Stimulates Atrial Natriuretic Peptide Release Research Articles

Receptors involved in moxonidine-stimulated atrial natriuretic peptide release from isolated normotensive rat hearts

Imidazoline I 1-receptors are present in the heart and may be involved in atrial natriuretic peptide (ANP) release. The following studies investigated whether moxonidine (an antihypertensive imidazoline I 1-receptor and α 2-adrenoceptor agonist) acts directly on the heart to stimulate ANP release, and to characterize the receptor type involved in this action. Perfusion of rat (200–225 g) isolated hearts with moxonidine (10 − 6 and 10 − 5 M), for 30 min, resulted in ANP release (83 ± 29 and 277 ± 70 ng/30 min, above basal, respectively), significantly ( P < 0.01) different from perfusion with buffer (− 6 ± 31 ng/30 min). ANP release stimulated by moxonidine (10 − 6 M) was inhibited by co-perfusion with the antagonists, AGN192403 (imidazoline I 1-receptor), phenoxybenzamine (α 2 > α 1-adrenoceptors), and prazosin (α 1 > α 2-adrenoceptors), but increased by rauwolscine (α 2-adrenoceptors). Perfusion with 10 − 5 M brimonidine (full α 2-adrenoceptor agonist) inhibited moxonidine-stimulated ANP release. Similarly, moxonidine (10 − 6 M) tended to reduce coronary flow, but significantly increased coronary flow in the presence of brimonidine, which was vasoconstrictive when perfused alone. Coronary flow was reduced by 10 − 5 M each, brimonidine > clonidine > moxonidine; while similar bradycardia was observed with clonidine and moxonidine, but not with brimonidine. In conclusion, these results argue in favor of moxonidine acting primarily on imidazoline I 1-receptors to release ANP, with both α 2-adrenoceptor and imidazoline I 1-receptors exerting inhibitory inter-relation. In contrast, the coronary vasodilatory effect of moxonidine requires full activation of α 2-adrenoceptor. The sympatholytic and ANP-releasing effects of moxonidine appear to be mediated by cardiac imidazoline receptors that may be differentially localized. Most importantly, moxonidine can stimulate ANP release from the heart without contribution of the central nervous system.

Hypertension induced by nitric oxide synthase inhibition activates the atrial natriuretic peptide (ANP) system

We assessed the effect of nitric oxide (NO) synthase inhibition on plasma atrial natriuretic peptide (ANP) concentration and content in some brain structures [neurohypophysis (NH), adenohypophysis (AH), medial basal hypothalamus (MHB) and olfactory bulb (OB)] in rats before and after blood volume expansion (BVE). Male Wistar rats were injected i.p. with N ϖ-nitro- l-arginine ( l-NNA), 25 mg/kg of body weight, 40 min before the experiment (acute treatment) or l-NNA at a dose of 25 mg/kg body weight, twice a day, for 4 days (chronic treatment). The acute treatment caused an increase in the blood pressure and plasma ANP concentration in rats under basal conditions and after BVE. A decrease in ANP content was observed in the OB and NH, whereas no significant changes were found in the AH or MBH. In chronically treated rats, we also found an increase in blood pressure and in plasma ANP concentration under basal conditions and after BVE. The ANP content increased in the OB, NH and AH. These results indicate that systemic NO synthase inhibition increases ANP concentration in plasma and in areas of the central nervous system. We hypothesize that ANP participates in the hypertension-induced by NO synthesis blockade acting by baroreceptors input to the brain to stimulate ANP release and synthesis that reduces NO prival hypertension.

Role of cAMP and calcium influx in endothelin-1-induced ANP release in rat cardiomyocytes.

The mechanism of endothelin-1 (ET-1)-induced atrial natriuretic peptide (ANP) release was studied in neonatal rat ventricular cardiomyocytes. These cells expressed a single high-affinity class of ETA receptor (dissociation constant = 54 +/- 18 pM, n = 3), but no ETB receptors. Incubation of cardiomyocytes with ET-1 led to concentration-dependent ANP release and prostacyclin production. ET-1-induced ANP release was affected by neither protein kinase C (PKC) inhibition or downregulation nor by cyclooxygenase inhibition, indicating that ET-1-stimulated ANP secretion is not a PKC-mediated, prostaglandin-dependent process. Furthermore, ET-1 significantly stimulated adenosine 3',5'-cyclic monophosphate (cAMP) production and increased cytosolic calcium concentration in these preparations. Both ET-1-induced calcium influx and ANP release were decreased by the cAMP antagonist Rp-cAMPS, the Rp diastereoisomer of cAMP. Moreover, ET-1-induced ANP secretion was strongly inhibited in the presence of nifedipine as well as in the absence of extracellular calcium. Thus our results suggest that ET-1 stimulates ANP release in ventricular cardiomyocytes via an ETA receptor-mediated pathway involving cAMP formation and activation of a nifedipine-sensitive calcium channel.

Oxytocin releases atrial natriuretic peptide by combining with oxytocin receptors in the heart.

Previous studies indicated that the central nervous system induces release of the cardiac hormone atrial natriuretic peptide (ANP) by release of oxytocin from the neurohypophysis. The presence of specific transcripts for the oxytocin receptor was demonstrated in all chambers of the heart by amplification of cDNA by the PCR using specific oligonucleotide primers. Oxytocin receptor mRNA content in the heart is 10 times lower than in the uterus of female rats. Oxytocin receptor transcripts were demonstrated by in situ hybridization in atrial and ventricular sections and confirmed by competitive binding assay using frozen heart sections. Perfusion of female rat hearts for 25 min with Krebs-Henseleit buffer resulted in nearly constant release of ANP. Addition of oxytocin (10(-6) M) significantly stimulated ANP release, and an oxytocin receptor antagonist (10(-7) and 10(-6) M) caused dose-related inhibition of oxytocin-induced ANP release and in the last few minutes of perfusion decreased ANP release below that in control hearts, suggesting that intracardiac oxytocin stimulates ANP release. In contrast, brain natriuretic peptide release was unaltered by oxytocin. During perfusion, heart rate decreased gradually and it was further decreased significantly by oxytocin (10(-6) M). This decrease was totally reversed by the oxytocin antagonist (10(-6) M) indicating that oxytocin released ANP that directly slowed the heart, probably by release of cyclic GMP. The results indicate that oxytocin receptors mediate the action of oxytocin to release ANP, which slows the heart and reduces its force of contraction to produce a rapid reduction in circulating blood volume.

Atrial natriuretic peptide in brain and pituitary gland.

The data reviewed establish the presence and important role in body fluid homeostasis of brain atrial natriuretic peptide (ANP) in all vertebrate-species examined. The peptide is localized in neurons in hypothalamic and brain stem areas involved in body fluid volume and blood pressure regulation, and its receptors are located in regions that contain the peptide. Most, if not all, of the actions of ANP are mediated by activation of particulate guanylyl cyclase with generation of guanosine 3',5'-cyclic monophosphate, which mediates its actions in brain as in the periphery. Although atrial stretch releases ANP from cardiac myocytes, the experiments indicate that the response to acute blood volume expansion is markedly reduced after elimination of neural control. Volume expansion distends baroreceptors in the right atria, carotid-aortic sinuses, and kidney, altering afferent input to the brain stem and hence the hypothalamus, resulting in stimulation via ANPergic neurons in the hypothalamus of oxytocin release from the neurohypophysis that circulates to the right atrium to stimulate ANP release. The ANP circulates to the kidney and induces natriuresis. Atrial natriuretic peptide also induces vasodilation compensating rapidly for increased blood volume by increased vascular capacity. Atrial natriuretic peptide released into hypophysial portal blood vessels inhibits release of adrenocorticotropic hormone (ACTH), thereby decreasing aldosterone release and enhancing natriuresis. Furthermore, the ANP neurons inhibit AVP release leading to diuresis and decreased ACTH release. Activation of hypothalamic ANPergic neurons via volume expansion also inhibits water and salt intake. These inhibitory actions may be partially mediated via ANP neurons in the olfactory system altering salt taste. Atrial natriuretic peptide neurons probably also alter fluid movement in the choroid plexus and in other brain vascular beds. Therefore, brain ANP neurons play an important role in modulating not only intake of body fluids, but their excretion to maintain body fluid homeostasis.

Cellular mechanism of angiotensin II-induced atrial natriuretic peptide release in rat right atrial tissue

This study presents an investigation of the mechanism of angiotensin II (Ang II)-induced atrial natriuretic peptide (ANP) release in superfused sliced right atria of rats. Ang II (0.1 μM) enhanced ANP release by 49 %. This phenomenon was significantly blocked by (Sara 1-Ileu 8) Ang II (1 μM) and losartan (0.1 μM). The use of neomycin (100 μM), a phospholipase-C inhibitor completely suppressed the effect of Ang II on ANP increase. To elucidate the intracellular mechanism of ANP released by Ang II, the role of protein kinase C (PKC) was determined by 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7) and phorbol ester : 4-β-phorbol 12-myristate-13-acetate (PMA). We observed that PMA (0.1 μM) stimulated ANP release whereas H-7 (10 μM), an inhibitor of PKC in the presence of Ang II, prevented ANP increase. The role of calcium was also evaluated with 8-(N-N-diethylamino)-ocytyl-3,4,5, trimethoxy-benzoate (TMB-8) (10 μM) and N-(6-aminohexyl)-5-chloro-1-naphtalene sulfonamide (W-7) (10 μM), which completely inhibited ANP release by Ang II. Pre-treatment with diltiazem (10 (μM), an antagonist of the Ca + channel, did not prevent ANP increase due to Ang II, but A23187 (5 μM) enhanced ANP release by Ang II. These results suggest that PKC and intracellular calcium play an important role in ANP release under the influence of Ang II in rat atrial tissue.

Endothelin antiserum decreases volume-stimulated and basal plasma concentration of atrial natriuretic peptide.

Endothelin-1 (ET-1) is the most powerful factor known to release atrial natriuretic peptide (ANP) in vivo and in cultured cardiac myocytes or preparations of atrium. We tested the role of endogenous ET-1 in the regulation of ANP release by passive immunization in anesthetized rats. Intravenous injection of antiserum against ET-1 was shown to decrease basal and volume-stimulated plasma concentrations of ANP, whereas control serum was without effect. Antiserum generated in rabbits cross-reacted 100% with endothelin-2 and -3. In pentobarbital-anesthetized Wistar rats treated with ET-1 antiserum, plasma ANP concentration measured by radioimmunoassay was reduced by 37% from starting level after 10 minutes and by 30% after 60 minutes. Control rat serum had no effect on plasma ANP. Rapid intravenous infusion of 8 mL of 0.9% NaCl caused a sixfold increase of plasma ANP concentration in control rats but only twofold in rats pretreated with ET-1 antiserum (P < .01). This effect of ET-1 antiserum was dose dependent. ET-1 antiserum changed neither blood pressure nor heart rate significantly in anesthetized rats. Pretreatment with ET-1 antiserum did not affect the initial hypotensive response to intravenous ET-1 0.5 nmol/kg but significantly attenuated the subsequent hypertensive response to endothelin. Endothelin may be a physiological modulator of both basal and stimulated ANP release.

Role of myocardial redox and energy states in ischemia-stimulated release of atrial natriuretic peptide

The correlations between myocardial redox and energy states and atrial natriuretic peptide (ANP) secretion were studied in the perfused rat heart by exposing the hearts to global and low-flow ischemia for varying periods. Atrial and ventricular energy states and immunoreactive ANP in the effluent perfusate were measured. The basal secretion rate of ANP was 2.7 ± 0.2 ng/min · g dry wt and it was stimulated 2.6 ± 0.4, 4.0 ± 0.6, 11.2 ± 2.1 and 13.3 ± 3.2-fold (means ± s.e.) at the time point of 2 min after 5, 10, 20 and 30-min periods of ischemia, respectively. The increase in ANP release during the post-ischemic period was statistically significant and showed positive linear correlation with the atrial and ventricular lactate/pyruvate ratios (r = 0.92 and 0.89, respectively) and negative non-linear correlation with the atrial and ventricular phosphorylation potentials (r = −0.97 and −0.94, respectively). In agreement with the enhanced release of ANP after global ischemia, low-flow ischemia also increased ANP release. Cellular damage was not evidently responsible for the increased secretion, because only ANP 1–28, the processed form of the peptide, was detected in the perfusates and no processing of exogenous proANP during or after ischemia was observed. These results indicate that myocardial ischemia stimulates ANP release and suggest that cellular redox and energy states may be linked to ANP release during ischemia/reperfusion. Thus, ANP release during and after ischemia in vivo may be due not only to atrial distention but also to changes in energy metabolism.

Calcitonin gene-related peptide(CGRP) stimulates the release of atrial natriuretic peptide(ANP) from isolated rat atria

The effect of calcitonin gene-related peptide(CGRP) on the release of atrial natriuretic peptide(ANP) was studied in spontaneously beating, isolated rat atria. CGRP stimulated the ANP release in a dose-dependent manner. When the atria were incubated with a combination of phentolamine, propranolol, and atropine, these antagonists blocked neither the rise in ANP release nor the positive chronotropic and inotropic effects of CGRP. Therefore, we conclude that CGRP stimulates ANP release as well as cardiac contractility independently of adrenergic and cholinergic receptors.

Atrial natriuretic peptide during supraventricular tachycardia and relation to hemodynamic changes and renal function

Changes in plasma levels of atrial natriuretic peptide (ANP) and arginine vasopressin were studied in 5 patients during and after a 30-minute period of induced supraventricular tachycardia (SVT). Immediately after the induction of SVT, plasma ANP levels began to increase, peaked at 32 minutes (+734% increase on average) and then gradually decreased. The mean plasma arginine vasopressin levels decreased during SVT, but the differences were not significant. When plasma ANP levels during SVT were compared with the simultaneously measured hemodynamic variables, a significant positive correlation (r = 0.73, p < 0.001) was observed between plasma ANP levels and pulmonary capillary wedge pressure. Induced SVT was associated with increased urinary sodium and potassium excretion, increased urine flow and increased free water clearance. Concomitantly, glomerular filtration rate significantly increased (+77%) with an increase in filtration fraction. Although no significant change was observed in plasma renin activity, plasma aldosterone concentrations decreased during and after SVT. These results suggest that increased left atrial pressure stimulates ANP release during SVT and that increased glomerular filtration rate and decreased aldosterone secretion by ANP, in addition to the inhibition of water reabsorption by decreased arginine vasopressin, may be responsible for natriuresis and diuresis associated with SVT.

Influence of sodium concentration in cerebrospinal fluid on plasma atrial natriuretic peptide in conscious rats.

1. To test the influence of a sodium (Na+) stimulus within the central nervous system on the release of atrial natriuretic peptide (ANP), we examined the effects of intracerebroventricular infusion of high Na+ artificial cerebrospinal fluid (CSF) on blood pressure, urinary Na+ excretion and plasma ANP levels in conscious Wistar rats. 2. Infusion of high Na+ (0.6 mol/l) CSF into the lateral ventricle at a rate of 1 microliter/min for 60 min significantly increased mean blood pressure and urinary Na+ excretion, while normal Na+ (0.15 mol/l) CSF had no effects. Plasma ANP levels were higher in the high Na+ CSF group than in the normal Na+ group (154 +/- 39 vs 52 +/- 19 pmol/l, P less than 0.05). 3. Interruption of the sympathetic nervous system and the vascular action of vasopressin with intravenous hexamethonium and d(CH2)5Tyr(Me)arginine vasopressin attenuated the pressor and natriuretic responses to intracerebroventricular high Na+ CSF. Plasma ANP levels in these rats did not differ significantly from those in rats which were similarly treated but were given normal Na+ CSF. 4. These results indicate that elevation of the CSF Na+ concentration without peripheral volume loading can stimulate ANP release into the circulation. ANP release due to central Na+ stimulus appears to be mediated by the sympathetic nervous system, vasopressin, and/or haemodynamic change caused by these factors.

A calmodulin antagonist (W-7) and a protein kinase C inhibitor (H-7) have no effect on atrial natriuretic peptide release induced by atrial stretch

The role of intracellular signals in the regulation of atrial natriuretic peptide (ANP) release was investigated using isolated rat left atria. Dibutyryl cyclic AMP and dibutyryl cyclic GMP had no effect on ANP release. Arginine vasopressin and phenylephrine, both of which activate the polyphosphoinositide system and consequently both the diacylglycerol-protein kinase C system and the Ca 2+-Ca 2+ receptor system, stimulated ANP release dose-dependently. The ANP release stimulated by phenylephrine was inhibited by N-(6-aminohexyl)-5-chloro-l-naphthalenesulfonamide (W-7), a calmodulin antagonist, and 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7), a protein kinase C inhibitor. Atrial stretch stimulated ANP release, but the release was not inhibited by W-7 or H-7. These results suggest that the mechanism responsible for phenylephrine-induced ANP release differs from that for stretch-induced ANP release.

Increased Plasma Levels of Atrial Natriuretic Peptide in Patients with Chronic Renal Failure: Effect of Noradrenaline Infusion

Basal plasma atrial natriuretic peptide (ANP) and blood pressure were measured in 11 patients with chronic renal failure before requirement of dialysis, 13 patients on chronic dialysis, and 28 control subjects (Study 1). Changes in ANP during noradrenaline infusion were determined in eight patients with chronic renal failure before dialysis, 12 patients on chronic dialysis, and 17 control subjects (Study 2). ANP was also measured in 14 healthy control subjects during angiotensin II infusion (Study 3). Study 1 showed a significantly greater ANP in patients before the stage of dialysis (median 23 pg/ml) and in dialysis patients (34 pg/ml) than in control subjects (19 pg/ml) P less than 0.01 for both. In Study 2, noradrenaline induced an increase in ANP in the non-dialysed patients (P less than 0.05) and in the control subjects (P less than 0.01), but not in the dialysis patients. According to Study 3, ANP was unchanged during angiotensin II infusion. Blood pressure was increased in all groups during noradrenaline and angiotensin II infusions. It can be concluded that ANP Is increased both in patients with chronic renal failure before requirement of dialysis and in patients on maintenance dialysis. It is suggested that noradrenaline stimulates ANP release.

The effect of short-lasting atrial pacing on the release of atrial natriuretic peptide, vasopressin, and methionine enkephalin in man.

In order to study the effect of atrial tachycardia on the release of atrial natriuretic peptide (ANP), AVP, and methionine enkephalin (M-Enk), plasma concentrations of these peptides in the right ventricle were determined in patients with various arrhythmias (N = 10) during cardiac catheterization and incremental atrial pacing. Each pacing (100 per min, the maximum rate for 1:1 atrioventricular conduction, and 200 per min) lasted 4 to 5 min. Plasma ANP was significantly increased from 53.1 +/- 12.2 in the resting condition to 168.9 +/- 59.9 pmol/l at a pacing rate of 200 beats per min (P less than 0.05); plasma AVP tended to decrease, but not significantly, and plasma M-Enk did not change at all. Pulse pressure in the right atrium (PPRA) and mean right atrial pressure (MRAP) tended to increase during the pacing, and at the rate of 200 beats per min PPRA was significantly higher than at the rate of 100 beats per min. Mean arterial blood pressure, plasma osmolality, and plasma sodium and potassium concentrations did not change significantly. There were significant correlations between plasma ANP and PPRA, MRAP and heart rate. These results indicate that atrial pacing stimulates ANP release with a rise in right atrial pressure, but does not influence M-Enk and AVP releases.

Plasma atrial natriuretic peptide in DOCA-NaCl-treated rats.

In order to assess the possible role of atrial natriuretic peptide (ANP) in the development of deoxycorticosterone (DOCA)-NaCl-induced hypertension, plasma immunoreactive ANP concentration was compared with sodium balance and blood pressure in NaCl- or DOCA-NaCl-treated rats. Both NaCl-and DOCA-NaCl-loading increased plasma ANP levels (to 86 +/- 8.1 and 105 +/- 12 pg ml-1 respectively; 47 +/- 6.7-60 +/- 4.6 pg ml-1 in controls), which were correlated to sodium intake and excretion. In DOCA-NaCl-treated rats, the highest ANP levels (105 +/- 12 pg ml-1) were found 4 weeks after the beginning of DOCA-NaCl treatments. Along with the development of DOCA-NaCl hypertension in 1-kidney-DOCA-NaCl-treated rats, however, plasma ANP concentration did not rise further. We conclude that secretion of ANP into the circulation is increased during DOCA-NaCl treatment. Elevated blood pressure does not stimulate ANP release in DOCA-NaCl-treated rats further.