Olli Arjamaa

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).

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.

Block of stretch-activated atrial natriuretic peptide secretion by gadolinium in isolated rat atrium.

1. Isolated superfused rat atrial preparations were used to study the mechanism of stretch‐induced atrial natriuretic peptide (ANP) secretion. The stretch of the atrial myocytes was induced by raising the intra‐atrial pressure. The secretion rates were analysed by measuring ANP concentrations from the superfusate fractions by radioimmunoassay. 2. The effect of gadolinium, a blocker of stretch‐activated ion channels, on stretch‐induced and basal ANP secretion was investigated by superfusing the atrial preparation with 5, 20 or 80 microM GdCl3. Gadolinium decreased stretch‐induced ANP secretion in a dose‐dependent manner, but did not affect basal secretion. 3. Because high concentrations of gadolinium may block voltage‐gated calcium channels, we tested whether the selective blockers of L‐type (diltiazem) and T‐type (NiCl2) calcium channels affect the stretch‐stimulated ANP release. Neither diltiazem at 3 microM nor NiCl2 at 50 microM affected stretch‐induced ANP release in paced atrial preparation. 4. Gadolinium, but not diltiazem, also inhibited stretch‐stimulated ANP secretion in non‐paced, quiescent atria. 5. The findings that ANP release is inhibited by Gd3+, but not by diltiazem or NiCl2, and that the stretch‐induced secretion in quiescent atria is also inhibited by Gd3+, suggest that stretch‐activated ion channels are involved in the regulation of stretch‐induced ANP release.

Pressor hormones regulate atrial-stretch-induced release of atrial natriuretic peptide in the pithed rat.

Atrial wall stretching is a known stimulus for atrial natriuretic peptide (ANP) secretion. The effects of the stimulation of autonomic nervous system, hemodynamic factors, and humoral factors (epinephrine, angiotensin, vasopressin, and brain extracts) on the release of ANP under basal conditions and during increased atrial pressure produced by acute volume loading in pithed rats were examined. In conscious rats, acute volume expansion by 0.9percent; of saline (4 ml) increased the plasma immunoreactive ANP (IR-ANP) concentrations by a factor of 4 (140±30 pg/ml vs. 521±140 pg/ml, p<0.001, n=8), whereas volume-induced ANP release was blocked in pithed rats (75±9 pg/ml vs. 99±13 pg/ml, NS, n=7). The ANP versus right atrial pressure curve shifted to the right, indicating that much smaller amounts of IR-ANP were released in pithed than in conscious rats for each given increase in right atrial pressure. Electrical vagal and sympathetic nerve stimulations or changes in heart rate had no effect on plasma IR-ANP concentrations and failed to restore the volume-load-induced release of ANP in pithed rats. When extracts of anterior pituitary lobe, brain cortex, or hypothalamus were infused, no effect on volume-expansion-induced plasma IR-ANP levels was seen. In contrast, acute volume expansion caused a fourfold increase in levels of circulating IR-ANP in pithed rats that received posterior pituitary extracts, and the ANP versus right atrial pressure curve shifted markedly to the left. Infusion of a V1 antagonist blocked the volume-expansion-induced ANP release produced by the posterior pituitary extract. When [Arg8]-vasopressin (0.025 or 0.05 μg/kg/ min) was infused to pithed rats, mean arterial pressure increased but basal plasma IR-ANP did not change significantly. However, acute volume expansion in the presence of vasopressin infusion (0.05 μg/kg/min) increased the amount of circulating IR-ANP by a factor of 4 (113±14 pg/ml vs. 414±43 pg/ml, p<0.001, n=8). Thus, for a given increase in right atrial pressure, a similar amount of IR-ANP was released in the pithed rat during the vasopressin infusion as in the normal conscious animal. V1 antagonist blocked the increase in mean arterial pressure as well as the increase of plasma IR-ANP produced by [Arg8]-vasopressin. In addition, volume expansion during intravenous epinephrine (1.75 μg/kg/min) and angiotensin (1.0 μg/kg/min) doubled plasma IR-ANP levels. These results indicate that pressor hormones, especially vasopressin, restore the ability of volume expansion to induce ANP release in the pithed rat. The combined effect of right atrial pressure and pressor hormones in regulating ANP release is a new mechanism by which humoral stimulation modulates the direct, mechanical-stimulus-induced hormone secretion.

Atrial natriuretic peptide (ANP) in rat gastrointestinal tract

Acid extracts of rat stomach and small intestine contained 8.6 ± 3.7 and 39 ± 15 ng/g of immunoreactive atrial natriuretic peptide (ANP). When studied by gel filtration and reverse‐phase high‐performance liquid chromatography, the stomach immunoreactivity consisted of multiple components, whereas the small intestine contained mostly proANP and ANP 1–28‐like material. These findings indicate that ANP may have a role in the physiology of the gastrointestinal tract, e.g. in the regulation of water and electrolyte absorption.

A Novel Cardiac Hormone Related to A-, B-, and C-Type Natriuretic Peptides

Mechanisms acting against accumulation of volume are important in pathophysiological situations with volume and salt overload, such as congestive heart failure. Osmoregulating animals that migrate between environments with high and low salinity are ideal models for studying the defence mechanisms against volume gain. We have now cloned and sequenced from salmon (Salmo salar) a cDNA encoding a novel vasorelaxant cardiac hormone of 29 amino acids which is produced by proteolytic processing of a 148-residue preprohormone. Structural and biological results, as well as its distribution indicate that it belongs to an unrecognized family related to natriuretic peptides, perhaps representing an ancestor of ANP and BNP. We have synthesized the 29-amino acid hormone and set up a specific radioimmunoassay. The distribution of the mRNA and peptide is strictly restricted to the heart, with high levels both in the atrium and ventricle in various fish species. The hormone relaxes aortic smooth muscle derived from salmon...

Effect of ryanodine on atrial natriuretic peptide secretion by contracting and quiescent rat atrium.

To elucidate the mechanism involved in the release of atrial natriuretic peptide (ANP), we studied the importance of ryanodine-sensitive Ca2+ release in stretch-secretion coupling. The experiments were made with a left atrial preparation, where the stretch of myocytes was induced by changing the intra-atrial pressure. When external pacing was not applied, the atrial preparation was not spontaneously contracting, and it was therefore possible to investigate the secretory mechanism in the quiescent atrium. The superfusate was collected in 2-min fractions and assayed for ANP immunoreactivity. Filtration analysis revealed that the major fraction in the superfusate in all experimental situations had a similar molecular weight as the ANP 1–28. Ryanodine (1.0 μM and 0.1 μM) inhibited stretch-stimulated ANP secretion dose dependently both in paced and nonpaced atrium, but did not have any effect on basal secretion. The present results support the notion that intracellular Ca2+ transients from the intracellular stores are essential for stretch-stimulated ANP secretion, independently from excitation and contraction. Basal ANP secretion is not inhibited by blocking ryanodine-sensitive Ca2+ channels, either in contracting or in non-contracting atria. In addition our results confirm that the principal stimulus for ANP secretion in response to atrial distension is the stretch of myocytes. Length shortening of myocytes is not essential for ANP release.

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.

Are the blood pressure and endocrine responses of healthy subjects exposed to cold stress altered by an acutely increased sodium intake

Abstract In the study reported here, we examined blood pressure and endocrine responses in cold conditions during salt load in young healthy subjects who had previously shown increased resting blood pressure during acutely increased sodium intake. Subjects (n=53) added 121 mmol sodium into their normal diet for 1 week. If their mean arterial pressure had increased by a minimum of 5 mmHg compared to the previous measure they were selected for subsequent experiments. The subjects (n=8) were given 121 mmol supplemental sodium · day−1 for 14 days. They were then put into a wind tunnel for 15 min (temperature −15 °C, wind speed 3.5 · ms−1). Their blood pressure increased (P < 0.05) during the cold exposure, independent of the sodium intake. Their mean (SEM) plasma noradrenaline increased from 3.58 (0.62) nmol · l−1 to 5.61 (0.79) nmol · l−1 (P < 0.05) when the subjects were given a normal diet, and from 2.45 (0.57) nmol · l−1 to 5.06 (0.56) nmol · l−1 (P < 0.05) when the subjects were given an elevated sodium diet. The starting concentrations and the endpoint concentrations were statistically similar. The plasma levels of the N-terminal fragment of pro-atrial natriuretic peptide decreased during the whole-body cold exposure: with the sodium load the change was from 256.6 (25.5) nmol · l−1 to 208.0 (25.3) nmol · l−1, and with the normal diet, from 205.8 (16.4) nmol · l−1 to 175.1 (16.1) nmol · l−1. The haematocrit and red blood cell count increased (P < 0.05) with normal and elevated sodium diet in cold conditions, but haemoglobin increased (P < 0.05) only with high salt in cold conditions. To conclude, acutely increased sodium intake does not change the blood pressure response or hormonal responses to exposure to acute cold stress in healthy subjects.

The concentration of atrial natriuretic peptide (ANP) is decreased in plasma but not in atria in hypophysectomised rats

To study the role of the pituitary gland in the release of Atrial Natriuretic Peptide (ANP) plasma and atrial concentrations were measured both in intact and in hypophysectomized rats. The plasma concentration of ANP (pg/ml) was significantly (p less than 0.01) decreased from 143 +/- 35 to 82 +/- 29 (mean +/- SD, n) while the tissue concentration (ng/wet tissue mg) remained unchanged, 192 +/- 46 and 194 +/- 39, respectively. The total atrial amount of ANP (ug) was, however, significantly (p less than 0.01) decreased from 29.7 +/- 7.8 to 17.0 +/- 3.3 after hypophysectomy. In intact animals, a volume load (1.1ml/100 body weight g 0.9% NaCl) resulted in 2-fold (p less than 0.001) increase in the plasma ANP levels whereas similar load had no effects on plasma ANP levels in hypophysectomized animals. In both groups, the right atrial pressure was increased from about 2 to about 6 mmHg. We conclude that in the absence of pituitary gland the right atrial pressure and the atrial ANP concentration do not change but plasma ANP levels and the response to volume stimulus are attenuated.