ATRIAL NATRIURETIC HORMONES ORIGINATING FROM THE N-TERMINUS OF THE ATRIAL NATRIURETIC FACTOR PROHORMONE

1. Four peptide hormones consisting of amino acids 1–30 (Long Acting Sodium Stimulator), 31–67 (Vessel Dilator), 79–98 (Kaliuretic Stimulator) and 99–126 (Atrial Natriuretic Factor [ANF]) originate from the same 126 amino acid ANF prohormone. 2. Each of these four peptide hormones circulates as a distinct peptide with vessel dilator and long acting sodium stimulator circulating at 10- to 24-fold higher concentrations than ANF while kaliuretic stimulator circulates at a three-fold higher concentration than ANF. 3. Each of these peptide hormones is released with an increase in central volume causing stretch of the atrium of the heart and with rapid heart beats greater than 125 beats/min. 4. Each of these peptide hormones lowers blood pressure, causes a diuresis and enhances sodium and/or potassium excretion. 5. In disease states which retain sodium and water such as congestive heart failure (CHF), each of these atrial peptides increases in the circulation proportionately to the severity of sodium retention, but of the radioimmunoassays to each of these hormones only the vessel dilator radioimmunoassay differentiates between mild (class I) CHF and healthy individuals.     

ATRIAL NATRIURETIC PEPTIDE FRAGMENTS IN DOGS WITH EXPERIMENTAL HEART FAILURE

1. This study in the canine arteriovenous (AV) fistula model of high-output heart failure (HOHF) evaluated the chronic temporal changes in plasma ANF and pro ANF 31–67 and their relationship to body-fluid balance and the renin-aldosterone axis. In addition, the haemodynamic, hormonal and renal excretory effects of synthetic pro ANF 31–67 infusions were examined in normal and AV fistula dogs with compensated HOHF. 2. Following the construction of the AV fistula, the dogs exhibited chronic parallel elevations in right atrial pressure and the plasma concentrations of ANF and pro ANF 31–67. The gradual increases in the two peptides were associated with a gradual decrease in plasma renin activity and the re-establishment of sodium balance. 3. In normal and compensated AV fistula dogs, synthetic pro ANF 31–67 produced similar significant reductions in arterial blood pressure, right atrial pressure and elevations in urinary sodium excretion. These effects were not associated with increases in plasma or urinary cyclic GMP (cGMP). 4. These results suggest that the elevation in the endogenous circulating levels of pro ANF 31–67 in the AV fistula dogs may represent one chronic adaptive mechanism to achieve body fluid homeostasis. Furthermore, via potentially different mechanisms of action, ANF and pro ANF 31–67 may coordinate and contribute to the regulation of haemodynamic and renal function during physiological and pathophysiological situations.     

SODIUM LOADS ENHANCE THE NATRIURETIC RESPONSES TO ATRIAL NATRIURETIC PEPTIDE AND NEUTRAL ENDOPEPTIDASE INHIBITORS IN CONSCIOUS CYNOMOLGUS MONKEYS

1. The effects of sodium supplements on the renal responses to human atrial natriuretic peptide (hANP 99–126) and to the selective inhibitors of neutral endopeptidase 3.4.24.11 (NEP) SQ 28 603 and candoxatrilat were determined in conscious monkeys. 2. When the monkeys’ diet was changed from 0.55% sodium to 1.1% sodium, the natriuretic response to 100 μmol/kg intravenous of SQ 28603 increased from 665 ± 64 to 1015 ± 224 μEq/3h. An acute oral load of 25 mEq sodium significantly increased the natriuresis stimulated by 300 pmol/kg, P.o., of SQ 28603 from 700 ± 332 μEq/3h in normal monkeys to 2437 ± 841 μEq/3h. Therefore, the non-human primate model was appropriate for investigating the effects of sodium loads on the urinary ANP and cGMP responses to exogenous ANP in the presence and absence of NEP inhibitors. 3. Graded intravenous infusions of saline increased basal urine volume and excretion of sodium and ANP. Salt supplements enhanced the diuretic, natriuretic and ANP responses to 0.3 nmol/kg intravenous of hANP 99–126 in monkeys treated with vehicle or 10 μmol/kg intravenous of candoxatrilat. The sodium and ANP excretions stimulated by hANP 99–126 were positively correlated with each other and with the calculated intravenous sodium load in the presence or absence of candoxatrilat. 4. SQ 28 603 and candoxatrilat (0.3 to 10 μmol/kg intravenous) each produced significant, dose-related potentiation of the natriuretic, cGMP and ANP responses to 0.3 nmol/kg intravenous of hANP 99–126 in monkeys receiving 5 mL/kg + 0.2 mL/min saline. In addition, the highest dose of SQ 28 603 produced significant depressor activity. 5. In conclusion, the increased natriuretic activity of hANP 99–126 in sodium loaded monkeys was mediated, in part, by increased ANP delivery to the guanylate cyclase linked ANP receptors in the distal renal tubules.     

RENAL EFFECTS OF ATRIAL NATRIURETIC FACTOR (99–126) IN POTASSIUM LOADED SHEEP

1. The renal response to renal arterial infusion of synthetic atrial natriuretic factor (ANF 99-126) was examined in conscious sheep following dietary K loading, and compared with the response in normal sheep. 2. Renal arterial infusion of ANF in K loaded sheep increased the excretion of Na and Ca, but did not affect the excretion of K. 3. The natriuretic effect of ANF was attenuated in K loaded animals, possibly as a consequence of the reduction in Na status which is associated with K loading.     

RENAL EFFECTS OF ATRIAL NATRIURETIC FACTOR (99–126) IN WATER-DEPRIVED SHEEP

1. The renal response to renal arterial infusion of synthetic human atrial natriuretic factor (ANF) (99-126) at 50 micrograms/h was examined in conscious sheep dehydrated by 48 h water deprivation and was compared with the response of normally hydrated animals. 2. Renal arterial infusion of ANF produced increases in the excretion of Na, K, Ca and urine in both dehydrated and normally hydrated animals, although the effect was significantly blunted in dehydrated animals compared with normally hydrated animals. 3. The attenuation of renal effects of ANF in dehydrated animals is probably due to the negative sodium and/or fluid balance of the dehydrated animals.     

ATRIAL NATRIURETIC FACTOR IN CHRONIC RENAL FAILURE: STUDIES IN MAN AND THE RAT

1. Plasma concentration and atrial content of atrial natriuretic factor (ANF) were measured in rats with chronic renal failure induced by subtotal nephrectomy. 2. Plasma ANF was higher, and atrial ANF content lower in rats with renal failure when compared with sham-operated controls. 3. Plasma renin activity (PRA) and ANF were elevated at 1 week following subtotal nephrectomy. After 1 month plasma ANF had risen further, but PRA was suppressed to below control values. 4. Plasma ANF was also measured in six patients with chronic renal failure undergoing routine haemodialysis. 5. Elevated plasma ANF levels in patients with renal failure were lowered by haemodialysis, although extraction of ANF across the dialysis membrane was negligible. 6. Secretion of ANF is increased in chronic renal failure in man and the rat, possibly mediated by increased intravascular volume.     

HYPOTENSIVE RESPONSE TO ATRIAL NATRIURETIC FACTOR IN CONSCIOUS RENAL HYPERTENSIVE BEAGLES

The hemodynamic responses to i.v. infusion of 0.3 and 0.6 microgram/kg per min of human atrial natriuretic factor (hANF [102-126]) in intact, conscious, one-kidney, perinephritic, hypertensive beagles were examined and compared with the responses in ganglionic-blocked dogs. Blood pressure and heart rate were not affected but plasma ANF-like immunoreactivity was increased by as much as 627%. After hexamethonium (20 mg/kg, i.v.) blockade, a dose-dependent hypotensive response of up to 29 mmHg with no change in heart rate was observed. It is concluded that the compensatory mechanisms of the neurally mediated baroreflex system masked the depressor actions of hANF.     

RAPID HAEMODYNAMIC RESPONSES TO ATRIAL NATRIURETIC FACTOR (99–126) IN CONSCIOUS SHEEP

1. Haemodynamic effects of 20 micrograms and 100 micrograms injection of atrial natriuretic factor 99-126 (ANF) were studied in conscious sheep. 2. ANF injection rapidly decreased blood pressure associated with a fall in total peripheral resistance, increased heart rate and cardiac output. These parameters returned to normal within 5 min of injection. 3. This study shows that ANF has an initial vasodilatory action to decrease blood pressure, which is different from the hypotensive mechanism seen with short-term infusion (60 min) of ANF in sheep.     

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.     

ADRENALECTOMY OVERCOMES THE BLUNTED NATRIURETIC RESPONSE TO ATRIAL NATRIURETIC PEPTIDE DURING HYPOCAPNIA IN RATS

1. Hypocapnia has been shown to blunt the natriuretic effect of atrial natriuretic peptide (ANP) independently of the renal nerves. In order to examine whether the adrenal glands are a limiting factor for the natriuretic effect of ANP, we evaluated the natriuretic responses of adrenalectomized rats to ANP infusion during hypocapnia. 2. Rats subjected to total adrenalectomy (ADX) or sham-operation (sham) were divided into hypocapnic and normo-capnic groups depending on their arteria. PCO₂ levels. 3. In sham rats, ANP infusion at a rate of 12 μg/kg per h resulted in a smaller increase in the fractional excretion of sodium during hypocapnia (mean±SEM: 1.02±0.40%, n = 10) than normocapnia (3.95±0.64%, n = 9; P < 0.001). The level of fractional excretion of sodium with ANP infusion during hypocapnia was not significantly different from the level in saline-infused hypocapnic sham rats (0.93 ±0.62%, n= 10). In hypocapnic ADX rats (n= 11), ANP induced greater increases in the fractional excretion of sodium (5.59±1.35%) than did saline infusion (1.04+1.02%, n= 10; P < 0.002). In the absence of adrenal glands, the magnitude of natriuresis after ANP infusion during hypocapnia and normocapnia (3.32 ±1.07%, n = 9) were the same. 4. We conclude that the natriuretic effect of ANP is blunted during hypocapnia in the presence, but not in the absence, of adrenal glands. Our data suggest that the adrenal glands have an important role in limiting the natriuretic effect of ANP.     

RENAL EFFECTS OF ATRIAL NATRIURETIC FACTOR (99–126) IN CONSCIOUS SODIUM-REPLETE SHEEP

1. The effect of renal arterial infusion of synthetic human atrial natriuretic factor (ANF (99-126] on renal function in the conscious euvolaemic sheep was characterized. ANF (99-126) was infused for 2 h at 5 and 50 micrograms/h into the renal artery of crossbred Merino ewes with chronically indwelling cannulae inserted in the renal artery. The effect on absolute and fractional excretion of Na, K, Ca, Cl and HCO3, glomerular filtration rate (GFR), effective renal plasma flow (ERPF) and free water clearance (CH2O) were measured. 2. Infusion at 50 micrograms/h produced a fourfold increase in Na and Cl excretion. Ca excretion increased eightfold, while K and HCO3 increased by small amounts. At the lower dose only Na, Cl and Ca excretion increased significantly. The changes in absolute excretion of each ion were closely mirrored by changes in fractional excretion. CH2O became more negative at both levels of infusion. Small changes in GFR were measured at both rates of infusion. No changes in ERPF or renin secretion were observed. 3. ANF (99-126) infusion at 50 micrograms/h for 1 h increased the excretion of Li, such that more than 70% of the change in Na excretion was associated with the changes in Li clearance. Changes in GFR accounted for less than 10% of change in Na excretion. 4. Following either long-term (50 micrograms/h for 6 h) or repeated short-term (20 micrograms/h for 30 min) infusions of ANF (99-126), the response of Na excretion was not sustained. The mechanisms of the tachyphylaxis remains undetermined. 5. ANF (99-126) is a powerful stimulus to the absolute and fractional excretion of Na, K, Ca, Cl and HCO3. The mechanism of action is not known, but appears to be related to changes in tubular function and/or a change in glomerulotubular balance.     

BLUNTED NATRIURETIC RESPONSE TO ENDOGENOUS ATRIAL NATRIURETIC PEPTIDE DURING RAPID CARDIAC PACING IN ANAESTHETIZED DOGS

1. We investigated whether diuresis and natriuresis induced by endogenous atrial natriuretic peptide (ANP) were blunted during rapid cardiac pacing. 2. Changes in plasma ANP, renal function and haemody-namics during rapid cardiac pacing were studied in anaesthetized closed-chest dogs. Dogs were paced via the right ventricle at a rate of 200 b.p.m. (moderate pacing) or 250 b.p.m. (severe pacing) for 180 min. 3. The maximal increases in plasma ANP and urinary excretion of cGMP during severe pacing were four- and three-fold higher, respectively, than those during moderate pacing. Despite the higher concentration of plasma ANP, the maximal increases in urine volume, urinary excretion of sodium and fractional excretion of sodium during severe pacing were similar to those during moderate pacing. Mean arterial pressure and renal vascular resistance were decreased only by severe pacing. The increase in total peripheral resistance during severe pacing was significantly smaller than that during moderate pacing. However, the glomerular filtration rate was kept at basal levels by both moderate and severe pacing. 4. These results suggest that there are certain mechanisms that counteract renal tubular sodium reabsorption induced by endogenous ANP under conditions of severe pacing. The suppression occurs at tubular sites but not at glomerular sites. One of the possibilities for the suppression is the decrease in renal perfusion pressure accompanied by decreases in peritubular capillary hydrostatic pressure.     

INTERRELATIONSHIP BETWEEN CORTISOL AND ATRIAL NATRIURETIC FACTOR IN THE IMMATURE OVINE FETUS

1. In chronically cannulated ovine fetuses (100-130 days of gestation) the infusion of cortisol (86.7 +/- 15 micrograms/h for 4 h) or human atrial natriuretic factor (ANF; 4.4 micrograms for 2 h) resulted in highly significant increases in the excretion of sodium, chloride, potassium and water. 2. Cortisol had no significant effect on fetal plasma ANF concentrations. All values are mean and s.e.m. Plasma immunoreactive ANF was 53 +/- 5 and 67.3 +/- 13 pmol/L in the 4 h saline infused fetuses, and 51.3 +/- 14.3 and 74 +/- 13.3 pmol/L in cortisol-infused fetuses (n = 7). A separate group of fetuses received 2 h infusions of saline or hANF (4.4 micrograms/h), and plasma IR-ANF values were measured (n = 3). The values, at 0, 60, 90 and 120 min were, respectively, 19.7 +/- 3, 17.3 +/- 0.7, 18.7 +/- 3.7 and 20.7 +/- 3.7 pmol/L in the saline infused group, and 25.3 +/- 5.3, 80.7 +/- 32.3, 123.3 +/- 4.3 and 100 +/- 15 pmol/L in the ANF-infused fetuses. 3. Blood cortisol concentrations, in fetuses infused for 4 h with 0.9% NaCl, were 3.1 +/- 0.8 nmol/L (n = 7); in fetuses infused with 0.9% NaCl for 2 h were 3.6 +/- 1 nmol/L (n = 3); in fetuses infused for 4 h with cortisol were 19.9 +/- 1.9 nmol/L (n = 7); and in fetuses infused with hANF for 2 h were 6.0 +/- 3.0 nmol/L (n = 5). 4. There was no effect of fetal hANF infusion on maternal or fetal blood aldosterone concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)     

ATTENUATED RENAL RESPONSE TO ATRIAL NATRIURETIC PEPTIDE INFUSION IN RATS WITH HEART FAILURE

1. The natriuretic and diuretic effects of three atrial natriuretic peptide (ANP) infusion rates were examined in rats 4 weeks after myocardial infarction induced by left coronary artery ligation. 2. The natriuretic and diuretic effects of ANP were observed in controls and rats with infarction, but the effects were significantly attenuated in the latter. 3. Rats with chronic left heart failure were less sensitive to the renal effects of ANP compared with controls. 4. Impaired sodium and water excretion in chronic heart failure may be due partly to an attenuated renal response to ANP.     

EFFECTS OF ATRIAL NATRIURETIC FACTOR ON CYCLIC GMP CONTENT IN THE RAT AORTIC SMOOTH MUSCLE: STUDIES ON THE ROLE OF MEMBRANE Na+,K+-ATPase

1. These studies were conducted to determine whether preservation of the functional integrity of the membrane, Na+,K+-stimulated ATPase is essential for the atrial natriuretic factor (r-ANF-8-33) to enhance guanosine 3',5'-monophosphate (cGMP) content in the rat aortic smooth muscle. In freshly dissected rat aortic tissues, levels of cGMP were estimated using radioimmunoassay. 2. ANF (0.1 mumol/L in Krebs-Henseleit media) produced significant elevation in cGMP levels in the aortic smooth muscle when compared with that incubated in the control media, whereas suppression of Na+-pump with ouabain (1.0 mmol/L) and/or K+-free media did not produce any significant changes in the basal cGMP level; these two experimental manoeuvres did not prevent enhancement of cGMP by ANF. 3. Incubation of the tissues in the media containing ouabain plus vasoconstrictor concentrations of norepinephrine (0.3 mumol/L) also did not alter basal cGMP levels and did not prevent the ability of ANF to elevate cGMP. 4. These studies demonstrate that the antagonism by ouabain, of vasorelaxant effects of ANF (as reported in the literature) are not due to the prevention of the ability of ANF to enhance cGMP levels in the arterial smooth muscle. It is proposed that such an antagonism may be related to the actions of ouabain and ANF on diverse, and perhaps independent, mechanisms which affect Ca2+-fluxes across the cell membrane.     

MECHANISMS OF THE PRESSOR RESPONSE TO INTRAVENOUS THYROTROPIN-RELEASING HORMONE IN THE RAT

1. The purpose of this study was to examine the contribution of the sympatho-adrenomedullary system to the blood pressure response to an intravenous bolus of thyrotropin-releasing hormone (TRH) in conscious medullectomized and sham-operated rats. 2. The peak pressor effect of 0.5 mg TRH was significantly increased in rats having no adrenal medulla (+ 24.2 ± 1.6 mmHg, mean ± s.e.m., P<0.01) as compared to sham-operated animals (+12.2 ± 3.0 mmHg). 3. Blockade of alpha-adrenergic receptors with phentolamine abolished the pressor effect of TRH in control rats (+ 2.1 ± 1.9 mmHg) but did not attenuate the blood pressure response of medullectomized rats (+ 21.5 ± 4.7 mmHg). In contrast, beta-blockade with propranolol blunted the blood pressure responsiveness of rats subjected to adrenal medullectomy (+ 12.4 ± 2.6 mmHg) but did not modify the effect of TRH in sham-operated controls (+ 10.9 ± 2.9 mmHg). 4. The direct in vitro effect of TRH on isolated mesenteric rat arteries was also evaluated. TRH did not induce contractions of isolated arteries. 5. These results suggest that in rats with intact adrenals, the pressor effect of intravenous TRH is mediated primarily by a stimulation of alpha-adrenergic receptors. Adrenal medullectomy appears to enhance the blood pressure response to intravenous TRH. Activation of cardiac beta-adrenocep-tors seems to contribute to the blood pressure increasing effect of intravenous TRH in medullectomized animals.     

EFFECT OF ATRIAL NATRIURETIC PEPTIDE ON CELLULAR ELEMENT CONCENTRATIONS IN RAT PROXIMAL TUBULES: EVIDENCE FOR INHIBITION OF THE SODIUM PUMP

1. In order to further define the action of atrial natriuretic peptide (ANP) on proximal tubular (PT) transport, combined clearance and electron microprobe X-ray (EMPX) experiments were performed on five male Wistar rats infused with ANP (0.16 nmol/kg per h) and nine control animals. 2. Electron microprobe X-ray analysis of PT cell electrolytes (mmol/ kg wet weight) revealed a similar [Na]_i in both the control and ANP treated groups (16.4 ± 0.4 vs 16.5 ± 0.4; P= 0.894). [Cl]_i was lower in the ANP treated animals (14.8 ± 0.3 vs 12.0 ± 0.3; P<0. 0001) as was [K]_i (131.4 ± 1.4 vs 114 ± 1.7; P<0.0001). The PT cells in the ANP treated group had a significant reduction in dry weight (20.1 ± 0.3 g%vs 19.0 ± 0.3 g%; P<0.024), indicating significant cell swelling. Thus, despite a normal [Na]_i, there was net accumulation of Na_i following ANP treatment. 3. These results are consistent with accumulation of Nai due to inhibition of the Na pump followed by cell swelling and subsequent regulatory volume decrease with exit of K and Cl. These results are the first to show the effect of ANP on PT intracellular electrolytes.     

EFFECTS OF VOLUME EXPANSION AND CONTRACTION ON PLASMA LEVELS OF ATRIAL NATRIURETIC PEPTIDE IN MAN

1. Effects of saline infusion and blood removal on atrial natriuretic peptide (ANP) in normal subjects were examined in order to better define the magnitude of acute central volume regulatory influences on ANP. 2. Plasma ANP levels increased progressively during volume expansion with saline infusion, increasing by 18% after 30 min and by 93% after 120 min, and did not change during recumbency alone. 3. Plasma ANP levels immediately after a standard blood donation performed semirecumbent were significantly lower than before blood donation; they fell by 18%. 4. The magnitude of the fall in ANP induced by blood donation correlated significantly with basal plasma ANP. 5. In man, ANP responds to both increases and decreases in central blood volume, consistent with a role for ANP in blood volume homeostasis.