Atrial natriuretic peptides: receptors and second messengers

What is next in nitric oxide research? From cardiovascular system to cancer biology

The broad role of nitric oxide (NO) and cyclic GMP in biochemistry and biology as important messenger molecules is evident from the numerous publications in this research field. NO and cGMP have been known as components of the key signaling pathway in regulating numerous processes such as vascular dilation, blood pressure, neurotransmission, cardiovascular function, and renal function. In spite of almost 150,000 publications with nitric oxide and cyclic GMP, there are few publications regarding the effects of these messenger molecules on gene regulation, cell differentiation and cell proliferation. Our research data with embryonic stem cells and several cancer cell lines suggest that nitric oxide, its receptor soluble guanylyl cyclase (sGC) and sGC's product cyclic GMP can regulate the processes of proliferation and differentiation. Furthermore, we have found that undifferentiated stem cells and some malignant tumors such as human glioma have decreased levels of sGC and translocation of the sGCβ1 subunit to the nucleus. We propose that sGC and cyclic GMP function as tumor suppressors. An understanding of the mechanisms of the translocation of the sGCβ1 subunit into the nucleus and the possible regulation of gene expression of NO and/or cyclic CMP could lead to novel and innovative approaches to cancer therapy and stem cell proliferation and differentiation.

Regulation of the G2/M transition in rodent oocytes

Regulation of maturation in meiotically competent mammalian oocytes is a complex process involving the carefully coordinated exchange of signals between the somatic and germ cell compartments of the ovarian follicle via paracrine and cell-cell coupling pathways. This review highlights recent advances in our understanding of how such signaling controls both meiotic arrest and gonadotropin-triggered meiotic resumption in competent oocytes and relates them to the historical context. Emphasis will be on rodent systems, where many of these new findings have taken place. A regulatory scheme is then proposed that integrates this information into an overall framework for meiotic regulation that demonstrates the complex interplay between different follicular compartments.

Role of nitric oxide on the vasorelaxant effect of atrial natriuretic peptide on rabbit aorta basal tone

The role of nitric oxide (NO) on the vasorelaxant effect of atrial natriuretic peptide (ANP) on the basal tone of rabbit aortic rings conditioned to angiotensin II (Ang II) was studied. ANP aortic relaxation and nitrite release were measured in the presence and absence of endothelium and a NO-synthase inhibitor. Ang II at 10(-8) M triggered a contractile response, conditioning the vessel to a vasorelaxant effect of ANP (10(-8) M). This effect was significantly enhanced by endothelium removal, NG-nitro-L-arginine methyl ester (L-NAME, 10(-4) M), and methylene blue (10(-5) M). ANP decrease of basal tone in Ang-II-sensitized aortic rings was improved when a higher concentration of Ang II was used (l0(-6) M). Basal and Ang-II-stimulated nitrite release were measured in stretched (S) and nonstretched (NS) aortic rings. Nitrite release was significantly increased in S rings (p < 0.001). L-NAME (10(-4) M) partially inhibited nitrite release in both basal and Ang-II-stimulated S aortic rings. In NS aortic rings, the NO inhibitor did not inhibit basal nitrite release but blunted the Ang-II-stimulated nitrite level. A significant negative correlation between nitrite release and the ANP vasorelaxant effect on basal tone was dependent on the Ang-II-sensitizing dose. The present results demonstrate that ANP relaxant effects on aortic basal tone are related to NO levels, which are regulated by S- and Ang-II-concentration-dependent NO generation and quenching.

Atrial natriuretic peptide mediated polyuria: pathogenesis of polyhydramnios in the recipient twin of twin-twin transfusion syndrome

We studied the role of atrial natriuretic peptide (ANP) in the pathophysiology of polyhydramnios in monochorionic (MC) twins with and without twin-twin transfusion syndrome (TTTS). Matched maternal, fetal blood samples and amniotic fluids (AF) were obtained in utero (n=12) and at birth (n=20) from MC twins with TTTS. Blood and amniotic fluid samples were also collected from non-TTTS MC twin pairs in utero (n=6) and at birth (n=20). In both groups cellular localization of ANP in the fetal kidney and heart was performed using anti ANP rabbit polyclonal antibody. Concentrations of ANP in pg/ml were determined by radioimmunoassay.In recipient fetuses, ANP levels were higher than the donors both in utero (P< 0.001) and at birth (P< 0.001). No such differences were found between the non-TTTS twins. In the TTTS group maternal ANP levels were lower than the non-TTTS group (P< 0.05). A linear relationship was found between fetal ANP levels and the AF volumes removed at fetal blood sampling (r(2)=0.68;P< 0.01, n=12). ANP was localized predominantly to the cytoplasm of the distal convoluted tubules of the fetal kidney and heart, and the intensity of immunostaining for ANP in kidney and heart were markedly greater in the recipient than the donor twin. No such differences were found between the twin pairs. These data suggest that polyhdramnios in the recipient twin occurs as a consequence of ANP mediated increase in fetal urine output and raises the possibility of direct fetal therapy with ANP blocking agents.

Effects of atrial natriuretic peptide in the gut

The intestinal tract is a target organ for atrial natriuretic peptide (ANP), characterized by various biologic activities, immunoreactivity, as well as specific binding sites for ANP. A review of previous studies reveals that ANP is an important regulator of water and nutrient intake, which acts via multiple signaling pathways including activation of guanylyl cyclase to produce its biologic responses. As a regulator, the peptide locally controls hydrosaline balance and acute systemic effects. Therefore, ANP could also act as a local mediator or paracrine effector of intestinal function.

Inactivation of atrial natriuretic factor-stimulated cyclic guanosine 3',5'-monophosphate (cGMP) in UMR-106 osteoblast-like cells

Previous studies have suggested a role of cyclic guanosine 3', 5'-monophosphate (cGMP) in the differentiation and proliferation of osteoblasts. We studied the effect of ANF (atrial natriuretic factor) on intracellular cGMP accumulation, cGMP efflux, and cGMP-phosphodiesterase (PDE) activity in UMR-106 osteoblast-like cells. ANF rapidly increased both intracellular cGMP and cGMP efflux. ANF-stimulated intracellular cGMP peaked at 2 min in the absence and at 10 min in the presence of 0.25 mM 3-isobutyl-1-methylxanthine. Probenecid, an antagonist of anion transport, blocked the efflux of cGMP (IC(50) = 0.1 mM), ruling out simple diffusion as a mechanism of the efflux. cGMP-PDE activity was increased threefold in crude homogenates from ANF-treated cells (IC(50) = 23 nM). ANF-evoked stimulation of cGMP-PDE activity was reached simultaneously with the peak in intracellular cGMP. Separation of the PDEs by Q-Sepharose chromatography revealed three cGMP-hydrolyzing peaks. The first peak was sensitive to the PDE5 (cGMP-specific PDE) isoenzyme-selective inhibitor zaprinast (IC(50) = 0.45 microM). The second peak was stimulated fourfold by the addition of calcium/calmodulin, indicating the presence of PDE1. The third peak was sensitive to the PDE2 (cGMP-stimulated PDE) isoenzyme-selective inhibitor 9-[2-hydroxy-3-nonyl]adenine (EHNA) (IC(50) = 3 microM), and was activated by over 300% in the presence of 4 microM cGMP. Our results show that ANF-stimulated cGMP is released from UMR-106 cells by a probenecid-sensitive mechanism. ANF also stimulates cGMP hydrolysis by activating cGMP-PDE activity. Three distinct cGMP-hydrolyzing PDEs, namely PDE5, PDE1, and PDE2, are present in the studied cells.

Atrial natriuretic peptide modifies arterial blood pressure through nitric oxide pathway in rats

The aim of the present study was to determine the relationship between the hypotensive effect of the atrial natriuretic peptide (ANP) and the nitric oxide (NO) pathway. N(G)-nitro-L-arginine methyl ester bolus (L-NAME, 1 mg/kg) reverted the decrease in mean arterial pressure induced by ANP administration (5 microg/kg bolus and 0.2 microg x kg(-1) x min(-1) infusion), and the injection of L-NAME before peptide administration suppressed the ANP hypotensive response. To confirm these findings, a histochemical reaction was used to determine NADPH-diaphorase activity (a NO synthase marker) in the endothelium and smooth muscle of aorta and arterioles of the small and large intestine. ANP increased aorta and arteriole endothelium staining after both in vivo administration and in vitro tissue incubation. In both cases, L-NAME prevented the ANP effect on NADPH-diaphorase activity. Tissues incubated with 8-bromoguanosine 3',5'-cyclic monophosphate mimicked ANP action. In addition, ANP administration increased urinary excretion of NO(x) end products. These findings indicate that ANP increases NO synthesis capability and NO production and suggest that the cGMP pathway may be involved. In conclusion, the NO pathway could be an intercellular messenger in the ANP endothelium-dependent vasorelaxation mechanism.

Atrial natriuretic peptide effect on NADPH-diaphorase in rat intestinal tract

Histochemical reaction of NADPH-diaphorase (NOS-NADPH-d) was used to identify NO synthesis. A 30-min 0.1 microg microg/kg/min ANP infusion led to about a 10% and 35% increase in small and large intestine enterocytes stain respectively. This increase was abolished by a bolus of 1 mg/kg L-NAME before ANP infusion in small intestine, and partially abolished it in colon. Incubation of small and large intestine with 0.5 microM ANP increased stain at about 20%. In both tissues the preincubation with 0.1 mM L-NAME abolished the ANP effect. Incubation with 0.1 mM 8-Br-cGMP enhanced staining about 70% and 30% in small and large intestine respectively. Our results show that ANP enhances NOS-NADPH-d activity, suggesting that ANP stimulates NO synthase in enterocytes by L-arginine-NO pathway. 8-Br-cGMP mimicked the effect of ANP described above. Therefore, the guanylate cyclase-coupled natriuretic receptors, NPR-A and NPR-B, probably mediate this ANP effect.

Relaxing effects of cyclic GMP and cyclic AMP-enhancing agents on the long-lasting contraction to endothelin-1 in the porcine coronary artery

In the coronary circulation, endothelin-1 (ET-1) evokes spasms which are difficult to treat when the endothelial integrity is compromised. This study compares several classes of relaxing agents on already established contractions to ET-1 in an in vitro model using ring segments of the porcine left descending coronary artery (pLAD). All segments were precontracted with 10 nmol/L ET-1. The calcium channel blocker isradipine was 300 times more potent than verapamil, but was only a partial relaxant; the maximal relaxation obtained was 52 +/- 2% (n = 6). Atrial natriuretic peptide (ANP) was an equally potent relaxant of the ET-1 contraction; however, it too was an incomplete relaxant, maximal relaxation being < 60%. A 50% relaxation of the ET-1 contraction was obtained with 0.28 +/- 0.24 mumol/L ANP, n = 4 (IC50). Comparison of cyclic nucleotide analogues revealed a 30 times higher potency for 8-bromo-cyclic guanosine monophosphate (8-Br-cGMP)(IC50 44 +/- 11 mumol/L, n = 6) than for 8-bromo-cyclic adenosine monophosphate (8-Bi-cAMP) (IC50 1600 mumol/L, n = 6). The cyclic nucleotide phosphodiesterase (PDE) inhibitor milrinone, a PDE 3-inhibitor with an IC50 2.4 +/- 1.8 mumol/L, (n = 6) was 10 times more potent than rolipram (PDE 4-inhibitor), zaprinast (PDE 5-inhibitor) and vinpocentine (PDE 1-inhibitor). Withdrawal of these analogues and inhibitors from segments continuously exposed to 10 nmol/l ET-1 revealed that vinpocentine and 8-Br-cGMP were irreversible relaxants, in contrast to milrinone and 8-Br-cAMP. In conclusion, this study has demonstrated that cGMP-enhancing agents, such as the naturally occurring ANP, the calcium channel blocker isradipine, and the synthetic inhibitor of PDE 3, were the most effective relaxants of ET-1 evoked contractions in pLAD in vitro.

Human natriuretic factor in cirrhotic patients undergoing orthotopic liver transplantation

We measured the plasma levels of atrial natriuretic factor (ANF) during orthotopic liver transplantation (OLT) in eight adult patients with cirrhosis and ascites. The aim of this study was to determine whether significant differences in ANF concentration may be detected during the individual phases of OLT and to correlate these changes with hemodynamics. In each patient a hemodynamic assessment was achieved using a Swan-Ganz fiber optic catheter for continuous monitoring of cardiac output (CO), systemic vascular resistance index (SVRI), right filling pressure as assessed by central venous pressure (CVP), and left filling pressure by means of pulmonary arterial wedge pressure (PAWP). During reperfusion a clear-cut increase in ANF values was observed (P < 0.05). Concurrently, an increase in CVP (P < 0.05) and a decrease in SVRI were observed without any significant increase in diuresis. These data suggest that ANF might play a role in the development of the reperfusion syndrome.

Edema of the nephrotic syndrome: the role of the atrial peptide system

The nephrotic syndrome is associated with an expanded interstitial volume and edema due to sodium and water retention. The mechanisms underlying these abnormalities have been only partially clarified. Renal hypoperfusion has been considered the key event that promotes avid sodium and water reabsorption by the kidney. Hypoperfusion results from hypovolemia, a consequence of urinary protein losses and decreased oncotic pressure. However, in some patients plasma volume is normal or even increased, suggesting that in such cases the cause of sodium and water retention might be independent of systemic events and possibly originates in the kidney. Experimental evidence is now available to support this, but the intrarenal mediator(s) that promote the abnormal salt retention are still not fully clear. Atrial natriuretic peptide (ANP), which increases sodium and water excretion, has been suspected to participate in fluid retention. This is consistent with experimental and human data of a markedly blunted natriuretic and diuretic response to systemic infusion of ANP in the nephrotic syndrome. Recent studies of the mechanisms of the blunted natriuretic and diuretic response to ANP documented an increased activity of renal sympathetic nerves, but the results are controversial. The altered response to ANP also may be related to a defect in the number and affinity of receptor-binding sites for the peptide. Evidence also is available of a possible defect at the level of intracellular cyclic guanosine monophosphate, the second messenger of ANP. The gene encoding for a cyclophilin-like protein, which is increased in sodium-retaining conditions, is upregulated in the kidneys of nephrotic rats, and the infusion of ANP further increases cyclophilin-like protein mRNA. Thus, multiple factors probably act in concert to induce edema formation in the nephrotic syndrome. In this review we specifically address the tubular insensitivity to the natriuretic and diuretic action of ANP, which could be an important initiating event and could possibly contribute to sustaining the edema.

The effect of nitric oxide-donating vasodilators on monocyte chemotaxis and intracellular cGMP concentrations in vitro

The effects of sodium nitroprusside (SNP) and 3-morpholino sydnonimine (SIN-1), isosorbide dinitrate (ISDN) and glyceryl trinitrate (GTN), and molsidomine (the inactive precursur of SIN-1) on monocyte chemotaxis and cyclic GMP (cGMP) concentration were studied. SNP and SIN-1 inhibited monocyte N-formyl-methionyl-leucyl-phenylalanine-stimulated migration and increased cGMP concentrations in a dose-dependent (> 10(-5) mol.l-1) and time-dependent manner. Furthermore, 8-bromo cGMP inhibited monocyte chemotaxis in a dose-dependent fashion. In contrast, ISDN, GTN and molsidomine did not alter monocyte migration or cGMP concentration. These results support earlier observations that nitric oxide inhibits monocyte function in vitro via a cGMP-mediated mechanism. The differential effects of the spontaneous and thiol-dependent NO-donating nitrovasodilators on monocyte function suggests that monocytes, like platelets, are not able to directly metabolise ISDN and GTN. If similar observations can be made in vivo, it is possible that certain nitrovasodilators might be used therapeutically to inhibit monocyte function, for example during atherogenesis.

Natriuretic peptide receptor mRNAs in the rat and human heart

Functional studies indicate that atrial natriuretic peptide (ANP), a member of the natriuretic peptide family, has direct effects on cardiac muscle cells. However, conventional ligand-binding studies designed to establish the presence of natriuretic peptide-binding sites in the heart have yielded conflicting results. There are discrepancies also between the latter and the receptor distribution predicted from the pattern of the mRNA transcripts localized by in situ hybridization. Here we have employed the technique of cDNA amplification with the polymerase chain reaction to confirm the presence of natriuretic peptide A, B, and C receptor mRNAs in rat and human cardiac tissue. In the rat heart, the distribution of the A and B receptor transcripts appears to be relatively homogeneous; in contrast, the C type mRNA is concentrated principally in the atria, with no difference between the left and right sides of the heart. A and B receptor DNA products were obtained after amplification of left, but not right, ventricular cDNA from the heart of a 16-yr-old male with cystic fibrosis; the yield of C receptor DNA was similar for both ventricles. If these mRNA transcripts are translated into functional receptors in the rat and human heart, ANP and the other natriuretic peptides may have direct effects on cardiac function, including regulation of natriuretic peptide release via a short feedback loop, modulation of contractility of the heart, or activation of cardiac reflexes.

Interaction of natriuretic peptides and cGMP production via the same receptor in mouse astrocytes

Atrial and brain natriuretic peptides have been found previously to bind to specific receptors on cultured mouse astrocytes and to stimulate cyclic guanosine 5-monophosphate (cGMP) production with similar dose dependency although brain natriuretic peptide (BNP) shows a greater maximal stimulatory effect. The present study provides evidence that both peptides work through the same pathway. No additive or synergistic effect was observed when astrocytes were exposed to both peptides. However, human ANF(99-126) at high concentrations partially inhibited porcine BNP induced cGMP production to the level seen with ANF alone. ANF could be viewed as a partial agonist of pBNP competing for the same effector sites. Differences in structure between human ANF(99-126) and porcine BNP may account for the difference in cGMP response. The interaction between the two peptides and the cGMP response does not reflect receptor binding affinities and is likely to be a post-binding event.

Atrial natriuretic peptide increases cyclic guanosine monophosphate immunoreactivity in the carotid body

The mammalian carotid body is a peripheral arterial chemoreceptor organ involved in the regulation of respiration, and in the modulation of blood pressure through reflex control of peripheral vascular resistance and cardiac output. In addition to its responsiveness to blood gases, the organ is also sensitive to hyperosmotic solutions, and we have recently shown that a systemic hormonal regulator of natriuresis and diuresis, atrial natriuretic peptide, is a potent inhibitor of chemoreceptor activity evoked by hypoxia in the cat carotid body. The present study demonstrates atrial natriuretic peptide immunoreactivity in type I cells of the carotid body, and shows further that a biologically active atrial natriuretic peptide fragment, atriopeptin III, increases cyclic guanosine monophosphate immunoreactivity in type I cells in a dose-dependent manner. Moreover, double-labeling techniques demonstrate co-existence of atrial natriuretic peptide immunoreactivity with the atriopeptin III-enhanced cyclic guanosine monophosphate reaction product. These findings indicate the probable existence of atrial natriuretic peptide receptors coupled to membrane-bound guanylate cyclase on the parenchymal type I cells. Our findings support the view that cyclic guanosine monophosphate functions as a second messenger in this organ, and may serve as a functional activity marker in identifying type I cells which respond to atrial natriuretic peptide.

Autoradiographic visualization of the natriuretic peptide receptor-B in rat tissues

Natriuretic peptide receptor-B (NPRB) was visualized in rat tissues by in vitro autoradiography, using its putative physiological agonist C-type natriuretic peptide (CNP). In initial studies, we determined that atrial natriuretic peptide (ANP) is not a suitable ligand for labeling the NPRB: in tissues reported to contain NPRB transcripts, CNP did not inhibit [125I]ANP binding except to NPRC sites. Therefore, to visualize the NPRB we used 125I[Tyr(o)]-CNP as a radioligand with an excess of NPRC-blocking peptide: C-ANP. With this approach we detected the highest number of NPRB-like sites in the pars intermedia of the pituitary gland. A large number of these sites were present in pituitary neural and anterior lobes, area postrema, adrenal medulla and cortex. A moderate NPRB population was observed in the subfornical organ, plexiform layer of the olfactory bulb and kidney. Low concentrations of NPRB were noted in the cerebellum and cerebrum but not in the choroid plexus and pia-arachnoid. Saturation experiments performed on cerebellum sections revealed a very low concentration (Bmax 4.8 fmol/mg protein) of high affinity (Kd 1.2 nM) NPRB-like sites. This study is the first demonstration of 125I[Tyr(o)]-CNP binding sites with characteristics of the NPRB in intact tissues.

Atrial natriuretic factor: its (patho)physiological significance in humans

The first human studies using relatively high-doses of ANF revealed similar effects as observed in the preceding animal reports, including effects on systemic vasculature (blood pressure fall, decrease in intravascular volume), renal vasculature (rise in GFR, fall in renal blood flow), renal electrolyte excretion (rises in many electrolytes), and changes in release of a number of different hormones. Whether all these changes are the result of direct ANF effects or secondary to a (single) primary event of the hormone remains to be determined. Certainly, it has been proven that more physiological doses of ANF fail to induce short-term changes in many of these parameters leaving only a rise in hematocrit, natriuresis and an inhibition of the RAAS as important detectable ANF effects in humans. This leads us to hypothesize that ANF is a "natriuretic" hormone with physiological significance. The primary function in humans is to regulate sodium homeostasis in response to changes in intravascular volume (cardiac atrial stretch). Induction of excess renal sodium excretion and extracellular volume shift appear to be the effector mechanisms. The exact mechanism of the natriuresis in humans still needs to be resolved. It appears however, that possibly a small rise in GFR, a reduction in proximal and distal tubular sodium reabsorption, as well as an ensuing medullary washout, are of importance. The pathophysiological role of ANF in human disease is unclear. One may find elevated plasma irANF levels and/or decreased responses to exogenous ANF in some disease states. Whether these findings are secondary to the disease state rather than the cause of the disease remains to be resolved. Therapeutic applications for ANF, or drugs that intervene in its production or receptor-binding, seem to be multiple. Most important could be the antihypertensive effect, although areas such as congestive heart failure, renal failure, liver cirrhosis and the nephrotic syndrome cannot be excluded. Although the data that have been gathered to date allowed us to draw some careful conclusions as to the (patho)physiological role of ANF, the exact place of ANF in sodium homeostatic control must still be better defined. To achieve this, we will need more carefully designed low-dose ANF infusion, as well as ANF-breakdown inhibitor studies. Even more promising, however, is the potential area of studies open to us when ANF-receptor (ant)agonists become available for human use.

Regulation of adrenal atrial natriuretic hormone receptor subtypes

Regulation of atrial natriuretic hormone (ANH) receptor binding and aldosterone suppression was studied in isolated adrenal glomerulosa cells from rats fed a high-salt (HS) or low-salt (LS) diet for 3 days. In plasma of HS rats, aldosterone levels were 5 times lower and immunoreactive ANH two times higher than in LS rats. Competitive binding studies showed the same affinity for human atrial natriuretic hormone (hANH) in both pools of cells, but receptor density was 50% higher on LS cells. A linear ANH analog that binds to non-guanylate-cyclase-coupled receptors did not show increased binding to LS cells. Cyclic GMP production in response to hANH was identical in both groups. The aldosterone-inhibitory effect of hANH on both groups of basal and angiotensin II-stimulated cells was also identical. Thus a short-term high-salt diet causes decreased density of ANH receptors in glomerulosa cells without changing biological activity of ANH. These results suggest that dietary salt content changes the number of ANH receptors and that non-guanylate-cyclase-coupled receptors contain at least two classes of receptors.

Natriuretic peptide receptors regulate endothelin synthesis and release from parathyroid cells

Cloned rat parathyroid cells (PTr cell line) that produce parathyroid hormone-related peptide plus endothelin 1 and primary cultures of human parathyroid cells were tested for growth and differentiation responses to atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP). High- and low-affinity binding sites for ANP were found on PTr cells; BNP appeared to bind to the same receptors with similar affinities. Either ANP or BNP stimulated production of cGMP and caused a 30% decrease in Na(+)-K(+)-Cl- cotransport. Each peptide increased synthesis and secretion of endothelin 1 by PTr cells in a dose-dependent fashion, but cell growth was not affected. Human parathyroid cells (normal and pathological) also responded to ANP or BNP with an increase in cGMP production. The finding of receptors for natriuretic hormones on parathyroid cells with consequent effects on release of endothelin 1 might be of relevance in understanding the clinical association between hyperparathyroidism and hypertension.

Characterisation of atrial natriuretic peptide receptors in bovine ventricular sarcolemma

Analysis of [125I]-ANP binding data in an isolated bovine ventricular sarcolemmal membrane fraction revealed a single high affinity binding site (Kd approximately 5 x 10(-11) M). The ring deleted ANP analogue des [QSGLG]-ANP (4-23)-NH2 bound with a 1000-fold lower affinity indicating the absence of C-type receptors in this preparation. ANP stimulated guanylate cyclase activity by up to 2-fold with half-maximal activation at approximately 10(-9) M. Crosslinking [125I]-ANP to its receptor with disuccinimidyl suberate (DSS) revealed two radiolabelled bands of 120 kDa and 65 kDa on non-denaturing SDS-PAGE. Radioactive signals from both bands were lost by reducing the sample with beta-mercaptoethanol prior to electrophoresis, in which case a radioactive fragment of less than 5 kDa migrated with the dye front. These results suggest that the binding of ANP to both high and low molecular weight "receptor" proteins may be associated with the hydrolysis of the peptide.

Atrial natriuretic peptide receptors and activation of guanylate cyclase in rat cardiac sarcolemma

Two classes of atrial natriuretic peptide (ANP) receptors are present in purified sarcolemmal membrane fractions isolated from rat ventricle. Scatchard analysis using [125I]-ANP reveals high affinity (Kd approximately 10(-11) M) and low affinity (Kd approximately 10(-9) M) binding sites. Basal guanylate cyclase activities associated with these membrane fractions range from 3.2 +/- 1.3 pmol/min/mg protein in the presence of Mg2+ to 129 +/- 17 pmol/min/mg protein in the presence of Mn2+. Millimolar concentrations of adenosine triphosphate (ATP) potentiates Mg2+- but not Mn2+-supported activity. Binding of ANP to the low affinity site but not the high affinity site results in a maximum 2-fold activation of Mn2+- and up to 6-fold activation of Mg2+/ATP supported guanylate cyclase activities.