Organization of the gene for iso-rANP, a rat B-type natriuretic peptide

Natriuretic peptides stimulate steroidogenesis in the fetal rat testis

To study the regulation of fetal testicular steroidogenesis in the rat, we examined effects of members of the natriuretic peptide family, that is, atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP), on testosterone production of dispersed Leydig cells of rat fetuses at Embryonic Day (E) 18.5. All three peptides stimulated testosterone production, with significant effect at concentrations > or =1 x 10(-8) mol/L of ANP, > or =1 x 10(-9) mol/L of BNP, and > or =1 x 10(-6) mol/L of CNP. Likewise, receptors for all three peptides (i.e., NPR-A, NPR-B, and NPR-C) were expressed in the fetal testis as early as E15.5. The natriuretic peptides had no effect on cAMP production by fetal Leydig cells. When tested in combination with two other peptides previously shown to stimulate fetal testicular steroidogenesis, vasoactive intestinal peptide and pituitary adenylate cyclase-stimulating polypeptide (PACAP-27), the combined effects did not differ significantly from the maximum effect with any one of the peptides alone. In conclusion, our present findings provide both functional and molecular evidences for NPR-A, NPR-B, and NPR-C in the fetal testis. Because ANP has previously been detected in fetal plasma and we now demonstrate the expression of BNP and CNP in fetal testes, these findings indicate involvement of the natriuretic peptides in endocrine and paracrine regulation during the early phase of fetal testicular steroidogenesis at E15.5--19.5 (i.e., before the onset of pituitary LH secretion).

Gene structure of a new cardiac peptide hormone: a model for heart-specific gene expression

Volume excess and mechanical load lead to the induction of the endocrine activity of the heart. The increased production and secretion of A- and B-type natriuretic peptides (ANP and BNP), in turn, unload the heart due to their physiological effects. To find out the mechanisms of cardiac-specific expression and sensitivity to mechanical stimuli of the natriuretic peptide genes, we have used salmon (Salmo salar) as our model organism, because osmoregulating fish have a particularly well developed defense mechanism against volume excess. We have previously cloned a complementary DNA from salmon heart encoding a novel vasorelaxant cardiac hormone, salmon cardiac peptide (sCP). Its production is restricted to the heart, and its release is very sensitive to mechanical load. We have now cloned the gene encoding sCP. The structure of the gene suggests that sCP may represent an ancestral form of the mammalian natriuretic peptides. Remarkably, despite the large phylogenetic distance, the sCP promoter is as effective as mammalian ANP promoters in cultured neonatal rat atrial cardiomyocytes. Therefore, structural and functional comparisons of the promoters of sCP and ANP provide an excellent means of identifying the elements and transcription factors required for atrial-specific gene expression and the regulation of the endocrine function of the heart. Isolation of the protein product of sCP gene from salmon atrium demonstrated that the storage form of sCP is the prohormone of 126 amino acids. The final processing of the prohormone appears to take place during exocytosis of the secretory granules, as the released and circulating form is the biologically active 29-amino acid sCP.

Altered regulation of natriuretic peptides in the rat heart by prenatal exposure to morphine

1. Both endogenous and exogenous opioids modulate blood pressure and cardiac function by stimulating cardiac synthesis of atrial natriuretic factor (ANF) and brain natriuretic peptide (BNP). Since morphine crosses the placental barrier, it could alter the ANF-BNP system in the fetal heart. The aim of this study was to characterize cardiac natriuretic peptides in normal rat development and in rats prenatally exposed to morphine. 2. Female rats received either saline or morphine (10 or 20 mg kg-1 day-1) via osmotic minipumps during gestation. The effects of this treatment were investigated in offspring at 1, 4 and 22 days of age. 3. During maturation, atrial ANF and ANF mRNA increased by 3-fold from birth to 3 weeks of age, but BNP and BNP mRNA tended to decrease. In the ventricles, both ANF and BNP content decreased at 3 weeks after birth, from 25.11 +/- 3.6 to 0.81 +/- 0.1 ng (mg protein)-1 (P < 0.001), and from 3.36 +/- 0.33 to 0.19 +/- 0.01 ng (mg protein)-1 (P < 0.001), respectively. However, whereas ventricular ANF mRNA decreased, BNP mRNA levels did not change during maturation. Prenatal exposure to morphine significantly increased ANF content in the left atria of 22-day-old rats, and in the right atria of 1-, 4- and 22-day-old rats compared with age-matched saline controls. In contrast, prenatal exposure to 20 mg kg-1 day-1 morphine significantly inhibited BNP and BNP mRNA in the ventricles at all ages studied. 4. These observations suggest that alterations in mRNA synthesis or stability and/or post-translational processing of ANF and BNP occur in the heart during maturation, and that prenatal exposure to morphine alters cardiac production, and possibly release, of both peptides.

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.

Elevated levels of natriuretic peptides in lungs of hamsters with genetic cardiomyopathy

Various alterations in the natriuretic peptide system have been observed in heart and plasma in humans and animals with heart failure. However, there is limited information about these hormones in hamster lung especially in those with genetic cardiomyopathy, a model of human congestive heart failure. Therefore, the aim of the present study was to investigate the content of the three natriuretic peptides (atrial natriuretic peptide (ANP); brain natriuretic peptide (BNP); C-type natriuretic peptide (CNP) and their gene expression in lungs of normal and cardiomyopathic hamsters. The presence of mRNA coding for ANP and BNP in lungs and heart was investigated by Northern blot and confirmed by reverse transcription polymerase chain reaction (RT-PCR). The peptide contents and plasma concentrations were determined by specific radioimmunoassays. Plasma ANP increased in hamsters with moderate to severe cardiomyopathy (aged 230 days) from control levels of 71.8+/-15.8 to 243.1+/-44.0 pg/ml (P < 0.01). Plasma BNP also increased from 79.7+/-23.5 to 227.9+/-51.6 pg/ml (P < 0.01). The levels of the three peptides in lungs of 30- and 120-day-old cardiomyopathic (CMO) hamsters were not different from their corresponding age-matched controls. However, lung ANP increased in 230-day-old CMO from 589+/-63 to 1624+/-219 pg/mg protein (P < 0.01). Lung BNP and CNP also increased from 332+/-35 to 531+/-55 pg/mg protein (P < 0.01) and from 118+/-21 to 224+/-29 pg/mg protein (P < 0.01), respectively. Lung ANP mRNA and BNP mRNA were significantly (P < 0.05) higher in 230-day-old hamsters than those detected in age-matched normal controls. Our data demonstrate that the hamster lungs produce ANP, BNP and CNP, and that this production is enhanced in moderate to severe cardiomyopathy. These findings imply that the lung natriuretic peptide system may participate in pulmonary function especially during cardiac dysfunction.

Molecular cloning of the complementary DNA and gene that encode mouse brain natriuretic peptide and generation of transgenic mice that overexpress the brain natriuretic peptide gene

Brain natriuretic peptide (BNP) is a cardiac hormone that occurs predominantly in the ventricle. To study the roles of BNP in chronic cardiovascular regulation, we isolated mouse BNP cDNA and genomic clones, and generated transgenic mice with elevated plasma BNP concentration. The mouse BNP gene was organized into three exons and two introns. Two BNP mRNA species were identified, which were generated by the alternative mRNA splicing. The ventricle was a major site of BNP production in mice. Mouse preproBNP was a 121- (or 120-) residue peptide, and its COOH-terminal 45-residue peptide was the major storage form in the heart. Transgenic mice carrying the human serum amyloid P component/mouse BNP fusion gene were generated so that the hormone expression is targeted to the liver. In the liver of these mice, considerable levels of BNP mRNA and peptide were detected, reaching up to 10-fold greater than in the ventricle. These animals showed 10- to 100-fold increase in plasma BNP concentration accompanied by elevated plasma cyclic GMP concentration, and had significantly lower blood pressure than their nontransgenic littermates. The present study demonstrates that these mice provide a useful model system with which to assess the roles of BNP in cardiovascular regulation and suggests the potential usefulness of BNP as a long-term therapeutic agent.

Radioimmunoassay for rat B-type natriuretic peptide (BNP-45)

Rat BNP-45 is the main circulating form of BNP in rat plasma. To understand the role of BNP in physiological and pathophysiological conditions, a specific radioimmunoassay (RIA) for the quantitative determination of the peptide in plasma and tissues is necessary. An assay using rBNP-45 as the standard in conjunction with antisera directed against this peptide has not been described in the literature, though some investigators have reported values ranging from 0.73-2.0 pmol/L using either BNP-26 or BNP-32 as the standard peptide. Unfortunately, these forms of BNP do not exist in rat plasma. In our studies, we have developed a specific RIA for rBNP-45 using rBNP-45 as the standard peptide and Tyro-rBNP-45 as the radioligand. We have used two specific antisera for assay purposes; one against rBNP-45, and the second to a peptide composed of the first 20 amino acids of rBNP-45 (rBNP[1-20]). The recovery of various amounts of rBNP-45 added to control plasma was 50-80% depending on the method of extraction and purification. The interassay and intraassay coefficients of variation were 12% and 6% respectively. Values obtained were similar for blood sampled by either cardiac puncture, decapitation, or aortic puncture. The method was used to measure rBNP-45 in the plasma of normal (WKY) and Spontaneously Hypertensive (SHR) rats. The values obtained were 5.46 +/- 0.43 and 19.6 +/- 2.36 pmol/L respectively. The rat atrial natriuretic peptide (ANP[99-126]) values in the same extracts were 23.2 +/- 0.45 and 51.6 +/- 3.16 pmol/L.

Circulatory and metabolic responses in awake dogs to infusion of iso-rANP/(rBNP)

We reported that a second rat atrial peptide, iso-atrial natriuretic peptide (iso-rANP(1-45)) and a potential putative homologue, iso-rANP(17-45) (identical with rat brain natriuretic peptide except for one amino acid) elicited circulatory and renal responses in anesthetized rats. In the present studies, low-dose intravenous infusions of iso-rANP(1-45) (6.3-25 pmol kg-1 min-1) and iso-rANP(17-45) (12.5-50 pmol kg-1 min-1) into conscious dogs produced subtle circulatory effects compared to control studies. Relative to oxygen consumption, cardiac output was lower and total peripheral resistance higher with both iso-rANP(1-45) and iso-rANP(17-45). Heart rate tended to be slightly lower relative to control studies during peptide infusions, and the highest infusion doses caused a decrease in mean arterial pressure. Plasma protein increased and plasma osmolality decreased with iso-rANP(1-45); infusion of iso-rANP(17-45) caused a decrease in the respiratory exchange ratio. The mechanism of action of iso-rANP may have been direct, via an active receptor. However, we previously reported for these same experiments that infusion of iso-rANP(1-45) and iso-rANP(17-45) increased plasma ANP and decreased plasma renin activity. Thus, circulatory changes during infusion of iso-rANP were consistent with an indirect mechanism related to increased endogenous ANP.

Chronic ethanol ingestion modifies the renin-aldosterone axis independent of alterations in the regulation of atrial natriuretic peptide

Using an animal model, we have investigated the effects of chronic ethanol ingestion on the regulation of atrial natriuretic peptide (ANP) synthesis and release. Male Sprague-Dawley rats were maintained for 6 weeks on a liquid diet of ethanol (up to 20% v/v) as part of a 2% solution of calf milk replacer. Weight-matched controls received an equal volume of ethanol-free solution, and normal animals drank ad libitum. All animals received rat chow throughout the experiment. This model produced physiologically relevant levels of blood ethanol, as concentrations at the time of sacrifice were 171.98 +/- 39.26 mg/dl. Plasma renin activity was significantly elevated in response to ethanol treatment, whereas circulating aldosterone concentration was reduced. No alterations in the plasma or atrial tissue levels of ANP were evident, although we did observe a significant increase in the ventricular tissue levels of ANP from 45.1 to 71.8 ng/g as a consequence of ethanol treatment. Levels of both atrial and ventricular ANP mRNA were not different between alcohol-treated and liquid-restricted control animals, although both groups showed significant increases in the amount of transcript in comparison with rats drinking ad libitum. No significant increases in either arterial blood pressure or heart/body weight ratio were observed for ethanol-treated rats. These results suggest that modifications in the renin-aldosterone axis can occur independently of alterations in the regulation of ANP under the influence of chronic ethanol ingestion.

Structure, expression, and genomic mapping of the mouse natriuretic peptide type-B gene

The structure of the mouse natriuretic peptide type-B (BNP) gene was determined by isolating and sequencing genomic clones. The mouse BNP gene was structurally similar to other natriuretic peptide genes and comprised three exons and two introns. Expression of the mouse BNP gene was found only in cardiac tissue as determined by ribonuclease protection analyses. Initiation of transcription was 31 bp downstream from a consensus TATA box as determined by primer extension analysis of cardiac RNA. Comparative DNA sequence analysis identified several DNA elements with potential transcriptional regulatory function. Comparative amino acid sequence analysis showed that the N-terminal portion of the mouse and rat BNP precursors was more conserved than the C-terminal 45-amino-acid sequence that constitute the bioactive BNP-45 peptide. The proteolytic processing site (RXXR-S) generating bioactive BNPs was highly conserved among all BNP precursors and was identical to the consensus site of furin, a calcium-dependent serine endoprotease. Finally, the BNP gene was mapped using recombinant inbred DNA and a polymerase chain reaction-based restriction fragment-length polymorphism assay to mouse chromosome 4 near the atrial natriuretic factor (Anf) locus. No recombination event between Bnp and Anf was evident in the 39 recombinant inbred and inbred strains examined. This physical linkage between the two natriuretic peptide genes expressed in cardiac tissue may be important for their transcriptional regulation.

Expression of natriuretic peptide genes in cardiac tissues of hypertensive rats

A quantitative assay using a reverse transcriptase-linked polymerase chain reaction has been developed for measuring the levels of rANP and iso-rANP mRNA. A linear correlation between total RNA template and amplified cDNA was obtained for the amplification of cDNA from both iso-rANP and rANP mRNAs even when both cDNAs were amplified in the same assay. Application of the assay showed that in contrast to rANP levels of iso-rANP transcript in hypertensive rats remained approximately the same in atrium but were increased 10-fold in ventricle compared to normal rats. Given the relative size of the ventricle the increase of iso-rANP in this tissue in SHR may be a major response to the hypertensive state.

Estradiol, progesterone and testosterone exposures affect the atrial natriuretic peptide gene expression in vivo in rats

To clarify the effects of sex hormones on the expression of atrial natriuretic peptide (ANP), ovariectomized and intact female rats were subcutaneously injected with estradiol, progesterone, a mixture of them or olive oil solvent; castrated and untouched male rats were subcutaneously injected with estradiol, testosterone or olive oil, once a day for 7 days. The relative rANP-mRNA contents of rat atrial were measured by molecular hybridization. rANP-cDNA was labeled with 32P as a probe. The results revealed that estradiol and progesterone increased ANP gene expression. Furthermore their effects were associated with administration dose of these hormones and it was shown that they are probably coordinated. The physiological amounts of estradiol and progesterone may maintain suitable levels of rANP-mRNA and androgen may also increase the ANP gene expression in vivo. These experiments suggested that female sex hormone may have a dual purpose in fluid balance.

Gene and precursor structures of human C-type natriuretic peptide

We have isolated the gene for human C-type natriuretic peptide (CNP) from a human genomic library using a cloned porcine CNP genomic DNA fragment as probe. Human CNP gene consists of at least two coding blocks and an intron, and encodes a 126-residue CNP precursor (human prepro-CNP). From a comparison of the amino acid sequences of porcine and rat prepro-CNPs, human prepro-CNP is found to be processed to generate 22-and 53-residue peptides (human CNP-22 and human CNP-53, respectively) as major endogenous CNPs in human. Interestingly, human CNP-53 has two amino acid substitutions as compared to the porcine and rat CNP-53s, whereas human CNP-22 is identical to the porcine and rat CNP-22s. Intravenous injection of human CNP-53 into anesthetized rats induces diuretic-natriuretic and hypotensive activities with same potencies as porcine CNP-53 does, although these activities were considerably lower (about 1/100) than those of human alpha-ANP.