Tachycardia releases atrial natriuretic peptide in the anesthetized rabbit

Practical application of natriuretic peptides in paediatric cardiology

It is still uncertain if cardiac natriuretic peptides are useful biomarkers in paediatric cardiology. In this review we identify four clinical scenarios in paediatric cardiology, where clinical decision-making can be difficult, and where we feel the paediatric cardiologists need additional diagnostic tools. Natriuretic peptide measurements could be that extra tool. We discuss and suggest N-terminal pro-B-type natriuretic peptide and B-type natriuretic peptide reference intervals for children without cardiovascular disease and cut-off points for the four specific paediatric heart conditions. We conclude that in premature neonates with persistent arterial ducts; in teenagers with tetralogy of Fallot and pulmonary regurgitation; and in children with heart transplants and potential allograft rejection cardiac peptides can provide the clinician with additional information, but in children with atrial septal defects the peptides are not helpful in guiding treatment or follow-up.

Changes in plasma atrial natriuretic peptide during exercise in healthy volunteers

Graded exercise was performed in three healthy volunteers. Plasma levels of immunoreactive atrial natriuretic peptide (iANP) were determined at different workloads. Unchanged or slightly decreased plasma levels of iANP were observed during light exercise, whereas at medium to high workloads a considerable increase in plasma levels of iANP was found. Factors responsible for the increase in plasma levels of iANP might include elevated right atrial pressure and increased plasma levels of epinephrine and norepinephrine.

Changes in circulating levels of atrial natriuretic factor (ANF) during orthotopic liver transplantation in humans

Atrial natriuretic factor (ANF) is a 28 amino acid peptide secreted by the atrial cardiocytes. Clearance is via the lung (50%) and the liver (25%). The main stimulus to ANF secretion is atrial distension but vasoconstrictors, sympathetic stimulation, catecolamines and tachycardia are able to enhance its circulating blood levels. ANF blood concentrations were measured during orthotopic liver transplantation in six postnecrotic cirrhotic patients. Significant increases in ANF blood levels occurred at the end of the anhepatic phase (P < or = 0.02 vs baseline) associated with low cardiac filling pressures (P < or = 0.02 vs baseline) and increased systemic vascular resistances (P < or = 0.02 vs preanhepatic phase). Aldosterone blood levels showed a similar behaviour, increasing significantly (P > or = 0.001 vs baseline) at the end of the anhepatic phase. ANF fell after reperfusion of the graft and returned towards baseline values at the end of the procedure. Since most of the total body clearance of ANF is performed by the lungs, its sharp increase at the end of the anhepatic phase could be considered a counterregulatory response to vasoconstricting stimulation and to fluid-sparing mechanisms in the presence of relative hypovolaemia. Its decrease after reperfusion could be related to volume normalization and partly to the enhanced clearance performed by the newly grafted liver.

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.

Effect of ryanodine on atrial natriuretic peptide secretion by contracting and quiescent rat atrium

To elucidate the mechanism involved in the release of atrial natriuretic peptide (ANP), we studied the importance of ryanodine-sensitive Ca2+ release in stretch-secretion coupling. The experiments were made with a left atrial preparation, where the stretch of myocytes was induced by changing the intra-atrial pressure. When external pacing was not applied, the atrial preparation was not spontaneously contracting, and it was therefore possible to investigate the secretory mechanism in the quiescent atrium. The superfusate was collected in 2-min fractions and assayed for ANP immunoreactivity. Filtration analysis revealed that the major fraction in the superfusate in all experimental situations had a similar molecular weight as the ANP 1-28. Ryanodine (1.0 microM and 0.1 microM) inhibited stretch-stimulated ANP secretion dose dependently both in paced and non-paced atrium, but did not have any effect on basal secretion. The present results support the notion that intracellular Ca2+ transients from the intracellular stores are essential for stretch-stimulated ANP secretion, independently from excitation and contraction. Basal ANP secretion is not inhibited by blocking ryanodine-sensitive Ca2+ channels, either in contracting or in non-contracting atria. In addition our results confirm that the principal stimulus for ANP secretion in response to atrial distension is the stretch of myocytes. Length shortening of myocytes is not essential for ANP release.

Extracellular fluid translocation in perfused rabbit atria: implication in control of atrial natriuretic peptide secretion

1. Transmural transport of 22Na+, 51Cr-EDTA, [3H]inulin and [14C]Dextran (57 kDa) was measured in perfused rabbit atria. The radiolabelled extracellular space (ECS) markers and [14C]Dextran were introduced into the pericardial space or atrial lumen. Atrial volume changes were induced by steps up and down in atrial pressure. 2. Basal rates of transmural transport of radiolabelled ECS markers across the atrial wall were relatively stable up to 70 min. Atrial stretch and release resulted in a rapid but transient, and reversible increase in the ECS fluid (ECF) translocation. The increased translocation of the ECF into the atrial lumen occurred within 15 s of the reduction of atrial distension and returned to the baseline level within 60 s. 3. Transmural transport of [3H]inulin across the atrial wall was bidirectional. 4. The clearance of radiolabelled ECS markers was molecular-size dependent. The transmural clearance of [3H]inulin was dependent on the distension-reduction volume changes induced by atrial stretch and release. Little transport of [14C]Dextran across the atrial wall was observed. 5. The ECF translocation across the atrial wall was not influenced by changes in external Ca2+ but was suppressed by low temperature. 6. Dynamic changes in the ECS of the atrium were observed in response to atrial distension and reduction. The ECS of the atrium increased on distension and decreased on reduction of atrial distension. 7. Reduction in atrial distension resulted in an increase in the secretion of immunoreactive atrial natriuretic peptide (ANP) which coincided with an increase in the translocation of the ECF. The secretion of immunoreactive ANP was a function of the translocation of the ECF. 8. It is suggested that atrial stretch and release may play a role in driving fluid flow within the interstitium and fluid translocation out of the interstitium. This fluid movement presumably leads to convective transport of released ANP into the atrial lumen.

Atrial natriuretic factor release during rapid ventricular pacing: interplay between autonomic and hemodynamic stimulants

Plasma levels of atrial natriuretic factor (ANF) and norepinephrine are markedly elevated during episodes of ventricular tachycardia. Although atrial distention appears to be the major stimulus for ANF release, reflex changes in autonomic tone might also contribute. Plasma ANF and norepinephrine levels, sinus node cycle length, systolic blood pressure, and mean right atrial pressure were therefore assessed during rapid right ventricular pacing at 150 beats/min for 10 minutes. In five patients (group 1) observations were made without autonomic blockade, and another five patients (group 2) had ventricular pacing after cardiac autonomic blockade. In group 1 systolic blood pressure fell during ventricular pacing from 122 +/- 4 to 105 +/- 5 mm Hg (p < 0.02), norepinephrine levels increased from 195 +/- 26 to 411 +/- 71 pg/ml (p < 0.02), and sinus node cycle length decreased from 936 +/- 99 to 688 +/- 58 msec (p < 0.02). Right atrial pressure was elevated from 2.6 +/- 0.6 to 7.4 +/- 0.6 mm Hg (p < 0.02), and ANF levels increased from 161 +/- 23 to 240 +/- 26 pg/ml (p < 0.05). Whereas systolic blood pressure, norepinephrine, sinus cycle length, and right atrial pressure returned promptly to baseline levels when ventricular pacing was stopped, ANF levels continued to rise (296 +/- 37 pg/ml; p < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Alterations in the secretion of atrial natriuretic factor in atria from aged rats

We measured plasma atrial natriuretic factor levels and atrial natriuretic factor secretion by isolated left atria from aging rats to determine the secretory response to stretch and adrenergic stimulation. Systolic arterial pressure and right atrial pressure were measured in vivo. Twenty-four hours later, atria were removed and studied in vitro in a perifusion system. After removal, stabilization at 0.7 g tension, and equilibration for 65 minutes, atria were stretched by increasing external tension for 20 minutes. After reequilibration atria were perifused with phenylephrine, 10(-5) M, for an additional 30 minutes. Right atrial pressure was not different between young (3 months) and aged (16-24 months) rats. Aged rats had higher plasma atrial natriuretic factor levels (52 +/- 8 versus 21 +/- 6 pmol/l; p less than 0.05) than young rats. Basal atrial natriuretic factor secretory rate in vitro was greater in atria from aged rats than young rats (875 +/- 35 versus 402 +/- 22 pg/min; p less than 0.05). Atria from aged rats had an increased response to phenylephrine compared with young rats (1,687 +/- 143 versus 788 +/- 113 pg/min; p less than 0.05) when means were adjusted for basal secretory rate. The secretory response to stretch was less than that of young rats (673 +/- 37 versus 773 +/- 27 pg/min), although this difference was not significant (p = 0.07). Atrial natriuretic factor secretion in response to adrenergic stimulation is increased with aging, and these secretory responses may contribute to increased plasma levels that occur during aging. In contrast to increased adrenergic responses, atrial natriuretic factor secretion after external stretch is not increased in aging rats.

The effect of hypovolemia on the renal and cardiovascular responses to atrial natriuretic factor (ANF) infusion

The renal and cardiovascular effects of ANF infusion have been examined in separate series of experiments; in conscious instrumented sheep following either hemorrhage (10 mL/kg body weight) or removal of 500 mL of plasma by ultrafiltration. Renal arterial infusion of hANF (99-126) at 50 micrograms/h increased sodium excretion from 99 +/- 30 to 334 +/- 102 (p less than 0.05) in normal animals, and from 77 +/- 31 to 354 +/- 118 mumol/min in hemorrhaged animals. Similarly in sheep following ultrafiltration, cardiac output and stroke volume were reduced by intravenous infusion of ANF (100 micrograms/h), although these effects were less marked than those observed in normal animals. The rapid modulation of natriuretic responses to ANF observed in volume expanded animals is not seen in this model of acute volume depletion suggesting that the mechanism through which the renal response to ANF is modulated in low sodium or volume states is not simply the reverse of that which produces rapid enhancement of response following blood volume expansion.

Exercise-induced increases in atrial natriuretic factor are attenuated by endurance training

Short-term exercise has been associated with increased plasma levels of atrial natriuretic factor, a potent dilating and natriuretic hormone. In this study, the effect of exercise training on atrial natriuretic factor release during short-term exercise was investigated in men without a history of cardiovascular or other major disease. A well trained group of 10 men who exercised an average of 6,618 kcal/week was compared with a minimally trained group of 9 men who exercised 1,479 kcal/week. Maximal oxygen uptake was 55.2 ml/kg per min in the well trained group and 42.5 ml/kg per min in the minimally trained group (p less than 0.05). Plasma for atrial natriuretic factor, norepinephrine and epinephrine was obtained at rest, at 4 min of exercise and at maximal exercise. Atrial natriuretic factor was lower at rest in the minimally trained than in the well trained men (23 vs. 35.9 pg/ml, p less than 0.05). At maximal exercise, atrial natriuretic factor increased 2.6 times the value at rest in minimally trained men (59.8 pg/ml, p less than 0.05 vs. rest), but did not change in well trained men (34 pg/ml). In minimally trained men at rest, at 4 min of exercise and at maximal exercise, plasma levels of atrial natriuretic factor correlated with heart rate, cardiac output, mean arterial pressure and plasma levels of norepinephrine and epinephrine; these correlations were not found in the well trained group. Thus, short-term exercise results in a significant increase in atrial natriuretic factor in minimally trained but not in well trained men.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

The inotropic effect of atrial natriuretic factor in the anesthetized rabbit

This study was designed to investigate whether atrial natriuretic factor (ANF) administered over the physiological, pathological and pharmacological range has a negative inotropic action on the heart. Anesthetized rabbits were infused with increasing doses of ANF (0.05, 0.25 and 0.5 micrograms kg-1 min-1), while measuring hemodynamic variables including the maximum rate of change of left ventricular pressure (dP/dtmax) as an index of inotropic state. Plasma levels of immunoreactive ANF (iANF) were measured to relate the hemodynamic changes to actual plasma levels of the peptide. Administration of ANF was associated with decreases in blood pressure, left ventricular pressure and dP/dtmax so that after 0.5 micrograms kg-1 min-1 infusion, these variables had decreased by 21 +/- 2 mmHg, 21 +/- 5.3 mmHg and 925 +/- 175 mmHg/s, respectively (P less than 0.01). There were no significant changes in right atrial pressure, left ventricular end-diastolic pressure or heart rate. Since dP/dtmax can be influenced by changing hemodynamic variables and baroreflex changes, a second group of rabbits was studied in which afterload and heart rate were held artificially constant. Again, in this group of rabbits, infusions of AFN led to decreasing inotropic state, so that at the highest infusion rate, a 14% decrease in dP/dtmax was observed (P less than 0.05). By comparison, hydralazine, a drug which causes active vasodilatation but no direct inotropic action, significantly (P less than 0.01) decreased blood pressure, left ventricular pressure and dP/dtmax when infused at a rate of 10 micrograms kg-1 min-1. However, in animals in which afterload was controlled, hydralazine did not affect any of the variables measured.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Atrial natriuretic factor levels in renal stone patients with idiopathic hypercalciuria and in healthy controls: the effect of an oral calcium load

Ionized calcium is a stimulator for the release of several peptide hormones. Atrial natriuretic factor (ANF) is a peptide hormone released from atrial tissue in response to atrial distension or volume expansion. In the present study, we have examined the effect of an oral calcium load in healthy controls and renal stone patients with idiopathic hypercalciuria. Our results demonstrated that ANF release increased in both groups in response to a calcium load. However, idiopathic hypercalciuric patients presented lower basal ANF levels in the presence of high calcitriol levels. The role of calcitriol on ANF release remains to be evaluated.

Atrial pacing stimulates secretion of atrial natriuretic polypeptide without elevation of atrial pressure in awake dogs with experimental complete atrioventricular block

To clarify whether or not tachycardia stimulates the secretion of atrial natriuretic polypeptide (ANP) without elevation of atrial pressure, we examined the effects of atrial pacing on ANP secretion in awake dogs with normal sinus rhythm and with complete atrioventricular block (CAVB), which was produced surgically by heat cauterization of His' bundle. In four dogs with normal sinus rhythm, atrial pacing increased the atrial rate from 146 +/- 20 to 260 +/- 10 beats/min, with marked elevation of right atrial pressure (from 0.2 +/- 0.1 to 3.9 +/- 0.8 mm Hg) and left atrial pressure (from 0.2 +/- 0.1 to 8.6 +/- 2.8 mm Hg). Along with the hemodynamic changes, the ANP level in plasma obtained from the coronary sinus was increased from 405 +/- 99 to 849 +/- 199 pg/ml (p less than 0.01). In five dogs with CAVB, the ANP level was also significantly increased from 730 +/- 82 to 1,137 +/- 35 pg/ml (p less than 0.01) by rapid atrial pacing (from 164 +/- 20 to 317 +/- 30 beats/min) in spite of the lack of any appreciable changes in either left or right atrial pressure. Furthermore, in the CAVB group, while the ventricular rate was increased by ventricular pacing from 52 +/- 5 to 146 +/- 16 beats/min, atrial pacing was performed simultaneously without atrioventricular (A-V) sequential form. Even under this condition, the ANP level was increased from 480 +/- 172 to 626 +/- 223 pg/ml (p less than 0.05) without any substantial changes in atrial pressure. Such effects of rapid atrial pacing on ANP secretion were suppressed after infusion of autonomic blocking agents.(ABSTRACT TRUNCATED AT 250 WORDS)

[Changes in renal function induced by anesthesia]

The rate of urine formation and its composition are influenced by the different drugs used during surgery. Anaesthetics act on renal function, not only directly, but also by producing changes in cardiovascular function and in neuroendocrine activity. Many factors may be incriminated: lowered blood pressure and cardiac output, increased sympathetic outflow (renal nerve stimulation and increased plasma catecholamines), increased release of renin, angiotensin and vasopressin. The effects of anaesthetics on the kidney go beyond a simple change in basal haemodynamics and include, for some drugs, an alteration in the ability for the kidney to autoregulate its blood flow and glomerular filtration rate. Studies on toad bladders showed a decrease in transport of water, sodium and organic anions. But, in fact, renal effects of anaesthetics in man and animals depend on the species, the anaesthetic and the method used to study the effect. Most barbiturates and inhalational anaesthetics tend to decrease renal blood flow (RBF) and glomerular filtration rate (GFR). These trends are gradually reversed during recovery. The effects of ketamine and diazepam are not clearly defined. Morphine and fentanyl decrease urine flow and GFR, whilst RBF increases or decreases, depending on whether a direct or indirect measurement technique was used. Muscle relaxants have little effect on renal function. Spinal and epidural anaesthesia only slightly decrease GFR and RBF in proportion to the decrease in mean arterial pressure. Obviously, the preexisting intravascular volume and the quantity of intravenous fluids given strongly influence the renal response to spinal and epidural anaesthesia. Some studies have shown that urine flow rate, creatinine clearance, urinary sodium excretion and RBF are reduced during mechanical ventilation with positive end-expiratory pressure. Surgery itself influences renal function by inducing alterations in prerenal haemodynamics. Operative stress leads to an increase in circulating catecholamines and angiotensin. Significant fluid shifts, excessive blood loss and redistribution of a third space may lead to a prerenal oliguric state, increasing secretion of vasopressin. Acute renal failure (ARF) is a frequently lethal complication of critical surgical illness, due to a variety of factors which interfere with glomerular filtration and tubular reabsorption, such as renal hypoperfusion or nephrotoxic insults. In fact, the initiating aggression ultimately culminates in the development of one or more of the maintenance factors (decreased tubular function, tubular obstruction, decreased GFR and RBF) that reduce urine flow and osmolar excretion. Good management during the perioperative period tends to minimize the risk of developing ARF.

Norepinephrine-induced atriopeptin release in conscious dogs is mediated by alterations in atrial pressure

This study was designed to determine whether the increase in atriopeptin secretion induced by an intravenous infusion of norepinephrine is mediated directly by adrenergic receptor stimulation or indirectly by the associated increase in atrial pressure. Norepinephrine was infused at 0.5 microgram.kg-1.min-1 for 30 min into both sham-operated (intact) and cardiac-denervated conscious dogs. The infusion increased mean arterial pressure in all dogs. On the other hand, left atrial pressure increased from 5.0 +/- 0.7 to 9.6 +/- 1.6 mmHg (p less than 0.01) in intact dogs, but decreased from 5.5 +/- 1.0 to 2.0 +/- 0.7 (p less than 0.01) in cardiac-denervated dogs. Right atrial pressure changes followed similar trends, but were not significant in the intact group. Plasma atriopeptin increased from 73 +/- 12 to 110 +/- 18 pg/ml (p less than 0.01) as left atrial pressure increased in intact dogs and decreased from 79 +/- 15 to 54 +/- 10 pg/ml (p less than 0.01) as left atrial pressure decreased in cardiac-denervated dogs. The changes in plasma atriopeptin correlated closely with the changes in left atrial pressure (r = 0.941, p less than 0.001) and to a lesser extent with the changes in right atrial pressure (r = 0.413, p less than 0.05). These results suggest that the change in plasma atriopeptin induced by infusion of norepinephrine into conscious dogs is mediated by the concomitant change in atrial pressures.

Plasmatic levels of atrial natriuretic peptide (ANP) in drowning. A pilot study

Bodies found in water may cause problems for forensic pathologists who have to differentiate drowning from postmortem immersion or fresh from salt water drowning. The exact physiopathology of drowning is still controversial and complementary tests can not exactly establish the exact cause of death if macroscopic findings at autopsy are not conclusive. We have employed atrial natriuretic peptide (ANP) as a marker in an experimental series of fresh and salt water drowning, comparing their results with a non-drowned control series. There are differences between the plasma basal levels of the control series (79 pg/ml) and the levels in animals drowned in fresh water (358 pg/ml, P less than 0.001) and between control and rabbits drowned in salt water (190 pg/ml, P less than 0.001). According to these values, there are also differences between fresh and salt water drowned animals (P less than 0.001). We propose this peptide as a new marker in cases of drowning, with the ability to differentiate drowning from postmortem immersion and fresh from salt water drowning.

Renal-endocrine adaptations to endogenous atrial natriuretic factor during tachycardia-induced reductions in renal perfusion pressure

Atrial pressure, atrial natriuretic factor (ANF), the renin-angiotensin-aldosterone system, and renal hemodynamic functions were examined during and after right ventricular pacing in anesthetized dogs (n = 9). Mean arterial pressure, cardiac output, and renal blood flow decreased during tachycardia while right and left atrial pressures increased. ANF markedly increased during tachycardia but urinary and fractional excretion of sodium were unchanged from control. Plasma renin activity was not increased during pacing despite the decrease in renal perfusion pressure. After tachycardia and restoration of mean arterial pressure to control, ANF declined but remained elevated above control despite a return of atrial pressure to control level. After tachycardia, urinary and fractional sodium excretion increased significantly in the absence of an increase in glomerular filtration rate. These findings support the following conclusions: 1) tachycardia increases ANF in association with increased atrial pressure; however, an elevation of ANF persists following tachycardia despite the absence of the persistent stimulus of elevated atrial pressures; 2) the increase in ANF during tachycardia may contribute to the absence of a decrease in sodium excretion and activation of the renin-angiotensin system that occurs with reduction in renal perfusion pressure; and 3) tachycardia-induced natriuresis may be dependent on an increase in ANF and the maintenance of renal perfusion pressure.

Influence of hypothermic cardiopulmonary bypass on atrial natriuretic factor levels

Atrial natriuretic factor (ANF) is a peptide released from the heart in response to atrial distension. This peptide causes diuresis, vasodilatation, decreased blood pressure, and antagonizes the renin-aldosterone and antidiuretic hormone neuraxes. The influence of cardiopulmonary bypass and cardiac surgery on the circulation and release of ANF is unknown. Plasma ANF concentrations were therefore determined in patients undergoing coronary artery revascularization (CABG) and mitral valve replacement (MVR). Peptide levels were unchanged following anaesthetic induction. Plasma ANF concentrations decreased significantly during hypothermic (less than or equal to 28 degrees C) cardiopulmonary bypass in both patient groups. After 60 minutes of cardiac bypass, ANF declined from (mean +/- SEM) 512 +/- 132 to 20 +/- 6 pg.ml-1 (P less than 0.05) during MVR, and from 178 +/- 41 to 110 +/- 48 pg.ml-1 during CABG (P less than 0.05). Rewarming during bypass was associated with an increase in ANF concentration in both groups. Heparin anticoagulation and protamine reversal had no effect on immunoreactive ANF levels. In patients undergoing CABG, there was a linear relationship between plasma ANF concentration (pg.ml-1) and right atrial pressure (mmHg) prior to cardiopulmonary bypass (r = 0.86, P less than 0.005). However, one and three hours after cardiopulmonary bypass there was no significant relationship between right atrial pressure and ANF plasma levels. These results suggest that reduction in plasma ANF concentration occurs during hypothermic cardiopulmonary bypass. Furthermore, the proportional relationship between atrial distension and circulating ANF concentration was altered following cardiac surgery.

The effect of calcium antagonists on atrial natriuretic peptide (ANP) release from the rat heart during rapid cardiac pacing

The diuresis associated with rapid atrial rhythms is a well recognized clinical entity (Wood, 1963). Atrial natriuretic peptide (ANP) levels are elevated during rapid atrial rhythms (Hirata et al., 1987), including during rapid atrial pacing (Rankin et al., 1986; Schiebinger and Linden 1986; Walsh et al., 1987), and may contribute to the associated diuresis. Calcium channel antagonists are often used to treat atrial tachycardias but the effect this may have on ANP secretion and subsequent compensatory responses, such as a diuresis, is unknown. Reported here are experiments demonstrating that the increase in ANP secretion that accompanies rapid atrial pacing of the isolated perfused rat heart is abolished by calcium channel antagonists. This effect is not limited to a single class of calcium channel antagonists and could be demonstrated with Verapamil and Nifedipine. Although extrapolation to the in vivo situation should always be done with care, the results reported here contribute towards clarifying the effect of the calcium channel antagonists on the ANP response to rapid heart rates.

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

Release of atrial natriuretic factor (ANF) following an elevation in heart rate is thought to be mediated primarily by a change in atrial stretch. To evaluate the direct effect of chronotropic stimulation on ANF release, isolated rat left atria were electrically paced (1-9 Hz) at constant resting tension (0.5-4 g), and the amount of immunoreactive ANF (IRANF) released at each frequency and tension was quantitated with a sensitive radioimmunoassay. Our results show that at controlled resting tensions greater than 1 g, chronotropic stimulation increased IRANF secretion in a manner dependent on the pacing frequency; rapid atrial rates (e.g., 8 and 9 Hz) were necessary to release ANF at tensions of 1 g or less. Resting tension influenced the magnitude of the secretory response to electrical stimulation. Release of IRANF with contraction frequency was transient in nature and, at high frequencies, was associated with a decrease in developed (systolic) tension in accordance with the negative force-frequency relation inherent in the rat heart. When evaluated at a single diastolic tension and pacing frequency, IRANF release was positively correlated with systolic tension. ANF released under in vitro conditions was approximately 3,000 Da, in agreement with the size of the physiologically circulating form. In atria from reserpinized rats, evidence for involvement of catecholamines in chronotropic-stimulated ANF release was suggested. The presence of lidocaine (5 x 10(-4) M) had no effect on rate-induced ANF secretion. Therefore, chronotropic stimulation releases ANF independently of changes in atrial stretch. The magnitude of this response depends on a combination of pacing frequency and diastolic tension.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma atrial natriuretic polypeptide as an index of left ventricular end-diastolic pressure in patients with chronic left-sided heart failure

To evaluate the relationship between plasma atrial natriuretic polypeptide (ANP), hemodynamic parameters, and plasma catecholamines and, in addition, to determine whether circulating ANP is metabolized in the pulmonary circulation, plasma concentrations of ANP were determined in 40 patients with chronic left-sided heart failure. After at least 30 minutes of bed rest with the patient in the supine position, blood samples were drawn simultaneously from both the main pulmonary artery (mPA) and the ascending aorta (Ao) before administration of contrast medium. The plasma ANP concentrations significantly decreased from the mPA to the Ao (135.3 +/- 18.1 pg/ml vs 127.4 +/- 19.4 pg/ml; mean +/- SEM, p less than 0.05). The plasma ANP level in the mPA correlated with the plasma norepinephrine level in the Ao (r = 0.71, p less than 0.01), right atrial pressure (r = 0.34, p less than 0.05), mean pulmonary capillary wedge pressure (r = 0.829, p less than 0.001), and left ventricular end-diastolic pressure (LVEDP) (r = 0.88, p less than 0.001). Of the various hemodynamic parameters and plasma catecholamine concentrations in the Ao, only LVEDP was found to be an independent and significant predictor of plasma ANP levels in the mPA. These results indicate that ANP released from the heart is regulated mainly by preload (LVEDP) in cases of left-sided heart failure and that circulating ANP is metabolized in the pulmonary circulation. In conclusion, the plasma ANP concentration may be a useful noninvasive index of LVEDP in patients with chronic left-sided heart failure.

Prohormone atrial natriuretic peptides 1-30, 31-67, and 99-126 increase in proportion to right ventricular pacing rate

To determine whether heart rate contributes to release of three new peptide hormones synthesized in the heart, right ventricular pacing at rates of 100, 125, 150, and 180 bpm was performed in six dogs with measurement of the plasma concentration of these peptides at each pacing rate while right atrial and systemic blood pressures were simultaneously monitored. These three peptides of the 126-amino-acid prohormone of atrial natriuretic factor (ANF), consisting of amino acids 1-30 (pro ANF 1-30), 31-67 (pro ANF 31-67), and 99-126 (ANF), increased incrementally at paced heart rates of 125, 150, and 180 bpm (r = 0.8, p less than 0.001). Right atrial pressure decreased with increasing heart rate but systemic blood pressure did not decrease until the heart rate was 180 bpm, at which time these peptides had obtained their maximal circulating concentrations. After pacing, mean right atrial pressure and levels of ANF returned to prepacing values within 30 minutes. Mean arterial blood pressure, on the other hand, increased throughout the 120-minute period after pacing. At 2 hours after pacing, levels of pro ANFs 1-30 and 31-67 were elevated compared with prepacing values. These data demonstrate that, at heart rates of 125 bpm and above, pro ANF 1-30, pro ANF 31-67, and ANF (99-126) are simultaneously and incrementally released in direct proportion to heart rate. The sustained elevation in pro ANFs 1-30 and 31-67 seen 2 hours after pacing suggests that they may contribute to the prolonged diuresis seen after cardiac pacing or tachycardia.

The influence of carotid sinus pressure on plasma atrial natriuretic peptide in anaesthetized rabbits

The effect of changes in carotid sinus perfusion pressure on plasma immunoreactive atrial natriuretic peptide (IR-ANP) was examined in anaesthetized rabbits, and the role of arterial pressure in mediating the changes in IR-ANP was assessed. Plasma IR-ANP was significantly greater (101.7 +/- 24.3 pg ml-1) when carotid sinus pressure was 60 mmHg than when it was 160 mmHg (27.1 +/- 8.6 pg ml-1). Mean arterial pressure (MAP) was significantly greater when carotid sinus pressure was controlled at 60 mmHg compared to when it was 160 mmHg, but right atrial pressure (RAP) was not significantly different at the two carotid sinus pressures. The administration of hexamethonium attenuated the changes in MAP and heart rate (HR) which occurred in response to alterations in carotid sinus pressure, and abolished the change in plasma IR-ANP. The results suggest that an inverse relationship exists between carotid sinus pressure and plasma IR-ANP, and that the release of ANP in response to a reduction of carotid sinus pressure is mediated by the associated haemodynamic changes.

Endothelin stimulates release of atrial natriuretic peptides in vitro and in vivo

The effect of endothelin (END) on the release of atrial natriuretic peptides (ANP) was studied in isolated rat atria and in conscious rats. END stimulates the ANP release in vitro in a dose-dependent manner. An increase in ANP plasma levels and cyclic GMP plasma levels was also observed in conscious rats after injection of END. When a monoclonal antibody directed against ANP was injected together with END the increase in cyclic GMP was completely blocked. From this study it is concluded that END is a potent secretagogue for ANP both in vitro and in vivo.

The release mechanism for atrial natriuretic factor during blood volume expansion and tachycardia in dogs

Atrial natriuretic factor (ANF) is released during blood volume expansion and tachycardia, but only blood volume expansion causes atrial distension, which presumably promotes ANF release. Our study was undertaken to search for a common release mechanism. In five anaesthetized, closed-chest dogs, plasma immunoreactive (IR) ANF was measured at three levels of blood volume, which were obtained by infusing a Ringer's solution. At each level of blood volume, plasma IR-ANF was measured at three pacing frequencies. Plasma IR-ANF increased as mean right atrial pressure (mRAP) was raised from 2 to 10 mmHg by volume expansion, whereas pacing tachycardia (at heart rates (HR) 50 +/- 3 and 98 +/- 1 beats min-1 above control) at each level of blood volume expansion increased plasma IR-ANF and systolic RAP (sRAP) at constant mRAP. Plasma IR-ANF was more strongly correlated to sRAP (r = 0.83) than to mRAP (r = 0.69), but the product sRAP x HR had the highest correlation coefficient (r = 0.86). According to the multiple regression equation: plasma IR-ANF = k1 + k2mRAP + k3sRAP + k4sRAP x HR, the product sRAP x HR had the highest coefficient of determination (r2 = 0.75) and was the only significant determinant. We conclude that atrial tension or stress, developing during each atrial systole, is an important determinant of ANF release. Since atrial diastolic and systolic dimensions do not increase during pacing tachycardia, ANF release is not dependent on atrial distension.

Calcitonin gene-related peptide(CGRP) stimulates the release of atrial natriuretic peptide(ANP) from isolated rat atria

The effect of calcitonin gene-related peptide(CGRP) on the release of atrial natriuretic peptide(ANP) was studied in spontaneously beating, isolated rat atria. CGRP stimulated the ANP release in a dose-dependent manner. When the atria were incubated with a combination of phentolamine, propranolol, and atropine, these antagonists blocked neither the rise in ANP release nor the positive chronotropic and inotropic effects of CGRP. Therefore, we conclude that CGRP stimulates ANP release as well as cardiac contractility independently of adrenergic and cholinergic receptors.

Atrial natriuretic factor release is enhanced by incremental atrial pacing

There is ample evidence from animal models indicating that secretion of atrial natriuretic factor (ANF) can be induced by an increase in atrial contraction frequency or atrial distension. The influence of these stimulatory signals on ANF secretion in humans has not been fully elucidated. We assessed the responses to graded right atrial pacing in 28 patients (aged 33 to 70 years) at rates of 100, 125, and 150 beats/min on right atrial pressure, left atrial size (by two-dimensional echocardiography in 9 of 28 patients), and circulating plasma ANF levels. At pacing rates of 125 and 150 beats/min, ANF levels increased from a baseline value of 64 +/- 9 fmol/ml (mean +/- SEM) to 89 +/- 13 fmol/ml (pp less than 0.05) and to 132 +/- 17 fmol/ml, respectively (p less than 0.001). Right atrial pressure increased from a baseline value of 4.1 +/- 0.7 mm Hg to 4.5 +/- 0.6 mm Hg at a pacing rate of 125 beats/min (p less than 0.05) and to 6.1 +/- 0.8 mm Hg at a pacing rate of 150 beats/min (p less than 0.001). Left atrial dimension increased from a baseline value of 44.5 +/- 3 mm to 49.5 +/- 3 mm at a pacing rate of 125 beats/min (p less than 0.05) and increased further to 52.5 +/- 3 mm at a pacing rate of 150 beats/min (p less than 0.001). No significant changes in atrial pressure or size or in plasma ANF were observed at a pacing rate of 100 beats/min.(ABSTRACT TRUNCATED AT 250 WORDS)

Responsiveness of atrial natriuretic factor to reduction in right atrial pressure in patients with chronic congestive heart failure

In patients with congestive heart failure, atrial natriuretic factor may serve as a counter-regulatory hormone, offsetting the vasoconstrictive and volume-retentive effects of the sympathetic nervous system, the renin-angiotensin-aldosterone system and vasopressin. Indeed, the plasma levels of atrial natriuretic factor and the vasoconstrictor hormones are often simultaneously elevated in these patients. It is not known, however, whether atrial natriuretic factor remains responsive to sudden reductions in atrial pressure in patients with chronic heart failure, or is unresponsive like the vasoconstrictor systems. To examine this issue, the plasma concentrations of atrial natriuretic factor and the vasoconstrictor hormones were measured in 20 normal subjects and 12 patients with chronic congestive heart failure during incremental lower body negative pressure, an intervention that lowers atrial pressure. In the normal subjects, incremental lower body negative pressure at -10, -20 and -40 mm Hg decreased central venous pressure and pulse pressure. At maximal lower body negative pressure, plasma atrial natriuretic factor levels decreased from 51 +/- 5 to 27 +/- 3 pg/ml (p less than 0.01), whereas increases occurred in plasma levels of norepinephrine (194 +/- 11 to 385 +/- 70 pg/ml, p less than 0.01), renin activity (1.4 +/- 0.2 to 3.9 +/- 0.1 ng/ml per h, p less than 0.01) and vasopressin (1.3 +/- 0.1 to 6.4 +/- 2.4 pg/ml, p less than 0.05). In the patients with congestive heart failure, lower body negative pressure also reduced central venous pressure. Baseline plasma atrial natriuretic factor levels were markedly elevated, averaging 438 +/- 138 pg/ml, and decreased to 317 +/- 87 pg/ml at maximal lower body negative pressure (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Verapamil impairs secretion of stimulated atrial natriuretic factor in humans

The adaptation of the secretory rate of atrial natriuretic factor to repeated adequate stimuli and the influence of the calcium antagonist verapamil on the release of atrial natriuretic factor were investigated in 16 patients. In eight patients (Group 1) right atrial pressure was abruptly increased by rapid right ventricular pacing for 4 min (stimulation I). After a 15 min interval, the identical stimulation was repeated (stimulation II). Eight patients (Group 2) underwent the same protocol but received 5 mg of verapamil intravenously after stimulation I. Pacing increased right atrial pressure in both groups identically by 70%. In Group 1, release of atrial natriuretic factor caused by the second stimulation (median 290 pg/ml over basal) was significantly (2.5-fold) larger than atrial natriuretic factor release induced by the first stimulation (median 116 pg/ml over basal). In the verapamil-treated patients (Group 2), the effect of right atrial pressure increase on release of atrial natriuretic factor was abolished after stimulation II. In both groups, changes in plasma concentrations of cyclic guanosine monophosphate corresponded to changes in atrial natriuretic factor concentrations. Thus, the myoendocrine cells are apparently capable of a fast upward regulation of their response to repeated secretory stimuli. Verapamil appears to block the stimulatory effect of a sudden increase in right atrial pressure upon release of atrial natriuretic factor.

Plasma atrial natriuretic polypeptide concentrations during and after reversion of paroxysmal supraventricular tachycardias

Plasma concentrations of immunoreactive atrial natriuretic polypeptide were raised in 22 of 23 patients with paroxysmal supraventricular tachycardia and in all seven patients with atrial flutter. Plasma concentrations of atrial natriuretic polypeptide rose soon after the onset of supraventricular tachycardia. A sample taken 30 minutes after reversion to sinus rhythm (pharmacological or non-pharmacological) showed a significant fall in 19 of the 23 patients with paroxysmal supraventricular tachycardia and all seven patients with atrial flutter. Because atrial natriuretic polypeptide has powerful natriuretic and diuretic properties, an increase may contribute considerably to the polyuria that is often associated with episodes of supraventricular tachycardia.

The effect of hemorrhage and resuscitation on serum levels of immunoreactive atrial natriuretic factor

To determine if atrial natriuretic factor (ANF) might have a role in blood volume regulation during hypovolemia, the serum level was measured before and after hemorrhage and resuscitation (RESUS) in a porcine shock model. Hemorrhage of 40% of the blood volume produced significant (p less than 0.01) decreases in mean arterial pressure, central venous pressure, and cardiac output and a significant increase in heart rate (HR), plasma renin activity, and catecholamines (p less than 0.01). Hemorrhage was also associated with a significant increase in ANF (p less than 0.05). All parameters except HR returned to baseline values with RESUS. The rise in ANF associated with moderate hemorrhage was unexpected, but may have been due to the profound tachycardia or decreased degradation during shock. Sodium excretion and urine flow increased significantly with RESUS without an associated increase in ANF, suggesting that ANF may not be involved in the early phase of postresuscitation diuresis.

Analysis of azide-insensitive Ca2+-dependent ATPase activity in atrial specimens from patients with coronary or valvular heart disease

The activity of the azide-insensitive Ca2+-dependent ATPase (highly enriched in myofibrillar ATPase activity) was studied in specimens of both right and left atria which were taken from patients with ischemic and/or valvular heart disease during coronary by pass and/or valvular substitution. A significantly lower enzymatic activity was found in atrial specimens from patients with left ventricular heart failure in comparison to the atrial fragments obtained from the patients with normal heart function. Such an inhibition reflected a significant increase in the Km of the enzyme for ATP and was associated with a concomitant reduction in Vmax, both more evident in the left atrial fragments. Moreover, tissue homogenates of atrial specimens from failing hearts exhibited a lower protein SH group content when compared to the atrial homogenates from the heart with normal left ventricular heart function.

Effect of beta 1-adrenoceptor blockade on plasma levels of atrial natriuretic peptide during exercise in normal man

Increased plasma levels of atrial natriuretic peptide (ANP) during exercise have been reported. To investigate the role of tachycardia as a stimulus for release of ANP during exercise the following study was undertaken. Graded exercise was performed in six healthy volunteers before and after beta 1-adrenoceptor blockade. Plasma levels of ANP were determined at different workloads in both cases. At rest and at all workloads during exercise plasma levels of ANP were higher after beta 1-adrenoceptor blockade than without. Therefore, it is unlikely that tachycardia is a major stimulus for secretion of ANP during exercise. It is suggested that increased right atrial pressure and/or pulmonary arterial blood pressure and increased plasma levels of catecholamines are important secretory stimuli for ANP during exercise.

Peptides in the mammalian cardiovascular system

Ample immunocytochemical evidence is now available demonstrating that several peptides are present in the mammalian cardiovascular system where they are localised to nerve fibres and myocardial cells. The neuropeptides (neuropeptide Y, calcitonin gene-related peptide, tachykinins and vasoactive intestinal polypeptide) are localised to large secretory vesicles in subpopulations of afferent or efferent nerves supplying the heart and vasculature of several mammals, including man. Although they often exert potent pharmacological effects on the tissues in which they occur their physiological significance has still to be established. They may act directly via specific receptors and/or indirectly by influencing the release and action of other cardiovascular transmitters. In marked contrast, atrial natriuretic peptide is produced by cardiac myocytes and considered to act as a circulating hormone.

Epinephrine increases plasma immunoreactive atrial natriuretic hormone levels in humans

Six normal human subjects each underwent sequential 80-min infusions of saline and epinephrine (EPI) at 0.55 and 2.75 micrograms X min-1 X m-2 to determine the role of EPI in the control of atrial natriuretic hormone (ANH) in humans. Plasma immunoreactive-ANH (IR-ANH) levels nearly doubled in response to the infusion of EPI at 0.55 microgram X min-1 X m-2 (P less than 0.05) and then plateaued; heart rate accelerated significantly (P less than 0.01) with increasing plasma EPI levels, whereas systolic blood pressure increased only with higher plasma EPI levels (P less than 0.05). To determine whether beta-adrenergic mechanisms mediate the EPI-induced increase in IR-ANH, six additional subjects each received infusions on two separate days of saline for 240 min and the beta-adrenergic antagonist propranolol followed by propranolol plus EPI at 2.75 micrograms X min-1 X min-2 each for 80 min. Neither saline nor propranolol plus EPI caused a significant increase in plasma IR-ANH. We conclude that EPI increases plasma IR-ANH through beta-adrenergic mechanisms in humans. beta-Adrenergic-mediated increases in plasma IR-ANH levels appear to be unrelated to changes in the heart rate.

Influence of isoproterenol on plasma immunoreactive atrial natriuretic peptide and plasma vasopressin in the anesthetized rabbit

Changes in levels of plasma immunoreactive atrial natriuretic peptide (IR-ANP) were measured in response to administration of isoproterenol in the anesthetized, vagotomized rabbit. A dose-dependent increase in plasma IR-ANP was seen in response to 10 min isoproterenol infusions between 0.1 and 10.0 micrograms/kg/min. The time course of these responses showed the maximum levels of IR-ANP to be attained 10 min after the cessation of infusion. In rabbits in which plasma vasopressin (AVP) levels were also measured, the maximum levels of AVP were attained during the infusion period. There was no correlation between levels of AVP and IR-ANP suggesting that AVP released into the plasma did not affect directly the release of IR-ANP. The changes in IR-ANP in response to isoproterenol were significantly reduced in rabbits which had been administered the beta-1-adrenoceptor blocking agent, atenolol. In six rabbits in which the vagi remained intact, the increases in IR-ANP were reduced and became significant only with 10 micrograms/kg/min isoproterenol infusion. The results demonstrate that isoproterenol infusion increases the level of plasma IR-ANP in the anesthetized rabbit and suggest that this is through an effect on the heart rather than on peripheral vessels.

Plasma atrial natriuretic peptide is increased during atrial pacing in conscious rabbits

The effect of increases in heart rate on plasma atrial natriuretic peptide (ANP) concentrations was investigated in conscious rabbits. Plasma ANP concentrations were significantly increased following abrupt increases in heart rate produced by atrial pacing at 400 beats/min. Pacing at 300 beats/min resulted in smaller increases in plasma ANP concentration. Stepwise increases in heart rate produced by atrial pacing at 250, 300, 350 and 400 beats/min resulted in increases in plasma ANP concentrations at 400 beats/min only. The increase in plasma ANP concentration during atrial pacing correlated significantly with the increase in heart rate achieved.