Influence of ANP on sympathetic nerve activity and chronotropic regulation of the heart

Glial cell activity in cardiovascular diseases and risk of acute myocardial infarction

Growing evidence indicates that the pathophysiological link between the brain and heart underlies cardiovascular diseases, specifically acute myocardial infarction (AMI). Astrocytes are the most abundant glial cells in the central nervous system and provide support/protection for neurons. Astrocytes and peripheral glial cells are emerging as key modulators of the brain-heart axis in AMI, by affecting sympathetic nervous system activity (centrally and peripherally). This review, therefore, aimed to gain an improved understanding of glial cell activity and AMI risk. This includes discussions on the potential role of contributing factors in AMI risk, i.e., autonomic nervous system dysfunction, glial-neurotrophic and ischemic risk markers [glial cell line-derived neurotrophic factor (GDNF), astrocytic S100 calcium-binding protein B (S100B), silent myocardial ischemia, and cardiac troponin T (cTnT)]. Consideration of glial cell activity and related contributing factors in certain brain-heart disorders, namely, blood-brain barrier dysfunction, myocardial ischemia, and chronic psychological stress, may improve our understanding regarding the pathological role that glial dysfunction can play in the development/onset of AMI. Here, findings demonstrated perturbations in glial cell activity and contributing factors (especially sympathetic activity). Moreover, emerging AMI risk included sympathovagal imbalance, low GDNF levels reflecting prothrombic risk, hypertension, and increased ischemia due to perfusion deficits (indicated by S100B and cTnT levels). Such perturbations impacted blood-barrier function and perfusion that were exacerbated during psychological stress. Thus, greater insights and consideration regarding such biomarkers may help drive future studies investigating brain-heart axis pathologies to gain a deeper understanding of astrocytic glial cell contributions and unlock potential novel therapies for AMI.

The Differences of Mechanisms in Antihypertensive and Anti-Obesity Effects of Eucommia Leaf Extract between Rodents and Humans

In the 1970s, Eucommia leaf tea, known as Tochu-cha in Japanese, was developed from roasted Eucommia leaves in Japan and is considered as a healthy tea. The antihypertensive, diuretic, anti-stress, insulin resistance improving, and anti-obesity effects of Eucommia leaf extract have been reported. However, the identification and properties of the active components as well as the underlying mechanism of action are largely unknown. In this review, we summarize studies involving the oral administration of geniposidic acid, a major iridoid component of Eucommia leaf extract which increases plasma atrial natriuretic peptide (ANP) on the atria of spontaneously hypertensive rats (SHR) by activating the glucagon-like peptide-1 receptor (GLP-1R). To achieve the antihypertensive effects of the Eucommia leaf extract through ANP secretion in humans, combining a potent cyclic adenosine monophosphate phosphodiesterase (cAMP-PDE) inhibitor, such as pinoresinol di-β-d-glucoside, with geniposidic acid may be necessary. Changes in the gut microbiota are an important aspect involved in the efficacy of asperuloside, another component of the Eucommia leaf extract, which improves obesity and related sequelae, such as insulin resistance and glucose intolerance. There are species differences of mechanisms associated with the antihypertensive and anti-obesity effects between rodents and humans, and not all animal test results are consistent with that of human studies. This review is focused on the mechanisms in antihypertensive and anti-obesity effects of the Eucommia leaf extract and summarizes the differences of mechanisms in their effects on rodents and humans based on our studies and those of others.

Exogenous ketosis suppresses diuresis and atrial natriuretic peptide during exercise

We have previously demonstrated that exogenous ketosis reduces urine production during exercise. However, the underlying physiological mechanism of this anti-diuretic effect remained unclear. Therefore, we investigated whether acute exogenous ketosis by oral ingestion of ketone ester (KE) during a simulated cycling race (RACE) affects the hormonal pathways implicated in fluid balance regulation during exercise. In a double-blind crossover design, 11 well-trained male cyclists participated in RACE consisting of a 3-h submaximal intermittent cycling (IMT_180') bout followed by a 15-minute time trial (TT_15') in an environmental chamber set at 28 °C and 60 % relative humidity. Fluid intake was adjusted to maintain euhydration. Before and during RACE, the subjects received either a control drink (CON) or the ketone ester (R)-3-hydroxybutyl (R)-3-hydroxybutyrate (KE), which elevated blood β-hydroxybutyrate to ~2-4 mM. Urine output during IMT_180' was ~20% lower in KE (1172 ± 557 ml) than in CON (1431 ± 548 ml, p < 0.05). Compared with CON, N-terminal pro-atrial natriuretic peptide (NT-pro ANP) concentration during RACE was ~20% lower in KE (p < 0.05). KE also raised plasma noradrenaline concentrations during RACE. Performance in TT_15' was similar between CON and KE. In conclusion, exogenous ketosis suppresses diuresis and downregulates α-natriuretic peptide activity during exercise.

Natriuretic peptides: new challenges — new solutions

Natriuretic peptides (NPs) are one of the most significant biomarkers, the practical use of which increases, and their diagnostic and prognostic value in patients with various chronic noncommunicable diseases is beyond doubt. Since the discovery of these markers, research has been actively carried out to study the biological and pathophysiological roles of NPs in a wide range of diseases, including hypertension and heart failure (HF). These studies showed that A-type and B-type NPs are hormones secreted by the heart in response to pre- or afterload, which prevent high blood pressure and fluid retention. In addition, C-type NPs are produced by the vascular endothelium and act as a local a  mediator with angioprotective properties. Since the NP system is a natural antagonist of the sympathoadrenal and renin-angiotensinaldosterone systems, it is interesting to study novel strategies to use new drug classes for hypertension. These drugs are neprilysin inhibitors, which destroys NPs; their action is to enhance the synthesis of endogenous peptides. Dual angiotensin receptor and neprilysin inhibition is widespread in clinical practice in patients with heart failure with reduced ejection fraction. Neprilysin inhibition has also been shown to be an effective strategy for hypertensive patients. The article discusses the role and value of NP system in  the dia - gnosis of heart failure and blood pressure regulation, and also considers new promising directions for neprilysin inhibition and activation of endogenous NP synthesis.

 

Sympathovagal imbalance decades after atrial septal defect repair: a long-term follow-up study

 

OBJECTIVES

Recent evidence suggests that patients with a corrected atrial septal defect (ASD) have higher morbidity and mortality. An abnormal autonomic regulation of the heart may be a part of the explanation for this. Our objective was to study heart rate variability (HRV) in adults with a corrected ASD as a prominent tool to investigate the autonomic regulation of the heart.

METHODS

Autonomic cardiac function was investigated in adults with either a surgically closed or percutaneously closed ASD and healthy control subjects. A 48-h Holter monitor was performed on each participant and HRV was assessed.

RESULTS

A total of 17 patients with surgically closed ASDs, 18 percutaneously closed ASDs and 18 controls were included. The mean age in the surgical group, percutaneous group and controls was 32 ± 9, 28 ± 7 and 32 ± 10 years, respectively. The mean time since closure was 19 ± 8 years for the surgical group and 15 ± 5 years for the percutaneous group. The surgically closed ASD patients showed decreased HRV in all six parameters studied when compared to the controls. Similarly, the percutaneously closed ASDs showed decreased HRV in three out of six parameters when compared to controls.

CONCLUSIONS

Adults with an ASD, whether closed surgically or percutaneously, have impaired HRV compared to their age- and sex-matched controls, more so in the patients with a surgically closed ASD.

Clinical trial registration number

ClinicalTrials.gov (identifier: NCT03565471).

Natriuretic peptides and neprilysin inhibition in hypertension and hypertensive organ damage

The family of natriuretic peptides (NPs) discovered in mammalian tissues including cardiac atrium and brain consists of three members, namely, atrial, B- and C-type natriuretic peptides (ANP, BNP, CNP). Since the discovery, basic and clinical studies have been vigorously performed to explore the biological functions and pathophysiological roles of NPs in a wide range of diseases including hypertension and heart failure. These studies revealed that ANP and BNP are hormones secreted from the heart into the blood stream in response to pre- or after-load, counteracting blood pressure (BP) elevation and fluid retention through specific receptors. Meanwhile, CNP was found to be produced by the vascular endothelium, acting as a local mediator potentially serving protective functions for the blood vessels. Because NPs not only exert blood pressure lowering actions but also alleviate hypertensive organ damage, attempts have been made to develop therapeutic agents for hypertension by utilizing this family of NPs. One strategy is to inhibit neprilysin, an enzyme degrading NPs, thereby enhancing the actions of endogenous peptides. Recently, a dual inhibitor of angiotensin receptor-neprilysin was approved for heart failure, and neprilysin inhibition has also been shown to be beneficial in treating patients with hypertension. This review summarizes the roles of NPs in regulating BP, with special references to hypertension and hypertensive organ damage, and discusses the therapeutic implications of neprilysin inhibition.

Natriuretic peptide analogues with distinct vasodilatory or renal activity: integrated effects in health and experimental heart failure

AIMS

Management of acute decompensated heart failure (ADHF) requires disparate treatments depending on the state of systemic/peripheral perfusion and the presence/absence of expanded body-fluid volumes. There is an unmet need for therapeutics that differentially treat each aspect. Atrial natriuretic peptide (ANP) plays an important role in blood pressure and volume regulation. We investigate for the first time the integrated haemodynamic, endocrine and renal effects of human ANP analogues, modified for exclusive vasodilatory (ANP-DRD) or diuretic (ANP-DGD) activities, in normal health and experimental ADHF.

METHODS AND RESULTS

We compared the effects of incremental infusions of ANP analogues ANP-DRD and ANP-DGD with native ANP, in normal (n = 8) and ADHF (n = 8) sheep. ANP-DRD administration increased plasma cyclic guanosine monophosphate (cGMP) in association with dose-dependent reductions in arterial pressure in normal and heart failure (HF) sheep similarly to ANP responses. In contrast to ANP, which in HF produced a diuresis/natriuresis, this analogue was without significant renal effect. Conversely, ANP-DGD induced marked stepwise increases in urinary cGMP, urine volume, and sodium excretion in HF comparable to ANP, but without accompanying vasodilatory effects. All peptides increased packed cell volume relative to control in both states, and in HF, decreased left atrial pressure. In response to ANP-DRD-induced blood pressure reductions, plasma renin activity rose compared to control only during the high dose in normals, and not at all in HF-suggesting relative renin inhibition, with no increase in aldosterone in either state, whereas renin and aldosterone were both significantly reduced by ANP-DGD in HF.

CONCLUSION

These ANP analogues exhibit distinct vasodilatory (ANP-DRD) and diuretic/natriuretic (ANP-DGD) activities, and therefore have the potential to provide precision therapy for ADHF patients with differing pathophysiological derangement of pressure-volume homeostasis.

Natriuretic Peptides and Normal Body Fluid Regulation

Natriuretic peptides are structurally related, functionally diverse hormones. Circulating atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are delivered predominantly by the heart. Two C-type natriuretic peptides (CNPs) are paracrine messengers, notably in bone, brain, and vessels. Natriuretic peptides act by binding to the extracellular domains of three receptors, NPR-A, NPR-B, and NPR-C of which the first two are guanylate cyclases. NPR-C is coupled to inhibitory proteins. Atrial wall stress is the major regulator of ANP secretion; however, atrial pressure changes plasma ANP only modestly and transiently, and the relation between plasma ANP and atrial wall tension (or extracellular volume or sodium intake) is weak. Absence and overexpression of ANP-related genes are associated with modest blood pressure changes. ANP augments vascular permeability and reduces vascular contractility, renin and aldosterone secretion, sympathetic nerve activity, and renal tubular sodium transport. Within the physiological range of plasma ANP, the responses to step-up changes are unimpressive; in man, the systemic physiological effects include diminution of renin secretion, aldosterone secretion, and cardiac preload. For BNP, the available evidence does not show that cardiac release to the blood is related to sodium homeostasis or body fluid control. CNPs are not circulating hormones, but primarily paracrine messengers important to ossification, nervous system development, and endothelial function. Normally, natriuretic peptides are not powerful natriuretic/diuretic hormones; common conclusions are not consistently supported by hard data. ANP may provide fine-tuning of reno-cardiovascular relationships, but seems, together with BNP, primarily involved in the regulation of cardiac performance and remodeling. © 2017 American Physiological Society. Compr Physiol 8:1211-1249, 2018.

The gut-renal axis: do incretin-based agents confer renoprotection in diabetes?

Diabetic nephropathy is the leading cause of end-stage renal disease worldwide, and is associated with a high risk of cardiovascular morbidity and mortality. Intensive control of glucose levels and blood pressure is currently the mainstay of both prevention and treatment of diabetic nephropathy. However, this strategy cannot fully prevent the development and progression of diabetic nephropathy, and an unmet need remains for additional novel therapies. The incretin-based agents--agonists of glucagon-like peptide 1 receptor (GLP-1R) and inhibitors of dipeptidyl peptidase 4 (DPP-4), an enzyme that degrades glucagon-like peptide 1--are novel blood-glucose-lowering drugs used in the treatment of type 2 diabetes mellitus (T2DM). Therapeutic agents from these two drug classes improve pancreatic islet function and induce extrapancreatic effects that ameliorate various phenotypic defects of T2DM that are beyond glucose control. Agonists of GLP-1R and inhibitors of DPP-4 reduce blood pressure, dyslipidaemia and inflammation, although only GLP-1R agonists decrease body weight. Both types of incretin-based agents inhibit renal tubular sodium reabsorption and decrease glomerular pressure as well as albuminuria in rodents and humans. In rodents, incretin-based therapies also prevent onset of the morphological abnormalities of diabetic nephropathy.

Synthesis of atrial natriuretic peptide in the rabbit inner ear

OBJECTIVES/HYPOTHESIS

Atrial natriuretic peptide (ANP) exhibits natriuretic, diuretic, and vasorelaxant activity, maintaining fluid and electrolyte homeostasis. Although ANP is mainly synthesized and secreted in/from atrium, ANP has been found in various tissues including the rat inner ear. The aim of this study was to identify the synthesis of ANP in lateral cochlear wall tissues and the presence of ANP in perilymph using rabbits.

STUDY DESIGN

In vivo study using a rabbit model.

METHODS

Expression of ANP in the rabbit inner ear tissue and the presence of perilymph were examined by radioimmunoassay and polymerase chain reaction using an ANP polyclonal antibody and rat ANP primers, respectively. Characteristics of ANP and pro-ANP present in the inner ear were also evaluated by high-performance liquid chromatography (HPLC), comparing to rat cardiac ANP and pro-ANP.

RESULTS

Immunoreactive ANP (ir-ANP) was found in the perilymph and lateral cochlear wall tissue of rabbits. The levels of ir-ANP in the perilymph were 5 to 16 ng/mL. Elusion profiles of HPLC showed two main peaks that were exactly matched with rat cardiac ANP and pro-ANP. Expression of ANP mRNA was also detected.

CONCLUSIONS

Our findings suggest that ANP and its precursor protein are synthesized in the ear tissues and secreted to perilymph. This hormone may play a role in control of water and/or ion homeostasis of the fluids in the ear that are responsible for normal hearing.

Expression of the vasoactive proteins AT1, AT2, and ANP by pregnancy-induced mouse uterine natural killer cells

Angiotensin II receptor type 1 (AT1) activation leads to vasoconstriction and type 2 receptor (AT2) leads to vasodilation. Atrial natriuretic peptide (ANP) antagonizes the effects of AT1. In human and murine pregnancies, uterine natural killer (uNK) cells closely associate with decidual blood vessels. Protein localization of AT1, AT2, and ANP to mouse uNK cells was examined between gestation days (gds) 6 and 12, the interval of uNK cell expansion. Percentages of uNK cells expressing AT1 or AT2 changed between gd6 and gd10. Atrial natriuretic peptide did not localize to uNK cells at gd6 or 8, but did colocalize to uNK cells at gd10 and 12, times immediately after spiral arterial modification. This is the first report of AT1, AT2, and ANP expression in uterine immune cells. Expression of these molecules suggests that uNK cells have the potential to contribute to the changes in blood pressure that occur between days 5 and 12 of pregnancy in mice.

Urinary responses to acute moxonidine are inhibited by natriuretic peptide receptor antagonist

We have previously shown that acute intravenous injections of moxonidine and clonidine increase plasma atrial natriuretic peptide (ANP), a vasodilator, diuretic and natriuretic hormone. We hypothesized that moxonidine stimulates the release of ANP, which would act on its renal receptors to cause diuresis and natriuresis, and these effects may be altered in hypertension. Moxonidine (0, 10, 50, 100 or 150 microg in 300 microl saline) and clonidine (0, 1, 5 or 10 microg in 300 microl saline) injected intravenously in conscious normally hydrated normotensive Sprague-Dawley rats (SD, approximately 200 g) and 12-14-week-old Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR) dose-dependently stimulated diuresis, natriuresis, kaliuresis and cGMP excretion, with these effects being more pronounced during the first hour post-injection. The actions of 5 microg clonidine and 50 microg moxonidine were inhibited by yohimbine, an alpha2-adrenoceptor antagonist, and efaroxan, an imidazoline I1-receptor antagonist. Moxonidine (100 microg) stimulated (P<0.01) diuresis in SHR (0.21+/-0.04 vs 1.16+/-0.06 ml h(-1) 100 g(-1)), SD (0.42+/-0.06 vs 1.56+/-0.19 ml h(-1) 100 g(-1)) and WKY (0.12+/-0.04 vs 1.44+/-0.21 ml h(-1) 100 g(-1)). Moxonidine-stimulated urine output was lower in SHR than in SD and WKY. Moxonidine-stimulated sodium and potassium excretions were lower in SHR than in SD, but not WKY, demonstrating an influence of strain but not of pressure. Pretreatment with the natriuretic peptide antagonist anantin (5 or 10 microg) resulted in dose-dependent inhibition of moxonidine-stimulated urinary actions. Anantin (10 microg) inhibited (P<0.01) urine output to 0.38+/-0.06, 0.12+/-0.01, and 0.16+/-0.04 ml h(-1) 100 g(-1) in SD, WKY, and SHR, respectively. Moxonidine increased (P<0.01) plasma ANP in SD (417+/-58 vs 1021+/-112 pg ml(-1)) and WKY (309+/-59 vs 1433+/-187 pg ml(-1)), and in SHR (853+/-96 vs 1879+/-229 pg ml(-1)). These results demonstrate that natriuretic peptides mediate the urinary actions of moxonidine through natriuretic peptide receptors.

Cardiac and intestinal natriuretic peptides: insights from genetically modified mice

Since the original discovery of atrial natriuretic peptide (ANP) more than two decades ago, the application of gene targeting technology in mice has provided new insights into the diverse physiological functions of natriuretic peptides and their membrane guanylyl cyclase (GC) receptors. Disruption of the genes for ANP or its receptor, GC-A, demonstrated that this system is not only essential for the maintenance of normal blood pressure and volume, but in addition exerts local antihypertrophic effects in the heart. Disruption of the genes encoding B-type (BNP) or C-type natriuretic peptides (CNP) or the CNP-receptor, GC-B, demonstrated that these "natriuretic" peptides are in fact unlikely to physiologically regulate renal sodium excretion but instead exert important autocrine/paracrine cGMP-mediated effects on cellular proliferation and differentiation in various tissues. Notably, the intestinal peptide uroguanylin, which activates a third guanylyl cyclase receptor (GC-C), exerts diuretic/natriuretic activity and links the intestine and kidney in an endocrine way to modulate renal function in response to oral salt load. Reviewed here is the physiology of cardiac and intestinal natriuretic peptides and their guanylyl cyclase receptors, with special focus on the information gained to date from genetically modified mice.

Sympathetic overactivity in patients with rheumatic mitral stenosis

BACKGROUND

Mitral stenosis may increase sympathetic nervous activity by increasing left atrial pressure and reducing cardiac output. And elevated sympathetic nerve activity may be a risk factor for the development of clinical manifestations of mitral stenosis. In this study, we assessed the autonomic nervous system activity in patients with mitral stenosis by heart rate variability analysis and defined factors affecting autonomic functions.

METHODS AND RESULTS

Fifty-four patients with rheumatic mitral stenosis were compared with an age- and gender-matched control group composed of 42 healthy individuals. SDNN, RMSSD, PNN50, and HF were lower; mean heart rate (HR), LF and LF/HF ratio were higher in the patients with mitral stenosis compared to the control group. SDNN was correlated positively with left ventricle ejection fraction (LVEF), negatively with mitral valve area, left atrial (LA) diameter, and duration of symptoms. RMSSD was correlated positively with mean transmitral gradient, negatively correlated with age; PNN50 was correlated negatively with mitral valve area and positively correlated with transmitral gradient. LF was positively and HF was negatively correlated with LA diameter; LF was correlated positively, and HF was negatively correlated with duration of symptoms. LF/HF ratio was positively correlated with LA diameter and duration of symptoms, negatively with LVEF and mean valve area.

CONCLUSION

As a result, sympathetic nervous system activity is increased in patients with mitral stenosis and sympathetic overactivity worsens their symptoms. Most significant factors that affect autonomic functions in these patients are left atrial dilatation and mitral valve area.

ANP potentiates nonarterial baroreflex bradycardia: evidence from sinoaortic denervation in rats

Previous findings indicate that atrial natriuretic peptide (ANP) enhances the reflex bradycardia arising from stimulation of cardiac mechanoreceptors and chemoreceptors, but not that from arterial baroreceptors. The present study tests this proposal by examining the effect of ANP on these reflexes in six chronically sinoaortic-denervated (SAD), and eight sham-operated (sham), conscious rats. Arterial baroreceptor-heart rate (HR) reflex function was examined by constructing full-range steady-state blood pressure (BP)-HR curves using alternating doses of pressor (methoxamine, 2-100 microg/kg) and depressor (nitroprusside, 1-50 microg/kg) agents. Nonarterial baroreceptor reflex function was assessed by the 'ramp' bradycardic response to the rapid BP rise after i.v. methoxamine (100 microg/kg bolus dose). The cardiopulmonary chemoreflex was evoked by i.v. injections of serotonin (1-20 microg/kg). These three tests were performed on each rat during infusions, in random order, of rat ANP (150 ng/kg/min i.v.) and saline vehicle. The ability of ANP to significantly enhance ramp reflex bradycardia was not diminished in SAD compared with sham rats (+54 +/- 12% vs. +42 +/- 15%, respectively). ANP also significantly enhanced cardiopulmonary chemoreflex bradycardia in both groups (+60 +/- 15% in SAD, +40 +/- 8% in sham). Neither the normal steady-state BP-HR response in sham rats nor the small residual response in SAD rats was enhanced by ANP (-1 +/- 7% in sham, -11 +/- 8% in SAD). We conclude that ANP enhances reflex bradycardias of nonarterial, probably cardiac mechanoreceptor, origin.

Estrogen receptors activate atrial natriuretic peptide in the rat heart

In this study, semiquantitative reverse transcription-PCR analysis showed that estrogen receptor alpha (ERalpha) and beta (ERbeta) mRNAs are developmentally regulated in the rat heart. We found that ERalpha mRNA was low in all heart chambers of 4-day-old rats, but was elevated in the atria (6- to 18-fold) and ventricles (3- to 4-fold) of adult rats. Western blotting analysis confirmed that these differences were efficiently translated into 67-kDa ERalpha protein. ERbeta mRNA was expressed at its highest level in the left atrium and was 3- to 4-fold lower in other heart chambers of 4-day-old animals. In adult rats ERbeta was decreased dramatically in the left atrium (20-fold) and, to a lesser extent in the other heart chambers (2- to 4-fold). Significant ER changes occurred already in the first week after birth. Accordingly, estrogen regulation in cells from neonatal hearts, as reported in several studies, may not correspond to that occurring in fully differentiated adult hearts, because of an altered degree of ER expression. In adult rats, ovariectomy decreases atrial ERalpha, the atria/body weight ratio, and atrial natriuretic peptide (ANP) transcription. Treatment of ovariectomized rats with 17-beta-estradiol (25 microg, 10 days, s.c.) reversed these changes. In addition, there was no effect of ovariectomy and 17-beta-estradiol supplementation on systolic blood pressure, but in ovariectomized rats a decreased heart rate followed 17-beta-estradiol administration. Similar to the effects on ERalpha in the atria, ovariectomy lowered plasma ANP levels, and 17-beta-estradiol administration restored ANP in the plasma of ovariectomized rats. Changes in plasma ANP correlated with changes in ANP content in the right atrium, as demonstrated by RIA. Increased ANP expression and secretion in response to ERalpha activation may be a protective mechanism in the heart.

Atrial natriuretic peptide is involved in renal actions of moxonidine

Moxonidine, an antihypertensive imidazoline compound, reduces blood pressure by selective activation of central imidazoline I(1)-receptors and inhibition of sympathetic nerve activity and by direct actions on the kidney, with both mechanisms resulting in diuresis and natriuresis. We hypothesized that the hypotensive and renal actions of moxonidine may be mediated by atrial natriuretic peptide (ANP), a cardiac peptide involved in pressure and volume homeostasis through its vasodilatory, diuretic, and natriuretic actions. Renal parameters were measured on an hourly basis over a period of 4 hours in conscious rats that received bolus intravenous injections of moxonidine (1 to 150 microg/300 microL saline). During the first hour, moxonidine dose-dependently stimulated diuresis, natriuresis, kaliuresis, and urinary cGMP, the index of ANP activity. Moxonidine (50 microg) significantly (P<0.001) stimulated urinary volume (0.35+/-0.04 versus 1.05+/-0.09 mL/h per 100 g), sodium (14. 3+/-2.5 versus 51.8+/-6.5 micromol/h per 100 g), potassium (10.5+/-2. 3 versus 32.3+/-3.2 micromol/h per 100 g), and cGMP (325+/-52 versus 744+/-120 pmol/h per 100 g). Pretreatment with a selective imidazoline receptor antagonist, efaroxan, dose-dependently inhibited moxonidine-stimulated renal parameters. Efaroxan (25 microg per rat) significantly inhibited moxonidine-stimulated diuretic and natriuretic effects and urinary cGMP excretion (744+/-120 versus 381+/-137 pmol/h per 100 g, P<0.02). The alpha(2)-adrenoceptor antagonist yohimbine (50 microg per rat) partially yet significantly inhibited moxonidine-stimulated diuresis and natriuresis but not cGMP excretion. Plasma ANP was dose-dependently increased by moxonidine and was inhibited by pretreatment with efaroxan (220.8+/-36.9 versus 100.3+/-31.7 pg/mL, P<0.03) but not by yohimbine. In conclusion, selective in vivo activation of imidazoline receptors by moxonidine is associated with dose-dependent diuresis, natriuresis, and kaliuresis as well as stimulated plasma ANP and urinary cGMP excretion, thus implicating ANP in the renal actions of moxonidine.

Cardiac afferents and neurohormonal activation in congestive heart failure

Cardiac chambers have afferent connections to the brainstem and to the spinal cord. Vagal afferents mediate depressor responses and become activated by volume expansion, increased myocardial contractility and atrial natriuretic factor. Sympathetic afferents, on the contrary, are activated by metabolic mediators, myocardial ischemia and cardiac enlargement. These opposite behaviors may lead to activation or suppression of the sympathetic nervous system and of the renin-angiotensin-aldosterone system. As cardiac diseases progress, the heart dilates, plasma norepinephrine increases, atrial natriuretic factor is released and the renin-angiotensin-aldosterone system is suppressed to maintain water and sodium excretion. This dissociation of the neurohormonal profile of cardiac patients, may be explained by coactivation of sympathetic afferents, by cardiac dilatation, and of vagal afferents by atrial natriuretic factor. In more advanced stages, atrial natriuretic factor suppression of the renin-angiotensin-aldosterone system is overridden by overt sympathetic activation and sodium and water retention ensues. Digitalis, angiotensin-converting enzyme inhibitors and beta-blockers selectively decrease cardiac adrenergic drive. A common mechanism of action, to all three groups of drugs, would be attenuation of sympathetic afferents and partial normalization of vagal afferents. Consequently, heart size and cardiac afferents emerge as the key factors to understand the pathophysiology and treatment of the syndrome of congestive heart failure.

ANP and bradycardic reflexes in hypertensive rats: influence of cardiac hypertrophy

In previous studies we demonstrated that in normotensive rats, but not in spontaneously hypertensive rats (SHR), atrial natriuretic peptide (ANP) enhances bradycardic reflexes through an action on cardiac vagal afferent pathways. The present study aimed to determine whether cardiac hypertrophy, hypertension, or a nonreversible genetic factor accounted for the insensitivity of SHR to ANP action on cardiac reflex pathways. SHR were treated with the angiotensin-converting enzyme (ACE) inhibitor perindopril (3 mg/kg per day) for 6 weeks from 4 to 9 weeks of age (SHR-S, n=10) or for 9 weeks from 4 to 12 weeks of age (SHR-L, n=10) or were untreated (SHR, n=10) to produce differential effects on blood pressure and left ventricle/body weight ratio (LV/BW). Untreated normotensive Wistar-Kyoto rats (WKY, n=10) were also studied. At 13 weeks of age, all rats were instrumented with aortic and jugular catheters, and at 14 weeks we measured heart rate reflexes to rapid intravenous infusions of methoxamine (100 microg/kg, cardiac baroreflex) and serotonin (5 to 60 microg/kg, von Bezold-Jarisch cardiac chemosensitive reflex), with either alpha-rat ANP (150 ng/kg per minute IV) or saline vehicle (270 microL/h IV) infusion. Perindopril treatment for 6-week (SHR-S) and 9-week (SHR-L) durations maintained blood pressure at normotensive levels in both groups. SHR-S exhibited a small degree of cardiac hypertrophy (LV/BW was 8% higher than in WKY but 11% less than in untreated SHR), but LV/BW was normalized in SHR-L (to within 1% of WKY LV/BW). In WKY, ANP significantly (P<0.05) enhanced bradycardic responses to both the cardiac baroreflex (by 42+/-10%) and von Bezold-Jarisch chemosensitive reflex (by 17+/-5%) activation but had no effect in SHR. The cardiac reflex action of ANP was restored in SHR-L (ANP enhanced reflex bradycardia by 28+/-12% and 36+/-8%, baroreflex and von Bezold-Jarisch reflex, respectively; P<0.05), but SHR-S, which developed some cardiac hypertrophy, remained unresponsive to ANP. Our results suggest that the inability of ANP to sensitize cardiac vagal (nonarterial) afferents in SHR was not due to an inherited irreversible component, or the hypertension per se, but was associated with the presence of cardiac hypertrophy. A functional consequence of hypertension-induced cardiac hypertrophy may be the inhibition of the cardioprotective action of ANP through cardiac vagal reflexes.

Inverse relationship of urinary cyclic GMP to blood pressure reactivity in the CARDIA study: vasodilatory regulation of sympathetic vasoconstriction. Coronary Artery Risk Development in Young Adults

OBJECTIVE

To determine whether urinary cyclic GMP (cGMP), which mediates the actions of the vasodilators nitric oxide and atrial natriuretic factor, is inversely related to blood pressure (BP) reactivity. In previous work, we found that urinary cGMP was inversely related to diastolic BP, but cGMP levels were higher among individuals presumed to have increased adrenergic activity, increased reactivity, and increased risk of hypertension (blacks, individuals with a family history of hypertension).

METHOD

We measured 24-hour urinary cGMP levels in a substudy of 529 individuals in the Coronary Artery Risk Development in Young Adults (CARDIA) study; the sample was 23 to 35 years of age and approximately balanced for race (black/white) and gender. BP reactivity to stressors (video game, star-tracing, cold pressor) was tested 3 years earlier. Baseline BP was included as a covariate in all analyses.

RESULTS

Diastolic BP reactivity to cold pressor was inversely related to cGMP excretion (p < .05); the relationship was strongest among black women with a family history of hypertension (partial r = -.33, p < .01). Systolic BP reactivity to star-tracing was also inversely related to cGMP (p < .01); the relationship for both star-tracing and video game stressors was strongest among black men (partial r values = -.25 and -.24, respectively; p values < .01).

CONCLUSIONS

The results indicate that vasodilatory activity may impact the BP response to stress through modulation of adrenergic activation, particularly among blacks.

Relationship of diastolic blood pressure with cyclic GMP excretion among young adults (the CARDIA Study): influence of a family history of hypertension. Coronary Artery Risk Development in Young Adults

BACKGROUND

Guanosine 3':5'-cyclic monophosphate (cGMP) is the second messenger of nitric oxide and atrial natriuretic factor, and mediates local vasodilatation. These vasodilatory factors are important in blood pressure regulation and possibly in the etiology of hypertension. Urinary cGMP levels among normotensive young adults have not previously been studied.

SUBJECTS

A subset of normotensive participants from the CARDIA study (n = 563), aged 23-35 years, was studied. The sample was approximately balanced for sex and race (black/white).

METHODS

Twenty-four-hour urinary cGMP levels were measured using an enzyme immunoassay; levels were adjusted for creatinine excretion. The blood pressure, smoking status, and risk factors for hypertension [including a family history of hypertension (FHH), the body mass index, education, alcohol intake, and sodium excretion] were also measured.

RESULTS

Women excreted more cGMP than did men, and blacks excreted more cGMP than did whites (both P < 0.0001). Excretion of cGMP was also greater among smokers (P < 0.001) and those with an FHH (P = 0.05), and was related directly and independently to sodium excretion (P < 0.02). The diastolic blood pressure (DBP) was related inversely to the excretion of cGMP among individuals without an FHH (r = -0.36, P < 0.001), but not among individuals with an FHH. In multiple regression analysis, the excretion of cGMP remained related significantly to the DBP and accounted for more variance in DBP than did any other variable among those without an FHH (delta R2 = 0.08, P < 0.001).

CONCLUSIONS

Urinary cGMP excretion is related inversely and independently to the DBP among those without an FHH but not among those with an FHH, suggesting that cGMP-related vasodilatation is impaired in those with an FHH. Sex differences in urinary excretion of cGMP are consistent with results from studies showing that estrogen increases the endothelial production of nitric oxide.

Direct chronotropic effects of atrial and C-type natriuretic peptides in anaesthetized dogs

1. The chronotropic effects of atrial natriuretic peptide (ANP) and C-type natriuretic peptide (CNP) were investigated using injections (50 micrograms in 1 ml of Tyrode solution as bolus over 1 min) directly into the sinus node artery of 21 anaesthetized and vagotomized dogs which had been pretreated with a beta-adrenoceptor antagonist. The injections were also repeated following: (a) alpha-adrenoceptor antagonism (prazosin) and muscarinic receptor antagonism (atropine); (b) inhibition of prostaglandin synthesis (indomethacin); (c) angiotensin II AT1 receptor antagonism (losartan); (d) histamine H1 (mepyramine) and H2 (cimetidine) receptor antagonism. 2. The results obtained indicate that ANP had no significant effect on the basal sinus rate, whereas CNP produced a slight but significant increase of 12 +/- 2 beats min-1. The effect of CNP was long-lasting (return to pre-injection levels after maximum effect in 17 +/- 3 min) and was not influenced by the various antagonists mentioned above. 3. During in vitro experiments on spontaneously beating right atria isolated from 6 dogs, the injection of CNP (50 micrograms in 1 ml of Tyrode solution) into the sinus node artery produced an increase in atrial rate of 14 +/- 1 beats min-1. 4. The results of this work indicate that CNP exerts a significant and prolonged positive chronotropic effect both in vivo and in vitro. Other studies are required to elucidate the mechanism of action of CNP on the heart conduction system, to ascertain the presence of natriuretic peptide receptor B in the region of the sinoatrial node and to determine the role of CNP in the control of heart rate.