Pertussis toxin attenuates atrial natriuretic factor-mediated inhibition of adenylate cyclase. Involvement of inhibitory guanine nucleotide regulatory protein

Cytoplasmic domain of natriuretic peptide receptor C constitutes Gi activator sequences that inhibit adenylyl cyclase activity

We have recently demonstrated that a 37-amino acid peptide corresponding to the cytoplasmic domain of the natriuretic peptide receptor C (NPR-C) inhibited adenylyl cyclase activity via pertussis toxin (PT)-sensitive G(i) protein. In the present studies, we have used seven different peptide fragments of the cytoplasmic domain of the NPR-C receptor with complete, partial, or no G(i) activator sequence to examine their effects on adenylyl cyclase activity. The peptides used were KKYRITIERRNH (peptide 1), RRNHQEESNIGK (peptide 2), HRELREDSIRSH (peptide 3), RRNHQEESNIGKHRELR (peptide 4), QEESNIGK (peptide X), ITIERRNH (peptide Y), and ITIYKKRRNHRE (peptide Z). Peptides 1, 3, and 4 have complete G(i) activator sequences, whereas peptides 2 and Y have partial G(i) activator sequences with truncated carboxyl or amino terminus, respectively. Peptide X has no structural specificity, whereas peptide Z is the scrambled peptide control for peptide 1. Peptides 1, 3, and 4 inhibited adenylyl cyclase activity in a concentration-dependent manner with apparent K(i) between 0.1 and 1 nm; however, peptide 2 inhibited adenylyl cyclase activity with a higher K(i) of about 10 nm, and peptides X, Y, and Z were unable to inhibit adenylyl cyclase activity. The maximal inhibitions observed were between 30 and 40%. The inhibition of adenylyl cyclase activity by peptides 1-4 was absolutely dependent on the presence of guanine nucleotides and was completely attenuated by PT treatment. In addition, the stimulatory effects of isoproterenol, glucagon, and forskolin on adenylyl cyclase activity were inhibited to different degrees by these peptides. These results suggest that the small peptide fragments of the cytoplasmic domain of the NPR-C receptor containing 12 or 17 amino acids were sufficient to inhibit adenylyl cyclase activity through a PT-sensitive G(i) protein. The peptides having complete structural specificity of G(i) activator sequences at both amino and carboxyl termini were more potent to inhibit adenylyl cyclase activity as compared with the peptides having a truncated carboxyl terminus, whereas the truncation of the amino-terminal motif completely attenuates adenylyl cyclase inhibition.

Enhanced expression of Gi proteins in non-hypertrophic hearts from rats with hypertension-induced by L-NAME treatment

OBJECTIVE

The objective of the present studies is to investigate if the enhanced expression of Gs alpha protein and their mRNA observed in various models of hypertensive rats is due to the expressed hypertrophy or hypertension.

METHODS

Hypertension, in Sprague-Dawley rats was induced by the oral administration of the arginine analog N(omega)-nitro-L-arginine methyl ester (L-NAME) in their drinking tap water for a period of 4 weeks. The control rats were given plain tap water only. The levels of inhibitory guanine nucleotide regulatory proteins (Gi alpha-2, Gi alpha-3), stimulatory guanine nucleotide proteins (Gs alpha) and G beta proteins were determined by immunoblotting, whereas the levels of Gi alpha-2, Gi alpha-3, Gs alpha and adenylyl cyclase type V enzyme mRNA were determined by Northern-blotting techniques. Adenylyl cyclase activity was determined by measuring [32P]cAMP formation from [alpha32P]ATP.

RESULTS

The systolic blood pressure was enhanced in L-NAME-treated rats compared to control rats (190 +/- 9.2 mmHg versus 121 +/- 6.3 mmHg); however, heart-to-body-weight ratio was not different in two groups. The levels of Gi alpha-2 and Gi alpha-3 proteins and their mRNA were significantly augmented in hearts from L-NAME-treated rats, however, the levels of Gs alpha and G beta were unaltered. In addition, the effect of low concentrations of GTPgammaS on forskolin (FSK)-stimulated adenylyl cyclase activity (receptor-independent functions of Gi alpha) was significantly enhanced in L-NAME-treated rats. However, the inhibitions of adenylyl cyclase exerted by oxotremorine, C-ANP(4-23) and angiotensin II (AII) (receptor-dependent function of Gi alpha) were completely attenuated in L-NAME-treated rats. On the other hand, cholera toxin stimulated GTP or GTPgammaS-sensitive adenylyl cyclase activity (Gs alpha function) to similar extent in control and L-NAME-treated rats, suggesting that Gs alpha functions were not altered by L-NAME treatment. However, the stimulatory effects of isoproterenol, glucagon, NaF on adenylyl cyclase were diminished in L-NAME-treated rats. In addition, FSK-stimulated enzyme activity was also diminished in L-NAME-treated rats without any changes in the mRNA levels of type V enzyme.

CONCLUSIONS

These results suggest that L-NAME hypertensive rats that do not express cardiac hypertrophy exhibit enhanced expression of Gi alpha protein and associated adenylyl cyclase activity.

Downregulation of atrial natriuretic peptide ANP-C receptor is associated with alterations in G-protein expression in A10 smooth muscle cells

Atrial natriuretic peptide (ANP) receptors A and B are guanylyl cyclase receptors, whereas ANP-C receptors are coupled to adenylyl cyclase through inhibitory guanine nucleotide (Gi) protein. ANP has been shown to downregulate ANP-A and -B receptors and cGMP response in various tissues. In the present studies, we have examined the regulation of ANP-C receptor-adenylyl cyclase signal transduction by ANP and [des(Gln(18),Ser(19),Gln(20),Leu(21), Gly(22))ANP(4-23)-NH(2)](C-ANP(4-23)) that interacts specifically with ANP-C receptor in A10 smooth muscle cells (SMC). Treatment of the cells with C-ANP(4-23) for 24 h resulted in a reduction in ANP receptor binding activity. [(125)I]ANP(99-126) bound to control and C-ANP(4-23)-treated cell membranes at a single site with dissociation constants of 33.7 +/- 6 and 35.0 +/- 4.5 pM and B(max) of 74.0 +/- 5.0 and 57.6 +/- 4.0 fmol/mg of protein, respectively. C-ANP(4-23) inhibited adenylyl cyclase activity in a concentration-dependent manner in control cells. A maximal inhibition observed was about 30-40% with an apparent K(i) of about 1 nM; however, this inhibition was completely attenuated in cells pretreated with ANP(99-126) or C-ANP(4-23) (10(-7) M). However, the inhibition of adenylyl cyclase by 17-amino acid peptide (RRNHQEESNIGKHRELR) (R17A) of cytoplasmic domain of ANP-C receptor was attenuated by about 50% but was not completely abolished by C-ANP(4-23) treatment. The attenuation of C-ANP(4-23)-mediated inhibition of adenylyl cyclase was dependent on the concentration and time of pretreatment of the cells with C-ANP(4-23). In addition, angiotensin II- (Ang II-) mediated inhibition of adenylyl cyclase ( approximately 30%) was also abolished by C-ANP(4-23) treatment, indicating that the desensitization elicited by ANP was heterologous. In addition, C-ANP(4-23) treatment decreased the expression of Gialpha-2 and Gialpha-3 proteins by about 40 and 60%, respectively, and their mRNA by 40%. However, the levels of Gi proteins were not altered when the cells were treated for shorter period of time (2-4 h) or with lower concentrations of C-ANP(4-23) (10(-10) M). On the other hand, the levels of Gsalpha but not of Gbeta were increased by about 35% by C-ANP(4-23) treatment. Furthermore, the stimulations exerted by GTPgammaS, isoproterenol, FSK, and NaF on adenylyl cyclase were also augmented in cells treated with C-ANP(4-23). These results indicate that C-ANP(4-23) treatment of A10 cells desensitizes ANP-C receptor-mediated inhibition of adenylyl cyclase which may be due to the downregulation of ANP-C receptor and decreased expression of Gialpha proteins to which these receptors are coupled.

Natriuretic peptides in fish physiology

Natriuretic peptides exist in the fishes as a family of structurally-related isohormones including atrial natriuretic peptide (ANP), C-type natriuretic peptide (CNP) and ventricular natriuretic peptide (VNP); to date, brain natriuretic peptide (or B-type natriuretic peptide, BNP) has not been definitively identified in the fishes. Based on nucleotide and amino acid sequence similarity, the natriuretic peptide family of isohormones may have evolved from a neuromodulatory, CNP-like brain peptide. The primary sites of synthesis for the circulating hormones are the heart and brain; additional extracardiac and extracranial sites, including the intestine, synthesize and release natriuretic peptides locally for paracrine regulation of various physiological functions. Membrane-bound, guanylyl cyclase-coupled natriuretic peptide receptors (A- and B-types) are generally implicated in mediating natriuretic peptide effects via the production of cyclic GMP as the intracellular messenger. C- and D-type natriuretic peptide receptors lacking the guanylyl cyclase domain may influence target cell function through G(i) protein-coupled inhibition of membrane adenylyl cyclase activity, and they likely also act as clearance receptors for circulating hormone. In the few systems examined using homologous or piscine reagents, differential receptor binding and tissue responsiveness to specific natriuretic peptide isohormones is demonstrated. Similar to their acute physiological effects in mammals, natriuretic peptides are vasorelaxant in all fishes examined. In contrast to mammals, where natriuretic peptides act through natriuresis and diuresis to bring about long-term reductions in blood volume and blood pressure, in fishes the primary action appears to be the extrusion of excess salt at the gills and rectal gland, and the limiting of drinking-coupled salt uptake by the alimentary system. In teleosts, both hypernatremia and hypervolemia are effective stimuli for cardiac secretion of natriuretic peptides; in the elasmobranchs, hypervolemia is the predominant physiological stimulus for secretion. Natriuretic peptides may be seawater-adapting hormones with appropriate target organs including the gills, rectal gland, kidney, and intestine, with each regulated via, predominantly, either A- or B-type (or C- or D-type?) natriuretic peptide receptors. Natriuretic peptides act both directly on ion-transporting cells of osmoregulatory tissues, and indirectly through increased vascular flow to osmoregulatory tissues, through inhibition of drinking, and through effects on other endocrine systems.

Altered expression of Gi-protein and adenylyl cyclase activity in hearts from one kidney one clip hypertensive rats: effect of captopril

OBJECTIVE

To investigate whether one kidney one clip (1K-1C) hypertensive rats associated with high levels of angiotensin II (Ang II) exhibit enhanced expression and functions of G proteins in the heart and whether the enhanced expression can be attributed to Ang II.

METHODS

The levels of G protein and G protein mRNA in hearts from 1K-1C hypertensive rats were determined by immunoblotting and Northern blotting techniques using specific antibodies and cDNA probes, respectively, for different isoforms of G proteins. Adenylyl cyclase activity, stimulated or inhibited by agonists, was determined to examine the function of G proteins.

RESULTS

The levels of Gialpha-2 and Gialpha-3 proteins and mRNA were significantly increased in hearts from 1K-1C hypertensive rats compared with control rats, whereas the levels of Gsalpha were unchanged. Guanosine 5'-[3'-thio] triphosphate (GTPgammaS), isoproterenol, glucagon, sodium fluoride (NaF) and forskolin (FSK) stimulated adenylyl cyclase activity in hearts from control and hypertensive rats to varying degrees; however, the stimulations were significantly less in hypertensive rats compared with control rats. On the other hand, the inhibitory effect of low concentrations of GTPgammaS on FSK-stimulated adenylyl cyclase activity (an index of Gi function) was significantly enhanced in hearts from 1K-1C hypertensive rats, whereas the inhibitory effect of C-ANF4-23 on adenylyl cyclase was increased and that of Ang II was decreased in hearts from 1K-1C hypertensive rats. Captopril, an angiotensin-converting enzyme inhibitor, restored the augmented levels of Gi proteins and also the altered stimulation and inhibition of adenylyl cyclase by GTPgammaS, stimulatory and inhibitory hormones, respectively, in hearts from hypertensive rats.

CONCLUSION

These data suggest that 1K-1C hypertensive rats exhibit enhanced expression of Gialpha proteins and associated functions that may be attributable to the enhanced levels of Ang II in this model of hypertension.

Structural and functional evolution of the natriuretic peptide system in vertebrates

The natriuretic peptide (NP) system consists of three types of hormones [atrial NP (ANP), brain or B-type NP (BNP), and C-type NP (CNP)] and three types of receptors [NP receptor (R)-A, NPR-B, and NPR-C]. ANP and BNP are circulating hormones secreted from the heart, whereas CNP is basically a neuropeptide. NPR-A and NPR-B are membrane-bound guanylyl cyclases, whereas NPR-C is assumed to function as a clearance-type receptor. ANP, BNP, and CNP occur commonly in all tetrapods, but ventricular NP replaces BNP in teleost fish. In elasmobranchs, only CNP is found, even in the heart, suggesting that CNP is an ancestral form. A new guanylyl cyclase-uncoupled receptor named NPR-D has been identified in the eel in addition to NPR-A, -B, and -C. The NP system plays pivotal roles in cardiovascular and body fluid homeostasis. ANP is secreted in response to an increase in blood volume and acts on various organs to decrease both water and Na+, resulting in restoration of blood volume. In the eel, however, ANP is secreted in response to an increase in plasma osmolality and decreases Na+ specifically, thereby promoting seawater adaptation. Therefore, it seems that the family of NPs were originally Na(+)-extruding hormones in fishes; however, they evolved to be volume-depleting hormones promoting the excretion of both Na+ and water in tetrapods in which both are always regulated in the same direction. Vertebrates expanded their habitats from fresh water to the sea or to land during evolution. The structure and function of osmoregulatory hormones have also undergone evolution during this ecological evolution. Thus, a comparative approach to the study of the NP family affords new insights into the essential function of this osmoregulatory hormone.

Angiotensin II enhances the expression of Gialpha in A10 cells (smooth muscle): relationship with adenylyl cyclase activity

In the present studies, we have investigated the effect of angiotensin II (AII) on guanine nucleotide regulatory protein (G protein) expression and functions in A10 smooth muscle cells. AII treatment of A10 cells enhanced the levels of inhibitory guanine nucleotide regulatory protein (Gi) as well as Gi mRNA and not of stimulatory guanine nucleotide regulatory protein (Gs) in a concentration-dependent manner as determined by immunoblot and Northern blot analysis, respectively. AII-evoked increased expression of Gialpha-2 and Gialpha-3 was inhibited by actinomycin D treatment (RNA synthesis inhibitor). The increased expression of Gialpha-2 and Gialpha-3 by AII was not reflected in functions, because the GTPgammaS-mediated inhibition of forskolin-stimulated adenylyl cyclase and the receptor-mediated inhibition of adenylyl cyclase by AII and C-ANP4-23 [des(Gln18, Ser19, Gln20, Leu21, Gly22) ANP4-23-NH2] were not augmented but attenuated in AII-treated A10 cells. The attenuation was prevented by staurosporine (a protein kinase C inhibitor) treatment. On the other hand, AII treatment did not affect the expression and functions of stimulatory guanine nucleotide regulatory protein (Gs), however, the stimulatory effects of 5'-O-(3-thiotriphosphate), isoproterenol, and N-ethylcarboxamide adenosine (NECA) on adenylyl cyclase activity were inhibited to various degrees by AII treatment. Staurosporine reversed the AII-evoked attenuation of isoproterenol- and NECA-stimulated enzyme activity. From these results, it can be suggested that AII, whose levels are increased in hypertension, may be one of the possible contributing factors responsible for exhibiting an enhanced expression of Gi protein in hypertension.

Gender and relaxation to C-type natriuretic peptide in porcine coronary arteries

Experiments were designed to determine whether or not relaxations of coronary arterial smooth muscle to C-type natriuretic peptide (CNP) vary according to gender, and if so, to determine mechanisms for the differences. Rings of coronary arteries without endothelium from sexually mature male and female Yorkshire pigs were suspended in organ chambers for measurement of isometric force. Cumulative concentration-responses to CNP (10(-9)-10(-7) M) were obtained in the absence and presence of either K+ channel blockers (charybdotoxin, apamine, or glibenclamide, 10(-7) M) or the clearance-receptor antagonist C-ANP (10(-6) M) during contractions to prostaglandin F2alpha (2 microM). Relaxations to CNP were significantly less in arteries from male compared with female pigs and were significantly attenuated by charybdotoxin and glibenclamide in both sexes. However, apamine reduced relaxations to CNP only in arteries from female pigs. C-ANP significantly potentiated relaxations to CNP only in arteries from male pigs. In separate experiments, cyclic guanosine monophosphate (cGMP) was measured by radioimmunoassay at specified times after the addition of CNP (10(-7) M). Peak increases in cGMP were greater and occurred earlier in arteries from female than from male pigs; these differences were eliminated by the phosphodiesterase inhibitor 3-isobutyl-1-methyl-xanthine (10(-4) M). These results demonstrate three mechanisms that contribute to gender differences in CNP-mediated relaxation of coronary arterial smooth muscle: activation of low conductance Ca2+-activated K+ channels, natriuretic peptide clearance receptors, and activity/regulation of phosphodiesterases.

Dual role for adenine nucleotides in the regulation of the atrial natriuretic peptide receptor, guanylyl cyclase-A

The ability to both sensitize and desensitize a guanylyl cyclase receptor has not been previously accomplished in a broken cell or membrane preparation. The guanylyl cyclase-A (GC-A) receptor is known to require both atrial natriuretic peptide (ANP) and an adenine nucleotide for maximal cyclase activation. When membranes from NIH 3T3 cells stably overexpressing GC-A were incubated with ATP, AMPPNP, or ATPgammaS, only ATPgammaS dramatically potentiated ANP-dependent cyclase activity. When the membranes were incubated with ATPgammaS and then washed, GC-A now became sensitive to ANP/AMPPNP stimulation, suggestive that thiophosphorylation had sensitized GC-A to ligand and adenine nucleotide binding. Consistent with this hypo- thesis, the ATPgammaS effects were both time- and concentration-dependent. Protein phosphatase stability of thiophosphorylation (ATPgammaS) relative to phosphorylation (ATP) appeared to explain the differential effects of the two nucleotides since microcystin, beta-glycerol phosphate, or okadaic acid coincident with ATP or ATPgammaS effectively sensitized GC-A to ligand stimulation over prolonged periods of time in either case. GC-A was phosphorylated in the presence of [gamma32P]ATP, and the magnitude of the phosphorylation was increased by the addition of microcystin. Thus, the phosphorylation of GC-A correlates with the acquisition of ligand sensitivity. The establishment of an in vitro system to sensitize GC-A demonstrates that adenine nucleotides have a daul function in the regulation of GC-A through both phosphorylation of and binding to regulatory sites.

Decreased expression of natriuretic peptide A receptors and decreased cGMP production in the choroid plexus of spontaneously hypertensive rats

Atrial natriuretic peptide receptor (ANP) subtypes and their signal transduction response were characterized in choroid plexus of spontaneously hypertensive (SHR) and normotensive (WKY) rats. We found two ANP receptor subtypes, guanylate cyclase coupled and uncoupled, in both rat strains. Binding of ANP was lower in SHR choroid plexus when compared to WKY. The lower ANP binding in SHR was the result of a decrease of binding to the guanylate cyclase-coupled receptor subtype A, a decrease that correlated well with the decreased ANP-induced cGMP formation in SHR. Forskolin stimulated cGMP production to the same extent in both strains. In WKY rats, ANP increased basal and forskolin-stimulated cAMP production; conversely, in SHR, ANP did not affect the basal level of cAMP and inhibited the forskolin-stimulated cAMP production. These results demonstrate differences in ANP receptor subtype expression, and ANP signal transduction in choroid plexus of hypertensive and normotensive rats, which is of possible significance to the central mechanisms of blood pressure control.

Potentiation of dipsogenic actions by centrally administered type-C natriuretic peptide in spontaneously hypertensive but not Wistar-Kyoto rats

1. The effects of type-C natriuretic polypeptides (CNP) on the central dipsogenic and pressor responses to angiotensin II (AngII) were studied by the administration of agents into the lateral cerebral ventricle under conscious and unrestrained conditions in normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive rats (SHR). 2. The fluid intake induced by AngII (25 ng) and water deprivation were potentiated after pretreatment with CNP in SHR but not in WKY rats. However, carbachol-induced water intake was not altered by pretreatment with CNP (2.5 micrograms) in either WKY rats or SHR. 3. In contrast, CNP did not influence the pressor responses to AngII in either WKY rats or SHR.

Atrial natriuretic peptide-C receptor and membrane signalling in hypertension

Atrial natriuretic peptide (ANP) regulates a variety of physiological parameters, including the blood pressure and intravascular volume, by interacting with its receptors present on the plasma membrane. ANP receptors are of three subtypes: ANP-A, -B and -C receptors. ANP-A and ANP-B receptors are guanylyl cyclase receptors, whereas ANP-C receptors are coupled to adenylyl cyclase inhibition or phospholipase C activation through inhibitory guanine nucleotide-regulating protein. Unlike other G protein-coupled receptors, ANP-C receptors have a single transmembrane domain and a short cytoplasmic domain of 37 amino acids, the cytoplasmic domain has a structural specificity like those of other single-transmembrane-domain receptors and 37 amino-acid cytoplasmic domain peptide is able to exert is inhibitory effect on adenylyl cyclase. The activation of ANP-C receptor by C-ANP(4-23) (a ring-deleted peptide of ANP) and C-type natriuretic peptide inhibits the mitogen-activated protein kinase activity stimulated by endothelin-3, platelet-derived growth factor and phorbol-12 myristate 13-acetate. C-ANP also inhibits mitogen-induced stimulation of DNA synthesis, indicating that the ANP-C receptor plays a role in cell proliferation through an inhibition of mitogen-activated protein kinase and suggesting that the ANP-C receptor might also be coupled to other signal transduction mechanism(s) or that there might be an interaction of the ANP-C receptor with some other signalling pathways. ANP receptor binding is decreased in most organs in hypertensive subjects and hypertensive animals. This decrease is consistent with there being fewer guanylyl cyclase-coupled receptors in the kidney and vasculature and selective inhibition of the ANP-C receptor in the thymus and spleen. Platelet ANP-C receptors are decreased in number in hypertensive patients and spontaneously hypertensive rats. ANP-A, -B and -C receptors are decreased in number in deoxycorticosterone acetate-salt-treated kidneys and vasculature; however, the responsiveness of adenylyl cyclase to ANP is augmented in the vasculature and heart and is attenuated completely in platelets. These alterations in ANP receptor subtypes may be related to the pathophysiology of hypertension. Several hormones such as angiotensin II, ANP and catecholamines, the levels of which are increased in hypertension, downregulate or upregulate ANP-C receptors and ANP-C receptor-mediated inhibition of adenylyl cyclase. It can be suggested that the antihypertensive action of several types of drugs such as angiotensin converting enzyme inhibitors, angiotensin type 1 receptor antagonists and beta2-adrenergic antagonists may partly be attributed to their ability to modulate the expression and function of the ANP-C receptor.

Thrombin inhibits atrial natriuretic peptide receptor activity in cultured bovine endothelial cells

Thrombin and the atrial natriuretic peptide (ANP) possess a number of functionally antagonistic properties in vascular endothelial cells. Thus, regulatory interactions that modulate the activity of one or the other could have important sequelae with regard to cardiovascular homeostasis. Thrombin treatment effected a dose- and time-dependent reduction in ANP receptor activity (maximal 70% to 80% inhibition) in cultured bovine aortic endothelial cells. This resulted from a decrease in total receptor number as well as a modest reduction in the affinity of the receptor for its ligand. The inhibition was largely confined to the type C receptor population, in that thrombin had no effect on maximal type A receptor-linked cGMP accumulation. The protein kinase C-activating phorbol ester 12-O-tetradecanoylphorbol 13-acetate effected a similar reduction in binding activity; however, suppression of protein kinase C activity did not reverse the thrombin effect. Pretreatment of endothelial cells with cycloheximide did not completely prevent the thrombin-dependent inhibition, and thrombin did not effect a reduction in type C receptor mRNA levels, findings that argue for a postsynthetic inhibitory locus. The inhibition of receptor activity was effectively irreversible in that suspension of protein synthesis blocked the recovery of receptor density on the cell surface. Reduction in type C receptor density was accompanied by modest increases in the stability of ANP in the culture medium and enhancement of the cellular cGMP response to the peptide, particularly at low ligand concentrations. These findings demonstrate a potentially important interaction between these two agonist systems in regulating endothelial cell function within the vascular wall.

Identification of cGMP-dependent protein kinase and its specific substrates in the anterior pituitary

In the anterior pituitary, cGMP is produced in response to a number of stimuli, but intracellular events distal to cGMP production are obscure. Since cGMP-dependent protein kinase (PKG) is a major effector of cGMP actions in other tissues we have determined whether PKG and its specific substrates might be present and responsive to external signals in the ovine anterior pituitary. Photoaffinity labelling with [32P]cGMP revealed a specific 78 kDa protein in ovine anterior pituitary that comigrated with purified bovine lung PKG-I. PKG in protein extracts from anterior pituitary or cultured anterior pituitary cells was enriched by DEAE ion-exchange chromatography and assayed for activity. Both tissue and cultured cells had a relatively high PKG activity by comparison with aortic smooth muscle (known high activity) and brain (known low activity). Subcellular distribution studies showed that in anterior pituitary, aortic and brain, PKG activity was present in both cytosol and triton-extracted membrane fractions, while in platelets the activity was associated with only the membrane fraction. To determine if this PKG might be responsive to extracellular signals an activity ratio assay was used. Incubation of cultured cells with atrial natriuretic peptide (ANP) and sodium nitroprusside, activators of membrane and cytosolic guanylate cyclases respectively, increased the activity of PKG. To determine events distal to PKG activation, a search for potential substrates of PKG was performed. Few substrates were detectable upon addition of purified PKG to tissue lysates due to the high background activity of endogenous protein kinases in the anterior pituitary. However, 19 substrates of PKG were detected in heat-stable and 14 in acid-soluble protein extracts of the anterior pituitary, in which background phosphorylation was almost abolished. After partial purification through Q-Sepharose ion-exchange chromatography some of these proteins were preferentially phosphorylated by addition of PKG-I, while the others were additionally substrates of exogenous cAMP-dependent protein kinase (PKA) or Ca2+ and phospholipid-dependent protein kinase (PKC). A 132-kDa substrate showed an identical phosphopeptide map to a PKG substrate previously described in vascular smooth muscle and platelets. These data demonstrate for the first time the presence of functional PKG activity and multiple PKG substrates in the anterior pituitary where they may play a role in mediating the intracellular actions of cGMP.

Cytoplasmic domain of natriuretic peptide receptor-C inhibits adenylyl cyclase. Involvement of a pertussis toxin-sensitive G protein

Natriuretic peptide receptor C (NPR-C) is a disulfide-linked homodimer with an approximately 440-amino acid extracellular domain and a 37-amino acid cytoplasmic domain; it functions in the internalization and degradation of bound ligand. The use of NPR-C-specific natriuretic peptide analogs has implicated this receptor in mediating the inhibition of adenylyl cyclase or activation of phospholipase C. In the present studies we have investigated the role of the cytoplasmic domain of NPR-C in signaling the inhibition of adenylyl cyclase. Polyclonal rabbit antisera were raised against a 37-amino acid synthetic peptide (R37A) corresponding to the cytoplasmic domain of NPR-C. Incubation of anti-R37A with rat heart particulate fractions blocked atrial natriuretic peptide-dependent inhibition of adenylyl cyclase. The cytoplasmic domain peptides R37A and TMC (10 residues of transmembrane domain appended on R37A) were equipotent in inhibiting adenylyl cyclase (Ki approximately 1 nM) in a GTP-dependent manner, whereas K37E (a scrambled peptide control for R37A) did not inhibit adenylyl cyclase activity. Prior incubation of membranes with pertussis toxin blocked R37A or TMC inhibition of cAMP production. Detergent solubilization of the rat heart particulate fraction destroyed natriuretic peptide inhibition of adenylyl cyclase, but TMC was able to inhibit cAMP production in a dose-dependent manner. Our results provide evidence that the 37-amino acid cytoplasmic domain of NPR-C is sufficient for signaling inhibition of adenylyl cyclase through a pertussis toxin-sensitive G protein.

Aberrant adenylyl cyclase/cAMP signal transduction and G protein levels in platelets from hypertensive patients improve with antihypertensive drug therapy

We have previously demonstrated a decreased expression of Gi alpha 2 protein in platelets from spontaneously hypertensive rats that was associated with an altered responsiveness of adenylyl cyclase to hormone stimulation and inhibition. In the present studies, we have used platelets from hypertensive patients and examined the hormonal regulation of adenylyl cyclase as well as the levels of G proteins and their modulation by antihypertensive drug therapy. We performed these studies in platelets from four groups of subjects: normotensive subjects (group 1), untreated mildly essential hypertensive patients (group 2), and treated moderately to severely hypertensive patients whose blood pressure was uncontrolled (group 3) or controlled with drug treatment (group 4). GTP gamma S, 5'-(N-ethylcarboxamido)adenosine (NECA), and prostaglandin E1 stimulated adenylyl cyclase activity to a greater extent in hypertensive patients (group 2). This effect was partially corrected (by approximately 50% to 80%) in the patients under antihypertensive drug therapy (groups 3 and 4). In addition, inhibition of adenylyl cyclase mediated by a ring-deleted analogue of atrial natriuretic factor (C-ANF4.23) observed in control normotensive subjects was blunted in hypertensive patients (group 2) and was not corrected in treated patients. Gi alpha levels determined by immunoblotting were in the same range for the four groups, whereas Gi alpha 2 and Gi alpha 3 levels were decreased by 70% and 60%, respectively, in hypertensive patients (group 2) compared with normotensive subjects. Antihypertensive drug therapy (groups 3 and 4) partially restored Gi alpha 2 levels toward normal (group 1) by about 60% and 70%, respectively; however, the reduced Gi alpha 3 levels in group 2 hypertensive patients were not improved in group 3 but were raised toward normal levels in group 4 by about 55%. These results suggest that the altered responsiveness of platelet adenylyl cyclase to hormones in hypertension and the normalization of the response with antihypertensive drug therapy could partly be due to the ability of the latter to modulate Gi alpha protein expression. These effects on platelet function may underlie the beneficial effects of antihypertensive agents on some of the complications of hypertension.

Atrial natriuretic peptide (ANP)-induced inhibition of glucagon secretion: mechanism of action in isolated rat pancreatic islets

ANP increases insulin levels in vivo. Because in vitro an ANP-induced increase in cGMP levels of islets of Langerhans was observed but no simultaneous increase in insulin release, secreted glucagon may be a candidate for this second messenger affected by ANP. The inhibitory effect of glucose on glucagon secretion was pronounced by 1.0 nM ANP at 3.0 mM glucose as well as at 5.6 and 8.3 mM glucose. Because in other tissues cGMP (the specific second messenger of ANP1 inhibits Ca2+ channels, the uptake of 45Ca2+ was investigated. ANP (1.0 nM) inhibited 45Ca2+ uptake, which was nearly completely abolished by a pertussis toxin (PT) pretreatment. The inhibition of 45Ca2+ uptake fits to inhibitory ANP effects on glucagon secretion but does not fit to insulin secretion. The glucagon secretion coupling cascade affected by ANP probably involves an increase in cGMP because 8-Br-cGMP (a membrane-permeable cGMP analogue) also decreased glucagon secretion. ANP(4-23), a truncated form of ANP, which is selective for the ANP clearance receptor, also inhibited glucagon secretion. HS-42-1, a guanylate cyclase receptor antagonist, tended to reverse the effect of ANP on glucagon release. The data indicate that in the presence of ANP, the in vivo homeostasis of glucose, though plasma insulin levels are increased, is not due to an ANP-mediated increase in glucagon secretion; ANP has a complex inhibitory effect on glucagon release. The data further indicate that the ANP-induced inhibition of glucagon secretion probably involves the cGMP system, an inhibition of Ca2+ uptake and the involvement of PT-sensitive G-proteins. Moreover, an involvement of the clearance receptor seems to be likely. ANP is a valuable tool for investigating glucagon secretion from pancreatic islets because paracrine effects of insulin can be excluded.

Atrial natriuretic peptide suppresses the transcription of its guanylyl cyclase-linked receptor

Atrial natriuretic peptide (ANP) treatment of rat aortic smooth muscle cells suppressed both 125I-ANP binding and ANP-dependent cGMP accumulation, suggesting reductions in the type C (NPR-C) and type A (NPR-A) natriuretic peptide receptor populations, respectively. NPR-A, but not NPR-C, mRNA levels were reduced in a dose-dependent fashion by ANP. The latter effect appeared to be due, at least in part, to suppression of NPR-A gene promoter activity. ANP effected a dose- and time-dependent reduction in a transiently transfected NPR-A luciferase reporter (-1575LUC). Analysis of 5' deletion mutants of the NPR-A promoter demonstrated that the ANP-dependent sequence lies between -1575 and -1290 relative to the transcription start site. Inhibition of the ANP promoter was also effected by brain natriuretic peptide, type C natriuretic peptide, and 8-bromo-cGMP, but not by the NPR-C-selective ligand cANF. In the case of 8-bromo-cGMP, the responsive element(s) was localized to the same 285-base pair region linked to the ANP effect above. These findings indicate that ANP autoregulates its own receptors in these cells and, at least in the case of NPR-A, it does so through suppression of receptor gene expression and receptor synthesis. This suppression may operate through a cGMP-dependent element located more than a kilobase upstream from the transcription start site.

C-type natriuretic peptide and brain natriuretic peptide inhibit adenylyl cyclase activity: interaction with ANF-R2/ANP-C receptors

C-type natriuretic peptide (CNP) and brain natriuretic peptide (BNP) are members of the natriuretic peptide family, which have been shown to interact with ANP-C/ANF-R2 receptors in addition to ANP-B receptor subtypes. The present study was undertaken to investigate if the interaction of CNP and BNP with ANP-C receptors results in the inhibition of adenylyl cyclase activity. CNP and BNP inhibited adenylyl cyclase activity in heart and brain striatal membranes in a concentration dependent manner with an apparent Ki between 0.1 and 1.0 nM. Maximal inhibition observed in heart membranes were about 25% and 35% for BNP and CNP respectively, however the inhibitions in brain striatal membranes were smaller (approximately 20%). The inhibition was dependent on the presence of guanine nucleotides and was attenuated by pertussis toxin treatment. In addition, CNP inhibited the stimulatory effect of isoproterenol on adenylyl cyclase, whereas CNP as well as BNP showed an additive effect with the inhibitory response of angiotensin II on adenylyl cyclase activity. When the combined effect of C-ANF4-23/BNP, C-ANF4-23/CNP and BNP/CNP at optimal concentrations was studied together on adenylyl cyclase activity, the percent inhibition remained the same for C-ANF4-23 and BNP or C-ANF4-23 and CNP, however, an additive inhibitory effect was observed for BNP and CNP. These results suggest that CNP and BNP like C-ANF4-23 interact with ANP-C receptors and result in the inhibition of adenylyl cyclase activity. On the other hand, CNP and BNP interact with the ANP-C receptor, however, the interaction may be different sites or there may be two subpopulations of ANP-C receptors specific for each of the peptides. These results indicate that BNP and CNP, like ANP and C-ANF4-23, inhibit the adenylyl cyclase/cAMP signal transduction system through an inhibitory guanine nucleotide regulatory protein, by interacting with ANP-C receptor subtypes.

Defective ANF-R2/ANP-C receptor-mediated signalling in hypertension

In the present studies we have shown that atrial natriuretic factor (peptide) receptor of ANF-R2/ANP-C type is coupled to adenylyl cyclase/cAMP signal transduction system through Gi-regulatory protein and is implicated in mediating some of the physiological responses of atrial natriuretic factor or peptide (ANP). ANF-R2/ANP-C receptor-mediated adenylyl cyclase inhibition was altered in hypertension. This alteration was tissue specific. In heart, aorta, brain and adrenal, the extent of inhibition of adenylyl cyclase by ANP was enhanced in SHR as compared to age-matched WKY, whereas in platelets, the ANP-mediated inhibition was completely attenuated. The enhanced inhibition of adenylyl cyclase by ANP was also observed in heart and aorta from DOCA-salt hypertensive rats. In addition, the augmented inhibition of adenylyl cyclase by ANP was observed in 2 weeks and older SHR but not in 3-5 days old SHR. Similarly, in DOCA-salt hypertensive rats, the enhanced inhibition of adenylyl cyclase by ANP was observed after 2 weeks of DOCA-salt treatment when the blood pressure was also enhanced, however one week older SHR but not in 3-5 days old SHR. Similarly, in DOCA-salt hypertensive rats, the enhanced inhibition of adenylyl cyclase by ANP was observed after 2 weeks of DOCA-salt treatment when the blood pressure and augmented ANP-mediated inhibition of adenylyl of DOCA-salt treatment did not result in an augmented blood pressure and augmented ANP-mediated inhibition of adenylyl cyclase, suggesting that blood pressure increase may be responsible for the enhanced responsiveness of ANP to adenylyl cyclase inhibition. However, in genetic model of hypertension, the increased inhibition of adenylyl cyclase by ANP at 2 weeks of age (when the blood pressure is normal) may be implicated in the pathogenesis of hypertension. The augmented inhibition of adenylyl cyclase in cardiovascular tissues from SHR and DOCA-salt hypertensive rats may be due to the upregulation of ANF-R2/ANP-C receptors or due to the amplification of post-receptor signalling mechanisms.

Prolonged antagonism of alpha 1-adrenergic vasoconstriction in the rat liver by atrial natriuretic peptide

BACKGROUND/AIMS

Vasodilator hormones that regulate hepatic circulation at physiological concentrations have not been sufficiently identified. The presence of atrial natriuretic peptide (ANP) and its receptors in the hepatic vascular bed suggest such vasorelaxing potential.

METHODS

Livers of male Sprague-Dawley rats were perfused in a flow-constant fashion. The selective alpha 1-adrenergic agonist phenylephrine (PE) (1.5 mumol/L) was infused from 30 to 36 minutes and again from 70 to 76 minutes after starting perfusion (n = 5). ANP (0.1 pmol/L to 200 nmol/L), des-(Gln18, Ser19, Gly20, Leu21, Gly22)-ANP fragment (C-ANP) (20 nmol/L), or 8-bromoguanosine 3',5'-cyclic monophosphate (8-Br-cGMP) (50 mumol/L) (each n = 4) were added from 20 to 40 minutes.

RESULTS

During the first infusion of PE, portal pressure increased from 3.7 +/- 0.5 to 12.1 +/- 0.8 cm H2O maximally (mean +/- SD) and increased again to 11.5 +/- 2.0 during the second PE infusion. ANP at physiological concentrations reduced both PE-induced increases of portal pressure in a dose-dependent fashion, reaching half-maximal effects around 20 pmol/L and maximal effects (about 50% inhibition of PE-induced vasoconstriction) at 40 pmol/L. The cGMP analogue 8-Br-cGMP showed the same long-lasting vasodilating effect as ANP. In contrast, C-ANP, which binds only to the ANP C-receptor, had no effects.

CONCLUSIONS

Physiological concentrations of ANP antagonize alpha 1-adrenergic vasoconstriction in the liver, suggesting an important function in the humoral regulation of hepatic circulation. The prolonged hemodynamic effect of ANP seems to be ANP A-receptor/guanylyl cyclase/cGMP-mediated.

Effects of ANP receptor antagonists on ANP secretion from adult rat cultured atrial myocytes

Atrial natriuretic peptide (ANP) is a hormone-secreted predominantly by atrial myocytes. ANP exerts many of its actions via activation of the particulate guanylyl cyclase receptor ANPR-A and the formation of guanosine 3',5'-cyclic monophosphate (cGMP), which serves as a second messenger in the target cells. Using membrane-permeable cGMP analogues (8-bromo-cGMP and dibutyryl- cGMP), we first tested the hypothesis that ANP secretion by adult rat cultured atrial myocytes can be modulated through the second messenger cGMP. Second, we examined the effects of two competitive ANPR-A receptor antagonists, namely HS-142-1 and anantin, on cGMP formation and ANP secretion from cultured atrial myocytes. Cultured atrial myocytes secreted large quantities of immunoreactive (ir) ANP under basal conditions. We found that cGMP analogues inhibited basal irANP secretion from cultured atrial myocytes, whereas HS-142-1 and anantin had stimulating effects. HS-142-1 and anantin reduced cGMP formation in cultured atrial myocytes at basal conditions. These results suggest an autoregulatory mechanism of ANP secretion by atrial myocytes in an autocrine/paracrine fashion.

Atrial natriuretic factor inhibits autophosphorylation of protein kinase C and A 240-kDa protein in plasma membranes of bovine adrenal glomerulosa cells: involvement of cGMP-dependent and independent signal transduction mechanisms

To clarify the intracellular signalling mechanisms of atrial natriuretic factor (ANF), we studied its effect on protein phosphorylation in plasma membranes of bovine adrenal cortical cells. ANF (1 x 10(-7) M) inhibited phosphorylation of the 78-kDa protein kinase C (PKC) and a 240-kDa protein in specific manner. In parallel experiments, cGMP (0.5 mM) inhibited phosphorylation of only the 78-kDa PKC but it did not affect phosphorylation of the 240-kDa protein. Phosphorylation of the 78-kDa PKC was enhanced in a Ca(2+)-/phospholipid-dependent manner. However, after prolonged preincubation of plasma membranes with Ca2+ (0.5 mM), the incorporation of 32P-radioactivity rapidly decreased in the 78-kDa PKC and subsequently increased in the 45- and 48-kDa protein bands due to Ca(2+)-dependent proteolytic degradation of 78-kDa PKC. Polyclonal antibodies against brain PKC were used to immunoblot and immunoprecipitate the 78-kDa PKC. Preincubation of plasma membranes with Ca2+ for varying times, followed by immunoblotting revealed a gradual loss of the immunoreactive 78-kDa PKC band in a time-dependent manner. Immunoprecipitation of phosphorylated 78-kDa PKC in plasma membranes showed that its phosphorylation was significantly inhibited in the presence of ANF as compared to control membranes, phosphorylated in the absence of ANF. The results in this present study document a new signal transduction mechanism of ANF at molecular level which possibly involves dephosphorylation of the 78-kDa PKC and a 240-kDa protein in a cGMP-dependent and -independent manner in bovine adrenal glomerulosa cell membranes.

Modulation of guanylate cyclase-coupled atrial natriuretic factor receptor activity by mastoparan and ANF in murine Leydig tumor cells: role of G-proteins

Mastoparan potently stimulated catalytic activity of guanylate cyclase-coupled atrial natriuretic factor receptor (GC-A/ANF-R), both in the plasma membranes and intact Leydig tumor (MA-10) cells. In plasma membrane preparations, a maximum of 5-fold GC catalytic activity was stimulated by 100 microM mastoparan and the half maximum stimulation (EC50) was achieved at 40 microM concentration. Mastoparan potentiated GC activity by more than 40%, above the level, stimulated by ANF. Mas 7, an active analog of mastoparan, stimulated the GC activity in a similar manner to mastoparan whereas Mas 17, an inactive analog, did not enhance GC activity. In membranes prepared from mastoparan-treated intact MA-10 cells, GC catalytic activity was enhanced by more than 4-fold as compared with untreated control cells. Pretreatment of membranes with either anti-Gs alpha or anti-Gi alpha antibodies had no effect on mastoparan-stimulated GC activity, however, anti-Go alpha antibodies inhibited the stimulatory effect of mastoparan by almost 50%. Agents known to modulate the effect of mastoparan such as EGTA (Ca2+ chelator), W7 (calmodulin inhibitor) and staurosporine (protein kinase C inhibitor) had no effect on the mastoparan-stimulated GC activity. Mastoparan enhanced the ANF-stimulated GC activity in detergent solubilized membrane preparations without a significant change in ANF-binding capacity. The data establish a role for mastoparan in the ANF-dependent stimulation of GC-A/ANF-R catalytic activity, both in the plasma membrane preparations and intact Leydig tumor (MA-10) cells. Furthermore, these findings provide new evidence that mastoparan (isolated from wasp venom) potently stimulates guanylate cyclase activity of GC-A/ANF-R by activating G-proteins.

Effect of PT-treatment on ANP-mediated inhibition of adenylate cyclase and amylase release in rat parotid gland

Effects of pertussis toxin (PT) treatment on atrial natriuretic peptide (ANP)-mediated inhibition of adenylate cyclase and amylase release were investigated in rat parotid gland. Adenylate cyclase activity stimulated by GTP gamma S in PT-treated membranes was much larger than that in normal membranes. ANP dose-dependently inhibited adenylate cyclase stimulated by GTP gamma S in control rat parotid membranes, however in membranes prepared from PT-injected (in vivo) rat parotid gland, ANP did not inhibit adenylate cyclase. ANP(10(-7)M) inhibited cAMP accumulation stimulated by forskolin (10(-6)M) in control rat parotid acinar cells by about 34%, however, in PT-treated cells, the inhibitory effect of ANP was attenuated completely. In control cells amylase release stimulated by isoproterenol (10(-6)M) and forskolin (10(-6)M) were also depressed by ANP (10(-7)M) by 27 and 30% respectively. The inhibitory response of ANP on amylase release was completely attenuated by PT-treatment. Gi was detected as a ADP-ribosylated 41-KDa protein by incubation of parotid membranes with PT and [alpha-32P]NAD. In rat parotid gland, these results suggested that ANP mediates adenylate cyclase/cAMP system and consequently reduces amylase release through ANP-C receptor coupled to Gi.

Influence of atrial natriuretic peptide on cyclic nucleotides and amylase release in rat parotid salivary gland in vitro

Atrial natriuretic peptide (ANP), sodium nitroprusside and hydroxylamine increased cGMP accumulation in rat parotid acinar cells both in the presence and absence of forskolin but in a different manner. On the other hand, ANP decreased forskolin-stimulated cAMP accumulation, although sodium nitroprusside and hydroxylamine had no effect on cAMP accumulation. Amylase release stimulated by forskolin, dibutyryl-cAMP or isoproterenol was depressed by ANP, whereas sodium nitroprusside and hydroxylamine did not evoke the inhibition of forskolin-stimulated amylase release. These results suggest that the inhibition of cAMP accumulation and of amylase release by ANP were not mediated via cGMP produced by guanylate cyclase-A.

Platelets from spontaneously hypertensive rats exhibit decreased expression of inhibitory guanine nucleotide regulatory protein. Relation with adenylyl cyclase activity

We have recently demonstrated an enhanced expression of inhibitory guanine nucleotide regulatory protein (Gi) in the heart and aorta from spontaneously hypertensive rats (SHR) as compared with control Wistar-Kyoto (WKY) rats; this enhanced Gi expression was associated with an increased inhibition of adenylyl cyclase by inhibitory hormones and decreased stimulation of adenylyl cyclase by stimulatory hormones. In the present studies, we have determined the levels of stimulatory and inhibitory guanine nucleotide regulatory proteins (Gs and Gi, respectively) in platelets from SHR by cholera toxin- and pertussis toxin-catalyzed ADP-ribosylations, respectively, as well as by immunoblotting techniques using specific antibodies for Gs and Gi. Cholera toxin catalyzed the ADP-ribosylation of a single protein of M(r) 45,000 in rat platelets from SHR and WKY rats, and the labeling of this band was not altered in SHR as compared with WKY rats. Pertussis toxin, on the other hand, catalyzed the ADP-ribosylation of a single protein band of M(r) 41,000 in platelets from SHR and WKY rats, and unlike the response in heart and aorta, the labeling of this band was significantly decreased in SHR as compared with WKY rats. Furthermore, immunoblotting experiments using AS/7 antibody, which is specific for Gi alpha-1 and Gi alpha-2, showed a decrease in Gi alpha-2 in platelets from SHR as compared with WKY rats. In addition, the inhibitory effects of angiotensin II and atrial natriuretic factor on adenylyl cyclase and cAMP levels were completely abolished in SHR platelets, whereas the stimulatory effects of GTP, N-ethylcarboxamide adenosine, prostaglandin E1, and forskolin on adenylyl cyclase and cAMP levels were enhanced.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of binding sites in rat for A, B and C-type natriuretic peptides

Binding studies, affinity cross-linking and guanylate cyclase assays allowed a comparison of receptors with which the rat forms of atrial/A-type natriuretic peptide (rANP), brain/B-type natriuretic peptide (rBNP) and C-type natriuretic peptide (rCNP) interact in rat kidney cortex and lung. This work represents the first study in which the rat form of BNP (= rBNP-45/iso-rANP(1-45)) has been used as a radiolabelled tracer to further characterize its receptors in these tissues. In addition, these studies stress the use of the same species of natriuretic peptide and assay system, an important experimental des ign given that BNPs show species-specific differences in structure. rBNP-45 bound with lower affinity to rANP (99-126) receptors, namely guanylate cyclase-linked receptor(s) and C-receptor. No receptor which interacted with only rBNP-45 was detectable in lung and kidney cortex. Since rBNP-45 interacted preferentially with the C-receptor and was less potent than rANP(99-126) in stimulating glomerular guanylate cyclase, rBNP-45 may signal through another second messenger in addition to cyclic GMP. Work with truncated analogues of this hormone pinpointed regions of this peptide which may contribute to receptor binding affinity and guanylate cyclase activation. CNP-22 bound to only a subset of ANP receptors and was least effective in stimulating glomerular guanylate cyclase, suggesting a differential mode of action from ANP.

A dramatic pH-dependent alteration in ANP receptor density: a note for using cultured cells

Culture media tend to become acidic when rapidly growing cells are cultured under batch conditions using a CO2/HCO3- buffer system. The effects of this inherent lowering of pH on cellular makeup of cultured cells, which have long been ignored, were examined by monitoring the pH and number of the atrial natriuretic peptide (ANP) receptors expressed on the cultured bovine endothelial cells. The Eagle's minimum essential medium was adjusted to three different pH values of 7.0, 7.4, and 7.7 and used for 48-h batch cultures. After this 48-h incubation, the pH values of the media were found to be 7.0, 7.1, and 7.4, respectively. These pH shifts had unexpectedly strong influences on the ANP receptor levels without affecting the affinity. Cells maintained in the slightly higher pH medium had a trace amount of the receptor (< 10 sites/cell), while those in the lower pH environment exhibited a large number of binding sites (40,000 sites/cell). Similar situations might occur in other cellular components and in other types of cells, and therefore, such possibilities should be kept in mind when cultured cell systems are used.

Atrial natriuretic peptide-induced inhibition of aldosterone secretion: a quest for mediator(s)

Atrial natriuretic peptide (ANP) inhibits aldosterone secretion evoked by its physiological secretagogues by a mechanism(s) likely to involve intracellular messengers. When one examines the results of various investigations so far, this premise, although not definitive yet, seems to be supported. Therefore a brief perspective on the cellular messengers of the various secretagogues is provided before the inquiry into the possible mechanism of action of ANP. The receptors of ANP in the adrenal cells have been identified and characterized. ANP inhibits adenylate cyclase in various tissues through an inhibitory G protein, which appears to explain in part the inhibitory effect of ANP on adrenocorticotropin-induced aldosterone secretion. However, there could be other possible effects of ANP as discussed. ANP probably inhibits aldosterone secretion evoked by angiotensin II and potassium by interfering with the appropriate changes in calcium flux and cell calcium concentration, concomitants of stimulation by these secretagogues. The potential modes of these effects are probed. The role of guanosine 3',5'-cyclic monophosphate, which is increased by receptor activation of guanylate cyclase by ANP and is thought to play a major role in the biological effects of ANP in some other tissues, remains controversial in the aldosterone-lowering effect of ANP, and this is also discussed extensively in this review.

The ANF-C receptor is not linked to adenylyl cyclase inhibition in bovine pulmonary artery endothelial cells

The possible inhibition of adenylyl cyclase activity by atrial peptides selective for the ANF-C receptor was investigated in bovine pulmonary artery endothelial cells. In these cells isoprenaline, guanine nucleotide and forskolin dose-dependently increased activity over basal levels. In the presence of rANF(99-126), these dose-dependent increases were not reduced, nor were they affected by the ANF-C receptor selective analogue C-ANF(102-121). Furthermore, the selective analogues rANF(103-123) and des[Cys105,Cys121]rANF104-126 had no effect on basal or stimulated adenylyl cyclase activity. It can be concluded that ANF-C receptors are not linked to inhibition of adenylyl cyclase in these cells.

Production of polyclonal antibody to the bovine adrenal atrial natriuretic factor-R1 receptor

A polyclonal antibody monospecific for an intracellular epitope of the atrial natriuretic factor (ANF)-R1 receptor was produced. The receptor protein (200 pmoles) was purified to homogeneity from bovine adrenal zona glomerulosa (BAZG), reduced, alkylated and digested with trypsin. The tryptic fragments were purified by reverse-phase h.p.l.c. on a C18 column. Based on the sequence of one of these fragments, a peptide was chemically synthesized, coupled to thyroglobulin and injected into rabbits. The antibody obtained was shown to be specific for the R1-type as no receptor was detected in bovine red blood cells (RBC) (which are devoid of ANF receptors) and in NIH-3T3 cell membranes (where only the R2-type is expressed). Several other tissues were screened and comparison of the immunoreactive receptor density estimates with those obtained by ANF binding yielded a correlation coefficient (r2) of 0.965. The minimal detectable dose was typically 3 fmoles/tube and the ED50 of the RIA was 30 fmoles/tube. Cyanogen bromide digestion of the receptor was essential for antigenic detection, indicating that the epitope is probably hindered due to the tertiary structure of the native protein. Moreover, location of the epitope in the kinase homology domain of the receptor, combined with partial tryptic digestion, suggests that the proteolysis-sensitive region of the receptor is located between the transmembrane-spanning domain and the amino acid 586. This method of production of antibodies should be useful to precisely map the amino acids involved in various functions of the receptor.

Selective activation of the B natriuretic peptide receptor by C-type natriuretic peptide (CNP)

The natriuretic peptides are hormones that can stimulate natriuretic, diuretic, and vasorelaxant activity in vivo, presumably through the activation of two known cell surface receptor guanylyl cyclases (ANPR-A and ANPR-B). Although atrial natriuretic peptide (ANP) and, to a lesser extent, brain natriuretic peptide (BNP) are efficient activators of the ANPR-A guanylyl cyclase, neither hormone can significantly stimulate ANPR-B. A member of this hormone family, C-type natriuretic peptide (CNP), potently and selectively activated the human ANPR-B guanylyl cyclase. CNP does not increase guanosine 3',5'-monophosphate accumulation in cells expressing human ANPR-A. The affinity of CNP for ANPR-B is 50- or 500-fold higher than ANP or BNP, respectively. This ligand-receptor pair may be involved in the regulation of fluid homeostasis by the central nervous system.

The protective effect of atrial natriuretic peptide (ANP) on cells damaged by oxygen radicals is mediated through elevated CGMP-levels, reduction of calcium-inflow and probably G-proteins

ANP increases cellular cGMP content in cultured hepatocytes and decreases Ca2(+)-inflow in a concentration- and time-dependent manner which explains a beneficial effect on hypoxia cell injury (25). Both observations are mimicked by SNP and 8-Br-cGMP and blocked by Ly 83583 indicating a cGMP-mediated mechanism. The protective effect was also inhibited by Pertussis Toxin (PT) without lowering the elevated cGMP-level. But PT in combination with ANP leads to a higher Ca2(+)-inflow. Stimulated Na(+)-inflows are also be lowered by ANP. Here, neither SNP can mimick nor PT can inhibit this effect. Our results now indicate that the beneficial effect by ANP at the cellular level is mediated through cGMP which decreases calcium-inflow. ANP seems to control Ca2(+)-channels direct via a PT-sensitive G-protein and indirect by a cGMP-mediated mechanism and Na(+)-channels cGMP-independent through a PT-insensitive G-protein, thus preventing cells on hypoxia and oxygen radicals.

Guanylyl cyclase-linked receptors

The guanylyl cyclase receptor family contains members that exist in both the particulate and soluble fractions of cell homogenates. Based on cloning studies, proteins with guanylyl cyclase activity contain a single transmembrane domain, or exist as heterodimers with no apparent transmembrane domains. The members containing the single transmembrane domain appear to act as cell surface receptors for peptides such as natriuretic peptides and bacterial heat-stable enterotoxins, while the heterodimeric forms are activated by nitric oxide. The concentrations of the intracellular messenger, cyclic GMP, then, are regulated by multiple primary signaling molecules, all of which appear to bind directly to the guanylyl cyclase enzyme.

Inhibition of hepatic adenylate cyclase by NADH

Adenylate cyclase activity in isolated rat liver plasma membranes was inhibited by NADH in a concentration-dependent manner. Half-maximal inhibition of adenylate cyclase was observed at 120 microM concentration of NADH. The effect of NADH was specific since adenylate cyclase activity was not altered by NAD+, NADP+, NADPH, and nicotinic acid. The ability of NADH to inhibit adenylate cyclase was not altered when the enzyme was stimulated by activating the cyclase was not altered when the enzyme was stimulated by activating the Gs regulatory element with either glucagon or cholera toxin. Similarly, inhibition of Gi function by pertussis toxin treatment of membranes did not attenuate the ability of NADH to inhibit adenylate cyclase activity. Inhibition of adenylate cyclase activity to the same extent in the presence and absence of the Gpp (NH) p suggested that NADH directly affects the catalytic subunit. This notion was confirmed by the finding that NADH also inhibited solubilized adenylate cyclase in the absence of Gpp (NH)p. Kinetic analysis of the NADH-mediated inhibition suggested that NADH competes with ATP to inhibit adenylate cyclase; in the presence of NADH (1 mM) the Km for ATP was increased from 0.24 +/- 0.02 mM to 0.44 +/- 0.08 mM with no change in Vmax. This observation and the inability of high NADH concentrations to completely inhibit the enzyme suggest that NADH interacts at a site(s) on the enzyme to increase the Km for ATP by 2-fold and this inhibitory effect is overcome at high ATP concentrations.

Alpha-adrenoceptor activation of polyphosphoinositide hydrolysis in the rat tail artery

alpha-Adrenoceptor coupling to polyphosphoinositide (PI) hydrolysis was studied in the rat tail artery. Inositol phosphate (IP) accumulation was stimulated by the non-selective alpha-adrenoceptor agonist norepinephrine and the alpha 1-adrenoceptor agonist phenylephrine. This stimulation was relatively dependent on extracellular Ca2+ and enhanced markedly in the presence of LiCl. In addition, norepinephrine- and phenylephrine-stimulated IP accumulation was relatively sensitive to blockade by prazosin, compared to rauwolscine. The putative alpha 2-adrenoceptor agonist UK 14304 also stimulated PI breakdown in a concentration-dependent manner, although this stimulation did not reach equilibrium at up to 10 mM and was relatively sensitive to prazosin, compared to rauwolscine, over the lower agonist concentrations. NaF stimulated IP accumulation independently of alpha-adrenoceptor activation. PI breakdown by alpha-adrenoceptor agonists and NaF was attenuated by N-ethylmaleimide but not pertussis toxin treatment. In addition, dithiothreitol blocked NaF-stimulated, but not alpha-adrenoceptor-mediated. PI breakdown. These results suggest the coupling of alpha 1-adrenoceptor, via phospholipase C, to PI hydrolysis in the rat tail artery. This study also provides evidence for the involvement of one or more non-Gi-like G-protein(s) in the signal transduction process.

Guanylyl cyclase is a heat-stable enterotoxin receptor

Plasma membrane forms of guanylyl cyclase have been shown to function as natriuretic peptide receptors. We describe a new clone (GC-C) encoding a guanylyl cyclase receptor for heat-stable enterotoxin. GC-C encodes a protein containing an extracellular amino acid sequence divergent from that of previously cloned guanylyl cyclases; however, the protein retains the intracellular protein kinase-like and cyclase catalytic domains. Expression of GC-C in COS-7 cells results in high guanylyl cyclase activity. In addition, heat-stable enterotoxin from E. coli, but not natriuretic peptides, causes marked elevations of cyclic GMP and is specifically bound by cells transfected with GC-C. The enterotoxin fails to elevate cyclic GMP in nontransfected cells or in cells transfected with the natriuretic peptide/guanylyl cyclase receptors. These results show that a heat-stable enterotoxin receptor responsible for acute diarrhea is a plasma membrane form of guanylyl cyclase.

Atrial natriuretic factor increases cyclic GMP and cyclic AMP levels in a directly photosensitive pineal organ

Atrial natriuretic factor (ANF) stimulates accumulation of cyclic GMP in a photosensitive organ, as evidenced for the first time in cultured trout pineals. Stimulation was rapid (within a few min), dose-dependent, and stronger in organs cultured in darkness than in those cultured under light. After 30 min in the dark, (i) cyclic AMP levels were slightly increased at 10(-7) mole/l of ANF, (ii) cyclic GMP and cyclic AMP increased dramatically after inhibition of the phosphodiesterases by isobutylmethylxanthine (IBMX), (iii) ANF and IBMX effects were more than additive on cyclic GMP, (iv) pertussis toxin decreased the cyclic GMP response to ANF. These responses were affected by light. The possibility that cyclic GMP might be a second messenger of both light and chemical (ANF) inputs, in pineal photoreceptor cells, is hypothetized.

Influence of anti-beta-receptor antibodies on cardiac adenylate cyclase in patients with idiopathic dilated cardiomyopathy

Autoantibodies against the cardiac beta 1-adrenoceptor are present in the sera of patients with idiopathic dilated cardiomyopathy and may modulate the responsiveness of cardiac beta-adrenergic pathways to agonists. The regulation of cardiac adenylate cyclase activity by autoantibodies was examined in 50 patients with dilated cardiomyopathy. Inhibition of isoproterenol-sensitive adenylate cyclase activity could be demonstrated by serum dilutions or IgG in 52% (26 of 50) of the patients; basal and NaF-stimulated activities, in contrast, were unaffected. In 14 patients, both ligand binding to beta-receptor and isoproterenol-sensitive adenylate cyclase activity were inhibited by 100-fold serum dilutions. Pretreatment of cardiac membranes with pertussis toxin did not affect inhibition of adenylate cyclase indicating that the effect of sera does not depend on Gi. The immunogenetic control of antireceptor antibodies was examined by comparing the distribution of HLA antigens in antibody-positive and antibody-negative patients. HLA-DR4 and HLA-DR1 were strongly associated with antibodies inhibiting ligand binding and adenylate cyclase activity (71% of patients with such antibodies typed as either DR4 or DR1). Conversely 58% of patients with HLA-DR4 and 71% of patients with HLA-DR1 antibodies showed inhibition of adenylate cyclase activity compared to 46% of those who lacked both HLA-DR4 and HLA-DR1 antibodies. These results strongly suggest that cardiac beta-adrenergic receptors and adenylate cyclase activity in dilated cardiomyopathy can be modulated by circulating autoantibodies, the presence of which is under the control of the major histocompatibility complex.

Receptor-effector coupling by G proteins

The primary structure of G proteins as deduced from purified proteins and cloned subunits is presented. When known, their functions are discussed, as are recent data on direct regulation of ionic channels by G proteins. Experiments on expression of alpha subunits, either in bacteria or by in vitro translation of mRNA synthesized from cDNA are presented as tools for definitive assignment of function to a given G protein. The dynamics of G protein-mediated signal transduction are discussed. Key points include the existence of two superimposed regulatory cycles in which upon activation by GTP, G proteins dissociate into alpha and beta gamma and their dissociated alpha subunits hydrolyze GTP. The action of receptors to catalyze rather than regulate by allostery the activation of G proteins by GTP is emphasized, as is the role of subunit dissociation, without which receptors could not act as catalysts. To facilitate the reading of this review, we have presented the various subtopics of this rapidly expanding field in sections 1-1X, each of which is organized as a self-contained sub-chapter that can be read independently of the others.

Roles of G proteins in coupling of receptors to ionic channels and other effector systems

Guanine nucleotide binding (G) proteins are heterotrimers that couple a wide range of receptors to ionic channels. The coupling may be indirect, via cytoplasmic agents, or direct, as has been shown for two K+ channels and two Ca2+ channels. One example of direct G protein gating is the atrial muscarinic K+ channel K+[ACh], an inwardly rectifying K+ channel with a slope conductance of 40 pS in symmetrical isotonic K+ solutions and a mean open lifetime of 1.4 ms at potentials between -40 and -100 mV. Another is the clonal GH3 muscarinic or somatostatin K+ channel, also inwardly rectifying but with a slope conductance of 55 pS. A G protein, Gk, purified from human red blood cells (hRBC) activates K+ [ACh] channels at subpicomolar concentrations; its alpha subunit is equipotent. Except for being irreversible, their effects on gating precisely mimic physiological gating produced by muscarinic agonists. The alpha k effects are general and are similar in atria from adult guinea pig, neonatal rat, and chick embryo. The hydrophilic beta gamma from transducin has no effect while hydrophobic beta gamma from brain, hRBCs, or retina has effects at nanomolar concentrations which in our hands cannot be dissociated from detergent effects. An anti-alpha k monoclonal antibody blocks muscarinic activation, supporting the concept that the physiological mediator is the alpha subunit not the beta gamma dimer. The techniques of molecular biology are now being used to specify G protein gating. A "bacterial" alpha i-3 expressed in Escherichia coli using a pT7 expression system mimics the gating produced by hRBC alpha k.