Effects of changes in water-sodium balance on levels of atrial natriuretic factor messenger RNA and peptide in rats

Responses of atrial mRNA, atrial peptide and plasma peptide of atrial natriuretic factor (ANF) to treatments to alter fluid volume were studied in rats using RNA dot hybridization assay and radioimmunoassay. Specific changes in the level of ANF mRNA relative to total atrial RNA were observed in atria from sodium restricted rats and water deprived then sodium loaded rats, demonstrating an association of change in water-sodium balance with the expression of ANF gene. The levels of mRNA and the immunoreactive ANF in plasma decreased to 30% and 15% of controls, respectively, on water-deprivation and then increased again to control levels after administering 1.8% NaCl solution, whereas atrial immunoreactive ANF increased to about twice the control on water-deprivation and decreased again after supplying NaCl solution, in parallel with the level of the hematocrit. These findings suggest that atrial ANF content is dependent more on ANF release than on biosynthesis.     

Atrial natriuretic peptide in hypoxia

A growing number of mammalian genes whose expression is inducible by hypoxia have been identified. Among them, atrial natriuretic peptide (ANP) synthesis and secretion is increased during hypoxic exposure and plays an important role in the normal adaptation to hypoxia and in the pathogenesis of cardiopulmonary diseases, including chronic hypoxia-induced pulmonary hypertension and vascular remodeling, and right ventricular hypertrophy and right heart failure. This review discusses the roles of ANP and its receptors in hypoxia-induced pulmonary hypertension. We and other investigators have demonstrated that ANP gene expression is enhanced by exposure to hypoxia and that the ANP so generated protects against the development of hypoxic pulmonary hypertension. Results also show that hypoxia directly stimulates ANP gene expression and ANP release in cardiac myocytes in vitro. Several cis-responsive elements of the ANP promoter are involved in the response to changes in oxygen tension. Further, the ANP clearance receptor NPR-C, but not the biological active NPR-A and NPR-B receptors, is downregulated in hypoxia adapted lung. Hypoxia-sensitive tyrosine kinase receptor-associated growth factors, including fibroblast growth factor (FGF) and platelet derived growth factor (PDGF)-BB, but not hypoxia per se, inhibit NPR-C gene expression in pulmonary arterial smooth muscle cells in vitro. The reductions in NPR-C in the hypoxic lung retard the clearance of ANP and allow more ANP to bind to biological active NPR-A and NPR-B in the pulmonary circulation, relaxing preconstricted pulmonary vessels, reducing pulmonary arterial pressure, and attenuating the development of hypoxia-induced pulmonary hypertension and vascular remodeling.     

Atrial natriuretic peptide lowers pulmonary arterial pressure in hypoxia-adapted rats

To test the hypothesis that atrial natriuretic peptide (ANP) has a direct vasodilator effect on the pulmonary vasculature that is enhanced in hypoxia-induced pulmonary hypertension in the rat, we determined the effects of ANP on mean pulmonary (MPAP) and systemic arterial pressure (MSAP) in intact conscious Sprague-Dawley rats exposed to 10% O2 or room air for 4 wk. Catheters were placed in the pulmonary artery through the right jugular vein by means of a closed-chest technique. MPAP and MSAP were monitored before and after intravenous injections of graded doses of ANP. ANP produced dose-related decreases in MPAP that were greater in the hypoxic group than in air controls. There were no significant between-group differences in the systemic depressor responses to ANP or in the ANP-induced reduction in cardiac output. ANP lowered MPAP significantly in isolated perfused lungs from both hypoxia-adapted and air control rats, and this effect was significantly greater in the hypoxic than the air control lungs. These data indicate that ANP lowers pulmonary arterial pressure in rats with hypoxia-induced pulmonary hypertension, mainly by a direct vasodilator effect on the pulmonary vasculature.     

Atrial natriuretic factor messenger ribonucleic acid and peptide in the human heart during ontogenic development

We have investigated the level of expression of the atrial natriuretic factor (ANF) gene in the human heart during ontogenic development by determining the concentrations of ANF messenger ribonucleic acid (ANF mRNA), of immunoreactive ANF (IR ANF) and of receptor reactive ANF (RR ANF), in myocardial samples of the various heart chambers. We found the level was high and almost identical in the left and right ventricles in utero. It gradually decreased during ontogenic development to reach the low adult levels, with a more rapid decrease in the right than in the left ventricle after birth. In the atria, ANF gene expression was high as early as the 13th week of gestation, was higher in the right than in the left atrium, and appeared little affected by ontogenic development.     

Atrial natriuretic peptide in acute hypoxia-induced pulmonary hypertension in rats.

To test the hypothesis that exogenous atrial natriuretic peptide (ANP) prevents the acute pulmonary pressor response to hypoxia, ANP (20-micrograms/kg bolus followed by 1-microgram.kg-1.min-1 infusion) or vehicle was administered intravenously to conscious rats beginning 3 min before exposure to hypoxia or room air for 90 min. Exogenous ANP abolished the acute pulmonary pressor response to hypoxia in association with marked and parallel increases in plasma ANP and guanosine 5'-cyclic monophosphate (cGMP) and with a significant increase in lung cGMP content. To examine whether endogenous ANP modulates the acute pulmonary pressor response to hypoxia, rats were pretreated with a monoclonal antibody (Ab) to ANP and exposed to hypoxia. Mean pulmonary arterial pressure (MPAP) in the Ab-treated rats was not different from control over the first 6 h of hypoxic exposure. Thereafter, the Ab-treated group had significantly higher MPAP than control. Our data suggest that 1) exogenous ANP blocks the pulmonary pressor response to acute hypoxia via stimulation of cGMP accumulation in the pulmonary vasculature, and 2) endogenous ANP may modulate the subacute, but not acute, phase of hypoxic pulmonary hypertension.     

Atrial natriuretic peptide in the sheep

To study atrial natriuretic peptide (ANP) physiology in the chronically catheterized pregnant sheep model we developed a heterologous radioimmunoassay for ovine ANP using an antiserum raised against 1-28 human ANP. This antiserum (Tor I) is specific for the aminoterminus of the human ANP molecule and shows little cross reaction with any carboxyterminus ANP fragments. Ovine ANP immunoreactivity was characterized using this antiserum and a commercially available carboxyterminus ANP antiserum obtained from Peninsula Laboratories. Each antiserum detected 2 peaks of immunoreactivity in ovine atrial extracts chromatographed on a Biogel P-10 column. The minor peak migrated at a position close to 125I-human ANP whereas the major peak represented a larger molecular weight species of ANP. Examination of gel filtration eluates of ovine plasma extracts showed one immunoreactive ANP peak using the Tor I assay system and 2 peaks with the Peninsula Laboratories assay. Plasma immunoreactive ANP levels were determined in 9 sheep using both radioimmunoassay systems. Mean (+/- SEM) levels were similar using the Peninsula Laboratories and the Tor I assay systems (57 +/- 8 pg/ml versus 43 +/- 4 pg/ml, P greater than 0.05). Using the Tor I antiserum, fetal plasma immunoreactive ANP levels were found to be significantly higher than maternal levels (188 +/- 17 versus 48 +/- 8 pg/ml, P less than 0.01) whereas pregnant and nonpregnant adult sheep had similar plasma immunoreactive ANP levels (48 +/- 8 versus 43 +/- 4 pg/ml, P greater than 0.05). Disappearance curves of synthetic human ANP from the plasma of maternal and fetal sheep were assessed using both immunoassay systems and found to be similar.     

Atrial natriuretic peptide induces natriuretic peptide receptor-cGMP-dependent protein kinase interaction

Circulating natriuretic peptides such as atrial natriuretic peptide (ANP) counterbalance the effects of hypertension and inhibit cardiac hypertrophy by activating cGMP-dependent protein kinase (PKG). Natriuretic peptide binding to type I receptors (NPRA and NPRB) activates their intrinsic guanylyl cyclase activity, resulting in a rapid increase in cytosolic cGMP that subsequently activates PKG. Phosphorylation of the receptor by an unknown serine/threonine kinase is required before ligand binding can activate the cyclase. While searching for downstream PKG partners using a yeast two-hybrid screen of a human heart cDNA library, we unexpectedly found an upstream association with NPRA. PKG is a serine/threonine kinase capable of phosphorylating NPRA in vitro; however, regulation of NPRA by PKG has not been previously reported. Here we show that PKG is recruited to the plasma membrane following ANP treatment, an effect that can be blocked by pharmacological inhibition of PKG activation. Furthermore, PKG participates in a ligand-dependent gain-of-function loop that significantly increases the intrinsic cyclase activity of the receptor. PKG translocation is ANP-dependent but not nitric oxide-dependent. Our results suggest that anchoring of PKG to NPRA is a key event after ligand binding that determines distal effects. As such, the NPRA-PKG association may represent a novel mechanism for compartmentation of cGMP-mediated signaling and regulation of receptor sensitivity.     

Atrial natriuretic peptide in infants and children

Atrial natriuretic peptide (ANP) was measured in the plasma of 192 normal infants and children aged 1 day to 18 years. Plasma ANP was high during postnatal adaptation, particularly in premature infants. In 96 infants and children aged 4 months to 18 years, plasma ANP was similar to values obtained in 7 healthy adult volunteers (23.9 +/- 11.9 vs. 25.7 +/- 4.6 fmol/ml). There was no significant relationship between ANP and age. ANP is elevated about twofold in full-term neonates being 3-4 days of age, and returned to normal thereafter. It is concluded that ANP is raised during the postnatal adaptation. This hormone is possibly involved in the postnatal volume contraction and may antagonize vasoconstrictor hormones that are elevated during the postnatal period.     

Endothelin stimulates accumulations of cellular atrial natriuretic peptide and its messenger RNA in rat cardiocytes

The effect of endothelin-1 (ET-1) on secretion and synthesis of rat atrial natriuretic peptide (rANP) as well as its mRNA levels was studied in primary cultures of neonatal rat atrial cardiocytes. ET-1 dose-dependently (10(-10)-10(-7) M) increased media and cellular rANP-like immunoreactivity as well as its cytoplasmic mRNA levels in rat cardiocytes during 24 hrs incubation. These results suggest that ET-1 directly stimulates expression of the rANP gene in cardiocytes, thereby leading to enhanced synthesis and secretion of rANP.     

Atrial natriuretic peptide clearance receptor agonist lowers pulmonary pressure in hypoxic rats

We demonstrated previously that intravenous administration of exogenous atrial natriuretic peptide (ANP) lowers mean pulmonary arterial pressure (MPAP) in hypoxia-adapted rats. To test the hypothesis that endogenous ANP may also lower MPAP in this model, C-ANP-(4-23), a ring-deleted analogue of ANP that binds to the biologically silent ANP clearance receptor (C-ANP receptor) but not to the ANP biological receptor (B-ANP receptor), was administered intravenously as a bolus injection (10 micrograms/kg) followed by an infusion (1 micrograms.kg-1.min-1 for 60 min) to rats adapted to hypoxia (10% O2) for 4 wk and to air control rats. C-ANP-(4-23) significantly lowered MPAP in hypoxic rats but not in air controls. A statistically insignificant reduction in mean systemic arterial pressure was found in both groups after C-ANP-(4-23) administration. C-ANP-(4-23) significantly (two- to threefold) increased endogenous plasma ANP levels in both groups; the increase was not significantly different between groups. Both basal and post-C-ANP-(4-23) levels of plasma ANP were greater in hypoxia-adapted animals than in air controls; the C-ANP-induced increase in plasma ANP was not significantly different between groups. These results suggest that the endogenous ANP may modulate pulmonary vascular tone in rats with hypoxic pulmonary hypertension.     

Atrial natriuretic peptide inhibits iodide uptake and thyroglobulin messenger ribonucleic acid expression in cultured bovine thyroid follicles

The involvement of atrial natriuretic peptide (ANP) in the regulation of thyroid gland is supported by the presence of high-affinity ANP receptors and the identification of the peptide in thyroid follicular cells. The aim of this work was to study the action of ANP on parameters of thyroid hormone biosynthesis and analyze the intracellular mechanism of the ANP action in cultured bovine thyroid follicles. The addition of ANP (0.1-10 nM) to the culture medium for 24 h inhibited the TSH (thyroid-stimulating hormone)-stimulated iodide uptake with a maximal inhibition at 1 nM ANP. When thyrocytes were incubated with 10 nM ANP the inhibitory effect slightly increased from 24 to 72 h. Thyroglobulin (Tg) mRNA expression was reduced by 1 and 10 nM ANP. After 24 h of treatment with the cGMP analogue, N(2),2'-O-dibutyrylguanosine 3':5'-cyclic monophosphate [(Bu)(2)cGMP] (0.1 and 1 mM), an inhibition of iodide uptake and Tg mRNA expression was obtained, evidencing a cGMP-mediated inhibitory signal in the thyroid cell. A reduction of the cAMP production was induced by incubation of thyroid follicles with 1 and 10 nM ANP for 24 h. Under a similar treatment the cGMP accumulation was increased only by 10 nM ANP. The inhibitory effect of ANP on Tg mRNA level was reverted in the presence of pertussis toxin, an inhibitor of the G(i)-protein-mediated reduction of the adenylate cyclase activity. These results indicate an inhibitory action of ANP on parameters of thyroid hormone biosynthesis. A G(i)-protein-mediated reduction of the cAMP production seems to be the main factor involved in the ANP action although a role of the cGMP pathway should not be discarded specially at high ANP levels.     

Atrial natriuretic peptide in the heart and pancreas

We used antisera to pure atrial natriuretic peptide to localise this peptide by immunocytochemistry in rat and human tissue. We showed that both rat and human atrial cardiocytes gave a positive reaction while ventricular cardiocytes were consistently negative. Peripheral islet cells in rat but not in human pancreas also showed positive staining for ANP. We showed by double labelling techniques that the ANP was present in the glucagon containing cells.     

Atrial natriuretic peptide and vasopressin in human plasma

Using a specific radioimmunoassay for atrial natriuretic peptide (ANP), plasma immunoreactive ANP was measured in 17 normal subjects and 83 patients with various diseases. Plasma ANP concentration in normal subjects was 14.1 +/- 1.7 pg/ml (mean +/- S.E.). Relatively high plasma ANP concentrations were detected in patients with diabetes mellitus, hyperthyroidism, atrial fibrillation and liver cirrhosis. Plasma ANP concentrations in the patients correlated positively with mean arterial blood pressure and plasma AVP concentrations. Plasma ANP concentrations in the patients also had positive correlations with left atrial dimension and left ventricular diastolic dimension determined by echocardiography. Another positive correlation was observed in the patients between plasma AVP concentrations and mean arterial blood pressure. These results suggest that ANP is a volume regulatory hormone but also that ANP may be involved in the blood pressure regulating system.     

Atrial natriuretic peptide concentrations in pre-eclampsia.

The concentration of plasma immunoreactive atrial natriuretic peptide is positively associated with right atrial and pulmonary capillary wedge pressure, suggesting that blood volume and hence atrial pressure govern its release. Expansion of plasma volume is a central physiological adjustment in normal pregnancy. Conversely, pregnancies complicated by pre-eclampsia are associated with a reduction in plasma volume and central venous pressure. A study was therefore undertaken to test the hypothesis that plasma atrial natriuretic peptide concentrations are low in pre-eclampsia owing to deficient secretion. Concentrations of the peptide were measured by a specific radioimmunoassay. The mean plasma immunoreactive atrial natriuretic peptide concentration in healthy pregnant women (n = 22; third trimester) was higher (56 (1 SD 29) ng/l) than in 25 young, non-pregnant controls (37 (19) ng/l). Concentrations in patients suffering from mild pre-eclampsia (n = 9) were higher (127 (60) ng/l) than in normal pregnant women, and in patients with severe pre-eclampsia (n = 6) concentrations were higher still (392 (225) ng/l). Despite failure of plasma volume expansion and low central venous and pulmonary capillary wedge pressures in pre-eclampsia this condition is associated with greatly increased plasma concentrations of plasma immunoreactive atrial natriuretic peptide, which increase still further with the severity of the disease. These findings are clear evidence that atrial pressure may not be the principal determinant of the release of the natriuretic peptide in pre-eclampsia.     

Atrial natriuretic peptide in acute mountain sickness

To test the hypothesis that elevated atrial natriuretic peptide (ANP) may be involved in altered fluid homeostasis at high altitude, we examined 25 mountaineers at an altitude of 550 m and 6, 18, and 42 h after arrival at an altitude of 4,559 m, which was climbed in 24 h starting from 3,220 m. In 14 subjects, symptoms of acute mountain sickness (AMS) were absent or mild (group A), whereas 11 subjects had severe AMS (group B). Fluid intake was similar in both groups. In group B, urine flow decreased from 61 +/- 8 (base line) to 36 +/- 3 (SE) ml/h (maximal decrease) (P less than 0.05) and sodium excretion from 7.9 +/- 0.9 to 4.6 +/- 0.7) mmol.l-1.h-1 (P less than 0.05); ANP increased from 31 +/- 4 to 87 +/- 26 pmol/l (P less than 0.001), plasma aldosterone from 191 +/- 27 to 283 +/- 55 pmol/l (P less than 0.01 compared with group A), and antidiuretic hormone (ADH) from 1.0 +/- 0.1 to 2.9 +/- 1.2 pmol/l (P = 0.08 compared with group A). These variables did not change significantly in group A, with the exception of a decrease in plasma aldosterone from 189 +/- 19 to 111 +/- 17 pmol/l (P less than 0.01). There were no measurable effects of elevated ANP on natriuresis, cortisol, or blood pressure. The reduced diuresis in AMS may be explained by increased plasma aldosterone and ADH overriding the expected renal action of ANP. The significance of elevated ANP in AMS remains to be established.(ABSTRACT TRUNCATED AT 250 WORDS)