Chronotropic stimulation: a primary effector for release of atrial natriuretic factor

Short-term hemodynamic and neuroendocrine effects of pimobendan and benazapril in dogs with myxomatous mitral valve disease and congestive heart failure

BACKGROUND

Pimobendan and benazepril are frequently used with diuretics to treat dogs in congestive heart failure (CHF) caused by myxomatous mitral valve disease (MMVD).

AIM

To compare the short-term effects of pimobendan versus benazepril on pump function, heart size, and neuroendocrine profile in dogs with CHF caused by MMVD.

ANIMALS

Sixteen client-owned dogs.

MATERIAL AND METHODS

Seven-day prospective single-blinded study of dogs stabilized on furosemide monotherapy, randomized to pimobendan (0.4-0.6 mg/kg/day) or benazepril (0.25-1.0 mg/kg/day). Dogs had first-pass radionuclide angiocardiography, and heart size was measured by radiography and echocardiography. Circulating neuroendocrine hormones were measured.

RESULTS

Baseline variables did not differ between treatment groups. Greater decreases in the pimobendan than in the benazepril group were found for heart rate (P = .001), heart rate-normalized pulmonary transit time (P = .02), left atrial size (P = .03), and systolic and diastolic left ventricular diameters (P < .001 and P = .03, respectively) and volumes (P < .001 and P = .02, respectively), whereas ejection fraction increased more (P = .02) in the pimobendan group. Of the neuroendocrine hormones, only N-terminal proatrial natriuretic peptide (NT-ProANP) differed (P = .04) between groups. Within groups, plasma aldosterone increased (P = .01), and NT-proANP (P = .01) and NT-proB-type (P = .02) natriuretic peptide decreased in the pimobendan group, and NT-proANP (P = .02) and plasma vasopressin (P = .01) decreased in the benazepril group.

CONCLUSIONS AND CLINICAL IMPORTANCE

Pimobendan improves short-term cardiac function more than benazepril in dogs with CHF caused by MMVD. Pimobendan treatment enables the heart to work at smaller end-systolic and diastolic dimensions while maintaining adequate forward stroke volume. Some of the treatment responses found in neuroendocrine profile might have therapeutic relevance.

Plasma Atrial Natriuretic Peptide (ANP) Concentration in Patients with Pheochromocytoma

The interaction between catecholamines (CA) and ANP is not clearly established. The effects of excess endogenous CA on ANP secretion can be investigated in patients with pheochromocytoma. We studied 27 patients with surgically and histologically proven pheochromocytoma (P) aged 19-70 years. In 16 of these patients plasma ANP study was repeated after surgical removal of the tumour. The control group (C) consisted of 20 healthy volunteers aged 21-48 years. Moreover, 42 patients with uncomplicated mild to moderate essential hypertension (EH) aged 18-48 years were also studied. In P higher plasma ANP concentration versus C, EH was found (51.9 +/- 8.1; 25.5 +/- 1.5; 19.3 +/- 1.5 fmol/ml, respectively). In 16 patients with P, increased plasma ANP level (mean 63.3 +/- 12.6 fmol/ml) declined after surgical removal of the tumour (mean 22.4 +/- 2.9 fmol/ml). In the P patients no relationship was found between plasma ANP and hormonal patterns of the tumour or between plasma ANP and plasma catecholamines, whereas significant positive correlations between plasma ANP and both systolic and diastolic blood pressure and heart rate were demonstrated. These results suggest that excess CA produced by the chromaffin tumour induce ANP secretion via stimulation of adrenergic receptors. However, influence of the haemodynamic changes evoked by CA cannot be excluded. It is suggested that increased secretion of ANP may be of some importance in maintaining blood pressure homeostasis in patients with pheochromocytoma.

Electrical and Structural Remodeling: Role in the Genesis and Maintenance of Atrial Fibrillation

Atrial fibrillation (AF) and congestive heart failure (CHF) are 2 frequently encountered conditions in clinical practice. Both lead to changes in atrial function and structure, an array of processes known as atrial remodeling. This review provides an overview of ionic, electrical, contractile, neurohumoral, and structural atrial changes responsible for initiation and maintenance of AF. In the last decade, many studies have evaluated atrial remodeling due to AF or CHF. Both conditions often coexist, which makes it difficult to distinguish the contribution of each. Because of atrial stretch in the setting of hypertension or CHF, atrial remodeling frequently occurs long before AF arises. Alternatively, AF may lead to electrical remodeling, that is, shortening of refractoriness due to the high atrial rate itself. In many experimental AF or rapid atrial pacing studies, the ventricular rate was uncontrolled. In those studies, atrial stretch due to CHF may have interfered with the high atrial rate to produce a mixed type of electrical and structural remodeling. Other studies have dissected the individual role of AF or atrial tachycardia from the role CHF plays in atrial remodeling. Atrial fibrillation itself does not lead to structural remodeling, whereas this is frequently produced by hypertension or CHF, even in the absence of AF. Primary and secondary prevention programs should tailor treatment to the various types of remodeling.

Atrial Natriuretic Peptides During Experimental Atrial Tachycardia:

INTRODUCTION

Atrial tachycardia and chronic heart failure (CHF) are associated with elevated levels of atrial natriuretic peptide (ANP) and its amino terminal part NT-ANP. Chronic high atrial rates may cause CHF due to a rapid ventricular response. The aim of this study was to establish the contribution of elevated atrial rate and of high ventricular rate, resulting in CHF, on ANP and NT-ANP levels during chronic atrial tachycardia.

METHODS AND RESULTS

Thirteen goats (AV-paced group) were subjected to 4 weeks of rapid AV pacing with an atrial and ventricular rate of 240 beats/min. Another five goats (A-paced group) were subjected to 4 weeks of atrial pacing at 240 beats/min while the ventricular rate was kept low and regular at 80 beats/min. Pacing was interrupted only for measurement of right atrial (RA) and left ventricular (LV) diameter and sampling for ANP, NT-ANP, and renin. In the AV-paced group, RA and LV diameter reached 152% and 109% of baseline values, respectively. Both ANP and NT-ANP (8.3 +/- 9.2 pmol/L and 0.5 +/- 0.4 nmol/L at baseline, respectively) increased progressively (53.1 +/- 37.9 pmol/L and 2.0 +/- 0.9 nmol/L, respectively, after 4 weeks). There was a significant correlation between the magnitude of atrial dilation and natriuretic peptide levels after 3 days. In A-paced goats, however, RA and LV diameters did not change. Furthermore, ANP and NT-ANP levels (9.1 +/- 6.0 pmol/L and 0.8 +/- 0.2 nmol/L at baseline, respectively) were unchanged after 4 weeks (5.3 +/- 3.4 pmol/L and 0.6 +/- 0.2 nmol/L, respectively).

CONCLUSION

Elevated levels of ANPs during chronic atrial tachycardia are related to a high ventricular rate rather than a high atrial rate alone. Rather than atrial tachycardia, the atrial hemodynamic burden is an important determinant of the sustained ANP response.

Effect of age and body weight on neurohumoral variables in healthy Cavalier King Charles spaniels

OBJECTIVE

To evaluate the effect of age and body weight on several neurohumoral variables that are commonly altered in heart failure in Cavalier King Charles Spaniels.

ANIMALS

17 healthy privately owned Cavalier King Charles Spaniels, 10 males and 7 females, ranging in age from 0.4 to 9.7 years, and ranging in body weight from 6.6 to 12.2 kg.

PROCEDURE

The clinical condition of the dogs was evaluated by physical examination, thoracic radiography, and echocardiography. Plasma nitrate and nitrite (P-NN), N-terminal atrial natriuretic and brain natriuretic peptides (NT-ANP and BNP, respectively), endothelin (ET-1), urine cyclic guanosine monophosphate (U-cGMP), and urine nitrate and nitrite (U-NN) concentrations were analyzed.

RESULTS

Plasma concentrations of NT-ANP and P-NN increased significantly with age, but plasma NT-ANP and P-NN also correlated significantly, irrespective of age. A modest increase of left atrial size did not explain the increase of NT-ANP and P-NN with age. Concentration of ET-1 correlated positively with heart rate; heart rate did not change with age. Weight had a negative impact on NT-ANP, P-NN, and U-cGMP concentrations and left atrial relative size.

CONCLUSIONS AND CLINICAL RELEVANCE

Age-matched controls are essential for evaluation of NT-ANP and P-NN concentrations and left atrial size. Weight may alter reference values of plasma NT-ANP, P-NN, and urine cGMP concentrations. Natriuretic peptides can be used as further evidence that heart failure exists. The increased plasma concentrations of NT-ANP (but not BNP) and P-NN with aging reflect neurohumoral physiologic changes that must be distinguished from pathologic changes in patients with heart failure.

Role of cell contractions in cAMP-induced cardiomyocyte atrial natriuretic peptide release

Incubation of spontaneously beating ventricular cardiomyocytes from neonatal rats with prostaglandin E(2) (0.1 microM) or forskolin (0.1 microM) simultaneously increased the rate of cellular contraction and atrial natriuretic peptide (ANP) secretion. Both responses were maximal within 10-20 min of application and were accompanied by three- to fourfold increases in cAMP formation. By contrast, a higher regimen of forskolin (10 microM) promoted a 20- to 30-fold increase in basal cAMP production, which was accompanied by the abolition of contractile activity and ANP release. Low regimens of forskolin (0.1 microM) doubled the occurrence of cytosolic Ca(2+) transients associated with monolayer contraction, whereas higher regimens of forskolin (10 microM) completely suppressed Ca(2+) transients. Moreover, in quiescent cultures that were pretreated with ryanodine, tetrodotoxin, nifedipine, or butanedione monoxime, prostaglandin E(2) (0.1 microM) and forskolin (0.1 microM) failed to elicit significant ANP secretion, suggesting that cAMP-elevating agents promote ANP secretion to a great extent via an increase in cellular contraction frequency in ventricular cardiomyocytes.

Oxytocin releases atrial natriuretic peptide by combining with oxytocin receptors in the heart

Previous studies indicated that the central nervous system induces release of the cardiac hormone atrial natriuretic peptide (ANP) by release of oxytocin from the neurohypophysis. The presence of specific transcripts for the oxytocin receptor was demonstrated in all chambers of the heart by amplification of cDNA by the PCR using specific oligonucleotide primers. Oxytocin receptor mRNA content in the heart is 10 times lower than in the uterus of female rats. Oxytocin receptor transcripts were demonstrated by in situ hybridization in atrial and ventricular sections and confirmed by competitive binding assay using frozen heart sections. Perfusion of female rat hearts for 25 min with Krebs-Henseleit buffer resulted in nearly constant release of ANP. Addition of oxytocin (10(-6) M) significantly stimulated ANP release, and an oxytocin receptor antagonist (10(-7) and 10(-6) M) caused dose-related inhibition of oxytocin-induced ANP release and in the last few minutes of perfusion decreased ANP release below that in control hearts, suggesting that intracardiac oxytocin stimulates ANP release. In contrast, brain natriuretic peptide release was unaltered by oxytocin. During perfusion, heart rate decreased gradually and it was further decreased significantly by oxytocin (10(-6) M). This decrease was totally reversed by the oxytocin antagonist (10(-6) M) indicating that oxytocin released ANP that directly slowed the heart, probably by release of cyclic GMP. The results indicate that oxytocin receptors mediate the action of oxytocin to release ANP, which slows the heart and reduces its force of contraction to produce a rapid reduction in circulating blood volume.

Plasma atrial natriuretic peptide in standardbred and Finnhorse trotters during and after exercise

To study the exercise-induced changes in atrial natriuretic peptide (ANP), a hormone with cardiovascular and renal effects, an incremental submaximal exercise test on a high-speed treadmill was carried out with Standardbred and Finnhorse trotters, the former bred for speed and the latter originally for heavy work. Standardbreds performed the 2 min exercise intervals at speeds of 6, 7, 8, 9 m s-1 and Finnhorses, according to their training status, at 5, 6, 7, 8 m s-1, 4, 5, 6, 7 m s-1 or 5, 6, 7 m s-1. Steady-state heart rate (HR) was reached within each 2 min interval. The increase in HR was linear and proportional to work intensity and physical condition and it peaked, average 204 beats min-1, during the last speed of the treadmill. Plasma ANP increased significantly and equally, by 27 +/- 4 pg mL-1, in both breeds and peaked at 5 min post-exercise. The rise in ANP during exercise showed good linearity with HR and increasing work intensity. The decrease of ANP after exercise was slow, which may be connected to the regulation of water and electrolytes. Interbreed differences in plasma ANP were not observed. The results suggest a role of ANP in cardiovascular control and fluid balance during and after exercise. In addition to other possible releasing factors during exercise, the increase in HR explains about 40% of the variability in the plasma ANP values.

Atrial natriuretic peptide and its potential role in pharmacotherapy

Atrial natriuretic peptide (ANP) is a 28 amino-acid polypeptide secreted into the blood by atrial myocytes after atrial pressure and distension. Although its role in humans is not clear, it can produce a variety of physiologic effects including vasodilatation, natriuresis, and suppression of the renin-angiotensin-aldosterone axis. These actions are potentially useful in a variety of pathologic states such as hypertension and congestive heart failure, and diverse methods to augment the effects of ANP in these states have been devised. The results are exciting and, despite some problems, may lead to the pharmacologic use of enhancement of ANP actions in several clinical disorders.

Role of prostaglandin-mediated cyclic AMP formation in protein kinase C-dependent secretion of atrial natriuretic peptide in rat cardiomyocytes

The role of endogenous prostaglandin production in phorbol diester-induced myocardial atrial natriuretic peptide (ANP) secretion was investigated in cultured spontaneously beating ventricular rat cardiomyocytes. Incubation of cells with 4 beta-phorbol 12-myristate 13-acetate (PMA; 0.1 microM) led to a rapid response in ANP release, a response accompanied by increases in cellular prostacyclin (PGI2) production, cyclic AMP (cAMP) formation and spontaneous contraction frequency. Although PMA-induced ANP secretion exhibited the pharmacological profile of a protein kinase C (PKC)-mediated event, the response was abolished in the presence of the cyclo-oxygenase inhibitors indomethacin (10 microM) and diclofenac (1 microM), indicating that endogenous prostaglandin production is responsible for PMA-induced ANP secretion in this system. Confirming this, PMA-induced ANP secretion was strongly correlated with endogenous formation of 6-oxo-prostaglandin F1 alpha (r = 0.93, P < 0.0005, n = 11), and exogenously applied PGI2, prostaglandin E2 (PGE2) or prostaglandin F2 alpha (PGF2 alpha) elicited simultaneous increases in cAMP formation, contraction frequency and ANP secretion in these cells. Furthermore, PMA-induced cAMP formation was abolished in the presence of either diclofenac or indomethacin, whereas the cAMP-elevating agent forskolin (0.1 microM) mimicked the secretory and chronotropic effect of PMA in these cells. A role for cAMP in PMA-induced ANP secretion was also apparent insofar as PMA-induced ANP release was substantially decreased in the presence of the Rp-diastereomer of 3',5'-cyclic adenosine monophosphorothioate (Rp-cAMPS; 10 microM), whereas the cAMP-mimetic agent dibutyryl cAMP (10 microM) provoked a rapid increase in ANP secretion in this system. Finally, the Ca(2+)-channel antagonist nifedipine (0.1 microM) severely decreased PGI2-, PGE2- and PMA-induced ANP secretion without affecting PGF2 alpha-induced peptide release, suggesting that PGI2 and/or PGE2, but not PGF2 alpha, are the prostanoids involved in PMA-induced ANP release. Taken together, these results suggest that PKC activation induces ANP secretion in spontaneously beating rat ventricular cardiomyocytes via an autocrine pathway involving increased PGI2 and/or PGE2 formation, a response leading to the activation of a myocardial adenylate cyclase and, subsequently, to that of a nifedipine-sensitive Ca2+ channel.

Response of atrial natriuretic factor to surgical pericardial drainage in patients with chronic pericardial effusion

Previous studies have demonstrated the importance of atrial transmural pressure in the secretion of atrial natriuretic peptide. These studies have been performed in patients with pericardial effusion and hemodynamic compromise. The response of atrial natriuretic peptide to the drainage of chronic pericardial effusion without clinical evidence of tamponade is unknown. We studied 13 patients with chronic abundant pericardial effusion but without hemodynamic compromise. Blood samples for hormonal determinations were drawn before and after surgical pericardiocentesis. Right atrial pressure was measured during the procedure. Drainage induced a significant increase of atrial natriuretic peptide (from 12 +/- 3.9 to 105 +/- 22.8 pmol/l, P < 0.001, mean +/- S.E.M.), correlated with the fall in right atrial pressure (from 7.65 +/- 1.18 to 4.31 +/- 1.46 mmHg, P < 0.05, r = 0.68, P = 0.01). This increase was inversely correlated with the rise of mean blood pressure after surgery (from 84 +/- 2.37 to 100 +/- 5.3 mmHg, P < 0.05, r = 0.65, P < 0.02). Plasma renin activity decreased after drainage (from 8.12 +/- 2.57 to 3.27 +/- 0.65 ng/ml/h, P < 0.05). Surgery induced an increase of plasma levels of aldosterone (from 811 +/- 241 to 1199 +/- 249 pmol/l, P < 0.05), which were reduced after pericardiocentesis (371 +/- 102 pmol/l, P < 0.02). In patients with chronic abundant pericardial effusion, surgical pericardiocentesis induced a significant increase of atrial natriuretic peptide, correlated with a fall in right atrial pressure. The increase of atrial natriuretic peptide was similar than in patients with tamponade, despite a moderate fall in right atrial pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Twenty-four-hour pattern of atrial natriuretic peptide in heart transplantation: evidence for lack of circadian rhythm. Temporal inter-relationships with plasma renin activity, aldosterone and cortisol

We have investigated the circadian rhythm of plasma atrial natriuretic peptide in 13 stable output heart transplanted patients, all without evidence of histological rejection and cardiac impairment, following antirejection therapy with Cyclosporine, Azathioprine and Prednisone. The 24-h pattern of plasma renin activity, plasma aldosterone and plasma cortisol has been studied as well. All the investigated variables were assayed six times over the 24-h span. The circadian time-qualified data were analyzed by ANOVA and Cosinor method. The 24-h mean levels of atrial natriuretic peptide, plasma renin activity and plasma aldosterone are significantly increased, while the concentrations of plasma cortisol are reduced in the heart transplanted recipients. ANOVA detected a significant within-day variability of all these humoral variables only in healthy subjects. A statistically significant circadian rhythm was validated by Cosinor procedure for all the investigated molecules in healthy subjects but not in heart transplanted patients. In our opinion, the increase of atrial natriuretic peptide is a counterregulatory mechanism aimed to compensate the cyclosporine-mediated activation of the renin-angiotensin-aldosterone system. The disappearance of the plasma renin activity, aldosterone and atrial natriuretic peptide circadian rhythm can be ascribed to the constant activation of the renin-angiotensin-aldosterone system. The hypocortisolism is due, in our opinion, both to glucocorticoid therapy and increase of plasma ANP concentration.

Distribution of atrial natriuretic peptides in the sand rat (Psammomys obesus) in comparison to that in the rat

Atrial natriuretic peptides (ANP) are a family of humoral compounds involved in water and salt homeostasis. Immunoreactive ANP (IR-ANP) was determined in the plasma and tissues of the rat and the sand rat (Psammomys obesus) using sensitive and specific radioimmunoassay. IR-ANP from the rat and the sand rat elute at identical retention times from reverse phase HPLC indicating that the same chemical entity is present in both species. IR-ANP highest levels were found, in both species, in the heart but it was also present in the adrenal gland, lung, kidney, liver, plasma and several loci in the central nervous system. The IR-ANP levels in the heart, adrenal gland, kidney, liver, cerebellum and cerebral cortex were lower in the sand rat compared to the rat. The plasma IR-ANP level of the diabetes-resistant sand rat was further decreased to about 10% of the level in the diabetes-resistant sand rat.

Release of atrial natriuretic factor

The cardiac atria synthesize and store a hormone termed atrial natriuretic factor (ANF). ANF is released into the systemic circulation, and the circulating 28 amino acid peptide can be measured by radioimmunoassay. The hormone participates in body fluid homeostasis through its effect on renal sodium excretion and by inducing a shift of circulating fluid to the interstitial space. Release of ANF is mainly regulated by mechanical changes in the left and right atrial wall. It has been demonstrated that ANF release is related to changes in atrial wall tension occurring during each atrial cycle, and therefore, release of ANF will increase with increasing heart rate. Not only the increase in wall tension during passive atrial distension (v wave), but also the increase in tension during atrial systole (a wave) are determinants of ANF release. The mechanochemical transducer is most likely located in the atrial myocytes, but its nature is unknown. There is no evidence to suggest that efferent cardiac nerves are essential in the regulation of ANF release. Humoral factors have been suggested as regulators of ANF release, particularly catecholamines and angiotensin II. A receptor-mediated direct stimulatory effect of alpha-adrenergic stimulation and an inhibitory effect of beta-adrenergic stimulation have been demonstrated, but these direct effects are small compared to the effect of changes in atrial wall tension. Circulating catecholamines and angiotensin II stimulate ANF release mainly through their haemodynamic effects.

Changes in plasma atrial natriuretic peptide concentration during heart transplantation

Examination of changes in plasma atrial natriuretic peptide (ANP) concentrations during heart transplantation may provide important information about factors influencing plasma ANP in patients with severe heart failure. Serial changes in plasma ANP during heart transplantation, and atrial content of ANP in native and donor atria, were measured in 12 patients. Preoperative plasma ANP was elevated in all patients (387 +/- 77 pg/mL), whereas atrial content of ANP in native atria was reduced (0.36 +/- 0.082 micrograms/mg protein). Preoperative plasma ANP did not correlate with hemodynamics, but was negatively correlated with creatinine clearance (r = -0.76, P < .01). Intraoperative plasma ANP prior to transplantation was strongly correlated with intraoperative plasma ANP after transplantation (r = 0.84, P < .001). Although postoperative plasma ANP was reduced from preoperative plasma ANP by 75%, these two measurements were also significantly correlated (r = 0.70, P < .02). Postoperative plasma ANP was not correlated with hemodynamics, but was negatively correlated with both creatinine clearance (r = -0.65, P < .05) and content of ANP in the native atria (r = -0.75, P < .01). Multiple linear regression analysis suggested that up to 85% of the variability of early postoperative plasma ANP could be accounted for by the variability in these latter two parameters. The decrease in native atrial ANP content, in the context of elevated plasma ANP concentration, is consistent with prior animal studies suggesting that severe heart failure induces cellular adaptations favoring accelerated ANP synthesis and secretion (with resultant reduction in tissue content).(ABSTRACT TRUNCATED AT 250 WORDS)

Postural changes in atrial natriuretic peptide (ANP) and fluid balance in conscious goats

To gain insight into how weak physiological stimuli suffice to release ANP, the effects of 15 degrees head-up and head-down tilt, 90 min each in succession, on plasma atrial natriuretic peptide (ANP), plasma cortisol, plasma and urinary Na, K and osmolality, plasma proteins and haematocrit were studied in five conscious goats before and after 2 wk daily training for the tilting procedure. In the trained goats the 15 degrees tilt did not affect the plasma ANP, cortisol or the urine excretion significantly. Total plasma proteins decreased significantly. In the untrained goats, on the other hand, an increasing trend was observed in the plasma ANP and cortisol as well as in the urine Na excretion during the head-up tilt. During the head-down tilt, the levels of ANP and Na excretion remained elevated. The plasma ANP was significantly increased after 40 min, by 28% as compared to the pre-tilting level. The plasma cortisol was first elevated, but then returned to the starting level. The results suggest that in the trained goats the responses to tilting were unmasked. Despite minor effects of the 15 degrees tilt in itself, increased plasma ANP seemed to be associated with increased natriuresis.

Right atrial predominance of atrial natriuretic peptide secretion in isolated perfused rat atria

In order to investigate the regulatory mechanism for the atrial release of atrial natriuretic peptide (ANP), a perfused rabbit atrial model was devised. In the present experiments, the effect of a reduction in atrial distension on the immunoreactive ANP (irANP) secretion was investigated and compared in the perfused right and left atria of rats. Elevations in right and left atrial pressure resulted in proportional increases in the volume of atrial distension-reduction which was larger in the right than in the left atria. The basal rate of irANP secretion was higher in the right than in the left atria. Increases in the volume of atrial distension-reduction resulted in proportional increases in irANP secretion in both atria. Increment in irANP secretion in response to a reduction in atrial distension was significantly higher in the right than in the left atria. Higher rate of irANP secretion in response to unit volume change was observed in the right atria. Increases in the volume of atrial distension-reduction resulted in accentuated irANP responses in the right atrium. IrANP content was significantly higher in the right than in the left atria. The results suggest that the right atrium is a predominant site in ANP secretion in rats.

Level and diurnal variations of hormones of interest to the cardiovascular system in patients with heart transplants

The lack of a nocturnal decrease in blood pressure in cyclosporine-treated cardiac transplant recipients may indicate abnormalities in the mechanism(s) responsible for circadian variability in other physiologic parameters such as in circulating hormones. This possibility was addressed through repeated determinations of circulating catecholamines, neuropeptide Y, pancreatic polypeptide, calcitonin gene-related peptide, plasma renin activity, aldosterone, atrial natriuretic factor and cortisol. The results from 10 patients with heart transplants were compared with those of 12 age-matched, healthy control subjects. Both groups were studied during 24-hour supine rest. There was no difference between patients and control subjects in mean levels of catecholamines, neuropeptide Y, pancreatic polypeptide and aldosterone. Patients had higher levels (+/- SD) of plasma renin activity (6.4 +/- 1.3 vs 2.6 +/- 0.4 ng/ml/hour, p less than 0.001), calcitonin gene-related peptide (47.7 +/- 9.9 vs 33.3 +/- 5.7 pmol/liter, p less than 0.01) and atrial natriuretic factor (93.0 +/- 56.7 vs 20.7 +/- 8.9 pg/ml, p less than 0.001) than control subjects, respectively. Cortisol was not detected in patients. Abnormal diurnal profiles in patients were found for calcitonin gene-related peptide, aldosterone and atrial natriuretic factor, and for pancreatic polypeptide, together with decreased levels, in patients with greater than 6 months follow-up. Except for hormones reflecting sympathetic nervous activity, all hormonal systems studied showed abnormalities in level or circadian rhythmicity, or both. The pancreatic polypeptide results suggest that parasympathetic neuropathy could develop in cyclosporine-treated heart transplant recipients.

Effects of arginine vasopressin and extracellular osmolarity on atrial natriuretic peptide release by superfused rat atria

This study investigated the characteristics of atrial natriuretic peptide (ANP) release from superfused sliced atria and ventricles of rats. Right atria spontaneously released more immunoreactive ANP (Ir-ANP: pg/min per mg tissue) (32 +/- 3) than did left atria (11 +/- 2) or right ventricles (1.5 +/- 0.5). Addition of 10(-9) to 10(-5) M of arginine vasopressin (AVP) to the superfusing fluid or increasing its osmolarity (290 to 490 mOsM) resulted in a significant increase of the Ir-ANP outflow from right atria. The effect of AVP was prevented by a specific V1 receptor antagonist, ([d(ch2)5Tyr(Me)]AVP). Superfusion with indomethacin (10(-5) M) did not alter spontaneous release but inhibited the peak levels of Ir-ANP induced by AVP (10(-5) M). Moreover, DDAVP, a specific V2 receptor agonist, did not induce Ir-ANP release. Ca(2+)-free medium alone or plus 1 mM EGTA induced a significant increase in basal Ir-ANP outflow. The Ir-ANP released chromatographed similarly to the standard alpha-rANP. These results suggest a specific stimulatory effect of AVP and osmolarity and a negative influence of extracellular Ca2+ on atrial spontaneous Ir-ANP release. It appears that the effect of AVP could be mediated by prostaglandin synthesis.

The relationship between atrial pressure, atrial dimensions and atrial natriuretic factor during pacing tachycardia in dogs

Acute supraventricular tachycardia is known to increase the plasma level of atrial natriuretic factor (ANF). The purpose of these experiments was to investigate if such an increase in plasma-ANF could be ascribed to changes in atrial pressure and atrial dimensions. Eight anaesthetized dogs were instrumented with atrial pressure catheters and sonomicrometers to measure left and right auricular and atrial free wall dimensions. An acute increase in atrial rate from 150 to 200 min-1 for 10 min did not change plasma-ANF or atrial haemodynamic variables. A further increase in atrial rate to 250 or 300 min-1 increased right and left atrial intracavitary pressures (P less than 0.01), both auricular diameters and right atrial free wall segment length (P less than 0.05). Left atrial free wall segment length remained unchanged. Plasma-ANF increased in all dogs (P less than 0.01). The change in plasma-ANF correlated well with changes in atrial pressures as well as with changes in atrial dimensions. These results support the hypothesis that release of ANF during acute atrial tachycardia may in part be attributed to atrial dilatation.

Atrial natriuretic factor release during acute infusion of isoproterenol in the conscious rat

The effect of beta-adrenergic stimulation on atrial natriuretic factor (ANF) release was studied in conscious rats. 20-min infusion of 85 or 850 ng kg-1 min-1 isoproterenol (ISO) resulted in positive inotropic and chronotropic responses and no elevation of atrial pressures. A slight increase in plasma ANF, together with a drop in blood pressure, were observed only in the group infused with the higher dose. During the infusion of 850 ng kg-1 min-1 ISO, there was no relationship between plasma ANF and any of the haemodynamic parameters, with the exception of mean arterial pressure (r = 0.72, P less than 0.05, n = 9). Larger doses (greater than 3 micrograms kg-1 min-1) were toxic. We conclude that beta-adrenergic stimulation is not an important stimulus for ANF release when diastolic resting tension is low.

Atrial strain is the main determinant of release of atrial natriuretic peptide

We studied the response of atrial natriuretic peptide to the hemodynamic and renin-aldosterone variations occurring in four patients who developed cardiac tamponade, either occurring in idiopathic fashion in one or secondary to metastatic involvement of the pericardium in three. Right atrial pressure, heart rate and arterial blood pressure were monitored and serial blood samples were taken before and over three hours after pericardiocentesis. During cardiac tamponade, normal levels of atrial natriuretic peptide (mean +/- SEM: 54 +/- 7.4 pg/ml) were observed in the plasma despite increased right atrial pressure (23 +/- 3.8 cm H2O) and heart rates (98 +/- 4.4). Removal of pericardial fluid (540 to 1160 ml) was associated at first with a 200% increase in plasma concentrations of atrial natriuretic peptide (108 +/- 8.8 pg/ml; P less than 0.001), then with a gradual decline toward normal levels, simultaneous with the normalization of right atrial pressure and heart rate. Activity of renin and concentrations of aldosterone in the plasma were increased during tamponade and returned gradually to normal after pericardiocentesis (3.8 +/- 0.9 to 1.2 +/- 0.3 ng/ml/h and 20 +/- 4.2 to 9 +/- 3.2 ng/dl, respectively; P less than 0.01). These data confirm that atrial strain, not intracavitary pressure in itself nor heart rate, is the main determinant of the acute release of atrial natriuretic peptide, which is associated with a suppressing effect on the renin-aldosterone system. In addition, our data indicate that secretion of atrial natriuretic peptide during cardiac tamponade is not stimulated by secondary hyperaldosteronism.

Sequential mechanism of atrial natriuretic peptide secretion in isolated perfused rabbit atria

It is well known that the secretion of atrial natriuretic peptide (ANP) is dependent on the atrial stretch. It has been claimed in this laboratory that the secretion of ANP occurs with a reduction in atrial distension. It was shown in the present experiment that the secretion of immunoreactive (ir) ANP occurs coincidently with a translocation of extracellular space marker (3-H)-inulin in the isolated perfused rabbit atria. Translocation of extracellular space fluid was observed with a reduction in atrial distension. The secretion of irANP into the atrial lumen occurs less than 15 sec of the reduction in atrial distension. It is therefore suggested that the incremental response of irANP secretion to the reduction in atrial distension is a sequential mechanism of ANP secretion, in which first is the release of ANP from the atrial myocytes into the extracellular space and then second is the translocation of ANP with extracellular space fluid into the atrial lumen with a reduction in atrial distension.

The effect of tachycardia on right atrial dynamics and plasma atrial natriuretic factor in anaesthetized rabbits

1. The effect of tachycardia on right atrial pressure (RAP) and dimensions (RAD) and plasma immunoreactive atrial natriuretic factor (IR-ANF) was examined in anaesthetized rabbits before and after the administration of atenolol. 2. Small increases in plasma IR-ANF occurred during pacing at 6 Hz and after the administration of atenolol (0.4 mg kg-1). A significantly greater increase in IR-ANF occurred during pacing in the presence of atenolol. 3. Despite a significant rise in mean RAP, systolic RAD (SRAD) and diastolic RAD (DRAD) did not change during tachycardia. Systolic RAD increased after the administration of atenolol while both SRAD and DRAD increased during pacing in the presence of atenolol. 4. Systolic right atrial wall stress (SRAS) increased during tachycardia, did not change after the administration of atenolol, and increased during pacing in the presence of atenolol. Minute SRAS followed a similar pattern of changes except that it decreased after atenolol. 5. Diastolic right atrial wall stress (DRAS) did not change during tachycardia, and increased both after atenolol and after pacing in the presence of atenolol. Tachycardia led to an increase in minute DRAS; a significantly greater increase in minute DRAS occurred during tachycardia in the presence of atenolol. 6. The absence of alterations in RAD during tachycardia suggests that the release of ANF in response to tachycardia is not due to simple mechanical stretch of the atria. Both systolic and diastolic atrial wall stress may be determinants of ANF release: the influence of systolic factors appears to predominate during tachycardia while diastolic factors appear to be the major determinants of the effects of atenolol.