosterone excretion 15.5 +/- 2.3 micrograms/day). Despite similarly low plasma renins, rise of serum aldosterone levels during A II infusion was significantly higher in group B patients than in group A patients and normotensive controls. Rise in mean arterial blood pressure, however, brought about by graded A II infusion was similar in both groups of hypertensives and in normotensive controls. The results demonstrate an increased adrenal sensitivity to A II in a subgroup of essential hypertensives only. A similar adrenal hypersensitivity to A II found by others in patients with hyperaldosteronism due to adrenal hyperplasia supports the hypothesis that the same mechanism underlies both disorders.

Multiplicative interaction between angiotensin II and K concentration in stimulation of aldosterone

The interaction of angiotensin II and plasma K concentration in stimulating aldosterone secretion was studied in a group of six dogs by determining the aldosterone response to three levels of angiotensin II while the dogs were maintained on three levels of K intake. The levels of angiotensin were 1) the endogenous level, 2) the concentration resulting from infusion of 5 ng X kg-1 X min-1, and 3) the concentration resulting from infusion of 10 ng X kg-1 X min-1. Each level was maintained for 5 days. The three rates of K intake were 10, 100, and 200 meq/day, each maintained for 3 wk. Data were analyzed from days 1, 2, and 5 of the angiotensin infusion periods. The regressions obtained from plotting plasma K (PK) versus aldosterone concentration on day 5 of each level of infusion were 1) for no infusion (endogenous PRA = 0.4 to 0.5 ng ANG I X ml-1 X h-1), aldosterone = 5.04 X PK - 16.56; 2) for 5 ng X kg-1 X min-1 infusion, aldosterone = 12.20 X PK - 39.09; and 3) for 10 ng X kg-1 X min-1 infusion, aldosterone = 35.50 X Pk - 119.31. Each regression was significantly different (P less than 0.001) from the other two. The plasma K axis intercepts, which are the points at which aldosterone secretion is zero, were 3.29, 3.20, and 3.36 for the regression from the 0, 5, and 10 ng X kg-1 X min-1 infusion rates, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Clinical syndromes associated with disorders of renal tubular chloride transport: excess and deficiency of a circulating factor?

Two contrasting patients are described, one with pseudo-Bartter's syndrome induced by frusemide abuse and the other a case of hyporeninaemic hypoaldosteronism. The clinical and biochemical features of these two conditions are the opposite of each other and, in the first patient, the effects of frusemide were antagonised by treatment with indomethacin while in the second frusemide itself corrected the syndrome. The decreased pressor sensitivity to infused angiotensin II seen in the patient with pseudo-Bartter's syndrome was corrected with indomethacin and the enhanced pressor sensitivity seen in hyporeninaemic hypoaldosteronism was reversed with frusemide. Frusemide, an agent which blocks chloride transport at the ascending limb of Henle's loop, was respectively thus the cause and the cure of these conditions. On the basis of this the suggestion is made that Bartter's syndrome and hyporeninaemic hypoaldosteronism represent respectively an excess and a deficiency of a circulating factor similar to frusemide capable of blocking renal tubular chloride transport.

Studies on acute glucose-induced aldosterone suppression: role of renin-angiotensin system

Glucose loading is known to cause acute suppression of plasma aldosterone and stimulation of plasma renin activity. The relative contribution of variations in circulating angiotensin II to the regulation of aldosterone secretion following glucose loading was assessed in ten normal subjects. The effects of a standard oral glucose loading test (100 g) on plasma concentrations of glucose, insulin, potassium, aldosterone, renin activity and cortisol were studied (a) under basal conditions, and (b) after inhibition of angiotensin II with the converting enzyme inhibitor captopril (50 mg t.i.d. during 3 days). Under basal conditions the acute increase in plasma glucose and insulin after glucose loading was accompanied by a significant decrease (P less than 0.01) in plasma cortisol and aldosterone and by a significant increase in plasma renin activity (P less than 0.01); plasma potassium was decreased slightly but not significantly. Following captopril treatment preloading plasma renin activity was increased significantly, most probably reflecting an effective reduction of angiotensin II. Glucose loading caused a similar suppression of plasma aldosterone, as observed under basal conditions. This observation suggests that renin activation does not substantially contribute to the acute regulation of plasma aldosterone after an oral glucose load.

Converting-enzyme inhibition corrects the altered adrenal response to angiotensin II in essential hypertension

Of patients with essential hypertension, 30% to 50% do not modulate adrenal and renovascular responsiveness to angiotensin II (AII) with changes in sodium intake. To define the role of AII in mediating these altered responses, the adrenal and renal vascular responses to AII infusion (0.3, 1.0, 3.0 ng/kg/min) were assessed on a sodium-restricted intake in 31 patients with essential hypertension and 13 normotensive controls before and after 72 hours of converting-enzyme inhibition. Forty percent of the hypertensive patients had a subnormal adrenal response to AII. There were no differences between the normal and abnormal responding hypertensive patients in a number of clinical and biochemical factors except that the "abnormal responders" had a significantly (p less than 0.03) greater control AII level (37 +/- 3 vs 29 +/- 3 pg/ml) and lower control plasma aldosterone level (14 +/- 2 vs 22 +/- 3 ng/dl) than the "normal responders." When a converting-enzyme inhibitor was administered, no change in adrenal responsiveness to AII occurred in the normotensive controls or the hypertensive normal responders. In the hypertensive abnormal responders, both the threshold sensitivity and the entire dose response curve was significantly (p less than 0.01) enhanced following short-term converting-enzyme inhibition. This increased sensitivity could not be explained by differences in AII increment with AII infusions, in basal aldosterone levels, or in blood pressure or basal AII response to converting-enzyme inhibition. Since they occurred whether captopril or enalapril (MK 421) were used, this phenomenon is likely to be a specific effect of converting-enzyme inhibition.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of aldosterone secretion during altered sodium intake

The interactions of the renin-angiotensin system with other factors in the regulation of aldosterone secretion were analyzed during altered sodium in the rat. During sodium restriction, the rise in aldosterone one secretion was accompanied by trophic changes in the adrenal glomerulosa zone including increased angiotensin II receptors and enzymes of early and late steps in the aldosterone biosynthetic pathway. All these effects of sodium restriction were reproduced by infusion of angiotensin II, and could be prevented by administration of the converting enzyme inhibitor, SQ 14,225. These findings indicate that the adrenal secretory and trophic responses to sodium restriction are mediated by angiotensin II. In hypophysectomized rats, the basal activities of the enzymes of the early aldosterone biosynthetic pathway were reduced, contributing to the blunted aldosterone responsiveness to sodium deficiency. However, sodium restriction for 6 days significantly increased adrenal glomerulosa angiotensin II receptors and enzymes of the early and late aldosterone biosynthetic pathway, indicating that the pituitary gland is not necessary for the adrenal effects of angiotensin II. In contrast to the prominent glomerulotropic actions of angiotensin II in rats on normal or low sodium intake, infusion of angiotensin II during high sodium intake did not increase blood aldosterone, angiotensin II receptors, or 18-hydroxylase activity, indicating that the trophic actions of the octapeptide are determined by the state of sodium balance. In recent studies, other factors including potassium, dopamine and somatostatin have been shown to potentiate or inhibit the actions of angiotensin II on the adrenal gland. The ability of such factors to influence the effects of angiotensin II could serve as a protective mechanism to modulate aldosterone responses to angiotensin II when elevations in the circulating level of the peptide occur in the absence of sodium deficiency.

Mechanisms in human renovascular hypertension

To clarify the pathophysiology of renovascular hypertension, we monitored intraarterial pressure continuously and measured hourly hormone levels for 24 hours under carefully controlled conditions in two hypertensive patients with unilateral renal artery occlusion. Comparison of the results with those obtained when the patients were normotensive 3 months after uninephrectomy indicated that, while the renin-angiotensin system played a central role in maintaining the hypertension, the sympathetic nervous system also contributed and, in addition, modulated short-term arterial pressure fluctuations. In the untreated state, the sympathetic regulation of renin secretion was heightened, and angiotensin II/aldosterone dose-responsiveness was augmented. It is suggested that these adaptive changes might serve to offset the tendency to severe sodium depletion and thence exacerbation of the hypertension.

The renin-angiotensin system and total body sodium and potassium in hypertensive women taking oestrogen-progestagen oral contraceptives

Measurements of total body sodium and potassium, and of components of the renin-angiotensin-aldosterone system, were made in a group of women who developed hypertension while taking oestrogen-progestagen oral contraceptives. The results were compared with similar measurements made in age-matched women with essential hypertension. Total body sodium and potassium were normal in both groups. Plasma renin-substrate was significantly elevated in the women taking oral contraceptives, while concentrations of active renin were similar and normal in both groups. Thus plasma angiotensin II was significantly elevated in the pill users; overall the product of renin and renin substrate concentrations correlated significantly with angiotensin II. The rise in plasma angiotensin II in conjunction with normal total body sodium could therefore contribute to the increase in blood pressure induced by oestrogen-progestagen oral contraceptives.

Angiotensin II receptors on human platelets

The investigation of the interaction between angiotensin II and its receptors in human subjects has been hampered by the inaccessibility of human tissue containing angiotensin II receptors. In order to find a more accessible angiotensin II-binding tissue, we studied angiotensin II binding to platelets in normal human volunteers. Platelet preparations purified on Ficoll: Isopaque gradients were incubated with 125I-angiotensin II (30 pm), with and without unlabeled angiotensin at 22 degrees C, separating bound from free hormone by microcentrifugation. Binding was linearly related to the number of platelets incubated, and, at 8 X 10(5) cells/microliters, specific binding ranged from 0.8 to 10%. Scatchard analysis indicated a binding site with a Kd of 2.4 +/- 0.3 X 10(-10) m which agreed well with the Kd by displacement analysis (3.1 X 10(-10) m). The relative binding potencies for angiotensin II and analogues were: angiotensin II = des-Asp1 an angiotensin II greater than [Sar1, Ala8] angiotensin II greater than des-Asp1-[Ile8] angiotensin II greater than angiotensin I. The effect of high and low sodium (Na) intake (200 vs. 10 mEq/day) on platelet angiotensin II binding was studied in nine subjects. Compared to low Na, high Na intake produced an 80% increase in the angiotensin II-binding capacity (P less than 0.01) with no significant change in binding affinity. We conclude that human platelets possess angiotensin receptors whose binding characteristics and modulation by dietary sodium resemble the properties of the receptors on "classical" animal angiotensin target tissues. The platelet may provide an accessible source of angiotensin receptors for a detailed study of angiotensin-receptor interaction in human tissue.

Low dose captopril in the treatment of severe refractory hypertension associated with renal failure

Six patients with severe refractory hypertension and chronic renal failure were treated with a low dose of captopril (mean daily dose 75 mg) in combination with dietary sodium restriction, frusemide and either metoprolol, labetalol or prazosin. Sustained control of blood pressure was achieved in all six patients. Adverse effects noted were severe hyperkalaemia (2 patients), skin rashes (2 patients) and taste disturbance (1 patient).

Renal vascular tachyphylaxis to angiotensin II: specificity of the response for angiotensin

Hypotheses concerning angiotensin's role in states characterized by severe and sustained renal vasoconstriction, must account for the poorly sustained renal response to this agent in healthy animals and man. To assess the specificity of renal vascular tachyphylaxis to angiotensin II (AII), renal blood flow was measured with an electromagnetic flowmeter in eight anesthetized dogs. Bolus injections of AII and norepinephrine into the renal artery were adjusted to produce at least a 50% reduction in renal blood flow, and were followed by a continuous infusion of AII sufficient to reduce renal blood flow acutely by 60 +/- 10%. The response to the continuous infusion was poorly sustained, blood flow returning to near baseline within 10 minutes: At this time the response to bolus administration of AII was lost, but the response to norepinephrine was sustained. At 30 minutes the response to norepinephrine was also reduced, and there was no response in three of the eight dogs. After stopping the AII infusion, renal vascular responsiveness to norepinephrine returned almost immediately, but 30-60 minutes were required before responsiveness to AII was restored. We conclude that there is true, specific renal vascular tachyphylaxis to AII--which may well reflect receptor modulation or occupation--and that with time an additional, non-specific vasodilator mechanism can come into play.

Aldosterone responsiveness to angiotensin II after sodium restriction in subjects with low renin essential hypertension

Plasma aldosterone (PA) responses to sodium restriction (25 mEq sodium/day for 4 days) and to graded angiotensin II (AII) infusions (2, 4 and 8 ng/kg/min each for 30 min) during a low sodium intake were studied in 14 subjects with low renin essential hypertension (LREH) versus 16 normotensive subjects. The PA response to sodium restriction in relation to changes in plasma renin activity (PRA) was estimated by the ratio of PA increment to PRA increment after sodium restriction (delta PA/delta PRA). In 8 of 14 LREH subjects, whose delta PA/delta PRA ratios were normal, the PA responses to the graded AII doses were similar to those in the normotensive subjects. However, in the remaining 6 LREH subjects whose delta PA/delta PRA ratios were high the PA responses to the graded AII doses were greater. Apparently some LREH subjects, whose delta PA/delta PRA ratios after sodium restriction were high, have an abnormally enhanced aldosterone responsiveness to AII under the condition of low sodium intake.

Simplified screening procedures for primary aldosteronism. Studies on the mechanism of the hyper-responsiveness to furosemide and standing

Screening tests for primary aldosteronism were compared in 22 patients with this disorder and 140 hypertensive controls. Adequate (93-100%) sensitivity and specificity were not provided by single tests ((1) serum K, (2) furosemide-stimulated PRA, or (3) plasma aldosterone concentration (PAC) after furosemide, or (4) after 2 liters 0.9% NaCl)), but were provided by combinations: (2) + (1) or (3); (2) + (1) or (4); and by a pressor response to saralasin + (3) or (4). Mechanism of the observed excessive rise in PAC "stimulated" by furosemide and standing for 2 h was studied in the same patients. The rise in "stimulated" PAC was (a) associated with a low PRA and not reproducible by angiotensin II infusions; (b) associated with a slight but significant rise in plasma cortisol and was reproducible by ACTH infusions. It is, therefore, attributed to the effects of slight ACTH release acting on adrenal tissue with super-sensitivity of aldosterone response to ACTH.

Plasma aldosterone response to angiotensin II in sodium-restricted elderly subjects with essential hypertension

The plasma aldosterone (PA) response to sodium restriction (25 mEq daily for 4 days) and to graded infusions of angiotensin II (AII, 2, 4 and 8 ng/kg/min each for 30 min) during a low-sodium intake were studied in 15 elderly subjects with mild essential hypertension versus 10 elderly normotensive subjects. The PA response to sodium restriction relative to changes in plasma renin activity (PRA) was estimated by the ratio of PA increment to PRA increment after sodium restriction (delta PA/delta PRA). THe PA response to graded AII infusions was determined by the increment of PA above the basal level after each dose of AII. In 10 of the 15 elderly hypertensive subjects whose PRAs responded normally to sodium restriction, the delta PA/delta PRA ratios and PA increments during the graded AII infusions were similar to those in the elderly normotensive subjects. However, in the remaining 5 elderly hypertensive subjects whose PRAs responded subnormally to sodium restriction, the delta PA/delta PRA ratios were high and the PA increments greater during the graded AII infusions. THe increments of mean blood pressure during the graded AII infusions were similar in the foregoing 10 of 15 hypertensive subjects, and significantly greater during the AII infusion rates of 4 and 8 ng/kg/min in the remaining 5 hypertensive subjects when compared with those in the normotensive subjects. Apparently some subjects with essential hypertension, whose PRAs response subnormally to sodium restriction, have an abnormally enhanced adrenal responsiveness to AII under the conditions of low-sodium intake.

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

This study was performed to evaluate the hypothesis that adrenal aldosterone output is modulated by daily potassium intake in man. Eight normotensive subjects, aged 24-38 yr, were fed 3 separate 150 mEq sodium-containing diets. Potassium content ranged from 300 mEq daily during the first week, to 80 mEq daily and 10 mEq daily for the subsequent 2 wk. On days 4 and 5 of each diet each subject took oral dexamethasone 0.5 mg B.I.D. to prevent endogenous ACTH secretion. On day 6 an infusion of angiotension II (AII) was administered in doses of 0.1, 0.3, 1.0, 3.0 and 10.0 ng/kg/min, each lasting 30 min. Blood pressure, pulse, and plasma aldosterone and potassium were measured throughout the infusions. All at infusion rates of 3 and 10 ng/kg/min, produced significant increases in plasma aldosterone values during liberal potassium intake but not when potassium intake was 10 mEq/day. We conclude that dietary potassium in man modulates All-mediated aldosterone secretion.

Response of aldosterone and 18-hydroxycorticosterone to angiotensin II in normal subjects and patients with essential hypertension, Conn's syndrome, and nontumorous hyperaldosteronism

Dose-response curves relating plasma angiotensin II (AII) concentration during AII infusion to blood pressure (BP), to plasma aldosterone, and to plasma 18-hydroxycorticosterone were compared in normal subjects and in patients with essential hypertension, Conn's syndrome, and nontumorous hyperaldosteronism. The BP response was steeper than normal in patients with Conn's syndrome and essential hypertension. Before infusion, mean plasma aldosterone concentration was approximately four-fold higher in Conn's syndrome than in the normal group, while that of 18-hydroxycorticosterone was ninefold higher. Neither increased significantly during AII infusion. In essential hypertension, both corticosteroids were within the normal range, but their responses to AII infusion were greater than normal. In the three subjects with non-tumorous hyperaldosteronism, plasma aldosterone and 18-hydroxycorticosterone concentrations were raised, and their responses to AII infusion resembled those found in essential hypertension and were different from those found in Conn's syndrome. This suggests that nontumorous hyperaldosteronism is not a variant of Conn's syndrome. In the response to AII and in other ways, it is indistinguishable from essential hypertension.

Studies of diurnal changes in plasma renin activity, and plasma noradrenaline, aldosterone and cortisol concentrations in man

Diurnal studies were performed on ten normal volunteers taking a normal sodium diet. Half-hourly blood samples were taken throughout 25 h and assayed for plasma renin activity (PRA) and the plasma concentrations of noradrenaline, aldosterone and cortisol. Sleep was recorded polygraphically and scored by standard criteria. Circadian rhythms were demonstrated for plasma cortisol, aldosterone and noradrenaline concentrations, but not for plasma renin activity. The nadir of the rhythm for the noradrenaline concentration appeared to be related to sleep itself rather than to any chronological index. Only PRA was effected by the stage of sleep, falling sharply during periods of REM sleep. Plasma cortisol and aldosterone concentrations showed a positive correlation over the 24 h. There was, however, no correlation between PRA and plasma aldosterone concentrations, except when the subjects arose after their night's recumbency. Plasma noradrenaline concentration did not correlate with the concentration of any of the other hormones measured.

Plasma aldosterone response to angiotensin II and potassium chloride infusions in hypopituitary patients

The acute adrenal response to potassium chloride and angiotensin II (A II) infusions was studied in hypopituitary patients and compared with normal subjects. The peripheral plasma levels of aldosterone (PAC), cortisol (F), potassium (K), and plasma renin activity (PRA), were measured at 09.00 h and after 60 and 120 min of infusion. All subjects were recumbent ad under balance conditions, receiving a daily dietary intake of 180 mmol of sodium and 80 mmol of potassium. Basal PAC in hypopituitary patients was not significantly different from that observed in the control group. Both normal and hypopituitary patients respond to A II infusion with significant increments. Under potassium chloride stimulus the PAC response in hypopituitary patients was only observed when cortisol (20 mg) was given 2 h prior to the infusion. When cortisol replacement was omitted the response KCl was not detected. These results suggest a permissive role of cortisol on glomerulosa response to potassium.

Effect of propranolol on aldosterone response to heat exposure in sodium-restricted men

To assess the role of the renin-angiotensin system in the control of aldosterone reease in response to heat exposure, 6 sodium restricted subjects were studied on three random experimental days: a control day and two heat exposure days (46C, 35 mbar, 90 min) with and without propranolol. Plasma aldosterone, renin activity, ACTH and K+ were determined from plasma samples taken every 20 min from 08:00 to 14:00. After propranolol administration, plasma aldosterone responsiveness to heat exposure increased, though plasma renin activity was depressed. Concurrently, propranolol reduced heat tolerance, leading to an increased ACTH and cortisol release in 3 of the subjects. Plasma levels of K+ were not significantly different during both heat exposure days. The enhanced response of plasma aldosterone may in part be related to the concurrent rises in ACTH, but a similar sustained aldosterone response was observed in subjects without ACTH release. Except for a significantly lower heart rate, propranolol induced no changes in hemodynamic and thermal response to heat. Plasma volume, blood pressure, sodium excretion, mean skin and rectal temperature rises and body weight losses were not modified by prior administration of the drug. The dissociation between aldosterone and renin activity after propranolol administration suggests that the aldosterone response to heat exposure is not primarily mediated by changes in activity of the renin-angiotensin system. Propranolol may lead to a reduced metabolic clearance rate of aldosterone or increase the sensitivity of the adrenal cortex to concomitant changes in the known stimuli, but the involvement of an additional factor in aldosterone regulation during heat exposure cannot be excluded.

Increased 18-hydroxycorticosterone responses to frusemide in essential hypertension

Plasma concentrations of angiotensin II (AII), aldosterone, 18-hydroxycorticosterone and cortisol were measured in seven patients with benign essential hypertension and in seven age-matched control subjects before, and at frequent intervals for 2 h after the intravenous injection of frusemide (40 mg). In the normal subjects, significant increases in the plasma levels of AII, aldosterone and 18-hydroxycorticosterone were apparent from 15 min after diuretic administration. The integrated responses of each hormone to frusemide administration were calculated. Aldosterone and AII responses to the diuretic were closely related, although three hypertensive patients had normal integrated aldosterone responses despite subnormal increases in the plasma concentrations of AII. The integrated 18-hydroxycorticosterone responses were greater in the hypertensive (median 970 nmol.h h-(1).1-(1)) than in the normal subjects (median 180 nmol.h-(1).1-(1)), P<0.05. Some patients with a raised blood pressure appear to have an enhanced adrenal corticosteroid response to frusemide; this probably reflects an increased sensitivity to angiotensin II.

Vascular reactivity to angiotensin and noradrenaline in rats maintained on a sodium free diet or made hypertensive with desoxycorticosterone acetate and salt (DOCA/salt)

The pressor response to angiotensin II (ATII) and to noradrenaline (NA), as well as the response of vascular beds (hindquarter and kidney) isolated and perfused with Krebs' solution, and the contractions of strips of thoracic aortae, portal veins to the same agents were measured in animals and organs taken from rats maintained on a sodium free diet or made hypertensive with DOCA/salt and in several groups of controls. The myoptropic effects of ATII and of 5HT were compared in stomach fundi. The main purpose of the study was to find out how a reduction or an increase of total body sodium and the associated changes of renin production can influence the vascular response to angiotensin and to catecholamines. Reduction of sodium in the diet was accompanied by no changes or a decrease of the responses of isolated vascular beds and tissues to ATII and NA; the pressor effect of ATII was also reduced, while that of NA was definitely increased. Treatment with DOCA/salt and the resulting hypertension were accompanied by an increased vascular response to ATII in vivo, to ATII and NA in the isolated hindquarter, while the other preparations (the perfused kidney, the thoracic aorta and the stomach fundus) showed a decreased response specific for ATII (in the kidney and the aorta) and to both ATII and 5HT (in the stomach). The responses of the portal vein to ATII and NA were unchanged. These results are discussed in relation to the changes of renin production, occuring in the two experimental conditions, and with respect to the various mechanisms currently considered for explaining the changes of vascular reactivity in hypertension.

Angiotensin II and renal hypertension in dog, rat and man: effect of converting enzyme inhibition

The role of the renin-angiotensin system in the pathogenesis of one-clip, two-kidney hypertension has been studied in man, dog and rat. Particular attention has been paid to peripheral plasma concentrations of angiotensin II in different circumstances; angiotensin II infusion has been combined with radioimmunoassay to construct angiotensin II/blood pressure dose-response curves. The effect of converting enzyme inhibitors has been studied, precautions being taken to avoid obtaining falsely high values for plasma angiotensin II because of cross-reaction with angiotensin I in these circumstances. The initial phase of one-clip, two-kidney hypertension is attributable to the direct pressor effect of the immediate rise in plasma angiotensin II. Subsequently, plasma angiotensin II is relatively lower, although blood pressure remains high. This upward resetting of the plasma angiotensin II/blood pressure relationship can be mimicked by infusing angiotensin II chronically at low dose. After reconstruction of a stenosed renal artery, or excision of a post-stenotic kidney, the angiotensin II/blood pressure relationship returns slowly to normal. In this second phase of one-clip, two-kidney hypertension, the long-term administration of saralasin, or of converting enzyme inhibitor, can also return arterial pressure to normal; brief administration of these drugs is less effective or ineffective. The results are compatible with, although they do not conclusively establish, an important slow pressor action of the renin-angiotensin system in the second phase of one-clip, two-kidney hypertension. This provides a rational basis for the use of captopril clinically in this condition.

Hyponatraemic hypertensive syndrome with renal-artery occlusion corrected by captopril

Malignant hypertension with severe hyponatraemia, hypokalaemia, depletion of sodium and potassium, and elevated blood levels of renin, angiotensin I, angiotensin II, aldosterone, and arginine vasopressin developed in a woman with renal-artery occlusion. Plasma angiotensin II was disproportionately high in relation to exchangeable sodium. Captopril, by inhibiting conversion of angiotensin I to angiotensin II, further elevated the blood levels of renin and angiotensin I but corrected all other abnormalities. Unilateral nephrectomy was subsequently curative.

Disorders of chloriuretic hormone secretion

Experimental evidence supports the existence of a circulating substance, natriuretic hormone, which augments electrolyte excretion. Because such a hormone probably acts by inhibiting chloride reabsorption in the thick, ascending limb of the loop of Henle it would more accurately be called chloriuretic hormone. Chloriuretic hormone must have an action which resembles that of loop diuretics such as frusemide and ethacrynic acid. An excess of chloriuretic hormone could explain all the manifestations of Bartter's syndrome, whereas a deficiency could account for Gordon's syndrome. Hyporeninaemic hypoaldosteronism may develop in subjects who are unable to increase chloriuretic hormone concentrations appropriately in response to progressive impairment of renal function.

Blood clearance rates of angiotensin II and its metabolites in sheep: presence of immunoreactive fragments in arterial blood

1. The blood clearance rates of exogenous (des-1-Asp)-angiotensin II and (des-1-Asp-2-Arg)-angiotensin II calculated from the arterial blood level during steady state infusion in conscious sodium-replete sheep were found to be 125 litre/h (s.d. = 27, n = 18) and 188 litre/h (s.d. = 23, n = 18), respectively. These blood clearance rates were in the same order as values for angiotensin II previously reported. 2. Using descending paper chromatography, immunoreactive fragments of angiotensin II in arterial blood extracts were adequately separated from angiotensin II. Quantitative determination of angiotensin II by direct immunoassay on whole blood extract using recovery tracer to correct for losses during extraction were on the average 31% (n = 9) higher than those obtained after the same blood extract had been subjected to chromatographic separation. 3. The blood clearance rates of exogenous 1-Asp-5-Ile-angiotensin II calculated from infusion rate/blood angiotensin II concentration before and after chromatographic separation were significantly different, being 103 litre/h (s.d. = 16, n = 18) and 134 litre/h (s.d. = 25, n = 9), respectively (P less than 0.001;t-test, 16 d.f.). 4. Our data demonstrate the presence of immunoreactive fragments of angiotensin II in arterial blood of sheep during steady-state infusion and indicate that direct immunoassay on arterial blood extracts without adequate separation may result in artificially higher values of angiotensin II in blood and consequently false value for its clearance rate.

Effect of the angiotensin II antagonist saralasin on plasma aldosterone concentration and on blood pressure before and during sodium depletion in normal subjects

The effect of the angiotensin II antagonist saralasin on plasma aldosterone, plasma angiotensin II and blood pressure was studied in six normal supine subjects both before and during sodium depletion. Before sodium depletion, infusion of saralasin produced no consistent changes; during sodium depletion, infusion of the angiotensin antagonist caused a fall in plasma aldosterone and an increase in plasma angiotensin II in each subject. It is concluded that angiotensin II plays a major part in stimulating aldosterone secretion during sodium depletion in man.

Suppression of plasma ACTH concentration by angiotensin II infusion in normal humans and in a subject with a steroid 17 alpha-hydroxylase defect

Six healthy subjects were infused with angiotensin II and plasma concentrations of angiotensin, ACTH and cortisol were measured before, during and after the infusion. In all cases the plasma ACTH concentration fell as plasma angiotensin increased and rose again, sometimes to higher than basal levels, when the angiotensin infusion was terminated. These effects were most marked at the highest rate of infusion (8 pmol/kg/min) and, at the lower rates (2 and 4 pmol/kg/min), there was some recovery of ACTH levels during the infusion period in some subjects. Plasma ACTH concentrations also fell when angiotensin was infused into a patient with high ACTH levels due to a steroid 17 alpha-hydroxylation defect. The inhibition of ACTH secretion is not due to a rise in plasma cortisol operating a negative feedback inhibition. It could be a direct effect of the infused angiotensin on the brain-hypothalamus-pituitary complex or an effect on the metabolism of ACTH.

Failure of renin suppression by angiotensin II in hypertension

Angiotensin II was infused at rates varying from 0.1 to 10 ng/kg per minute into 49 subjects with hypertension and 26 normotensive subjects and changes in blood pressure, plasma angiotensin II, and plasma renin activity (PRA) were determined after 20 and 30 minutes at each dose. Similar dose-related increases in angiotensin II and blood pressure occurred with a threshold of 1 ng/kg per minute in the normotensive and hypertensive subjects. Whereas angiotensin II induced a significant, dose-related decrement in renin activity in the normotensive subjects, with a threshold of 1.0 ng/kg per minute, no significant change in renin activity occurred in either the normal-renin or high-renin hypertensive subjects. In a separate study, nine normotensive and six hypertensive sodium-restricted subjects were given a converting enzyme inhibitor, SQ 20881, 30 microgram/kg. Despite a significantly greater fall in blood pressure (P less than 0.006) and angiotensin II concentration (P less than 0.045) in the hypertensive subjects, they did not have a greater rise in plasma renin activity. We conclude that angiotensin II reduces renin release in normal man at infusion rates that yield plasma angiotensin II levels within the physiological range but has a strikingly reduced influence on renin release in hypertension. In high-renin hypertension due to renal artery stenosis or nephrosclerosis, renin release is presumed to be relatively autonomous because of a dominant, intrarenal mechanism. The mechanism in normal-renin essential hypertension is not clear, but the abnormality could well be related to the pathogenesis of the hypertension.

Effect of prolonged low-dose infusions of ile5-angiotensin ii on blood pressure, aldosterone and electrolyte excretion in sodium replete man

Two healthy young males on a constant normal sodium diet (135 mM/day) were infused for 132 h with 3 ng/kg/min of angiotensin II. Plasma angiotensin II levels were therby raised to the range of moderate sodium depletion. Plasma aldosterone and the urinary excretion rate of aldosterone-18-glucuronide were markedly increased during the whole infusion period and returned to control levels after the infusion was stopped. A slight tendency of aldosterone secretion to decrease towards the end of infusion was probably due to sodium retention (appr. 200 mM and 350 mM respectively) and to a fall in plasma potassium by approximately 0.5 mM/1. Plasma aldosterone during infusion, maintained circadian variations similar to those of cortisol. Plasma cortisol patterns were unaffected by angiotensin II. Blood pressure increased gradually during angiotensin II infusion, reflecting changes in sodium balance. The results, differing from those of studies in dog and sheep, support the assumption that angiotensin II is an important regulator of aldosterone secretion in man rather than a merely permissive factor.

Effects of angiotensin II on steroid metabolism and hepatic blood flow in man

Metabolic clearance rates (MCR) of aldosterone, cortisol, 11-deoxycorticosterone (DOC), corticosterone, and progesterone were simultaneously measured by constant infusion in eight control subjects before and during angiotensin II infusion in subpressor (3 ng/min per kg) and pressor (22 ng/min per kg) doses. Plasma levels of aldosterone and cortisol, the heat-labile protein-bound fraction of aldosterone, and hepatic blood flow (HBF) (as estimated by the fractional clearance of indocyanine green) were determined concomitantly. Angiotensin II in a subpressor dose produced a significant decrease of the MCR of aldosterone (by 23%), cortisol (by 16%), DOC (by 26%), corticosterone (by 14%) and progesterone (by 33%). The pressor dose further decreased the respective MCR by 37%, 21%, 40%, 28%, and 42% of the baseline value. Plasma aldosterone levels rose by 317% with subpressor and by 434% with pressor doses. HBF decreased by 18% with subpressor and by 33% with pressor doses of angiotensin II. Furthermore, there were significant negative correlations between the MCR of each steroid and the respective values of the fractional clearance of indocyanine green. We conclude that angiotensin II, by its vasoconstrictive action on the splanchnic vascular bed, decreases the MCR of aldosterone, cortisol, DOC, corticosterone, and progesterone. This decrease has to be taken into account when considering the stimulatory effect of angiotensin II on various plasma steroid concentrations.

Aldosterone production during dietary sodium restriction and beta-adrenergic blockade

The role of the renin-angiotensin system in mediating the aldosterone response to sodium depletion was examined by administration of propranolol during dietary sodium restriction. The beta-adrenergic antagonist prevented the expected increase of plasma renin activity in response to sodium restriction in six of twelve studies. Plasma angiotensin II concentration failed to increase in four of five subjects in whom the renin response was abolished. Despite unchanged or decreased plasma renin activity and plasma angiotensin II concentration, plasma aldosterone concentration increased significantly in response to dietary sodium restriction. The increase in aldosterone production could not be attributed to changes in plasma sodium or potassium concentration or increased ACTH secretion. It is suggested that the aldosterone response to sodium restriction is mediated not only by increased plasma renin activity and angiotensin II concentration, but also another mechanism, possibly related to increased adrenal sensitivity to angiotensin during sodium depletion.