Partial vasopressin 1a receptor agonism reduces portal hypertension and hyperaldosteronism and induces a powerful diuretic and natriuretic effect in rats with cirrhosis and ascites

The vasopressin system has emerged as a therapeutic focus for lowering portal hypertension and reducing splanchnic vasodilation in patients with refractory ascites. Clinically available vasopressin agonists are limited by preferential selectivity for V1 receptors that also have steep concentration-response curves with potential risks of excess vasoconstriction and/or complete antidiuretic effects. OCE-205 is a novel, selective, partial V1a receptor agonist with mixed agonist/antagonist activity and no V2 receptor activation at therapeutic doses. We carried out two studies assessing the in vivo effects of OCE-205 in different rat models of cirrhosis and ascites. In a carbon tetrachloride rat cirrhosis model, OCE-205 administration produced a marked reduction in portal hypertension and hyperaldosteronism, along with robust diuretic and natriuretic effects. These effects were accompanied by marked decreases in ascites volume, with three of five animals experiencing total mobilization of ascites. There was no evidence of fluid overload or sodium or water retention, confirming OCE-205's lack of V2 receptor activity. In a second, corroborative study using a bile duct ligation rat model of ascites, OCE-205 produced significant decreases in ascites volume and body weight and a significant increase in urine volume versus vehicle. Urine sodium excretion increased significantly after the first administration of OCE-205 relative to vehicle; however, repeat administration over 5 days did not lead to hyponatremia. Thus, in separate in vivo models, the mixed agonist/antagonist OCE-205 demonstrated relevant and expected endpoint findings consistent with its known mechanism of action and in vitro pharmacology without apparent unwanted effects or nonspecific toxicities.

Atrial pronatriodilatin: a precursor for natriuretic factor and cardiodilatin

Numerous peptides isolated from rat heart atria, including two containing 33 and 73 amino acids, were isolated and shown to exhibit natriuretic activities. Here, we describe the purification and partial amino acid sequence of a 106-residue peptide containing the previously sequenced 33- and 73-amino-acid ANF peptides. The determined sequence is a novel one and is not significantly homologous to any known protein or segment thereof. In fact, this sequence shows significant homology only to another novel partial sequence obtained from sequence analysis of a porcine peptide, called cardiodilatin, also found in heart atria. This relationship is taken as evidence that ANF and cardiodilatin are part of the same precursor molecule which would contain at the very least 126 amino acids.

The kidney in chronic liver disease: circulatory abnormalities, renal sodium handling and role of natriuretic peptides

Patients with chronic liver disease (Läennec's cirrhosis) present sodium chloride retention, leading to fluid accumulation within the extracellular space (edema) and specially in the abdomen (ascites). This article reviews the pathogenesis of the hemodynamic abnormalities observed in these patients, particularly the hypothesis of "primary arterial vasodilation", with an increased nitric oxide production by endothelial cells playing a major role in the pathogenesis of vasodilation. Since excessive renal sodium reabsorption precedes ascites formation, two hypotheses are analyzed with respect to the handling of renal sodium in chronic liver disease: the underfilling and overflow theories. Furthermore, the role of natriuretic peptides is reviewed, the increment in atrial natriuretic peptide observed in well compensated cirrhotic patients being considered as a compensatory response to volume expansion, although with renal resistance to this peptide in early stages of the disease. This review ends with an integrated explanation of the circulatory disturbances, renal sodium retention and renal resistance to atrial natriuretic peptide resulting in the sodium and water abnormalities observed in chronic liver disease.

Rapid ventricular pacing in the dog: pathophysiologic studies of heart failure

We examined rapid ventricular cardiac pacing as a means of inducing heart failure in the dog to establish the sequence and nature of physiologic compensation in this preparation. Seven animals paced at 250 beats/min for 3 weeks (VP1 group) showed an increase in cardiac size from 78.5 +/- 9.5(SD) to 105.8 +/- 13.0 cm2, a reduction in mean arterial pressure from 149 +/- 7 to 130 +/- 21 mm Hg, a fall in cardiac index from 196 +/- 57 to 125 +/- 37 ml/kg/min, and an increase in left ventricular filling pressure from 6 +/- 5 to 22 +/- 9 mm Hg and in right atrial pressure from 2 +/- 2 to 5 +/- 3 mm Hg. An additional series of six animals (VP2 group) was paced until a clear biologic end point for heart failure was reached (average 5.3 +/- 1.9 weeks) and they showed similar but more advanced changes compared with the VP1 group. The changes in cardiac size and hemodynamics in the VP1 and VP2 groups were significantly different from those in parallel studies of 10 sham-operated animals. Plasma norepinephrine and renin activity were unchanged in sham-operated animals, whereas in the VP1 group, plasma norepinephrine rose from 338 +/- 118 to 764 +/- 567 pg/ml (p less than .05), but plasma renin activity did not change. In the VP2 group norepinephrine rose from 471 +/- 285 to 999 +/- 425 pg/ml (p less than .025) and plasma renin rose from 2.1 +/- 1.5 to 8.0 +/- 7.1 ng/ml/hr (p less than .05). There was an excellent correlation between plasma norepinephrine and renin activity before the animals were killed in both the VP1 and VP2 groups (r = .88, p less than .001). No change was evident in atrial natriuretic factor content, as determined by bioassay, in sham-operated or VP1 group animals. However, there was a significant reduction in atrial natriuretic activity from the right atrium that was inversely correlated with the level of right atrial pressure in the VP2 group.

Stimulation of guanylate cyclase by atrial natriuretic factor in isolated human glomeruli

A 23 amino acid synthetic peptide fragment of atrial natriuretic factor (ANF) stimulated guanylate cyclase activity in isolated human glomeruli in a concentration- and time-dependent manner. ANF activated particulate guanylate cyclase whereas it had no effect on soluble guanylate cyclase. These results demonstrate that the glomerulus is a target structure for ANF in humans. They also suggest that ANF-induced increase in glomerular filtration rate is due to a direct effect of this peptide on the glomerular cells mediated by activation of glomerular guanylate cyclase.

Demonstration and characterization of alpha-human atrial natriuretic factor in human plasma

This paper describes a highly specific and sensitive radioimmunoassay for alpha-human atrial natriuretic factor (alpha-hANF), the C-terminal 28-amino-acid residue portion of human prepro-ANF in human plasma. A novel extraction and prepurification procedure allowed for detection of levels of immunoreactive-alpha-hANF as low as 0.5 fmol/ml. In normotensive subjects, levels in the range 1-23 fmol/ml (mean = 8.9 fmol/ml) were found. Combined gel permeation and HPLC analysis demonstrated that this ir-alpha-hANF was comprised virtually exclusively of authentic 28-residue alpha-hANF. No evidence for occurrence of larger precursor forms in human plasma was acquired. A heterogenous group of hypertensive patients displayed considerably higher levels (mean = 62.2 fmol/ml), of interest in view of the hypotensive properties of ANF.

Cardiovascular investigations of an endogenous digoxin-like factor

A circulating factor with digoxin immunoreactivity has been demonstrated. Elevated levels of this substance appear to be present after volume expansion and salt loading, and in some forms of hypertension. The potentially causative role for this factor in hypertension can be demonstrated by the normalization of blood pressure after antidigoxin antibody infusions in low-renin and sodium-dependent hypertension. The possibility that renal excretory defects may be the initiating event to elevate endogenous digoxin is suggested by studies with normotensive humans and monkeys with renal disease. In the latter case cardiovascular deficits were noted that were analogous to those detected in renal hypertensive monkeys with elevated endogenous digoxin. Considered together, these results suggest the existence of a natriuretic and hypertensive substance that plays a role in body fluid homeostasis and blood pressure regulation.

[Effect of natriuretic factor on sodium secretion in the small intestine]

In dogs with Tiri-Vella fistula, the natriuretic factor increases the secretion of the intestinal fluid and its content of sodium. In rats with increased content of natriuretic factor, the water and sodium absorption diminishes in the intestine whereas the secretion of 22Na into the intestine lumen is increased.

Molecular forms of atrial natriuretic polypeptides in mammalian tissues and plasma

A highly sensitive radioimmunoassay detecting for all the atrial natriuretic polypeptides isolated so far from human and rat (hANPs and rANPs) has been established by using an antiserum raised against alpha-hANP, since the antiserum recognizes the subsequence flanked by two cysteine residues (positions 7 and 23) in alpha-hANP and crossreacts with human as well as rat ANPs. By using the radioimmunoassay combined with gel chromatography or high performance liquid chromatography, it was revealed that ANP immunoreactivity in human atria is composed of alpha-(28 residues), beta-(56 residues: alpha-hANP dimer) and gamma-hANP(126 residues) in various ratios. In rat, porcine and bovine atrial tissues, however, gamma-rANP(126 residues) was found to be a major component. In contrast with atrial ANP, plasma ANP was found to be predominantly alpha-form in rat.

Atrial natriuretic polypeptides (ANP): rat atria store high molecular weight precursor but secrete processed peptides of 25-35 amino acids

A radioimmunoassay was developed for rat atrial natriuretic polypeptides. Using the radioimmunoassay and gel filtration in reducing and dissociating conditions, extracts of rat atria were found to contain mainly a 15000-dalton immunoreactive material, probably corresponding to pronatriodilatin. However, when isolated beating atria were incubated in plasma-free conditions, the secreted immunoreactive material consisted almost exclusively of 2500-3500-dalton peptide(s). These results show that rat atrial cells secrete a low molecular weight natriuretic peptide which is highly active, but store the less active large molecular weight form(s).

Ninhibin: a sperm factor attenuates the atrial natriuretic factor mediated inhibition of adenylate cyclase: possible involvement of inhibitory guanine nucleotide regulatory protein

The effect of ninhibin, a sperm factor extracted from bovine sperm, was studied on adenylate cyclase from rat aorta. Ninhibin treatment of the membranes activated adenylate cyclase in a concentration dependent manner. The maximal activation (approximately equal to 4-fold) was obtained at 2micrograms ninhibin and at 10 min of treatment at 37 degrees C. On the other hand, in untreated control membranes, ninhibin at 2 micrograms could stimulate adenylate cyclase by about 50-60% only. In addition, ninhibin potentiated the guanine nucleotide-, isoproterenol- and forskolin (FSK)-stimulated adenylate cyclase activities and also attenuated the GTP gamma s and atrial natriuretic factor (ANF)-mediated inhibition of enzyme activities. Furthermore the inhibition of isoproterenol- and FSK-stimulated adenylate cyclase activities by ANF was also abolished by ninhibin. These data indicate that ninhibin which has been suggested to inactivate or inhibit Ni-guanine nucleotide regulatory protein can also attenuate the ANF-receptor mediated inhibition of adenylate cyclase in rat aorta suggesting an involvement of Ni-guanine nucleotide regulatory protein in the coupling of ANF receptors to adenylate cyclase.

Mechanism of action of natriuretic fraction of urine

The nature of calcium dependence of natriuretic fraction (NF)-induced contractions in the rat smooth muscle anococcygeus and a possible correlation between the effect of NF on sodium transport and its natriuretic potency was investigated. NF and noradrenaline-induced contractions were partially dependent on external calcium concentration. Ouabain and potassium (K) free solution (KoPSS)-induced contractions were totally dependent on external calcium and were more effectively inhibited by calcium antagonists than those of NF and noradrenaline suggesting pharmacomechanical coupling as the mode of calcium dependence of NF. Like other agonists which contract by pharmacomechanical coupling, NF stimulated sodium transport (86rubidium uptake) in dog saphenous vein. Such stimulation correlated positively (r = 0.495, n = 17, P less than 0.05) with the natriuretic potency of NF. NF, like noradrenaline, contracts smooth muscle by pharmacomechanical coupling and NF-induced natriuresis is associated with stimulation of the sodium pump.

Vascular receptor binding activities and cyclic GMP responses by synthetic human and rat atrial natriuretic peptides (ANP) and receptor down-regulation by ANP

Biological activities of a variety of synthetic human (h) and rat (r) atrial natriuretic peptide (ANP) and related peptides as assessed by receptor binding and cyclic GMP response, and regulation of vascular ANP receptors were studied in rat aortic vascular smooth muscle cells (VSMC) in culture. alpha-hANP1-28 and alpha-hANP7-28 equally inhibited the binding of 125I-labeled-alpha-hANP to its vascular receptors, whereas Met(O)12-alpha-hANP1-28 was less potent and reduced and carboxymethylated (RCM)-alpha-hANP1-28 was ineffective. rANP5-27 and rANP5-28 were equipotent in receptor binding, whereas rANP5-25 had somewhat less potent effect and rANP8-28 fragment was ineffective. alpha-hANP1-28, alpha-hANP7-28, rANP5-27 and rANP5-28 similarly stimulated intracellular cyclic GMP formation, whereas rANP5-25 showed less stimulatory effect, and RCM-alpha-hANP1-28, Met12-sulfoxide and rANP fragment were ineffective. Pretreatment with unlabeled alpha-hANP (3.2 X 10(-9) and 3.2 X 10(-8)M) for 24 hrs resulted in a substantial reduction (55 and 75%) of total receptor number without changing the affinity of ANP receptors. These results suggest that the common ring structure formed by the disulfide bond in the molecule is critical for receptor binding and subsequent biological actions, and that a hydrophobic amino acid located at the position of 12, and (24-26) residues at the C-terminal side, but not (1-6) at the N-terminal side, of the disulfide bridge may play a part in modulating receptor binding and/or biological functions. The present study also indicates "down-regulation" of vascular ANP receptors by homologous ligand.

Role of natriuretic factor in central nervous system (CNS)-induced natriuresis

The presence of a natriuretic factor in the plasma of rats in which a 350 mM Na (high Na) artificial cerebrospinal fluid (CSF) was infused into the lateral ventricle was tested. Blood was obtained from control rats and rats which received an infusion of high Na CSF intraventricular (IVT) for 15 min. The plasma was incubated for 30 min at room temperature, acidified, placed in a boiling-water bath, and then centrifuged. The plasma supernate was assayed for natriuretic activity in pentobarbital anesthetized bioassay rats. Sodium excretion increased 6.5 +/- 1.1 mueq/kg X min in rats which received an infusion of a control saline solution, 13.3 +/- 3.2 mueq/kg X min in rats which received infusion of control plasma supernates, and 32.1 +/- 8.3 mueq/kg X min in those rats which received plasma supernates from rats infused with high Na CSF IVT. Blood pressure was unchanged in all groups. The increment in sodium excretion elicited by plasma supernate from the high Na IVT group was significantly greater than that elicited by either control saline solution or control plasma extracts. Therefore, it is concluded that a heat-stable and nonpressor natriuretic factor is present in the plasma of rats infused IVT with high Na CSF.

Diuretic and natriuretic actions of extract prepared from rat erythrocytes

Freeze-dried preparation of rat erythrocytes was extracted into distilled water and after heat treatment the extract was chromatographed on Sephadex G-25 column. The effluent fractions were pooled, freeze-dried, dissolved in Ringer's solution and bioassayed for diuretic and natriuretic responses in anaesthetized non-diuretic rats. The active fractions potently enhanced urine output (about 9-fold control) and urinary sodium excretion (about 7-fold control). The responses started to develop after a lag period of approximately 10-20 min, peaked between 60-80 min post-injection and then diminished. The augmentation of sodium excretion persisted longer than the increased diuresis. The biological activity of the extract was retained after dialysis in a tube with nominal molecular weight cut-off below 1,000.

The increase of cGMP by atrial natriuretic factor correlates with the distribution of particulate guanylate cyclase

We have demonstrated previously that atrial natriuretic factor (ANF) augments urinary, plasma and kidney cGMP levels but has no significant effect upon cAMP. Using cGMP as a marker, we searched for specific target sites involved in the action of ANF in the dog kidney, and observed no change of cGMP in the proximal tubules, a 2-fold increase over basal levels in the thick loop of Henle and a 3-fold elevation in the collecting duct. The most striking action on cGMP occurred in the glomeruli with a rise of up to 50-fold being evident at 1-2 min. after the addition of ANF. The results obtained in the absence or presence of a phosphodiesterase inhibitor support the notion that the effects of ANF were exerted at the level of guanylate cyclase stimulation rather than cGMP phosphodiesterase inhibition. The action of sodium nitroprusside (SNP), a direct stimulator of soluble guanylate cyclase, differed from that of ANF. The ability of the factor to enhance cGMP levels was correlated with the distribution of particulate guanylate cyclase. This study identifies the glomeruli and the distal part of the nephron as specific targets of ANF and implicates particulate guanylate cyclase as the enzyme targetted for the expression of its action.

Structural identification of beta- and gamma-human atrial natriuretic polypeptides

Atrial natriuretic polypeptides (ANPs) of varying chain length have been identified recently in human and rat atrial tissue. Their potent natriuretic-diuretic activities indicate their key role in the regulation of extracellular fluid volume and electrolyte balance. Furthermore, human and rat cDNAs encoding their precursor have been cloned and identified. Natriuretic-diuretic activity in human atrial extract comprises three distinct components (alpha, relative molecular mass (Mr) approximately 3,000; beta, Mr approximately 6,000; gamma, Mr approximately 13,000). However, only the 3,000-Mr peptide, alpha-human atrial polypeptide (alpha-hANP), comprising 28 amino acids, has so far been identified. We report here the purification and sequence analysis of two novel hANPs of higher Mr, beta- and gamma-hANP, both of which exhibit natriuretic and hypotensive activity. gamma-hANP, composed of 126 amino acids, carries the alpha-hANP sequence at its carboxy terminus. The identification of gamma-hANP reveals that the peptide, being the largest form of hANP, is processed directly from a 151-residue precursor by removal of a 26-residue signal peptide. In contrast, beta-hANP (56 residues) comprises an anti-parallel dimer of alpha-hANP; such a dimeric peptide possessing bioactivity has never been found in the tissue as an endogenous entity.

Release of natriuretic factor from rat heart-lung preparation by atrial distension

The hypothesis that an increase in blood volume results in the release of an atrial natriuretic factor (ANF) via atrial stretch was examined using an isolated rat heart-lung preparation. The heart lung preparation was perfused with a fluorocarbon emulsion for two periods of 20 min. During the first period the venous return reservoir was placed at a level of 1-2 cm above the right atrium [low central venous pressure (low CVP)] and at a level 7-12 cm above the heart (high CVP) for the second period. The perfusate used for each period was collected and saved for later analysis of natriuretic activity using anesthetized rats as a bioassay. Samples of either low or high CVP perfusate were infused into the abdominal aorta of the assay rats while urine was collected from a bladder catheter. A 3-min infusion of the high CVP perfusate at a rate of 0.5 ml/min resulted in an increase in urine flow from 10 to 28 microliter/min (P less than 0.01), sodium excretion from 0.14 to 1.34 mueq/min (P less than 0.01), and potassium excretion from 0.17 to 0.81 mueq/min (P less than 0.01). Infusion of the low CVP perfusate failed to produce a significant diuresis, natriuresis, or kaliuresis. These results are consistent with the hypothesis that ANF may be a hormone involved in the control of blood volume.

Structure-activity relationships of atrial natriuretic factor (ANF). II. Effect of chain-length modifications on vascular reactivity

The effect of the length of ANF peptides on the inhibition of the norepinephrine-induced contraction was studied. Starting from the 26 residues ANF (Arg101-Tyr126), shorter N- and/or C-terminal fragments were produced, either by N-terminal chemical cleavage or C-terminal enzymatic digestion of ANF or both respectively. The N-terminal removal of Arg101 did not modify the inhibitory response. Further N-terminal truncation up to des-Arg101-Arg102-Ser103-Ser104 ANF still produced a marked inhibitory effect on norepinephrine. In contrast C-terminal cleavage had a much more pronounced effect. Since des-Tyr126 ANF, des-Arg125-Tyr126 ANF and des-Phe124-Arg125-Tyr126 ANF exhibit much lower activities than the parent ANF. Finally, when the 5 residues C-terminal to Cys121 are removed, the resulting molecule is almost inactive. These data indicate that the C-terminal segment of ANF may modulate the binding of ANF to its receptor(s). Relatively, the N-terminal region seems to be much less important.

Nucleotide sequences of the human and mouse atrial natriuretic factor genes

Mouse and human atrial natriuretic factor (ANF) genes have been cloned and their nucleotide sequences determined. Each ANF gene consists of three coding blocks separated by two intervening sequences. The 5' flanking sequences and those encoding proANF are highly conserved between the two species, while the intervening sequences and 3' untranslated regions are not. The conserved sequences 5' of the gene may play an important role in the regulation of ANF gene expression.

Radioimmunoassay for alpha-human and rat atrial natriuretic polypeptide

Using a synthetic common carboxy-terminal fragment of alpha-human atrial natriuretic polypeptide (alpha-hANP) and alpha-rat atrial natriuretic polypeptide (alpha-rANP), we have produced an antiserum for alpha-ANP(17-28) and established a radioimmunoassay (RIA) for alpha-ANP that recognizes alpha-hANP and alpha-rANP equally. High performance gel permeation chromatography coupled with the RIA revealed that alpha-hANP-like immunoreactivity (alpha-hANP-LI) in the human atrium consists of three major components, gamma-hANP, alpha-hANP and another. On the other hand, alpha-rANP-LI in the rat atrium comprised at least two components, 13K alpha-rANP-LI and 3K-5K alpha-rANP-LI, which were presumably gamma-rANP and beta-rANP, respectively. Thus, considerable amounts of gamma-hANP and gamma-rANP are present in human right auricles and rat atria, respectively. The RIA established in this study provides a useful tool to investigate the pathophysiological significance of alpha-ANP and related peptides in cardiovascular disorders and shows that gamma-ANP is not only a precursor of alpha-ANP but acts as an important hormone.

Observations on a low molecular weight natriuretic and Na-K-ATPase inhibitory material in urine

Natriuretic and Na-K-ATPase inhibitory material prepared from urine by gel filtration on G25 Sephadex was previously found to be of low molecular weight, polar and non-peptide. Although activity appeared to depend on an amino group, tests and radioenzymatic assays for catecholamines suggested that these were not implicated in the natriuretic activity. Further purification of the material included solvent extraction, cation exchange and high performance liquid chromatography. At each stage, fractions were assayed for natriuretic activity, stimulation of G6PD and inhibition of Na-K-ATPase in cytochemical assays, and for digoxin-like activities i.e. inhibition of dog kidney Na-K-ATPase (Sigma), displacement of 3H ouabain bound to cell membranes and cross reaction with antidigoxin antibody. The crude material possessed all activities, but with successive purifications the activities separated from each other and were thus due to different substances. Analyses for catecholamines with HPLC and electrochemical detection revealed that the natriuretic activity was due to dopamine.

Natriuretic and smooth muscle responses to human urinary natriuretic hormone in rats: relation to blood pressure and kallikrein excretion in patients

Human urine contains a small molecular weight natriuretic substance and similar material isolated from the kidney inhibits Na/K ATPase. Such action on smooth muscle in blood vessels would cause contraction. Human urinary natriuretic material caused contraction of the smooth muscle in the rat anococcygeus muscle and this activity correlated with its natriuretic activity. Known vasoactive substances could not explain the activity of the natriuretic factor when tested on the anococcygeus muscle. The best correlation with blood pressure of the patient was with the log of the ratio of natriuretic activity divided by the kallikrein excretion. A normotensive woman with severe renal failure had very high kallikrein excretion as well as increased natriuretic activity and her data fitted the same correlation as the hypertensives' data.

Diuretics. Clinical pharmacology and therapeutic use (Part I)

25 years have elapsed since the introduction of the first effective oral diuretic, chlorothiazide. Diuretics are now amongst the most widely prescribed drugs in clinical practice worldwide. Availability of these drugs has not only brought therapeutic benefit to countless numbers of patients but it has at the same time provided valuable research tools with which to investigate the functional behaviour of the kidney and other electrolyte-transporting tissues. Despite many remaining gaps in our knowledge of the biochemical processes involved in diuretic drug action, available compounds can be divided into 5 groups on the basis of their preferential effects on different segments of the nephron involved in tubular reabsorption of sodium chloride and water. Firstly, there is heterogeneous group of chemicals that share the common property of powerful, short-lived diuretic effects that are complete within 4 to 6 hours. These agents act on the thick ascending limb of Henle's loop and are known as 'high ceiling' or 'loop' diuretics. The second group are the benzothiadiazines and their many related heterocyclic variants, all of which localise their effects to the early portion of the distal tubule. The third group comprises the potassium-sparing diuretics which act exclusively on the Na+-K+/H+ exchange mechanisms in the late distal tubule and cortical collecting duct. The action of drugs in groups 2 and 3 is prolonged to between 12 and 24 hours. The fourth group consists of diuretics that are chemically related to ethacrynic acid but have the unusual property of combining within the same molecule the property of saluresis and uricosuria. These compounds have actions, to different individual extents, in the proximal tubule, thick ascending limb, and early distal tubule and are known as 'polyvalent' diuretics. Finally, there is a mixed group of weak or adjunctive diuretics which includes the vasodilator xanthines such as aminophylline, and the osmotically active compounds such as mannitol. Available evidence on the molecular mechanisms of action of diuretics in each group is reviewed. The haemodynamic, humoral and physical factors involved in control of electrolyte and fluid handling by the kidney in normal conditions and pathological states are discussed in relation to rational choices of different diuretics in the treatment of various oedematous and non-oedematous conditions.

The concept of the natriuretic hormone and its relation to hypertension

Acute volume expansion increases the intrathoracic blood volume thus endowing the plasma with an increased capacity to cause a natriuresis, to inhibit Na-K-ATPase and stimulate vascular reactivity. It is not known whether these changes, which stem from a common stimulus are due to a change in the concentration of one substance or several. It is proposed that in essential hypertension a genetic abnormality of the kidney causes a difficulty in excreting sodium. This leads to an initial blood volume expansion which causes the observed rise in the plasma's capacity to inhibit sodium transport, and to the increased vascular tone. Eventually the increase in tone of the arterial smooth muscle causes the blood pressure to rise, while the increase in tone of the smooth muscle of the veins diminishes venous compliance thus causing a shift of blood from the periphery to the chest thus providing the stimulus for the persistent rise in the plasma's capacity to increase vascular reactivity, even though total blood volume may have returned to normal.

The role of endogenous inhibition of Na-K-ATPase in human hypertension--sodium pump activity as a determinant of peripheral vascular resistance

High sodium intake in the presence of an intrinsic or acquired defect in renal sodium excretion will result in extracellular fluid volume (ECFV) expansion which is accompanied by decreased baroreceptor reflex sensitivity. We have shown that ECFV-expansion also stimulates the secretion of an endogenous inhibitor of the Na-K-ATPase enzyme and high activity of this sodium transport inhibitor was detected in plasma of patients with primary aldosteronism, the most classical type of volume-dependent hypertension. Thus, vasoconstriction due to inhibition of sodium pump activity of the vascular smooth muscle cell may contribute to the pathogenesis of human arterial hypertension. In analogy, ouabain (8.5 micrograms/kg) when administered i.v. to healthy volunteers inhibited RBC - Na-K-ATPase by 49% and significantly increased peripheral vascular resistance by 24 - 36%. The calcium entry blocker nifedipine (10 mg orally) completely prevented ouabain-induced vasoconstriction suggesting that the action of ouabain was mediated by a rise in intracellular calcium. High potassium intake partially abolished the vasoconstrictor effect of ouabain and also significantly increased baroreceptor reflex sensitivity. The results of these studies support the concept that inhibition of the sodium and potassium pump of vascular smooth muscle cells by a yet putative endogenous inhibitor of Na-K-ATPase (natriuretic hormone) may represent a crucial mechanism in the pathogenesis of at least certain forms of essential and secondary hypertension in man.

Evidence for the existence of atrial natriuretic factor-containing neurons in the rat brain

Immunoreactive atrial natriuretic factor- (ANF-)positive nerve fibers and cell bodies were observed in the preoptic area, hypothalamus, mesencephalon, and pons of rats. In colchicine-treated animals a large number of immunoreactive ANF-positive cell bodies were seen in the organum vasculosum of the lamina terminalis, in several hypothalamic nuclei (e.g. periventricular, arcuate, and ventral premammillary nuclei), and in the dorsolateral tegmental nuclei of the pons. Varicose nerve fibers containing ANF were generally observed in the vicinity of the cells. These findings indicate that a widespread network of ANF-containing neurons is present in the brain.

Further characterization of the endogenous natriuretic and digoxin-like immunoreacting activities in human urine: effects of changes in sodium intake

In the present study natriuretic activity and digoxin-like immunoreacting activity (DLIA) were determined in small molecular weight (MW) fractions of urine from healthy subjects during low (35 mmol/day) and high (greater than 400 mmol/day) sodium intake by bioassay and by a radioimmunoassay for digoxin, respectively. After gel filtration of urine on a Sephadex G-25 column the natriuretic activity appeared in the post-salt fraction SIV, whereas DLIA was present in small amounts in the salt fraction SIII and, with consistently higher activity, in the post-salt fraction SIV. Natriuretic activity significantly increased and DLIA decreased in fraction SIV with high sodium intake, but total urinary excretion of DLIA remained unaltered during changes in sodium intake. In addition, anion-exchange and reverse-phase chromatography revealed that DLIA is not specifically related to the natriuretic activity but also reflects unspecific binding of various urine constituents to this digoxin antibody. Although the antibody binds a natriuretic material, this radioimmunoassay is thus unsuitable to determine the endogenous natriuretic activity in urine fractions. Whereas they elute differently on reverse-phase chromatography, amino acid analyses revealed that both the natriuretic factor directly purified from the post-salt fraction SIV and the natriuretic material bound to the digoxin antibody have in common four amino acids at similar molar ratios. The physicochemical properties as evidenced by chromatographic and electrophoretic studies as well as enzymatic inactivation suggest that the low MW natriuretic factor(s) in human urine may be associated with a small peptide(s) of weak acidic nature.

Inhibition of aldosterone production in the adrenal glomerulosa by atrial natriuretic factor

Several forms of the polypeptide atrial natriuretic factor (ANF) have been isolated recently from rat and human atria and identified; they are probably associated with the secretory granules of atrial tissue. The potent ability of ANFs to increase urine sodium content is mediated by their direct action on the kidney. We report here the high intrinsic activity of a synthetic replicate of one form of this molecule, ANF(8-33)(ref. 7), to inhibit directly basal aldosterone secretion and its ability to antagonize the stimulatory effects of adrenocorticotropin (ACTH) and angiotensin II (AN-II) on the secretion of aldosterone by rat adrenoglomerulosa cells in vitro. Our results suggest that ANF is of clinical importance in the management of aldosterone-dependent hypertension by modifying the adrenocortical response to endogenous ACTH and AN-II.

Effects of atrial extract on blood pressure and urinary excretion of electrolytes and prostaglandin E2 in rats

The mechanism of natriuresis following administration of atrial extract is not fully understood. This study related the changes in urinary excretion of electrolytes induced by atrial or ventricular extract to those in urinary excretion of prostaglandin E2 (PGE2) in rats. Both atria were removed from Wistar rats and homogenized in a 1 N acetic acid solution. After centrifugation, the supernatant was lyophilized and then resolved in a 0.1 N acetic acid solution. The apical portions of heart ventricles were similarly prepared, and served as controls. One hundred microliter of the solution containing the extract of 10 mg of tissue were injected intravenously into assay rats, the carotid artery and the urinary bladder of which were cannulated to measure blood pressure and urinary output, respectively. Urinary PGE2 was determined by radioimmunoassay. The atrial extract rapidly lowered blood pressure without changing heart rate. This hypotensive effect was maintained for at least 30 minutes. When the urinary samples obtained before and 10 minutes after the injection of the atrial extract were compared, urinary volume and excretion of sodium (UNaV) and potassium (UKV) were increased by 8, 24 and 3-fold, respectively. Urinary excretion of PGE2 (UPGEV) was increased from 5.3 +/- 0.9 (SE) to 16.3 +/- 2.9 ng/kg/10 min (p less than 0.001). There was a significant positive correlation between UNaV and UPGEV in all the samples (r = 0.72, p less than 0.001), and between the changes in UNaV and those in UPGEV after administration of the atrial extract (r = 0.62, p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Structure-activity relationships of atrial natriuretic factor (ANF). I. Natriuretic activity and relaxation of intestinal smooth muscle

Atrial natriuretic factor-related peptides were produced from synthetic ANF (101-126) either by chemical hydrolysis at the N-terminal end or by treatment with carboxypeptidases at the C-terminal end. The biological activities of these peptides were characterized in vivo by a natriuretic bioassay and in vitro by relaxation of contracted intestinal smooth muscle (chick rectum). In the natriuretic assay, the removal of Asn122, Ser123 and Phe124 at the C-terminal end alters considerably the renal response. Deletion or extension of amino acids at the N-terminal end affects only slightly the natriuretic activity. Removal of N-terminal or C-terminal amino acids decreases the relaxant activity of ANF on the chick rectum. In both bioassays, simultaneous deletions of residues at both ends drastically affect the activity in an additive manner.

Effects of auriculin (atrial natriuretic factor) on blood pressure, renal function, and the renin-aldosterone system in dogs

Auriculin is a potent vasoactive and natriuretic peptide that was recently isolated and purified from rat atrial tissue. Since this peptide could be of great importance for renal, cardiovascular, and volume homeostasis, its functional properties have been characterized in dogs. The effects of synthetic auriculin on renal function, mean blood pressure, plasma renin activity, renin secretory rate, and plasma aldosterone levels were determined. Auriculin was administered intravenously as a prime (1.0 microgram/kg body weight) and constant infusion (0.1 microgram per minute/kg body weight for one hour) to five anesthetized dogs. In addition, two conscious dogs were used to verify some of the results obtained in anesthetized dogs. Auriculin decreased mean blood pressure from 134 +/- 5 to 122 +/- 4 mm Hg (p less than 0.05, paired t test) and increased glomerular filtration rate (25.5 +/- 2.7 to 32.4 +/- 4.1 ml per minute per kidney, p less than 0.05), diuresis (0.21 +/- 0.03 to 1.06 +/- 0.14 ml per minute per kidney, p less than 0.05), natriuresis (38 +/- 0.6 to 187 +/- 35 mueq per minute per kidney, p less than 0.05), and kaliuresis (14.8 +/- 1.6 to 35.7 +/- 6.3 mueq per minute per kidney, p less than 0.05). These effects were sustained throughout the infusion of auriculin and were entirely reversible. Renal plasma flow increased transiently for one to two minutes, and then returned to or below control levels. Urine osmolality decreased by 40 percent (p less than 0.05) whereas free water clearance remained unchanged (p less than 0.05). Auriculin reversibly decreased plasma renin activity (11.6 +/- 2.3 to 3.6 +/- 1.2 ng/ml per hour, p less than 0.05), renin secretory rate (895 +/- 313 to 255 +/- 28 ng per hour per minute, p less than 0.05), and plasma aldosterone levels (8.4 +/- 1.6 to 3.6 +/- 0.7 ng/dl, p less than 0.05), whereas plasma cortisol levels remained unchanged. These results demonstrate that auriculin has a unique combination of functional properties, increasing glomerular filtration rate, diuresis, and natriuresis, without a sustained increase in total renal blood flow, and lowering blood pressure, plasma renin levels, renin secretory rate, and plasma aldosterone levels. These properties suggest an important potential role for atrial natriuretic peptides in the regulation of renal function, extracellular volume, and blood pressure.

Synthesis and renal activity of rat atrial granules depend on extracellular volume

Extracellular fluid volume (by 22Na) and extent of 4-h [3H]fucose incorporation into atrial-specific granules were measured in deoxycorticosterone acetate (DOCA)/salt-loaded or Na-deficient rats. The natriuretic potency of extracts from their atria was also measured in assay rats. DOCA/salt-treated animals had a significantly greater extracellular volume, a significantly greater degree of fucose uptake, and a significantly more potent diuretic and natriuretic effect than did Na-deficient rats. These observations, together with the known decrease in atrial granularity with DOCA treatment, suggest that a chronic increase in extracellular fluid volume is associated with increased synthesis and metabolism of atrial natriuretic factor. They also confirm the finding reported by others that granularity and natriuretic potency are not always directly related. It may be that visible granules represent a peptide storage form that requires further processing to become natriuretic.

Rat atrial natriuretic factor: isolation, structure and biological activities of four major peptides

Four peptides possessing both natriuretic activity and smooth muscle relaxant activity were isolated from rat atrium and their amino acid sequences determined. The four peptides designated ANF-I, ANF-II, ANF-III and ANF-IV containing 35, 31, 30 and 25 amino acid residues, respectively, were obtained in a molar ratio of 4:60:20:16. The predominant species ANF-II, which may represent the native form of ANF, has the following sequence: (H2N)-G-P-R-S-L-R-R-S-S-C-F-G-G-R-I-D-R-I-G-A-Q-S-G-L-G-C-N-S-F-R-Y-(COO H) in which Cys-10 and Cys-26 are disulfide linked. Cleavage of the aspartyl linkage at position 16 by staphylococcal protease caused complete inactivation, indicating that the ring conformation is essential. The dose-response relationships determined for the four peptides in relaxing norepinephrine-induced contraction of rabbit thoracic aorta showed half-maximum relaxation at concentrations ranging from 1.5 X 10(-9) to 2.5 X 10(-9) M. Comparable dose-response relationships were observed in relaxation of carbacol-induced contraction of chick rectum strips as tested with ANF-II and ANF-IV.

Vasodilator profile of synthetic atrial natriuretic factor

The vasodilator profile of synthetic atrial natriuretic factor (ANF) was characterized using isolated vascular preparations. Nanomolar concentrations of ANF relaxed rabbit aortic rings contracted by serotonin, histamine, methoxamine or angiotensin II. The synthetic peptide was most effective (IC50 = 1.3 X 10(-10) M) in relaxing the tonic, intrinsic contractions of the rabbit facial vein. ANF was poorly active against K+-contracted aortic rings or the phasic contractions of the rat portal vein. A similar vasodilator profile was obtained for sodium nitroprusside but not papaverine, hydralazine, adenosine or nifedipine. This first demonstration of the vascular activity of synthetic ANF depicts this substance as a nonselective vasodilator of agonist-induced contractions. The observed similarities in the vasodilator activity of ANF and sodium nitroprusside suggest a common mechanism of action.

Atrial natriuretic factor is a circulating hormone

The radioimmunoassay of atrial natriuretic factor (ANF) has been applied for determination of immunoreactive ANF (IR-ANF) in rat plasma. Immunoreactive ANF has been extracted from rat plasma by immunoaffinity column on Sepharose-4B anti-ANF or by Vycor glass. The mean concentrations of IR-ANF in ether anesthetized rats were found to be 1.61 +/- 0.14 ng/ml in female and 1.25 +/- 0.21 ng/ml in male rats when extracted on Sepharose-4B anti-ANF, and 1.21 +/- 0.10 ng/ml in females and 1.02 +/- 0.11 ng/ml in males when extracted by Vycor glass. A close linear correlation has been observed between the plasma IR-ANF concentrations in aorta and jugular vein. The described results indicate that atrial cardiocytes secrete atrial natriuretic factor into plasma. The heart is, therefore, an endocrine organ.

Atrial natriuretic factor in rat hypothalamus, atria and plasma: determination by specific radioimmunoassay

A rapid and reproducible radioimmunoassay method was developed for rat atrial natriuretic factor (ANF)-IV. The method is also applicable to human atrial peptide. ANF was detected in rat hypothalamus (5.03 pmoles/g tissue), right (86.8 pmoles/mg tissue) and left atria (52.5 pmoles/mg tissue), and plasma (156 fmoles/ml). After high salt intake immunoreactive ANF in atria and plasma increased significantly, while a significant decrease was observed in hypothalamus. Gel chromatography revealed high and low molecular weight ANF in atria and hypothalamus while only a low molecular weight form was found in plasma.

Actions of synthetic atrial natriuretic factor on bovine adrenal glomerulosa

Synthetic atrial natriuretic factor (ANF) inhibited aldosterone production by suspensions of bovine adrenal glomerulosa cells. Inhibition by ANF was most pronounced when basal aldosterone production was measured. The effects of angiotensin II (AII), N6,O2'-dibutyryl-adenosine 3':5'-cyclic monophosphate (dibutyryl cyclic AMP), and elevated potassium were also inhibited by ANF. Inhibition could be partially overcome by high doses of agonist. Inhibition was localized to the early pathway of aldosteronogenesis, to a step before cholesterol side-chain cleavage. ANF had no effect on binding of AII to receptors, on the stimulation by AII of phospholipid turnover, or on the alteration by AII of calcium fluxes.

Sodium transport inhibition, cell calcium, and hypertension. The natriuretic hormone/Na+-Ca2+ exchange/hypertension hypothesis

Sodium plays a critical role in the etiology of essential hypertension, but the mechanism by which excess dietary sodium actually leads to the elevation of blood pressure is not understood. The hypothesis described shows how an excessive sodium load can lead to the development of hypertension. The underlying factor must be a genetic or acquired deficiency or limitation in renal sodium excretion that may be undetectable by standard renal function tests. The resultant tendency towards sodium, water, and extracellular fluid volume expansion is compensated by the secretion of a natriuretic hormone that promotes sodium excretion by inhibiting sodium pumps in the kidney tubule cells. The hormone also inhibits sodium pumps in other cells, including vascular smooth muscle cells, causing intracellular sodium to increase. Then, because the vascular smooth muscle cells contain a Na+-Ca2+ exchange transport system in their plasma membranes, more calcium than normal is delivered to these cells. This causes the increased contractility and reactivity that underlies the increased vascular tone and peripheral vascular resistance that elevates the blood pressure.

A digoxin-like immunoreactive substance in preeclampsia

A study has been undertaken of levels of an endogenous substance which immunologically cross reacts with digoxin and may be the putative natriuretic hormone. The possibility that this substance may play a role in the pathophysiology of preeclampsia has been studied by measuring the plasma concentrations in clinically healthy and preeclamptic pregnant patients. A significant difference (p less than 0.001) between these two groups has been found.