Role of NPR-C natriuretic receptor in nitric oxide system activation induced by atrial natriuretic peptide

Atrial natriuretic peptide (ANP) exerts its hypotensive, natriuretic and diuretic effects, almost in part, through the activation of nitric oxide synthase (NOS). The aim was to investigate the natriuretic receptor type and the signaling cascade involved in NOS activation induced by ANP. Male Wistar rats were sacrificed and NOS activity was determined in kidney, aorta and heart with L-[U14C]-arginine, as substrate. ANP and cANP (4-23), a selective NPR-C ligand, increased NOS activity in all tissues. ANP induced a more marked activation in aorta and kidney than cANP (4-23), but no difference in atria NOS activation was observed. NOS activity induced by both peptides was blunted by nifedipine (L-type channel blocker) and calmidazolium (calmodulin antagonist) in heart and aorta. In kidney, nifedipine and calmidazolium abolished NOS activity stimulated by cANP (4-23) but only partially inhibited NOS activity elicited by ANP. Gi inhibition with pertussis toxin abolished NOS activity stimulated by ANP and cANP in atria but only partially inhibited the increased NOS activity induced by ANP and cANP in kidney, aorta and ventricle. Our results show that NPR-C receptor would mediate the activation of NOS by ANP in atria. In kidney, aorta and ventricle, NOS activation would also involve NPR-A and/or B. ANP would interact with NPR-C coupled via Gi to activation Ca2+ -dependent NOS.     

The role of atrial natriuretic peptide in the immune system

Atrial natriuretic peptide (ANP) is a hormone predominately produced by the heart atria which regulates the water and salt balance as well as blood pressure homeostasis. Being expressed in various parts of the immune system a link of the peptide to the immune system has been proposed. In fact, this review focus on effects of ANP in the immune system and reports about the role of the peptide in innate immune functions as well as in the adaptive immune response.     

Nocturnal fluctuations of plasma atrial natriuretic peptide in normotensive and hypertensive men

Plasma levels of immunoreactive atrial natriuretic peptides (ANP) were measured at 10-min intervals during night-sleep in 4 normotensive and in 4 moderate, essential hypertensive subjects. The mean ANP levels ranged from 24.3 to 27.9 pg.ml-1 for the normal subjects. These mean levels were not significantly different in the hypertensive subjects (range: 26.3 to 37.2 pg.ml-1). Fluctuations, often of small amplitude, were observed around this mean, without any defined periodicity. Changes in plasma ANP were not associated with changes in heart rate. Analysis of the ANP profiles and the concomitant sleep stage patterns did not reveal any temporal relationship between ANP fluctuations and specific sleep stages or waking periods. The ANP profiles did not differ between the groups, which indicates no abnormality in ANP secretion in moderate essential hypertension.     

The effect of atrial natriuretic peptide on intestinal electrolyte transport.

The effect of atrial natriuretic peptide (ANP) on rat small intestinal electrolyte transport was examined. In vivo, intravenous administration of rat ANP(99-126) induced diuresis and natriuresis in conjunction with a significant decrease in intestinal water (basal, 37.1 +/- 5.7 versus ANP 28.5 +/- 6.0 microliters/cm per 20 min, P less than 0.05) and Na+ (4.0 +/- 0.7 versus 2.8 +/- 0.9 mumol/cm per 20 min, P less than 0.05) absorption (n = 9). In vitro, in Ussing chambers, in both jejunum and ileum, addition of 1.0 microM ANP to short circuited, stripped tissue produced a maximal increase in short circuit current and stimulated net Cl- secretion due to a significant increase in the unidirectional serosal to mucosal flux (JCl-sm: jejunum 17.4 +/- 1.3 versus 19.8 +/- 1.3 microEq/cm2 per h, P less than 0.01, n = 6; ileum 13.4 +/- 0.5 versus 17.2 +/- 0.6, P less than 0.01, n = 6) which was inhibited by the calcium channel antagonist verapamil (82 +/- 26%, P less than 0.05) and by the 5-HT2 receptor antagonist cinanserin (72 +/- 44%, P less than 0.05). Guanylate cyclase activity was stimulated by ANP in intact epithelium, but not in isolated crypt and villus enterocytes.     

The sympathetic nervous system is involved in variant Creutzfeldt-Jakob disease

Prion epizoonoses spread from animals consumed by humans raise the question of which pathways lead to prion neuroinvasion after oral exposure of humans. Here we show that neurons of sympathetic ganglia of patients with variant Creutzfeldt-Jakob disease (vCJD) accumulate the abnormal isoform of the protein prion. This observation shows the involvement of the sympathetic nervous system in the pathogenesis of vCJD and suggests a role for GUT-associated sympathetic neurons in prion propagation in humans after oral contamination.     

Natriuretic effect of atrial natriuretic peptide in diabetes insipidus rats

Washout of the solute concentration gradient in the renal medullary interstitium has been suggested to play a role in mediating the natriuretic response to atrial natriuretic peptide (ANP). The purpose of this study was to determine the effects of ANP 8-33 on sodium excretion in Brattleboro diabetes insipidus (DI) rats, in which medullary tonicity is known to be decreased as compared to Long-Evans (LE) control rats. Basal urine osmolality (Uosm) was significantly lower in DI rats as compared to LE rats (123 +/- 6 vs 673 +/- 38 mOsm/kg). Infusion of ANP 8-33 at a rate of 4 micrograms/kg/hr for 60 min resulted in a significantly greater increase in UnaV (delta 6.1 +/- 1.2 vs delta 2.9 +/- 0.7 microEq/min) and urine flow (delta 40 +/- 12 vs delta 8 +/- 7 microliter/min) in the LE rats than in the DI rats. The greater natriuresis occurred in the LE rats despite no significant change in Uosm. Fractional lithium reabsorption (an indicator of proximal sodium reabsorption) decreased similarly in both groups. Infusion of ANP had no effect on mean arterial pressure in LE and DI groups. In summary, infusion of ANP in the DI rat resulted in a significant natriuresis, albeit less than in LE rats. The natriuresis in the LE rats occurred despite no significant change in Uosm. These data suggest that mechanisms other than medullary washout are responsible for the natriuretic effects of ANP.     

The effect of sulfinpyrazone on the response to sympathetic nervous system stimulation

The purpose of this study was to determine if sulfinpyrazone has a direct action on sympathetic nerve endings to prevent release of the transmitter. Pre-synaptic and post-synaptic events as well as direct sympathetic nervous system stimulation were tested in 15 α-chloralose anesthetized cats before and 1 hour after sulfinpyrazone (100 mg·kg⁻¹, i.v.). Heart rate response to cardiac accelerator nerve stimulation or to increasing doses of isoproterenol was not significantly depressed by sulfinpyrazone. In addition, no alteration in the reflex activation of the sympathetic nervous system in response to histamine was observed following sulfinpyrazone. Both norepinephrine and epinephrine levels were similar to those levels reported previously by Smith and Robinson (7) for untreated cats. We conclude sulfinpyrazone has no direct depressing effect on the sympathetic nerve endings and that this mechanism cannot explain the reported beneficial effect of sulfinpyrazone on coronary occlusion induced arrhythmias.     

Placental barrier to atrial natriuretic peptide in rats

Transplacental passage of 125I-labelled synthetic rat atrial natriuretic peptide (ANP) was investigated in 20-day pregnant rats under pentobarbitone anesthesia. Although significant quantities of radioactivity were detected in the fetal plasma after maternal injections and in the maternal plasma after fetal injections of 125I-labelled synthetic ANP, no fraction of the placentally transferred radioactivity was due to intact ANP. Despite a rapid maternal and fetal metabolism of ANP, both maternal and fetal plasma radioactivity remained relatively stable for at least 3 h and less than 10% of the injected radioactivity was excreted in the maternal urine during a 90-min period. It is concluded that ANP is not transported in either direction across the placenta in rats.     

Mild dehydration and atrial natriuretic peptide in young and elderly subjects.

The influence of mild dehydration on plasma levels of atrial natriuretic peptide (ANP) was studied in both young (aged 18 to 25 years) and elderly (aged 72 to 86 years) subjects. We expected that dehydration would lower ANP concentrations due to the ensuing volume contraction. A different response of the ANP hormonal system in the elderly might help to explain the observation that elderly subjects are more predisposed to dehydration as compared to young subjects. Dehydration was induced by restriction of fluid intake to 25% of normal for one day. During the study, urinary osmolality increased while osmolar clearance and body weight decreased. Basal ANP concentrations were higher in the elderly subjects. However, these levels did not change during the dehydration study neither in the young nor in the elderly subjects. This may be explained by the activation of counter-regulatory systems preventing a decrease in central blood volume and hence a decrease in ANP concentration.     

Central nervous system mediated vasodepressor action of atrial natriuretic factor

Administration of 20, 4 or 2.5 micrograms/kg of atriopeptin III (AT III) into the fourth ventricle of the brain of spontaneously hypertensive rats produced a 13, 14 and 7 mm Hg decrease in MAP respectively, while 1 microgram/kg had no effect on MAP and was significantly different from 20 or 4 micrograms/kg (p less than 0.025). In contrast, injection of AT III 20 micrograms/kg into the lateral ventricle did not produce a change in MAP. To examine an interaction of AT III with the opioidergic system, the opiate antagonist, naloxone HCl, 10 micrograms, was given by ICV injection 10 minutes prior to AT III, and significantly prevented the depressor response to AT III (p less than 0.025 compared with AT III alone). Injection of specific anti-sera to beta-endorphin failed to prevent the AT III-induced depressor response. Our results demonstrate that AT III can act within the central nervous system to decrease the MAP of rats, most likely at a locus in proximity to the fourth ventricle of the brain. Further, an interaction with the central opioidergic nervous system underlies the central effects of AT III.     

Atrial natriuretic peptide in the central nervous system of the rat

1. Studies of the presence of atrial natriuretic peptide immunoreactivity and receptor binding sites in the central nervous system have revealed unusual sites of interest. 2. As a result, numerous studies have appeared that indicate that brain atrial natriuretic peptide is implicated in the regulation of blood pressure, fluid and sodium balance, cerebral blood flow, brain microcirculation, blood-brain barrier function, and cerebrospinal fluid production. 3. Alteration of the atrial natriuretic peptide system in the brain could have important implications in hypertensive disease and disorders of water balance in the central nervous system.     

Reversible pegylation prolongs the hypotensive effect of atrial natriuretic peptide

Natriuretic peptides (NP), including atrial natriuretic peptide (ANP), induce potent natriuresis and vasodilation and thereby generate hypotension in vivo. Despite intensive efforts, clinical application of NP as an antihypertensive agent is limited because of their short biological half-life and poor bioavailability. Recently, we have developed a strategy that facilitates slow release of peptides from PEG-peptide inactive conjugates, based on reversible pegylation. Peptides prepared by this approach undergo slow, spontaneous chemical hydrolysis at physiological conditions, releasing the native active peptide/protein drug from the inactive conjugates over prolonged periods. A PEG chain of 30 kDa was linked covalently to the alpha-amino side chain of the hormone via a MAL-Fmoc-NHS spacer, yielding PEG 30-Fmoc-ANP, a prodrug that releases the native hormone upon incubation at physiological conditions. Bolus administration of native ANP to Wistar rats receiving adrenaline yields a short, transitory effect in lowering blood pressure (BP), reaching a maximum at 2 min, and then returning to control values after 12 to 25 min. In contrast, administration of PEG 30-Fmoc-ANP lowered BP following a lag period of 50 min, and maintained low BP for a period exceeding 60 min. Saline or PEG 30-Fmoc-Alanine were not effective in lowering BP in Wistar rats. These results show that the novel compound, PEG 30-Fmoc-ANP, is a reversible pegylated prodrug derivative that facilitates a prolonged BP lowering effect in rats and may be considered as a candidate for development into an antihypertensive drug.     

Effect of experimental obesity and subsequent weight reduction upon circulating atrial natriuretic peptide

The effect of obesity and weight reduction upon circulating concentrations of atrial natriuretic peptide was assessed in an experimental model of the disease. Obese rats weighing in excess of 750 g were compared with formerly obese animals subjected to a 15-week period of caloric restriction resulting in a 40% reduction in body weight. Mean adipocyte size was significantly reduced with weight loss, as was estimated body fat. Mean arterial blood pressure remained normotensive for both groups, but a significant reduction in heart rate was associated with weight reduction. Circulating atrial natriuretic peptide was significantly elevated in the lean rats, which also exhibited decreased plasma renin activity and a negative sodium balance. Analysis of heart to body weight ratios implied that an obesity-associated, volume-induced cardiac hypertrophy remained even after the normalization of body fat. These results suggest that the diuresis and natriuresis accompanying weight reduction may be facilitated by atrial natriuretic peptide, which was elevated in part due to a persistent left ventricular hypertrophy following the transition from the obese to lean condition.     

The effect of atrial natriuretic peptide on small intestinal contractility and transit.

Isometric tension in response to ANF (10(-10) to 10(-4) M) was recorded from longitudinally and circularly oriented rat jejunal smooth muscle strips. Conscious, fasted rats received an IV infusion of 1.25 nmol ANF/100 g body weight in 0.5 ml normal saline and controls received saline alone. Five minutes later 10 muCi Na2 51CrO4 in 0.5 ml saline was instilled through a jejunostomy. Fifteen minutes later animals were sacrificed, and the gut divided into 8 equal segments of small intestine, cecum and remaining colon. The radioactivity of each segment was measured and a geometric center of transit determined for each group. ANF induced relaxation of longitudinally oriented strips (Tm = -72.3 +/- 10.7 mN/g, ED50 7.3 +/- 3.6 x 10(-8) M), and contraction of circularly oriented strips (Tm = 35.0 +/- 5.0 mN/g, ED50 1.3 +/- 1.0 x 10(-8) M). This response was unaffected by 10(-6) M tetrodotoxin. The geometric mean center of transit was significantly (p less than 0.001) further aboral in ANF-treated compared to control animals (intestinal segment 4.2 +/- 0.2 vs. 3.2 +/- 0.2).