Alignment of rat cardionatrin sequences with the preprocardionatrin sequence from complementary DNA

Emerging pathways of communication between the heart and non-cardiac organs

The breakthrough discovery of cardiac natriuretic peptides provided the first direct demonstration of the connection between the heart and the kidneys for the maintenance of sodium and volume homeostasis in health and disease. Yet, little is still known about how the heart and other organs cross-talk. Here, we review three physiological mechanisms of communication linking the heart to other organs through: i) cardiac natriuretic peptides, ii) the microRNA-208a/mediator complex subunit-13 axis and iii) the matrix metalloproteinase-2 (MMP-2)/C-C motif chemokine ligand-7/cardiac secreted phospholipase A2 (sPLA2) axis – a pathway which likely applies to the many cytokines, which are cleaved and regulated by MMP-2. We also suggest experimental strategies to answer still open questions on the latter pathway. In short, we review evidence showing how the cardiac secretome influences the metabolic and inflammatory status of non-cardiac organs as well as the heart.

Characterization of a long-acting recombinant human serum albumin-atrial natriuretic factor (ANF) expressed in Pichia pastoris

The cardiac hormone atrial natriuretic factor (ANF) combines pharmacological properties of drugs used to treat essential hypertension (EH), congestive heart failure (CHF) and acute myocardial infarction (AMI). Treatment of CHF or AMI patients with an intravenous (iv) infusion of the circulating form of ANF (ANF(99-126)) produces significant clinical improvement. The short half-life (5 min) and peptide nature of ANF impose logistic restrictions for chronic administration. To increase its half-life, we fused ANF and human serum albumin (HSA) mini-genes by recombination in Pichia pastoris. The activity of three configurations of the fusion protein was tested in vitro and in vivo. The fusion protein that comprised of C-terminus HSA connected to N-terminus ANF via a hexaglycine linker showed the best outcome; it increased cGMP production in vitro. In vivo an iv bolus of HSA-ANF into mice increased significantly plasma cGMP levels and lowered blood pressure (BP) for up to 6 h hence successfully extended ANF half-life in plasma while retaining its biological activity. HSA-ANF represents the basis for development in the chronic therapeutic use of ANF.

Involvement of endogenous ouabain-like compound in the cardiac hypertrophic process in vivo

The Na(+)/K(+)-ATPase inhibitor ouabain has been shown to trigger hypertrophic growth of cultured cardiomyocytes; however, the significance of endogenous ouabain-like compound (OLC) in the hypertrophic process in vivo is unknown. Here we characterized the involvement of OLC in left ventricular (LV) hypertrophy induced by norepinephrine (NE) and angiotensin II (Ang II) infusions in rats. Administration of NE (300 microg/kg/h) via subcutanously implanted osmotic minipumps for 72 h resulted in a significant increase in left ventricular weight to body weight (LVW/BW) ratio (P<0.001) and a substantial up-regulation of atrial natriuretic peptide (ANP) gene expression (13.2-fold, P<0.001). NE infusion induced a transient increase in plasma OLC levels at 12 h (P<0.05), which returned to control levels by 72 h. Adrenalectomy markedly reduced both basal and NE-induced increase in plasma OLC levels. LVW/BW ratio was not modulated by adrenalectomy; however, ANP gene expression was blunted by 44% (P<0.01) and 47% (P<0.05) at 12 and 72 h, respectively. In agreement, adrenalectomy reduced up-regulation of ANP without affecting LV mass in rats infused with Ang II (33 microg/kg/h). Administration of exogenous ouabain (1 nM to 100 microM) for 24 h had no effect on ANP gene expression in cultured neonatal rat ventricular myocytes. However, the up-regulation of ANP mRNA levels induced by the alpha-adrenergic agonist phenylephrine (1 microM) was markedly enhanced by ouabain (100 microM) (5.6-fold vs. 9.6-fold, P<0.01). These data show that OLC as an adrenal-derived factor may be required for the induction LV ANP gene expression during the hypertrophic process.

Endothelin-1 and angiotensin II contribute to BNP but not c-fos gene expression response to elevated load in isolated mice hearts

The early events in the cardiac hypertrophic process induced by hemodynamic load include activation of B-type natriuretic peptide (BNP) and c-fos gene expression. However, it is unknown whether stretch acts directly or through local paracrine factors to trigger changes in cardiac gene expression. Herein we studied the involvement of endothelin-1 (ET-1) and angiotensin II (Ang II) in load-induced activation of left ventricular BNP and c-fos gene expression using an in vitro stretch model in isolated perfused adult mice hearts. Two-hour stretch induced by increasing coronary flow rate from 2 to 5 ml/min increased the expression of BNP and c-fos genes by 1.9- and 1.5-fold, respectively (P<0.001 and P<0.05). A mixed ET(A/B) receptor antagonist bosentan attenuated the BNP gene expression response to load by 58% (P<0.005). A similar 53% inhibition was observed with the selective ET(A) receptor blocker BQ-123 (P<0.05). Type 1 Ang II receptor antagonist CV-11974 decreased the activation of BNP gene expression by 50% (P<0.05). In contrast, the activation of c-fos gene expression was not inhibited by antagonists of ET(A/B) and AT(1) receptors. Our results show that ET-1 and Ang II play a key role in the induction of BNP, but not c-fos gene expression in response to load in intact adult murine hearts.

Determinants of natriuretic peptide gene expression

The cardiac natriuretic peptides (NP) atrial natriuretic factor or peptide (ANF or ANP) and brain natriuretic peptide (BNP) are polypeptide hormones synthesized, stored and secreted mainly by cardiac muscle cells (cardiocytes) of the atria of the heart. Both ANF and BNP are co-stored in storage granules referred to as specific atrial granules. The biological properties of NP include modulation of intrinsic renal mechanisms, the sympathetic nervous system, the rennin-angiotensin-aldosterone system (RAAS) and other determinants, of fluid volume, vascular tone and renal function. Studies on the control of baseline and stimulated ANF synthesis and secretion indicate at least two types of regulated secretory processes in atrial cardiocytes: one is stretch-stimulated and pertussis toxin (PTX) sensitive and the other is Gq-mediated and is PTX insensitive. Baseline ANF secretion is also PTX insensitive. In vivo, it is conceivable that the first process mediates stimulated ANF secretion brought about by changes in central venous return and subsequent atrial muscle stretch as observed in acute extracellular fluid volume expansion. The second type of stimulation is brought about by sustained hemodynamic and neuroendocrine stimuli such as those observed in congestive heart failure.

Ghrelin induces vasoconstriction in the rat coronary vasculature without altering cardiac peptide secretion

We administered ghrelin, a novel growth hormone-releasing hormone, to isolated perfused rat hearts, coronary arterioles, and cultured neonatal cardiomyocytes to determine its effects on coronary vascular tone, contractility, and natriuretic peptide secretion and gene expression. We also determined cardiac levels of ghrelin and whether the heart is a source of the circulating peptide. Ghrelin dose dependently increased coronary perfusion pressure (44 ± 9%, P < 0.01), constricted isolated coronary arterioles (12 ± 2%, P < 0.05), and significantly enhanced the pressure-induced myogenic tone of arterioles. These effects were blocked by diltiazem, an L-type Ca2+ channel blocker, and bisindolylmaleimide (Bis), a protein kinase C (PKC) inhibitor. Interestingly, coinfusion of ghrelin with diltiazem completely restored myocardial contractile function that was decreased 30 ± 3% ( P < 0.01) by diltiazem alone. In contrast, combination of ghrelin with diltiazem or Bis did not significantly alter atrial natriuretic peptide (ANP) secretion, which was decreased 40% ( P < 0.01) and 50% ( P < 0.05) by these agents alone, respectively. Administration of ghrelin to cultured cardiomyocytes had no effect on ANP or B-type natriuretic peptide secretion or gene expression. Detectable amounts of low-molecular-weight ghrelin were present in cardiac tissue extracts but not in isolated heart perfusate. Thus we provide the first evidence that ghrelin has a coronary vasoconstrictor action that is dependent on Ca2+ and PKC. Furthermore, the data obtained from diltiazem infusion suggest that ghrelin has a role in regulation of contractility when L-type Ca2+ channels are blocked. Finally, the observation that immunoreactive ghrelin is found in cardiac tissue suggests the presence of a local cardiac ghrelin system.

Adrenomedullin modulates hemodynamic and cardiac effects of angiotensin II in conscious rats

We examined whether adrenomedullin, a vasoactive peptide expressed in the heart, modulates the increase in blood pressure, changes in systolic and diastolic function, and left ventricular hypertrophy produced by long-term administration of ANG II or norepinephrine in rats. Subcutaneous administration of adrenomedullin (1.5 μg·kg−1·h−1) for 1 wk inhibited the ANG II-induced (33.3 μg·kg−1·h−1 sc) increase in mean arterial pressure by 67% ( P < 0.001) but had no effect of norepinephrine-induced (300 μg·kg−1·h−1 sc) hypertension. Adrenomedullin enhanced the ANG II-induced improvement in systolic function, resulting in a further 9% increase ( P < 0.01) in the left ventricular ejection fraction and 19% increase ( P < 0.05) in the left ventricular fractional shortening measured by echocardiography, meanwhile norepinephrine-induced changes in systolic function were remained unaffected. Adrenomedullin had no effect on ANG II- or norepinephrine-induced left ventricular hypertrophy or expression of hypertrophy-associated genes, including contractile protein and natriuretic peptide genes. The present study shows that adrenomedullin selectively suppressed the increase in blood pressure and augmented the improvement of systolic function induced by ANG II. Because adrenomedullin had no effects on ANG II- and norepinephrine-induced left ventricular hypertrophy, circulating adrenomedullin appears to act mainly as a regulator of vascular tone and cardiac function.

Cardiac gene expression of natriuretic substances is altered in streptozotocin-induced diabetes during angiotensin II-induced pressure overload

OBJECTIVE

To gain insight into the cardiac adaptive mechanisms in diabetes, we studied whether angiotensin II (Ang II) alters expression of the atrial natriuretic peptide (ANP), B-type natriuretic peptide (BNP) and adrenomedullin (AM) genes in the left ventricle of the diabetic rat heart.

METHODS

Diabetes was induced by streptozotocin (STZ; 60 mg/kg body weight intravenously). During the last 24 h of 2.5 or 7 weeks of treatment of male Wistar rats with STZ or vehicle, Ang II (33 microg/kg per h) was administered via osmotic minipumps.

RESULTS

Diabetes was associated with an increased left ventricular weight to body weight (LV/BW) ratio, an index of left ventricular hypertrophy, at week 7 but not at week 2.5, and with increased ANP mRNA content at 2.5 weeks, but not with altered expression of the AM and BNP genes. Mean arterial pressure and LV/BW ratio were increased by Ang II in all groups except in the 7-week diabetic group. Levels of ANP mRNA were increased fourfold (P < 0.001) and threefold (P < 0.05) by Ang II at 2.5 and 7 weeks in control animals, respectively, and 11-fold (P < 0.001) and sevenfold (P < 0.001) at 2.5 and 7 weeks in diabetic animals, respectively. Ang II increased ventricular concentrations of BNP mRNA in control and diabetic animals at 2.5 weeks (1.3-fold, P < 0.001; and 1.6-fold, P < 0.001) and at 7 weeks (1.3-fold, P < 0.05; and 1.8-fold, P < 0.001), respectively. Left ventricular levels of adrenomedullin mRNA were increased by treatment with Ang II for 24 h in 2.5-week diabetic animals.

CONCLUSION

Ang II markedly increased the levels of natriuretic peptide mRNAs in the left ventricle of normal and diabetic rat hearts, whereas it increased adrenomedullin mRNA levels only in 2.5-week diabetic rats and failed to cause hypertension in 7-week diabetic rats. Left ventricular levels of ANP and BNP mRNA were increased by Ang II in diabetic animals more than the additive effects of diabetes and Ang II alone, showing that Ang II induced an amplified response with respect to cardiac concentrations of ANP and BNP in diabetes.

Effects of adrenomedullin on hypertrophic responses induced by angiotensin II, endothelin-1 and phenylephrine

We examined whether adrenomedullin (AM), a vasoactive peptide with significant expression and binding sites in the heart, modulates the hypertrophic response in cultured neonatal rat ventricular myocytes. Myocyte hypertrophy was induced by treating the cells with angiotensin II (Ang II), endothelin-1 (ET-1) or alpha-adrenergic agonist, L-phenylephrine (PHE). All treatments resulted in a hypertrophic response as reflected by increased protein synthesis and expression of atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP) genes. AM treatment resulted in a complete inhibition of the Ang II-induced increase in ANP and BNP gene expression and secretion. In contrast, no inhibitory effect was seen in either ET-1-induced natriuretic peptide gene expression or PHE-induced ANP and BNP gene expression and secretion. AM had only a modest effect on basal levels of natriuretic peptide secretion and gene expression. When AM mRNA levels in isolated neonatal rat myocytes treated for 48 h with Ang II, ET-1 or PHE were measured, only Ang II induced a consistent increase in AM gene expression. These results indicate that AM is not invariably associated with attenuation of the hypertrophic response but its effect is dependent on the stimulus activating myocyte hypertrophy. AM may form an important autocrine/paracrine growth-inhibitory loop in Ang II-induced myocyte hypertrophy.

Angiotensin II induced cardiac hypertrophy in vivo is inhibited by cyclosporin A in adult rats

Recently, the calcium-calmodulin-dependent calcineurin pathway has been defined as a central pathway for the induction of cardiac hypertrophy. The purpose of this study was to determine if cardiac hypertrophy in animals chronically treated with angiotensin II (AngII), could be prevented by blocking this pathway with cyclosporin A (CsA). Female Wistar rats were treated with AngII by subcutaneous infusion and injected twice a day with CsA (25 mg/kg) for 7 days. In the AngII treated group there was a 30% increase in the heart/body weight ratio (p < 0.05 vs. control). The increase in heart weight was blocked with CsA. Substantial increases in ANF and betaMHC gene expression were detected in the AngII treated animals, which were either attenuated or blocked with CsA treatment. Thus, this study demonstrates that CsA does prevent the development of cardiac hypertrophy in AngII treated rats, suggesting that the calcium-calmodulin-dependent calcineurin pathway is associated with angiotensin II induced hypertrophy in vivo.

Overexpression of sarcolemmal calcium pump attenuates induction of cardiac gene expression in response to ET-1

The function of the plasma membrane calmodulin-dependent calcium ATPase (PMCA) in myocardium is unknown. PMCA is localized in caveolae, 50- to 100-nm membrane invaginations, which also contain receptors for endothelin-1 (ET-1) and various other ligands. PMCA has been suggested to play a role in regulation of caveolar signal transduction. We studied the effects of the hypertrophic agonist ET-1 and increased coronary perfusion pressure on cardiac synthesis of B-type natriuretic peptide (BNP) in transgenic rats overexpressing the human PMCA 4CI in isolated perfused heart preparation. ET-1 infusion for 2 h increased BNP mRNA levels twofold in left ventricles (LV) of nontransgenic rats, whereas no increase was noted in PMCA rat hearts. Similar responses were seen in adrenomedullin and c-fos mRNA levels, and in immunoreactive BNP secretion. Increased mechanical load produced by elevated perfusion pressure induced similar 1.5- to 1.6-fold increases in LV BNP mRNA in both nontransgenic and PMCA rat hearts. These results show that cardiac overexpression of PMCA attenuates ET-1-stimulated early induction of cardiac gene expression, suggesting that PMCA may modulate myocardial growth responses.

Differential regulation of cardiac adrenomedullin and natriuretic peptide gene expression by AT1 receptor antagonism and ACE inhibition in normotensive and hypertensive rats

OBJECTIVE

To study the effects of long-term treatment with the type 1 angiotensin (AT1) receptor antagonist losartan and the angiotensin-converting enzyme (ACE) inhibitor enalapril, on cardiac adrenomedullin (ADM), atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP) gene expression.

METHODS

Spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats were given losartan (15 mg/kg per day) or enalapril (4 mg/kg per day) orally for 10 weeks. The effects of drugs on systolic blood pressure, cardiac hypertrophy, ANP, BNP and ADM mRNA and immunoreactive-ANP (IR)-ANP, IR-BNP and IR-ADM levels in the left ventricle and atria were compared.

RESULTS

Losartan and enalapril treatments completely inhibited the increase of systolic blood pressure occurring with ageing in SHR. The ratio of heart to body weight was reduced in both losartan- and enalapril-treated SHR and WKY rats. Treatment with losartan or enalapril reduced left ventricular ANP mRNA and IR-ANP in both strains, and ventricular BNP mRNA levels in SHR rats. Inhibition of ACE, AT1 receptor antagonism, changes in blood pressure or cardiac mass had no effect on left ventricular ADM gene expression in SHR and WKY rats. In addition, atrial IR-ANP and IR-ADM levels increased in SHR whereas IR-BNP levels decreased in WKY and SHR rats in response to drug treatments.

CONCLUSIONS

Our results show that ventricular ADM synthesis is an insensitive marker of changes in haemodynamic load or cardiac hypertrophy. Furthermore, the expression of ADM, ANP and BNP genes is differently regulated both in the left ventricle and atria in response to AT1 receptor antagonism and ACE inhibition.

Variable renal atrial natriuretic factor gene expression in hypertension

We have previously established the existence of atrial natriuretic factor (ANF) gene expression within the renal parenchyma. Neither the role nor the regulation of this extracardiac source of ANF is clearly defined. To determine whether renal ANF gene expression, similar to cardiac expression, is linked to the activity of the renin-angiotensin system (RAS), we compared renal ANF gene expression in rats after suprarenal aortic banding, a hypertension model associated with activation of RAS, and in the deoxycorticosterone acetate (DOCA)-salt model, which is characterized by depression of RAS. Renal ANF mRNA was measured with a quantitative competitive reverse transcription polymerase chain reaction method. DOCA-salt hypertension significantly reduced the expression of renal ANF. In contrast, aortic banding significantly increased renal ANF expression. In both cases, ANF gene expression in the heart increased. Ramipril treatment at 10 micrograms/kg of aortic-banded rats, a treatment that specifically affects local RAS but maintains hypertension, normalized renal ANF mRNA levels. Altogether, these results suggest that renal ANF gene expression is modulated by local RAS and is independent of circulating RAS and hypertension per se. The marked decrease of renal ANF mRNA in DOCA-salt hypertension suggests a pathogenic role for renal ANF gene downregulation by decreasing the sodium excretory mechanism mediated by the local expression of ANF acting on receptors found in the inner medullary collecting ducts. In aortic banding, renal ANF gene expression upregulation suggests a local compensatory function consistent with the consensus role of natriuretic peptides in the modulation of RAS, thus ameliorating the sodium-retaining effects of renal underperfusion.

Coronary pressure as a determinant of B-type natriuretic peptide gene expression in isolated perfused adult rat heart

The role of coronary flow in the regulation of ventricular B-type natriuretic peptide (BNP) gene expression was studied in isolated perfused rat heart preparation. The increase of coronary flow from 5 ml/min to 20 ml/min for 2 h resulted in a 132+/-6 mm Hg increase in aortic perfusion pressure. The changes in BNP mRNA and immunoreactive BNP (IR-BNP) levels in response to hemodynamic stress were compared to those of c-fos and adrenomedullin (ADM) gene expression. The increase of coronary flow resulted in 1.5-fold increases in the left ventricular BNP mRNA (P < 0.001) and IR-BNP (P < 0.05) levels in 2-month old rats. There was also a 1.5-fold (P < 0.05) increase in ventricular c-fos mRNA levels, whereas ADM mRNA levels decreased by 74% (P < 0.001) in the left ventricle. In 18-month old rats, the increase in coronary flow decreased left and right ventricular BNP mRNA levels by 18% (P < 0.05) and 39% (P < 0.001), respectively. There were no changes in IR-BNP peptide and c-fos mRNA levels, whereas ADM mRNA levels decreased by 46% (P < 0.001) in the left ventricles. The results show that increased aortic perfusion pressure results in differential expression of cardiac genes including up-regulation of ventricular BNP and c-fos gene expression and down-regulation of ADM gene expression. Furthermore, aging seems to elevate the threshold at which hemodynamic stress of the heart results in a response at BNP gene level.

Tissue-specific regulation of renal and cardiac atrial natriuretic factor gene expression in deoxycorticosterone acetate-salt rats

Atrial natriuretic factor (ANF) is expressed in several noncardiac tissues where it may have an autocrine or paracrine function. Such function may be expected of locally synthesized ANF in the renal parenchyma. Previous investigations of the existence of ANF mRNA in the renal parenchyma have yielded conflicting results. The investigations reported here were designed to detect and measure ANF mRNA in normal rats and in rats subjected to a deoxycorticosterone acetate (DOCA)-salt treatment schedule known to strongly activate cardiac ANF gene expression. The expression of the renal ANF gene was measured using a newly developed quantitative competitive reverse transcription-polymerase chain reaction (QC-RT-PCR). This method uses an internal competitor that serves as an internal standard and makes the procedure independent of measurement relative to housekeeping genes. It was found that renal ANF mRNA levels were 10(7) times lower than those found in left or right atria, but immunoreactive (ir) renal ANF concentration by specific radioimmunoassay was 10(4) times lower than that of atrial irANF levels. Reverse-phase high-performance liquid chromatography analysis revealed that more than 99% of renal irANF is processed ANF(99-126). This finding suggests that most of the irANF measured in kidney extracts likely originates from atrial sources. Left atrial ANF mRNA levels after 1 week of DOCA-salt treatment was significantly higher than that of control rats ([21.06+/-2.99] x 10(-l5) mol/microg total RNAversus [8.59 +/-1.26] x 10(-5) mol/microg total RNA, P<.05). However, renal ANF mRNA levels in DOCA-salt rats were significantly decreased compared with those of control rats ([1.64+/-0.34] x 10(-22) mol/microg total RNA versus [3.96+/-0.61]x 10(-22) mol/microg total RNA, P<.05). These results indicate that (1) renal ANF mRNA can be consistently and specifically demonstrated after reverse transcription and PCR amplification; (2) renal and cardiac ANF synthesis are regulated in a tissue-specific, opposite manner during DOCA-salt treatment; and (3) the finding that renal ANF mRNA is downregulated by DOCA-salt treatment together with previous findings suggest the need for further investigation into the role of renal ANF mRNA downregulation in the pathogenetic mechanism that leads to volume expansion and hypertension after chronic DOCA-salt treatment.

BNP gene expression is specifically modulated by stretch and ET-1 in a new model of isolated rat atria

We have assessed the effects of stretch or endothelin-1 (ET-1) on atrial natriuretic factor (ANF) and brain natriuretic peptide (BNP) secretion and gene expression using a new model of isolated right atria from the rat. This model allows for comparatively long-term in vitro study of adult tissue while retaining the anatomic conformation of the atrium. Stretch and ET-1 resulted in a transient stimulation of ANF and BNP secretion, with an initially larger proportional increase in ANF release. Stretch and ET-1 induced a marked increase in BNP gene expression after 1.5 and 4 h, respectively; the increase in BNP mRNA levels was maintained throughout the 8-h experimental period. Stretch and ET-1 also stimulated c-myc and Egr-1 mRNA levels, two markers of mechanical and receptor-mediated transcriptional activation. The selective response of BNP gene to stretch and ET-1 and the distinct responses of ANF and BNP secretion indicate that the atrial cardiocytes have the capability to individually regulate the synthesis of its endocrine products. This suggests that each hormone plays a specific role in the response of the heart to hemodynamic or neuroendocrine imbalances.

Endothelin-1 is involved in stretch-induced early activation of B-type natriuretic peptide gene expression in atrial but not in ventricular myocytes: acute effects of mixed ET(A)/ET(B) and AT1 receptor antagonists in vivo and in vitro

BACKGROUND

The precise role of paracrine and autocrine factors in mechanical load-induced activation of cardiac gene expression is unknown. Here we report the effects of endothelin-1 (ET-1) and angiotensin II (Ang II) receptor antagonism on acute pressure overload-induced activation of cardiac B-type natriuretic peptide (BNP) gene expression in spontaneously hypertensive rats (SHRs) in vivo and on mechanical stretch-induced increase in atrial BNP gene expression in vitro.

METHODS AND RESULTS

Acute pressure overload produced in conscious SHRs by infusion of arginine8-vasopressin (0.05 microg x kg(-1) x min(-1)) for 2 hours resulted in an increase in BNP mRNA levels in the left ventricle as well as in the atrium. Bolus injections of bosentan (mixed ET(A)/ET(B) receptor antagonist, 10 mg/kg I.V.) but not losartan (AT1 receptor antagonist, 10 mg/kg I.V.) blocked the increase of the BNP mRNA levels produced by pressure overload in the left atria, whereas the elevation of BNP mRNA levels was similar (a 1.9-fold increase) in the left ventricles of vehicle-, losartan-, and bosentan-infused SHRs. In an isolated perfused rat heart preparation, infusion of bosentan (1 micromol/L) for 2 hours inhibited the mechanical stretch-induced increase in BNP mRNA levels in the right atria, whereas an AT1 receptor antagonist, CV-11974 (10 nmol/L), had no effect.

CONCLUSIONS

The findings of the present study demonstrate that Ang II and ET-1 are not obligatorily required for stretch to trigger the increased BNP gene expression in ventricular myocytes in vivo. In contrast, mechanical load on the atrial myocytes did initiate an ET-1-dependent expression of BNP gene showing that endogenous ET-1 production differentially regulates BNP gene expression in atrial and ventricular myocytes.

Involvement of transcriptional and posttranscriptional mechanisms in cardiac overload-induced increase of B-type natriuretic peptide gene expression

The induction of atrial and ventricular B-type natriuretic peptide (BNP) gene expression is one of the earliest events occurring during hemodynamic overload. To examine the molecular mechanisms for increased BNP gene expression during cardiac overload, we studied the induction of the BNP gene expression compared with that of atrial natriuretic peptide (ANP) in a modified perfused rat heart preparation. An increase in right atrial pressure of 5 mm Hg resulted in a 1.4-fold (P < .05) and 2.2-fold (P < .01) increase in BNP mRNA levels after 1 and 2 hours, respectively, whereas ANP mRNA levels remained unchanged. Stretching for up to 2 hours also significantly increased right atrial immunoreactive BNP (ir-BNP) levels (from 15.8 +/- 2.2 to 20.1 +/- 1.2 ng/mg, P < .05). Actinomycin D (10 micrograms/mL), a transcriptional inhibitor, completely inhibited the stretch-induced increase in atrial BNP mRNA levels at 1 hour (P < .05) and 2 hours (P < .001), whereas a protein synthesis inhibitor, cycloheximide (90 micrograms/mL), had no effect on basal or direct mechanical stretch-induced increase in right atrial BNP mRNA levels. Furthermore, we examined the role of tyrosine kinase and protein kinase C activities in acute mechanical stretch-induced increase in BNP synthesis. Tyrosine kinase inhibitors lavendustin A (1 mumol/L) and tyrphostin A25 (3 mumol/L) and protein kinase C inhibitors staurosporine (30 nmol/L) and chelerythrine (1 mumol/L) prevented the stretch-induced increase in right atrial ir-BNP concentrations at 2 hours. In addition, chelerythrine inhibited the increase of right atrial BNP mRNA levels stimulated by cardiac overload. These resuls demonstrate that the early increase of BNP mRNA levels by mechanical stretch results from increased transcriptional activation and is independent of protein synthesis. Our results also suggest that protein kinase C and tyrosine kinases activities may be involved in coupling cardiac overload to alterations in atrial BNP synthesis.

Synthesis and secretion of natriuretic peptides in the hypertensive TGR(mREN-2)27 transgenic rat

To examine the pathophysiological mechanisms in transgenic rats carrying the murine Ren-2d renin gene, we studied atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) gene expression and secretion in 12-week-old hypertensive TGR(mREN-2)27 and normotensive Sprague-Dawley rats. Hypertension and marked left ventricular hypertrophy in TGR(mREN-2)27 rats were associated with high baseline plasma levels of immunoreactive ANP (148 +/- 18 versus 34 +/- 3 pmol/L, hypertensive versus normotensive rats; P < .001), whereas plasma immunoreactive BNP levels did not differ significantly between the strains (19 +/- 4 versus 12 +/- 3 pmol/L, P = .06). ANP mRNA and immunoreactive ANP levels in the left ventricular endocardial and epicardial layers in TGR(mREN-2)27 rats were about 20 to 40 times higher (P < .001) than those in normotensive rats. There were no statistically significant differences between atrial and ventricular BNP mRNA levels, but left ventricular immunoreactive BNP concentrations were twofold higher in hypertensive TGR(mREN-2)27 than in normotensive rats. Infusion of [Arg8]-vasopressin (0.05 microgram/kg per minute IV, for 2 hours) in normotensive rats produced rapid increases (twofold, P < .05 to .01) in left ventricular BNP mRNA and immunoreactive BNP levels, whereas ventricular BNP mRNA and peptide levels did not change significantly in hypertensive rats. The increase in left atrial BNP mRNA levels in response to acute pressure overload was also significantly smaller in the hypertensive than normotensive rats (3.5-fold versus 5.2-fold, P < .01). Furthermore, the proportional but not absolute (in picomoles per liter) increase in plasma immunoreactive ANP was smaller in transgenic rats in response to acute saline and [Arg8]-vasopressin infusions (0.9% NaCl: 1.9-fold increase versus 4.4-fold increase in normotensive rats, P < .001; [Arg8]-vasopressin: 2.2-fold versus 4.8-fold increase, P < .001). These results show that baseline and cardiac overload-induced increases in BNP synthesis are markedly attenuated in transgenic rats carrying the murine Ren-2d renin gene. In addition, acute volume and pressure overload produced a smaller proportional increase in ANP secretion in hypertensive rats than normotensive rats. These alterations in the natriuretic peptide system may contribute to the pathogenesis of hypertension and cardiovascular complications in the TGR(mREN-2)27 rat.

Role of calcium in stretch-induced release and mRNA synthesis of natriuretic peptides in isolated rat atrium

To investigate the role of Ca2+ in stretch-induced synthesis and release of atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP) isolated superfused rat atria were stretched by raising intra-atrial pressure. The immunoreactive (ir-) ANP and BNP concentrations were analysed by radioimmunoassay and the corresponding mRNA levels were quantified by Northern blot and dot blot analyses. Stretch-induced ir-ANP release and a rise in BNP mRNA levels increased at high (3.0 mM) compared to low (0.5 mM) extracellular Ca2+ concentration ([Ca2+]o). Moreover, the adaptation of stretch-induced ir-ANP release was dependent on [Ca2+]o. Atrial BNP mRNA levels were increased by stretch also in non-paced, electrically silent atria, where voltage-activated Ca2+ channels are not activated. The stretch-induced rise in BNP mRNA was blocked by gadolinium (80 microM), but not by the L-type channel blocker diltiazem (3.0 microM). This study indicates that both the stretch-secretion coupling of ir-ANP release and the pressure-stimulated synthesis of BNP mRNA are Ca2+-dependent processes. Gadolinium inhibits the stretch-stimulated rise in BNP mRNA levels in contracting and non-contracting atria, which is similar to its ability to block stretch-activated ir-ANP release, suggesting the involvement of Ca2+-permeable stretch-activated channels.

Evidence for load-dependent and load-independent determinants of cardiac natriuretic peptide production

BACKGROUND

In hypertension with cardiac hypertrophy, the specific contributions to increased production of the cardiac natriuretic peptides (NP) atrial natriuretic factor (ANF) and brain natriuretic peptide (BNP) by load and the hypertrophic process are not known. In the present work we determine ANF and BNP synthesis and secretion in the aortic-banded rat treated with dosage schedules of the ACE inhibitor ramipril that result in the prevention or regression of both hypertension and hypertrophy (high dosage) or in the prevention or regression of hypertrophy alone with persistent hypertension (low dosage). Myosin heavy chain (MHC) isoform switch was studied as an indicator of ventricular cardiocyte hypertrophy as well as the levels of collagen III mRNA as a measure of changes in extracellular matrix.

METHODS AND RESULTS

Ramipril was administered for 6 weeks just after suprarenal aortic banding, or rats were banded for 6 weeks, after which ramipril was administered during the following 6 weeks. Banding caused an increase in blood pressure, left ventricular weight-to-body weight ratio, plasma and ventricular NP, ventricular NP mRNA, collagen III, and beta-MHC mRNA. Ramipril at 1 mg/kg normalized all these parameters while ramipril at 10 micrograms/kg normalized left ventricular weight-to-body weight ratio but not blood pressure. Plasma and ventricular NP content and mRNA levels were partially normalized by ramipril (10 micrograms/kg). Ramipril (10 micrograms/kg) prevented increased collagen III mRNA levels but did not affect beta-MHC mRNA levels.

CONCLUSIONS

(1) NP production and secretion in aortic-banded rats are independently related to increased blood pressure and hypertrophy. (2) A load-dependent component is more important than a load-independent component in regulating left ventricular NP production. (3) ANF production is more sensitive than BNP production to the load-independent component. (4) Low-dose ramipril treatment reverses hypertrophy and the increased collagen III expression but does not reverse the increased beta-MHC isoform expression, suggesting that these are independently regulated processes. (5) Aortic banding and ACE inhibition do not affect atrial NP production and content.

Atrial natriuretic peptide and its mRNA in heart and brain of vasopressin-deficient Brattleboro rats

To understand the secretion and synthesis of atrial natriuretic peptide we measured immunoreactive atrial natriuretic peptide from plasma, heart tissues and brain areas, and ANP mRNA was determined from heart auricles and ventricles of vasopressin-deficient Brattleboro rats (DI) and from desmopressin treated Brattleboro rats (DI+DDAVP). Long-Evans rats (LE) served as controls. DI+DDAVP rats were given for 3 days sc. injections of 0.5 micrograms 1-desamino-8-D-arginine vasopressin in 1 mL saline twice a day. The rats were housed in single metabolic cages and urinary output and water intake were measured daily. All the body and organ weight parameters were similar in the three groups when the rats were killed. No change was seen in the plasma ANP level between the groups. The right ventricle of DI+DDAVP rats had significantly (P < 0.05) higher concentration of ANP than LE rats (15.8 +/- 4.4 vs. 3.4 +/- 0.6 ng mg-1 tissue). The left ventricle of DI and DI+DDAVP had significantly (P < 0.05) lower amounts of ANP mRNA than LE rats (0.5 +/- 0.2 vs. 1.3 +/- 0.2 and 0.5 +/- 0.1 vs. 1.3 +/- 0.2 arbitrary units). In the hypothalamus, the ANP concentration was significantly (P < 0.05) lower both in DI and in DI+DDAVP rats than in LE rats (9.3 +/- 1.3 vs. 14.5 +/- 1.6 and 6.1 +/- 0.6 vs. 14.5 +/- 1.6 pg mg-1 tissue). To conclude, although the water intake and urinary output of DI rats were changed towards normal with desmopressin treatment, the heart ventricular and hypothalamic ANP did not parallel the change.(ABSTRACT TRUNCATED AT 250 WORDS)

Atrial natriuretic peptide during water deprivation or hemorrhage in rats. Relationship with arginine vasopressin and osmolarity

The concentrations of atrial natriuretic peptide (ANP) in atria, hypothalami and plasma were investigated in relation to the variations of the plasma endogenous immunoreactive arginine vasopressin (Ir-AVP) during water deprivation or hemorrhage in normal conscious Wistar rats. Furthermore, the in vitro and in vivo effect of extracellular hyperosmolarity on ANP release from right atrium and hypothalamus was examined. Water deprivation elevated circulating immunoreactive ANP (Ir-ANP: pg/ml) to 153 +/- 7 (24 h); 174 +/- 1 (48 h) from the control level (109.6 +/- 7.8). This increase in Ir-ANP concentration which correlated with atrial (r = -0.93) or hypothalamic (r = -0.87) Ir-ANP content decrease, was associated with significantly enhanced levels of plasma Ir-AVP, plasma sodium, osmolarity and hematocrit. An acute volume depletion by hemorrhage significantly reduced plasma Ir-ANP (67 +/- 8.4 pg/ml) from the sham operated level (140 +/- 18 pg/ml). Plasma Ir-AVP was elevated dramatically (207.4 +/- 53.4 pg/ml) compared with the sham operated level (8.8 +/- 2.6 pg/ml). These results, indicating the lack of correlation between plasma Ir-ANP and Ir-AVP in vivo, suggest that the ANP secretion, which is regulated mainly by plasma volume, may be modulated by a change in plasma osmolarity. Extracellular hyperosmolarity stimulated the ANP release from superfused sliced normal rat atria and hypothalami.

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.

Immunoactive iso-ANP/BNP in plasma, tissues and atrial granules of the rat

We have developed a specific radioimmunoassay (RIA) for iso-rANP(1-45)/rBNP(51-95) using antiserum produced against a peptide comprise of the first 20 amino acid sequence of this cardiac hormone. Using this RIA, we found that atria contained the highest amount of iso-rANP(1-45) (567.37 pmol/g) which is about 140-fold higher than ventricles (4.32 pmol/g). With the exception of the lung and kidney, all other tissues had negligible amounts. The plasma level was 1.4 fmol/ml and the only molecular form found was the 45 amino acid form. HPLC analysis of extracts of isolated, purified atrial granules revealed that, like atrial natriuretic peptide (ANP), iso-rANP/rBNP is also stored in these granules. However, while ANP is stored predominantly as pro-ANP(1-126) and cleaved during or after release, iso-ANP/BNP is stored as the 45 amino acid form and may be processed prior to storage in the granules.

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.

Passive mechanical stretch releases atrial natriuretic peptide from rat ventricular myocardium

Ventricular hypertrophy is characterized by augmentation of synthesis, storage, and release of atrial natriuretic peptide (ANP) from ventricular tissue, but the physiological stimulus for ANP release from ventricles is not known. We determined the effect of graded, passive myocardial stretch on ANP release in isolated, arrested, perfused heart preparations after removal of the atria in 13-20-month-old Wistar-Kyoto (WKY) rats and spontaneously hypertensive rats (SHR). By this age, ANP gene expression was increased in the hypertrophic ventricular cells of SHR, as reflected by elevated levels of immunoreactive ANP and ANP mRNA and the increased ANP secretion (SHR, 93 +/- 14 pg/ml, n = 22; WKY rats, 22 +/- 2 pg/ml, n = 20; p less than 0.001) from perfused ventricles after removal of the atria. The release of ANP from ventricles was examined at two levels of left ventricular pressure by increasing the volume of the intraventricular balloon for 10 minutes. Stretching of the ventricles produced a rapid but transient increase in ANP secretion. As left ventricular pressure rose from 0 to 14 and 26 mm Hg in WKY rats and from 0 to 13 and 27 mm Hg in SHR, increases in ANP release into the perfusate of 1.4 +/- 0.1-fold and 1.5 +/- 0.2-fold (p less than 0.05) in WKY rats and 1.1 +/- 0.1-fold and 1.6 +/- 0.2-fold (p less than 0.05) in SHR, respectively, were observed. There was a highly significant correlation between the left ventricular pressure level and the maximal concentration of ANP in the perfusate during stretching (p less than 0.001, r = 0.59, n = 42), as well as between the maximal ANP concentrations in perfusate during stretching and the ventricular weight/body weight ratios of the corresponding animals (r = 0.38, p less than 0.05, n = 42). High performance liquid chromatographic analysis revealed that the ventricles both before and during stretch primarily released the processed, active, 28-amino acid ANP-like peptide into the perfusate. These results indicate that stretching is a direct stimulus for ventricular ANP release and show that ANP is also a ventricular hormone.

Effect of phorbol ester on the release of atrial natriuretic peptide from the hypertrophied rat myocardium

1. To determine the cellular mechanisms of atrial natriuretic peptide (ANP) release from ventricular cardiomyocytes, the secretory and the cardiac effects of a phorbol ester, 12-O-tetradecanoyl-phorbol-13-acetate (TPA), known to stimulate protein kinase C activity in heart cells, were studied in isolated, perfused heart preparations from 2- and 21-month-old Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats. TPA was added to the perfusion fluid for 30 min at a concentration of 46 nM after removal of atrial tissue. Additionally, atrial and ventricular levels of immunoreactive ANP (IR-ANP) and ANP mRNA, the distribution of ANP within ventricles as well as the relative contribution of atria and ventricles in the release of ANP were studied. 2. Ventricular hypertrophy that gradually developed in hypertensive rats resulted in remarkable augmentation of ANP gene expression, as reflected by elevated levels of immunoreactive ANP and ANP mRNA. The total amount of IR-ANP in the ventricles of the SHR rats increased 41 fold and ANP mRNA levels 12.9 fold from the age of 2 to 21 months. At the age of 21 months, levels of IR-ANP and ANP mRNA in the ventricles of SHR rats were 5.4 fold and 3.7 fold higher, respectively, than in the normotensive WKY rats. Immunohistochemical studies demonstrated ANP granules within the hypertrophic ventricles of the old SHR rats, but not within normal ventricular tissue. 3. In isolated perfused heart preparations, the severely hypertrophied ventricular tissue of SHR rats after atrialectomy secreted more ANP into the perfusate than did the control hearts. Interestingly, the ANP release from the hypertrophied ventricles of the old SHR rats increased considerably (from 413 + 30 to the maximum of 623 + 75 pgml-1, F = 10.8, P < 0.001, two-way analysis of variance), whereas only a small increase was seen in old WKY rats and no effect was observed in young animals of either strain. When intact rat hearts (without atrialectomy) were used, infusion of phorbol ester also increased the ANP secretion into the perfusate in young animals. 4. Our present results indicate that the phorbol ester TPA increases the release of ANP from the hypertrophied, but not from normal rat myocardium. Thus, hypertrophied rat ventricular myocytes appear to possess the cellular mechanisms necessary to secrete ANP by a regulated pathway. The results further suggest that protein kinase C activity may be involved in the the regulation of ANP secretion from ventricular cells, as has been shown earlier for atrial myocytes.

Adrenal cortex contributes to the regulation of NaCl-stimulated ANP release in the rat

The present study documents the effects of hypophysectomy and the effects of dexamethasone substitution on the NaCl-stimulated release and on the basal secretion rates of ANP from the rat atria in vitro. We also measured the concentration of mRNA in the atria after hypophysectomy. Rats (n = 12) were subjected to hypophysectomy by a parapharyngeal approach. One group of rats (n = 6) received dexamethasone 0.2 mg s.c. daily for 4 weeks, while the other group was left unsubstituted. After 4 weeks, the atrial block (n = 10) was excised, placed in an organ bath (field stimulation 4 s-1, 20 V, 1 ms; resting tension = 5 mN) and superfused (7 ml min-1) either with a physiological buffer solution (295 mosmol kg-1) or with a hyperosmotic NaCl solution (330 mosmol kg-1). The atria from the hypophysectomized rats did not respond to the stimulus: the concentration of ANP in the 1-min samples of the perfusate was under 100 pg ml-1. Dexamethasone treatment significantly (P less than 0.05) increased the ANP concentration to a maximum of 165 +/- 17 (mean +/- SEM) pg ml-1 during the superfusion while the control concentration was 110 +/- 19 pg ml-1. The ANP mRNA/18 S RNA ratios did not differ between the atria of hypophysectomized and control rats. In conclusion, glucocorticoids are required in the stimulus-induced release of ANP and the impaired release of ANP after hypophysectomy does not depend on an impaired synthesis of ANP.

Release of atrial natriuretic peptide from rat myocardium in vitro: effect of minoxidil-induced hypertrophy

1. Ventricular hypertrophy is characterized by stimulation of ventricular synthesis of atrial natriuretic peptide (ANP). To examine the role of ventricular ANP levels in the secretion of ANP into the circulation, atrial and ventricular levels of immunoreactive-ANP (IR-ANP) as well as ANP messenger RNA (mRNA), and the release of IR-ANP from isolated perfused hearts, both before and after atrialectomy, were measured simultaneously in control and minoxidil-treated Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats. 2. IR-ANP levels in the ventricles of untreated, 12 month-old SHR with severe ventricular hypertrophy were increased when compared to age-matched WKY rats. Minoxidil treatment for 8 weeks in both strains resulted in a decrease in mean arterial pressure and increases in ventricular weight to body weight ratios, plasma IR-ANP concentrations (in WKY from 133 +/- 20 to 281 +/- 34 pg ml-1, P less than 0.01; in SHR from 184 +/- 38 to 339 +/- 61 pg ml-1, P less than 0.05), and in ventricular IR-ANP contents (in WKY: 53%; in SHR: 41%). A highly significant correlation was found between ventricular IR-ANP content and ventricular weight to body weight ratio (r = 0.59, P less than 0.001, n = 26). 3. When studied in vitro, in isolated perfused heart preparations, the hypertrophied ventricular tissue after atrialectomy secreted more ANP into the perfusate than ventricles of the control hearts; ventricles contributed 28%, 22%, 18% and 15% of the total ANP release to perfusate in the minoxidil-treated SHR, control SHR, minoxidil-treated WKY and control WKY, respectively. A significant correlation was found between the IR-ANP release from ventricles and ventricular weight to body weight ratio (r = 0.56, P < 0.01, n = 24). 4. These studies demonstrate that the ventricles contribute substantially to the circulating level of ANP, and that the amount released depends on the degree of ventricular hypertrophy.

Processing of pro-hormone precursor proteins

Peptide-hormones and other biologically active peptides are synthesized as higher molecular weight precursor proteins (pro-proteins) which must undergo post-translational modification to yield the bioactive peptide(s). These post-translational enzymatic events include limited endoproteolysis and may include other modifications of the generated peptide such as limited exopeptidase digestion, N-terminal acetylation, C-terminal amidation, and formation of N-terminal pyroglutamyl residues (pyrrolation). The secretory vesicle hypothesis, one of the major hypotheses regarding processing, states that the initial endoproteolytic event occurs upon formation of the secretory vesicle (or granule) or within the secretory vesicle from which the bioactive peptides are released. Two different endoproteinases which are likely to be physiologically relevant processing enzymes of pro-atrial natriuretic factor and pro-gonadotropin releasing hormone precursor protein, respectively, have recently been discovered in our laboratory and are discussed as model enzymes in the context of this hypothesis. The results indicate that the precursor protein and its complement of processing enzymes are co-packaged into the secretory granule. Evidence is presented to support the idea that the specific sequence and conformation (secondary structural features) of the processing recognition site within the precursor protein likely contribute in large part to the basis for limited endoproteolysis. In the pro-hormones studied, the recognition site is an extended sequence of five to seven residues which likely exists as a beta-turn at the surface of the precursor protein. By extending our results to appropriate protein sequences in the National Biomedical Research Foundation database, we are suggesting that in addition to the doublet of basic amino acids, the primary processing recognition site in pro-hormone precursor proteins often contains a monobasic amino acid or a strongly polar residue (Glu or Asp) in close sequence proximity to the doublet of basic residues.

Regulation of ventricular atrial natriuretic peptide release in hypertrophied rat myocardium. Effects of exercise

Left ventricular hypertrophy is characterized by stimulation of ventricular synthesis of atrial natriuretic peptide (ANP). This study was designed to test the hypothesis that the increased ventricular ANP levels participate in the release of ANP into the circulation. Swimming was used as a physiologic model to induce ANP release from the heart, and atrial and ventricular levels of immunoreactive ANP (IR-ANP) and ANP messenger RNA (mRNA) were measured simultaneously in the spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats at rest and after swimming. IR-ANP concentration in the left ventricle of 1-year-old SHR with severe left ventricular hypertrophy was increased in association with the augmentation of ANP mRNA levels, whereas right ventricular levels of ANP were reduced in SHR compared with normotensive controls. A 30-minute exercise in hypertensive and in normotensive rats resulted in marked increases in mean arterial pressure, heart rate, plasma catecholamine levels, blood lactate levels, and plasma IR-ANP concentration. The increased ANP secretion was associated with a decrease in left (34-39%) and right (24%) ventricular concentration of IR-ANP; transmurally, this depletion of ventricular IR-ANP was greatest (28%) in the endocardial layer of the left ventricle of SHR. No significant differences were noted in total atrial and left or right auricular IR-ANP concentration between SHR and WKY rats or between the resting and swimming rats. When studied in vitro with an isolated, perfused heart preparation, the hypertrophic ventricular tissue after atrialectomy secreted more ANP into the perfusate than did control hearts; in SHR, ventricles contributed 28% of the total ANP release to perfusate, and in normotensive control rats, ventricles contributed 8%. These studies show that stimulated release of ANP is associated with depletion of endocardial left ventricular stores. The amount of ANP released in vitro and in vivo correlated with the degree of hypertrophy of the ventricle. Finally, the phorbol ester, known to increase ANP secretion from intact perfused hearts, had only a limited effect on ANP release after atrialectomy, suggesting that the secretion of ANP from ventricular cells may be mainly of the constitutive type.

Isolation and characterization of iso-rANP, a new natriuretic peptide from rat atria

Using a specific radioimmunoassay we have isolated and sequenced a new 45-amino acid peptide from rat atria which exhibits similar physiological and pharmacological properties to rat atrial natriuretic peptide (rANP). We have termed the new peptide iso-rANP, because of its functional and structural similarities to rANP. Amino acid sequence differences show that iso-rANP is genetically distinct from rANP. Iso-rANP has a single disulfide bond between residues 23-39 and this portion of the peptide shows substantial homology to rANP and to porcine brain natriuretic peptide (BNP). Little homology is evident at the N- and C-termini of iso-rANP and ANP. Iso-rANP is equipotent with rANP in eliciting diuresis, natriuresis and hypotension in the bioassay rat.

Expression, purification, and in vivo activity of atrial natriuretic factor prohormone produced in Escherichia coli

Atrial muscles of the heart are known to produce polypeptide hormones called atrial natriuretic factors (ANF) which have potent diuretic and hypotensive action. These hormones are synthesized as a larger protein precursor called pro atrial natriuretic factor or proANF which contains the biologically active ANF sequences at its C-terminus. Rat proANF (representing amino acids -1 to 128 of the coding sequence) was expressed in a soluble form in Escherichia coli. A simple purification procedure was developed which consists of boiling E. coli cell extracts in 1 M acetic acid and subjecting the supernatant to reversed-phase HPLC. The effect of intravenous administration of the purified recombinant proANF on mean arterial blood pressure was examined. The displacement dose-response curves obtained demonstrated that proANF exhibits similar, albeit less potent, physiological activity than ANF.

Biochemical studies of soluble atrial natriuretic peptide (ANP) receptors from rat olfactory bulb and vascular smooth muscle cells

1. Aim. The biochemical characteristics of atrial natriuretic peptide receptors (ANP-R) derived from rat vascular smooth muscle (A-10 cell line) and central nervous system (CNS; olfactory bulb) tissue were compared. 2. Method and Results. ANP-Rs from each source were solubilized with 40 to 65% efficiency utilizing the nonionic detergent Lubrol-PX. Upon solubilization, the ANP-R from each source maintained the ability to bind 125I-ANP (99-126) with a high affinity; Scatchard analysis indicated that the VSMC ANP-R displayed a Kd for the radioligand of approximately 10 pM, whereas the olfactory receptor possessed a Kd of about 165 pM. The Bmax values for the soluble VSMC and olfactory ANP-Rs were 285 and 30 fmol/mg protein, respectively. Competition binding studies indicated that the VSMC ANP-R bound ANP(99-126), ANP(103-126), and ANP(103-123) with similar affinities, whereas the olfactory ANP-R was much more sensitive to changes in the COOH-terminal structure of the competing peptide. The soluble ANP-Rs from VSMC and olfactory were chromatographically indistinguishable on phenyl-, DEAE-, and wheat germ agglutinin-agarose columns. However, the ANP-Rs could be distinguished using GTP-agarose; the olfactory ANP-R was capable of binding to the resin, whereas the VSMC ANP-R was not. 3. Conclusions. Coupled with other studies, these data suggest that the A10 VSMC ANP-R observed in this study may not be coupled to guanylate cyclase and may represent a receptor serving a clearance function, whereas a significant proportion of the olfactory CNS ANP-R appears to be associated with GTP-binding proteins, likely particulate guanylate cyclase, and probably represents a coupled form of the receptor.

Optimization of atrial natriuretic factor radioimmunoassay

The development and long-term performance of a radioimmunoassay method for cardionatrin (C I = ANF 99-126) is described. The method was evaluated for accuracy, specificity and precision using different protocols and in various species. The antiserum raised against C I cross-reacts 100% with both human and rat C I and 122% with cardionatrin IV (C IV = ANF 1-126). Dextran-coated charcoal (DCC) and double antibody (DA) disequilibrium protocols were used for the separation of free from antibody-bound radioactivity. The sensitivity, coefficient of variation within assay and between assay for DCC was 4.6 pg/tube, 3.1% and 13.8% respectively, and 1.5 pg/tube, 3.8% and 9.1% for the DA. The mean normal plasma C I levels in human, rat and dog after plasma acidification and extraction using the DA method was 49.7 +/- 4.0; 253.6 +/- 19.6 and 59.9 +/- 3.4 pg/mL respectively.

Atrial natriuretic peptide in the central nervous system of the rat

1. Studies of the presence of atrial natriuretic peptide immunoreactivity and receptor binding sites in the central nervous system have revealed unusual sites of interest. 2. As a result, numerous studies have appeared that indicate that brain atrial natriuretic peptide is implicated in the regulation of blood pressure, fluid and sodium balance, cerebral blood flow, brain microcirculation, blood-brain barrier function, and cerebrospinal fluid production. 3. Alteration of the atrial natriuretic peptide system in the brain could have important implications in hypertensive disease and disorders of water balance in the central nervous system.