Natriuretic and hypotensive effects of brain natriuretic peptide (BNP) in spontaneously hypertensive rats

Cardiac natriuretic peptide deficiency sensitizes the heart to stress induced ventricular arrhythmias via impaired CREB signaling

AIMS

The cardiac natriuretic peptides (atrial natriuretic peptide [ANP] and B-type natriuretic peptide [BNP]) are important regulators of cardiovascular physiology, with reduced natriuretic peptide (NP) activity linked to multiple human cardiovascular diseases. We hypothesized that deficiency of either ANP or BNP would lead to similar changes in left ventricular structure and function given their shared receptor affinities.

METHODS AND RESULTS

We directly compared murine models deficient of ANP or BNP in the same genetic backgrounds (C57BL6/J) and environments. We evaluated control, ANP deficient (Nppa-/-) or BNP deficient (Nppb-/-) mice under unstressed conditions and multiple forms of pathological myocardial stress. Survival, myocardial structure, function and electrophysiology, tissue histology, and biochemical analyses were evaluated in the groups. In vitro validation of our findings was performed using human derived induced pluripotent stem cell cardiomyocytes (iPS-CM). In the unstressed state, both ANP and BNP deficient mice displayed mild ventricular hypertrophy which did not increase up to 1 year of life. NP-deficient mice exposed to acute myocardial stress secondary to thoracic aortic constriction (TAC) had similar pathological myocardial remodeling but a significant increase in sudden death. We discovered that the NP-deficient mice are more susceptible to stress induced ventricular arrhythmias using both in vivo and ex vivo models. Mechanistically, deficiency of either ANP or BNP led to reduced myocardial cGMP levels and reduced phosphorylation of the cAMP response element-binding protein (CREBS133) transcriptional regulator. Selective CREB inhibition sensitized wild type hearts to stress induced ventricular arrhythmias. ANP and BNP regulate cardiomyocyte CREBS133 phosphorylation through a cGMP-dependent protein kinase 1 (PKG1) and p38 mitogen activated protein kinase (p38 MAPK) signaling cascade.

CONCLUSIONS

Our data show that ANP and BNP act in a non-redundant fashion to maintain myocardial cGMP levels to regulate cardiomyocyte p38 MAPK and CREB activity. Cardiac natriuretic peptide deficiency leads to a reduction in CREB signaling which sensitizes the heart to stress induced ventricular arrhythmias.

TRANSLATIONAL PERSPECTIVE

Our study found that ANP or BNP deficiency leads to increased sudden death and ventricular arrhythmias after acute myocardial stress in murine models. We discovered that ANP and BNP act in a non-redundant fashion to maintain myocardial cGMP levels and uncovered a unique role for these peptides in regulating cardiomyocyte p38 MAPK and CREB signaling through a cGMP-PKG1 pathway. Importantly, this signaling pathway was conserved in human cardiomyocytes. This study provides mechanistic insight into how modulating natriuretic peptide levels in human heart failure patients reduces sudden death and mortality.

Different Susceptibility of B-Type Natriuretic Peptide (BNP) and BNP Precursor (proBNP) to Cleavage by Neprilysin: The N-Terminal Part Does Matter

BACKGROUND

Protease neprilysin is known to be responsible for the degradation of natriuretic peptides. A recent heart failure (HF) drug, LCZ696 (EntrestoTM), that combines a neprilysin inhibitor and an angiotensin II receptor inhibitor was suggested to augment circulating B-type natriuretic peptide (BNP) concentrations, making the results of BNP measurements diagnostically ambiguous. Because the main form of measured BNP in HF patients is represented by its uncleaved precursor, proBNP, it is important to know the susceptibility of proBNP to cleavage by neprilysin.

METHODS

BNP 1–32 and nonglycosylated and glycosylated forms of proBNP 1–108 were incubated with neprilysin for different time periods. BNP immunoreactivity was analyzed using 2 sandwich immunoassays: one utilizing monoclonal antibody (mAb) KY-BNP-II (epitope 14–21) as capture with mAb 50E1 (epitope 26–32) for detection and a single-epitope sandwich BNP (SES-BNP) immunoassay specific to the epitope 11–17. Mass-spectrometry was applied to determine the sites of BNP cleavage.

RESULTS

In contrast to BNP, both forms of proBNP were resistant to degradation by neprilysin. The SES-BNP assay was much less susceptible to the BNP cleavage by neprilysin compared with the immunoassay utilizing antibodies specific to the region 14–21, comprising the site Arg17-Ile18, known as the site of BNP cleavage by neprilysin.

CONCLUSIONS

These findings suggest that modulation of neprilysin activity by specific inhibitors may not greatly influence the circulating concentrations of immunoreactive BNP, mostly represented in HF by proBNP, which is not susceptible to neprilysin. The different susceptibility of the BNP regions to neprilysin-dependent degradation highlights the importance of the choice of epitopes for reliable BNP immunodetection.

Natriuretic peptides in cardiometabolic regulation and disease

In the 30 years since the identification of the natriuretic peptides, their involvement in regulating fluid and blood pressure has become firmly established. Data indicating a role for these hormones in lifestyle-related metabolic and cardiovascular disorders have also accumulated over the past decade. Dysregulation of the natriuretic peptide system has been associated with obesity, glucose intolerance, type 2 diabetes mellitus, and essential hypertension. Moreover, the natriuretic peptides have been implicated in the protection against atherosclerosis, thrombosis, and myocardial ischaemia. All these conditions can coexist and potentially lead to heart failure, a syndrome associated with a functional natriuretic peptide deficiency despite high circulating concentrations of immunoreactive peptides. Therefore, dysregulation of the natriuretic peptide system, a 'natriuretic handicap', might be an important factor in the initiation and progression of metabolic dysfunction and its accompanying cardiovascular complications. This Review provides a summary of the natriuretic peptide system and its involvement in these cardiometabolic conditions. We propose that these peptides might have an integrating role in lifestyle-related metabolic and cardiovascular disorders.

Blood borne hormones in a cross-talk between peripheral and brain mechanisms regulating blood pressure, the role of circumventricular organs

Accumulating evidence suggests that blood borne hormones modulate brain mechanisms regulating blood pressure. This appears to be mediated by the circumventricular organs which are located in the walls of the brain ventricular system and lack the blood-brain barrier. Recent evidence shows that neurons of the circumventricular organs express receptors for the majority of cardiovascular hormones. Intracerebroventricular infusions of hormones and their antagonists is one approach to evaluate the influence of blood borne hormones on the neural mechanisms regulating arterial blood pressure. Interestingly, there is no clear correlation between peripheral and central effects of cardiovascular hormones. For example, angiotensin II increases blood pressure acting peripherally and centrally, whereas peripherally acting pressor catecholamines decrease blood pressure when infused intracerebroventricularly. The physiological role of such dual hemodynamic responses has not yet been clarified. In the paper we review studies on hemodynamic effects of catecholamines, neuropeptide Y, angiotensin II, aldosterone, natriuretic peptides, endothelins, histamine and bradykinin in the context of their role in a cross-talk between peripheral and brain mechanisms involved in the regulation of arterial blood pressure.

Effects of brain natriuretic peptide on pituitary-adrenal activation in rats

The aim of this study was to characterize the effects of brain natriuretic peptide (BNP) on the hypothalamo-pituitary-adrenal (HPA) responses to different stress paradigms (ether stress, electric shock and restraint). Rats were subjected to the stressful stimuli after intracerebroventricular administration of BNP (32.5 ng-6.5 microg) and plasma corticosterone was used as an indicator of the HPA activation. BNP did not modify the basal secretion, but inhibited the stress-induced rise in plasma corticosterone in a dose-dependent manner. BNP proved most effective in decreasing the corticosterone response to ether stress and attenuated the electric shock and restraint-induced HPA activation to a lesser extent. These results confirm the view that BNP takes part in the regulation of the HPA system.

Gene expression and synthesis of natriuretic peptides by cultured human glomerular cells

BACKGROUND

Atrial natriuretic peptide, brain natriuretic peptide and C-type natriuretic peptide belong to a family of hormones that have natriuretic and vasodepressor activity and may play a pathophysiologic role in hypertension, heart failure and renal failure. Whereas immunoreactive human forms of these three natriuretic peptides are found in renal tubules, it is not clear whether they are derived from the systemic circulation or from local production.

OBJECTIVE

To examine the gene expression of natriuretic peptides in cultured human glomerular cells.

MATERIALS AND METHODS

We sought to determine the presence of messenger RNA encoding for these natriuretic peptides using polymerase chain reaction following reverse transcription. The polymerase chain reaction products were confirmed by direct sequencing. Atrial natriuretic peptide, brain natriuretic peptide and C-type natriuretic peptide in cell-culture supernatants were measured by radioimmunoassays (with detection limits of 2.1, 2.1 and 0.21 pmol/l, respectively).

RESULTS

Atrial natriuretic peptide messenger RNA was not found in mesangial or glomerular epithelial cells (despite stimulation with tumor necrosis factor-alpha) except when the cells were cultured with a high concentration of fetal bovine serum (> 10%). Similarly, this peptide was not detected in supernatant unless the cells were cultured with fetal bovine serum at concentrations of > 10%. Brain natriuretic peptide messenger RNA was readily detected in cultured mesangial and glomerular epithelial cells with a lower concentration in the former. Brain natriuretic peptide was not found in the supernatant of resting mesangial cells but became detectable when incubated with tumor necrosis factor-alpha or fetal bovine serum. C-type natriuretic peptide messenger RNA was detected in mesangial and glomerular epithelial cells with a higher concentration in the latter. C-type natriuretic peptide was detected in the supernatant of resting glomerular epithelial cells and levels rose when incubated with increasing concentrations of tumor necrosis factor-alpha or fetal bovine serum. However, C-type natriuretic peptide was not detected in the supernatant of resting mesangial cells and remained undetectable following incubation with tumor necrosis factor-alpha or fetal bovine serum.

CONCLUSION

Our results suggest differences in the synthesis of natriuretic peptides between glomerular mesangial and epithelial cells.

Brain natriuretic peptide-mediated changes in the extracellular neurotransmitter turnover in the rostral ventrolateral medulla

Changes in the rostral ventrolateral medullary neurotransmitter levels and associated cardiovascular functions in response to local administration of brain natriuretic peptide were investigated in urethane-anesthetized Sprague-Dawley rats. Unilateral injections of various doses of brain natriuretic peptide into the rostral ventrolateral medulla region led to significant reductions in both blood pressure and heart rate. To identify the changes occurring in the extracellular neurochemical profile, brain natriuretic peptide was perfused at the rate of 1.5 microliters/min for a period of 1 h through a microdialysis probe implanted stereotaxically into the rostral ventrolateral medulla area and the dialysate was assayed every 15 min for both catechols and indoleamine. Both norepinephrine and epinephrine concentrations were significantly reduced. Dihydroxyphenylacetic acid concentration showed no significant change in response to brain natriuretic peptide perfusion. On the other hand, serotonin turnover estimated by the measurement of its metabolite (5-hydroxyindoleacetic acid) concentration increased during the perfusion of brain natriuretic peptide. Blood pressure and heart rate also showed significant reduction during the perfusion of brain natriuretic peptide. These results suggest that brain natriuretic peptide may be relevant in the central regulation of cardiovascular functions by modulating monoamine neurotransmitters.

The normal and pathological physiology of brain water

The physicochemical properties of water enable it to act as a solvent for electrolytes, and to influence the molecular configuration and hence the function--enzymatic in particular--of polypeptide chains in biological systems. The association of water with electrolytes determines the osmotic regulation of cell volume and allows the establishment of the transmembrane ion concentration gradients that underlie nerve excitation and impulse conduction. Fluid in the central nervous system is distributed in the intracellular and extracellular spaces (ICS, ECS) of the brain parenchyma, the cerebrospinal fluid, and the vascular compartment--the brain capillaries and small arteries and veins. Regulated exchange of fluid between these various compartments occurs at the blood-brain barrier (BBB), and at the ventricular ependyma and choroid plexus, and, on the brain surface, at the pia mater. The normal BBB is relatively permeable to water, but considerably less so to ions, including the principal electrolytes Brain fluid regulation takes place within the context of systemic fluid volume control, which depends on the mutual interaction of osmo-, volume-, and pressure-receptors in the hypothalamus, heart and kidney, hormones such as vasopressin, renin-angiotensin, aldosterone, atriopeptins, and digitalis-like immunoreactive substance, and their respective sites of action. Evidence for specific transport capabilities of the cerebral capillary endothelium, for example high Na+K(+)-ATPase activity and the presence at the abluminal surface of a Na(+)--H+ antiporter, suggests that cerebral microvessels play a more active part in brain volume regulation and ion homoeostasis than do capillaries in other vascular beds. The normal brain ECS amounts to 12-19% of brain volume, and is markedly reduced in anoxia, ischaemia, metabolic poisoning, spreading depression, and conventional procedures for histological fixation. The asymmetrical distributions of Na+ K+ and Ca2+ between ICS and ECS underlie the roles of these cations in nerve excitation and conduction, and in signal transduction. The relatively large volume of the CSF, and extensive diffusional exchange of many substances between brain ECS and CSF, augment the ion-homeostasing capacity of the ECS. The choroid plexus, in addition to secreting CSF principally by biochemical mechanisms (there is an additional small component from the extracellular fluid), actively transports some substances from the blood (e.g. nucleotides and ascorbic acid), and actively removes others from the CSF. In contrast with CSF secretion, CSF reabsorption is principally a biomechanical process, passively dependent on the CSF-dural sinus pressure gradient. Pathological increases in intracranial water content imply development of an intracranial mass lesion. The additional water may be distributed diffusely within the brain parenchyma as brain oedema, as a cyst, or as increase in ventricular volume due to hydrocephalus. Brain oedema is classified on the basis of pathophysiology into four categories, vasogenic, cytotoxic, osmotic and hydrostatic. The clinical conditions in which brain oedema presents the greatest problems are tumour, ischaemia, and head injury. Peritumoural oedema is predominantly vasogenic and related to BBB dysfunction. Ischaemic oedema is initially cytotoxic, with a shift of Na+ and CI- ions from ECS to ICS, followed by osmotically obliged water, this shift can be detected by diffusion-weighted MRI. Later in the evolution of an ischaemic lesion the oedema becomes vasogenic, with disruption of the BBB. Recent imaging studies in patients with head injury suggest that the development of traumatic brain oedema may follow a biphasic time course similar to that of ischaemic oedema. Hydrocephalus is associated in the great majority of cases with an obstruction to the circulation or drainage of CSF, or, occasionally, with overproduction of CSF by a choroid plexus papilloma. In either case, the consequence is a ris

BNP plasma levels during acute volume expansion and chronic sodium loading in normal men

Since the isolation of porcine brain natriuretic peptide (pBNP) in 1988, many aspects of this new endocrine and paracrine system have been elucidated. However, the precise role of human BNP (hBNP) in the regulation of sodium balance and blood pressure in men is still unclarified. We investigated the impact of acute volume expansion and high sodium intake on ANP and BNP plasma levels of 21 healthy male subjects. Following acute infusion of 2000 ml 0.9% saline into a peripheral vein over 30 minutes in a lying position, ANP levels in plasma increased from 110 +/- 14 pg/ml to 199 +/- 18 pg/ml (p < 0.001), while BNP levels remained constant (26 +/- 3 pg/ml before, 28 +/- 3 pg/ml after volume expansion). In a second experiment we investigated the change of ANP and BNP plasma levels after five days of controlled sodium intake (Here blood tests were taken from the probands in an upright position). ANP levels averaged 42 +/- 5 pg/ml after low and 73 +/- 14 pg/ml after high sodium intake. Before starting the diet, the ANP fasting blood level was 56 +/- 6 pg/ml. During both diets, plasma BNP was not altered significantly, but we found a slight insignificant increase after high sodium intake. At the end of each diet we tested ANP and BNP secretion by a physical exercise in an upright position. After high salt intake, BNP plasma levels rose from 32 +/- 4 pg/ml at rest to 45 +/- 7 pg/ml (p < 0.025) after 15 minutes of exercise, while ANP rose from 73 +/- 14 pg/ml to 120 +/- 20 pg/ml (p < 0.001). These results suggest a different regulation of ANP and BNP in normal subjects. ANP secretion is modulated by volume loading and by low or high salt intake while BNP responses to physical exercise.

Effects of brain natriuretic peptide-45, a circulating form of rat brain natriuretic peptide, in spontaneously hypertensive rats

Rat brain natriuretic peptide-45 (rat BNP-45) has recently been isolated from rat heart and shown to be a circulating form of rat BNP. We investigated the effects of rat BNP-45 in anesthetized spontaneously hypertensive rats (SHR) and Wistar-Kyoto rats (WKY) and compared them with those of rat alpha-atrial natriuretic peptide (alpha-ANP). BNP-45 was a potent natriuretic and hypotensive agent in both strains. The effects were comparable with those of alpha-ANP and were far greater than those of porcine BNP-26 reported previously. In SHR blood pressure decreased more than in WKY following injection of the highest dose (2.0 nmol/kg) of BNP-45 or alpha-ANP. However, WKY were more susceptible than SHR to BNP-45 for diuresis, natriuresis and urinary cGMP excretion. Moreover, a high dose of BNP-45 led to a prolonged lowering of blood pressure and urinary cGMP excretion compared to alpha-ANP, and these features were prominent in WKY. BNP-45 disappeared more slowly than alpha-ANP when the two peptide (2.0 micrograms) were injected i.v. in WKY. Thus, rat BNP-45 and alpha-ANP had comparable hypotensive and natriuretic potency; however, the action and plasma half-life of rat BNP-45 were more prolonged.

Origin of porcine brain natriuretic peptide-like immunoreactive innervation of the middle cerebral artery in the rat

We recently demonstrated that porcine brain natriuretic peptide-like immunoreactive (pBNPir) fibers innervate parts of the cardiovascular system, including the arteries comprising the circle of Willis. To determine the origin of this innervation, we used the retrograde fluorescent tracer Fast Blue dye combined with pBNP immunocytochemistry. Cells which project to the middle cerebral artery and were also pBNPir were found in the trigeminal, pterygopalatine and superior cervical ganglia bilaterally but not in the geniculate or otic ganglia. The majority of these double-labelled cells were found in the ipsilateral trigeminal (46%) and superior cervical ganglia (34%). A pBNP-like substance may be a natural vasodilator in sympathetic, sensory and to a lesser extent parasympathetic neurons innervating the cerebrovascular system.