Brenda J. Leckie

Radio-immunoassay for plasma alpha human atrial natriuretic peptide: a comparison of direct and pre-extracted methods.

Plasma atrial natriuretic peptide (ANP) concentrations were measured by both direct radio-immunoassay and with pre-extraction of the peptide from plasma using C18 reverse phase columns. Peptide concentrations were measured in normal subjects (including a group of eight volunteers who received an intravenous infusion of 0.9% NaCl solution), patients with renal failure (including a group with end-stage disease undergoing renal dialysis) and patients with a spectrum of cardiac dysfunction. The overall correlation of results from direct and extracted assay methods was good. However, absolute values from extracted assays were significantly lower than from parallel direct assays. This discrepancy was due to interference from platelets and from another, as yet unidentified, plasma component demonstrated by gel filtration experiments. Extraction of the peptide from plasma by C18 columns largely eliminated these sources of interference and was particularly important for accurate measurement of peptide concentrations within the normal range. Plasma peptide concentrations were elevated in cardiac and renal failure, fell with renal dialysis and rose in normal subjects challenged with an intravenous isotonic fluid load. These findings suggest that ANP participates in the regulation of body fluid volumes and arterial pressure.

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.

Captopril in the management of hypertension with renal artery stenosis: Its long-term effect as a predictor of surgical outcome

Fifteen patients with hypertension and unilateral renal artery disease were treated with captopril alone; 10 came to operation and were later assessed postoperatively with no drug treatment. Captopril caused both immediate and sustained decreases in plasma angiotensin II and aldosterone, with increases in plasma active renin and blood angiotensin I concentrations. Decrements in systolic and diastolic pressure 2 hours after the first dose of captopril were closely correlated with the initial decreases in plasma angiotensin II. Blood pressure was decreased by long-term captopril therapy irrespective of whether plasma angiotensin II was abnormally high before treatment. The long-term response of both systolic and diastolic pressure correlated well with the response to surgery. By contrast, the blood pressure decrease 2 hours after the initial dose of captopril variously underestimated and overestimated the decrease during prolonged use of the drug and did not relate to surgical outcome. In patients who, before treatment, had secondary aldosteronism, hyponatremia, hypokalemia and sodium and potassium deficiency, captopril corrected these abnormalities. In the remaining patients, long-term captopril therapy did not alter exchangeable sodium, plasma sodium or total body potassium, although plasma potassium levels increased.

Atrial natriuretic peptides and renin release

The relationship between endogenous plasma concentrations of atrial natriuretic peptide and renin was examined in resting normal subjects and patients with cardiac impairment. To test the hypothesis that atrial natriuretic peptide inhibits renin secretion, intravenous infusions of atrial natriuretic peptide were administered to normal volunteers, patients with end-stage renal failure, and conscious dogs in both sodium-replete and sodium-depleted states. Plasma atrial natriuretic peptide and renin were inversely related in normal subjects (r = -0.52, n = 140, p less than 0.001), but a weak positive association between these two variables was observed in patients with cardiac impairment (r = 0.32, n = 60, p less than 0.02). Low doses of both 26- and 28-amino-acid human atrial natriuretic peptide (2 pmol/kg/minute for two hours) given to sodium-replete normal subjects halved plasma renin compared with time-matched placebo values (19 +/- 4 and 18 +/- 3 versus 36 +/- 8 microU/ml, p less than 0.001 for both). Incremental doses of synthetic atrial natriuretic peptide suppressed plasma renin below time-matched placebo values in both sodium-replete (maximal suppression 1.2 +/- 0.4 versus 8.6 +/- 1.4 microU/ml, p less than 0.001) and sodium-depleted (maximal suppression 18.9 +/- 4.9 versus 51 +/- 13 microU/ml, p less than 0.05) dogs. This effect was initially apparent at low doses of atrial natriuretic peptide (1 pmol/kg/minute), and renin suppression was maximal, in both states, with lesser doses of atrial natriuretic peptide than those at which maximal natriuresis was observed. Atrial natriuretic peptide administered to patients with end-stage renal failure (10 pmol/kg/minute for one hour) caused no change in plasma renin. These data confirm that atrial natriuretic peptide inhibits renin secretion in a dose-related manner and suggest that this action of the peptide is modified by both the baseline sodium status and renal function of the recipient.

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.

Haemodynamic effects of captopril in acute left ventricular failure complicating myocardial infarction.

Activation of the renin-angiotensin system in acute myocardial infarction may have important haemodynamic consequences. The effects of captopril were assessed in nine patients with acute left ventricular failure complicating myocardial infarction. Plasma angiotensin II was elevated at 16.8 (3.6) pmol/1 (mean [SE]) including high levels in three of four patients in the absence of any previous therapy, including diuretics. Repeated low doses of captopril were administered to reduce pulmonary capillary wedge pressure <14 mm Hg or to a maximum total dose of 25 mg. Right atrial pressure fell from 12.4 (0.9) to 9.4 (0.7) mm Hg p < 0.001, pulmonary arterial pressure from 32.7 (3) to 26.4 (2.2) p = 0.01. and pulmonary capillary wedge pressure from 25.7 (2.9) to 19.9 (2.2) p = 0.01. Despite a fall in systemic vascular resistance from 1.540 (110) to 1.330 (76) dyn/s/cm5 and mean arterial pressure from 84.8 (3.9) to 76.7 (2.7) p = 0.001, changes in cardiac output were small: 3.8 (0.3) to 4.2 (0.3) NS. Angiotensin II fell in all patients even after only 3.125 mg to a mean of 3.6 (1.0). These improvements occurred whether basal angiotensin II was elevated or normal, and in the presence or absence of diuretic therapy. At 24 hours, seven patients received captopril in the maximum titrated dose of the previous day. Haemodynamic changes at one hour were of similar magnitude to those during incremental dosing. These results suggest that reduction of angiotensin II exerts beneficial haemodynamic effects in heart failure complicating acute myocardial infarction.

A transition-state analogue inhibitor of human renin (H.261): test in vitro and a comparison with captopril in the anaesthetized baboon.

H.261, a new transition state inhibitor of human renin with an IC50 of 6.9 X 10(-10) M, was given by intravenous infusion to six anaesthetized baboons. The inhibitor was infused first at 0.1 mumol/kg/h for 15 min, then at 1.0 mumol/kg/h for a further 15 min. After a recovery period of 2 h in which the animals received 5% dextrose, they were infused with captopril, 25 mumol/kg/h for 15 min. At both rates of infusion H.261 markedly and significantly reduced the enzymatic action of renin in plasma, the blood concentration of angiotensin I, the plasma concentration of angiotensin II and mean arterial pressure. All changes reverted towards or to control values in the subsequent control period. Captopril also lowered plasma angiotensin II concentration and mean arterial pressure markedly and significantly but, as expected for an inhibitor of the angiotensin I-converting enzyme, plasma active renin concentration and blood angiotensin I concentration increased. The changes of angiotensin II and arterial pressure were similar with captopril and H.261.

Peptide Inhibitors of Renin

Three experiments are described using new substrate analogue inhibitors of renin. The first experiment shows that introduction of a reduced isostere in the scissile peptide bond of an analogue greatly increases its ability to inhibit renin of a particular species. However, different species of renin substrate have different amino acids in their scissile bond and variation here also greatly influences the affinity of renin and substrate and hence of renin and substrate analogues. Finally, substitution of amino acids in the C-terminal adjacent to the scissile bond influences the affinity and efficacy of substrate analogues as inhibitors. In our second experiment a peptide inhibitor of dog renin, H.77, was used in an affinity column to produce a one-stage, 2000-fold, and complete purification of human renin. In our third experiment infusion of H.77 was used to lower circulating concentrations of angiotensin I and angiotensin II in conscious sodium-deplete dogs. Captopril was then given in addition to H.77 but blood pressure did not fall further, suggesting that captopril lowers blood pressure wholly or partly by reducing angiotensin II within the circulation and in extravascular sites.

New Inhibitors of Human Renin Tested in vitro and in vivo in the Anaesthetized Baboon

A new inhibitor of human renin (H. 189) is described. It is a decapeptide analogue of human renin substrate with the amino acid, statine, substituted for leucine in the scissile bond. Its inhibitory potency as shown by IC50 is 1.0 X 10(-8) M with human plasma renin and 1.5 X 10(-8) M with baboon plasma renin. It is less effective with dog and rat renin, but its inhibitory potency with human renin is similar to that of another inhibitor of ours (H. 142) having a reduced isostere in the scissile bond. H. 189 has some inhibitory effect on cathepsin D (IC50 6.5 X 10(-5) M) but H. 142 has no discernible effect. Pepstatin, on the other hand, was highly effective against cathepsin D (IC50 1.2 X 10(-8) M). H. 142 and H. 189 were infused intravenously at 10 mg/kg/h in four anaesthetized salt-deplete baboons (Papio hamadryas). The activity of renin in plasma decreased markedly as did the circulating concentration of its products, angiotensin I and angiotensin II.

The purification of a high-molecular-weight, enzymatically inactive renin precursor from human kidney.

A high-molecular-weight enzymatically inactive form of renin has been purified to homogeneity from human kidney. It has an Mr of 48 000 as determined by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate and an Mr of 51 000 by gel filtration on Sephadex G100. It is activated by treatment with trypsin and reversibly activated by exposure to acid. We conclude that this material represents a human prorenin.