NATRIURETIC AND HYPOTENSIVE EFFECTS OF α-HUMAN ATRIAL NATRIURETIC POLYPEPTIDE IN ANAESTHETIZED DOCA-SALT HYPERTENSIVE RATS

Both natriuretic and hypotensive effects of alpha-human atrial natriuretic polypeptide (alpha-hANP) were investigated in anaesthetized DOCA-salt hypertensive rats and control rats. An intravenous injection of two doses (0.3 and 3.0 micrograms/kg body weight) of alpha-hANP produced a rapid and marked increase in natriuresis and fall in blood pressure in DOCA-salt rats. Natriuretic and hypotensive effects of alpha-hANP in DOCA-salt rats were significantly greater than those in the control rats. It is suggested that DOCA-salt rats may have an enhanced natriuretic and hypotensive responsiveness to alpha-hANP.     

COMPARISON OF THE EFFECTS OF OUABAIN AND BRAIN NATRIURETIC PEPTIDE IN SALINE-LOADED SHEEP

1. It has been claimed that ouabain is an endogenous hormone that may be pivotal in the pathogenesis of some forms of hypertension and may exaggerate natriuresis in situations characterized by volume overload. We compared the haemodynamic, renal and endocrine effects of ouabain (at approximately 187 ng/kg per min for 2 h) with those of brain natriuretic peptide (BNP; at 5 pmol/kg per min for 2 h) in nine saline-loaded sheep in a balanced, randomized, single-blind, placebo-controlled crossover study. 2. Brain natriuretic peptide infusion reduced mean arterial pressure whereas ouabain infusion caused no change. Haematocrit rose steadily during BNP infusion but fell during ouabain infusion. Neither ouabain nor BNP affected urine volume, sodium, potassium or creatinine excretion. Mean heart rate declined during the ouabain and placebo infusions, but was not altered during BNP infusion. Endogenous ouabain concentrations were not detectable at baseline or during BNP or placebo infusions, but rose to concentrations of 11 ± 1.3 nmol/L during the ouabain infusion. 3. These results suggest that ouabain is not an endogenous hormone present at physiologically relevant concentrations. Furthermore, ouabain does not cause natriuresis during saline-loading in sheep and is therefore unlikely to be responsible for the exaggerated natriuresis seen in some forms of hypertension.     

CAPTOPRIL ANTAGONIZES THE HYPOTENSIVE ACTION OF ATRIAL NATRIURETIC PEPTIDE IN THE ANAESTHETIZED RAT

Atrial natriuretic peptide (8-33; ANP) caused a prolonged hypotensive response following intravenous injection in anaesthetized rats. This response was abolished by captopril treatment and restored by concomitant angiotensin II infusion. These results suggest that ANP exerts its hypotensive action in the anaesthetized rat by the antagonism of the vasoconstrictor action of endogenous angiotensin II.     

POTENTIATION OF DIPSOGENIC ACTIONS BY CENTRALLY ADMINISTERED TYPE-C NATRIURETIC PEPTIDE IN SPONTANEOUSLY HYPERTENSIVE BUT NOT WISTAR-KYOTO RATS

1. The effects of type-C natriuretic polypeptides (CNP) on the central dipsogenic and pressor responses to angiotensin II (AngII) were studied by the administration of agents into the lateral cerebral ventricle under conscious and unrestrained conditions in normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). 2. The fluid intake induced by AngII (25 ng) and water deprivation were potentiated after pretreatment with CNP in SHR but not in WKY rats. However, carbachol-induced water intake was not altered by pretreatment with CNP (2.5 μg) in either WKY rats or SHR. 3. In contrast, CNP did not influence the pressor responses to AngII in either WKY rats o. SHR.     

RENAL DENERVATION POTENTIATES THE NATRIURETIC AND DIURETIC EFFECTS OF ATRIAL NATRIURETIC PEPTIDE IN ANAESTHETIZED RABBITS

1. The role of the renal nerves in modulating the action of atrial natriuretic peptide (ANP) in the kidney was studied by comparing the responses to ANP in innervated and surgically denervated kidneys in anaesthetized rabbits. 2. A low dose of ANP (0.05 μg/kg per min, i.v.) was used to minimize the confounding effects of systemic hypotension. 3. The natriuretic and diuretic responses to ANP were significantly greater in denervated kidneys than in kidneys with intact innervation. Sodium excretion from denervated kidneys rose by 7.49 ± 3.11 μmol/min in response to ANP (-55%, P<0.05) compared to 0.84 ± 0.59 μmol/min (-28%, NS) in innervated kidneys. Urine flow increased markedly in denervated kidneys by 73.2 ± 29.9 μmol/min (-60%, P<0.05) but not in innervated kidneys. 4. Fractional sodium excretion increased significantly in denervated kidneys in response to ANP (median 2.3% to median 3.0%, P<0.05). 5. Renal blood flow, glomerular filtration rate (GFR) and glomerular capillary pressure were unchanged in response to ANP in either denervated or innervated kidneys. Pre-glomerular vascular resistance fell in denervated kidneys during ANP infusion. 6. The natriuresis and diuresis observed in the denervated kidneys, due to an increased fractional excretion of sodium without increases in GFR or glomerular capillary pressure, is consistent with effects of ANP on tubular reabsorption of sodium. 7. Thus, ANP produced a natriuresis and diuresis at a low dose in denervated but not in innervated kidneys. This indicates that reflex activation of renal nerves may antagonize the renal effects of ANP.     

EFFECTS OF ARGININE VASOPRESSIN, ANGIOTENSIN II AND ENDOTHELIN-1 ON THE RELEASE OF BRAIN NATRIURETIC PEPTIDE IN VIVO AND IN VITRO

1. The stimulatory effects of the vasoactive peptides arginine vasopressin (AVP), angiotensin II (AII) and endothelin-1 (ET-1) on the release of brain natriuretic peptide (BNP) were investigated in anaesthetized rats and in cultured rat atrial and ventricular cardiocytes. 2. A bolus injection of AVP induced a dose-dependent increase in plasma immunoreactive (ir)-BNP concentration in rats. AII induced a rapid and transient elevation in the ir-BNP level, while the increase produced by ET-1 was long-lasting. The elevation of the plasma ir-BNP concentration after stimulation by these three vasoconstrictors appeared to be paralleled by the elevation in mean blood pressure. 3. In the in vitro study, the rat atrial and ventricular cardiocytes both secreted ir-BNP into the medium in a time-dependent manner. ET-1 clearly stimulated the secretion of ir-BNP in both atrial and ventricular cardiocytes. In contrast, AVP and AII had no stimulatory effect in vitro. 4. Reverse-phase high performance liquid chromatography of the rat plasma and culture medium revealed a single major ir-BNP component that corresponded to synthetic rat BNP-45. 5. These observations indicate that AVP, AII and ET-1 stimulate the release of ir-BNP (probably rat BNP-45) through a change in blood pressure. In addition, ET-1 may also induce ir-BNP release through direct stimulation. As a cardiac hormone secreted from ventricles as well as atria, rat BNP may play a role in the regulation of blood pressure against the pressor effects of AVP, AII and ET–1.     

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.     

HYPOTENSIVE EFFECTS OF CAPTOPRIL ADMINISTERED CENTRALLY IN INTACT CONSCIOUS SPONTANEOUSLY HYPERTENSIVE RATS AND PERIPHERALLY IN ANEPHRIC ANAESTHETIZED SPONTANEOUSLY HYPERTENSIVE RATS

1. The hypotensive response to captopril in anaesthetized spontaneously hypertensive rats (SHR) is not modified by bilateral nephrectomy performed 1 or 24 h previously. 2. Intracerebroventricular injection (i.c.v.) of captopril (2 mg kg^?1) significantly lowered blood pressure of conscious SHR over a 7-h period of observation but there was no significant blood pressure response to i.c.v. vehicle, or to intravenous captopril (2 mg kg^?1) in SHR. 3. There was no significant blood pressure response to captopril (2 mg kg”') i.c.v. in the normotensive Wistar Kyoto controls (NT-WK). 4. These results indicate that captopril can lower the blood pressure of SHR by mechanisms independent of the kidneys or the circulating renin-angiotensin system. 5. The hypotensive effect of central captopril in SHR but not in the NT-WK suggests biochemical differences between the brains of the two rat strains.     

EFFECTS OF ANGIOTENSIN AND NORADRENALINE ON ATRIAL NATRIURETIC PEPTIDE LEVELS IN MAN

1. A significant positive correlation was found between changes in circulating noradrenaline (NA) levels and changes in atrial natriuretic peptide (ANP) levels during NA infusion and clonidine administration. 2. A significant positive correlation was also found between changes in arterial blood pressure and changes in ANP level during infusion of angiotensin II and of NA. 3. Two patients with very high circulating NA levels due to phaeochromocytoma, but receiving alpha- and beta-blockade, did not have clearly elevated ANP. A third not receiving medications and aged 73 years had elevated levels. 4. Atrial natriuretic peptide response to NA and angiotensin II may be mediated by changes in blood pressure levels or increased noradrenergic and angiotensinergic receptor activity in the atria or both. Atrial natriuretic peptide may have a role in blood pressure regulation in both normotensive and hypertensive man.     

ASSOCIATION OF THE BRAIN NATRIURETIC PEPTIDE GENE WITH BLOOD PRESSURE AND HEART WEIGHT IN THE RAT

1. Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are important in the control of body fluid homeostasis, blood pressure (BP) regulation and vascular remodelling. The genes for these peptides may, therefore, be involved in the pathogenesis of genetic hypertension. We have previously described a quantitative trait locus (QTL) for BP in the ANP gene region on rat chromosome 5. We have now assessed the possibility that this QTL lies at the closely linked BNP locus. 2. Intra-arterial BP and heart weight were measured in 12-week-old (n = 207) and 24-week-old (n= 88) F2 rats derived from crosses between Wistar-Kyoto normotensive rats and spontaneously hypertensive rats. We designed polymerase chain reaction primers to amplify a microsatellite in the BNP gene from genomic DNA. Analysis of variance was used for cosegregation analysis. Linkage mapping and localization of QTL was performed using the Mapmaker computer package. 3. A significant correlation was found between genotype for the BNP gene and systolic BP (P < 0.001) in 12-week-old rats. The ANP gene, but not the BNP gene, was associated with systolic BP in 24 week rats. There was no segregation of heart weight with BNP genotype at 12 or 24 weeks of age. The BNP gene mapped approximately 20 cM from the ANP gene in our rat hybrids, away from the previously described QTL. There was evidence for a second BP locus near to but distinct from the BNP gene. 4. These results suggest that BP QTL are present in the natriuretic peptide gene region but that the ANP and BNP genes themselves have no major effect on BP in this cross.     

IN VIVO AND IN VITRO EFFECTS OF ATRIAL NATRIURETIC PEPTIDE ON RENIN RELEASE

1. This study investigated the effect of atrial natriuretic peptide on renin release from the kidney. The in vitro direct effect was examined in the animal experiment using renal cortical slices of rat, and the in vivo effect was observed in the human infusion study. 2. In the in vitro experiments, alpha-human atrial natriuretic peptide (alpha-hANP) ranging 10(-9) to 10(-6) mol/L did not change the basal renin release rate from the renal cortical slices (-9% at 10(-6) mol/L, NS). Isoproterenol (10(-6) mol/L) increased renin release by 40% (P < 0.001), whereas angiotensin II (10(-6) mol/L) suppressed it by 48% (P < 0.001). However, alpha-hANP did not affect the stimulative effect of isoproterenol or the inhibitory effect of angiotensin II. 3. Also in the human study, infusion of 25 ng/kg per min alpha-hANP failed to change the plasma renin activity in normotensive subjects (-4%) or patients with essential hypertension (+5%), or even in patients with raised renin levels such as renovascular hypertension (+10%) or congestive heart failure (-13%). 4. These results put forth negative views on the direct involvement of atrial natriuretic peptide in renin release from the juxtaglomerular apparatus.     

PLASMA CONCENTRATION OF BRAIN NATRIURETIC PEPTIDE IS RELATED TO DIASTOLIC FUNCTION IN HYPERTENSION

1. The plasma brain natriuretic peptide (BNP) concentration is elevated in patients with essential hypertension and normal systolic function. This may be related to left ventricular hypertrophy or diastolic dysfunction, both of which commonly occur in hypertension. 2. Echocardiography was performed on 32 patients with newly diagnosed untreated mild-to-moderate hypertension (19 men, 13 women; mean±SD age 51±15 years; diastolic blood pressure 99±12mmHg; systolic blood pressur. 153.2±18.0 mmHg; plasma creatinine 86±15 μmol/L; creatinine clearance 92.2±20.5 mL/min; left ventricular mass index 116±28 g/m²; left ventricular ejection fraction 66±9%). A 15 mL peripheral venous blood sample was obtained at the time of echocardiography for radioimmunoassay of BNP. 3. Sixteen patients had abnormal Doppler transmitral flow (E/A rati. < 1) and a higher median plasma BNP concentration compared with those patients with E/A ≥1 (12.9 vs 5.9 pmol/L, respectively; P = 0.006). The plasma BNP level correlated significantly with E/A ratio (r = -0.50; P = 0.035). Multivariate analysis showed that the E/A ratio is related to plasma BNP, independent of age and blood pressure. 4. Our results suggest that the plasma BNP level is influenced by diastolic dysfunction. Further studies are needed to determine whether assay of plasma BNP helps to identify patients with diastolic dysfunction.     

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.     

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.     

EFFECT OF EXERCISE ON PLASMA ADRENOMEDULLIN AND NATRIURETIC PEPTIDE LEVELS IN MYOCARDIAL INFARCTION

1. We investigated the effect of exercise on plasma adreno-medullin, atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) concentrations and studied the relationship between these peptides and haemodynamic parameters in nine patients with old myocardial infarction (MI) and in eight normal subjects. 2. The exercise protocol consisted of two fixed work loads (40 and 80 W) for 4 min each and venous blood samples were taken at rest, during each exercise stage and after exercise while monitoring the mean arterial pressure (MAP) and heart rate (HR). In MI, pulmonary arterial pressure (PAP), pulmonary capillary wedge pressure (PCWP), left ventricular end-diastolic pressure (LVEDP) and cardiac output (CO) were measured throughout exercise. 3. Adrenomedullin levels did not significantly increase with exercise. Adrenomedullin levels correlated with PAP and PCWP at rest (P < 0.05). Atrial natriuretic peptide levels correlated with PAP, PCWP and LVEDP throughout exercise (P < 0.05) but, on multiple regression analysis, PCWP correlated only with ANP (P < 0.01). Brain natriuretic peptide levels correlated with LVEDP throughout exercise (P < 0.01) and its increment correlated closely with basal BNP levels at rest (P < 0.01). 4. These results suggest that adrenomedullin does not respond to the acute haemodynamic changes of exercise, whereas ANP responds to it and PCWP is the major stimulus factor. Brain natriuretic peptide responds to exercise in proportion to the basal synthesis of BNP in patients with left ventricular dysfunction and LVEDP may play a role in increasing BNP during exercise.