Effects of angiotensin and noradrenaline on atrial natriuretic peptide levels in man

Phospholipase C signaling tonically represses basal atrial natriuretic factor secretion from the atria of the heart

The cardiac hormone atrial natriuretic factor (ANF or ANP) plays significant, well-established roles in a large number of physiological and pathophysiological processes, including water and electrolyte balance, blood pressure regulation, and cardiovascular growth. Understanding the regulation of its production and secretion by atrial cardiomyocytes is incomplete. We have previously established a significant role of G(i/o) protein signaling in modulating ANF secretion as promoted by stretch of the atrial myocardium. In the present study, we investigated the role of G(q) protein signaling and its relationship to G(i/o) protein signaling using pharmacological manipulation of proximal effectors of G(αq) in an ex vivo model of spontaneously beating rat atria. Phospholipase C (PLC) and protein kinase C (PKC) inhibitors dramatically increased basal secretion of ANF. Furthermore, although atrial wall stretch is a potent stimulus for secretion, stretch unexpectedly reduced ANF secretion to basal levels under PLC and PKC inhibitory conditions. Inhibition of the inositol triphosphate receptor did not appear to affect basal secretion but dose-dependently blocked stretch-secretion coupling. The results obtained demonstrate that the PLC and PKC signaling cascades play important albeit unexpected roles in the regulation of basal and stimulated ANF secretion and suggest interplay between the G(q) and G(i/o) protein signaling pathways.

Plasma Atrial Natriuretic Peptide (ANP) Concentration in Patients with Pheochromocytoma

The interaction between catecholamines (CA) and ANP is not clearly established. The effects of excess endogenous CA on ANP secretion can be investigated in patients with pheochromocytoma. We studied 27 patients with surgically and histologically proven pheochromocytoma (P) aged 19-70 years. In 16 of these patients plasma ANP study was repeated after surgical removal of the tumour. The control group (C) consisted of 20 healthy volunteers aged 21-48 years. Moreover, 42 patients with uncomplicated mild to moderate essential hypertension (EH) aged 18-48 years were also studied. In P higher plasma ANP concentration versus C, EH was found (51.9 +/- 8.1; 25.5 +/- 1.5; 19.3 +/- 1.5 fmol/ml, respectively). In 16 patients with P, increased plasma ANP level (mean 63.3 +/- 12.6 fmol/ml) declined after surgical removal of the tumour (mean 22.4 +/- 2.9 fmol/ml). In the P patients no relationship was found between plasma ANP and hormonal patterns of the tumour or between plasma ANP and plasma catecholamines, whereas significant positive correlations between plasma ANP and both systolic and diastolic blood pressure and heart rate were demonstrated. These results suggest that excess CA produced by the chromaffin tumour induce ANP secretion via stimulation of adrenergic receptors. However, influence of the haemodynamic changes evoked by CA cannot be excluded. It is suggested that increased secretion of ANP may be of some importance in maintaining blood pressure homeostasis in patients with pheochromocytoma.

Pharmacokinetic and pharmacodynamic effects of midodrine on blood pressure, the autonomic nervous system, and plasma natriuretic peptides: a prospective, randomized, single-blind, two-period, crossover, placebo-controlled study

BACKGROUND

Midodrine is an alpha-agonist prodrug of desglymidodrine (DGM) that has been reported to be of clinical benefit in patients with neurocardiogenic syncope. Its effects may be mediated not only by its hypertensive properties but also by its neurohumoral influences independent of blood pressure (BP).

OBJECTIVE

The present study aimed to simultaneously characterize the effects of midodrine on BP, plasma catecholamines, plasma atrial natriuretic peptide (ANP), and power spectral analysis of heart rate (HR) in healthy volunteers.

METHODS

This was a prospective, randomized, single-blind, 2-period, crossover study in which a single, oral, 5-mg dose of midodrine was compared with placebo. The washout period between midodrine and placebo was 1 week. The study parameters included plasma DGM (as measured by high-performance liquid chromatography [HPLC]); systolic and diastolic BP (as measured with an oscillometric monitor); HR; plasma catecholamines (measured by HPLC); plasma ANP, also known as venous return (measured by a radio-immunoassay); and low- and high-frequency HR variation (calculated from computerized 5-minute electrocardiographic recordings). All study parameters were measured simultaneously 12 times just before and over a period of 8 hours after drug administration.

RESULTS

Fifteen healthy nonsmoking male subjects (14 white, 1 black; mean [SD] age, 28.6 [4.7] years; weight, 74.5 [16.4] kg; seated BP, 109.9 [9.0]/73.6 [9.5] mm Hg; seated HR, 63.8 [8.4] bpm) were randomized. No significant effects of midodrine on BP were observed. At Cmax, midodrine decreased norepinephrine from 188.4 (30.6) to 162.5 (29.8) pg/mL (P = 0.011) and HR from 57.2 (7.3) to 54.9 (6.6) bpm (P = 0.022). A significant correlation was found between DGM concentration and HR ( varphi -0.61; P = 0.014). A DGM-related increase in plasma ANP (+29.6 [90.0] fmoL/mL) was observed.

CONCLUSION

This study in healthy male volunteers found that midodrine has sympatholytic influences that are independent of BP but related to augmented venous return.

Circadian rhythm of ANP, aldosterone and PRA in normotensive IUGR

OBJECTIVE

Atrial natriuretic peptide (ANP) increases are reported during normal pregnancy, but the relation to arterial pressure and the renin-angiotensin system is debatable. We assessed whether normotensive pregnancies with intrauterine growth retardation (IUGR) present an alteration of maternal ANP levels.

DESIGN

A total of 11 pregnant women with IUGR, in the absence of any other maternal or fetal pathology, entered the study during the third trimester. They were compared with 12 healthy pregnant women of similar age and characteristics. We monitored all subjects for blood pressure (BP), ANP, aldosterone and plasma renin activity (PRA), under the same conditions for 24 h. All subjects were submitted to the same regimen of life; with homogeneous dark : light periods, salt intake and meal times.

METHODS

BP was monitored at 20 min intervals for 24 h and blood tests performed at six time points during the 24 h. EDTA plasma samples were immediately centrifuged. Hormone assays were performed by radioimmunoassay. Koch's nonparametric two-way analysis of variance (ANOVA) was used to compare the hormone time-dependent profiles in the two groups. Circadian rhythms were assessed by cosinor analysis.

RESULTS

The IUGR group was characterized by higher ANP values compared to normal pregnancy, (205 +/- 24 versus 146 +/- 21 pg/ml: P < 0.05) but not significant differences were shown for PRA, aldosterone and BP circadian rhythms.

CONCLUSIONS

This study shows higher ANP values in human pregnancy complicated by IUGR, with presence of normal BP, aldosterone and PRA profiles.

Ageing and blood pressure regulation: dose-response relationships for angiotensin, blood pressure, atrial natriuretic peptide and aldosterone in normal subjects of varying ages

1. Infusion of increasing doses of angiotensin II (AII) in normal subjects sequentially increased blood pressure, aldosterone and atrial natriuretic peptide (ANP) levels. 2. The slope of ANP response to AII was positively correlated with basal ANP and with the slope of blood pressure response to AII (pressor slope) but not with age. 3. This is consistent with the response of ANP to AII being mediated partly by the rise in blood pressure, independent of ageing. 4. As expected in a selected normotensive population, there was no correlation between basal blood pressure and age, but pressor slope was positively correlated with age. 5. Thus, dose-response relationships may be an index of age-induced alterations in pressure regulatory mechanisms.

Elevation of plasma atrial natriuretic peptide occurs during adrenaline infusion in hypertensive but not normotensive subjects

The levels of plasma atrial natriuretic peptide in response to graded adrenaline infusion were determined in six patients with essential hypertension and six healthy normotensive subjects (controls). Basal plasma adrenaline concentration was similar in both groups and rose progressively and to a similar level during adrenaline infusion. Plasma noradrenaline rose in both groups and to the same extent during the 26 and 39 ng/kg/min adrenaline infusion rates. Basal plasma atrial natriuretic peptide levels were higher in the hypertensives than in the controls. Graded adrenaline infusion had no effect on atrial natriuretic peptide levels in the controls but significantly raised atrial natriuretic peptide levels in the hypertensives. Systolic blood pressure rose progressively during adrenaline infusion at a lower infusion rate in the hypertensives than in the controls. Similarly, while heart rate rose during adrenaline infusion in both groups, there was a greater rise in the hypertensives. The increased cardiovascular responsiveness to adrenaline infusion in patients with essential hypertension may explain why plasma atrial natriuretic peptide levels rose only in this group and not the normotensive subjects.

Plasma immunoreactive atrial natriuretic peptide and vasopressin after ethanol intake in man

To study the mechanisms of alcohol-induced diuresis, the plasma concentration of immunoreactive atrial natriuretic peptide and arginine vasopressin, serum sodium and osmolality, plasma renin activity and aldosterone, urinary sodium and volume, free water clearance, blood pressure and heart rate were measured in seven healthy men after oral intake of ethanol (1.5 g kg-1 in 6 h). Serum ethanol levels increased to 27 +/- 4 mmol l-1 (mean +/- SD) in 30 min and remained detectable for 14 h. Serum osmolality rose from 280 +/- 10 to 340 +/- 4 mosm kg-1 in 2 hours (P less than 0.01) and was 300 +/- 4 at 14 h (P less than 0.01). Formation of hypotonic urine began after the alcohol intake and resulted in a net loss of 0.9 +/- 0.1 kg water in 2 h. Free water clearance increased from -3.4 +/- 1.4 to 2.8 +/- 1.5 ml min-1 in 2 h (P less than 0.01). Plasma immunoreactive arginine vasopressin decreased from 5.7 +/- 2.1 to 3.3 +/- 1.3 ng l-1 (P = 0.05) in 30 min and increased to 17 +/- 25 and 12 +/- 10 ng l-1 at 6 and 12 h, respectively (P less than 0.05 for both). Plasma immunoreactive atrial natriuretic peptide levels decreased from 17 +/- 9 to the minimum of 11 +/- 3 ng l-1 in 2 h (P less than 0.01) and returned to the initial levels in 6 h. Serum sodium, plasma renin activity and plasma aldosterone increased maximally by 4 +/- 2, 165 +/- 153 and 143 +/- 101% (P less than 0.01 each) during 1-6 h.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of angiotensin II on atrial natriuretic peptide in pregnancy

To clarify the effects on angiotensin II and blood pressure on plasma atrial natriuretic peptide in pregnancy, angiotensin II was infused in 37 healthy, normotensive pregnant women in week 32 of gestation. Levels of atrial natriuretic peptide were determined with the women in a sitting position 3 days before the test and in the left lateral recumbent position immediately before and after the test. Levels of atrial natriuretic peptide were not influenced by parity or salt intake. They were found to be significantly higher in the left lateral recumbent position than in the sitting position. There was also a significant increase in atrial natriuretic peptide levels during angiotensin II infusion. A dose of angiotensin II less than 1.5 ng/kg/min did not affect atrial natriuretic peptide levels. Rise in atrial natriuretic peptide levels was minimally correlated with rise in mean arterial pressure, as caused by acute vasoconstriction from angiotensin II infusion. Therefore it is suggested that, in pregnancy, the response of atrial natriuretic peptide to angiotensin II is mediated not only by changes in blood pressure but also by angiotensinergic receptor activity in the atria.

Norepinephrine-induced atriopeptin release in conscious dogs is mediated by alterations in atrial pressure

This study was designed to determine whether the increase in atriopeptin secretion induced by an intravenous infusion of norepinephrine is mediated directly by adrenergic receptor stimulation or indirectly by the associated increase in atrial pressure. Norepinephrine was infused at 0.5 microgram.kg-1.min-1 for 30 min into both sham-operated (intact) and cardiac-denervated conscious dogs. The infusion increased mean arterial pressure in all dogs. On the other hand, left atrial pressure increased from 5.0 +/- 0.7 to 9.6 +/- 1.6 mmHg (p less than 0.01) in intact dogs, but decreased from 5.5 +/- 1.0 to 2.0 +/- 0.7 (p less than 0.01) in cardiac-denervated dogs. Right atrial pressure changes followed similar trends, but were not significant in the intact group. Plasma atriopeptin increased from 73 +/- 12 to 110 +/- 18 pg/ml (p less than 0.01) as left atrial pressure increased in intact dogs and decreased from 79 +/- 15 to 54 +/- 10 pg/ml (p less than 0.01) as left atrial pressure decreased in cardiac-denervated dogs. The changes in plasma atriopeptin correlated closely with the changes in left atrial pressure (r = 0.941, p less than 0.001) and to a lesser extent with the changes in right atrial pressure (r = 0.413, p less than 0.05). These results suggest that the change in plasma atriopeptin induced by infusion of norepinephrine into conscious dogs is mediated by the concomitant change in atrial pressures.

Levels of atrial natriuretic peptide are not always consistent with atrial pressure: is there alternative regulation as evidenced in Gordon's and Bartter's syndromes?

1. In Bartter's syndrome, atrial pressures were low, consistent with volume contraction, while atrial natriuretic peptide (ANP) levels were unexpectedly elevated. Infusion of normal saline increased both right atrial pressure (RAP) and ANP levels, while administration of prostaglandin inhibitors raised RAP, probably due to volume expansion, but ANP levels fell paradoxically. 2. In Gordon's syndrome, atrial pressures were unexpectedly low or normal despite volume expansion, while ANP levels were normal. Pressor infusions of angiotensin II either raised right and left atrial pressures (LAP) without increasing ANP, or increased ANP without increasing atrial pressures. 3. In these two syndromes, atrial pressures and ANP levels were poorly correlated, leading to the proposal that other regulators of ANP may be important.

Increased release of alpha-atrial natriuretic peptide during controlled mechanical ventilation with positive end-expiratory pressure in humans

The objective of this study was to test the hypothesis that a release of alpha-artrial natriuretic peptide (ANP) is depressed resulting in the reduction of urinary output in patients receiving controlled mechanical ventilation (CMV) with positive end-expiratory pressure (PEEP). Five normovolemic patients with no apparent cardiac, renal, endocrine, or pulmonary dysfunctions were included in this study. After the patients were mechanically ventilated using a volume-cycled ventilator with zero cmH(2)O PEEP for one hour, hemodynamic variables were measured. Urine and blood samples were collected after the measurements. Plasma alpha-ANP levels were determined on blood samples taken from radial artery using specific radioimmunoassay. Then PEEP levels were changed to 5, 10, 15 and, finally, 0 cmH(2)O in four consecutive one-hour periods. At the end of each period, the measurements and collection of the samples were repeated. With increasing levels of PEEP, central venous pressure (CVP), pulmonary artery wedge pressure (PAWP), and heart rate were pressure-dependently increased. On the other hand, cardiac output and urinary output were decreased. Plasma levels of alpha-ANP were also increased by the institution of PEEP. These changes occurred in a pressure-dependent fashion. Urinary sodium excretion, potassium excretion, fractional excretion of sodium and free water clearance remained unchanged. It is concluded that a release of alpha-ANP was augmented rather than depressed with PEEP. This suggests that a decrease in urinary excretion in patients with PEEP may not be due to a reduced release of alpha-ANP.

Altering angiotensin levels by administration of captopril or indomethacin, or by angiotensin infusion, contributes to an understanding of atrial natriuretic peptide regulation in man

1. Plasma atrial natriuretic peptide (ANP) levels were positively correlated with plasma renin activity (PRA) levels, when blood volume and blood pressure (BP) were not raised in normal subjects (NLS) or patients with postoperative aldosterone-producing adenoma (APA), Bartter's syndrome (BS), Addison's disease, anorexia nervosa, diuretic abuse or salt-losing congenital adrenal hyperplasia. 2. Angiotensin II infusion raised ANP levels in NLS, and patients with BS, pre- and postoperative APA, only when BP rose, suggesting that this effect might be mediated by the rise in BP. 3. Captopril lowered aldosterone and ANP levels in renal artery stenosis, but falling BP levels could mediate this effect. Captopril lowered aldosterone and BP in BS, but did not lower ANP, perhaps because angiotensin remained elevated. 4. Indomethacin lowered ANP when PRA was initially normal or raised (NLS and BS), but not when PRA was suppressed (APA). This effect could not be mediated by BP, which rose, but could be mediated by renin-angiotensin, which fell. 5. Factors other than central blood volume and atrial stretch may modulate ANP levels. Plasma angiotensin II may be such a factor, and may exert an important influence at high levels, especially when blood volume is low.

Response of atrial natriuretic peptide to adrenaline and noradrenaline infusion in man

1. Atrial natriuretic peptide (ANP) levels were significantly increased during both adrenaline and noradrenaline infusions, in the physiological range, in normal subjects and in patients with essential hypertension. 2. During adrenaline infusion significant increases in both circulating adrenaline and noradrenaline levels were observed. Mean arterial pressure was unaltered. Changes in heart rate were not significant. 3. During noradrenaline infusion, significant increases in circulating noradrenaline and mean arterial pressure were also observed. Heart rate and plasma adrenaline levels were unaltered. 4. Fluctuations in sympathetic nervous system activity may be involved in the regulation of ANP via adrenoceptor stimulated release of ANP. Other known regulators such as atrial stretch and increasing heart rate may modify this response.