The sequence of an atriopeptigen: A precursor of the bioactive atrial peptides

Processing of pro-hormone precursor proteins

Peptide-hormones and other biologically active peptides are synthesized as higher molecular weight precursor proteins (pro-proteins) which must undergo post-translational modification to yield the bioactive peptide(s). These post-translational enzymatic events include limited endoproteolysis and may include other modifications of the generated peptide such as limited exopeptidase digestion, N-terminal acetylation, C-terminal amidation, and formation of N-terminal pyroglutamyl residues (pyrrolation). The secretory vesicle hypothesis, one of the major hypotheses regarding processing, states that the initial endoproteolytic event occurs upon formation of the secretory vesicle (or granule) or within the secretory vesicle from which the bioactive peptides are released. Two different endoproteinases which are likely to be physiologically relevant processing enzymes of pro-atrial natriuretic factor and pro-gonadotropin releasing hormone precursor protein, respectively, have recently been discovered in our laboratory and are discussed as model enzymes in the context of this hypothesis. The results indicate that the precursor protein and its complement of processing enzymes are co-packaged into the secretory granule. Evidence is presented to support the idea that the specific sequence and conformation (secondary structural features) of the processing recognition site within the precursor protein likely contribute in large part to the basis for limited endoproteolysis. In the pro-hormones studied, the recognition site is an extended sequence of five to seven residues which likely exists as a beta-turn at the surface of the precursor protein. By extending our results to appropriate protein sequences in the National Biomedical Research Foundation database, we are suggesting that in addition to the doublet of basic amino acids, the primary processing recognition site in pro-hormone precursor proteins often contains a monobasic amino acid or a strongly polar residue (Glu or Asp) in close sequence proximity to the doublet of basic residues.

Ultrastructural aspects of atrium development: demonstration of endocardial discontinuities and immunolabeling of atrial natriuretic factor in the Syrian hamster

The endocardium ultrastructure of 13 embryonic day old hamsters was examined, especially in relationship with the atrial myocytes. The endothelial morphology was described, including the junctional attachments and their relationships with subjacent atrial myocytes. Characteristic atrial myocytes organelles were identified: myofibrils, atrial granules, lipidic inclusions, and polysomes. Immunogold labeling demonstrated that atrial natriuretic factor (ANF)-containing granules were already present in the differentiating cardiomyocytes, even before the myofibrils were completely organized. At this stage of development, while the endothelium was a narrow barrier between the blood and the cardiomyocytes, it displayed fenestrations, but also epithelial discontinuities. In addition it also contains immunoreactive-ANF products. In light of the current knowledge about ANF processing it was proposed that the endocardium lining could be an obligated passageway for transport or activating proANF into ANF before its release into the blood stream. In addition the endocardial gaps could suggest that, until about 13 to 14 days of fetal development, heart atrial tissue could be more susceptible to the effects of pathogenetic compounds than in a later state of development.

Atrial natriuretic peptide in dehydrated Long-Evans rats and Brattleboro rats

Atrial natriuretic peptide (ANP) was measured by radioimmunoassay in atrial and plasma extracts from normal Long-Evans (LE) rats and Brattleboro-strain diabetes insipidus (DI) rats. LE rats, dehydrated for 72 hours, had an increased plasma osmolality and plasma vasopressin. They also demonstrated a higher atrial immunoreactive ANP (IR-ANP) content than hydrated animals (72 hr dehydration: 178.2 +/- 30.4 micrograms/g wet weight atria, mean +/- SE, control: 60.4 +/- 8.2; P less than 0.001). Plasma IR-ANP in dehydrated LE rats tended to be lower than hydrated LE but this was not statistically significant [72 hr dehydration: 61.9 +/- 5.9 pg/ml, control: 82.4 +/- 8.2]. IR-ANP concentration in atrial extracts from DI rats, without detectable plasma vasopressin levels but with increased plasma osmolality, was not different from that in hydrated LE rats (DI: 100.6 +/- 13.2 micrograms/g). There was also no significant difference between plasma IR-ANP in DI and hydrated LE rats (DI: 100.2 +/- 11.9 pg/ml). The atrial IR-ANP concentration in DI rats was decreased by infusion with either arginine-vasopressin (AVP) or 1-deamino-8-arginine vasopressin (DDAVP), and plasma IR-ANP was increased significantly by both infusions (AVP: 171.3 +/- 18.1 pg/ml, DDAVP: 179.5 +/- 24.6). Thus, changes in atrial and plasma IR-ANP concentration appeared to be associated with changes in water balance but not with plasma AVP levels, indicating that the changes in volume may be a more important factor controlling ANP release in vivo than vasopressin itself.

Organization of atriopeptin-like immunoreactive neurons in the central nervous system of the rat

The atrial natriuretic peptide, atriopeptin, is a circulating hormone that plays an important role in the regulation of fluid and electrolyte homeostasis. Several recent studies have shown that atriopeptin-like immunoreactivity is present within the central nervous system as well as peripheral tissues. In the present report, we describe in detail the organization of atriopeptin-like immunoreactive (APir) perikarya and fibers in the central nervous system of the rat. The most prominent collection of APir perikarya was found in the hypothalamus, adjacent to the anteroventral tip of the third ventricle. Additional groups of APir perikarya were observed along the wall of the third ventricle and in the paraventricular and arcuate nuclei. Separate, smaller groups with distinctive morphology were seen in the lateral hypothalamic area, in the supra-mammillary, medial, and lateral mammillary nuclei, medial habenular nucleus, bed nucleus of the stria terminalis, and the central nucleus of the amygdala. In the pons and brain-stem, APir neurons were observed in the pedunculopontine and laterodorsal tegmental nuclei, as well as in the ventral tegmental area, Barrington's nucleus, the parabrachial nucleus, and the nucleus of the solitary tract. The densest terminal fields of APir fibers were found in the paraventricular nucleus of the hypothalamus, the bed nucleus of the stria terminalis, the median eminence, and the interpeduncular nucleus. The presence of atriopeptin immunoreactivity within the central nervous system suggests that atriopeptin may function as a central neuromediator. Potential functions of this candidate neuromediator deduced from its anatomical distribution are discussed, including the possibility that atriopeptin may function as both a central neuromediator and a systemic hormone in the regulation of the cardiovascular system.

Colocalization of atriopeptin-like immunoreactivity with choline acetyltransferase- and substance P-like immunoreactivity in the pedunculopontine and laterodorsal tegmental nuclei in the rat

Atriopeptin, the atrial natriuretic peptide, is a circulating hormone that plays an important role in the regulation of fluid and electrolyte balance. Immunohistochemical studies have shown that large, multipolar atriopeptin-like immunoreactive (APir) neurons are present in areas of the midbrain corresponding to the large neurons of the pedunculopontine tegmental (PPT) and lateral dorsal tegmental (TLD) nuclei, all of which can be stained immunohistochemically for choline acetyltransferase-like immunoreactivity (ChATir). A subpopulation of these cholinergic PPT and TLD neurons are also known to contain substance P-like immunoreactivity (SPir). Using an immunofluorescent technique that allows simultaneous localization of two antigens, we have studied the relationship between APir, SPir and ChATir in the pontine tegmentum of the rat. We have found that the large multipolar APir neurons of the pontine tegmentum are identical to the ChATir neurons of the PT and TLD nuclei and a subpopulation of the APir neurons in PPT and TLD neurons are also SPir.

Identification of the cardiac and circulating form of atriopeptin in rabbit

Analysis of peptides purified from high and low molecular weight fractions of rabbit atrial extracts indicates that the sequence of the first 30 residues of rabbit atriopeptigen exhibits 80% homology with the rat peptide, and that the low molecular weight rabbit peptide (28 residues) is identical to rat atriopeptin 28 (AP 28). The effects of infused 1-deaminoarginine8-vasopressin (dAVP) and phenylephrine, volume expansion, and water immersion on AP release into the circulation of the rabbit was studied. Neither dAVP, nor water immersion elevated right atrial pressure (RAP) or plasma AP levels in the anesthetized rabbits. Phenylephrine induced a sustained increase in systemic blood pressure and right atrial pressure which was accompanied by elevated plasma AP immunoreactivity which appeared to be identical to rat AP-28 on HPLC. There is obviously a preferential conservation of the AP sequence, since the C-terminal peptide is exactly the same in rabbit, rat and mouse and differs from human, dog, cow and pig only by the single substitution of an isoleucine for a methionine residue.

George E. Brown memorial lecture. Role of atrial peptides in body fluid homeostasis

Extracts of mammalian atria, but not ventricles, induce marked diuresis, natriuresis, and reduction in blood pressure when infused systemically in rats and dogs. These extracts also inhibit aldosterone biosynthesis and renal renin release. Natriuretic peptides, 21 amino acids and longer, have been isolated from atria of rodents and man, and share a nearly homologous amino acid sequence at the carboxyterminus. Natriuretic activity resides in a 17-amino acid ring formed by a disulfide bridge, and the C-terminal Phe-Arg appears necessary for full biological potency. The deoxyribonucleic acid-encoding atrial natriuretic peptides have been cloned and the gene structure elucidated. Reduction of the diuretic and natriuretic responses to an acute volume load by right atrial appendectomy first suggested a role for atrial peptides in the physiological response to plasma volume expansion. Subsequently, release of peptides with natriuretic and spasmolytic properties from isolated heart preparations in response to right atrial distension was demonstrated by bioassay and radioimmunoassay. The presence of these peptides in normal rat and human plasma in concentrations of 20-100 pM, and the findings of increased levels in response to acute and chronic plasma volume expansion, rapid atrial tachyarrhythmias, systemic hypertension, congestive heart failure, and renal insufficiency imply that they play an important role in body fluid homeostasis. The mechanisms by which atrial peptides increase renal salt and water excretion are as yet unclear. Renal vascular effects have been consistently demonstrated, and limited evidence for direct actions on tubule ion transport has also been reported recently. In vitro, these peptides cause precontracted vascular and nonvascular smooth muscle to relax, mediated by a direct action on smooth muscle cells. Specific receptors for these peptides have been characterized in crude membranes prepared from whole kidney homogenates and adrenal glomerulosa cells, in intact glomeruli and cultured glomerular mesangial cells, and in intact bovine aortic smooth muscle and endothelial cells. Natriuretic peptides stimulate cyclic guanosine monophosphate accumulation in target tissues, and augment particulate guanylate cyclase activity in membrane fractions, suggesting that cyclic guanosine monophosphate is the second messenger mediating their cellular action.

Atriopeptin III improves renal functions in a ureter-obstructed rat kidney model

The renal hemodynamic and excretory effects of atriopeptin III were studied in normal rat kidneys and in kidneys made dysfunctional by the application and release of a 24 h unilateral ureteral obstruction (UO24h), which decreased baseline glomerular filtration rate (GFR) by 80%. Atriopeptin III (0.5 nmol/kg i.v.) decreased blood pressure (10-15%) for more than 30 min, and increased urine flow rate and sodium excretion in normal and diseased kidneys for ca. 15 min. An initial enhancement of renal blood flow (ca. 20%) was apparent for less than 5 min. Atriopeptin III (bolus injection) temporarily enhanced the GFR 2-3-fold in the diseased (UO24h) kidneys, whereas no changes of GFR were noted in control kidneys. When atriopeptin III was continuously infused at a rate of 0.1 nmol/kg per min, GFR in UO24h kidneys increased from 0.28 +/- 0.08 ml/g per min to a stable level of 0.82 +/- 0.10 ml/g per min. Again, GFR in the control kidneys remained unaffected (1.25 +/- 0.08 ml/g per min). The enhancement of GFR in the UO24h kidney was associated with large increases of urine flow rate and sodium excretion.

Atrial natriuretic polypeptides in the specific atrial granules of the rat heart: immunohistochemical and immunoelectron microscopical localization and radioimmunological quantification

Atria of several mammalian species contain atrial natriuretic polypeptides (ANP) with natriuretic, diuretic, and vasodilating activity. In the present studies ANP were localized and quantitated in different parts of the heart by immunocytochemical and radioimmunological methods. The concentration of immunoreactive ANP as determined by quantitative radioimmunoassay in rat heart atria was a follows (ng/mg, mean +/- SD, n = 5): right auricle (688 +/- 156), left auricle (556 +/- 156), right atrium (334 +/- 60), and left atrium (93 +/- 36). The staining intensities in immunohistochemical localizations were consistent with the quantitative data. The location of the peptides was sarcoplasmic and granular. The highest concentration of ANP was found in the perinuclear area of the atrial myocyte sarcoplasm, but some staining was also seen in the periphery of the cells. The indirect immunoelectron microscopical gold method showed that ANP are located in the specific atrial granules supporting previous findings.

Release of atriopeptin in the rat by vasoconstrictors or water immersion correlates with changes in right atrial pressure

Vasopressin, its 1-deamino analog (dAVP), angiotensin II, and phenylephrine, administered intravenously, increased plasma atriopeptin immunoreactivity in chloral hydrate-anesthetized rats. A continuous one hour infusion of either dAVP or phenylephrine caused a sustained elevation in: a) systemic blood pressure; b) right atrial pressure; c) left ventricular end diastolic pressure; and d) plasma atriopeptin immunoreactivity. While continuous infusion of angiotensin II also produced a sustained elevation in left ventricular end diastolic pressure, the changes in right atrial pressure and plasma atriopeptin were only transient. These data suggest that plasma atriopeptin most closely correlates with right atrial pressure. Consistent with this hypothesis, we found that the release of atriopeptin directly correlated with changes in right atrial pressure in anesthetized, water-immersed rats.

Atrial natriuretic factor: a hormone produced by the heart

Systematic studies on the significance of the secretory-like morphological characteristic of cardiac atrial muscle cells of mammals led to the finding that these cells produce a polypeptide hormone. This hormone, described in 1981 as atrial natriuretic factor (ANF), is diuretic (natriuretic), hypotensive, and has an inhibitory effect on renin and aldosterone secretion. Thus, ANF probably intervenes in the short- and long-term control of water and electrolyte balance and of blood pressure. Phylogenetically, ANF appears early, suggesting different functions for this peptide in accordance with each species' environment. Knowledge of the properties of the hormone should provide insights into the pathophysiology of important clinical entities and lead to the development of new pharmaceutical products.

Rat platelets activate high molecular weight atrial natriuretic peptides in vitro

Evidence suggests that the more biologically active low molecular weight forms (less than 10,000) of rat atrial natriuretic peptides are proteolytically derived from a less active precursor of higher molecular weight. Conversion and activation could occur within the myocyte as well as during circulation. The present study found that in vitro rat blood and platelets were capable of converting the high molecular weight atrial natriuretic peptides (greater than 10,000) to low molecular weight atrial natriuretic peptides within minutes and that enhanced biological activity attended the conversion. Rat high molecular weight peptides were partially purified by gel filtration, lyophilized, and reconstituted in Krebs-Ringer bicarbonate buffer. One milliliter of fresh rat blood was incubated with the high molecular weight peptides at 37 degrees C for 2 minutes. After centrifugation, the supernatant was fractionated on Sephadex G-75. Natriuretic activity was determined by bioassay in anesthetized rats. In contrast to the results following incubation of high molecular weight peptides in Krebs-Ringer bicarbonate buffer alone, which showed that 95% of the natriuretic activity remained in the high molecular weight peptide region, the natriuretic activity of the blood-treated high molecular weight peptides eluted almost exclusively in the low molecular weight peptide region, which indicates conversion. Blood was separated into plasma, erythrocytes, lymphocytes, and platelets. Conversion of high to low molecular weight peptides occurred only after incubation with platelets. Compared with control high molecular weight peptides, rat platelet-treated high molecular weight peptides had significantly greater activity in relaxing histamine-contracted rabbit aortic smooth muscle (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Sodium ion stimulates the release of atrial natriuretic polypeptides (ANP) from rat atria

The release of atrial natriuretic polypeptides from spontaneously beating isolated rat atria was found to be sensitive to the increase in the concentration of sodium ion. The osmotic pressure, when produced by pharmacologically inactive choline chloride, also increased the release of ANP but substantially less than the sodium ion. Sodium ion and osmotic pressure stimulated the release of ANP in the hyperosmotic but not in the hypo-osmotic range. Neither stretch nor several neurotransmitters tested had any effects on the rate of ANP secretion.

Inhibition of aldosterone biosynthesis by atriopeptins in rat adrenal cells

The effect of synthetic atriopeptins on basal and stimulated aldosterone secretion was determined in isolated adrenal glomerulosa cells of the rat. Neither atriopeptin I (1-21) or III (1-24, i.e., the Phe-Arg-Tyr carboxy-terminal extension of atriopeptin I) altered basal aldosterone release. However, if the cells were prepared from adrenals of sodium-depleted rats, the basal aldosterone release was increased by 9-fold, compared with cells from normal rats. This elevated release was inhibited by 32% by atriopeptin I and atriopeptin III. Atriopeptin III was more potent than atriopeptin I. Angiotensin II and adrenocorticotropin stimulated the release of aldosterone in a concentration-related manner. Both atriopeptin I and atriopeptin III inhibited the stimulation by the peptides. Atriopeptin I inhibited angiotensin II- and adrenocorticotropin-induced aldosterone production by 50% at concentrations of 12 and 11 nM, respectively, and 0.5 and 0.2 nM, respectively, for atriopeptin III. Potassium-stimulated aldosterone production was also inhibited by atriopeptin I and atriopeptin III with 50% inhibition at concentrations of 10 and 0.4 nM, respectively. Shorter peptides (1-20, 1-19, and 3-19) were equipotent to atriopeptin I (1-21) as inhibitors of angiotensin II-induced steroidogenesis. To determine the site at which atriopeptins inhibit aldosterone synthesis, we used cyanoketone to inhibit 3 beta-hydroxy-dehydrogenase and dissociate the early and late pathways. Angiotensin II (2 nM) increased the synthesis of pregnenolone (early pathway), as well as the conversion of [3H]corticosterone to [3H]aldosterone (late pathway). Atriopeptin III inhibited basal pregnenolone synthesis by 36% and completely blocked angiotensin II-stimulated synthesis. The peptide similarly inhibited the late pathway.(ABSTRACT TRUNCATED AT 250 WORDS)

Atrial natriuretic polypeptides (ANP): rat atria store high molecular weight precursor but secrete processed peptides of 25-35 amino acids

A radioimmunoassay was developed for rat atrial natriuretic polypeptides. Using the radioimmunoassay and gel filtration in reducing and dissociating conditions, extracts of rat atria were found to contain mainly a 15000-dalton immunoreactive material, probably corresponding to pronatriodilatin. However, when isolated beating atria were incubated in plasma-free conditions, the secreted immunoreactive material consisted almost exclusively of 2500-3500-dalton peptide(s). These results show that rat atrial cells secrete a low molecular weight natriuretic peptide which is highly active, but store the less active large molecular weight form(s).

Conversion of atriopeptin II to atriopeptin I by atrial dipeptidyl carboxy hydrolase

We are examining the substrate specificity of atrial dipeptidyl carboxyhydrolase, a membrane-bound metallo enzyme that we isolated from bovine atrial tissue homogenates. This enzyme readily removes the dipeptide, Phe-Arg, from Bz-Gly-Ser-Phe-Arg, a stand-in substrate for atriopeptin II, one of several atrial natriuretic factors. We now report that the atrial enzyme cleaves the C-terminal dipeptide, Phe-Arg, from atriopeptin II to form atriopeptin I. The km (pH 7.5) is 25 microM and the ratio of relative Vmax/km as a measure of substrate specificity indicates that atriopeptin II is a 240-fold better substrate than Bz-Gly-His-Leu. Only Phe-Arg was detected as a hydrolysis product, indicating that sequential cleavage of Asn-Ser from atriopeptin II does not occur, and that atriopeptin I is not a substrate. Bz-Gly-Asn-Ser was as good a substrate for the atrial enzyme as Bz-Gly-His-Leu, but Bz-Cys(bzl)-Asn-Ser was not hydrolyzed. This result suggests that the presence of an intact disulfide bond or an S-alkylated residue in the P1 position of a substrate (as in atriopeptin I) prevents hydrolysis by the atrial enzyme. Comparative studies were made with the angiotensin I converting enzyme. Atriopeptin II was not a substrate. The stand-in substrates for atriopeptin I, Bz-Cys(bzl)-Asn-Ser and Bz-Gly-Asn-Ser were barely hydrolyzed, which by itself suggests that atriopeptin I is not a substrate of the angiotensin converting enzyme. Our results strongly suggest that atriopeptin II is converted to atriopeptin I and that hydrolysis is mediated by the atrial enzyme. The angiotensin I converting enzyme plays no role in processing these peptides. We suggest that the atrial enzyme be named atrial peptide convertase.

Alignment of rat cardionatrin sequences with the preprocardionatrin sequence from complementary DNA

Mammalian atria contain peptides that promote the excretion of salt and water from the kidney. When rat atrial tissue is extracted under conditions known to inhibit proteolysis, four natriuretic peptides, cardionatrins I to IV, are consistently isolated. These peptides derive from a common precursor, preprocardionatrin, of 152 amino acids, whose sequence was determined by DNA sequencing of a complementary DNA clone. Amino acid sequencing located the start points of cardionatrins I, III, and IV in the overall sequence. Cardionatrin IV most closely resembles procardionatrin because it begins immediately after the signal sequence at residue 25. Cardionatrin III begins at residue 73, and cardionatrin I, sequenced previously, begins at residue 123. Compositional analysis indicated that each of these cardionatrins extends up to tyrosine at position 150 but lacks the terminal two arginine residues.

Differential structure-activity relationships of atrial peptides as natriuretics and renal vasodilators in the dog

Natriuretic-diuretic and vasodilator activities of synthetic atriopeptin (AP)-related peptides were examined in the anesthetized dog. We have selected, the naturally occurring, APIII as the reference compound for comparison with various related peptides. APIII is a 24 amino acid peptide with the sequence ser-ser-cys-phe-gly-gly-arg-ile-asp-arg-ile-gly-ala-gln-ser-gly-leu-gly- cys-asn-ser-phe-arg-tyr-OH. APII, another peptide isolated from atrial extracts, lacks the C-terminal arg- of APIII. N-terminal amino acid extensions on APIII or APII, exhibited enhanced natriuretic-diuretic effectiveness. Furthermore, the maximum response obtained by ser-leu-arg-arg-APIII and arg-arg-APIII were significantly higher and the dose-response curve was not parallel to that obtained with APIII. In contrast, there were no significant qualitative or quantitative differences between the renal blood flow responses produced by the N-terminal extended peptides and APII or APIII. These results suggest a heterogeneity of AP receptors in vascular and renal tubular tissues.

Comparative vascular pharmacology of the atriopeptins

The atriopeptins are potent relaxants of norepinephrine-constricted aortic strips or are dilators of renal blood vessels in isolated perfused rat kidneys that are constricted by norepinephrine. This vasorelaxant property of the atriopeptins requires the presence of phenylalanine arginine (i.e., atriopeptin II, III, or ser-leu-arg-arg atriopeptin III) residues in the carboxy terminus which are considerably more effective than atriopeptin I (the 21 amino acid peptide which lacks the phe-arg C-terminus) or the core peptide (residues 3-19). However, these artificially in vitro precontracted preparations do not accurately predict the vascular effectiveness of the atriopeptins in intact rats. Intravenous administration of the atriopeptins (including atriopeptin I) to anesthetized rats produces concentration-dependent hypotension, a selective decrease in renal resistance in low doses (determined with microspheres), and pronounced diuresis. At higher doses, atriopeptins increase blood flow in other vascular beds. On the other hand, in the anesthetized dog, injection (intraarterially) of the phe-arg-containing peptides produces a concentration-dependent increase in both renal blood flow and sodium excretion, whereas atriopeptin I is inactive. Although there is a species difference in responsiveness to atriopeptin I, these data demonstrate a direct correlation between the renal vasodilation and diuresis produced by this novel family of atrial peptides.

Atriopeptin-immunoreactive neurons in the brain: presence in cardiovascular regulatory areas

Antisera to atriopeptin III and to a cyanogen bromide fragment of the precursor molecule atriopeptigen were prepared and used to examine the distribution of atriopeptin-like immunoreactive material in the heart and brain of the rat. Granules of this material were seen in myocytes throughout the right and left atria and were densest in the perinuclear region. The distribution of atriopeptin-like immunoreactive material in the heart is consistent with previous reports of atrial secretory granules. In the brain neurons containing the material were observed in the hypothalamus and the pontine tegmentum. Atriopeptin in the brain may serve as a neurotransmitter in neural systems controlling blood volume and composition, the same physiological functions regulated by blood-borne atriopeptin.

Biochemical studies of rat atrial natriuretic factor

The natriuretic substances were purified from rat atrium (ANF, atrial natriuretic factor) and were shown to be identical with the inhibitor of norepinephrine-induced contraction of smooth muscle. Their four native forms were isolated. Amino acid sequence analyses showed they are peptides with 35, 31, 30 and 25 amino acid residues respectively and contain a ring structure consisting of 17 amino acid residues and a disulfide bridge. The presence of a high molecular weight prohormone was shown. cDNA coding for the precursor was cloned and used to deduce the amino acid sequence of the preprohormone. Genomic DNA for ANF was cloned and the presence of two introns were found. Several ANF peptides were synthesized. Structure-function studies showed that the ring structure is essential for the activity. Antibodies produced against the synthetic 25 amino acid residue ANF were used to develop radioimmunoassay. The presence of ANF in rat plasma was demonstrated as evidence that ANF is a circulating hormone. ANF was also found in the hypothalamus of rats. The quantitative determination of the synthetic ability of ANF has been determined by the application of ANF cDNA for the quantification of ANF messenger RNA. Immunohistochemical methods localized ANF in cardiac atriocytes, gonadotrophs in anterior pituitary and adrenal medulla (chromaffin cells). A strong immuno-reactivity was found in dark cells of the collecting ducts of the kidney. ANF increases cyclic GMP in target cells suggesting that cyclic GMP may be the intracellular mediator of ANF action.

Structure-activity relationships of atrial natriuretic factor (ANF). I. Natriuretic activity and relaxation of intestinal smooth muscle

Atrial natriuretic factor-related peptides were produced from synthetic ANF (101-126) either by chemical hydrolysis at the N-terminal end or by treatment with carboxypeptidases at the C-terminal end. The biological activities of these peptides were characterized in vivo by a natriuretic bioassay and in vitro by relaxation of contracted intestinal smooth muscle (chick rectum). In the natriuretic assay, the removal of Asn122, Ser123 and Phe124 at the C-terminal end alters considerably the renal response. Deletion or extension of amino acids at the N-terminal end affects only slightly the natriuretic activity. Removal of N-terminal or C-terminal amino acids decreases the relaxant activity of ANF on the chick rectum. In both bioassays, simultaneous deletions of residues at both ends drastically affect the activity in an additive manner.

Atriopeptin release from the isolated perfused rabbit heart

Mammalian atrial extracts have been shown to contain bioactive peptides which exert natruiretic, diuretic, and smooth muscle relaxant effects. These extracts include several low molecular weight (less than 5,000 Mr) atrial peptides (atriopeptins) which exhibit identical sequences over a central core region which are derived from the high molecular weight peptide (atriopeptigen) precursor which has been purified and sequenced. In the current study we found that extracts of rabbit atria possess both high and low molecular weight bioactive atrial peptides, however, the coronary venous effluent obtained from the isolated perfused rabbit heart only contained the low molecular weight peptide. This trypsin labile activity causes a dose-dependent relaxation of rabbit aorta and chicken rectum assay strips. Separation of the bioactivity with gel filtration chromatography and reversed phase HPLC indicates the heart releases a single substance similar to atriopeptin III. There was no evidence that atriopeptigen was released from the isolated perfused rabbit heart. We suggest that atriopeptigen is proteolytically processed in the atria to an atriopeptin which is subsequently the released form of the atrial peptide.

Atrial natriuretic factor in rat hypothalamus, atria and plasma: determination by specific radioimmunoassay

A rapid and reproducible radioimmunoassay method was developed for rat atrial natriuretic factor (ANF)-IV. The method is also applicable to human atrial peptide. ANF was detected in rat hypothalamus (5.03 pmoles/g tissue), right (86.8 pmoles/mg tissue) and left atria (52.5 pmoles/mg tissue), and plasma (156 fmoles/ml). After high salt intake immunoreactive ANF in atria and plasma increased significantly, while a significant decrease was observed in hypothalamus. Gel chromatography revealed high and low molecular weight ANF in atria and hypothalamus while only a low molecular weight form was found in plasma.