A sensitive radioimmunoassay of alpha human atrial natriuretic polypeptide using monoclonal antibody recognizing human form ring structure

A monoclonal antibody (C351) against alpha human atrial natriuretic polypeptide (alpha hANP) recognizing human form ring structure was established and applied to a radioimmunoassay of plasma alpha hANP. The minimum detectable amount in terms of 10% radioligand displacement relative to zero dose were 0.28 fmol/tube, corresponding to 0.7 fmol/ml in plasma after extraction using Sep-Pak C18 cartridges. When the mean plasma levels at recumbent position in fasted morning were compared in 10 young (less than 30 years) and 10 elderly (greater than or equal to 50 years) healthy subjects taking normal sodium diet, it was slightly higher in the latter (3.2 +/- 0.4 vs 4.7 +/- 0.5 fmol/ml, mean +/- SE, p less than 0.05). After i.v. infusion of hypertonic saline (2.5% NaCl) at a rate of 0.24 ml/kg/min for 20 min in 6 normal subjects (26 to 35 years), it was increased from 4.1 +/- 0.4 to 5.9 +/- 0.7 fmol/ml (p less than 0.01). In 6 patients with essential hypertension (34 to 57 years), it was elevated with high salt intake, i.e. 3.3 +/- 0.3, 3.9 +/- 1.03 and 7.6 +/- 1.5 fmol/ml under 34, 170 and 340 mEq NaCl/day for 7 days, respectively. From these results, the radioimmunoassay of plasma IR-alpha hANP using MAb C351 seems to be quite suitable to detect rather small changes at low plasma concentrations and to investigate a physiological importance of alpha hANP in man.     

Pharmacokinetics of synthetic alpha-human atrial natriuretic polypeptide in normal men; effect of aging

We reported that plasma human atrial natriuretic polypeptide (hANP) in healthy aged men was significantly higher than that in young men, presumably due to a diminished cellular response to endogenous hANP in the aged subjects (J. Clin. Endocrinol. Metab., 64 (1987) 81). To examine the effect of age on the metabolic clearance rate for hANP, synthetic alpha-hANP (2 micrograms/kg) was administered intravenously to healthy young (n=6) and aged (n=4) men. The plasma hANP was measured by a direct radioimmunoassay. The disappearance of alpha-hANP from plasma was characterized by a biexponential decay curve, in both groups. There was no difference of the initial phase between young and aged groups (young vs aged; 1.2 +/- 0.2 min vs 2.9 +/- 1.0 min), while the second phase of alpha-hANP disappearance in the aged group was significantly prolonged compared with that in the young individuals (young vs aged; 17.3 +/- 3.9 min vs 34.3 +/- 3.0 min, P less than 0.05). We tentatively conclude that the reduced metabolic clearance rates for hANP were responsible, in part, for the high plasma concentrations in the aged men.     

The lipid-mobilizing effect of atrial natriuretic peptide is unrelated to sympathetic nervous system activation or obesity in young men

We recently demonstrated that natriuretic peptides and especially the atrial natriuretic peptide (ANP) are powerful lipolytic agents on isolated human fat cells. To search for a possible influence of obesity on ANP responsiveness, we compared the lipolytic effects of human ANP (h-ANP) on isolated subcutaneous abdominal adipose tissue (SCAAT) fat cells from young healthy lean and obese men. The lipid-mobilizing effects of an intravenous infusion of h-ANP was studied, as well as various metabolic and cardiovascular parameters that were compared in the same subjects. h-ANP (50 ng/min/kg) was infused iv for 60 min. Microdialysis probes were inserted in SCAAT to measure modifications of the extracellular glycerol concentrations during h-ANP infusion. Spectral analysis of blood pressure and heart rate oscillations that were recorded using digital photoplethysmography were used to assess changes in autonomic nervous system activity. h-ANP induced a marked and similar increase in glycerol and nonesterified fatty acids, and a weak increase in insulin plasma levels in lean and obese men. Plasma norepinephrine concentrations rose similarly during h-ANP infusion in lean and obese men. The effects of h-ANP infusion on the autonomic nervous system were similar in both groups, with an increase in the spectral energy of the low-frequency band of systolic blood pressure variability and a decrease in the spectral energy of the high-frequency band of heart rate. In SCAAT, h-ANP infusion increased extracellular glycerol concentration and decreased blood flow similarly in both groups. The increase in extracellular glycerol observed during h-ANP infusion was not modified when 0.1 mM propranolol was added to the microdialysis probe perfusate to prevent beta-adrenoceptor activation. These data show that ANP is a potent lipolytic hormone independent of the activation of the sympathetic nervous system, and that obesity did not modify the lipid-mobilizing effect of ANP in young obese subjects.     

Effects of atrial natriuretic peptide infusion and its metabolism in patients with chronic renal failure.

Synthetic alpha-human atrial natriuretic peptide (hANP) was infused continuously at a rate of 80 ng/kg/min for 20 min into normal volunteers and patients with chronic renal failure (CRF) receiving hemodialysis. Blood pressure (BP) decreased significantly both in normals and in patients with CRF. The magnitude and the duration of the decrease, however, were greater in patients with CRF. The plasma aldosterone concentration (PAC) decreased significantly in normals and only minimally in patients with CRF. The half time (T1/2) of plasma hANP in patients with CRF (M +/- SE: 4.5 +/- 0.5 min) was longer than that in normals (1.8 +/- 0.2 min). Moreover, the metabolic clearance rate in patients with CRF (64 +/- 7 ml/kg/min) was less than in normals (150 +/- 20 ml/kg/min). Thus, the T1/2 in plasma of hANP in patients with CRF was noticeably longer than in a normal control group. These findings suggest that hANP suppresses PAC regardless of electrocyte imbalances and/or volume change induced by kidney dysfunction and that the kidney may be important in degrading hANP.     

Clonidine and morphine increase atrial natriuretic peptide secretion in anesthetized rats

In order to determine whether the activity of central alpha 2-adrenergic and opioid receptors influence plasma atrial natriuretic peptide (ANP) levels, clonidine and morphine were infused into the lateral cerebral ventricle for 45 min in anesthetized Sprague-Dawley rats. The central administration of a low dose of clonidine (10 ng/min) caused a significant increase in plasma ANP without changing arterial blood pressure or central venous pressure. Pretreatment with yohimbine (5 micrograms/min) completely blocked the effect of clonidine. Central infusion of morphine (100 ng/min) also elevated plasma ANP levels and naloxone (5 micrograms/min) blunted this effect. Intravenous infusion of the same dose of clonidine or morphine did not affect plasma ANP levels. Moreover, the effect of clonidine on plasma ANP was partially blocked by pretreatment with naloxone (5 micrograms/min). These results suggest that central alpha 2-adrenergic and opioid receptors may be involved in ANP secretion.     

Pharmacokinetic analysis of the plasma disappearance of ovine follitropin and analogues in the male rat

Ovine follitropin (oFSH) and its chemically deglycosylated (DG) or the asialo analogue were injected as a single bolus (2.5 micrograms) to intact mature male rats and plasma levels measured by radioimmunoassay. Semi-logarithmic plots of the plasma disappearance curves were all biphasic. For all substances tested, the data were best fitted to a biexponential equation and the following parameters were calculated: the half-lives of the distribution and elimination phases and the metabolic clearance rate (MCR) at steady state. It was found that the distribution phase of oFSH was much slower than for deglycosylated (DG) alpha + beta recombinant, DG-oFSH and asialo-oFSH. The elimination half-lives for asialo-oFSH and DG alpha + beta recombinant were much shorter than for oFSH and DG-oFSH. oFSH had the lowest MCR, followed by DG alpha + beta, DG-oFSH and asialo-oFSH. This study clearly shows that sugar residues internal to sialic acid also play a significant part in the clearance of the hormone.     

Identification of an atrial natriuretic peptide specific receptor in human kidney

A specific receptor for human atrial natriuretic peptide (h-ANP) was identified in the human kidney using the radioligand binding assay. Samples were prepared from non-malignant renal tissues obtained at nephrectomy of patients with renal carcinoma. Binding studies using [125I]hANP were performed at 0 degree C for 20 minutes and terminated by a rapid filtration technique. Scatchard plot analysis revealed [125I]hANP bound to a single class of binding site (Kd = 0.4 +/- 0.2 nM) with a density of 16 +/- 4 fmol/mg protein in the renal cortex (n = 7). The binding was rapid and maximal binding was obtained within 20 minutes after the start of incubation. Radioligand displacement was observed in a dose dependent fashion when cold hANP was entered into the reaction mixture. However, unrelated agents, such as angiotensin II or 1-epinephrine, did not affect the binding. This is the first time characterization of the hANP receptor in the human kidney has been conducted using a Scatchard plot analysis.     

Atrial natriuretic factor inhibits the CRH-stimulated secretion of ACTH and cortisol in man.

Corticotrophic secretion of ACTH is stimulated by corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP), and suppressed by glucocorticoids. In vitro and preclinical studies suggest that atrial natriuretic factor (ANF) may be a peptidergic inhibitor of pituitary-adrenocortical activity. The aim of this study was to elucidate a possible role of ANF as a modulator of ACTH release in humans. A bolus injection of 100 micrograms human CRH (hCRH) during a 30 min intravenous infusion of 5 micrograms/min human alpha atrial natriuretic factor (h alpha ANF) was administered at 19:00 to six healthy male volunteers. In comparison to saline, a blunted CRH-stimulated secretion of ACTH (mean maximum plasma level +/- SD 45 min after hCRH: saline 46.2 +/- 14.2 pg/ml, h alpha ANF 34.6 +/- 13.8 pg/ml, p-value = 0.007) and a delayed rise (10 min) in cortisol were detected. The maximum plasma cortisol levels remained nearly unchanged between saline and h alpha ANF administration (mean maximum plasma level +/- SD 60 min after hCRH: saline 182 +/- 26 ng/ml, h alpha ANF 166 +/- 54 ng/ml). No effects of h alpha ANF on basal cortisol levels were observed; in contrast, basal ACTH plasma levels were slightly reduced. Basal blood pressure and heart rate remained unaffected. In the control experiment, infusion of 3 IU AVP in the same experimental paradigm increased basal and stimulated ACTH and cortisol levels significantly in comparison to saline. These observations suggest that intravenously administered haANF inhibits the CRH-stimulated release of ACTH in man.     

Plasma disappearance half-time and metabolic clearance rate of exogenous human growth hormone-releasing hormone-(1-44)-NH2 in normal subjects

By means of human growth hormone-releasing hormone (hGHRH)-RIA using an antiserum directed toward the C-terminal region of hGHRH-(1-44)-NH2, the plasma disappearance half-time and metabolic clearance rate (MCR) of immuoreactive hGHRH (IR-hGHRH) were examined in normal subjects after an iv bolus injection of synthetic hGHRH-(1-44)-NH2 (1 microgram/kg BW). The disappearance of IR-hGHRH from plasma was characterized by a biexponential decay curve, with initial distribution and subsequent metabolic t1/2 value of 5.0 +/- 1.0 and 29.6 +/- 5.4 min (mean +/- SE), respectively. The MCR of IR-hGHRH was 6.1 +/- 1.2 ml/min X kg. The volume of distribution of IR-hGHRH was 3.3 +/- 0.4 liters. The molecular size of the plasma IR-hGHRH was not different from that of hGHRH-(1-44)-NH2 for at least 90 min after the injection of hGHRH-(1-44)-NH2 when examined by gel-filtration chromatography. This prolonged clearance of hGHRH from human plasma relative to that of other hypothalamic hormones may in part explain the sustained plasma GH rises after hGHRH injection in man.     

Plasma growth hormone responses to repetitive administrations of growth hormone releasing factor in patients with pituitary dwarfism

Plasma growth hormone (GH) responses to the repetitive administrations of synthetic human pancreatic growth hormone releasing factor (hpGRF-44) were studied in 15 patients with GH deficiency (11 diagnosed as idiopathic and 4 diagnosed as secondary to hypothalamo-pituitary tumor). hpGRF-44 was administered by single iv bolus (2 micrograms/kg), repetitive im (100 micrograms, twice a day), and/or repetitive iv infusion (2.5 micrograms/min for 90 min, once a day) for three to six consecutive days. Three of the eleven idiopathic GH deficient patients had plasma GH responses to both single iv bolus injection and repetitive administrations by im, or iv infusion of hpGRF. In four of the remaining eight, who had not had peak plasma GH levels above 5 ng/ml to a single iv bolus of the peptide, repetitive administrations of hpGRF-44 by im injection and/or iv infusion induced GH responses to the peptide. In the four patients with secondary GH deficiency, three had plasma GH response to hpGRF administration but one patient, who had indications of pituitary disorder, did not show any plasma GH response to either single iv injection or repetitive administrations of hpGRF-44. These data show that repetitive administrations of hpGRF-44 can induce plasma GH responses in some GH deficient patients who do not respond to a single iv bolus of the peptide.     

Age differences in renal response to atrial natriuretic peptide in rabbits

Plasma levels of atrial natriuretic peptide (ANP) and the effect of exogenous ANP on renal function have been studied in newborn and adult rabbits. In order to investigate an age difference in responsiveness to ANP, we studied the renal effects of alpha-human ANP (1-28) administered at the same dose per kg body weight in adult and neonatal rabbits. Plasma basal ANP levels were similar in 18 newborn (4- to 11-day-old) compared to 7 adult rabbits (150 +/- 16 and 151 +/- 28 pg/ml, resp.). Eleven newborn and 11 adult rabbits were anesthetized and mechanically ventilated. After a control period, each animal received an hANP loading dose (3 micrograms/kg i.v.), followed by an infusion of 0.3 micrograms/kg/min. Blood gases remained stable throughout the experiment in both groups. Mean blood pressure decreased in newborn (28.5 +/- 0.8 to 26.2 +/- 1.0 mmHg) and adult (92 +/- 3 to 84 +/- 3 mmHg) animals. Percent hANP-induced changes in renal functions in newborn and adult rabbits were, respectively: urine flow rate: -21 +/- 4% and +57 +/- 8%; urinary sodium excretion: +4 +/- 7% and +81 +/- 11%; glomerular filtration rate (GFR): -19 +/- 4% and -4 +/- 6%; renal blood flow (RBF): -22 +/- 4% and -11 +/- 5%. As expected, diuresis and natriuresis increased in adult rabbits. Failure of hANP to increase natriuresis and diuresis in newborn rabbits could be related to the marked decrease in GFR, receptor immaturity and/or interactions with other hormonal systems.     

Amino acid sequence of atrial natriuretic peptides in human coronary sinus plasma

Two atrial natriuretic peptides were purified from pooled human coronary sinus plasma by Sep-Pak extraction, immunoaffinity chromatography and reverse phase HPLC. The amino acid sequences of the two peptides were homologous with 99-126 human atrial natriuretic peptide (hANP) and 106-126 hANP, the latter being most probably linked to 99-105 ANP by the disulphide bond. The molar ratio of the peptides in plasma, as assessed by radioimmunoassay was 10:3.     

Effect of human atrial natriuretic peptide on blood pressure after sodium depletion in essential hypertension.

Human atrial natriuretic peptide was infused over four hours in three patients with essential hypertension. When the patients had a sodium intake of 200 mmol (mEq) daily an infusion of 0.5 micrograms atrial natriuretic peptide/min caused no significant change in blood pressure, whereas an infusion of 1.0 micrograms/min caused a gradual decrease in blood pressure and an increase in heart rate. After two to three hours of infusion with the higher dose two patients showed a sudden decrease in heart rate, with symptomatic hypotension. When the same patients had an intake of 50 mmol sodium daily their blood pressure was more sensitive to infusion of atrial natriuretic peptide; one patient again developed symptomatic hypotension, this time during an infusion of 0.5 micrograms/min. During all infusions distinct natriuresis occurred irrespective of whether blood pressure was affected. Prolonged, relatively low dose infusions of atrial natriuretic peptide can cause unwanted symptomatic hypotension. The effect on blood pressure is enhanced after sodium depletion, and blood pressure should be monitored carefully during longer infusions of atrial natriuretic peptide in patients with essential hypertension.     

Comparative biological activities of alpha-rat and alpha-human atrial natriuretic peptide in the inhibition of arginine vasopressin release in conscious rats

The comparative biological activities of intracerebroventricular (icv) injection of alpha-rat and alpha-human atrial natriuretic peptide (rANP and hANP, respectively) in the arginine vasopressin (AVP) release in conscious rats and the binding properties of these peptides to their specific receptors have been investigated. An icv injection of 5 micrograms rANP inhibited the AVP release induced by osmotic and hemorrhagic stimuli. In contrast, 20 micrograms of hANP was needed to exert an inhibitory effect on the AVP release. The receptor binding studies were carried out by using rat hypothalamic membrane preparations. The binding studies revealed that the potency of rANP was greater than that of hANP in displacing radioligand from its binding sites. Scatchard analysis revealed that the dissociation constant for rANP was significantly lower than that for hANP (0.52 +/- 0.04 vs 1.20 +/- 0.16 nM, P less than 0.01). The binding capacity of these peptides was similar. These results suggest that the greater biological potency of rANP compared with hANP in the inhibition of AVP release is caused by the difference in the binding potency of these peptides.     

Significant increase in plasma immunoreactive atrial natriuretic polypeptide concentration during head-out water immersion

To investigate the physiological regulatory mechanism of human atrial natriuretic polypeptide (hANP) secretion, plasma hANP was measured by a direct radioimmunoassay during head-out total body water immersion (WI) in normal men. Five healthy men were immersed in water for 1 hr. Urine volume and Na excretion were significantly increased during WI. Plasma hANP increased significantly during WI peaking at 30 min. and returned toward the baseline after WI. Plasma renin activity and norepinephrine were suppressed occasionally during WI. Plasma ADH did not change throughout the study period. Maximal increments in plasma hANP correllated with that in urine output or urinary Na excretion during WI. These data suggest that acute central hypervolemia caused by WI increases hANP secretion and that this increase may participate in the diuretic response to WI.     

Atrial natriuretic peptides elevate cyclic GMP levels in primary cultures of rat ependymal cells

The aim of this study was to examine the effect of atrial natriuretic peptides on primary cultures of ependymal cells, as measured by changes in intracellular levels of cyclic GMP. Incubation of ependymal cells with rat atrial natriuretic peptide-(1-28) (rANP) elicited a 30-fold increase in ependymal cGMP content within 1 min and more than a 100-fold increase within 10 min to a plateau value of approximately 30 pmol/mg protein. The C-type natriuretic peptide (CNP) elicited a similar increase in cGMP levels; however the maximal effect was observed within 1 min and the levels subsequently dropped by 90% to a low plateau within 10 min. A comparison of the concentration-response curves for rANP, human ANP-(1-28) (hANP) and CNP showed that rANP, hANP and CNP had similar effects, with regards to elevation of cGMP levels at high concentrations, but with differing EC50 values. These results demonstrate the presence of a heterogenous population of functional ANP receptors in cultured ependymal cells suggesting that ANP may regulate specific ependymal cell activity.     

Identification and partial characterization of immunoreactive and bioactive atrial natriuretic peptide from eel heart

Summary Cardiocytes positive for human atrial natriurectic peptide (hANP) were identified histochemically in the eel atrium, but they were not found in the ventricule. Secretory granules were frequently observed in atrial cardiocytes by electron microscopy, but the number of such granules was quite small in the ventricle. Immunogold cytochemistry revealed that immunoreactive ANP (IR-ANP) in atrial cardiocytes was localized in these granules. In spite of poor immunostaining of the eel ventricle, an acid extract of the ventricle contained 25±4 ng·g tissue^-1 (n=9) of IR-ANP when the level of IR-ANP was measured by radioimmunoassay (RIA) for hANP. This level was one eight of that measured in atrial extracts (203±13 ng·g tissue^-1, n=9). Plasma contained 116.7±18.6 pg·ml^-1 (n=9) of IR-ANP. An extract of eel hearts decreased arterial pressure in eels and quail as did hANP. The level of ANP in the extract, as measured by an eel vasodepressor bioassay, was much greater than that measured by RIA for hANP. The immunoreactive and bioactive ANP in the heart extract are identical since the vasodepressor activity disappeared after IR-ANP was absorbed by excess antibodies raised against hANP. Chromatography on Sephadex G-75 generated a major peak of IR-ANP at a position that corresponded to a molecular weight of 14 kD and minor peaks at 3–7 kD from both plasma and heart extract. Reverse phase HPLC of plasma and heart extract generated several peaks of IR-ANP at positions more hydrophilic than those of mammalian ANPs. These results show that eel hearts contain immunoreactive and bioactive ANPs which are distinctly different from hANP. These ANPs are synthesized both in the atrium and in the ventricle, and they are secreted into the circulation mostly in the larger molecular form. The atrial ANP may be stored in the granules and secreted upon exposure of eels to certain stimuli, but the ventricular ANP may be secreted constitutively into the circulation without prior storage in the granules.Abbreviations ANP atrial natriuretic peptide - BSA bovine serum albumin - IR-ANP immunoreactive ANP - PBS phosphate-buffered saline - RIA radioimmunoassay     

Infusion of a novel peptide, calcitonin gene-related peptide (CGRP) in man. Pharmacokinetics and effects on gastric acid secretion and on gastrointestinal hormones

Calcitonin gene-related peptide (CGRP) is a recently discovered widespread regulatory peptide which is encoded in the same gene as calcitonin. We assessed the effect of systemic infusion of synthetic rat CGRP at low dose (range 0.32-2.56 pmol/kg per min) on submaximal pentagastrin-stimulated gastric secretion and on gastrointestinal hormones. To assess its pharmacokinetic parameters in man the MCR and plasma half-life were estimated by the continuous infusion method. Gastric acid output and pepsin secretion were significantly reduced by CGRP (-29% of basal, P less than 0.01 and -40% of basal, P less than 0.005, respectively). There was a significant fall in basal levels of gastrin (-39%, P less than 0.001); gastric inhibitory peptide (-44.7%, P less than 0.001); enteroglucagon (-25%, P less than 0.001) and neurotensin (-33%, P less than 0.05). There was no significant change in plasma levels of insulin, motilin, pancreatic polypeptide or glucose. Suppression of gastric secretion and the fall in gastrointestinal hormones was prolonged and basal levels were not re-established after stopping the CGRP infusion. The disappearance curve of immunoreactive CGRP from the plasma was bi-exponential. The plasma half-life of immunoreactive CGRP was calculated as 6.9 +/- 0.9 min for the fast decay and 26.4 +/- 4.7 min for the slow decay. The calculated MCR was 11.3 +/- 1.2 ml/kg per min. Except for flushing of the face no untoward effects were observed. The results of this study suggest the possibility that CGRP could play a role in the regulation of gastric secretion and gastrointestinal hormone release.     

Circadian variation and age dependence of human atrial natriuretic peptide levels in hospitalized patients

Circadian variation of plasma levels of human atrial natriuretic peptide (hANP) was studied in 8 patients less than or equal to 65 a of age (mean +/- SD = 43.8 +/- 13 a; 5 females, 3 males) and in 15 patients greater than 65 years of age (mean +/- SD = 81.4 +/- 5.7 a; 9 females, 6 males). Intraindividual variation was up to 40% relative to the day's mean level in both groups. A significant elevation of hormone levels in the evening was observed in patients greater than 65 years of age (P less than 0.002), no circadian rhythm could be detected in patients less than or equal to 65 years of age. All patients except the two eldest had average plasma levels of hANP within our normal range of 3-75 ng/l (N = 106; mean +/- SD = 29.9 +/- 15.3), found in healthy persons up to 65 years of age. We propose, that in the elderly hANP levels rise during the day by edema because of latent renal and/or cardial insufficiency. At rest, in the evening and during the night edema is eliminated by the well known nycturia, which might well be facilitated or at least partially caused by elevated levels of hANP.     

Effects of synthetic atrial natriuretic factor (ANF) on renin-aldosterone axis in the conscious newborn calf

The effects of synthetic atrial natriuretic factor (ANF) on the renin-aldosterone axis were studied in fifteen 4-7 day-old male milk-fed calves divided into 3 groups of 5 animals each. Synthetic ANF intravenous (i.v.) administration (1.6 micrograms/kg body wt over 30 min) induced a transient significant fall in plasma renin activity (from 2.5 +/- 0.3 to 1.7 +/- 0.3 ng angiotensin l/ml/h; P less than 0.05) but failed to reduce basal plasma aldosterone levels in the first group of animals. Administration (i.v.) of angiotensin II (AII) (0.8 micrograms/kg body wt for 75 min) was accompanied by a progressive fall in plasma renin activity (from 2.2 +/- 0.3 to 0.8 +/- 0.1 ng angiotensin l/ml/h; P less than 0.01) and by an increase in plasma aldosterone levels (from 55 +/- 3 to 86 +/- 5 pg/ml; P less than 0.01) both in the second and the third groups; addition of ANF to AII infusion (AII: 0.5 mu/kg body wt for 45 min; AII: 0.3 micrograms/kg body wt and ANF 1.6 micrograms/kg body wt during 30 min) in the third group did not modify plasma renin activity or AII-stimulated plasma aldosterone levels when compared to the AII-treated group. These findings show that in the newborn calf ANF is able to reduce plasma renin activity but fails to affect basal and AII-stimulated plasma aldosterone levels, suggesting that the zona glomerulosa of the newborn adrenal cortex is insensitive to a diuretic, natriuretic and hypotensive dose of the atrial peptide.