Control of enzymatically inactive renin in man under various pathological conditions: implications for the interpretation of renin measurements in peripheral and renal venous plasma

1. Human plasma contains two types of renin: one is active in its native form (active renin), the other has renin-like activity after exposure to low pH (inactive renin). Reactions of acid-activated plasma renin and kidney renin with either homologous or heterologous substrate showed identical Km values.

2. Peripheral venous values for active and inactive renin in essential hypertension (n = 22), renovascular hypertension (n = 14), primary aldosteronism (n = 12), adrenal insufficiency (n = 6) and control subjects (n = 13) were directly correlated. But the percentage of renin that was active varied widely.

3. After bilateral nephrectomy in 12 patients both active and inactive plasma renin fell, but did not completely disappear. Estimates of half-life in two patients were 30–80 min for active renin and 150–165 min for inactive renin.

4. Renal vein to peripheral vein ratios of active and inactive renin in ten patients with essential hypertension (19 determinations) ranged from 0·96 to 1·60 and from 0·68 to 1·44 respectively with mean values (±sem) of 1·21 ± 0·04 and 1·06 ± 0·05.

5. The renal vein to peripheral vein ratio of active renin on the affected side in 13 out of 17 patients with renovascular hypertension was above the range found in essential hypertension. Six of them also had an elevated ratio of inactive renin on that side, which indicated renal release of this form of renin into the circulation. But, in contrast to the renal vein to peripheral vein ratio of active renin, the mean value of the ratio of inactive renin on the affected side was not significantly higher than on the contralateral side. The results suggest a renal mechanism not only for controlling the total quantity of circulating renin but also for modulating its degree of activation.

Plasma thyroxine, 3,3',5-triiodothyronine and 3,3',5'-triiodothyronine during beta-adrenergic blockade in hyperthyroidism

Plasma thyroxine (T4), 3,3',5-triiodothyronine (T3) and 3,3',5'-triiodothyronine (rT3) were measured in 16 patients with Graves' disease. Patients were studied under the following conditions: first without any treatment, then, during beta-adrenergic blockade with propranolol, and finally after euthyroidism had been attained by carbimazole. During propranolol T3/T4 ratio decreased, whereas T4 remained unchanged. After carbimazole T3/T4 ratio returned to its pretreatment value. rT3/T4 ratio showed opposite changes. These results suggest that peripheral conversion of T4 into T3 and rT3 in hyperthyroidism is, at least partly, dependent on the functional status of the beta-adrenergic system. Suppressed peripheral conversion of T4 into T3 during beta-adrenergic blocking agents may contribute to the beneficial effects of these drugs in thyrotoxicosis.

Haemodynamic characteristics of low-renin hypertension

1. Intra-arterial pressure, cardiac output, renal blood flow and glomerular filtration rate were measured in 19 patients with low-renin hypertension and in 30 patients with normal-renin hypertension. 2. Cardiac output and renal blood flow were significantly lower in low-renin hypertension. Total peripheral and renal vascular resistance were markedly higher in this group. 3. Plasma renin concentration correlated inversely with both total peripheral and renal vascular resistance as well as with age. Multiple regression analysis indicated that part of the relationship between renin and haemodynamic variables did not depend on age. Furthermore, plasma renin concentration did not decrease with age in a group of 40 normotensive control subjects of similar age to the hypertensive patients. 4. The results provide further confirmation that renin decreases as hypertension progresses.

Follow-up of renin in essential hypertension

1. Twenty-three subjects with essential hypertension were followed for a period of up to 7 years. They were untreated during the investigations. 2. Plasma renin concentration was found to decrease temporarily in some subjects. The ultimate change was a gradual rise. In those subjects who suffered myocardial infarction renin tended to rise more sharply. 3. A relationship was established with the rise in renal vascular resistance, which almost invariably occurred over the years.

Inactive renin in human plasma

Human plasma contains renin, which is enzymatically active at neutral pH (active renin), and a non-dialysable factor, which has renin-like activity after treatment at low pH (inactive renin). In vitro activated plasma-renin and purified human renal renin showed identical enzyme-kinetic properties. Quantitative estimations of inactive renin in renal venous plasma from 5 patients with renal-artery stenosis demonstrated its release by the kidney. Acute stimulation of renin release by isoprenaline, tilting, or diazoxide in 13 normotensive individuals and in 9 patients with essential hypertension increased active plasma-renin and reduced inactive plasma-renin. Inactive plasma-renin was increased and active plasma-renin decreased during suppression of renin release by propranolol in 12 patients with essential hypertension. In 55 patients with various disorders, inactive and active plasma-renin were directly correlated. However, the concentration of inactive renin, for a given value of active renin, varied widely from patient to patient. These results indicate that so-called inactive renin is indeed physiologically related to active renin. They also suggest that inactive renin can be activated not only in vitro, but also in vivo. Different renin assays measure different relative amounts of active and inactive renin. This may call for reinterpretation of results obtained by various methods, especially in situations where changes in plasma concentrations of the two forms of renin are in opposite directions.

Follow-up of renin in essential hypertension

In a prospective study of patients with essential hypertension, plasma renin levels showed a progressive increase with longer follow-up. This was associated with a parallel increase in renal vascular resistance. Arterial blood pressure and plasma volume did not change significantly during follow-up. In patients where the hypertension was complicated by myocardial infarction there was a comparatively greater increase in renin levels and renal vascular resistance which may be attributable to chronic reduction of cardiac output.

Dissociation of renin and aldosterone during dehydration: studies in a case of diabetes insipidus and adipsia

The regulation of aldosterone was studied in a child with diabetes insipidus and adipsia, associated with holoprosencephaly. Plasma ADH was low and unresponsive to dehydration. Plasma renin concentration ranged from 52 to 1350 ng ml-1 h-1 at various degrees of hydration, and plasma aldosterone ranged from 4.7 to 104 ng/100 ml. Despite these wide ranges, the levels of the two hormones were not correlated. The aldosterone-renin ratio (log.) was inversely related to the plasma sodium concentration, while the plasma renin concentration (log.) was directly related to plasma sodium. Reduced values of both extracellular fluid volume (radiosulphate and sodium spaces) and total exchangeable sodium were measured when plasma sodium was elevated. Sodium depletion at the time when the patient was in a dehydrated state appeared to be caused, at least partly, by defective renal sodium conservation. Thus, in the dehydrated state, the patient showed the following unusual combination of abnormalities: hypernatraemia, sodium depletion, hyperreninaemia, and low to normal plasma aldosterone. The abnormal aldosterone-renin ratio was probably not caused by an intrinsic adrenal abnormality, since high levels of aldosterone were measured as long as a certain degree of hydration had been achieved with or without exogenous ADH, and since plasma cortisol was normal and responsive to exogenous ACTH. The results suggest that the responsiveness of the adrenal cortex to angiotensin may vary with extracellular sodium concentration. The direction of this effect, that is, suppression of aldosterone with increased sodium concentration, is not different from what is observed under experimental conditions, when extra-cellular sodium concentration is raised by infusions of hyperosmolar saline.

Comparison between the effects of ST 600 and clonidine

The effects of the antihypertensive imidazoline compounds 2-(5 fluor-0-toluidine)-2-imidazoline hydrochloride (ST 600) and 2-(2, 6 dichlorophenylamine)-2-imidazoline hydrochloride (ST 155, clonidine, Catapres) on intra-arterial pressure, cardiac output, stroke volume, heart rate, total peripheral resistance, renal blood flow, glomerular filtration rate, renal vascular resistance, plasma volume, plasma renin and aldosterone concentration were studied in five patients with essential hypertension. The antihypertensive action of both compounds was similar and was accompanied by a reduction in heart rate and in cardiac output, total peripheral resistance being unchanged. There was no significant decrease in renal blood flow and glomerular filtration rate. Plasma volume and plasma concentrations of renin and aldosterone also did not change significantly. In the face of similar reductions in blood pressure, no differences were observed between cardio-renal haemodynamic responses after ST 600 and clonidine. However ST 600 had a longer lasting effect (8-12 hours).

Comparison between the effects of ST 600 and catapres

The effects of the antihypertensive imidazoline compounds 2-(5 fluor-0-toluidine)-2-imidazoline hydrochloride (ST 600) and 2-(2, 6 dichlorophenylamine)-2-imidazoline hydrochloride (ST 155, catapres) on intra-arterial pressure, cardiac output, stroke volume, heart rate, total peripheral resistance, renal blood flow, glomerular filtration rate, renal vascular resistance, plasma volume, plasma renin and aldosterone concentration were studied in five patients with essential hypertension. The antihypertensive action of both compounds was similar and was accompanied by a reduction in heart rate and in cardiac output, total peripheral resistance being unchanged. There was no significant decrease in renal blood flow and glomerular filtration rate. Plasma volume and plasma concentrations of renin and aldosterone also did not change significantly. In the face of similar reductions in blood pressure, no differences were observed between cardio-renal haemodynamic responses after ST 600 and catapres. However ST 600 had a longer lasting effect (8-12 hours).

Is low-renin hypertension a stage in the development of essential hypertension or a diagnostic entity?

A study of the frequency distribution of plasma-renin concentration in 81 patients with essential hypertension produced no evidence of a distinct sub-population with low renin levels. An arbitrary dividing line was used, therefore, to define low-renin hypertension (36% of patinets). Patients in this group were older than those with normal renin levels, and there was a significant negative correlation between renin and age among all patients. Low-renin hypertension was not characterized by increased exchangeable sodium, but exchaneable postassium was significantly lower than in patients with normal plasma-renin. This difference became insignificant when five patients in the low-renin group with persistent hypokalaemia were excluded. It is concluded that low-renin hypertension does not represent a separate diagnostic entity but that plasma-renin falls with age in essential hypertension.

Interrelationships between blood pressure, renin, renin substrate and blood volume in terminal renal failure

1. The effect of haemodialysis on blood pressure was assessed in forty-six patients with end-stage kidney disease, in relation to measurements of plasma renin concentration (PRC), plasma renin substrate (PRS) and blood volume. Parallel measurements of PRC and blood volume were carried out before and during the period of regular dialysis treatment. PRS was measured during this period only.

2. Both before and during regular dialysis PRC was higher in relation to blood volume in patients whose hypertension persisted after fluid withdrawal than in cases who became normotensive. PRS levels were similar in both groups.

3. The decrease of blood volume by regular dialysis led to a rise in plasma renin both in patients with controllable hypertension and in those with persistent hypertension. After at least 5 months of regular dialysis, the acute removal of 1.5 litres of body fluid caused a similar percentage rise in PRC in six patients with persistent hypertension and twelve patients with controllable hypertension. In three other patients with controllable hypertension and very low PRC, PRC was unresponsive both to the acute and the chronic volume depletion.

4. The pressor response to exogenous angiotensin in anephric patients, in whom no renin could be detected in the plasma, was decreased after ultrafiltration. This was caused by an increase in the threshold dose and not by a decrease in the slope of the dose-response relationship. This effect of ultrafiltration was due to fluid loss itself rather than to a rise in endogenous angiotensin.

5. The results indicate that the presence of hypertension in the face of fluid loss reflects a disturbance in renin release which is rather associated with the pathologic state of the kidney than with an excessive stimulus to the kidney. The decrease in sensitivity to angiotensin caused by fluid loss is presumably overridden by the effect of a high renin level.

Renin suppression in hypertension in relation to body fluid volumes, patterns of sodium excretion and renal haemodynamics

1. In patients with essential hypertension plasma renin concentration (PRC) was assessed in relation to age, plasma volume, extracellular fluid volume, aldosterone secretion rate, renal vascular resistance, filtration fraction and excess sodium excretion after acute salt loading.

2. Up to a calculated renal vascular resistance of 20 000 dyn s cm−5, PRC was found to be inversely related with the above-mentioned variables except plasma volume, extracellular fluid volume and aldosterone secretion rate.

3. It is concluded that the phenomenon of renin suppression is not dependent on mineralocorticoid excess. Instead, a decrease in PRC together with hypernatriuresis appears to reflect a progressive switch in intrarenal haemodynamic relationships. Renin suppression should therefore be considered to be a feature of progressive, but still uncomplicated, essential hypertension.