Atrial natriuretic factor elicits an endothelium-independent relaxation and activates particulate guanylate cyclase in vascular smooth muscle

Effect of beta-adrenergic receptor blockade on atrial natriuretic peptide in essential hypertension

Plasma levels of atrial natriuretic peptide (ANP) were measured in 32 untreated subjects with essential hypertension and in 31 patients undergoing long-term treatment with beta-blockers. Patients receiving beta-blockers had significantly higher mean plasma ANP levels (72.0 +/- 36.0 [SD] pg/ml) than did untreated hypertensive subjects (39.8 +/- 15.8 pg/ml; p less than 0.01) and healthy normotensive controls (33.9 +/- 16.6 pg/ml; n = 61, p less than 0.01), while the mean plasma ANP concentration in untreated hypertensive subjects was not statistically different from that in control subjects. Administration of atenolol, 50 mg/day, for 4 weeks to 10 untreated subjects resulted in a significant (p less than 0.001) rise in plasma ANP levels (from 38.8 +/- 9.5 to 68.7 +/- 20.6 pg/ml). In 31 patients undergoing long-term treatment with beta-blockers, multivariate regression analysis revealed that age, pretreatment mean blood pressure, and plasma concentration of cyclic 3',5'-guanosine monophosphate (cGMP) were significant predictors of plasma ANP levels. These results suggest that beta-adrenergic receptor blockade in patients with essential hypertension elevates plasma ANP levels with a concomitant rise in cGMP concentrations, and that increased ANP in plasma may play a role in the compensatory mechanism that operates in response to beta-adrenergic receptor blockade.

Effects of 8-bromo-cGMP on Ca2+ levels in vascular smooth muscle cells: possible regulation of Ca2+-ATPase by cGMP-dependent protein kinase

The effects of 8-bromo-cGMP on intracellular calcium concentrations in cultured rat aortic smooth muscle cells were studied. Both angiotensin II and depolarizing concentrations of K+ stimulated Ca2+ accumulation in the cytoplasm. The increase in Ca2+ due to angiotensin II was associated with an increase in inositol phosphates, while that due to K+ was not. Preincubation of cells with 8-bromo-cGMP (100 microM) caused an inhibition of peak Ca2+ accumulation to either angiotensin II or K+. To probe the mechanism of action of cGMP in vascular smooth muscle, the effects of cGMP-dependent protein kinase on Ca2+-ATPase from the cultured cell particulate material were investigated. Ca2+-activated ATPase was stimulated approximately equal to 2-fold by exogenous calmodulin and up to 4-fold by low concentrations of purified cGMP-dependent protein kinase. The inclusion of both calmodulin and cGMP-dependent protein kinase resulted in an additive stimulation of Ca2+-ATPase. Stimulation of Ca2+-ATPase activity was observed at all Ca2+ concentrations tested (0.01-1.0 microM). cAMP-dependent protein kinase catalytic subunit and protein kinase C were either ineffective or less effective than cGMP-dependent protein kinase in stimulating the Ca2+-ATPase from rat aortic smooth muscle cells. These results suggest a possible mechanism of action for cGMP in mediating decreases in cytosolic Ca2+ through activation of a Ca2+-ATPase and the subsequent removal of Ca2+ from the cell.

Angiotensin decreases cyclic GMP accumulation produced by atrial natriuretic factor

Atrial natriuretic factor (ANF) produced rapid increases in cyclic GMP (cGMP) in cultured aortic smooth muscle cells. Angiotensin II (ANG II) markedly decreased the accumulation of cGMP that was evoked by ANF. Arginine vasopressin and ATP, which evoke transient increases in free Ca2+ similarly to ANG II, also inhibited cGMP accumulation. The effect of the calcium mobilizing neurohormones was mimicked by the divalent cation ionophore, A23187. The cyclic nucleotide phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine, prevented ANG II from inhibiting ANF-evoked cGMP accumulation. ANG II also inhibited cGMP accumulation induced by nitroprusside, a compound that activates cytosolic guanylate cyclase. These findings support the hypothesis that ANG II decreases cGMP accumulation by stimulating cGMP hydrolysis, apparently via a Ca2+-activated cGMP phosphodiesterase.

Clinical application of atrial natriuretic polypeptide in patients with congestive heart failure: beneficial effects on left ventricular function

Synthetic alpha-human atrial natriuretic polypeptide was infused in patients with congestive heart failure (CHF) (New York Heart Association class III or IV) and in those without CHF. The infusion of atrial natriuretic polypeptide (ANP) at a rate of 0.1 microgram/kg/min significantly decreased pulmonary capillary wedge pressure and increased stroke volume index in all of the patients with CHF, whereas it decreased pulmonary capillary wedge pressure but caused no significant change in stroke volume index in the patients without CHF. Concomitant significant reductions in total systemic resistance were observed in both groups of patients. The ANP infusion significantly increased the urine volume, the excretion of sodium, and endogenous creatinine clearance in the patients without CHF. In the patients with CHF, it also showed a tendency to increase all these variables, but the urine volume did not correlate with the reduction in pulmonary capillary wedge pressure. The ANP infusion also decreased plasma aldosterone concentrations in these patients, although no significant difference was observed in the decrement of the plasma aldosterone concentration in the patients with and those without CHF. These findings indicate that the ANP infusion improves left ventricular function in patients with CHF, and suggest that this improvement results mainly from the vasodilating activity of ANP.

An activator of protein kinase C (phorbol dibutyrate) attenuates atrial-natriuretic-factor-stimulated cyclic GMP accumulation in smooth-muscle cells

Rat thoracic aortic smooth-muscle cells (A-10; A.T.C.C. CRL 1476) displays a high density of vasopressin and atrial-natriuretic-factor (ANF) receptors and a low density of beta-adrenergic receptors. ANF stimulates cGMP (cyclic GMP) accumulation in a time- and dose-dependent fashion. Pretreatment of these cells with phorbol dibutyrate (PDBu), a known activator of protein kinase C, attenuated ANF-stimulated cGMP accumulation without affecting basal cGMP concentrations. This effect was concentration-dependent and was observed as early as 2 min after treatment. 4 alpha-Phorbol 12, 13-didecanoate (alpha PDD), which does not activate protein kinase C, did not inhibit the cGMP accumulation. PDBu pretreatment did not affect the density and affinity of ANF receptors. These data suggest that PDBu, presumably via activation of protein kinase C, might stimulate phosphorylation of a key regulatory protein in the ANF/cGMP pathway.

Atrial natriuretic factor and sodium nitroprusside increase cyclic GMP in cultured rat lung fibroblasts by activating different forms of guanylate cyclase

We used cultured rat lung fibroblasts to evaluate the role of particulate and soluble guanylate cyclase in the atrial natriuretic factor (ANF)-induced stimulation of cyclic GMP. ANF receptors were identified by binding of 125I-ANF to confluent cells at 37 degrees C. Specific ANF binding was rapid and saturable with increasing concentrations of ANF. The equilibrium dissociation constant (KD) was 0.66 +/- 0.077 nM and the Bmax. was 216 +/- 33 fmol bound/10(6) cells, which corresponds to 130,000 +/- 20,000 sites/cell. The molecular characteristics of ANF binding sites were examined by affinity cross-linking of 125I-ANF to intact cells with disuccinimidyl suberate. ANF specifically labelled two sites with molecular sizes of 66 and 130 kDa, which we have identified in other cultured cells. ANF and sodium nitroprusside produced a time- and concentration-dependent increase in intracellular cyclic GMP. An increase in cyclic GMP by ANF was detected at 1 nM, and at 100 nM an approx. 100-fold increase in cyclic GMP was observed. Nitroprusside stimulated cyclic GMP at 10 nM and at 1 mM a 500-600-fold increase in cyclic GMP occurred. The simultaneous addition of 100 nM-ANF and 10 microM-nitroprusside to cells resulted in cyclic GMP levels that were additive. ANF increased the activity of particulate guanylate cyclase by about 10-fold, but had no effect on soluble guanylate cyclase. In contrast, nitroprusside did not alter the activity of particulate guanylate cyclase, but increased the activity of soluble guanylate cyclase by 17-fold. These results demonstrate that rat lung fibroblasts contain ANF receptors and suggest that the ANF-induced stimulation of cyclic GMP is mediated entirely by particulate guanylate cyclase.

Identification of atrial natriuretic factor receptor of neuroblastoma N4TG1 cells: binding characteristics and photoaffinity labeling

We have found specific receptors for atrial natriuretic factor (ANF) in cultured neuroblastoma cells (N4TG1) of peripheral ganglionic origin. Scatchard analysis of the displacement binding revealed noninteracting, single-class binding sites with a KD of 1 X 10(-10) M and a density (Bmax) of 110,000-150,000 sites/cell. The cell-bound 125I-ANF was displaced by unlabeled ANF in a dose-dependent manner. Hormones unrelated to ANF such as angiotensins, adrenocorticotropic hormone, or arginine vasopressin were ineffective in displacing the cell-bound radioactivity. Using azidobenzoyl-125I-ANF as a photoaffinity ligand, an ANF receptor with an apparent Mr of 138,000 was identified by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and autoradiography. The addition of unlabeled ANF (1 microM) to the incubation medium completely abolished the labeling of this protein band, but atriopeptin I (1 microM) or angiotensins I, II, and III (each 1 microM) were not effective in inhibiting the affinity labeling. The treatment of the neuroblastoma cells with ANF stimulated intracellular cyclic GMP levels in a dose-dependent manner with an EC50 of 5 nM. ANF (1 X 10(-7) M) stimulated cyclic GMP accumulation in less than 5 min by 30-fold as compared to the controls.

Evidence for lack of a role of cGMP in effect of alpha-hANP on aldosterone inhibition

To investigate the role of guanosine 3',5'-cyclic monophosphate (cGMP) in the inhibitory effect on aldosterone production of alpha-human atrial natriuretic polypeptide (alpha-hANP) we first compared the effects of the peptide with those of sodium nitroprusside (SNP) on the production of aldosterone and cGMP in dispersed adrenal capsular cells of rats, second, examined the effects of derivatives of cGMP on the production of aldosterone, and, third, studied the influence of potassium on the effects of alpha-hANP on the production of aldosterone and cGMP. alpha-hANP at concentrations of 3 X 10(-8) to 3 X 10(-7) M decreased the production of aldosterone in a dose-dependent manner, while markedly increasing the production of cGMP. On the other hand, although SNP at concentrations of 10(-5) to 10(-3) M increased the production of cGMP in a dose-dependent manner, it caused no significant changes in the production of aldosterone. Neither dibutyryl cGMP nor 8-bromo-cGMP affected the production of aldosterone in the adrenal cells. Although the aldosterone-inhibitory effect of alpha-hANP was lost in the potassium-free medium, the cGMP-stimulatory effect of the peptide was not altered by adding potassium to the incubation medium at concentrations of 0-5 meq/l. These results suggest that cGMP plays a minor role in the inhibitory effect of alpha-hANP on the production of aldosterone and that the production of cGMP stimulated by the peptide is not directly involved in the decrease in aldosterone production in adrenal capsular cells of rats.

Atrial natriuretic peptide effects on cGMP and cAMP contents in microdissected glomeruli and segments of the rat and rabbit nephrons

A microradioimmunoassay has been developed in order to measure the changes in cGMP cell content induced in vitro by atrial natriuretic peptides (ANP) in either glomeruli or defined portions of tubules microdissected from collagenase treated rat and rabbit kidneys. When tested at 0.1 microM or 1 microM, all ANP analogues used produced in rat glomeruli a 20-25 fold increase in cGMP accumulation compared to basal values. Threshold responses were obtained with about 1 nM ANP and apparent Ka values ranged between 5 and 50 nM. Atriopeptin III led to similar results in glomeruli isolated from rabbit. Under the same experimental conditions, no cGMP could be detected in any ANP-treated nephron segment from the rat kidney (namely, from the proximal convoluted tubule up to the outer medullary collecting tubule) nor in cortical collecting tubules isolated from the rabbit kidney. Moreover, ANP did not alter the forskolin-induced increase in cAMP content in glomeruli or collecting tubules, nor the AVP-induced increase in cAMP content in collecting tubules. Our data confirm the marked effect of ANP on cGMP generation by isolated glomeruli from rat and rabbit; however, they are not compatible with a direct action of ANP stimulating cGMP generation in tubules or inhibiting vasopressin-induced cAMP generation in collecting tubules.

Hemodynamic changes and renal plasma flow in early heart failure: implications for renin, aldosterone, norepinephrine, atrial natriuretic peptide and prostacyclin

Vasoconstrictory and vasodilatory hormone systems may be important in the regulation of peripheral vascular resistance and renal hemodynamics in the early phase of heart failure. The activity of the renin-angiotensin-aldosterone system (RAAS), the sympathetic nervous activity, and, as possible counterregulating systems, the activity of prostacyclin and atrial natriuretic peptide (ANP) were studied in 6 conscious dogs during the first 4 days of congestive heart failure in relation to hemodynamic changes and renal plasma flow. Congestive heart failure was induced by rapid right ventricular pacing, which caused a considerable decrease of cardiac output (-38%; p less than 0.05), oxygen saturation of the mixed venous blood (-13%; p less than 0.05), and mean arterial pressure (-24 mm Hg; p less than 0.05) on the 4th day. Mean pulmonary arterial pressure and mean pulmonary capillary wedge pressure increased (+4 mm Hg; p less than 0.05 and +7 mm Hg, respectively; p less than 0.05). Renal plasma flow was slightly reduced (N.S.), renal vascular resistance did not change. Peripheral vascular resistance showed a significant increase only on the 1st day. Sympathetic nervous activity was stimulated (from 175 +/- 31 pg/ml to 391 +/- 100 pg/ml; p less than 0.05), while plasma renin concentration was significantly suppressed on the 4th day (from 3.3 +/- 0.4 ngAI/ml/h to 1.9 +/- 0.5 ngAI/ml/h; p less than 0.05), and plasma aldosterone levels were decreased (from 108 +/- 12 pg/ml to 76 +/- 12 pg/ml; p less than 0.05). ANP increased 3-fold (p less than 0.05) and 6-keto-prostaglandin F1 alpha increased in 4 out of 6 dogs.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of atrial natriuretic factor on cyclic guanosine monophosphate and cyclic adenosine monophosphate accumulation in microdissected nephron segments from rats

Atrial natriuretic factor (ANF) (1 microM) markedly increased cyclic guanosine monophosphate (cGMP) content in microdissected glomeruli (35-fold) and in microdissected inner medullary collecting ducts (IMCD) (20-fold). ANF caused little or no increase in cGMP content in other nephron segments. The threshold concentration for increased cGMP accumulation by ANF was 0.1-1 nM in IMCD, which is in the range reported for rat plasma. Sodium nitroprusside (1 mM), which selectively stimulates soluble guanylate cyclase, increased cGMP content in glomeruli but not in IMCD. ANF did not alter cAMP accumulation in the absence or presence of vasopressin (AVP) or parathyroid hormone (PTH) in outer and inner medullary tubule suspensions, or in microdissected proximal convoluted tubules (PCT), medullary thick ascending limbs (MAL) or IMCD. These data are compatible with the hypothesis that cGMP is a second messenger for a physiologic action of ANF in the inner medullary collecting duct. ANF apparently activates membrane-bound guanylate cyclase in this segment.

Atrial natriuretic factors stimulate accumulation and efflux of cyclic GMP in C6-2B rat glioma and PC12 rat pheochromocytoma cell cultures

Atrial natriuretic factors (ANFs) were tested for their effects on cyclic GMP production in two neurally derived cell lines, the C6-2B rat glioma cells and the PC12 rat pheochromocytoma cells. These cell lines were selected because both are known to possess high amounts of the particulate form of guanylate cyclase, a proposed target of ANF in peripheral organs. Previous studies from our laboratory have shown that ANF selectively activates particulate, but not soluble, guanylate cyclase in homogenates of a variety of rat tissues and that one class of ANF receptor appears to be the same glycoprotein as particulate guanylate cyclase. In the present study we found that four analogs of ANF stimulate cyclic GMP accumulation in both C6-2B and PC12 cells with the rank order of potency being atriopeptin III = atriopeptin II greater than human atrial natriuretic polypeptide greater than atriopeptin I. Atriopeptin II (100 nM) for 20 min elevated cyclic GMP content in C6-2B cells fourfold and in PC12 cells 12-fold. Atriopeptin II (100 nM) for 20 min also stimulated the efflux of cyclic GMP from both C6-2B cells (47-fold) and PC12 cells (12-fold). Accumulation of cyclic GMP in both cells and media was enhanced by preincubation with the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (250 microM). After 20 min of exposure to atriopeptin II, cyclic GMP amounts in the media were equal to or greater than the amounts in the cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Studies on the effect of different inhibitors of arachidonic acid metabolism on glyceryltrinitrate-induced relaxation and cGMP elevation in bovine vascular tissue

This study was performed in order to investigate the possible involvement of arachidonic acid metabolites in the mediation of glyceryltrinitrate (GTN)-induced relaxation in isolated bovine mesenteric artery (BMA) and vein (BMV) and in bovine coronary artery (BCA). Concentration-effect curves for GTN were established on the different types of vessels, precontracted with 2.5 microM phenylephrine (BMA) or K+-depolarization (BMV and BCA), in the presence or absence of different inhibitors of the arachidonic acid metabolism. The used inhibitors of arachidonic acid metabolism were: 10 microM quinacrine (a phospholipase A2 inhibitor), 100 microM acetylsalicylic acid, 20 microM indomethacin (cyclooxygenase inhibitors), 3 mM tranylcypromine (inhibitor of PGI2 synthesis), 50 microM nordihydroguairetic acid and 40 microM BW 755C (lipoxygenase inhibitors). In addition, SKF 525A (10 microM) was tested on BMA; this agent is considered to block the cytochrome P450-dependent monooxygenase pathway. The effect on the endogenous levels of cyclic nucleotides after treatment with some of the inhibitors were also measured. Quinacrine and acetylsalicylic acid had no statistical significant effect (P greater than 0.05) on the pD2-value for GTN-induced relaxation in BMA, BMV and BCA. The results obtained with indomethacin were very variable. This drug was almost completely without effect on the GTN induced relaxation in BCA. In BMA a significant potentiation of the relaxant response was obtained (P = 0.002), while in BMV a significant inhibition was seen (P = 0.049).(ABSTRACT TRUNCATED AT 250 WORDS)

Nitrates. Mode of action at a cellular level

The nitrates used therapeutically in angina pectoris and congestive heart failure are, from a chemical point of view, organic nitroesters. Their principal pharmacological effect is vascular smooth muscle relaxation, leading to vasodilation, which explains their therapeutic effects. Several mechanisms have been proposed for their mode of action at the cellular level, in order to explain vascular smooth muscle relaxation. Today, there is strong evidence that organic nitroesters stimulate the enzyme guanylate cyclase in the smooth muscle cell. This enzyme produces a cyclic nucleotide, cyclic guanosine-3',5'-monophosphate (cGMP), which in turn eventually lowers the free calcium concentration in the cytosol to induce relaxation. The exact mechanism by which the organic nitroesters stimulate guanylate cyclase is still obscure. Preliminary results from our laboratory indicate that there may be more than one mechanism responsible for the activation of the enzyme. Knowledge of the mode of action at the cellular level is probably important in order to understand the mechanism(s) behind the development of tolerance towards the organic nitroesters.

Atrial natriuretic factor: physiologic actions and implications in congestive heart failure

Atrial natriuretic factor (ANF) represents a newly recognized hormone of cardiac origin. This peptide is synthesized by the myocardial cells of both atria and released by atrial stretch. The hormone promotes sodium and water excretion by the kidney, inhibits the renin-angiotensin-aldosterone system, and reduces systemic arterial pressure. Specific receptors for ANF are present in the kidney, adrenal glands, vascular smooth muscle, platelets and central nervous system. Congestive heart failure is characterized by increased circulating levels of ANF; however, there appears to be an attenuation in the renal response to the hormone. Recent investigations have reported the effect of systemic administration of synthetic ANF to normal individuals and those with congestive heart failure. The hormone may promote a significant natriuresis and diuresis in addition to reducing arterial pressure and inhibiting renin and aldosterone secretion. Substantial questions remain as to the full physiologic significance and therapeutic potential of this hormone.

The vasodilator potency of atrial natriuretic peptide in man

The vasodilating potency of alpha-human atrial natriuretic peptide (alpha-hANP) was investigated in the forearms of 16 normotensive subjects, 22 to 48 (mean 28) years old, with the use of venous occlusion plethysmography. alpha-hANP, 0.005 to 1.5 micrograms/min/100 ml forearm volume (FAV), infused in nine dose steps into the brachial artery increased forearm blood flow (FAF; ml/min/100 ml FAV) from 2.8 +/- 0.4 (SEM) to a maximum of 9.6 +/- 1.1. Forearm vascular resistance (mean arterial pressure/FAF) decreased by 72%. The alpha-hANP dose that produced a 50% vasodilator response was 0.093 +/- 0.016 microgram/min/100 ml FAV (n = 11) and it resulted in a venous plasma concentration of ANP (pANP) of 115 +/- 7 pmol/liter (normal 2 to 80; radioreceptor assay). Intraindividually, the maximum dose of alpha-hANP induced an increase in FAF that was 60% of the maximum response to sodium nitroprusside (14.1 +/- 1.8). Combined infusions (n = 9) of maximum forearm vasodilator doses of alpha-hANP and nitroprusside increased FAF to 22.7 +/- 3.4; this additive vasodilator effect of alpha-hANP and nitroprusside is consistent with their different actions on the guanylate cyclase system. In man, the direct vasorelaxant effect of alpha-hANP occurs at concentrations within the upper normal range of pANP, suggesting a physiologic vasodilator role for alpha-hANP.

The endothelium and cyclic guanosine monophosphate in hyperthyroid-induced hypertension

We investigated the role of endothelium derived relaxing factor (EDRF) and cyclic guanosine monophosphate (cGMP) in the altered vascular reactivity of hyperthyroidism (HT). Rats were given daily injections of triiodothyronine (T3), 50 micrograms/100 g body weight for two weeks, and they had significantly higher serum levels of T3 compared to untreated, control rats (493 +/- 82 vs. 58 +/- 7 ng/dl, p less than 0.05) and significant elevations in their systolic blood pressure (188 +/- 6 vs 126 +/- 3 mm Hg, p less than 0.05). Vascular reactivity was studied in isolated muscle baths; cGMP was measured by RIA. There were no differences in contractile responses to phenylephrine (PE) in isolated aortae from the HT and control rats, but aortae from the HT rats contracted with PE relaxed less to acetylcholine (Ach); the calcium ionophore, A23187; and sodium nitroprusside (SNP). Sensitivity to atrial natriuretic factor (ANF) and 8-Br cGMP was unaltered. Blood vessels from HT rats generated significantly less cGMP in response to Ach, SNP, and ANF. Treatment of the hypertension in the HT rats which hydralazine or propranolol restored the vascular relaxation response to Ach but not SNP; cGMP responses remained blunted. These data suggest that endothelium dependent vasodilators may induce relaxation independent of elevations of cGMP in aortae from HT rats.

Angiotensin II-induced vasopressin release is attenuated by central atrial natriuretic factor

Administration of angiotensin II (Ang II) into the cerebral ventricles (icv) of rats elicits vasopressin release and an increase in blood pressure. The effect of atrial natriuretic factor (ANF) on these actions of ANG II was studied in conscious spontaneously hypertensive rats. The magnitude and time course of the blood pressure increase following ANG II (50 and 100 ng) were not altered by ANF, icv. However, vasopressin levels which were stimulated from 10.8 +/- 1.5 to 62.1 +/- 6.4 pq/ml by ANG II (100 ng) were significantly suppressed by combined administration of ANG II (100 ng) and ANF (3 ug/kg) (33.0 +/- 4.3 pg/ml). The injection of ANF alone into the cerebral ventricles had no effect on resting blood pressure or vasopressin levels. Peripheral administration of ANF was unable to attenuate the ANG II-induced vasopressin release. These data suggest that there exists a central interaction of ANF and ANG II within the brain which cannot be mimicked by peripheral administration of ANF.

The involvement of atriopeptins in blood pressure regulation

The atriopeptins are newly discovered cardiac-derived peptides whose observed actions suggest a role in volume homeostasis and blood pressure regulation. Studies in animal models are underway to pinpoint pathogenetic mechanisms involved in the evolution of hypertension, some of which may well be shared by humans with "essential" hypertension. Preliminary observations indicate that circulating atriopeptin levels are altered in human disease. It is anticipated that exogenously administered atriopeptin may be a helpful pharmacological tool in the management of patients with volume overload and hypertension.

Atrial natriuretic factor receptor on cultured Leydig tumor cells: ligand binding and photoaffinity labeling

Atrial natriuretic factor (ANF) is a peptide hormone discovered recently from the heart atrium that possesses potent natriuretic and vasorelaxant activities. Recently we found that ANF markedly stimulates intracellular cGMP and almost completely inhibits cAMP accumulation in testicular interstitial tumor cells [Pandey, K. N., Kovacs, W. J., & Inagami, T. (1985) Biochem. Biophys. Res. Commun. 133, 800-806]. These actions of ANF suggest the presence of ANF receptors in testicular interstitial cells. In this study, cultured murine Leydig tumor cells have been shown to contain specific binding sites for ANF. Saturation binding studies indicated a single class of binding sites with a Kd of 5 X 10(-9) M at a density of 2 X 10(6) sites/cell. The binding of mono[125I]iodo-ANF (125I-ANF) was competed by unlabeled ANF in a dose-dependent manner. Hormones unrelated to ANF such as angiotensin I, bovine luteinizing hormone, and human chorionic gonadotropin were not able to compete against 125I-ANF. The binding of 125I-ANF was rapid, reaching maximum levels in 15 min at 4 degrees C. At 37 degrees C, the cell-bound 125I label was quickly decreased. Pretreatment of cells with NH4Cl, chloroquine, or NaN3 resulted in significant increases in maximum levels of the cell-bound 125I radioactivity. A photoaffinity reagent for ANF receptor was prepared by reacting ANF with succinimido 4-azidobenzoate, and resultant 4-azidobenzoyl- (AZB-) ANF was purified by high-performance liquid chromatography (HPLC). AZB-ANF was radioiodinated by use of chloramine T and purified again by HPLC.(ABSTRACT TRUNCATED AT 250 WORDS)

Vasopressin-mediated inhibition of atrial natriuretic factor-stimulated cGMP accumulation in an established smooth muscle cell line

Rat thoracic aortic smooth muscle cells (line A10, ATCC CRL 1476) display a high density of atrial natriuretic factor (ANF) receptors. ANF stimulated the accumulation of cGMP in these cells in a time- and dose-dependent fashion. These cells are known to display a high density of vasopressin receptors of the vascular V1 subtype. These vasopressin receptors mediate inhibition of isoproterenol-stimulated cAMP accumulation and stimulation of inositol phosphate accumulation and calcium fluxes. Addition of [8-arginine]vasopressin ([Arg8]VP) to these cells inhibited ANF-stimulated cGMP accumulation. Inhibition of cGMP accumulation was dependent on the concentration of [Arg8]VP, with half-maximal and maximal effects occurring at 0.4 and 10 nM, respectively. [Arg8]VP did not have significant effects on basal cGMP levels. The inhibition by [Arg8]VP appears to be mediated by V1 receptors, since the V2 renal receptor agonist [1-desaminocysteine,8-D-arginine]vasopressin was ineffective. Also, the selective V1 antagonist [1-(beta-mercapto-beta,beta-cyclopentamethylenepropionic acid),2-(O-methyltyrosine),8-arginine]vasopressin and the mixed V1/V2 antagonist [1-(beta-mercapto-beta,beta-cyclopentamethylenepropionic acid),2-(O-ethyl-D-tyrosine),4-valine,8-arginine]vasopressin blocked the [Arg8]VP-mediated effect, whereas the selective V2 antagonist [1-(beta-mercapto-beta,beta-cyclopentamethylenepropionic acid), 2-D-isoleucine,4-valine,8-arginine]vasopressin was minimally effective. These data show that in rat aortic smooth muscle cells, V1 receptors are negatively coupled to guanylate cyclase. These data also suggest that the vasoconstrictor activity of [Arg8]VP might involve inhibition of ANF-receptor-mediated vascular relaxation through inhibition of cGMP accumulation in addition to its effects on isoproterenol-mediated cAMP accumulation and inositol phosphate accumulation and calcium fluxes.

Atriopeptin II-induced relaxation of rabbit aorta is potentiated by M&B 22,948 but not blocked by haemoglobin

We examined the effects of haemoglobin (which inhibits the vascular responses to stimulation of soluble guanylate cyclase) and of M&B 22,948 (which selectively inhibits cyclic GMP phosphodiesterase) on the relaxation induced in rabbit aorta by the atrial natriuretic peptide, atriopeptin II (which stimulates particulate guanylate cyclase). Pretreatment with M&B 22,948 (100 microM) produced a 2.3 fold potentiation of atriopeptin II-induced relaxation of endothelium-denuded rings of rabbit aorta. Pretreatment with haemoglobin (10 microM) had no effect on the relaxation or the 10.9 fold increase in cyclic GMP content induced by atriopeptin II in endothelium-denuded rings of rabbit aorta. The potentiation by M&B 22,948 suggests a causal role for cyclic GMP in mediating atriopeptin II-induced vasodilatation of rabbit aorta. The inability of haemoglobin to block the atriopeptin II-induced rise in cyclic GMP suggests that it does not block stimulation of particulate guanylate cyclase. Thus, it is unlikely that a ferrous haem-containing receptor site is involved in the activation of the particulate form of guanylate cyclase as it is with soluble guanylate cyclase.

cGMP-dependent protein kinase enhances Ca2+ current and potentiates the serotonin-induced Ca2+ current increase in snail neurones

Protein phosphorylation catalysed by cyclic AMP-dependent, Ca2+/calmodulin-dependent and Ca2+/diacylglycerol-dependent protein kinases is important both in the modulation of synaptic transmission and in the regulation of neuronal membrane permeability (for reviews see refs 5-7). However, there has previously been no evidence for the involvement of cyclic GMP-dependent protein kinase (cGMP-PK) in the regulation of neuronal function. Serotonin induces an increase of Ca2+ current in a group of identified ventral neurones of the snail Helix aspersa. This effect is probably mediated by cGMP because it is mimicked by the intracellular injection of cGMP or the application of zaprinast, an inhibitor of cGMP-dependent phosphodiesterase. We have now found that the effect of either serotonin or zaprinast on the Ca2+ current is potentiated by the intracellular injection of cGMP-PK. Moreover, the intracellular injection of activated cGMP-PK (cGMP-PK + 1 microM cGMP) greatly enhances the Ca2+ current of the identified ventral neurones seen in the absence of serotonin. These results indicate that cGMP-PK has a physiological role in the control of the membrane permeability of these neurones.

Steroidogenic effect of atrial natriuretic factor in isolated mouse Leydig cells is mediated by cyclic GMP

The effects of different atrial natriuretic peptides on cyclic GMP formation and steroidogenesis have been studied in Percoll-purified mouse Leydig cells. Rat atrial peptides rANP (rat atrial natriuretic peptide), rAP-I (rat atriopeptin I) and rAP-II (rat atriopeptin II), in the presence of a phosphodiesterase inhibitor, stimulated cyclic GMP formation in a concentration-dependent manner. In the presence of saturating concentrations of the peptides, a 400-600 fold stimulation of cyclic GMP accumulation was observed. Among the peptides, rAP-II appeared to be the most potent. ED50 values (concentration causing half-maximal effect) for rAP-II, rANP and rAP-I were 1 X 10(-9) M, 2 X 10(-9) M and 2 X 10(-8) M respectively. A parallel stimulation of cyclic GMP formation and testosterone production by the cells was observed after incubation of the cells with various concentrations of rAP-II. In the presence of a saturating concentration of rAP-II (2 X 10(-8) M), maximum stimulation of intracellular cyclic GMP content was obtained within 5 min of incubation. Testosterone production by mouse Leydig cells could be stimulated by 8-bromo cyclic GMP in a concentration-related manner. At a 10 mM concentration of the cyclic nucleotide, steroidogenesis was stimulated to a similar extent as that obtained with a saturating concentration of human chorionic gonadotrophin (5 ng/ml). On the basis of these results we conclude that cyclic GMP acts as a second messenger in atrial-peptide-stimulated steroidogenesis in mouse Leydig cells. The steroidogenic effect of atrial peptides appears to be species-specific, since none of these peptides stimulated testosterone production by purified Leydig cells of rats, though in these cells a 40-60-fold stimulation of cyclic GMP formation in response to each of the three peptides was observed. However, 8-bromo cyclic GMP could stimulate testosterone production in rat Leydig cells. Therefore we conclude that the lack of steroidogenic response in rat Leydig cells to atrial-natriuretic-factor-stimulation results from an insufficient formation of cyclic GMP in these cells. This species difference would appear to result from a lower guanylate cyclase activity in rat Leydig cells.

In vivo evidence that cGMP is the second messenger for atrial natriuretic factor

cGMP generation has been associated with many of the vascular and endocrine actions of atrial natriuretic factor (ANF) in vitro. To examine the role of cGMP as a second messenger for the renal hemodynamic action of ANF in vivo, we measured glomerular filtration rate (GFR) and cGMP concentration in systemic artery, renal vein, and urine as well as in Bowman's space and end-proximal tubule (by free-flow micropuncture) after administration of ANF. ANF increased GFR by 45% and simultaneously induced a greater than 5-fold increase of cGMP concentration in glomerular ultrafiltrate (Bowman's space) when compared to controls. There was no significant increase in either systemic artery or renal vein cGMP concentration. Thus, the source of increased Bowman's space cGMP is not from the blood via filtration but rather from either glomerular mesangial or epithelial cells, which are not in direct contact with the circulation. Although a small amount of tubular handling of cGMP occurred along the length of the nephron, the augmented cGMP production from the glomerulus accounted for most of the 10- to 12-fold higher urinary cGMP excretion observed after ANF administration. Intrarenal arterial infusion of dibutyryl cGMP, but not dibutyryl cAMP, increased GFR in a dose-dependent fashion (from 10 to 1000 microM) by a mechanism similar to that of ANF--an increase in glomerular hydraulic pressure. Thus, ANF markedly stimulated glomerular production of cGMP, which coincided with a marked increase in GFR. Since dibutyryl cGMP itself was capable of increasing GFR, cGMP is the likely second messenger for ANF in vivo.

Effect of atrial natriuretic factor on angiotensin II-induced hypertension in rats

To assess the physiological role of atrial natriuretic factors in blood pressure regulation, we studied the effect of chronic infusion of a synthetic atrial natriuretic factor of 25 amino acid residues (Arg 102-Tyr 126) in rats with angiotensin II-induced hypertension. Rats were studied while on a normal sodium diet or during sodium loading with 1% NaCl solution used as drinking water. Systolic blood pressure decreased slightly during combined infusion of synthetic atrial natriuretic factor, 150 micrograms/kg/day, and angiotensin II, 900 micrograms/kg/day. This effect was sustained for 3 days in rats receiving a regular sodium intake (p less than 0.01) and during sodium loading (p less than 0.01). Administration of synthetic atrial natriuretic factor to rats made hypertensive by a 3-day infusion of angiotensin II reduced blood pressure slightly, but not to control levels, and this effect was sustained for the remaining 3 days of the experiment in both dietary groups. These results indicate that a nonhypotensive dose of synthetic atrial natriuretic factor can modulate the vasopressor effect of angiotensin II. Thus, the attenuating effect may be involved in blood pressure regulation independently of sodium metabolism, although its actual physiological importance remains undetermined.

Atrial natriuretic factor and cyclic guanosine 3',5'-monophosphate in vascular smooth muscle

To elucidate the molecular mechanism of the vascular action of atrial natriuretic factor (ANF), we investigated the effects of synthetic ANF and sodium nitroprusside on the levels of intracellular cyclic nucleotides and prostacyclin (measured as its stable metabolite 6-keto-prostaglandin F1 alpha) in cultured vascular smooth muscle cells from rat mesenteric artery and, in some experiments, from rat renal artery. Both ANF and sodium nitroprusside increased intracellular cyclic guanosine 3',5'-monophosphate (cGMP) levels in a dose-dependent manner but did not affect cyclic adenosine 3',5'-monophosphate levels or 6-keto-prostaglandin F1 alpha synthesis. The stimulatory effect of ANF and sodium nitroprusside on cGMP levels were additive. Neither the deprivation of extracellular Ca2+ nor calcium entry blockers affected ANF-stimulated cGMP levels. Preincubation of ANF or sodium nitroprusside with kallikrein attenuated only the effect of ANF on cGMP levels. The effect of kallikrein was abolished by serine protease inhibitors. In contrast, the oxidant methylene blue inhibited the effect of sodium nitroprusside on cGMP levels, but not that of ANF. The stimulatory effect of ANF on cGMP levels was greater in cells from renal artery than in those from mesenteric artery. These results in cultured vascular smooth muscle cells further support the hypothesis that cGMP mediates the vasorelaxant action of ANF.

Stimulation of cyclic GMP synthesis in human cultured glomerular cells by atrial natriuretic peptide

Recently a stimulatory effect of atrial natriuretic peptide (ANP) on the particulate guanylate cyclase system has been reported in the glomeruli from different species. Using cultures of homogeneous human glomerular cell lines, we found that rat and human ANP stimulated markedly cGMP formation in epithelial cells with a threshold dose of 1 nM. A 20-fold increase was obtained at 5 microM. Stimulation was also present but less substantial (2-fold at 5 microM) in mesangial cells. cGMP was formed rapidly and released in the medium. ANP and sodium nitroprusside, an activator of soluble guanylate cyclase, had additive effects on cGMP formation. ANP did not inhibit cAMP formation in both cell lines. These results demonstrate that, at least in the human species, epithelial cells represent the main target of ANP in the glomerulus. Synthesis of cGMP in the glomerular epithelial cells in response to ANP also suggests that the excess of urinary cGMP produced by the kidney which is observed after ANP administration is of glomerular rather than of tubular origin.

Effect of 8-bromo-cyclic guanosine monophosphate (cGMP) on coronary artery constriction in isolated rabbit hearts

The vasodilator 8-bromo-guanosine 3':5'-monophosphate (8-bromo-cGMP) effectively counteracts vasopressin-induced coronary artery constriction in a supported perfused working rabbit heart. In this preparation, the coronary arteries remain in contact with the beating heart. The obtuse marginal artery and portions of the left anterior descending coronary artery were deprived of endothelium. Perfusion was carried out with Krebs-Henseleit solution, oxygenated with a disposable infant oxygenator. The internal diameter of large coronary arteries was determined by color arteriography (injection of patent blue dye and gated photography). The effect of vasopressin with and without the addition of 8-bromo-cGMP on cardiac performance (cardiac output, left ventricular systolic pressure, left ventricular end-diastolic pressure, maximal rate of rise in left ventricular pressure [dP/dtmax], mean aortic pressure) and large coronary vessel and total coronary vascular resistance was determined in nine experiments. In addition, changes in coronary sinus partial pressure of carbon dioxide (PCO2) and pH were observed. Vasopressin alone caused a significant decline in coronary flow, myocardial oxygen consumption and coronary sinus pH. Cardiac performance declined, probably because of myocardial ischemia. Large coronary vessel and total coronary vascular resistance rose. The vasodilator 8-bromo-cGMP strongly inhibited the vasoconstrictor action of vasopressin, counteracted the increase in large and total coronary vascular resistance, prevented the fall in myocardial oxygen consumption and eliminated changes in pH or PCO2 of coronary sinus effluent. Because of the elimination of myocardial ischemia by 8-bromo-cGMP, cardiac performance was normalized.(ABSTRACT TRUNCATED AT 250 WORDS)

Atrial natriuretic factor stimulates Na/K/Cl cotransport in vascular smooth muscle cells

Atrial natriuretic factor (ANF) is a collective term used to describe a group of peptides isolated from mammalian atria which have vasorelaxant activity as well as diuretic and natriuretic activity. Recently, ANF peptides have been shown to bind to specific receptors on vascular smooth muscle cells (VSMC) and to cause an elevation in cGMP levels. We have previously demonstrated that VSMC possess a prominent, cyclic-nucleotide-sensitive Na/K/Cl cotransport system. In the present study, the effects of the ANF peptide rat atriopeptin III (rAP III) were measured on Na/K/Cl cotransport of VSMC by using primary cultures derived from rat thoracic aorta. It was found that rAP III caused a marked elevation of Na/K/Cl cotransport. Maximal stimulation occurred at 100 nM, and the dose of rAP III required for half-maximal potassium influx (K1/2) was 9 nM. We also investigated the effect of rAP III on cGMP levels in VSMC. It was found that rAP III increased cGMP in a dose-dependent manner, with a K1/2 value of 10 nM. Finally, we measured the effect of the permeable cGMP analog 8-bromo-cGMP on Na/K/Cl cotransport. It was found that 8-bromo-cGMP stimulated cotransport to the same extent as did a saturating dose of rAP III (K1/2 = 0.2 microM). Saturating doses of rAP III and 8-Br-cGMP in combination did not stimulate cotransport in an additive manner, suggesting that rAP III probably does not elevate cGMP via inhibition of phosphodiesterase. These findings suggest that activation of Na/K/Cl cotransport via elevations in cGMP may be associated with ANF-mediated vasorelaxation and/or ANF-mediated diuresis and natriuresis.

Atrial natriuretic peptide inhibits renin release from juxtaglomerular cells by a cGMP-mediated process

We have examined the effect of a synthetic analogue of human alpha-atrial natriuretic peptide (ANP), APII, on renin release in cultured renal juxtaglomerular cells (JGA cells). Using cell cultures containing 80-90% renal juxtaglomerular cells, we found that ANP (10(-13)-10(-9) M) strongly inhibited renin release from the cells in a dose-dependent fashion (ki, 10 pM) to about 10% of control. Inhibition of renin release by ANP was paralleled by an increase in cellular cGMP levels; while in the presence of the cGMP-phosphodiesterase inhibitor M&B 22948 (1 mM), concentrations of ANP lower by a factor of 100 were required to obtain the same effects on renin release and cGMP levels. The guanylate cyclase inhibitor methylene blue (10 microM), on the other hand, shifted the dose-response curves for renin release and cGMP levels to 100-fold higher concentrations of ANP. Neither the influx of 45Ca into the cells nor the intracellular quin-2 signal, which is a measure for changes of intracellular Ca concentration, was in any way altered by ANP. Our results suggest that ANP inhibits renin release from juxtaglomerular cells by a cGMP-dependent process that does not involve changes in intracellular calcium.

Atrial pressure and secretion of atrial natriuretic factor into the human central circulation

Atrial natriuretic factor, a peptide found in mammalian cardiac atria, has natriuretic and vasodilatory properties that may be important in the regulation of intravascular volume. To study factors related to its release in human subjects, intracardiac pressures and plasma atrial natriuretic factor concentrations in the central circulation were measured in 34 patients with a variety of cardiovascular disorders. Plasma atrial natriuretic factor concentration increased from the inferior vena cava to the right atrium (76 +/- 24 to 162 +/- 37 pg/ml, p less than 0.001) and from the vena cava to the aorta (76 +/- 24 to 177 +/- 46 pg/ml, p less than 0.001). Mean right atrial pressure was positively correlated with atrial natriuretic factor concentration in the pulmonary artery (r = 0.58, p less than 0.001), and mean pulmonary capillary wedge pressure was positively correlated with concentration in the aorta (r = 0.64, p less than 0.001). In six patients whose atrial natriuretic factor concentrations were measured at two different levels of atrial pressure, increased atrial pressure was accompanied by increased atrial natriuretic factor concentration in the pulmonary artery (p less than 0.01) and aorta (p less than 0.01). Atrial natriuretic factor levels measured in fresh myocardium from a patient undergoing cardiac transplantation showed tissue concentrations in the atria 500-fold higher than tissue concentrations in the ventricles. These data document that atrial natriuretic factor is found in human atrial myocardium and suggest that it may be released in response to increased atrial pressure. Such a secretory release mechanism is consistent with the hypothesis that atrial natriuretic factor plays a role in the regulation of circulatory volume.

Cyclic GMP-dependent protein kinase relaxes skinned fibers from guinea pig taenia coli but not from chicken gizzard

The effect of cGMP and cGMP-dependent protein kinase (cG-PK) on contraction and relaxation was studied in skinned smooth muscle fibers from guinea pig taenia coli and chicken gizzard. At a fixed [Ca2+] relaxation was significantly enhanced by activated cG-PK in fibers from guinea pig taenia coli, but not in those from chicken gizzard. The Ca2+-requirement for half maximal tension maintenance was shifted to the right. Relaxation was associated with a decline in phosphorylated myosin light chain-2 from 34% to 25%. Similarly to relaxation activated cG-PK inhibited tension development only in fibers from taenia coli. These results suggest that mammalian and chicken smooth muscle fibers respond differently to cG-PK.

Testosterone production by mouse Leydig cells is stimulated in vitro by atrial natriuretic factor

The synthetic atrial peptides, rat atrial natriuretic peptide, atriopeptin I and atriopeptin II, stimulated testosterone production by mouse Leydig cells in a time- and concentration-dependent manner. The maximum stimulation of the steroidogenesis in response to the peptides was 6-10-fold over the basal level, as compared with 20-24-fold stimulation obtained with saturating concentrations of hCG. The stimulation of steroidogenesis by the most potent peptide, atriopeptin II, was markedly enhanced in the presence of the phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine, suggesting an involvement of cyclic nucleotides. However, neither basal nor hCG-stimulated levels of cAMP were altered by the peptide, though testosterone production in response to submaximal concentrations of hCG was increased in the presence of atriopeptin II. The nature of the second messenger involved and the mechanism of action of the atrial peptides may be elucidated by further research in progress.