Similar baroreflex bradycardic actions of atrial natriuretic peptide and B and C types of natriuretic peptides in conscious rats

Restorative effect of atrial natriuretic peptide or chronic neutral endopeptidase inhibition on blunted cardiopulmonary vagal reflexes in aged rats

Arterial baroreflex function diminishes with age, but whether cardiopulmonary vagal reflexes are similarly altered with physiological aging has not been fully elucidated. In this study, predominantly cardiac high pressure mechanoreceptor-activated (ramp baroreflex) and cardiopulmonary chemoreceptor-activated (von Bezold-Jarisch reflex) vagal reflexes in conscious, instrumented rats were impaired by 30% to 40% (P<0.05) in 24-month-old (n=12) compared with 6-month-old rats (n=12). To determine whether this is a restorable deficit, the influence of atrial natriuretic peptide (ANP), either by infusion or blockade of its breakdown, was studied. ANP infusion was previously shown to enhance Bezold-Jarisch reflex and ramp baroreflex bradycardia in young adult rats. The present study confirmed that vagal reflex augmentation by ANP (50 pmol/kg per minute) also occurs in old rats (increased by 60+/-18% (Bezold-Jarisch reflex) and 91+/-15% (ramp baroreflex; P<0.05). Direct vagal stimulation in anesthetized animals showed that the target for ANP was not the cardiac vagus itself in old rats (n=7), although in young rats only, we confirmed the published finding that ANP enhances vagal bradycardia (by 58+/-14%, n=7). Neutral endopeptidase 24.11 degrades ANP and several other peptides. The neutral endopeptidase inhibitor candoxatrilat (5 mg/kg per day IV for 7 to 9 days) restored vagal reflex bradycardia in old rats (n=6) to levels similar to those in young neutral endopeptidase inhibitor-treated rats (n=6). Impaired cardiopulmonary vagal reflex control of heart rate is thus a feature of normal aging, and this deficit may be ameliorated by either ANP infusion or chronic neutral endopeptidase inhibition.

Effect of systemic B-type natriuretic peptide on cardiac vagal motoneuron activity

Intravenous B-type natriuretic peptide (BNP) enhances the bradycardia of reflexes from the heart, including the von Bezold-Jarisch reflex, but its site of action is unknown. The peptide is unlikely to penetrate the blood-brain barrier but could act on afferent or efferent reflex pathways. To investigate the latter, two types of experiment were performed on urethane-anesthetized (1.4 g/kg iv) rats. First, the activity was recorded extracellularly from single cardiac vagal motoneurons (CVMs) in the nucleus ambiguus. CVMs were identified by antidromic activation from the cardiac vagal branch and by their barosensitivity. Phenyl biguanide (PBG), injected via the right atrium in bolus doses of 1-5 mug to evoke the von Bezold-Jarisch reflex, caused a dose-related increase in CVM activity and bradycardia. BNP infusion (25 pmol.kg(-1).min(-1) iv) significantly enhanced both the CVM response to PBG (n = 5 rats) and the reflex bradycardia, but the log-linear relation between those two responses over a range of PBG doses was unchanged by BNP. The reflex bradycardia was not enhanced in five matched time-control rats receiving only vehicle infusions. In five other rats the cervical vagi were cut and the peripheral right vagus was stimulated supramaximally at frequencies of 1-20 Hz. The bradycardic responses to these stimuli were unchanged before, during, and after BNP infusion. We conclude that systemic BNP in a moderate dose enhances the von Bezold-Jarisch reflex activation of CVM, in parallel with the enhanced reflex bradycardia. That enhancement is due entirely to an action before the vagal efferent arm of the reflex pathway.

Influence of atrial fibrillation on cardiac brain natriuretic peptide release during haemodynamic stress in heart failure

BACKGROUND

The determinants of release of brain natriuretic peptide (BNP) in heart failure (HF) are incompletely understood, particularly, the effect of heart rhythm and haemodynamic stress.

AIMS

To investigate the effect of haemodynamic stress on cardiac BNP release in HF and differentiate this response for atrial fibrillation (AF) and sinus rhythm (SR).

METHODS

In 18 HF patients (ejection fraction<40%, 9 in AF and 9 in SR) haemodynamics and BNP levels were measured from arterial and coronary sinus samples at baseline, after 10 min of 20 degrees passive head up tilt (HUT) and after 10 min of isometric handgrip (IHG) exercise. From these data, we calculated a transcardiac BNP gradient and compared results between the AF and SR cohort.

RESULTS

During haemodynamic stress in both groups, there were no significance differences in left sided filling pressures. At baseline, there were no differences in BNP measurements between the SR and AF group. The transcardiac BNP gradient increased significantly in the SR (p=0.02) but not the AF cohort, after HUT. During IHG exercise, there was a significant decrease in cardiac BNP release in the AF cohort (p=0.03) but not the SR cohort.

CONCLUSION

These data imply in HF, cardiac rhythm influences cardiac BNP release in response to haemodynamic stress.

Brain natriuretic peptide and cardiac autonomic function in type 2 diabetic patients

The present study tested the hypothesis that increased plasma brain natriuretic peptide (BNP) levels are related to cardiac autonomic dysfunction in type 2 diabetic patients. A total of 32 consecutive Japanese patients with type 2 diabetes were assigned to either a high-BNP (>or=18 pg/ml) group (n=12; age 57+/-13 years, mean+/-S.D.) or a normal-BNP (<18 pg/ml) group (n=20; 59+/-10 years). No patient had any overt structural heart disease. Cardiac autonomic function was assessed by measurements of baroreflex sensitivity (BRS), heart rate variability (HRV) and cardiac (123)I-metaiodobenzylguanidine (MIBG) scintigraphic findings. BRS was lower (p<0.005) in the high-BNP group than in the normal-BNP group. However, the components of HRV, and the early and delayed myocardial uptake of (123)I-MIBG and percentage washout rate of (123)I-MIBG were not significantly different between the groups. The plasma level of BNP negatively correlated with BRS (r=0.35, p=0.049). These findings suggest that increased plasma BNP levels were related to cardiac reflex parasympathetic dysfunction in our Japanese type 2 diabetic patients.

Cardiac hypertrophy in transgenic rats expressing a dominant-negative mutant of the natriuretic peptide receptor B

Natriuretic peptides (NP) mediate their effects by activating membrane-bound guanylyl cyclase-coupled receptors A (NPR-A) or B (NPR-B). Whereas the pathophysiological role of NPR-A has been widely studied, only limited knowledge on the cardiovascular function of NPR-B is available. In vitro studies suggest antiproliferative and antihypertrophic actions of the NPR-B ligand C-type NP (CNP). Because of the lack of a specific pharmacological inhibitor, these effects could not clearly be attributed to impaired NPR-B signaling. Recently, gene deletion revealed a predominant role of NPR-B in endochondral ossification and development of female reproductive organs. However, morphological abnormalities and premature death of NPR-B-deficient mice preclude detailed cardiovascular phenotyping. In the present study, a dominant-negative mutant (NPR-BDeltaKC) was used to characterize CNP-dependent NPR-B signaling in vitro and in transgenic rats. Here we demonstrate that reduced CNP- but not atrial NP-dependent cGMP response attenuates antihypertrophic potency of CNP in vitro. In transgenic rats, NPR-BDeltaKC expression selectively reduced NPR-B but not NPR-A signaling. NPR-BDeltaKC transgenic rats display progressive, blood pressure-independent cardiac hypertrophy and elevated heart rate. The hypertrophic phenotype is further enhanced in chronic volume overload-induced congestive heart failure. Thus, this study provides evidence linking NPR-B signaling to the control of cardiac growth.

Cardioprotective functions of atrial natriuretic peptide and B-type natriuretic peptide: a brief review

1. If one was to design a hormone to protect the heart, it would have a number of features shown by the cardiac natriuretic peptides atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP). These hormones are made in cardiomyocytes and are released into the circulation in response to atrial and ventricular stretch, respectively. Atrial natriuretic peptide and BNP can reduce the preload and after-load in normal and failing hearts. They reduce blood volume over the short term by sequestering plasma and over the longer term by promoting renal salt and water excretion and by antagonizing the renin-angiotensin-aldosterone system at many levels. Each of these actions affords indirect benefit to a volume- or pressure-threatened heart. 2. Recent studies have identified additional modes of action of the natriuretic peptides that may also confer cardioprotective benefits, especially in heart disease. The emerging findings are: (i) that ANP and BNP antagonize the cardiac hypertrophic action of angiotensin II and continue working under conditions where endothelial nitric oxide (NO) function is compromised, such as in the presence of high glucose in diabetes; (ii) they potentiate the bradycardia caused by inhibitory ('autoprotective') cardio-cardiac reflexes; and, furthermore, (iii) BNP can suppress cardiac sympathetic nerve activity in humans, including those with heart failure. Thus, it appears that natriuretic peptides can shift sympathovagal balance in a beneficial direction (away from the sympathetic). The vagal reflex and antihypertrophic actions of the peptides are mediated by particulate guanylyl cyclase (pGC) natriuretic peptide receptors. 3. The multiple synergistic actions of the natriuretic peptides make them and their pGC receptors attractive targets for therapy in heart disease. Encouragingly, exogenous natriuretic peptides remain effective even when endogenous peptide levels are raised, as is the case in heart failure. They also remain effective in disease states where other protective mechanisms, such as the NO system, have become ineffective, offering yet further encouragement for the therapeutic use of the natriuretic peptides.

Do natriuretic peptides modify arterial baroreflexes in sheep?

While atrial and B-type natriuretic peptides (ANP and BNP) have been shown to enhance reflex responses attributed to cardiac vagal afferents, their effects on arterial baroreceptor reflex function remain controversial. The actions of C-type natriuretic peptide (CNP) in this regard are unknown. To clarify their actions on arterial baroreflexes, we tested whether i.v. infusions of ANP, BNP or CNP at 10 pmol kg(-1) min(-1) modified the steady-state mean arterial blood pressure-heart rate (MAP-HR) relationship in conscious sheep. At this dose, all three natriuretic peptides are known to enhance the cardiac chemoreflex response to phenylbiguanide (Bezold-Jarisch reflex). Sigmoid MAP-HR relationships were constructed from the steady-state responses to alternating injections of vasopressor (phenylephrine, 1-15 microg kg(-1)) and vasodepressor agents (nitroprusside, 1-15 microg kg(-1)) in the absence and presence of infused ANP, BNP or CNP (tested in random order at least 1 week apart). No parameter of the steady-state baroreflex relationship was significantly altered by infusion of any of the three natriuretic peptides. We conclude that in conscious sheep, normal arterial baroreceptor-HR reflex function prevails in the presence of moderate doses of ANP, BNP or CNP.

A randomized crossover study to evaluate Ro 115-1240, a selective alpha1A/1L-adrenoceptor partial agonist in women with stress urinary incontinence

OBJECTIVES

To investigate the potential therapeutic benefits of the selective alpha1A/1l-adrenoceptor partial agonist Ro 115-1240 in women with mild-to-moderate stress urinary incontinence (SUI).

PATIENTS AND METHODS

Thirty-seven women with mild-to-moderate SUI were enrolled in a randomized, placebo-controlled crossover study. Patients received 1.5 mg Ro 115-1240 twice daily or matching placebo for 2 or 4 weeks. Voiding diaries were used to record the number of SUI episodes, urge incontinence episodes and pads used. Sitting blood pressures and heart rate were recorded at each visit.

RESULTS

Ro 115-1240 was associated with a significantly lower mean weekly number of SUI episodes than placebo (8.4 vs 6.0; P= 0.0079), a 28% relative improvement over placebo. There was also a significantly lower mean number of pads used and wet pads changed/week with Ro 115-1240 than with placebo (P = 0.0055 and 0.0066, respectively). The most frequently reported treatment-emergent adverse events were scalp tingling, headache, chills, piloerection, and pruritus. Generally these events were transient and mild to moderate. There was a slightly lower mean sitting heart rate with Ro 115-1240 than with placebo, but no difference in mean systolic or diastolic blood pressure between treatments.

CONCLUSIONS

This study suggests that selective alpha1A/1l-adrenoceptor partial agonists have the potential to improve the symptoms of SUI with little or no cardiovascular effect. These results are encouraging and a randomized controlled trial of Ro 115-1240 in a larger population with SUI is warranted to substantiate these findings.

Expression of brain natriuretic peptide in the rat heart studies during heart growth and in relation to sympathectomy

Brain natriuretic peptide (BNP) might be of importance during heart development and is described to be increasingly expressed in congestive heart failure and to affect the progress of this condition. However, details in the normal expression of BNP are still unclear in various parts of the adult and growing heart, including the conduction system. In this study, we investigated the expression of BNP in relation to that of atrial natriuretic peptide (ANP) in the growing as well as in the adult rat heart. The effects of chemical sympathectomy in adult rats were also examined. Contrary to previous BNP immunohistochemical studies, the BNP antiserum was preabsorbed with an excess of ANP before staining to abolish the crossreactivity with ANP. There was a pronounced BNP immunoreaction in the auricles, the trabeculated ventricular walls, and the peripheral parts of the conduction system at 0-1 days postnatally. The degree of immunoreaction gradually decreased with increasing age. A similar developmental pattern was seen concerning ANP expression, but the magnitude of the latter clearly exceeded that for BNP. Immunoreaction for BNP was never detected in the atrioventricular (AV) node and AV bundle at any stage. In contrast to the situation for ANP previously observed, no obvious changes in BNP immunoreaction patterns were observed in response to sympathectomy. This is the first study to thoroughly demonstrate the expression of BNP in the various regions of the rat heart during growth and in the normal and sympathectomized adult stage. The observations are related to possible functions of natriuretic peptides in the growing and adult heart.

Guanylyl cyclase receptors mediate cardiopulmonary vagal reflex actions of ANP

Atrial natriuretic peptide (ANP) potentiates vagal cardiopulmonary reflexes due to chemosensory (Bezold-Jarisch [B-J] reflex) or mechanosensory (ramp baroreflex) activation. The ANP receptor mediating these actions is unknown. We examined the role of particulate guanylyl-cyclase (pGC) receptors in ANP-induced enhancement of cardiopulmonary vagal reflexes. Cardiopulmonary baroreceptor reflex function was assessed by bradycardic responses to ramp blood pressure rises after rapid intravenous methoxamine (100 micro g/kg bolus dose). The B-J reflex was evoked by 3 intravenous doses of serotonin (1 to 10 micro g/kg). In conscious, chronically instrumented rats (n=9), these tests were performed on each animal during randomized infusions of rat ANP (150 ng/kg per minute IV), saline (270 micro L/h IV), the pGC receptor antagonist HS-142-1 (3 mg/kg IV), or combined HS-142-1+ANP treatment. HS-142-1 alone attenuated normal B-J reflex (by 33+/-8%, P<0.05) but not ramp baroreflex responses. As we showed previously, ANP enhanced baroreflex and B-J reflex bradycardia (by approximately 140% and approximately 30%, respectively, P<0.05), compared with saline infusion. These ANP effects were completely blocked by HS-142-1, demonstrating that the cardiopulmonary vagal reflex actions of ANP occurred through pGC natriuretic peptide receptors. Additionally, we have provided evidence for the first time that pGC natriuretic peptide receptors are essential for the full expression of the B-J reflex but not for that of cardiopulmonary vagal baroreflexes. This tonic interaction between pGC natriuretic peptide receptors and cardiopulmonary chemosensitive receptors may be important during pathophysiological activation of B-J reflex, such as with myocardial infarction.

Exercise capacity and brain natruiretic peptide in hypertension

Brain natriuretic peptide (BNP) is increased and it is also released during exercise in hypertension, but its biologic role is unclear. However, since BNP is released from the left ventricule and it is known to reduce left ventricular filling pressure, it is possible that it exerts a favorable effect on exercise performance. We studied the relationship between endogenous BNP release and exercise capacity in hypertension with reference to left ventricular hypertrophy (LVH). Cardiopulmonary exercise study was carried out in 2 groups of hypertensives, 24 [16 men, aged 50 (SD 11) years] of whom had echocardiographic LVH and 25 [16 men, aged 41 ( 12)] who did not have LVH. In multiple regression analyses, the major determinants of peak oxygen uptake (VO max) were age (-), sex (male), peak exercise systolic BP (+) and post-exercise BNP (-). For the predicted adjusted %VO max as a measure of individualised exercise capacity, the significant predictors were the exercise-induced BNP rise (-) (p = 0.0003) and peak exercise systolic BP (+) (p = 0.001). In other words, subjects with greater myocardial dysfunction had a greater rise in BNP during exercise. LVH did not however relate to exercise capacity. The baseline, post-exercise and the % rise in BNP (pmol/L) with exercise were not statistically different in those with LVH compared with those without (median values of 11.2, 14.6 and 133% versus 10.6, 11.5 and 120% respectively). Similarly, there were no significant differences in exercise capacity between the groups: exercise time, oxygen uptakes at ventilatory threshold and at peak exercise (VO max) were 10.8 (2.5) min, 15.8 (4.6) and 33.6 (7.6) ml/min/kg in the LVH group against 11.4 (2.9) min, 18.6 (5.2) and 36 (11.1) ml/min/kg in the non-LVH group respectively. The estimated VO max was not different from that predicted from age, sex, weight and height in either group suggesting preserved exercise capacity in these subjects as a whole. In Conclusion, BNP may potentially act as a homeostatic mechanism that helps to limit exercise incapacity in hypertension irrespective of LVH.

Vascular mechanisms of sudden death in hypertrophic cardiomyopathy, including blood pressure responses to exercise

Approximately a third of patients with hypertrophic cardiomyopathy fail to increase blood pressure appropriately during exercise, a consequence of an inappropriate vasodilator response in nonexercising beds, leading to an exaggerated fall in systemic vascular resistance. The precise mechanism responsible for this abnormal vascular control in hypertrophic cardiomyopathy is still unclear, but is thought to be secondary to enhanced cardiac baroreceptor activity. However, alternate or synergistic mechanisms, including enhanced release of brain natriuretic peptide, may be involved. Normal exercise blood pressure responses have been shown to have a high (97%) negative predictive accuracy for sudden death during an average follow-up of approximately 3 years, providing considerable reassurance. Patients with abnormal blood pressure responses on exercise were at markedly increased risk of sudden cardiac death, although the positive predictive accuracy during this brief follow-up period was low (15%). It is likely that vascular instability may act as a trigger for sudden cardiac death in patients with an underlying electrophysiologic substrate. Recent evidence suggests that this vascular instability may also result in hypotension during ordinary daily activity, or even at rest, and may be an important cause of syncope in hypertrophic cardiomyopathy. Further studies are required to identify mechanisms of attenuating or reversing this vascular instability. Such measures might have the potential to improve symptoms of recurrent syncope and perhaps reduce the risk of sudden cardiac death.

ANP, BNP, and CNP enhance bradycardic responses to cardiopulmonary chemoreceptor activation in conscious sheep

We demonstrated previously that atrial natriuretic peptide (ANP) enhances reflex bradycardia to intravenous serotonin [5-hydroxytryptamine (5-HT)] (von Bezold-Jarisch reflex) in rats. To determine whether 1) ANP affects this cardiopulmonary vagal reflex in another species and 2) B-type (BNP) and C-type (CNP) natriuretic peptides share with ANP the ability to modulate this reflex, we used intravenous phenylbiguanide (PBG), a 5-HT(3) agonist, as the stimulus to evoke a von Bezold-Jarisch reflex (dose-related, reproducible bradycardia) in conscious adult sheep (n = 5). Three doses of PBG (13 +/- 3, 20 +/- 3, and 31 +/- 4 microg/kg) injected into the jugular vein caused reflex cardiac slowing of -7 +/- 1, -15 +/- 2, and -36 +/- 3 beats/min, respectively, under control conditions. These doses of PBG were repeated during infusions of ANP, BNP, or CNP (10 pmol. kg(-1). min(-1) iv), or vehicle (normal saline). Each of the natriuretic peptides significantly (P < 0.05) enhanced the sensitivity of bradycardic responses to PBG by 94 +/- 8% (ANP), 142 +/- 55% (BNP), and 61 +/- 16% (CNP). Thus not only did ANP sensitize cardiopulmonary chemoreceptor activation in a species with resting heart rate close to that in humans, but BNP and CNP also enhanced von Bezold-Jarisch reflex activity in conscious sheep.