Corticotropin-releasing activity of alpha-melanotropin.

Synthetic alpha-melanotropin stimulated the release of immunoreactive adrenocorticotropin from primary cultures of rat anterior pituitary cells. The effect of the alpha-melanotropin was dose-dependent. Cells incubated with synthetic arginine-vasopressin and alpha-melanotropin simultaneously produced an amount of adrenocorticotropin that was greater than the sum of the amount that the cells produced in response to each peptide added separately. Other peptides structurally similar to alpha-melanotropin, such as, beta-, gamma 1-, gamma 2-, and gamma 3-melanotropin, were also tested for adrenocorticotropin-releasing activity. Only the gamma 3-melanotropin demonstrated a statistically significant effect. A vasopressin preparation (Pitressin, Parke-Davis) purified from posterior pituitaries and previously shown to contain some alpha-melanotropin was much more potent in releasing adrenocorticotropin than the synthetic vasopressin.

Control of left ventricular mass by moxonidine involves reduced DNA synthesis and enhanced DNA fragmentation.

BACKGROUND AND PURPOSE: Left ventricular hypertrophy (LVH) is a maladaptive process associated with increased cardiovascular risk. Regression of LVH is associated with reduced complications of hypertension. Moxonidine is an antihypertensive imidazoline compound that reduces blood pressure primarily by central inhibition of sympathetic outflow and by direct actions on the heart to release atrial natriuretic peptide, a vasodilator and an antihypertrophic cardiac hormone. This study investigated the effect of moxonidine on LVH and the mechanisms involved in this effect. EXPERIMENTAL APPROACH: Spontaneously hypertensive rats were treated with several doses of moxonidine (s.c.) over 4 weeks. Blood pressure and heart rate were continuously monitored by telemetry. Body weight and water and food intake were measured weekly. Measurements also included left ventricular mass, DNA content, synthesis, fragmentation, and apoptotic/anti-apoptotic pathway proteins. KEY RESULTS: The decrease in mean arterial pressure stabilized at approximately -10 mm Hg after 1 week of treatment and thereafter. Compared to vehicle-treated rats (100%), left ventricular mass was dose- and time-dependently reduced by treatment. This reduction remained significantly lower after normalizing to body weight. Moxonidine reduced left ventricular DNA content and inhibited DNA synthesis. DNA fragmentation transiently, but significantly increased at 1 week of moxonidine treatment and was paralleled by elevated active caspase-3 protein. The highest dose significantly decreased the apoptotic protein Bax and all doses stimulated anti-apoptotic Bcl-2 after 4 weeks of treatment. CONCLUSIONS AND IMPLICATIONS: These studies implicate the modulation of cardiac DNA dynamics in the control of left ventricular mass by moxonidine in a rat model of hypertension.

Lung is an important source of atrial natriuretic factor in experimental cardiomyopathy.

The regulation of water and electrolyte homeostasis is multifactorial and includes the heart and kidneys as important regulatory centers. Within the heart, a recently discovered hormone, atrial natriuretic factor (ANF), has been implicated in the maintenance of water and salt balance. Primarily found in mammalian atria, ANF has been detected in low amounts in several tissues, including lungs. A disorder of the ANF system has been demonstrated in genetically cardiomyopathic hamsters, a model for human congestive cardiomyopathy. Atrial ANF gene expression and storage are decreased during development of this disease, while paradoxically, circulating levels of ANF are increased. We have hypothesized that an extracardiac source may contribute to ANF production in these pathological conditions. In this paper we provide evidence that ANF synthesis is stimulated in the lungs of hamsters during development of cardiomyopathy as revealed by increased ANF mRNA and peptide levels. Furthermore, we show that ANF synthesized in lungs is secreted and has identical chromatographic and biological properties to circulating ANF. The increased production of ANF in lungs may be physiologically important in preventing pulmonary edema. Alternatively, during cardiac dysfunction, lungs may play a compensatory role by increasing their contribution to plasma ANF levels.

Atrial natriuretic factor during atrial fibrillation and supraventricular tachycardia.

Plasma immunoreactive atrial natriuretic factor was measured in 10 patients with chronic atrial fibrillation before and after cardioversion to sinus rhythm, and in 14 patients during electrophysiologic evaluation of paroxysmal supraventricular tachycardia. The mean plasma concentration of atrial natriuretic factor in atrial fibrillation was 138 +/- 48 pg/ml and decreased to 116 +/- 45 pg/ml 1 hour after cardioversion to sinus rhythm (p less than 0.005). The mean plasma concentration of atrial natriuretic factor increased from 117 +/- 53 pg/ml in sinus rhythm to 251 +/- 137 pg/ml during laboratory-induced supraventricular tachycardia (p less than 0.005). Right atrial pressures were recorded in 12 patients; the baseline atrial pressure was 4.3 +/- 1.9 mm Hg and increased to 7.4 +/- 3.6 mm Hg during supraventricular tachycardia (p less than 0.005). A modest but significant linear relation was noted between the changes in plasma atrial natriuretic factor and right atrial pressure measurements during induced supraventricular tachycardia (r = 0.60, p less than 0.05). In conclusion, changes in atrial rhythm and pressure may be an important factor modulating the release of atrial natriuretic factor in the circulation and raised levels of this hormone may be a contributing factor for the polyuria and the hypotension associated with paroxysmal supraventricular tachyarrhythmias.

Pregnancy alters nitric oxide synthase and natriuretic peptide systems in the rat left ventricle.

Cyclic guanosine monophosphate (cGMP), which is implicated in cardiac cell growth and function, is synthesized by cytoplasmic soluble guanylyl cyclase (GC) stimulated via nitric oxide (NO) and by particulate membrane-bound GC activated via natriuretic peptides. We investigated possible cGMP elevation in the left ventricle (LV) of rats developing physiologic LV hypertrophy during gestation. Furthermore, expression of estrogen receptors (ER) and oxytocin receptors (OTR) was evaluated because their activation stimulates NO and atrial natriuretic peptide (ANP) release from the heart. Compared with nonpregnant controls, Sprague-Dawley rats on day 7 of gestation had similar heart weights, but, on days 14 and 21, ventricular mass increased by 12% and 28% respectively (P< 0.05). LV cGMP concentration was elevated at day 14 of gestation (3.25 +/- 0.12 vs 4.65 +/- 0.17 pmol/g wet weight, P< 0.01) but decreased at day 21 (2.45 +/- 0.09 pmol/g, P< 0.05) to increase again on postpartum day 1 (6.01 +/- 0.15 pmol/g) and day 4 (9.21 +/- 1.79 pmol/g). Changes in endothelial nitric oxide synthase (eNOS), inducible NOS (iNOS), OTR and ERalpha, but not ERbeta, proteins paralleled the pregnancy-related cGMP changes in the LV. In contrast, ANP mRNA of the LV remained at control level throughout gestation but increased postpartum, whereas brain natriuretic peptide (BNP) expression declined at term and increased postpartum. The particulate GC natriuretic peptide receptors (GC-A and GC-B) transcripts were already lower at day 14 of gestation. Natriuretic peptide clearance receptor (NPR-C) transcript was not altered on days 7 and 14, but increased at term. We conclude that cGMP concentration in the rat LV is influenced by both NOS and natriuretic peptide systems and may be involved in the changes of LV contractility and hypertrophy that occur during rat gestation.

A murine monoclonal antibody against rat atrial natriuretic factor (ANF) which cross-reacts with mouse ANF.

A monoclonal antibody (MAb), 2H2, against rat synthetic atrial natriuretic factor (ANF) (Arg101-Tyr126) recognizes native ANF related peptides. The lack of reactivity of 2H2 with amino-terminal truncated ANF peptides implicates the two amino terminal arginine residues of ANF in the 2H2 epitope. Similarly, poor immunoreactivity of human ANF indicates the participation of isoleucine 110. Arginines 101 and 102 and isoleucine 110 may thus participate in a conformational epitope recognized by 2H2 or alternatively, substitution for, or elimination of these residues may alter the conformation of the 2H2 epitope. The MAb shows little cross-reactivity with extracts of rabbit atria but recognizes ANF related peptides in mouse and hamster atrial extracts. 2H2 also identifies immunoreactive ANF in histological sections of rat, mouse and hamster atria.

Is clonidine-induced diuresis mediated by atrial natriuretic factor?

The effect of intracerebroventricular (ICV) administration of clonidine on urine output, urinary sodium excretion, urinary cGMP, and plasma immunoreactive atrial natriuretic factor (IR-ANF) was studied in conscious, normally hydrated rats. Clonidine treatment evoked a significant dose-dependent increase in urine output. A 20-fold elevation was noted after the highest clonidine dose (2 micrograms/rat). The observed diuresis was accompanied by enhanced sodium excretion, which with the highest dose (2 micrograms) of clonidine increased from 1.6 +/- 0.36 to 39.4 +/- 10.5 meq/liter (P less than 0.001). Plasma IR-ANF rose from 30.7 +/- 8.8 to 113.3 +/- 32.3 pg/ml plasma 5 min after the 0.5 micrograms clonidine dose (P less than 0.05), and urinary cGMP excretion was augmented from 8.49 +/- 4.29 to 27.7 +/- 5.0 pmol/min 1 h after 0.5 micrograms clonidine (P less than 0.05). Pretreatment with peripherally administered anti-ANF serum abolished the diuretic effect of intracerebroventricularly administered clonidine; urine output decreased from 1.49 +/- 0.41 to 0.42 +/- 0.21 ml/h. The urinary cGMP level after anti-ANF serum treatment fell from 25.0 +/- 7.56 to 7.1 +/- 3.5 pmol/min (P less than 0.05). Peripheral pretreatment with the alpha 2-antagonist yohimbine or the opioid antagonist naloxone partially abolished clonidine's diuretic impact: urine output dropped from 1.91 +/- 0.55 to 0.42 +/- 0.18 and 0.46 +/- 0.18 ml/h (P less than 0.05), respectively. At the same time, plasma IR-ANF decreased from 113.3 +/- 32.2 to 30.3 +/- 11.4 after yohimbine and to 24.6 +/- 12.1 pg/ml after naloxone treatment (P less than 0.05). These data suggest that ANF may be involved in the mechanism of diuresis of centrally applied clonidine, which appears to enhance ANF release through its central stimulation of opiate and alpha 2-adrenergic receptors.

B-type natriuretic peptide receptor expression and activity are hormonally regulated in rat ovarian cells.

Natriuretic peptides form a family of structurally related peptides known to regulate salt and water homeostasis and to cause vasodilation. Synthesis of atrial (ANP), brain (BNP), and C-type (CNP) natriuretic peptides occurs mainly in the heart and brain and has been identified recently in the female reproductive tract. The expression of ANP and CNP as well as their cognate guanylyl cyclase receptors (NPR-A and NPR-B, respectively) have been detected in the rat ovary. We have shown previously that the expression of the natriuretic peptides and their receptors in the rat ovary appears to be modulated by the estrous cycle. In the present study we have evaluated the expression of the natriuretic peptide system (peptide and receptor) in ovarian cells (granulosa and thecal-interstitial cells) obtained from immature female rats treated with either diethylstilbestrol (DES), an estrogen analog, or equine CG (eCG), a gonadotropin that possesses both LH and FSH activity. Using a whole cell RRA, we found that CNP binding was increased by 2-fold in granulosa cells taken from animals treated with either DES or eCG. Semiquantitative RT-PCR revealed that granulosa cells from DES- or eCG-treated animals have increased levels of NPR-B messenger RNA (mRNA) transcripts, which was in good agreement with the increased binding. The activity of the receptors was assessed by ligand-dependent stimulation of cGMP release. CNP, but not ANP, stimulated the release of cGMP from granulosa cells obtained from DES-treated, but not from eCG-treated, animals. The relative levels of CNP mRNA in granulosa cells were unaltered by either DES or eCG treatment. In contrast, CNP mRNA levels were increased more than 2-fold, but only in theca-interstitial from the eCG-treated animals. Our results indicate that CNP and NPR-B are expressed in the ovary, and their expression is responsive to hormonal treatments. Furthermore, expression of these components of the natriuretic peptide system appears to be compartmentalized, with CNP being derived from the extrafollicular compartment and acting, through NPR-B, on the granulosa cells.

Estrogens directly down-regulate receptors for angiotensin II in anterior pituitary cell culture without decreasing target cell responsiveness.

Chronic estradiol treatment in vivo has been shown to reduce the density of receptors for angiotensin II (ANG II) in the anterior pituitary lobe (AP). We studied whether estradiol is directly involved in the down-regulation of ANG II receptors, using AP cells in culture. Binding affinity and density of ANG II receptors were measured in disrupted AP cells with the radiolabeled antagonist [125I]Sar1, Ile8-ANG II ([125I]SARILE). Estradiol treatment (10 nM) for either 48 or 96 h caused a marked reduction (approximately 70%) in the density of receptors for ANG II in cultured AP cells, with no change in the dissociation constant of [125I]SARILE (Kd, 0.5 +/- (SE) 0.1 nM). In the AP, specific binding sites for ANG II are present in lactotrophs and ANG II has been shown to release prolactin (PRL). In AP cells treated with estradiol for 48 h, dose-response curves revealed that ANG II still increased PRL release (P less than 0.01). The average net PRL release (ANG II-stimulated minus basal) was greater in estradiol-treated cells than in controls, whereas the half-maximal stimulation dose (ED50) of ANG II was the same (0.07 +/- 0.04 nM). These results suggest that estrogens are directly involved in the modulation of ANG II receptors in the AP, causing marked receptor down-regulation without decreasing target cell responsiveness.

The atrial natriuretic peptide system in rat ovaries.

The atrial natriuretic peptide (ANP) gene is expressed in several extracardiac tissues where ANP is thought to be involved in autocrine or paracrine regulation. The current studies were designed to characterize the ANP system in rat ovaries. ANP content in rat ovaries was estimated by RIA to be 240 +/- 70 pg/mg protein. HPLC revealed the presence of the 28-amino acid circulating peptide as well as the 126-amino acid prohormone, suggesting that the ovaries are a site of ANP synthesis. Indeed, ANP messenger RNA was detected in this tissue by RNase mapping. ANP present in ovarian extracts displaced [125I]ANP from bovine adrenal receptors (R1 class) in a dose-dependent manner and in parallel to the synthetic peptide, indicating that it possesses biological activity. Immunocytochemical studies localized ANP to interstitial cells surrounding the follicles; weaker but specific staining was also observed in the ovum. High affinity ANP receptors (dissociation constant, 0.30 +/- 0.06 nM; maximum binding capacity, 160 +/- 40 fmol/mg protein) were identified in ovarian membranes. Unlabeled ANP but not c-atrial natriuretic factor (a specific agonist of ANP clearance receptors) competed with binding of [125I]ANP to ovarian membranes in a dose-dependent manner, suggesting that ovarian ANP receptors are predominantly of the R1 class. This was confirmed by cross-linking studies with [125I]ANP, which detected a single protein band with a molecular size of about 120 kilodaltons, corresponding to that of the guanylate cyclase-coupled R1 class of receptor. Consistent with the presence of biologically active receptors, ANP markedly enhanced cGMP accumulation (by 15-fold) in ovarian cells. The presence of both local ANP synthesis and high affinity transducing receptors in the ovaries indicates that the peptide plays a local role in ovarian growth or steroidogenesis.

ANF mRNA is detected by the polymerase chain reaction technique in the chorioidea and bodies but not in the retina of the rat eye.

Specific proANF mRNA was demonstrated by the polymerase chain reaction (PCR) technique in ciliary body and chorioidea tissue extracts but not in the retina of the rat eye. However, immunoreactive (IR) ANF was detected in all of these tissues by a specific and sensitive radioimmunoassay (RIA). Since ANF seems to be involved in the maintenance of intraocular pressure, the regulation of ANF gene expression in these distinct eye tissues could be an important factor in the pathogenesis of glaucoma.

Regulation of renal atrial natriuretic peptide receptors in pregnant sheep.

These studies were designed to characterize the atrial natriuretic peptide (ANF) receptor subtypes (guanylyl cyclase GC-A and GC-B and ANF-C) in normal sheep kidneys and to evaluate alterations in receptor kinetics during pregnancy. Kidneys were obtained from 12 nonpregnant and 12 pregnant sheep during late gestation and maintained on a 100 mmol/day salt intake. Competition binding receptor assays using [125I]human ANF showed that inner medullary membranes are exclusively of the GC-A subtype. The maximum binding capacity (Bmax, 109 +/- 12 vs. 89 +/- 18 fmol/mg protein) and dissociation constant (Kd, 240 +/- 70 vs. 324 +/- 99 pM) are not altered by pregnancy. Specific binding of glomerular membranes to [125I]Tyr-C-type natriuretic peptide, which shows the highest affinity toward GC-B receptors, was observed, but this binding was abolished when ANF-C receptors were saturated with excess C-ANF-(101-121), suggesting that [125I]Tyr-C-type natriuretic peptide binding was mediated by ANF-C receptors. Binding of [125I]human ANF to glomerular membranes revealed that glomerular ANF receptor number was reduced during pregnancy (1040 +/- 212 vs. 335 +/- 42 fmol/mg protein; P = 0.001), but binding affinity was not changed. The reduced number was mainly due to a decrease in ANF-C receptor density (832 +/- 213 vs. 260 +/- 31 fmol/mg protein; P = 0.005). Autoradiography of whole kidney frozen sections produced similar findings. These studies demonstrate that GC-B receptors are absent from renal glomeruli and inner medulla, and that ANF receptor subtypes are differentially regulated in the pregnant sheep kidney, suggesting a role for ANF in the altered volume and pressure homeostasis of pregnancy.

Atrial natriuretic peptide in rats with experimental cirrhosis of the liver without ascites.

Plasma levels of atrial natriuretic peptide (ANP) have been measured in eight sodium-retaining cirrhotic nonascitic rats and eight control animals before and after extracellular volume expansion by isotonic saline infusion (30 ml/kg BW, 20 min). In addition, disappearance of [125I]ANP was studied in six control and six cirrhotic rats. The effect of infusing synthetic rat ANP-(1-28) (1 microgram) on mean arterial pressure and renal function has been also studied. In basal conditions, cirrhotic rats showed higher ANP plasma levels than control animals (71.1 +/- 16.6 vs. 43.9 +/- 5.1 pg/ml; P less than 0.05). After extracellular volume expansion, ANP increased in both control and cirrhotic rats; the increase in cirrhotic was higher than that in control rats (88 +/- 27% vs. 33 +/- 8%; P less than 0.05). Disappearance of iodinated ANP from plasma was identical in control and cirrhotic rats. ANP infusion induced a larger decrease in mean arterial pressure in control (21 +/- 5%) than in cirrhotic rats (9 +/- 2.5%). ANP induced comparable increases in glomerular filtration rate and renal plasma flow in both groups of animals. However, diuretic and natriuretic effects were markedly impaired in cirrhotic animals. Thus, urinary flow increased by 91 +/- 18 microliters/min in control animals, but only by 37 +/- 7 microliters/min in cirrhotic animals. Fractional sodium excretion increased to 1.7 +/- 0.44% in controls and to 0.54 +/- 0.12% in cirrhotic rats (P less than 0.05). It is concluded that the defect in renal handling of sodium in cirrhotic rats is not due to a lack of ANP synthesis or release. In addition, these animals show an impaired renal response to ANP.

Cardiac and plasma atrial natriuretic factor in experimental congestive heart failure.

The cardiac and plasma levels of immunoreactive (IR-) atrial natriuretic factor (ANF) in cardiomyopathic hamsters with moderate and severe congestive heart failure were measured, compared with those of controls, and correlated by HPLC analysis of IR-ANF in atria, ventricles, and plasma and with the ultrastructure of atrial and ventricular cells. Congestive heart failure in the hamster produced a significant increase in plasma IR-ANF, a significant decrease in atrial IR-ANF, and a marked increase in ventricular IR-ANF. The HPLC pattern of IR-ANF was of the high mol wt type in atria and ventricles of control and cardiomyopathic animals. High mol wt forms of ANF appeared in the plasma of animals with severe congestive heart failure, but not in controls. Severe congestive heart failure produced a tremendous increase in the size of the Golgi complex, with a decrease in the number and size of secretory granules in atrial cardiocytes. Ventricular cardiocytes also showed a less marked increase in the size of the Golgi complex. Secretory-like granules indistinguishable from lysosomes were present in about 1% of ventricular cardiocytes of control hamsters; in hamsters with severe congestive heart failure, secretory granules, identical to those of atrial cardiocytes, were present in greater number in about 20% of ventricular cardiocytes. Immunocytochemistry (immunogold technique) revealed that secretory granules containing IR-ANF are not present in control ventricular cardiocytes but are localized in relatively large number in about 20% of ventricular cardiocytes in hamsters with severe congestive heart failure. These results suggest that the increased IR-ANF levels (including the high mol wt forms) in animals with congestive heart failure may come from hypersecretion of both atrial and ventricular cardiocytes.

Specific receptor-mediated inhibition by synthetic atrial natriuretic factor of hormone-stimulated steroidogenesis in cultured bovine adrenal cells.

The effect of synthetic atrial natriuretic factor (ANF) on adrenal steroidogenesis has been studied in primary culture of bovine adrenal cells. ANF-(8-33) produced a potent 40-70% inhibition of angiotensin II-, ACTH-, PGE1-, and forskolin-stimulated secretion of aldosterone production from zona glomerulosa cells with an ED50 of 120 pM. An equipotent inhibitory effect of the natriuretic factor on cortisol production was also observed in cultured zona fasciculata cells. Nicotine-stimulated secretion of catecholamines from medullary cells was only slightly inhibited by the factor at doses above 10 nM. [125I]iodo-ANF-(8-33) binding to glomerulosa membranes displayed an apparent affinity of 100-150 pM for specific receptor sites and was not inhibited by angiotensin II or ACTH. Conversely, the natriuretic factor had no affinity for angiotensin II receptor sites. The results demonstrate that part of the natriuretic effect of this new factor might be due to inhibition of adrenal steroidogenesis by action through a distinct receptor.

Internalization and lysosomal association of [125I]angiotensin II in norepinephrine-containing cells of the rat adrenal medulla.

The morphological localization of [125I]angiotensin II (AII) in the rat adrenal medulla (AM) was studied by light- and electron-microscopic radioautography in vivo. With light microscopy the presence of binding sites for AII in both norepinephrine-containing (NE) and epinephrine-containing (E) cells was confirmed. With electron microscopy, it was found that AII binds to the cell surface of NE cells, is progressively internalized, and is associated with lysosomes and Golgi complex within 20 min, whereas in E cells AII seems to be internalized earlier and recycled back to the cell surface within 5 min without any appreciable association with intracellular organelles. These results suggest different intracellular pathways for AII in NE and E cells of the rat AM.

Fate of [125I]angiotensin II in adrenal zona glomerulosa cells.

Binding and internalization of [125I]angiotensin II (AII) were studied by morphological and biochemical methods in rats in vivo. Light microscope radioautography demonstrated that [125I]AII binds specifically to adrenal zona glomerulosa (ZG) cells. Ultrastructural radioautographic analysis revealed that [125I]AII binds to the cell surface, clusters in coated pits, is internalized in coated vesicles, and is transported by receptosomes to lysosomes in less than 20 min. Biochemical analysis revealed that as much as 40% of the adrenal radioactive uptake behaves as native [125I]AII as shown by electrophoresis, immunoprecipitation and radioligand binding studies. These results indicate that the effects of AII on the secretion of aldosterone by ZG cells are mediated by cell surface phenomena and not by binding to intracellular organelles involved in steroidogenesis. They also indicate that the half-life of AII bound to receptors and internalized seems to be much longer (min) than in the systemic circulation (sec).

Characterization and distribution of natriuretic peptide receptors in the rat uterus.

Atrial natriuretic peptide (ANP) receptors were characterized in rat uterus. The binding of [125I]ANP to uterine membranes was completely competed for by increasing concentrations of unlabeled ANP (Kd = 0.39 nM) and brain natriuretic peptide (Kd = 1.24 nM) and partially by C-type natriuretic peptide (CNP; Kd = 80.4 nM), but not by C-ANF. Also, [125I]Tyr-CNP bound to uterine membranes was completely competed by unlabeled CNP (Kd = 1.12 nM). Cross-linking of [125I]ANP to uterine membranes revealed the presence of one band of 130 kilodaltons, corresponding to the guanylyl cyclase (GC-A and/or GC-B) subtypes of natriuretic peptide receptors. The presence of messenger RNA coding for genes of both GC-A and GC-B receptors was shown by quantitative reverse transcriptase polymerase chain reaction. Furthermore, ANP and, to a lesser degree, CNP stimulated the production of cGMP in rat uterus. Autoradiographic studies localized the highest binding of [125I]ANP in the endometrium, whereas [125I]Tyr-CNP binding was distributed in the endometrium as well as in the myometrium. These results demonstrate that rat uterine ANP receptors are of the guanylyl cyclase-coupled subtypes. The uterus is a target of natriuretic peptides where ANP induces its biological effects through the production of cGMP.

Atrial natriuretic factor immunoreactivity in human fetal lung tissue and perfusates.

Immunoreactive atrial natriuretic factor (ANF) was detected in human fetal homogenates and perfusates using a sensitive and specific radioimmunoassay for the 28 amino acid (C-terminal) fragment. Three peaks of ANF immunoreactive material were found in the lung homogenates. With high performance liquid chromatography, the elution characteristics of the first immunoreactive peak were the same as those of circulating human ANF. The other two peaks have not been characterized, although one had a position similar to the 126 amino acid rat prohormone (Asn 1-Ile 110-Tyr 126). The time course of release of immunoreactive ANF by perfused human fetal lungs was also studied. It is suggested that ANF may play a role in early pulmonary function.

Atrial natriuretic peptide is involved in renal actions of moxonidine.

Moxonidine, an antihypertensive imidazoline compound, reduces blood pressure by selective activation of central imidazoline I(1)-receptors and inhibition of sympathetic nerve activity and by direct actions on the kidney, with both mechanisms resulting in diuresis and natriuresis. We hypothesized that the hypotensive and renal actions of moxonidine may be mediated by atrial natriuretic peptide (ANP), a cardiac peptide involved in pressure and volume homeostasis through its vasodilatory, diuretic, and natriuretic actions. Renal parameters were measured on an hourly basis over a period of 4 hours in conscious rats that received bolus intravenous injections of moxonidine (1 to 150 microg/300 microL saline). During the first hour, moxonidine dose-dependently stimulated diuresis, natriuresis, kaliuresis, and urinary cGMP, the index of ANP activity. Moxonidine (50 microg) significantly (P<0.001) stimulated urinary volume (0.35+/-0.04 versus 1.05+/-0.09 mL/h per 100 g), sodium (14. 3+/-2.5 versus 51.8+/-6.5 micromol/h per 100 g), potassium (10.5+/-2. 3 versus 32.3+/-3.2 micromol/h per 100 g), and cGMP (325+/-52 versus 744+/-120 pmol/h per 100 g). Pretreatment with a selective imidazoline receptor antagonist, efaroxan, dose-dependently inhibited moxonidine-stimulated renal parameters. Efaroxan (25 microg per rat) significantly inhibited moxonidine-stimulated diuretic and natriuretic effects and urinary cGMP excretion (744+/-120 versus 381+/-137 pmol/h per 100 g, P<0.02). The alpha(2)-adrenoceptor antagonist yohimbine (50 microg per rat) partially yet significantly inhibited moxonidine-stimulated diuresis and natriuresis but not cGMP excretion. Plasma ANP was dose-dependently increased by moxonidine and was inhibited by pretreatment with efaroxan (220.8+/-36.9 versus 100.3+/-31.7 pg/mL, P<0.03) but not by yohimbine. In conclusion, selective in vivo activation of imidazoline receptors by moxonidine is associated with dose-dependent diuresis, natriuresis, and kaliuresis as well as stimulated plasma ANP and urinary cGMP excretion, thus implicating ANP in the renal actions of moxonidine.