Interaction of angiotensin converting enzyme inhibition and atrial natriuretic factor

Natriuretic Peptides and Normal Body Fluid Regulation

Natriuretic peptides are structurally related, functionally diverse hormones. Circulating atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are delivered predominantly by the heart. Two C-type natriuretic peptides (CNPs) are paracrine messengers, notably in bone, brain, and vessels. Natriuretic peptides act by binding to the extracellular domains of three receptors, NPR-A, NPR-B, and NPR-C of which the first two are guanylate cyclases. NPR-C is coupled to inhibitory proteins. Atrial wall stress is the major regulator of ANP secretion; however, atrial pressure changes plasma ANP only modestly and transiently, and the relation between plasma ANP and atrial wall tension (or extracellular volume or sodium intake) is weak. Absence and overexpression of ANP-related genes are associated with modest blood pressure changes. ANP augments vascular permeability and reduces vascular contractility, renin and aldosterone secretion, sympathetic nerve activity, and renal tubular sodium transport. Within the physiological range of plasma ANP, the responses to step-up changes are unimpressive; in man, the systemic physiological effects include diminution of renin secretion, aldosterone secretion, and cardiac preload. For BNP, the available evidence does not show that cardiac release to the blood is related to sodium homeostasis or body fluid control. CNPs are not circulating hormones, but primarily paracrine messengers important to ossification, nervous system development, and endothelial function. Normally, natriuretic peptides are not powerful natriuretic/diuretic hormones; common conclusions are not consistently supported by hard data. ANP may provide fine-tuning of reno-cardiovascular relationships, but seems, together with BNP, primarily involved in the regulation of cardiac performance and remodeling. © 2017 American Physiological Society. Compr Physiol 8:1211-1249, 2018.

Expression and distribution of brain natriuretic peptide in human right atria

OBJECTIVES

We investigated expression of brain natriuretic peptide (BNP) as well as atrial natriuretic peptide (ANP) and their genes in human right atria. Their relations with atrial pressure were also examined.

BACKGROUND

The BNP plays a roll in electrolyte-fluid homeostasis such as ANP. The tissue level is reported to be elevated in the failing ventricles. However, expression and transmural distribution of BNP in the atria remain unclear.

METHODS

Expression of ANP and BNP was immunohistochemically investigated in the right atrial (RA) specimens from 21 patients who had undergone cardiac surgery. The mRNA of specimens were quantitatively measured by Northern blot analysis and also evaluated by in situ hybridization. In addition, plasma levels of ANP and BNP were measured in the patients.

RESULTS

The BNP immunoreactivity was diffusely seen in RA tissue of patients with mean RA pressure (mRAP) of 5 mm Hg or more, but it was noted only in the subendocardial half of the atria of those with mRAP less than 5 mm Hg. There was a significant correlation between the incidence of BNP-positive myocytes and mRAP (r = 0.850, p < 0.0001). Conversely, ANP-positive myocytes were found diffusely in all cases. In Northern blot analysis, the mRNAs levels of ANP and BNP in the atrial tissue were positively correlated with the mRAP (ANP, p = 0.775, p < 0.005 and BNP, p = 0.771, p < 0.005). In situ hybridization confirmed these findings. The mRNA levels were significantly correlated to each other (r = 0.845, p < 0.0002). Plasma ANP and BNP levels were elevated in the patients compared with that in controls; however, none were significantly correlated with the mRAP.

CONCLUSIONS

Expression of BNP and BNP mRNA is augmented in the atria with increased pressure, and distributed predominantly in the subendocardial side. The level of BNP mRNA was well correlated with that of ANP mRNA. Thus, these two genes might be commonly regulated in response to atrial pressure.

Influence of angiotensin converting enzyme inhibition and angiotensin II type 1 receptor antagonism on renal sodium and water handling and albuminuria during infusion of atrial natriuretic factor into healthy volunteers

BACKGROUND

Atrial natriuretic factor increases urinary sodium and water excretion. It also causes an increase in albuminuria. Angiotensin converting enzyme inhibition attenuates the effects of atrial natriuretic factor on renal sodium and water handling; however, it is not known whether this effect is mediated by the accompanied decrease in blood pressure or by suppression of the renin-angiotensin system.

OBJECTIVE

To test the hypothesis that atrial natriuretic factor mediates natriuresis and diuresis by inhibiting angiotensin II, by studying the effects of the angiotensin converting enzyme inhibitor enalapril and the angiotensin II type 1 receptor antagonist losartan. In addition, the effects of these drugs on atrial natriuretic factor-induced albuminuria were examined.

DESIGN AND METHODS

We investigated the effects of enalapril and losartan on atrial natriuretic factor-induced changes in urinary excretion of sodium, water and albumin from eight healthy volunteers. Measurements of systemic and renal haemodynamics in these subjects were performed before and during a 2 h infusion of atrial natriuretic factor [0.01 microg/kg per min (low dose) for the first 60 min and 0.02 microg/kg per min (high dose) for the second 60 min]. Measurements were performed after 5 days of pretreatment with placebo, 50 mg losartan or 20 mg enalapril daily.

RESULTS

Mean arterial pressures during the clearance study were 84.6 +/- 1.7 mmHg after placebo, 84.0 +/- 2.2 mmHg after losartan treatment and 80.0 +/- 2.5 mmHg after enalapril treatment (P < 0.05). Plasma renin activity was significantly increased both by losartan and by enalapril treatments. Neither enalapril nor losartan treatment attenuated atrial natriuretic factor-induced changes in renal haemodynamics. After placebo pretreatment, fractional urinary excretion of sodium increased significantly during infusion of atrial natriuretic factor. Losartan treatment did not influence the increase in urinary excretion of sodium during infusion of atrial natriuretic factor, whereas enalapril treatment significantly attenuated this increase (P < 0.01). Atrial natriuretic factor significantly increased albuminuria. Neither losartan nor enalapril treatment reduced atrial natriuretic factor-induced albuminuria.

CONCLUSIONS

Enalapril treatment lowered blood pressure and attenuated the atrial natriuretic factor-induced increase in urinary excretion of sodium. In contrast, the angiotensin II type 1 receptor antagonist losartan, at a dosage that did not lower blood pressure, did not attenuate the increase in urinary excretion of sodium. These data indicate that atrial natriuretic factor increases natriuresis and diuresis independently of angiotensin II. The increase in albuminuria during infusion of atrial natriuretic factor was not influenced by enalapril and losartan treatments.

Responses of plasma concentrations of A type natriuretic peptide and B type natriuretic peptide to alacepril, an angiotensin-converting enzyme inhibitor, in patients with congestive heart failure

BACKGROUND

Plasma concentrations of A type or atrial natriuretic peptide (ANP) and B type or brain natriuretic peptide (BNP) are increased in patients with congestive heart failure (CHF).

OBJECTIVE

To examine the haemodynamic and hormonal responses, especially of ANP and BNP, to oral administration of an angiotensin-converting enzyme (ACE) inhibitor in patients with CHF and in controls.

PATIENTS

12 patients with CHF and 11 controls.

METHODS

Haemodynamic variables and plasma concentrations of ANP, BNP, and other hormones were serially measured for 24 hours after alacepril (37.5 mg) was given by mouth.

RESULTS

Pulmonary capillary wedge pressure and systemic vascular resistance decreased significantly in both groups. The cardiac index increased only in the CHF group. In patients with CHF pulmonary capillary wedge pressure, systemic vascular resistance, and cardiac index were significantly changed from 1 to 12 hours after alacepril administration. Plasma ANP and BNP decreased significantly after alacepril was given to the CHF group: neither concentration changed in the control group. In the CHF group plasma ANP was significantly lower between 1 and 6 hours and was highly significantly correlated with pulmonary capillary wedge pressure. Plasma BNP, however, was significantly lower between 6 and 24 hours after alacepril and was not correlated with pulmonary capillary wedge pressure.

CONCLUSIONS

The response of plasma BNP after alacepril administration occurred later and lasted longer than the plasma ANP response. This may indicate that the mechanisms of synthesis, secretion, or degradation of the two peptides are different.

Influence of angiotensin converting enzyme inhibition on relation of atrial natriuretic peptide concentration to atrial pressure in heart failure

OBJECTIVE

To examine the relation between haemodynamics and atrial natriuretic peptide concentration during short term angiotensin converting enzyme inhibition.

DESIGN

Patients were randomly allocated to receive placebo or one of three doses of the angiotensin converting enzyme inhibitor ramipril.

SETTING

Cardiac units of two tertiary referral hospitals.

SUBJECTS

38 Patients with stable congestive heart failure caused by ischaemic heart disease.

METHODS

Data were collected over a 24 hour period and assessed with the aim of distinguishing between the haemodynamic effects on plasma concentrations of atrial natriuretic peptide and the direct effects of the study drug, vasopressin concentrations, and angiotensin converting enzyme activity.

RESULTS

Pulmonary capillary wedge pressure was the main predictor of the plasma concentration of atrial natriuretic peptide. A higher plasma concentration of this peptide with a given pulmonary capillary wedge pressure was found after 24 hours of treatment with 2.5 mg and 5 mg of ramipril. Plasma concentration of the active metabolite, change in arginine vasopressin concentration or degree of angiotensin converting enzyme inhibition did not significantly predict change in plasma concentration of atrial natriuretic peptide or in the ratio of atrial natriuretic peptide concentration to pulmonary capillary wedge pressure.

CONCLUSIONS

A gradual increase in plasma concentration of atrial natriuretic peptide with a given pulmonary capillary wedge pressure, occurs during short term high degree inhibition of angiotensin converting enzyme. The causative mechanisms are yet to be identified. Such a change in the relation between central haemodynamics and atrial natriuretic peptide concentration may contribute to the beneficial effects of angiotensin converting enzyme inhibition in patients with congestive heart failure due to ischaemic heart disease.

Atrial natriuretic peptide and its potential role in pharmacotherapy

Atrial natriuretic peptide (ANP) is a 28 amino-acid polypeptide secreted into the blood by atrial myocytes after atrial pressure and distension. Although its role in humans is not clear, it can produce a variety of physiologic effects including vasodilatation, natriuresis, and suppression of the renin-angiotensin-aldosterone axis. These actions are potentially useful in a variety of pathologic states such as hypertension and congestive heart failure, and diverse methods to augment the effects of ANP in these states have been devised. The results are exciting and, despite some problems, may lead to the pharmacologic use of enhancement of ANP actions in several clinical disorders.

Negative chronotropic effect of the atrial natriuretic peptide in an anesthetized dog model

The effects of atrial natriuretic peptide (ANP) on heart rate and on the chronotropic response induced by angiotensin II were evaluated. The action of angiotensin II, injected into the sinus node artery, on heart rate was determined in anaesthetized and vagotomized dogs pretreated with beta-adrenoceptor antagonist (n = 9), before and after the infusion of physiological and pharmacological doses (10 and 50 ng/kg per min) of ANP. ANP plasma concentrations were determined by radioimmunoassay. Compared to the baseline concentration values (117 +/- 3 pg/ml), a slight increase was produced by the lower dose (293 +/- 45 pg/ml), whereas a significant augmentation was noted with the higher dose of ANP (1024 +/- 255 pg/ml). The basal heart rate and mean systemic arterial pressure were decreased (26.3 and 13.5%) during the intravenous infusion of the physiological dose of the peptide and were significantly reduced (32.5 and 29.2%, P < 0.05) by the administration of the pharmacological dose of ANP. Angiotensin II had a positive chronotropic effect (29 +/- 3 beats/min) that was significantly inhibited by the pharmacological dose of ANP. Our results suggest that ANP might have a role in the control of heart rate.

Early treatment with captopril after acute myocardial infarction

OBJECTIVES

To determine the effects of early treatment with captopril on haemodynamic function, neuroendocrine biochemistry, left ventricular structure, clinical outcome, and exercise capacity over one year from acute myocardial infarction.

DESIGN

Randomised, double blind, placebo controlled comparison of captopril and placebo.

SETTING

Coronary care units and cardiology departments of two university teaching hospitals in Glasgow.

PATIENTS

99 haemodynamically stable patients with acute myocardial infarction, selected on clinical grounds as being at risk of late ventricular dilatation.

INTERVENTION

Captopril or identical placebo started between six and 24 hours after start of symptoms and continued for 12 months. Target maintenance dose was 25 mg three times a day.

MAIN OUTCOME MEASURES

(a) Acute haemodynamic effects of treatment; (b) neuroendocrine biochemistry from admission to two months; and (c) change in echocardiographic measures of left ventricular size, clinical outcome, and exercise capacity after 12 months of treatment with a separate analysis of the effects of one month of treatment withdrawal on left ventricular volumes.

RESULTS

Captopril caused acute reductions in mean (SEM) pulmonary artery pressure (2.48 (0.69) mm Hg) and systemic vascular resistance (260 (103)) dyn.s.cm-5). Over the first 10 hours captopril reduced mean arterial pressure by 12.1 (2.4) mm Hg compared with 3.8 (1.9) mm Hg in the placebo group. No patient had to be withdrawn from the captopril group because of hypotension. From day 1 onwards systolic and diastolic arterial pressures in the captopril treated group were slightly but not significantly lower than on placebo. There was no difference in the incidence of ventricular or supraventricular arrhythmia with treatment. Captopril prevented the day 3 peak in angiotensin II that occurred in the placebo group (peak concentration (interquartile range): 10.1 (4.8-19.4) pg/ml v 16.8 (4.3-46.3) pg/ml)) but had no effect on atrial natriuretic factor, arginine vasopressin, or catecholamines. Plasma atrial natriuretic factor remained above normal in both groups at two months after infarction. After one year left ventricular volume indices had increased less on captopril than on placebo: left ventricular end systolic volume index 5.4 ml/m2 v 14.7 ml/m2 (95% confidence interval (95% CI) of difference -14.6 to -3.9; p = 0.0011); left ventricular end diastolic volume index 8.4 ml/m2 v 19.0 ml/m2 (95% CI of difference, -17.0 to -4.2; p = 0.0016). Withdrawal of captopril for one month did not affect ventricular volumes. There was no difference in exercise capacity.

CONCLUSIONS

Captopril started between six and 24 hours after acute myocardial infarction is not associated with significant hypotension. It suppresses activation of the renin angiotensin system but has no effect on plasma concentrations of other neurohormones. Atrial natriuretic factor remains raised at two months after myocardial infarction. Captopril significantly decreases left ventricular dilatation. This effect is not lost after one month of treatment withdrawal and is thus due to an alteration of left ventricular structure and not to a short lived haemodynamic action of captopril. Long-term treatment with captopril does not result in improved aerobic exercise capacity after acute myocardial infarction.

Enalaprilat in acute myocardial infarction: tolerability and effects on the renin-angiotensin system

We treated 48 patients with intravenous enalaprilat within 24 hours from the onset of acute myocardial infarction. Concomitant therapy included thrombolytic treatment (29), intravenous metoprolol (34), intravenous nitroglycerin (16) and intravenous furosemide (15). The first 40 patients included had systolic blood pressure at baseline greater than or equal to 110 mmHg. Intravenous bolus injections of 0.2-1.2 mg (mean 1.0 mg) enalaprilat in one hour were given to 20 patients and an intravenous infusion of 1 mg over two hours was administered to another 20 patients, as well as to a separate group of 8 patients with systolic blood pressure between 100-109 mmHg at baseline. The infusion was stopped in five cases when the systolic blood pressure fell below 100 and 90 mmHg, respectively, in the two infusion groups. No hypotensive reactions were symptomatic. Blood pressure decreased from a mean of 134/82, 131/79 and 106/72 mmHg to a minimum of 117/71, 118/73 and 97/63 mmHg, respectively, in the three groups. Almost complete suppression of plasma angiotensin converting enzyme activity was achieved within 30 minutes. No significant changes were found in plasma levels of angiotensin II, renin activity or atrial natriuretic peptide between baseline and 24 hours. Treatment was continued with oral enalapril 2.5-10 mg/day, which was generally well tolerated. We conclude that intravenous and oral enalapril added to conventional therapy in the early phase of acute myocardial infarction is well tolerated in selected patients, but should be carefully titrated.

Comparison of the effects of ANF 99-126 and ANF 103-126 on captopril-induced renin release in man

OBJECTIVE

The aim was to examine the effects of atrial natriuretic factor (ANF) 99-126 and ANF 103-126, an N-terminal shortened analogue of the peptide, on the plasma renin activity response to captopril, an inhibitor of angiotensin-converting enzyme.

DESIGN

Two protocols were performed. In the first protocol, subjects were studied on three occasions. Captopril 25 mg was given and a 60 minute infusion of 5% D-glucose (placebo), or ANF 99-126 3 or 10 pmol/kg/min, was administered in a single blind randomized manner. The second protocol was divided in two parallel phases comparing ANF 103-126 either 3 or 10 pmol/kg/min to placebo.

SUBJECTS

Thirty-three salt-replete healthy male volunteers aged 21-39 years were studied in the supine position.

MEASUREMENTS

Plasma renin activity, plasma ANF 99-126 and ANF 103-126 levels, heart rate and blood pressure were measured.

RESULTS

Compared to placebo infusion, the rise in plasma renin activity after captopril was attenuated by ANF 99-126 infusion (from 755% of baseline to 294% by ANF 99-126 3 pmol/kg/min and from 755 to 202% by 10 pmol/kg/min; P less than 0.03 and P less than 0.01 respectively). The comparable findings with ANF 103-126 were 492 to 218% (3 pmol/kg/min) and 645 to 364% (10 pmol/kg/min) (P less than 0.01 and P less than 0.01 respectively).

CONCLUSIONS

The results, taken in conjunction with previous findings, suggest that atrial natriuretic factor inhibits in a non-selective manner the renin response to all secretagogues so far tested in man. The current results also suggest that the anti-renin action of atrial natriuretic factor does not depend on the first four N-terminal amino acids of the native peptide.

Mixed inhibitors of angiotensin-converting enzyme (EC 3.4.15.1) and enkephalinase (EC 3.4.24.11): rational design, properties, and potential cardiovascular applications of glycopril and alatriopril

Angiotensin-converting enzyme (ACE) and enkephalinase, two cell surface metallopeptidases, are responsible for angiotensin II formation and atrial natriuretic factor (ANF) degradation, respectively, and thereby play a critical role in the metabolism of hormonal peptides exerting essentially opposite actions in cardiovascular regulations. To affect simultaneously both hormonal systems by a single molecular structure, we have designed glycoprilat and alatrioprilat [(S)-N-[3-(3,4-methylene-dioxyphenyl)-2-(mercaptomethyl)-1-oxoprop yl] glycine and -alanine, respectively]. In vitro the two compounds inhibit both ACE and enkephalinase activities with similar, nanomolar potencies, and in vivo, glycopril and alatriopril, the corresponding diester prodrugs, occupy the two enzyme molecules in lung at similar low dosages (0.2-0.5 mg/kg of body weight, per os). The high potency of these compounds is attributable to interaction of the methylenedioxy group with the S1 subsite of ACE and of the aromatic ring with the S1' subsite of enkephalinase. In rodents, low doses of these mixed inhibitors exert typical actions of ACE inhibitors--i.e., prevention of angiotensin I-induced hypertension--as well as of enkephalinase inhibitors--i.e., protection from 125I-ANF degradation or enhancement of diuresis and natriuresis following acute extracellular volume expansion. In view of the known counterbalanced physiological actions of the two hormonal peptides, whose metabolism is controlled by ACE and enkephalinase, mixed inhibitors of the two peptidases show promise for the treatment of various cardiovascular and salt-retention disorders.

Differential effects of angiotensin II and noradrenaline on tubular rejection of sodium produced by ANP in rats

The effect of Atrial Natriuretic Peptide (ANP) on renal tubular sodium handling (FENa) in the presence of converting enzyme inhibition (CEI), the AII antagonist Saralasin (SAR), noradrenaline (NA) and angiotensin II (AII) infusions was investigated. FENa and increases in FENa produced by ANP were significantly lower with CEI (p less than 0.03 and 0.0001) or SAR (p less than 0.02 and 0.02) against control (Vehicle + ANP). Mean arterial pressures (MAP) were also reduced. Returning MAP to 107 +/- 2 mmHg with NA (+CEI+ANP), did not change FENa (1.22% +/- 0.16 to 1.25% +/- 0.18, p greater than 0.66) whereas without CEI but with ANP (MAP 113 +/- 2 mmHg) FENa was significantly increased by NA (2.34% +/- 0.36, p less than 0.02). With AII+CEI+ANP, MAP was restored to 110 +/- 5 mmHg, and FENa was highly significantly increased (0.99% +/- 0.20 to 3.04% +/- 0.39, p less than 0.0003) in excess of that expected due to pressure effects alone. It is concluded the additional effect of AII on FENa (3.04 versus 1.25% with NA) at equivalent perfusion pressures are due to a separate additive phenomenon of AII and ANP both causing tubular rejection of Na.

Effects of low dose infusion of atrial natriuretic factor on acute inhibition of angiotensin converting enzyme in normal man

1. We have studied the effects of low dose infusions of atrial natriuretic factor (human ANF (99-126), 1.95 pmol/min per kg) on angiotensin converting enzyme (ACE) inhibitor-induced haemodynamic and hormonal changes in healthy subjects. 2. ACE inhibitor (captopril 25 mg, administered orally) was given against a background infusion of physiological saline (placebo day) or ANF (experimental day). 3. Compared with the placebo observations, ANF enhanced the fall in plasma aldosterone concentrations induced by captopril (P less than 0.05). 4. The rise of plasma renin activity following administration of ACE inhibitor which was observed during placebo infusion was abolished by ANF (P less than 0.05). 5. The responses of systemic blood pressure and heart rate to the converting enzyme inhibition were not affected by the infusion of ANF. 6. These results suggest that variations in endogenous circulating ANF may influence, in part, the response of these hormonal levels during ACE inhibition.

Enkephalinase (EC 3.4.24.11) inhibitors: protection of endogenous ANF against inactivation and potential therapeutic applications

Atrial natriuretic factor (ANF) is a cardiac hormone exerting potent cardiovascular and renal effects but its poor intestinal absorption and rapid inactivation have prevented so far its therapeutic utilisation. However inhibition of endogenous ANF metabolism progressively emerges as a novel therapeutic approach in cardiovascular and renal disorders. The critical role played by enkephalinase (membrane metalloendopeptidase, EC 3.4.24.11) in ANF inactivation was deduced from the effects of inhibitors. These compounds not only protect partially exogenous ANF from hydrolysis by some tissue preparations in vitro but also, in vivo, they increase the half-life of the exogenous hormone in plasma and, even more markedly, its recovery in intact form in kidney, a major target organ. In addition, enkephalinase inhibitors increase by two- to three-fold the circulating level of endogenous ANF, even when the latter is already markedly elevated, such as in patients with chronic heart failure. Finally, enkephalinase inhibitors induce a series of ANF-like responses such as natriuresis, diuresis or increase in cGMP excretion which are attributable to the hormone. These pharmacological observations, as well as preliminary clinical trials, suggest that enkephalinase inhibitors may represent a novel class of therapeutic agents with potential applications in congestive heart failure, essential hypertension and various sodium-retaining states.