Plasma renin activity and angiotensin pressor dose in hypertension correlation and diagnostic implications

Altered adrenal sensitivity to angiotensin II in low-renin essential hypertension

Low-renin essential hypertension (LREH) describes a widely recognized classification validated by clinical features, including salt-sensitive blood pressure and diuretic responsiveness. Classic physiological teaching has cited normal plasma aldosterone concentration despite suppressed renin as evidence for adrenal supersensitivity to angiotensin II (Ang II). We studied 94 patients with LREH, 242 normal-renin hypertensives, and 135 normal subjects as controls. Low-renin hypertensives did not differ significantly from the other groups in either basal or Ang II-stimulated aldosterone concentrations on a high-sodium diet. Stimulated with a low-sodium diet, LREH patients demonstrated the smallest rise in basal aldosterone secretion. Ang II responsiveness was also subnormal: the rise in aldosterone after Ang II infusion in LREH (613+/-39 pmol/L), although greater than in nonmodulators (180+/-17 pmol/L; P=0.001), was less than either the patients with intact modulation (940+/-53 pmol/L; P=0.001) or normotensives (804+/-50 pmol/L; P<0.05). Blacks with LREH demonstrated an even lower response than low-renin whites ((388+/-50 versus 610+/-47 pmol/L; P=0.0001). In contrast, the rise in systolic blood pressure with Ang II infusion on a low-salt diet was greatest among LREH patients (P=0. 001). Patients with LREH and nonmodulators were equally salt-sensitive. These results indicate that the adrenal response in LREH is normal on a high-salt diet but becomes progressively more abnormal as sodium control mechanisms are stressed. The factors that mediate enhanced adrenal response to Ang II with sodium restriction may be defective, suggesting the existence of alternative physiological mechanisms for sodium homeostasis in the low-renin state.

Effects of sodium salts on pressor reactivity in salt-sensitive men

Blood pressure in patients with essential hypertension is raised by sodium chloride but not by nonchloride sodium salts. Although a high sodium chloride diet is known to augment the pressor response to norepinephrine and angiotensin II, the effect of nonchloride sodium salts on pressor responsiveness has not been studied so far. To examine whether sodium chloride and nonchloride sodium salts evoke different pressor responses to these agonists, we performed graded norepinephrine and angiotensin II infusions in salt-sensitive (n = 7) and salt-resistant (n = 8) normotensive subjects. The subjects were given a low salt diet (20 mmol/day) for 3 weeks, to which a supplement of 200 mmol sodium per day, provided as either sodium chloride or sodium citrate, or a placebo was added for 1 week each. We found that, although sodium chloride raised mean arterial blood pressure in the salt-sensitive subjects (p less than 0.005), sodium citrate did not. However, under both sodium salts pressor response to norepinephrine and angiotensin II was significantly greater than under placebo (p less than 0.02). Furthermore, with both sodium salts, pressor response in the salt-sensitive subjects was greater than in the salt-resistant subjects (p less than 0.01). This study thus demonstrates that, although blood pressure in salt-sensitive individuals is raised by sodium chloride only, both sodium chloride and sodium citrate evoke similar increases in pressor response to norepinephrine and angiotensin II. Since pressor response increased with both sodium salts but resting blood pressure increased only with sodium chloride, enhanced pressor responsiveness alone cannot account for the sodium chloride-induced rise in resting blood pressure.

Antihypertensive mechanism of diuretic treatment with chlorthalidone. Complementary roles of sympathetic axis and sodium

Twenty-three patients with untreated mild to moderate essential hypertension had on the average an abnormally increased cardiovascular pressor responsiveness to exogenous norepinephrine (NE), while plasma and urinary NE, exchangeable body sodium and blood volume were normal. An increased pressor responsiveness to angiotensin II in these patients was associated with a tendency for low plasma renin activity (PRA). Compared to placebo conditions, treatment with chlorthalidone, 100 mg/day, for 6 weeks significantly decreased blood pressure and exchangeable sodium in these hypertensive patients but not in ten normal subjects; blood volume and heart rate were unchanged in both groups. Chlorthalidone induced a marked increase in PRA, but only a mild increase in angiotensin II pressor dose. In contrast, the diuretic caused a greater increase in NE pressor dose than in plasma NE in the hypertensive group, thus improving the disturbed relationship between plasma NE and NE responsiveness in these patients. No significant modification of plasma NE and NE responsiveness occurred in diuretic-treated normal subjects. In addition to sodium and the renin-angiotensin system, the sympathetic regulatory axis seems to be involved in the antihypertensive mechanism of chlorthalidone. Thiazide-like diuretics may decrease blood pressure in essential hypertension in part by lowering an abnormally high cardiovascular NE responsiveness without causing an equivalent increase in circulating NE.

Correction of altered noradrenaline reactivity in essential hypertension by indapamide

Fourteen patients with untreated mild to moderate essential hypertension had, on average, an abnormally high cardiovascular reactivity to exogenous noradrenaline and angiotensin II, while plasma noradrenaline, renin activity, exchangeable body sodium, and blood volume were normal. Treatment with a low dose of indapamide (2.5 mg/day) for 6 weeks decreased blood pressure by 10% in these hypertensive patients but not in 13 normal control subjects. Plasma or blood volume and exchangeable sodium were not changed significantly; nevertheless, the latter, and body weight, tended to be decreased slightly. Though a mild reduction in extracellular sodium in both normal and hypertensive subjects appears possible, it may not fully explain per se the blood pressure-lowering effect of indapamide in essential hypertension. Indapamide induced a mild decrease in angiotensin II pressor responsiveness in normal or hypertensive subjects, but a possible depressor influence from this change was probably antagonized by a concomitant pronounced increase in plasma renin activity. In hypertensive patients, the abnormally high noradrenaline reactivity was corrected by indapamide without an accompanying increase in endogenous plasma noradrenaline levels. Indapamide-induced changes in blood pressure correlated with those in noradrenaline pressor dose. It was concluded, therefore, that indapamide may decrease blood pressure in essential hypertension at least in part by lowering an abnormally high cardiovascular noradrenaline reactivity without causing an equivalent increase in adrenergic nervous activity.

Pindolol and systolic time intervals in patients with hypertension

Two studies of systolic time intervals (STIs) in patients with mild to moderate hypertension (HBP) revealed that no mean change in systolic intervals occurred with pindolol therapy, although some patients had significant alterations in their STIs. Pindolol responders with normal pretreatment preejection period to left ventricular ejection time (PEP/LVET) ratios had a significant increase in this ratio following pindolol therapy, whereas those with abnormal pretreatment PEP/LVET ratios had improvement in this ratio on administration of the drug. Patients on propranolol showed no change in PEP/LVET ratio. Propranolol administration slowed heart rate and lengthened Q-S2, S1-S2, and LVET, however, without altering the Q-S2 and LVET index, indicating that the changes were caused by the effect of propranolol on the heart rate alone. Chlorthalidone in high doses significantly reduced the Q-S2 index and the S1-S2 index, indicating that these changes were not caused by alteration of the heart rate. The second study suggests that STIs may provide a predictive clue for clinical response to pindolol. Patients with normal cardiac function (group I) are more likely to respond to pindolol than are those with abnormal cardiac function (group II). Directionally opposite changes in STIs in the two subgroups suggest different mechanisms for changing cardiac function. Pindolol's dual role as a beta-blocking agent with intrinsic sympathomimetic activity is proposed as a possible explanation, beta-blocking effects predominating in group I and sympathomimetic activity balancing the beta effect in group II.

Antihypertensive mechanism of the diuretic muzolimine in mild renal failure. Roles of sodium and cardiovascular norepinephrine responsiveness

Eighteen patients with mild impairment of renal function (glomerular filtration rate 65 +/- 5 ml/min:m +/- SEM) and hypertension (168/105 +/- 6/3 mmHg) were shown on average to have abnormally increased cardiovascular pressor responsiveness to infused norepinephrine (NE; p less than 0.05), whereas plasma and urinary NE, exchangeable body sodium and blood-volume did not differ significantly from normal. A slightly increased pressor responsiveness to angiotensin II was associated with a tendency to low plasma renin activity (PRA). Compared to placebo conditions, treatment with the loop-diuretic muzolimine in a mean dose of 35 +/- 2 mg/day for six weeks decreased blood-pressure and exchangeable sodium (p less than 0.05), and NE pressor responsiveness was restored to normal values, whilst plasma and urinary NE were not significantly changed. This was consistent with improvement of the initially abnormal relationship between NE levels and NE responsiveness factors. In contrast, the pressor dose of angiotensin II and PRA were increased to an approximatively similar extent during muzolimine treatment. These observations suggest that removal of body sodium and a decrease in NE reactivity without an equivalent increase in sympathetic nervous activity may be important complementary factors in the antihypertensive mechanisms of diuretic treatment in patients with mild renal functional impairment.

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.

Exaggerated pressor responsiveness to norepinephrine in nonazotemic diabetes mellitus

Pressor responses to norepinephrine (NE) or angiotensin II (AII) were studied in 27 diabetic patients without heart or renal failure and in 27 normal subjects. Mean plasma or 24-hour urinary sodium, blood volume and preinfusion plasma NE levels were similar in diabetic and normal subjects; exchangeable sodium was higher (p less than 0.02) and preinfusion plasma renin activity (PRA) was slightly lower in diabetic patients. The NE pressor and threshold doses were lower in diabetic patients than in normal subjects (76 versus 141 and 16 versus 41 ng/kg/min, respectively; p less than 0.05). The AII pressor dose also tended to be lower in diabetic patients (7.2 versus 11.9 ng/kg/min; p less than 0.05), but the AII threshold dose did not differ between the two groups (1.1 versus 1.6 ng/kg/min). These findings were similar in the diabetic subgroup without or with retinopathy (N = 13 and 14, respectively) and in those with normal or high blood pressure (N = 17 and 10, respectively). These observations suggest that in nonazotemic diabetes mellitus increases in AII pressor responsiveness are associated with a concomitant reduction in PRA. However, cardiovascular pressor responsiveness to NE tends to be exaggerated despite normal plasma NE levels and this alteration may occur already in the normotensive stage of diabetes mellitus. Cardiovascular hyperresponsiveness in diabetic subjects may be related to excess body sodium or structural alterations in the vasculature, or both.

Correction of altered noradrenaline reactivity in essential hypertension by indapamide

Fourteen patients with untreated mild to moderate essential hypertension had on average an abnormally high cardiovascular reactivity to exogenous noradrenaline and angiotension II, while plasma noradrenaline, renin activity, exchangeable body sodium, and blood volume were normal. Treatment with a low dose of indapamide (2.5 mg/day) for six weeks decreased blood pressure by 10% in these hypertensive patients but not in 13 normal control subjects. Plasma or blood volume and exchangeable sodium were not changed significantly; nevertheless, the latter, and body weight, tended to be decreased slightly. Though a mild reduction in extracellular sodium in both normal and hypertensive subjects appears possible, it may not per se fully explain indapamide's blood pressure-lowering effect in essential hypertension. Indapamide induced a mild decrease in angiotensin II pressor responsiveness in normal or hypertensive subjects, but a possible depressor influence from this change was probably antagonised by a concomitant pronounced increase in plasma renin activity. In hypertensive patients, the abnormally high noradrenaline reactivity was corrected by indapamide without an accompanying increase in endogenous plasma noradrenaline levels. Indapamide-induced changes in blood pressure correlated with those in noradrenaline pressor dose. It was concluded, therefore, that indapamide may decrease blood pressure in essential hypertension at least in part by lowering an abnormally high cardiovascular noradrenaline reactivity without causing an equivalent increase in adrenergic nervous activity.

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.

Pressor factors and cardiovascular pressor responsiveness in borderline hypertension

The role of various pressor factors and cardiovascular responsiveness to norepinephrine or angiotensin II in the pathogenesis of borderline hypertension was evaluated. Exchangeable body sodium, blood volume, plasma renin activity, norepinephrine or dopamine levels, and norepinephrine or epinephrine excretion rates were similar between 24 patients with borderline hypertension (mean age 34 +/- 4 (SEM) years and 22 normal subjects matched for age; the patients had a slight increase in supine plasma epinephrine. Pressor doses of norepinephrine or angiotensin II were significantly lower (p less than 0.01 and 0.001, respectively) in the borderline hypertensive group. These findings suggest that borderline hypertension may be maintained by inappropriately increased cardiovascular response to norepinephrine and angiotensin II in the presence of normal sympathetic and renin activity and a normal body sodium-volume state.

Age-rated profile of cardiovascular reactivity to norepinephrine and angiotensin II in normal and hypertensive man

The interrelationships among age, cardiovascular pressor reactivity to intravenously infused norepinephrine (NE) or angiotensin II, and endogenous plasma NE or renin (PRA) levels were evaluated i 31 normal subjects and 37 patients with essential hypertension. In normal subjects both angiotensin II pressor dose and PRA decreased progressively with aging. Angiotensin pressor dose correlated positively with PRA (r = 0.41, P < 0.025) and inversely with age (r = -0.46, P < 0.02). NE pressor dose and basal plasma NE were also positively correlated (r = 0.53, P < 0.005), but the two factors remained largely unchanged with aging. Findings in essential hypertension differed in certain aspects. Angiotensin II pressor dose did not correlate with either basal PRA or age; and pressor doses of NE and angiotensin II tended to be lower in some patients than in normal subjects. These findings indicate that aging is accompanied by a physiologic increase in cardiovascular reactivity to angiotensin II, probably due to a concomitant decrease in circulating renin. The dissociation between angiotensin pressor dose and PRA in essential hypertension suggests an interference from an other factor.

Recent pathogenic aspects in essential hypertension and hypertension associated with diabetes mellitus

The sympathetic system, the body sodium-fluid volume state, the renin-angiotensin system, functional and structural characteristics of the heart and blood vessels, and some other components are important complementary factors in blood pressure regulation. A deviation from the normal equilibrium among these components, with a persisting non-physiologic increase in pressor factor(s) or in the basal vascular tone and/or cardiovascular reactivity to pressor factors, leads to hypertension. This review discusses recent observations and concepts regarding the pathogenesis of essential hypertension and hypertension associated with diabetes mellitus. It focuses on the roles of various pressor factors as well as cardiovascular pressor responsiveness in the genesis of high blood pressure and in the antihypertensive mechanism of diuretic treatment.

Sympathetic nervous system and blood-pressure control in essential hypertension

In normotensive subjects an inverse correlation was observed between an index of sympathetic nervous activity (the plasma-noradrenaline concentration during physical exercise) and reactivity to exogenous noradrenaline. This relationship was invariably disturbed in age-matched patients with essential hypertension. Multiple-regression analysis revealed a highly significant correlation between the combination of both factors and the height of mean arterial blood-pressure (r=0.91). The findings suggest that sympathetic nervous activity and pressor response to noradrenaline together form an important determinant of the arterial blood-pressure level. An inverse relationship could be demonstrated between plasma-renin concentration and pressor response to angiotensin II in normotensives, and this relationship was unchanged in hypertensive patients. Therefore angiotensin II does not appear to contribute directly to high blood-pressure.

The pressor response to angiotensin II in patients with low renin essential hypertension

The etiology of low renin essential hypertension (LREH) has not been established with certainty, but mineralocorticoid excess has been implicated frequently in its pathogenesis. The finding of several investigators of a normal exchangeable sodium space and extracellular fluid volume, however, does not support this hypothesis. To evaluate the possible role of sodium and water retention in LREH, the pressor response to infused angiotensin II (A II) was determined and compared to that of normal subjects and that of subjects with normal renin essential hypertension (NREH). This approach was based on the known suprasensitivity of vascular receptors to A II in situations in which sodium and water compartments are expanded as they are, for example, in proven hypermineralocorticoid states such as primary aldosteronism. In this study, we found that subjects with LREH demonstrated no increased pressor response to graded doses of A II; this suggests that LREH is not primarily mediated by sodium and water retention.

Postural changes in plasma renin activity and responses to vasoactive drugs in a case of Shy-Drager syndrome

A male aged 47 years with gross autonomic insufficiency as part of the Shy-Drager syndrome is described. He did not sweat normally when warmed, and his circulatory responses to mental arithmetic, the Valsalva manoeuvre, and head-up tilt were abnormal indicating severe sympathetic failure. During head-up tilt there was a rise in plasma renin activity and plasma aldosterone. It is argued that plasma renin activity is not dependent on sympathetic nervous activity and may be mediated by renal baroreceptors. These rises may help sustain the blood pressure in such patients during repeated head-up tilts. Infusions of L-noradrenaline and angiotension produced greater hypertension, and injections of isoprenaline greater hypotension than in controls. Although it is difficult to exclude the possibility that one factor in this may be hypersensitivity of receptors in blood vessel walls, the principal factor is likely to be the absence of those baroreflexes of which the efferent pathways are in the sympathetic nervous system.

Enhanced aldosterone response to angiotensin II in human hypertension

Adrenal and vascular responsiveness to graded doses of angiotensin II (A II) were recorded for seven normal subjects and 12 patients with essential hypertension while in balance on an intake of 200 mEq sodium/100 mEq potassium. Patients with essential hypertension had been previously studied and known to have normal responses of plasma renin activity to sodium restriction and upright posture. A II was administered for 30 minutes at rates of 0.1, 0.3, 1, and 3 ng/kg per minute and plasma aldosterone responses were assessed 20 and 30 minutes later; blood pressure was monitored at intervals of 1 minute during infusion of A II at each rate. A significant increment in plasma aldosterone occurred at an infusion rate of 0.3 ng/kg per minute in patients with hypertension. This change was not seen until the infusion rate reached 1.0 ng/kg per minute in the normotensive control subjects. Even at an A II infusion rate of 1 ng/kg per minute, the increment in plasma aldosterone levels in normotensive subjects (4.2 +/- 0.6 ng/dl) was significantly less (P less than 0.001) than that in patients with essential hypertension (19 +/- 3 ng/dl). In both groups, a significant rise in mean arterial blood pressure occurred at an A II dose of 0.3 ng/kg per minute, but the pressor response of the hypertensive group was significantly greater at the highest infusion rate (3 ng/kg per minute) (P less than 0.05). Thus, enhanced adrenal and pressor responsiveness to infused A II was observed in the hypertensive subjects, suggesting a change in A II receptor affinity.

Vascular reactivity to norepinephrine and hemodynamic parameters in borderline hypertension

Pressor response to norepinephrine, cardiopulmonary blood volume, and hemodynamic parameters were studied in 41 borderline hypertensive patients in comparison with 42 permanent essential hypertensive patients and 28 normal subjects. Borderline hypertensive subjects had high cardiac index (p less than 0.0001), normal total peripheral resistance, and low total blood volume (p less than 0.005). The ratio between cardiopulmonary blood volume (CPBV) and total blood volume (TBV) was significantly higher in comparison with normal subjects (p less than 0.01) and permanent hypertensive subjects subjects (p less than 0.0001). The pressor dose of norepinephrine was elevated (p less than 0.0001) and was directly correlated with the basal values of the cardiac output (p less than 0.005), the cardiopulmonary blood volume (p less than 0.001), and the CPBV/TBV ratio (p less than 0.01). None of these results was observed in permanent hypertensive subjects: the only significant result was a negative correlation between the pressor dose of norepinephrine and the basal diastolic arterial pressure (p less than 0.0001). This study provides evidence that the cardiac output elevation in borderline hypertensive subjects was related to increased venous return and enhanced sympathetic venous tone.

Plasma renin activity in patients with coarctation of the aorta. A comment of the pathogenesis prestenotic hypertension

In 10 patients with coarctation of the aorta, plasma renin activity was measured after recumbency and orthostatism in peripheral venous blood and, in some cases, in renal venous blood also. In nine of these patients, basic plasma renin values and those obtained under stimulatory conditions lay within normal range, and only in one patient were the values elevated. However, since this patient displayed the lowest gradient of mean arterial blood pressure proximal and distal to the stenosis, it can be regarded as improbable that the elevation in plasma renin activity was caused by the coarctation. Plasma renin activity in the renal venous blood of both kidneys, which was measured in three patients, showed no elevation.

On the basis of observations of other authors and of our own studies it can be concluded that the kidneys can contribute to the development of prestenotic hypertension in acute constriction of the aorta in experiments with animals, but in chronic coarctation in animals or in man the humoral or renal theory has no importance to the pathogenesis of hypertension.