Prolonged administration of human atrial natriuretic peptide in healthy men. Reduced aldosteronotropic effect of angiotensin II

Effects of angiotensin and atrial natriuretic peptide on the cerebral circulation

The purpose of the present study was to determine effects of angiotensin (ANG) II on the cerebral circulation. We measured the pial artery pressure (PAP) and CBF in anesthetized rabbits. ANG II (5 micrograms/min) was infused into each carotid artery, and systemic arterial pressure was maintained constant. During infusion of ANG II, there was a significant increase in CBF and fall of PAP, with no change in the large artery resistance (LAR) and a significant decrease in the small vessel resistance (SVR). To investigate whether prostaglandin modulated the ANG II-induced increase in CBF, indomethacin was administered (10 mg/kg i.v.) in another group of animals. Indomethacin itself reduced PAP and increased LAR significantly without changing CBF or SVR. Indomethacin did not attenuate the effects of ANG II on the cerebral circulation. The CMRO2 was assessed during ANG II intracarotid infusion in another group of rabbits. CMRO2 did not change during infusion of ANG II. We also investigated effects of alpha-atrial natriuretic peptide (ANP) on the cerebral circulation. Infusion of ANP (1 microgram/min) decreased LAR by 28% (p less than 0.05) without altering SVR. Administration of ANG II after ANP tended to reduce LAR (p greater than 0.05), with a significant decrease in SVR. The results of the present study suggest that high doses of ANG II can produce cerebral vasodilatation, particularly of small vessels. Blood-borne ANP dilated the large arteries of the cerebral circulation selectively and neither interfered with nor reversed the ANG II-induced increase in CBF.

Atrial natriuretic factor mRNA and binding sites in the adrenal gland

The factor inhibiting aldosterone secretion produced by the adrenal medulla may be atrial natriuretic factor (ANF), since the latter abolishes aldosterone release in response to a number of secretagogues, including angiotensin II and K+. In this study we have shown that cells in the adrenal medulla contain ANF mRNA and therefore have the potential to synthesize this peptide. The presence of binding sites for ANF predominantly in the adrenal zona glomerulosa suggests that, if ANF is synthesized in the medulla and transferred to the cortex, it may affect mineralocorticoid status.

The diuretic and natriuretic action of human atrial natriuretic peptide in humans: lack of effect of exogenous insulin

The interaction of exogenous, synthetic human atrial natriuretic peptide [(hANP) (ANF 99-126)] and of exogenous insulin was investigated in six healthy men and in seven type I diabetic patients using the euglycemic clamp technique. A primed-continuous (1.0 mU/kg x min) infusion of biosynthetic human insulin was administered to acutely raise and maintain plasma insulin concentrations at approximately 75 to 100 microU/mL during four hours while plasma glucose concentrations were maintained constant at the fasting level by a variable infusion of glucose. In healthy men a decrease in natriuresis (P less than .01) was seen during a euglycemic clamp study without exogenous hANP. No changes in diuresis and natriuresis were seen during a control experiment without exogenous insulin and glucose. Both in healthy men and in the type I diabetics sequential IV bolus doses of hANP of 100, 200, and 400 micrograms induced an increase in urine flow (P less than .01) and in natriuresis (P less than .01). In healthy men these effects were comparable to those achieved by hANP in the absence of induced hyperinsulinemia. It is concluded that the antinatriuretic action of insulin is of no major relevance in counteracting the pharmacologic action of hANP in healthy men. The effects of pharmacologic doses of hANP on diuresis and natriuresis in patients with type I diabetes mellitus is comparable to that in healthy men.

Cardiac and vascular effects of atrial natriuretic factor and sodium nitroprusside in healthy men

To assess the contribution of venous effects to the hemodynamic changes caused by atrial natriuretic factor (ANF), the cardiac and peripheral effects of ANF were compared with those induced by the venoarterial vasodilator sodium nitroprusside. On 3 different days, eight healthy subjects received 2-hour infusions of either ANF, sodium nitroprusside, or placebo, by a single-blind crossover design. ANF was administered at a rate of 15 ng/kg/min for hour 1 and 50 ng/kg/min for hour 2; each infusion rate was preceded by a 50-micrograms bolus. The lower ANF infusion rate increased plasma cGMP fourfold, but only modest cardiovascular effects (small decreases in left ventricular end-diastolic and end-systolic volumes) were noted. At the higher ANF infusion rate, left ventricular volumes and intravascular volume, as indirectly assessed by changes in hematocrit levels, decreased further, which resulted in decreases in stroke volume, cardiac index, and systolic blood pressure. No evidence for arterial vasodilation (no decrease in diastolic blood pressure, total peripheral resistance, or forearm resistance) was obtained, and no increase in sympathetic activity was noted. In contrast, sodium nitroprusside caused arterial vasodilation, an increase in cardiac index, and significant increases in sympathetic activity. We conclude that short-term increases in plasma ANF within the physiologic range primarily affect the venous vascular bed (by decreasing intravascular volume or by venodilation) without increasing sympathetic activity.

Renal interaction of atrial natriuretic peptide with angiotensin II: glomerular and tubular effects

1. The possible interactions between the renal effects of atrial natriuretic peptide (ANP) and angiotensin II (AII) were studied in normal sodium-replete human subjects. Recent investigations have suggested that ANP inhibits the pressor and volume-retaining effects of activation of the renin-angiotensin system. Thus, ANP may attenuate the effects of AII on renal haemodynamics or tubular transport. 2. ANP (0.1 micrograms/kg per min, 60 min) was intravenously infused into eight normal human subjects with and without pretreatment with enalapril (20 mg, per oral), an inhibitor of the converting enzyme, and during infusion of AII (10 mg/kg per min). 3. ANP infusion alone caused increases in the urine volume (from 96 +/- 23 to 229 +/- 44 mL/h, P less than 0.05) and urinary sodium excretion (from 11.5 +/- 1.6 to 20.9 +/- 4.2 mEq/h, P less than 0.05). These changes were accompanied by an increase in the glomerular filtration rate (from 127 +/- 9 to 158 +/- 9 mL/min, P less than 0.05). ANP infusion after enalapril administration lowered the mean blood pressure (from 76 +/- 2 to 71 +/- 3 mmHg, P less than 0.05) to a level similar to that observed during ANP infusion alone (from 84 +/- 2 to 74 +/- 2 mmHg, P less than 0.01), but did not result in a significant diuresis (from 139 +/- 23 to 174 +/- 51 mL/h) or natriuresis (from 19.7 +/- 2.5 to 14.3 +/- 3.4 mEq/h, P less than 0.05). This combined treatment with a converting enzyme inhibitor and ANP reduced both the glomerular filtration rate (160 +/- 9 to 141 +/- 10 mL/min) and the renal plasma flow (from 775 +/- 49 to 570 +/- 45 mL/min, P less than 0.01). 4. The antinatriuretic effects of exogenous AII were reversed by superimposed ANP infusion (urinary sodium excretion: from 4.8 +/- 1.0 to 24.3 +/- 5.2 mEq/h, P less than 0.01). Under these conditions, the glomerular filtration rate increased (from 114 +/- 6 to 156 +/- 7 mL/min, P less than 0.05) to levels similar to those observed with ANP infusion alone. In addition the increased tubular sodium reabsorption induced by AII was inhibited by concomitant ANP infusion (fractional proximal tubular sodium reabsorption: from 90.7 +/- 3.5 to 80.3 +/- 16.6%, P less than 0.05, fractional post-proximal tubular sodium reabsorption: from 91.5 +/- 9.8 to 87.6 +/- 8.8%, P less than 0.05).(ABSTRACT TRUNCATED AT 400 WORDS)

Atrial natriuretic peptide in man

The heart is the major source of atrial natriuretic peptides (ANP). A propeptide is stored in atrial myocytes. In normal humans, atrial distension secondary to volume overload and/or increased atrial pressures are thought to stimulate the secretion of biologically active alpha-ANP (ANF-[99-126], 28 amino residues) into the circulation. Plasma immunoreactive ANP (irANP) rises in response to acute sodium-volume loading, the central shift of volume produced by lying down or by immersion, acute increases in blood pressure (BP), dynamic exercise, or the administration of glucocorticoids or mineralocorticoids. Plasma irANP also rises with aging. Synthetic alpha-ANP infused acutely i.v. can lower BP, reduce plasma volume by an extravascular shift, cause baroreflex-mediated sympathetic activation, directly inhibit adrenal steroidogenesis and lower plasma aldosterone and cortisol, directly inhibit renal renin release, elevate plasma insulin; diuresis, free water clearance and natriuresis increase already in response to low alpha-ANP doses that raise plasma irANP within the physiological-pathological range. It follows that in addition to direct influences on cardiovascular and renal function, the ANP system may comprise a cardio-adrenal feedback mechanism and perhaps also modulate insulin and the release of ADH. The major although yet unproven physiological role of the ANP system may be the protection of the heart against volume and/or pressure overload. The pathophysiological, diagnostic and therapeutic aspects of elevated plasma irANP values, ANP measurements, or administration of synthetic ANP, respectively, in various diseases are currently under intense study and of great potential interest.

Splanchnic disposal of human atrial natriuretic peptide in humans

In healthy men (n = 6), the splanchnic fractional extraction of human atrial natriuretic peptide (hANP), as determined by the hepatic venous catheter technique, was 75% under basal conditions resulting in a splanchnic uptake of hANP of 8.5 +/- 5.0 pmol/min. In spite of a drop (P less than .05) in splanchnic fractional extraction to about 50%, splanchnic uptake of hANP rose to 56 to 99 pmol/min (P less than .01) when pharmacologic plasma concentrations of hANP were induced during a bolus (100 micrograms)-primed intravenous (IV) infusion (100 micrograms/h; time, one hour) of hANP. This was accompanied by a fall in estimated hepatic blood flow (P less than .05), in pulmonary arterial pressure (P less than .01), and, in each individual, in systemic BP. Total metabolic clearance rates, splanchnic clearance rates, and production rates of hANP were 4.5 +/- 2.2 L/min, 0.4 +/- 0.1 L/min, and 46.1 +/- 20.1 pmol/min, respectively. Thus, in healthy men, the splanchnic area accounts for approximately 10% of total hANP clearance.

Effects of incremental infusions of atrial natriuretic factor on aldosterone, renin, and blood pressure in humans

To evaluate the physiological effects of human atrial natriuretic factor-(99-126) (ANF), we infused ANF, 0.1, 0.3, and 1.0 micrograms/min, or placebo for 125 minutes on different days into six sodium-deprived normal men. During the last 45 minutes of infusion, angiotensin II, 6 ng/kg/min, was infused. Blood pressure, heart rate, plasma concentrations of ANF, aldosterone, and cortisol, and plasma renin activity (PRA) were measured before and during infusion. Steady state mean plasma ANF levels during infusion were 26.2 (placebo), 68.8 (0.1 micrograms ANF/min), 221 (0.3 micrograms ANF/min), and 648 pg/ml (1.0 microgram ANF/min). Systolic blood pressure fell significantly (with 1.0 microgram ANF/min), and diastolic pressure tended to rise in a dose-dependent manner, while heart rate was unchanged. PRA and plasma aldosterone fell during ANF infusion in a dose-dependent manner (significant with 0.3 and 1.0 microgram ANF/min infused). The blood pressure-raising and aldosterone-stimulating effects of angiotensin II were blunted by ANF (significant only with 1.0 microgram ANF/min). It is concluded that effects of ANF on blood pressure and the renin-aldosterone system occur with plasma ANF levels close to the physiological range, as well as with slightly elevated ANF levels, as observed in congestive heart failure and renal insufficiency.

Physiologic changes during supraventricular tachycardia and release of atrial natriuretic peptide

Plasma levels of atrial natriuretic peptide (ANP) increase markedly during supraventricular tachycardia (SVT). Although natriuresis associated with SVT may be secondary to the augmented secretion of ANP, whether or not physiologic changes other than natriuresis can be attributed to the release of ANP has not been determined. In the present study, plasma ANP levels in 10 patients with SVT were found to be significantly (p less than 0.05) increased, from 37 +/- 11 pg/ml (mean +/- standard error of the mean) during the control period to 160 +/- 54 pg/ml at 60 minutes after the induction of SVT. Urinary sodium excretion, although insignificant, tended to increase during the 30-minute period after SVT termination. The filtration fraction determined by the ratio of creatinine to para-aminohippurate clearance significantly increased during SVT. An increase in capillary permeability seemed to have occurred as there was a rise of hematocrit, the changes of which showed a different time course from that of the urine volume. The ratio of plasma aldosterone concentration to plasma renin activity significantly decreased during SVT. As the same effects are observed after ANP infusion, these changes were attributed to ANP activity.

Atrial natriuretic peptide, angiotensin II and aldosterone in plasma in chronic glomerulonephritis during basal conditions and during exercise

Atrial natriuretic peptide (ANP), angiotensin II (AII) and aldosterone (Aldo) in plasma were determined at supine rest in 16 normotensive and 9 hypertensive patients with chronic glomerulonephritis and in 18 control subjects (Study 1). In addition, 12 of the normotensive, 7 of the hypertensive patients and 11 of the control subjects were studied with the same parameters after an exercise test (Study 2). Study 1 showed that supine ANP, AII and Aldo did not differ significantly between the groups. In Study 2, ANP increased after exercise in the normotensive patients (8.4 vs. 11.4 pmol/l (median), p less than 0.05) and control subjects (7.6 vs. 9.3 pmol/l, p less than 0.02) but not in the hypertensives (7.6 vs. 7.9 pmol/l, p greater than 0.10), and after exercise ANP was increased in the normotensive patients compared with the controls (p less than 0.02). After exercise, an enhanced increase of Aldo was found in the hypertensives but not in the normotensive patients compared with the controls, whereas the increase of AII did not differ significantly between the groups. It is concluded that patients with chronic glomerulonephritis and relatively well preserved renal function do not have major abnormalities of ANP at rest or during exercise. In the normotensive patients, however, ANP increased to a higher level than in the controls, but the difference was small and further studies are needed to define the role of ANP in blood pressure regulation of early stage chronic glomerulonephritis.

Atrial natriuretic factor inhibits ACTH stimulated aldosterone, but not cortisol, secretion in man

The effects of human ANF 99-126 on the aldosterone and cortisol responses to ACTH infusion were studied in 8 normal volunteers. ACTH infusion caused a significant rise in aldosterone and cortisol on each study day. On the day that ANF was concomitantly infused the aldosterone, but not the cortisol, response to ACTH was significantly attenuated. These results show that a pharmacological dose of ANF selectively inhibits ACTH mediated mineralocorticoid as opposed to glucocorticoid release in man. These results support in vitro and in vivo findings from animal experiments. These findings also compliment previous studies showing that ANF inhibits ANG II stimulated aldosterone release in normal subjects.

Prolonged administration of human atrial natriuretic peptide in healthy men: evanescent effect on diuresis in spite of simultaneous infusion of norepinephrine

In healthy, sodium and fluid replete men (n = 6) a transient increase in diuresis is seen during the prolonged infusion of human atrial natriuretic peptide (hANP: 50 micrograms h-1 for 6 h). In order to evaluate whether this evanescent diuretic effect of hANP is due to the peptide's hypotensive action, the latter was compensated for by the concomitant infusion of norepinephrine (NE: 50 ng kg-1 min-1). The decrease in hANP-induced diuresis towards the end of the 6-h infusion was not prevented by the additional infusion of norepinephrine, which also failed to influence hANP-stimulated natriuresis. Plasma concentrations of hANP, which were continuously elevated to about double of basal concentrations during the infusion of hANP, were not affected by exogenous norepinephrine. These data demonstrate that the transient character of hANP-induced diuresis cannot be offset by counterbalancing the peptide's hypotensive effect.

An update on the physiology of atrial natriuretic factor

The atrial natriuretic factor (ANF) has pharmacological actions resulting in lower atrial and arterial pressures. Atrial distention stimulates ANF release, suggesting that ANF is an effector limb of a feedback loop for controlling cardiac filling pressure. To test this hypothesis it will be necessary to determine whether physiological atrial distention releases ANF in sufficient amounts to exert biological actions. Immunoblockade of endogenous ANF and attenuation of ANF release by atrial ablation inhibited volume-induced natriuresis in rats. Infusion of ANF in rats at doses mimicking those observed during experimental volume expansion produced a natriuresis sufficient to partly account for the volume-induced response. Infusion of ANF at doses expected to change plasma ANF levels minimally decreased arterial pressure in hypertensive rats over 7 days. In dogs, some studies suggest that increased plasma ANF levels following experimental changes in atrial pressure were not sufficient to exert acute cardiovascular or renal actions, whereas others support such a notion and indicate that ANF inhibited barostimulated renal renin release. This last action could alter arterial pressure in the long term by allowing sodium equilibrium at lower renal arterial pressure. Infusion of ANF in humans that produced plasma levels in the upper physiological range caused increased sodium excretion and decreased plasma renin activity. Although data are exiguous, justifying neither acceptance nor rejection of the hypothesis that ANF functions physiologically to regulate body fluid volume and arterial pressure, the current evidence slightly favors acceptance.