Atrial natriuretic peptide inhibits neurohypophysial hormones' release in the rat (in vitro and in vivo studies).

Intracerebroventricular hANP (50 nmol) inhibits release of vasopressin and oxytocin following dehydration as well as after haemorrhage. 10 nmol/L hANP markedly inhibits vasopressin and oxytocin release in vitro from the neurointermediate lobes both under basal condition as well as during stimulation with excess (56 mM) potassium. It is suggested that ANP may serve as a modulator of vasopressin and oxytocin release. The respective processes are localized, at least in part, at the neurohypophysial level.     

IGF-I does not affect the net increase in GH release in response to arginine

Arginine stimulates growth hormone (GH) secretion, possibly by inhibiting hypothalamic somatostatin (SS) release. Insulin-like growth factor I (IGF-I) inhibits GH secretion via effects at the pituitary and/or hypothalamus. We hypothesized that if the dominant action of IGF-I is to suppress GH release at the level of the pituitary, then the arginine-induced net increase in GH concentration would be unaffected by an IGF-I infusion. Eight healthy young adults (3 women, 5 men) were studied on day 2 of a 47-h fast for 12 h (35th-47th h) on four occasions. Saline (Sal) or 10 microg. kg(-1). h(-1) recombinant human IGF-I was infused intravenously for 5 h from 37 to 42 h of the 47-h fast. Arginine (Arg) (30 g iv) or Sal was infused over 30 min during the IGF-I or Sal infusion from 40 to 40.5 h of the fast. Subjects received the following combinations of treatments in random order: 1) Sal + Sal; 2) Sal + Arg; 3) IGF-I + Sal; 4) IGF-I + Arg. Peak GH concentration on the IGF-I + Arg day was ~45% of that on the Sal + Arg day. The effect of arginine on net GH release was calculated as [(Sal + Arg) - (Sal + Sal)] - [(IGF-I + Arg) - (IGF-I + Sal)]. There was no significant effect of IGF-I on net arginine-induced GH release over control conditions. These findings suggest that the negative feedback effect of IGF-I on GH secretion is primarily mediated at the pituitary level and/or at the hypothalamus through a mechanism different from the stimulatory effect of arginine.     

Ethnic differences in adrenocorticotropic hormone, cortisol and corticotropin-releasing hormone during pregnancy

Significant ethnic disparities exist in reproductive outcomes. A potential contributing factor may be the functioning of the hypothalamic-pituitary-adrenal (HPA) axis and placenta during pregnancy. In the present study, levels of cortisol, ACTH and CRH were determined longitudinally from the plasma of 310 African American, Hispanic and non-Hispanic White women at 18-20, 24-26 and 30-32 weeks' gestation. During pregnancy, African American women exhibited lower levels of cortisol than non-Hispanic women and higher levels of ACTH than Hispanic women. The trajectory of CRH increase also differed by ethnicity, with African Americans exhibiting the lowest levels both early and late in pregnancy. Higher levels of cortisol at 18-20 weeks were associated with higher levels of CRH at 30-32 weeks among the African American and Hispanic women, but not among non-Hispanic women. Ethnic differences persisted when adjusting statistically for sociodemographic and biomedical factors. The findings are consistent with the possibility that ethnic disparities in adverse birth outcomes may be due, in part, to differences in HPA axis and placental function.     

Effect of atrial natriuretic peptide on acetylcholine-induced release of corticotropin-releasing factor from rat hypothalamus in vitro

Effect of rat atrial natriuretic peptide (rANP) on acetylcholine-induced release of corticotropin-releasing factor (CRF) from the rat hypothalamus was studied in vitro using perifusion method. Perifused acetylcholine at 100 and 1000 ng/ml evoked significant CRF release, whereas norepinephrine at 10, 100 and 1000 ng/ml did not show a definite effect on CRF release. Continuous administration of alpha-rANP(1-28) (20ng/ml) inhibited the acetylcholine (100ng/ml)-induced CRF release. It is likely that ANP is involved in the regulation of CRF release.     

Atrial natriuretic peptide inhibits the stimulation of aldosterone secretion by ACTH in vitro and in vivo

Previous studies have shown that atrial natriuretic peptide (ANP) inhibits the secretion of aldosterone by isolated adrenal glomerulosa cells stimulated by angiotensin II, ACTH and potassium in vitro and by angiotensin II in conscious unrestrained rats. In this study we investigated further the effects of synthetic ANP on the dose-response curve of aldosterone secretion stimulated by ACTH in vitro. ANP displaced the dose-response curve of aldosterone to ACTH to the right with a significant change in EC50. A similar effect of ANP was reproduced in vivo in conscious unrestrained rats. There was no significant effect of ANP on the corticosterone response to ACTH in vivo. ANP is a potent regulator of aldosterone secretion which may modulate the effects of ACTH on the adrenal in vitro and in vivo.     

Altered regulation of renal nitric oxide and atrial natriuretic peptide systems in angiotensin II-induced hypertension

The present study was aimed to determine whether there is an altered role of local nitric oxide (NO) and atrial natriuretic peptide (ANP) systems in the kidney in association with the angiotensin (Ang) II-induced hypertension. Male Sprague-Dawley rats were used. Ang II (100 ng·min?1·kg?1) was infused through entire time course. Thirteenth day after beginning the regimen, kidneys were taken. The protein expression of NO synthase (NOS) and nitrotyrosine was determined by semiquantitative immunoblotting. The mRNA expression of components of ANP system was determined by real-time polymerase chain reaction. The activities of soluble and particulate guanylyl cyclases were determined by the amount of cGMP generated in responses to sodium nitroprusside and ANP, respectively. There developed hypertension and decreased creatinine clearance in the experimental group. The protein expression of eNOS, nNOS and nitrotyrosine was increased in the cortex, while that of iNOS remained unaltered. The urinary excretion of NO increased in Ang II-induced hypertensive rats. The catalytic activity of soluble guanylyl cyclase was blunted in the glomerulus in Ang II-induced hypertensive rats. The mRNA expression of ANP was increased in Ang II-induced hypertensive rats. Neither the expression of NPR-A nor that of NPR-C was changed. The protein expression of neutral endopeptidase was decreased and the activity of particulate guanylyl cyclase was blunted in the glomerulus and papilla in Ang II-induced hypertensive rats. In conclusion, the synthesis of NO and ANP was increased in the kidney of Ang II-induced hypertension, while stimulated cGMP response was blunted. These results suggest desensitization of guanylyl cyclase in the kidney of Ang II-induced hypertensive rats, which may contribute to the associated renal vasoconstriction and hypertension.     

Atrial natriuretic peptide during water deprivation or hemorrhage in rats. Relationship with arginine vasopressin and osmolarity.

The concentrations of atrial natriuretic peptide (ANP) in atria, hypothalami and plasma were investigated in relation to the variations of the plasma endogenous immunoreactive arginine vasopressin (Ir-AVP) during water deprivation or hemorrhage in normal conscious Wistar rats. Furthermore, the in vitro and in vivo effect of extracellular hyperosmolarity on ANP release from right atrium and hypothalamus was examined. Water deprivation elevated circulating immunoreactive ANP (Ir-ANP: pg/ml) to 153 +/- 7 (24 h); 174 +/- 1 (48 h) from the control level (109.6 +/- 7.8). This increase in Ir-ANP concentration which correlated with atrial (r = -0.93) or hypothalamic (r = -0.87) Ir-ANP content decrease, was associated with significantly enhanced levels of plasma Ir-AVP, plasma sodium, osmolarity and hematocrit. An acute volume depletion by hemorrhage significantly reduced plasma Ir-ANP (67 +/- 8.4 pg/ml) from the sham operated level (140 +/- 18 pg/ml). Plasma Ir-AVP was elevated dramatically (207.4 +/- 53.4 pg/ml) compared with the sham operated level (8.8 +/- 2.6 pg/ml). These results, indicating the lack of correlation between plasma Ir-ANP and Ir-AVP in vivo, suggest that the ANP secretion, which is regulated mainly by plasma volume, may be modulated by a change in plasma osmolarity. Extracellular hyperosmolarity stimulated the ANP release from superfused sliced normal rat atria and hypothalami.     

Atrial dynamics and release of atrial natriuretic factor in vivo.

In anaesthetized rabbits blood volume was altered by infusion and withdrawal of donor blood over the range of +60 to -40% of the blood volume. Right and left atrial pressures were measured and it was shown that sonomicrometry allowed adequate measurement of phasic changes in atrial dimensions. Plasma immunoreactive atrial natriuretic peptide concentration changed in a nonlinear fashion with changes in blood volume, and was linearly related to both peak systolic and peak diastolic right and left atrial wall stress. It was not possible to make the distinction between distension (diastolic stress) or tension (systolic stress) as the major determinant of ANF release in response to changes in blood volume.     

Using GFP to image peptide hormone and neuropeptide release in vitro and in vivo

Traditionally, peptide secretion by endocrine cells and neurons was studied by measuring changes in release in response to experimental perturbations. Now it is possible to directly view dense core vesicles (DCVs), secretory apparatus proteins and individual exocytotic events by imaging fluorescent proteins in living cells. Fundamental insights into peptide release by cultured cells have been made with wide field, confocal and total internal reflection (also called evanescent wave) microscopes. Researchers have also used a variety of fluorescent protein constructs that vary in spectra, pH sensitivity, inducibility, and age dependence. Most recently, these approaches have been applied to transgenic animals so that hormone and neuropeptide release can be studied in vivo.     

Atrial natriuretic peptide concentration and natriuretic hormone activity in plasma of patients with chronic renal failure

To elucidate further the possible role of atrial natriuretic peptide (ANP) and hypothetical natriuretic hormone (NH) in volume and BP regulation in chronic renal failure (CRF) we measured plasma ANP, digitalis-like substances (DLS) and Na+-K+-ATPase activity (using 86Rb influx into RBC) in 9 patients with CRF before and after hemodialysis. Volume expansion between consecutive dialyses led in all patients to the elevation of plasma ANP (83.4 +/- 14.2 pmol/l) reaching in some overhydrated subjects and/or patients with concomitant cardiac insufficiency concentration greater than 150 pmol/l. Reduced 86Rb influx into RBC before hemodialysis (37.7 +/- 4.9% of controls) was accompanied by higher DLS concentrations (201 +/- 32 pmol/l). Ultrafiltration during hemodialysis with ECFV reduction lowered both ANP and DLS concentrations to 28.1 +/- 9.4 pmol/l and to 151 +/- 23 pmol/l, respectively, and abolished partly the inhibition of Na+-K+-ATPase activity (64.9 +/- 7.6% of controls). These changes corresponded to the degree of ECFV alteration. Our results suggest that both natriuretic principles are activated during ECFV expansion in CRF, probably as a corrective mechanism, with a tendency to normalize when ECFV is reduced during hemodialysis.     

Transgenic overexpression of intraislet ghrelin does not affect insulin secretion or glucose metabolism in vivo

Whereas ghrelin is produced primarily in the stomach, a small amount of it is produced in pancreatic islets. Although exogenous administration of ghrelin suppresses insulin secretion in vitro or in vivo, the role of intraislet ghrelin in the regulation of insulin secretion in vivo remains unclear. To understand the physiological role of intraislet ghrelin in insulin secretion and glucose metabolism, we developed a transgenic (Tg) mouse model, rat insulin II promoter ghrelin-internal ribosomal entry site-ghrelin O-acyl transferase (RIP-GG) Tg mice, in which mouse ghrelin cDNA and ghrelin O-acyltransferase are overexpressed under the control of the rat insulin II promoter. Although pancreatic desacyl ghrelin levels were elevated in RIP-GG Tg mice, pancreatic ghrelin levels were not altered in animals on a standard diet. However, when Tg mice were fed a medium-chain triglyceride-rich diet (MCTD), pancreatic ghrelin levels were elevated to ～16 times that seen in control animals. It seems likely that the gastric ghrelin cells possess specific machinery to provide the octanoyl acid necessary for ghrelin acylation but that this machinery is absent from pancreatic β-cells. Despite the overexpression of ghrelin, plasma ghrelin levels in the portal veins of RIP-GG Tg mice were unchanged from control levels. Glucose tolerance, insulin secretion, and islet architecture in RIP-GG Tg mice were not significantly different even when the mice were fed a MCTD. These results indicate that intraislet ghrelin does not play a major role in the regulation of insulin secretion in vivo.     

Atrial natriuretic peptide and angiotensin II binding sites in cerebral capillaries of spontaneously hypertensive rats

1. We carried out investigations on specific atrial natriuretic peptide (ANP) and angiotensin II (ANG) binding sites in capillaries isolated from the cerebral cortex of spontaneously hypertensive rats (SHR), an animal model of human essential hypertension, and also from Wistar Kyoto rats (WKY). 2. In an equilibrium binding study done in the presence of increasing concentrations of the radiolabeled ligands, the binding of 125I-rat alpha-ANP (1-28) [ANF-(99-126)] (125I-rANP) and 125I-ANG (5-L-isoleucine) (125I-ANG) to the cerebral capillaries was single and of a high affinity. 3. The maximum binding capacity (Bmax) and dissociation constant (Kd) in the 125I-rANP binding of 20-week-old, hypertensive SHR was significantly lower than in age-matched, normotensive WKY. Conversely, a significant increase in the Bmax of 125I-ANG binding of adult SHR was observed, with a significant decrease in the Kd. 4. There was no differences in the Bmax of 125I-rANP and 125I-ANG binding between 4-week-old, prehypertensive SHR and age-matched WKY. However, there was a significant decrease in the Kd of 125I-rANP binding of SHR. 5. As a dramatic change in the binding kinetics of 125I-rANP and 125I-ANG was noted in the cerebral capillaries of adult sustained-hypertensive SHR, the possibility that ANP and ANG play a role in the etiology of dysfunction of the blood-brain barrier complicated with hypertension, by interacting with specific receptors, would have to be considered.     

In vivo and in vitro studies on the effect of adrenocorticotropic hormone or cortisol on the pituitary response to gonadotropin releasing hormone

Experiment I: Hyperadrenal states were induced in intact heifers (N = 3) or adrenalectomized (ADRX) heifers (N = 3) by constant infusion of ACTH (20.8 micrograms, 1-24 ACTH/h) or hydrocortisone succinate (HS) (30 mg/h), respectively. Control infusions consisted of the saline vehicle. All infusions began on Day 2 of a normal estrous cycle. Exogenous gonadotropin releasing hormone (GnRH) was given as a 100-micrograms bolus i.v. on Days 7, 9, and 11 (intact) or 5, 7, and 9 (ADRX) of the cycle. In intact heifers, the cumulative luteinizing hormone (LH) response was reduced (P less than 0.05) by the ACTH treatment. In ADRX heifers, the HS treatment did not alter the cumulative response but did alter the qualitative response with a time X treatment interaction (P less than 0.01). The LH response in the HS-ADRX animals had a slower onset and lower peak concentrations with a more prolonged response. Experiment II: Dispersed bovine pituitary cells were prepared and incubated at concentrations of 2 X 10(6) viable cells in 2.0 ml per dish. Cells were exposed to cortisol at concentrations of 0.01, 0.10, 0.21 and 1.03 X 10(-6) M for time periods of 8, 14, 20 or 26 h for basal LH secretion studies and 10, 16, 22 and 28 h for GnRH-stimulated LH secretion. Both dosage of cortisol and length of exposure had a depressing effect on basal LH release. The cortisol pretreatment also decreased (P less than 0.001) the LH release following addition of GnRH (8.5 X 10(-8) M) in cultures at all dosages and exposure times of cortisol. However, there was no decrease in LH or protein content of cells. These experiments indicate a direct action of cortisol on the pituitary gland to depress both basal and stimulated LH release.     

Endothelin stimulates release of atrial natriuretic peptides in vitro and in vivo

The effect of endothelin (END) on the release of atrial natriuretic peptides (ANP) was studied in isolated rat atria and in conscious rats. END stimulates the ANP release in vitro in a dose-dependent manner. An increase in ANP plasma levels and cyclic GMP plasma levels was also observed in conscious rats after injection of END. When a monoclonal antibody directed against ANP was injected together with END the increase in cyclic GMP was completely blocked. From this study it is concluded that END is a potent secretagogue for ANP both in vitro and in vivo.     

Effect of natriuretic peptides on in vitro stimulated adrenocorticotropic hormone release and pro-opiomelanocortin mRNA expression by the fetal rat pituitary gland in late gestation

OBJECTIVE AND METHODS: We investigated the effects of individual natriuretic peptides (atrial natriuretic peptide, ANP; brain natriuretic peptide, BNP, and C-type natriuretic peptide, CNP) on rat corticotropin-releasing factor stimulated adrenocorticotropic hormone (ACTH) secretion by the pituitary gland of 21-day-old rat fetuses in vitro and on pro-opiomelanocortin gene expression using in situ hybridization. RESULTS: Graded concentrations of ANP, BNP, or CNP (10(-10), 10(-9), and 10(-8) mol/l) induced a log dose dependent inhibition of ACTH secretion induced by rat corticotropin-releasing factor (10(-10) mol/l). These natriuretic peptides showed equipotent effects on a molar basis. Moreover, ANP, BNP, or CNP at 10(-10) mol/l reduced significantly the pituitary pro-opiomelanocortin mRNA expression. In addition, the immunoreactive ANP, BNP, and CNP cells were localized in the anterior lobe, but not in the intermediate lobe of the fetal pituitary gland. CONCLUSIONS: These data suggest that the fetal pituitary gland may be both a source and a target for natriuretic peptides that might control ACTH synthesis and release via an endocrine and/or paracrine mechanism. The natriuretic peptides could participate, as well as glucocorticoids, in the control of the corticotropin-stimulating activity of the fetal rat in late gestation.     

Physiologically regulated transgenic ABCA1 does not reduce amyloid burden or amyloid-beta peptide levels in vivo

ABCA1-deficient mice have low levels of poorly lipidated apolipoprotein E (apoE) and exhibit increased amyloid load. To test whether excess ABCA1 protects from amyloid deposition, we crossed APP/PS1 mice to ABCA1 bacterial artificial chromosome (BAC) transgenic mice. Compared with wild-type animals, the ABCA1 BAC led to a 50% increase in cortical ABCA1 protein and a 15% increase in apoE abundance, demonstrating that this BAC supports modest ABCA1 overexpression in brain. However, this was observed only in animals that do not deposit amyloid. Comparison of ABCA1/APP/PS1 mice with APP/PS1 controls revealed no differences in levels of brain ABCA1 protein, amyloid, Abeta, or apoE, despite clear retention of ABCA1 overexpression in the livers of these animals. To further investigate ABCA1 expression in the amyloid-containing brain, we then compared ABCA1 mRNA and protein levels in young and aged cortex and cerebellum of APP/PS1 and ABCA1/APP/PS1 animals. Compared with APP/PS1 controls, aged ABCA1/APP/PS1 mice exhibited increased ABCA1 mRNA, but not protein, selectively in cortex. Additionally, ABCA1 mRNA levels were not increased before amyloid deposition but were induced only in the presence of extensive Abeta and amyloid levels. These data suggest that an induction of ABCA1 expression may be associated with late-stage Alzheimer's neuropathology.     

Atrial natriuretic peptide (ANP) does not inhibit the basal vascular tone present in the in situ blood-perfused dog gracilis muscle

This study evaluated the effects of rat ANP(5-28) infusion into the blood-perfused dog gracilis muscle at concentrations ranging from 30 to 10,000 pg/ml. The vasculature of gracilis muscles from anesthetized beagle dogs was isolated and pump-perfused at constant flow with blood utilizing an extracorporeal circuit. Maximal vasodilatory capacity was determined by adenosine injection. ANP was infused into the arterial circuit to produce increasing arterial blood concentrations. Each infusion lasted 10 min. Systemic arterial pressure, central venous pressure, cardiac output and heart rate did not change during ANP infusion into the gracilis vasculature. ANP at arterial blood concentrations up to 10,000 pg/ml did not produce significant vasodilation although the vasculature showed pronounced vasodilation in response to adenosine. In vitro experiments showed that ANP had much less vasorelaxant activity in dog femoral artery and saphenous vein than in rabbit aorta. Therefore, rat ANP(5-28) at concentrations within and well above physiological and pharmacological ranges does not inhibit the basal vascular tone present in the innervated, blood-perfused dog gracilis muscle in situ.     

Estradiol, progesterone and testosterone exposures affect the atrial natriuretic peptide gene expression in vivo in rats.

To clarify the effects of sex hormones on the expression of atrial natriuretic peptide (ANP), ovariectomized and intact female rats were subcutaneously injected with estradiol, progesterone, a mixture of them or olive oil solvent; castrated and untouched male rats were subcutaneously injected with estradiol, testosterone or olive oil, once a day for 7 days. The relative rANP-mRNA contents of rat atrial were measured by molecular hybridization. rANP-cDNA was labeled with 32P as a probe. The results revealed that estradiol and progesterone increased ANP gene expression. Furthermore their effects were associated with administration dose of these hormones and it was shown that they are probably coordinated. The physiological amounts of estradiol and progesterone may maintain suitable levels of rANP-mRNA and androgen may also increase the ANP gene expression in vivo. These experiments suggested that female sex hormone may have a dual purpose in fluid balance.