NATRIURETIC AND HYPOTENSIVE EFFECTS OF BRAIN NATRIURETIC PEPTIDE IN ANAESTHETIZED DOCA-SALT HYPERTENSIVE RATS

1. Both natriuretic and hypotensive effects of brain natriuretic peptide (BNP), a novel peptide identified in porcine brain, were investigated in anaesthetized DOCA-salt rats and control rats. 2. An intravenous injection of two different doses (0.5 and 5.0 nmol/kg) of BNP produced a rapid and marked natriuresis and hypotension in DOCA-salt rats. 3. In particular, significant differences of responsiveness were observed between DOCA-salt and control rats when administered the lower dose of BNP. 4. It was suggested that DOCA-salt rats might be relatively more susceptible to BNP.     

NATRIURETIC EFFECT OF CHRONICALLY ADMINISTERED α-HUMAN ATRIAL NATRIURETIC POLYPEPTIDE IN SODIUM DEPLETED OR REPLETED CONSCIOUS SPONTANEOUSLY HYPERTENSIVE RATS

Hypotensive and natriuretic effects of chronically administered alpha-human atrial natriuretic polypeptide (alpha-hANP) were investigated in conscious spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) in both sodium depletion and repletion. Systolic blood pressure was significantly reduced in SHR and WKY in both sodium deplete and replete states. Urinary sodium excretion was significantly increased in SHR and tended to be increased in WKY on sodium repletion, but remained unchanged on sodium depletion. It is suggested that extracellular fluid volume may be an important determinant factor of the natriuretic action of ANP but may not affect the hypotensive effect.     

Involvement of sympathetic nerves in cardiosuppressive effects of alpha-human atrial natriuretic polypeptide (alpha-hANP) in anesthetized rats

In order to clarify the hypotensive mechanisms of alpha-human atrial natriuretic polypeptide (alpha-hANP), hemodynamic responses were investigated in anesthetized rats. Mean blood pressure (MBP), cardiac index (CI) and total peripheral resistance index (TPRI) were measured before and after infusion of alpha-hANP (30 ng/min per 100 g body weight) in intact, nephrectomized, atropinized or chemically sympathectomized rats. MBP was significantly reduced in all rats, associated with a decrease in CI. TRPI decreased slightly in sympathectomized rats but was unchanged in the other rats. The alpha-hANP-induced reduction in MBP of sympathectomized rats was significantly less than that of intact rats (9.5 +/- 1.2 vs. 20.9 +/- 2.6%; P less than 0.01). Similarly, the decrease in CI of sympathectomized rats was significantly less than that of intact rats (5.0 +/- 1.8 vs. 21.7 +/- 5.3%; P less than 0.05). Changes in MBP and CI of nephrectomized, atropinized or intact rats did not differ significantly. It is suggested that neither fluid loss nor parasympathetic nerves play an important role in the acute hemodynamic response. However, sympathetic nerves might be involved in the cardiosuppressive action of alpha-hANP.     

Antihypertensive action of K in rats with DOCA-salt-induced hypertension

We investigated the role of renal sympathetic tone and central noradrenergic neurons in the mechanism for natriuretic and antihypertensive effects of potassium supplement in DOCA-salt hypertensive rats. Systolic blood pressure of DOCA-salt rats continued to rise during 4 weeks of DOCA-salt (1% NaCl) treatment. In contrast, 0.2% KCl or 1% KCl supplement attenuated the development of the hypertension dose-relatedly. One percent KCl supplement attenuated sodium retention and prevented the increased sodium space in DOCA-salt rats. Whereas norepinephrine turnover rate in the kidney of DOCA-salt rats after the 4-week treatment was markedly accelerated, it was normalized by 0.2% KCl or 1% KCl supplement. On the other hand, norepinephrine turnover rates in the hypothalamus and medulla oblongata of DOCA-salt rats were delayed, while they were restored by the KCl supplement. These results suggest that potassium supplement in DOCA-salt rats attenuated the development of the hypertension and that it may be attributed to the restoration of sodium retention and thereby volume expansion. It appears that the restoration of either the increased renal sympathetic tone or the decreased noradrenergic nerve activity in the brain-stem may be involved in the natriuretic and antihypertensive effects of potassium supplements in DOCA-salt hypertensive rats.     

Hemodynamic effects of alpha-human atrial natriuretic polypeptide (alpha-hANP) in rats

alpha-Human atrial natriuretic polypeptide (alpha-hANP) was infused in anesthetized Wistar rats, and its hemodynamic effects were investigated by measuring mean blood pressure (MBP), heart rate, cardiac index (CI), stroke volume index (SVI) and total peripheral resistance index (TPRI). alpha-hANP caused a reduction in MBP (approximately 20%) associated with decreases in CI and SVI but caused no change in TPRI. These results suggested that alpha-hANP may lower blood pressure mainly by decreasing cardiac output.     

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.     

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 immunoreactive atrial natriuretic peptide levels after subcutaneous alpha-hANP injection in normal humans

The plasma hormone and urine effects of 100 micrograms of alpha-human atrial natriuretic peptide (alpha-hANP) given by subcutaneous (s.c.) injection, were studied in eight healthy male volunteers. A control s.c. injection was administered on a separate day, and the study was single-blind. The peak immunoreactive atrial natriuretic peptide (IR-ANP) level of 29.4 +/- 4.4 pmol/L reached at 5 min was threefold higher than on the control day. Area under the IR-ANP response curve was approximately 1/32 of that after the same dose of alpha-hANP given by intravenous (i.v.) injection. alpha-hANP s.c. injection was not associated with significant effects on plasma renin activity (PRA), plasma aldosterone, or urine electrolyte excretion. It was concluded that only a small percentage of intact alpha-hANP is absorbed into the circulation after s.c. injection. With the dose of peptide and the injection vehicle chosen, alpha-hANP had little or no biological effect.     

Essential oil of Croton nepetaefolius decreases blood pressure through an action upon vascular smooth muscle: studies in DOCA-salt hypertensive rats

Experiments tested the hypothesis that hypotensive effects of intravenous (i.v.) treatment with the essential oil of Croton nepetaefolius (EOCN) result from its vasodilatory effects directly upon vascular smooth muscle. In both deoxycorticosterone-acetate (DOCA)-salt hypertensive and uninephrectomised control, conscious rats, i.v. bolus injections of EOCN (1 to 50 mg/kg) decreased mean aortic pressure (MAP) and heart rate (HR) in a dose-related manner. Treatment with DOCA-salt significantly enhanced EOCN-induced decreases in MAP without affecting bradycardia. Likewise, both maximal percent and absolute decreases in MAP elicited by i.v. hexamethonium (30 mg/kg), a ganglion blocker, were significantly greater in DOCA-salt hypertensive than in control rats. In DOCA-salt hypertensive rats, i.v. pretreatment with hexamethonium (30 mg/kg) reduced the bradycardia elicited by EOCN (50 mg/kg) without affecting the enhancement of EOCN-induced hypotension. In isolated thoracic aorta preparations from DOCA-salt hypertensive rats, EOCN (1-300 micrograms/ml) induced a concentration-dependent reduction of phenylephrine-induced contraction. Arteries from DOCA rats showed increased sensitivity to EOCN, as evidenced by the significant decrease in the IC50 for EOCN-induced reduction of phenylephrine-induced contraction (16.4 +/- 3.6 vs. 112.9 +/- 23.4 micrograms/ml in uninephrectomized controls). These results show that i.v. treatment with EOCN dose-dependently decreases blood pressure in conscious DOCA-salt hypertensive rats, and this action is enhanced when compared with uninephrectomized controls. This enhancement appears to be related mainly to an increase in EOCN-induced vascular smooth muscle relaxation rather than to enhanced sympathetic nervous system activity in this hypertensive model. Thus, the hypothesis that EOCN may be a direct vasorelaxant agent is supported by the results of the present study.     

Effects of a synthetic human atrial natriuretic polypeptide on regional blood flow in rats

The effects of alpha-hANP on systemic hemodynamics and regional blood flow were examined in conscious WKY and SHR. An intravenous infusion of alpha-hANP (3 micrograms/kg per min) resulted in a rapid and marked fall of blood pressure and of total peripheral resistance but with no change in cardiac output in both strains. alpha-hANP decreased vascular resistance in most organs and there was a redistribution of renal blood flow. Thus, the acute hypotensive effect of alpha-hANP is probably related to a vasodilator action.     

Renal effect of atrial natriuretic polypeptide: comparison with standard saluretics

The effects of synthetic alpha-human atrial natriuretic polypeptide (alpha-hANP) on renal hemodynamics and urine production were examined and compared with those of furosemide and trichlormethiazide (TCM) in anesthetized rats. The diuretic effect of alpha-hANP was brief but more powerful. Also, alpha-hANP produced a rapid and acute decrease in blood pressure (BP) accompanied by an increase in renal blood flow (RBF), while furosemide and TCM had no effect on either BP or RBF. Both the free water reabsorption rate (TCH2O) and free water production (CH2O) increased with administration of alpha-hANP but the linear relationship between CH2O and osmolar clearance (Cosm) and that between TCH2O and Cosm were not affected. Furosemide, on the other hand, affected the relationship between TCH2O and Cosm and between CH2O and Cosm. TCM affected only the relationship between CH2O and Cosm. These results suggest that the renal site of action and the mechanism of action of alpha-hANP differ from those of furosemide and TCM.     

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.     

Role of endothelin-1 and the ETA receptor in the maintenance of deoxycorticosterone acetate-salt-induced hypertension.

1. To search for a possible role for endothelin-1 (ET-1) in deoxycorticosterone acetate (DOCA)-salt-induced hypertension, we examined changes in concentration of ET-1 in vascular and renal tissue in DOCA-salt hypertensive rats and evaluated the antihypertensive effect of the ETA receptor antagonist, FR139317. 2. There was an increase in aortic immunoreactive-ET (IR-ET) concentrations in association with hypertension-induced treatment. There were no significant changes in ET-1 levels in the kidney with DOCA-salt treatment. 3. In DOCA-salt hypertensive rats, a significant correlation (r = 0.83, P < 0.01) was found between aortic IR-ET concentrations and systolic blood pressure. 4. High-performance liquid chromatography analysis of the aortic extract from DOCA-salt rats revealed one major component corresponding to the elution position of synthetic ET-1. 5. The intravenous bolus injection of FR139317 (10 mg kg-1) produced a slight decrease in blood pressure in the control rats and in the DOCA-salt hypertensive rat, FR139317 had a more pronounced hypotensive effect. 6. We propose that ET-1 production in vascular tissues is increased in DOCA-salt hypertensive rats. In addition, our study indicates the pathophysiological importance of increased endogenous ET-1 in the maintenance of DOCA-salt-induced hypertension, through interaction of the peptide with ETA receptors.     

The renal, cardiovascular and hormonal actions of human atrial natriuretic peptide in man; effects of indomethacin.

The renal, cardiovascular and hormonal effects of intravenous infusion of alpha-human atrial natriuretic polypeptides (alpha-hANP) at the concentrations of 0.0125, 0.025, 0.05, 0.1 microgram kg-1 min-1 for 20 min was studied in six male volunteers before and after indomethacin administration (150 mg day-1, three times daily for 3 days). Dose-dependent diuresis and natriuresis were observed in all subjects between the concentrations of 0.025 and 0.1 microgram kg-1 min-1, which were not influenced by indomethacin. Diastolic blood pressure decreased significantly (P less than 0.05) at the higher dose (0.05 microgram kg-1 min-1) of alpha-hANP, which was attenuated by indomethacin pretreatment. The plasma concentration of the immunoreactive alpha-hANP was 73.7 +/- 25 pg ml-1 on the control in subjects taking 200 mEq day-1 of sodium, and significant diuresis occurred when plasma concentration reached approximately 330.5 +/- 74.4 pg ml-1. alpha-hANP infusion caused a dose-dependent increase in cyclic GMP, no significant changes in plasma aldosterone and 18-hydroxycorticosterone, which were not influenced by indomethacin pretreatment. Plasma renin did not change in response to alpha-hANP infusion, which was significantly decreased (P less than 0.05) after indomethacin pretreatment. These results support that the renal effects of alpha-hANP may be exerted by prostaglandin-independent mechanisms. The renal effects occur at lower doses, and cardiovascular changes occur at higher doses of alpha-hANP.     

Antihypertensive property of a novel uricosuric diuretic,S-8666, in rats

The antihypertensive, diuretic, and toxicological effects of S-8666 were studied in rats. At doses of more than 60 mg/kg/day, p.o., S-8666 was antihypertensive in DOCA-salt hypertensive rats with a potency corresponding to 1/20 of that of trichlormethiazide. The antihypertensive effect was dose-dependent, and, at the higher doses, S-8666 had a more potent depressor effect than trichlormethiazide. The antihypertensive activity of S-8666 was shown predominantly by the S-(?)-enantiomer, with the R-(+)-enantiomer being only slightly active. A similar dose-dependent antihypertensive effect was shown by furosemide, tienilic acid, or indacrinone, but the therapeutic index (LD₅₀/minimum effective dose) of S-8666 was the highest among them. The acute diuretic and natriuretic effects of S-8666 after oral administration of 60–200 mg/kg to DOCA-salt hypertensive rats correlated well with its antihypertensive effect, although such diuretic and natriuretic effects in normotensive rats had no effect on blood pressure. S-8666, like trichlormethiazide, showed a prophylactic effect on the development of hypertension in DOCA-salt hypertensive rats, salt-loaded spontaneously hypertensive rats (SHR), and salt-loaded Dahl-S rats, though it was much less potent. Moreover, the combination of S-8666 with captopril in SHR enhanced the depressor effect of captopril like hydrochlorothiazide. These results indicate that S-8666 can be an effective nonthiazide diuretic for use as an antihypertensive agent.     

Enhanced hypotensive effects of the essential oil of Ocimum gratissimum leaves and its main constituent, eugenol, in DOCA-salt hypertensive conscious rats

The cardiovascular effects of intravenous (i.v.) treatment with the essential oil of Ocimum gratissimum (EOOG) and its main constituent, eugenol (Eug) were investigated in the experimental model of deoxycorticosterone acetate (DOCA-salt)-hypertensive rats. In both conscious DOCA-salt hypertensive rats and their uninephrectomized controls, i.v. bolus injections of EOOG (1 - 20 mg/kg) or Eug (1 - 10 mg/kg) induced dose-dependent hypotension and bradycardia. Treatment with DOCA-salt significantly enhanced the maximal decreases in mean aortic pressure (MAP) elicited by hexamethonium (30 mg/kg, i.v.) as well as the hypotensive responses to both EOOG and Eug without affecting the bradycardia. However, the enhancement of EOOG-induced hypotension in hypertensive rats remained unaffected by i.v. pretreatment with either hexamethonium (30 mg/kg) or methylatropine (1 mg/kg). These results show that i.v. treatment with EOOG or Eug dose-dependently decreased blood pressure in conscious DOCA-salt hypertensive rats, and this action is enhanced when compared with uninephrectomized controls. This enhancement appears related mainly to an increase in EOOG-induced vascular smooth relaxation rather than to enhanced sympathetic nervous system activity in this hypertensive model.     

Effects of brain mineralocorticoid receptor blockade on blood pressure and renal functions in DOCA-salt hypertension

In normotensive rats, we have previously demonstrated a role of brain mineralocorticoid receptors in blood pressure and renal function control. In the present study, the coordinate cardiovascular and renal effects of brain mineralocorticoid receptor blockade were examined by intracerebroventricular (i.c.v.) administration of a selective mineralocorticoid receptor antagonist (RU28318; 3,3-oxo-7 propyl-17-hydroxy-androstan-4-en-17yl-propionic acid lactone) in rats with hypertension induced by deoxycorticosterone acetate (DOCA) and salt. DOCA pellets were implanted s.c. in male Wistar rats given 0.9% NaCl as drinking solution 3 or 5 weeks before assessment of the effects of i.c.v. injection of RU28318 on cardiovascular and renal functions. Changes in expression of brain angiotensinogen, atrial natriuretic peptide (ANP) and mineralocorticoid receptor mRNA in specific brain areas in 3-week DOCA-salt rats were evaluated by in situ hybridization. The rise in systolic blood pressure induced by DOCA-salt treatment was most marked during the first 3 weeks. At 3 and 5 weeks after implantation of the DOCA-pellets a single i.c.v. injection of 10 ng of RU28318 significantly decreased systolic blood pressure during 24 h as assessed at 2, 8 and 24 h, while heart rate was not altered. Increased urinary excretion of water and electrolytes was observed in 3- and 5-week DOCA-salt rats during the period 0-8 h after i.c.v. injection of RU28318 while the suppressed plasma renin activity was not affected. The expression of brain angiotensinogen, ANP and mineralocorticoid receptor mRNA was not altered by 3-week DOCA-salt treatment, but 3 h after i.c.v. injection of RU28318, mineralocorticoid receptor mRNA expression in hippocampal cell fields responded with an increase of about 40%. In conclusion, these results demonstrate that in rats with hypertension induced by DOCA-salt, brain mineralocorticoid receptor blockade affects renal function and blood pressure regulation.     

猪脑钠素及其类似物的合成

以固相多肽合成方法合成了猪的二十六肽脑钠素(BNP)和它的一个类似物(Mpr⁴,D-Ala^6,13)-BNP(4-24)-NH₂,保护肽用HF裂解除去保护基、在碱性条件下空气氧化形成二硫桥后,粗产物经凝胶过滤和高效液相色谱分离纯化,均有与天然脑钠素相同的活性。合成肽相对于树脂初始取代量的产率分别为9.56%和11.03%。     

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.     

Chronic treatment with mianserin prevents DOCA-salt hypertension in rats--evidence for the involvement of central 5-HT2 receptors

Central 5-HT2A receptors have been implicated in central volume control by activating a central angiotensinergic pathway to cause the release of vasopressin. Interestingly, to induce DOCA-salt hypertension in rats vasopressin release is required. Thus the present experiments were carried out to determine whether continuous blockade of these receptors over 20 days, with the non-selective 5-HT2 receptor antagonist mianserin would prevent the development of deoxycorticosterone acetate (DOCA)-salt hypertension. Mianserin, given i.c.v. 90 or 60 microg twice daily for 20 days prevented the development of hypertension in conscious rats receiving DOCA-salt but did not affect blood pressure in rats on salt alone. Further, the dose of 30 microg given i.c.v. twice daily had no effect nor did the vehicle, polyethylene glycol (PEG), on the development of the hypertension. Mianserin 90 microg twice daily i.c.v. was also shown to prevent the increase in fluid intake, urinary flow and sodium excretion caused by DOCA-salt treatment. These data indicate that this action of mianserin is not due to an intrinsic hypotensive action but an action which involves interference with the mechanism by which DOCA-salt treatment causes hypertension. Thus the data overall support the view that to induce hypertension with DOCA-salt a central 5-HT-containing pathway needs to be activated, which then activates 5-HT2 receptors to cause the release of vasopressin which has previously been shown to be responsible for the initiation of DOCA-salt treatment hypertension.