Atrial natriuretic factor immunoreactivity in human fetal lung tissue and perfusates

Expression patterns of natriuretic peptides in pre-hibernating and hibernating anatolian ground squirrel (Spermophilus xanthoprymnus) lung

Anatolian ground squirrel (Spermophilus xanthoprymnus) is a true hibernator. This animal transiently reduces pulmonary function during hibernation. Continuance of pulmonary function is very important to survive ground squirrels during the hibernation. Natriuretic peptides may be key players in the modulation of pulmonary hemostasis. However, NPs' role in pulmonary function during hibernation remains unclear. We aimed to investigate the localization and distribution of atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP) and C-type natriuretic peptide (CNP) in squirrel lungs during pre-hibernation and hibernation periods using immunohistochemistry. Our immunohistochemical data indicate that ANP, BNP, and CNP were produced by the mucosal epithelium of terminal and respiratory bronchioles, smooth muscle cells in the lamina propria of terminal bronchioles and vascular smooth muscle cells, alveolar type II cells, and macrophages. ANP immunoreactivity was weaker than BNP and CNP immunoreactivities in these cells. The results also demonstrate that the number of ANP, BNP and CNP positive alveolar type II cells tended to increase, although statistically non-significant, during the hibernation period, but the expression of NPs in other pulmonary cells is unaffected by hibernation. This study firstly investigates ANP, BNP and CNP distribution in the Anatolian ground squirrel lung. However, further studies are required to dissect their functional roles during the hibernation.

Ontogeny of atrial natriuretic peptide and its receptor in the lung: effects on perinatal surfactant release

During the transition at birth to air breathing, regulation of surfactant release from alveolar type II (ATII) cells is critical. Atrial natriuretic peptide (ANP) stimulates natriuretic peptide receptor-A (NPR-A) and increases intracellular cGMP. We examined the changes in ANP and NPR-A in respiratory epithelium during the perinatal period using immunohistochemistry and studied the effect of ANP on surfactant release from ATII cells isolated from fetal and newborn lambs. NPR-A mRNA was detected in the fetal lung by Northern Blot and RT-PCR. At 100 d gestation (term 145 d), ANP staining was absent and NPR-A staining was weak in cuboidal epithelial cells. ANP and NPR-A staining was prominent in ATII cells at 136 d gestation and was undetectable postnatally. ANP stimulated (maximal effect at 10(-10)M) surfactant release from both late gestation fetal and neonatal ATII cells. Protein kinase G inhibition significantly blocked this release. We conclude that ANP stimulates surfactant release in isolated perinatal ATII cells by a cGMP-dependent mechanism. ANP and NPR-A expression in ATII cells is greatest in late gestation and declines sharply postnatally. We speculate that increased activity of the ANP/NPR-A pathway in late gestation may prime the surfactant system, preparing the lung for air breathing.

Expression and secretion of the atrial natriuretic peptide in human adipose tissue and preadipocytes

OBJECTIVE

Atrial natriuretic peptide (ANP) is a secretory hormone displaying diuretic, natriuretic, and vasorelaxant activities. Recently, its lipolytic activity has been reported. Since the expression of ANP in adipose tissue has not been documented, we used real-time reverse transcriptase polymerase chain reaction (RT-PCR) to investigate the expression of ANP in human adipose tissue and preadipocytes.

RESEARCH METHODS AND PROCEDURES

RNA was extracted from the human adipose tissue of severely obese premenopausal women as well as from human preadipocytes. For human preadipocytes, two cell systems were investigated: the human preadipose immortalized (Chub-S7) cells, a well-characterized human preadipose cell line, and primary preadipocytes derived from the stromal vascular fraction of the human adipose tissue. We measured the mRNA of ANP, of corin (a transmembrane serine protease involved in the conversion of pro-ANP to ANP) and of uncoupling protein 2 (UCP2; a control gene known to be ubiquitously expressed). The expression of ANP was also investigated using immunofluorescence and radioimmunoassay in Chub-S7 cells and human primary preadipocytes in culture.

RESULTS

Our results indicate that ANP and corin are expressed at the mRNA level in human adipose tissue and preadipocytes. Immunofluorescence experiments demonstrated that pro-ANP was expressed in Chub-S7 cells. In addition, ANP secretion could be measured in Chub-S7 cells and human primary preadipocytes in culture. Rosiglitazone, a selective peroxisome proliferator-activated receptor type gamma (PPAR-gamma) agonist promoting adipocyte differentiation, was found to modulate both ANP expression and secretion in preadipocytes.

DISCUSSION

Our findings suggest the existence of an autocrine/paracrine system for ANP in the human adipose tissue whose implications in lipolysis and cardiovascular function need to be further explored.

Guanylyl cyclases, nitric oxide, natriuretic peptides, and airway smooth muscle function

Airway smooth muscle (ASM) plays an important role in asthma pathophysiology through its contractile and proliferative functions. The cyclic nucleotides adenosine 3',5'-cyclic monophosphate (cAMP) and guanosine 3',5'-cyclic monophosphate (cGMP) are second messengers capable of mediating the effects of a variety of drugs and hormones. There is a large body of evidence to support the hypothesis that cAMP is a mediator of the ASM's relaxant effects of drugs, such as beta2-adrenoceptor agonists, in human airways. Although most attention has been paid to this second messenger and the signal transduction pathways it activates, recent evidence suggests that cGMP is also an important second messenger in ASM with important relaxant and antiproliferative effects. Here, we review the regulation and function of cGMP in ASM and discuss the implications for asthma pathophysiology and therapeutics. Recent studies suggest that activators of soluble and particulate guanylyl cyclases, such as nitric oxide donors and natriuretic peptides, have both relaxant and antiproliferative effects that are mediated through cGMP-dependent and cGMP-independent pathways. Abnormalities in these pathways may contribute to asthma pathophysiology, and therapeutic manipulation may complement the effects of beta2-adrenoceptor agonists.

Pulmonary natriuretic peptide system during rat development

Maturational changes in the rat lung natriuretic peptide system were studied postnatally in 1-, 4-, and 22-day-old rats. Lung atrial natriuretic factor (ANF) content increased significantly from day 1 to day 4 (712+/-188 vs. 1905+/-520 pg/mg protein; p<0.01) but decreased to 532+/-41 pg/mg protein, on day 22. These changes paralleled ANF messenger RNA (mRNA) detected by reverse transcribed polymerase chain reaction (RT-PCR). Rat pulmonary development also was associated with quantitative and qualitative alterations in ANF receptors. Competitive-binding radioreceptor assays of lung membranes with 125I-ANF and increasing concentrations of unlabeled ANF revealed that the natriuretic peptide receptor-binding sites (Bmax) progressively increased with age from 112 +/-21 fmol/mg protein at day 1 to 211+/-16 (p<0.02) and 326+/-62 fmol/mg protein (p<0.04) in 4- and 22-day-old rats, respectively. Autoradiographic studies of 125I-tyr(0)CNP binding to lung sections revealed that the levels of the natriuretic peptide receptor B (NPR-B) were undetectable. On the other hand, binding of 125I-ANF increased with age, and the higher binding at 4 days was mainly due to increased density of the clearance receptor-C (NPR-C), and at 22 days due to increased natriuretic peptide receptor-A (NPR-A). The increase in natriuretic peptide binding was confirmed at the level of synthesis, where RT-PCR revealed that NPR-A mRNA significantly increased (p<0.01) in 22-day-old rats. In conclusion, these studies demonstrate that the rat pulmonary natriuretic peptide system is altered during development. The altered synthesis of lung natriuretic peptides and their receptors may play a role in the postnatal adaptation of pulmonary circulation.

ANF system in the newborn piglet pulmonary vessels

The atrial natriuretic factor (ANF) induces diuresis, natriuresis, and vasodilation. Although it was originally found to be secreted from the atria, ANF synthesis has been demonstrated in other organs. The adult lung is not only the first target organ for ANF, but it also expresses the ANF gene and synthesizes, releases, and clears ANF from the circulation. We have shown the presence of ANF in human fetal lungs and also demonstrated that these lungs can release bioactive ANF. However, the role of the ANF system in the newborn lung is unknown. Therefore we studied the ANF system in pulmonary vessels (arteries and veins dissected from the hilum down to a 100-microm diameter), in isolated perfused lungs, and in the plasma from pulmonary artery and vein of 1- and 7-day-old piglets. High-performance liquid chromatography (HPLC) revealed the presence of both the mature peptide and the ANF prohormone in pulmonary vein microsomes, but in pulmonary arteries, only the mature form was identified. Furthermore, in the veins, the ANF content tended to be higher in 7- than in 1-day-olds. ANF caused a dose-dependent decrease in perfusion pressure (p < 0.05). In veins and arteries, most of the ANF receptors were of the type A guanylate cyclase as opposed to clearance receptors. Interestingly, the ANF receptors were fewer in veins, where synthesis takes place, than in arteries (p < 0.05). Significant circulating ANF plasma levels were measured by radioimmunoassay in both pulmonary artery and vein. However, there was no site difference in ANF plasma levels, suggesting that ANF is cleared and synthesized in the pulmonary vessels. In conclusion, the entire ANF system is present in the newborn piglet pulmonary vessels. The paucity of clearance receptors compared with functional receptors potentiates the role of ANF in the regulation of postnatal pulmonary vascular resistance.

Atrial natriuretic peptide in peripheral organs other than the heart

The heart atria represent the major site of synthesis of atrial natriuretic peptide (ANP) in mammals including man, and its function as a regulator of water and salt homeostasis has been repeatedly suggested. However, more recently ANP has been located in organs not intimately related to cardiovascular physiology, e.g. the adrenals, lungs, and gut, as well as tissues belonging to the lymphatic, reproductive or endocrine systems. Thus, ANP might serve many more physiological roles than originally thought, but the functional significance of ANP in these non-cardiac tissues is presently poorly understood.