Atriopeptin II lowers cardiac output in conscious sheep

ANP in regulation of arterial pressure and fluid-electrolyte balance: lessons from genetic mouse models

The recent development of genetic mouse models presenting life-long alterations in expression of the genes for atrial natriuretic peptide (ANP) or its receptors (NPR-A, NPR-C) has uncovered a physiological role of this hormone in chronic blood pressure homeostasis. Transgenic mice overexpressing a transthyretin-ANP fusion gene are hypotensive relative to the nontransgenic littermates, whereas mice harboring functional disruptions of the ANP or NPR-A genes are hypertensive compared with their respective wild-type counterparts. The chronic hypotensive action of ANP is determined by vasodilation of the resistance vasculature, which is probably mediated by attenuation of vascular sympathetic tone at one or several prejunctional sites. Under conditions of normal dietary salt consumption, the hypotensive action of ANP is dissociated from the natriuretic activity of the hormone. However, during elevated dietary salt intake, ANP-mediated antagonism of the renin-angiotensin system is essential for maintenance of blood pressure constancy, inasmuch as the ANP gene "knockout" mice (ANP -/-) develop a salt-sensitive component of hypertension in association with failure to adequately downregulate plasma renin activity. These findings imply that genetic deficiencies in ANP or natriuretic receptor activity may be underlying causative factors in the etiology of salt-sensitive variants of hypertensive disease and other sodium-retaining disorders, such as congestive heart failure and cirrhosis.

Effects of atrial natriuretic peptide on left ventricular function in hypertension

Atrial natriuretic peptide (ANP) has natriuretic and vasodilator actions that lower arterial pressure and may be beneficial to hypertensive patients. To assess the effects of ANP on left ventricular function in patients with hypertension, we compared it with the pure vasodilator nitroprusside. Simultaneous left ventricular micromanometer pressure and radionuclide volume were obtained at baseline, during nitroprusside infusion, during a second baseline period, and during ANP infusion in 10 patients with hypertension. Mean arterial pressure fell during ANP and nitroprusside. Heart rate and plasma norepinephrine levels increased by similar amounts during the two agents, whereas cardiac index and stroke volume index were unchanged during both. Peak positive left ventricular dP/dt fell similarly during ANP and nitroprusside, but left ventricular dP/dt at a developed pressure of 40 mm Hg, a less load-dependent index of contractility, was unchanged during both. The relation between end-systolic pressure and volume during ANP infusion was not shifted leftward or rightward from that during nitroprusside infusion, indicating no inotropic effect. Both ANP and nitroprusside shortened at time constant of isovolumic relaxation calculated by the logarithmic method but did not change the time constant calculated by the derivative method. Peak filling rate was unchanged from baseline during both agents. ANP did not shift the end-diastolic pressure-volume point away from the relation constructed from baseline and nitroprusside points. We conclude that ANP has no direct effect on myocardial contractile or diastolic function in patients with hypertension.

Sympathectomy fails to reveal prominent vasodilation by atrial natriuretic factor

Reflex activation of the sympathetic nervous system may conceal direct vasodilatory actions of atrial natriuretic factor and mediate atrial natriuretic factor-induced increases in total peripheral resistance. We determined whether peripheral sympathectomy would enhance the hypotensive actions of atrial natriuretic factor and convert the increase in total peripheral resistance to peripheral vasodilation. Sympathectomized rats studied included 1) conscious rats treated with 6-hydroxydopamine alone (partially sympathectomized) and 2) conscious anephric rats sympathectomized with adrenal demedullation and 6-hydroxydopamine (totally sympathectomized), with vascular tone returned to levels of sham-operated (control) rats with norepinephrine infusion. Sympathectomized rats and appropriate control rats received rat atrial natriuretic factor infusion (0.5 microgram/kg/min) or vehicle for 1 hour. Atrial natriuretic factor infusion lowered mean arterial pressure and increased hematocrit in control rats but not in partially sympathectomized rats. Changes in cardiac output and total peripheral resistance were not significantly different between control and partially sympathectomized rats. In totally sympathectomized rats, atrial natriuretic factor lowered mean arterial pressure more than in control rats; changes in cardiac output were nearly identical in both groups, but there were no changes in total peripheral resistance from control levels in the totally sympathectomized group. Changes in plasma volume and central venous pressure were similar in totally sympathectomized rats and control rats. These findings suggest that reflex sympathetic activity largely mediated atrial natriuretic factor-induced increases in total peripheral resistance but failed to reveal an atrial natriuretic factor-mediated sustained vasodilation in the absence of sympathetic reflexes. Furthermore, atrial natriuretic factor decreased cardiac output, central venous pressure, and plasma volume independent of the sympathetic nervous system.

Atrial natriuretic peptide attenuates the development of pulmonary hypertension in rats adapted to chronic hypoxia

To test the hypothesis that chronic infusion of atrial natriuretic peptide (ANP) instituted before hypoxic exposure attenuates the development of pulmonary hypertension in hypoxia adapted rats, ANP (0.2 and 1.0 microgram/h) or vehicle was administered intravenously via osmotic minipump for 4 wk beginning before exposure to 10% O2 or to room air. Low dose ANP increased plasma ANP levels by only 60% of vehicle controls after 4 wk and significantly decreased mean pulmonary arterial pressure (MPAP) (P less than 0.01), the ratio of right ventricular weight to body weight (RV/BW) (P less than 0.01), and the wall thickness of small (50-100 microns) pulmonary arteries (P = 0.01) in hypoxia-adapted rats. ANP did not alter any of these parameters in air-control rats. High dose ANP increased plasma ANP levels by 230% of control and produced greater reductions in MPAP (P less than 0.001) and RV/BW) (P less than 0.05), but not in pulmonary arterial wall thickness, than the low dose. Neither dose of ANP altered mean systemic arterial pressure in either hypoxic or normoxic rats. The data demonstrate that chronic infusion of exogenous ANP at a dose that does not affect MPAP or RV weight in air-control rats attenuates the development of pulmonary hypertension and RV enlargement in rats adapted to chronic hypoxia.

Atrial natriuretic factor in chronic obstructive lung disease with pulmonary hypertension. Physiological correlates and response to peptide infusion

To investigate the physiological role of atrial natriuretic factor (ANF) in patients with hypoxic pulmonary hypertension secondary to chronic obstructive lung disease (COLD), we infused synthetic alpha-human ANF in seven such patients, and investigated the physiological correlates to circulating peptide levels in 24 patients with COLD. ANF infusion, at incremental rates of 0.01, 0.03, and 0.1 micrograms/kg.min, increased basal plasma immunoreactive (ir) ANF (136 +/- 38 pg/ml) by 3-, 10-, and 26-fold, respectively, and reduced pulmonary artery pressure (from 33 +/- 3 to 25 +/- 2 mmHg, P less than 0.001) and systemic arterial pressure (from 88 +/- 4 to 79 +/- 4 mmHg, P less than 0.001) in a dose-related fashion. Cardiac index increased by 13.5% (P less than 0.01) while heart rate was unchanged. Cardiac filling pressures decreased at 0.1 micrograms/kg.min ANF. Pulmonary and systemic vascular resistance fell by 37% (P less than 0.001) and 19% (P less than 0.001), respectively. Arterial oxygenation was impaired during ANF infusion, suggesting partial reversal of hypoxic pulmonary vasoconstriction. Plasma renin activity remained unchanged but aldosterone fell by 44% (P less than 0.01). The levels of plasma irANF in 24 patients correlated directly with the degree of hemoconcentration (r = 0.67, P less than 0.001), respiratory acidosis (r = -0.65, P less than 0.001), and pulmonary hypertension (r = 0.52, P less than 0.01). The results suggest that ANF may serve as a potent pulmonary vasodilator involved in the circulatory homeostasis of patients with COLD.

Vascular responses to local atrial natriuretic peptide infusion in man

1. The effect on skin and muscle blood flow of arterial infusion of atrial natriuretic peptide (ANP) directly into the forearm circulation, and on venous tone of direct infusion into a dorsal hand vein, was studied in normal subjects. 2. ANP produced a dose-dependent increase in both skin and muscle blood flow, but at equivalent doses, produced no dilatation of noradrenaline-preconstricted dorsal hand veins. These findings indicate that ANP acting locally, is an arterioselective dilator in the upper limb circulation in normal man. 3. Measurements of ANP in venous plasma during arterial infusion suggest marked clearance of ANP across the forearm vascular bed. Such peripheral clearance may, at least in part, account for the short plasma half-life of this peptide. 4. The lowest dose of ANP infused was calculated to produce plasma levels similar to those found in patients with heart failure. The findings with this dose suggest that, in heart failure, circulating levels of ANP may be within a range capable of influencing peripheral vascular resistance directly.

Systemic and regional vascular effects of atrial natriuretic peptide in a rat model of chronic heart failure

To characterize the systemic and regional vascular effects of atrial natriuretic peptide (ANP) in chronic heart failure, central hemodynamics, regional blood flow and plasma ANP levels were determined in a rat model of myocardial infarction and failure and in sham-operated animals. Measurements were made in the conscious state before and after intravenous rANP [99-126] (8 micrograms bolus followed by continuous infusion of 1.0 microgram/kg/min). With this protocol, ANP significantly decreased cardiac output, right atrial, left ventricular end-diastolic and arterial pressures and there were increases in heart rate, systemic and intestinal vascular resistances in sham animals. Renal blood flow per gram of tissue was unchanged with ANP, but when expressed as a percentage of cardiac output, increased significantly, indicating a preferential renal vasodilatory effect of ANP. In rats with infarction and failure, this dose did not alter cardiac output or arterial pressure, but decreased right atrial and left ventricular blood flow. Although significantly reduced as compared to the control group, renal blood flow was not improved with ANP in the heart failure group. ANP plasma levels of the heart failure group were elevated at baseline (p less than 0.01), and increased 5-10 times after infusion of rANP. Thus, in rats with chronic heart failure, the renal vascular effects of ANP are blunted, which may, in part, explain the failure of ANP to restore the altered volume homeostasis in heart failure despite elevated ANP plasma levels. However, the effects on venous return were preserved which, in turn, improved cardiac performance via a reduction of preload.

Effect of atrial natriuretic factor on transmural myocardial blood flow distribution in the dog

These studies were designed to define the effect of atrial natriuretic peptide (ANP) on coronary flow. ANP was infused as a bolus directly into the left circumflex coronary artery in doses ranging from 0.05 to 5 micrograms in nine open-chest, anesthetized dogs. Coronary flow was measured with an electromagnetic flowmeter. Regional transmural myocardial blood flow and distribution were measured with 11.3 +/- 0.25 micron radionuclide-labeled microspheres. No significant change was noted in systemic hemodynamics (heart rate, arterial pressure, left atrial pressure, or cardiac output) during the course of the studies. ANP produced a transient vasodilatation of coronary resistance vessels and increased flow by 41% after both the 2.5 and 5 micrograms doses. The vasodilatation occurred uniformly throughout the ventricular wall so that the endocardial/epicardial flow ratio remained constant. There was no evidence of coronary vasoconstriction. The peak vasodilatation response occurred 28 +/- 7 sec after the beginning of the infusion of ANP and lasted approximately 3 min. These data support the hypothesis that ANP administration is associated with a vasodilator response in the coronary resistance vessels that may be modulated through either the release of another vasodilator substance or another mechanism.

Coronary vasoconstrictor effects of atriopeptin II

Atrial natriuretic peptides lower arterial pressure, cardiac filling pressure, and cardiac output. In isolated, Langendorff-perfused guinea pig hearts, atriopeptin II, the 23-amino acid atrial natriuretic peptide, is also a potent coronary vasoconstrictor. The median effective dose for atriopeptin II in guinea pig hearts is 26 nanomoles, the threshold constrictor dose is 5 nanomoles, and flow nearly ceases at a dose of 100 nanomoles in perfused hearts at constant pressure. Similar concentrations of atriopeptin II also cause coronary vasoconstriction in rat and dog heart preparations. The disulfide bridge is necessary for vasoconstrictor activity; reduction of this bridge abolishes the activity, as it does the other biological activities of atrial natriuretic peptides.