Area postrema and sympathetic nervous system effects of vasopressin and angiotensin II.

1. Precise control over the cardiovascular system requires the integration of both neural and humoral signals related to blood volume and blood pressure. Humoral signals interact with neural systems, modulating their control over the efferent mechanisms that ultimately determine the level of pressure and volume. 2. Peptide hormones such as angiotensin (Ang)II and arginine vasopressin (AVP) act through circumventricular organs (CVO) to influence cardiovascular regulation. 3. The area postrema (AP), a CVO in the brainstem, mediates at least some of the central actions of these peptides. Vasopressin appears to act in the AP to cause sympathoinhibition and a shift in baroreflex control of the sympathetic nervous system (SNS) to lower pressures. These effects of AVP and the AP appear to be mediated by alpha2-adrenoceptor and glutamatergic mechanisms in the nucleus tractus solitarius. 4. In contrast to AVP AngII has effects in the AP to blunt baroreflex control of heart rate and cause sympathoexcitation. The effects of chronic AngII to increase activity of the SNS may be due to AP-dependent activation of neurons in the rostral ventrolateral medulla.

Effects of enalaprilat on neointimal growth of cultured rabbit aorta following balloon injury.

Our objective was to determine if the ability of an angiotensin-converting enzyme (ACE) inhibitor to attenuate neointima formation in balloon-damaged vessel is expressed in an isolated organ culture model of neointimal growth. In vivo balloon angioplasty in combination with in vitro organ culture was used to produce a unique model of vascular neointima formation. Aortic segments were cultured in medium containing a broad concentration range of the ACE inhibitor enalaprilat (0-100 microM). Cell proliferative indices and neointima:media thickness ratios were determined from vessel segments after 1, 4, and 7 days in culture. We observed no significant effect on either parameter at any dose of enalaprilat. Linear regression analysis on the rate of increase in intima to media thickness ratios during the 7 days of culture also showed no effect of enalaprilat at any concentration. We conclude that enalaprilat has no effect on neointimal growth or cell proliferation in this vascular organ culture model, and it is suggested that ACE inhibitors may act by mechanisms other than local converting enzyme inhibition to attenuate neointimal growth in rabbits following vascular ballooning in vivo.

Enhanced neointimal growth in cultured rabbit aorta following in vivo balloon angioplasty.

We have used in vivo balloon catheterization in combination with in vitro organ culture to develop a model system for vascular neointima formation. A Fogarty balloon catheter was used to deendothelialize and rupture the internal elastic lamina of aortae in adult rabbits. After three d of recovery, aortae were harvested, divided into segments, and placed into organ culture. We obtained a daily index of cell proliferation in cultured vessels using [3H]thymidine incorporation into DNA. Also, segments were collected and processed for routine histology or immunohistochemistry. Aortic segments that had undergone ballooning 3 d before harvest and then cultured exhibited diffuse neointimal growth after several d in vitro, whereas those from sham-operated (nonballooned) rabbits showed generally only a single endothelial cell layer that is characteristic of normal intima. Aortae that were harvested, balloon-damaged in vitro, and then cultured exhibited no neointimal growth. The neointima that developed in cultured segments from in vivo ballooned rabbits was primarily of smooth muscle cell origin as determined by positive immunostaining for alpha-smooth muscle actin. The intima:media thickness ratios were significantly higher in aortic segments from ballooned rabbits at harvest and after 4 or 7 d in culture compared with those from nonballooned rabbits. Also, the [3H]thymidine index was higher in the in vivo ballooned aorta compared to non-ballooned or in vitro ballooned vessel. We conclude that ballooning in vivo followed by exposure to blood-borne elements produces an enhanced proliferative response in cultured vessels that is distinct from other in vitro models of neointimal growth.

Angiotensin hypertension.

1. One of the most important issues in the field of hypertension research centres on the therapeutic use of inhibitors of the renin-angiotensin system (RAS). Inhibitors of the RAS have potent anti-hypertensive effects, even in experimental models of hypertension and in human essential hypertension, where the activity of the peripheral RAS is low or normal. 2. It is suggested here that determining the mechanisms by which activation of the peripheral RAS produces hypertension will help us determine the anti-hypertensive effects of these inhibitors in low/normal renin-angiotensin hypertension. 3. Three hypotheses describing the hypertensive effects of angiotensin are discussed. The first hypothesis involves the direct vasoconstrictor effects of angiotensin. The second hypothesis suggests that chronic angiotensin produces hypertension by increasing Na+ reabsorption leading to volume expansion and hypertension. The final hypothesis suggests that, in angiotensin-induced hypertension, the increased Na+ reabsorption is not associated with volume expansion but, rather, is associated with an increase in vascular tone resulting from an interaction between angiotensin and the nervous system. 4. It is also hypothesized that the interaction between angiotensin and the nervous system produces a differential activation of sympathetic outflow that spares the kidney.

Fos-like immunoreactivity in the medulla after acute and chronic angiotensin II infusion.

Acute and chronic angiotensin (Ang) II hypertension are reported to have different mechanisms that involve differential contributions of the peripheral vasculature and the nervous system. Acute Ang II hypertension is mediated primarily by Ang acting at vascular smooth muscle, whereas chronic Ang II hypertension appears to have a neural component. In our experiments, the transition from a peripheral to a neural effect occurs over 10 hr of Ang II infusion in rats. To identify the role of the central nervous system in this transition, we measured Fos immunoreactivity, an indicator of neural activity, in the nucleus of the solitary tract (NTS), caudal ventrolateral medulla (CVL) and rostral ventrolateral medulla (RVL) in normal, sinoaortic denervated (SAD) and sham SAD rats after 2- or 18-hr Ang II infusion (50 ng/kg/min intravenously). Vehicle (5% dextrose) was infused in normal rats as control. Comparable increases in arterial pressure were produced by 2- and 18-hr Ang II infusion in all groups. Fos was increased in the NTS in sham SAD rats by 2- and 18-hr Ang II infusion (P < .05 vs. vehicle control). In the CVL, only 2-hr Ang II infusion was associated with increased Fos in normal and sham SAD rats (P < .05 vs. vehicle control) but not in SAD rats. In the RVL, 18-hr Ang II infusion elevated Fos in all groups (P < .05 vs. vehicle control). Activation of NTS during Ang II infusion is baroreceptor mediated and independent of infusion duration. Acute Ang II infusion produced a baroreceptor-mediated activation of the CVL, a region associated with baroreflex sympathoinhibition. Chronic Ang II infusion produced a baroreceptor-independent activation of the RVL, a brain area associated with sympathoexcitation, suggesting a centrally mediated increase in sympathetic outflow that may be associated with chronically infused Ang II.

Acute and chronic angiotensin hypertension: neural and nonneural components, time course, and dose dependency.

We examined the mechanisms mediating hypertension in conscious rats during acute and chronic infusion of angiotensin II (ANG II) at pressor doses (50, 100, and 200 ng.kg-1.min-1). Trimethaphan-induced blood pressure reduction was inversely related to the acute dose of ANG II, consistent with a constrictor action of ANG II on vascular smooth muscle and withdrawal of sympathetic tone. During chronic ANG II infusion, the entire increase in mean arterial pressure (MAP) was inhibited by trimethaphan, consistent with neural mediation. During acute ANG II hypertension, the AT1-specific receptor blocker losartan induced a large fall in MAP (64 +/- 4 mmHg) in ganglion-blocked (chlorisondamine) rats, whereas, during chronic ANG II hypertension, losartan had only a small hypotensive effect (11 +/- 3 mmHg). To determine the time course of the change from vascular smooth muscle action to neural action, we measured MAP in response to trimethaphan during the first 24 h of ANG II infusion. After 5 h, the minimal MAP in response to trimethaphan was significantly higher than that before ANG II. After 10 h of infusion, trimethaphan decreased MAP to pre-ANG II levels. That is, the neural component was fully active after only 10 h of infusion in rats. Finally, chronic administration of ANG II resulted in a dose-related increase in MAP that, at all doses, was completely inhibited by trimethaphan. These findings are consistent with ANG II acting primarily on vascular smooth muscle during acute infusion and via neural pathways during chronic treatment. The transition from direct smooth muscle to indirect neural action is rapid in rats (< 10 h), and the MAP and neural responses to ANG II are dose related during chronic hypertension.

In vivo pharmacology of SC-51316, a nonpeptidic angiotensin II receptor antagonist.

The depressor activity of a novel nonpeptidic angiotensin II (AII) receptor antagonist, SC-51316 (2,5-dibutyl-2,4-dihydro-4-[[2-(1H-tetrazol-5-yl)[1,1'-biphenyl]-4 '- yl]-methyl]-3H-1,2,4-triazol-3-one), is described. In anesthetized, ganglion-blocked rats, intravenous administration of SC-51316 inhibited the pressor response to an infusion of AII. To determine antihypertensive efficacy, conscious, spontaneously hypertensive rats were administered SC-51316 (30 mg/kg intragastrically) daily for 5 days. Blood pressure was reduced in a similar manner to that observed with the angiotensin converting enzyme inhibitor enalapril (10 mg/kg intragastrically). SC-51316 had no effect on heart rate. In conscious, sodium-deficient dogs, administration of SC-51316 (30 mg/kg orally) or enalapril (10 mg/kg orally) lowered blood pressure similarly over a 24 h observation period. Thus, SC-51316 antagonizes the activity of AII in vivo and is an orally active, antihypertensive agent.

Differential effects of renin-angiotensin system blockade on atherogenesis in cholesterol-fed rabbits.

To investigate the mechanism by which angiotensin-converting enzyme (ACE) inhibition attenuates atherogenesis, we have studied the effects of a non-sulfhydryl ACE inhibitor, enalapril, and an angiotensin receptor antagonist, SC-51316, in cholesterol-fed rabbits. After 3 mo of enalapril treatment (10 mg/kg per d, p.o.) the percent plaque areas in the thoracic aortas of treated animals were significantly reduced (controls: 86.8 +/- 3.5%; treated: 31.1 +/- 8%, P < 0.001). Aortic cholesterol content was also reduced (controls: 31.4 +/- 3.2 mg/g tissue; treated: 7.4 +/- 1.8 mg/g, P < 0.001). Enalapril had no significant effect on plasma lipid levels or conscious blood pressure. In a second study, the angiotensin II receptor antagonist SC-51316 was administered at a dose equivalent to enalapril at blocking angiotensin pressor effects in vivo (30 mg/kg per d, p.o.). Evaluation after 3 mo indicated no significant attenuation of aortic atherosclerosis. These results demonstrate that: (a) enalapril attenuates atherogenesis without affecting either blood pressure or plasma lipid levels; (b) antioxidant activity, found with sulfhydryl-containing ACE inhibitors, is not necessary for reducing plaque formation; and (c) the attenuation of atherogenesis by ACE inhibition may not be due to blockade of the renin-angiotensin system. Alternatively, one must consider the multiple effects of ACE inhibition on other hormone systems, such as bradykinin, or the possibility that alternate angiotensin II receptors may be involved in atherosclerosis.

In vitro pharmacology of a nonpeptidic angiotensin II receptor antagonist, SC-51316.

The properties of a novel nonpeptidic angiotensin II (AII) receptor antagonist, 2,5-dibutyl-2,4-dihydro-4-([2-(1H-tetrazol-5-yl)(1,1'-biphenyl) -4'-yl]methyl)-3H-1,2,4-triazol-3-one (SC-51316), are described. SC-51316 inhibited [125I]AII binding selectively to the AT1 receptor with IC50 values of 3.6 and 5.1 nM in rat adrenal cortical and rat uterine membrane preparations, respectively. The compound was a competitive and reversible antagonist of AII-mediated contraction of rabbit aortic rings with a pA2 of 8.86. In addition, SC-51316 inhibited AII-induced aldosterone release from rat adrenal zona glomerulosa cells and blocked inhibition of renin release by AII from rat kidney slices with pA2 values of 8.62 and 8.9, respectively. The agent (0.1 mM) did not inhibit angiotensin-converting enzyme or plasma renin activity. These data demonstrate that SC-51316 is a potent AII receptor antagonist which may prove to be useful as a pharmacologic tool for studying the role of the renin-angiotensin system in cardiovascular diseases.

Reversal of low dose angiotension hypertension by angiotensin receptor antagonists.

During acute angiotension II (Ang II) infusion (200 ng/kg/min i.v.) into anesthetized rats, mean arterial pressure rose from 124 +/- 1 to 154 +/- 2 mm Hg. The peptidic Ang II antagonist saralasin lowered arterial pressure in a dose-dependent manner. The maximal decrease in pressure was similar to that observed after the Ang II infusion was discontinued. The nonpeptide Ang II antagonist, 4'-[( 2-butyl-4-chloro-5-(hydroxymethyl)-1H-imidazole-1-yl] methyl) [1,1'-biphenyl] -2-carboxylic acid (SC-48742), lowered acutely elevated arterial pressure to a level similar to that on discontinuation of the angiotensin infusion. Chronic (8 days) infusion of Ang II (20 ng/kg/min i.v.) increased mean arterial pressure from 116 +/- 3 to 164 +/- 7 mm Hg, which then decreased to 121 +/- 6 mm Hg on termination of the infusion. Saralasin (10 micrograms/kg/min, a maximally effective dose during acute angiotensin infusion) decreased mean arterial pressure from 168 +/- 7 to 141 +/- 3 mm Hg, a pressure significantly higher (p less than 0.05) than the pressure observed after the angiotensin infusion was discontinued. SC-48742 decreased mean arterial pressure from 167 +/- 7 to 127 +/- 3 mm Hg, a pressure not statistically different from the minimum pressure observed after the angiotensin infusion was terminated. The mechanism of blood pressure elevation during acute high dose or chronic low dose Ang II infusion is different, the latter having a significant neural component as measured by the response to trimethaphan. The peptidic antagonist saralasin was fully effective in lowering acute angiotensin hypertension but only partially effective during chronic hypertension.(ABSTRACT TRUNCATED AT 250 WORDS)

Hemodynamic basis for the depressor activity of zaprinast, a selective cyclic GMP phosphodiesterase inhibitor.

The present investigation was undertaken to define the hemodynamic mechanisms by which the selective cyclic GMP phosphodiesterase inhibitor zaprinast (M&B 22,948; 2-o-propoxy-phenyl-8-azapurin-6-one) lowers mean arterial pressure (MAP). Anesthetized rats were instrumented with electromagnetic or pulsed-Doppler flow probes to measure cardiac output or regional blood flow, respectively, and catheters to record MAP, left ventricular pressure and administer drugs. Zaprinast (0.33-2.0 mg.min-1.kg-1) produced dose-dependent decreases in MAP (maximum = -39 +/- 6 mm Hg) and total peripheral resistance (maximum = -45 +/- 5%) when infused i.v. The greatest reductions in regional vascular resistance were observed in the mesenteric bed, with smaller decreases in the hindquarters and renal beds. Cardiac output increased (maximum = 22 +/- 7%) during the infusion of zaprinast; however, heart rate was only minimally affected. These increases in cardiac output were still evident in animals pretreated with beta adrenergic and muscarinic receptor antagonists. At the lowest dose, zaprinast tended to increase maximal first derivative of left ventricular pressure, whereas, at higher doses maximal first derivative of left ventricular pressure was depressed or unchanged. Additional studies were performed to examine the effect of the cyclic GMP phosphodiesterase inhibitor on the pressor and vasoconstrictor activity of phenylephrine and angiotensin II. The increases in MAP and total peripheral resistance produced by these agents were attenuated markedly during continuous infusion of zaprinast (1 mg.min-1.kg-1). These data suggest that zaprinast lowers MAP by decreasing peripheral vascular resistance and by antagonizing the actions of endogenous neurohumoral vasoconstrictor systems.

Chronic atriopeptin regulation of arterial pressure in conscious hypertensive rats.

Acute coadministrations of an inhibitor of endopeptidase 24.11 (thiorphan) and a ligand (SC-46542) selective for the non-guanylate cyclase-linked atriopeptin binding sites increases urinary sodium excretion to a greater degree in conscious spontaneously hypertensive rats than in normotensive Wistar-Kyoto rats. In the present study, we examined the effects of chronic 10-day intravenous infusions of SC-46542 (des[Phe106,Gly107,Ala115,Gln116] atriopeptin-(103-126] (0.1 mg/kg/hr), thiorphan (1.5 mg/kg/hr), and atriopeptin-(103-126) (100 ng/hr) alone or in combination on direct recording of mean arterial pressure in conscious spontaneously hypertensive rats. During an 11-day time-control infusion of isotonic saline vehicle (100 microliters/hr), mean arterial pressure remained stable. Chronic infusion of atriopeptin-(103-126) decreased mean arterial pressure progressively over the first 3 days; then mean arterial pressure progressively rose to control level over the following 3 days and remained at control level for the remainder of the experiment. Similarly, coinfusions of atriopeptin-(103-126) and SC-46542 or thiorphan, SC-46542 and thiorphan, or the triple infusion of atriopeptin-(103-126), SC-46542, and thiorphan had only transient effects on mean arterial pressure during 10-day infusions. SC-46542 alone had no effect on mean arterial pressure. Similarly, thiorphan alone had no effect on mean arterial pressure except at doses that blocked the acute pressor response to angiotensin I. Chronic infusions of atriopeptin-(103-126), SC-46542, and thiorphan alone or in combination are not effective long-term treatments for hypertension in spontaneously hypertensive rats.

Atriopeptin regulation and renal function in conscious spontaneously hypertensive rats.

We examined the interaction of a non-guanylate cyclase-linked atriopeptin (AP) binding site ligand, SC-46542 (des[Phe106,Gly107,Ala115,Gln116]AP-(103-126], and an endopeptidase 24.11 inhibitor, thiorphan, on mean arterial pressure, urinary sodium excretion, urinary cyclic guanosine monophosphate (cGMP) excretion, plasma cGMP concentration, and plasma AP immunoreactivity (ir) in conscious spontaneously hypertensive rats (SHR) and normotensive Wistar-Kyoto rats (WKY). Compared to vehicle control rats, coadministration of SC-46542 and thiorphan increased urinary sodium excretion in SHR from 2.1 +/- 0.3 to 11.6 +/- 0.7 microEq/min/100 g body weight and in WKY from 1.6 +/- 0.4 to 4.4 +/- 0.4 microEq/min/100 g body weight, and increased urinary cGMP excretion in SHR from 2.7 +/- 0.5 to 79.0 +/- 17.5 pmol/min/100 g body weight and in WKY from 7.0 +/- 3.0 to 72.4 +/- 10.6 pmol/min/100 g body weight. The change in urinary sodium excretion was greater in SHR than WKY. The coadministration of SC-46542 and thiorphan had greater effects on urinary sodium excretion and urinary cGMP excretion than administration of either compound alone. Coadministration of thiorphan and SC-46542 had no effect on glomerular filtration rate or plasma cGMP concentration, suggesting that the urinary cGMP excretion response was nephrogenous. Compared to vehicle control rats, plasma APir was increased during coadministration of SC-46542 and thiorphan in both SHR (998 +/- 76 v 5.10 +/- 116 pg/mL) and WKY (775 +/- 36 v 414 +/- 36 pg/mL).(ABSTRACT TRUNCATED AT 250 WORDS)

Central and peripheral actions of a nonpeptidic angiotensin II receptor antagonist.

Nonpeptidic imidazole derivatives were recently reported to be angiotensin II receptor antagonists with acute blood pressure-lowering activity. In the present study, we characterized the angiotensin II receptor antagonist properties of one such derivative, 4'-([2-butyl-4-chloro-5-(hydroxymethyl)-1H-imidazol-1-yl]methyl)- [1,1'-biphenyl]-2-carboxylic acid (IMI). In receptor binding studies, IMI displaced bound [125I]angiotensin II from rat uterine membranes with an IC50 of 0.17 microM. In isolated rabbit aortic rings, IMI shifted the angiotensin II concentration-response curve to the right in a parallel and concentration-dependent manner. A Schild plot of these data indicated a pA2 of 7.13 +/- 0.16 and a slope of 0.94 +/- 0.06. In rat kidney slices, IMI shifted the concentration-response curve for angiotensin II-induced inhibition of renin release to the right. Antagonism of the angiotensin II pressor response by IMI was dose dependent and reversible in ganglion-blocked, anesthetized rats. The water intake and pressor responses to intracerebroventricular angiotensin II (100 pmol) were inhibited by intracerebroventricular IMI (25 or 50 nmol) in conscious Sprague-Dawley rats. Similarly, the drinking and pressor responses to intravenous angiotensin II were blocked by intravenous IMI in conscious rats. IMI alone had no effects on mean arterial pressure or drinking when administered either intravenously or intracerebroventricularly. IMI decreased mean arterial pressure throughout 5 days of infusion in spontaneously hypertensive rats. In summary, IMI was a full competitive antagonist without partial agonist activity in peripheral tissues and the central nervous system. Moreover, the chronic administration of this angiotensin II receptor antagonist was antihypertensive in spontaneously hypertensive rats.

Effects of long-term increases in plasma ANP on angiotensin II-induced hypertension.

In vitro studies have suggested that atrial natriuretic peptide (ANP) is a potent antagonist of the vasoconstrictor actions of angiotensin II (ANG II). The purpose of this study was to determine the long-term effects of physiological increases in circulating levels of ANP on arterial pressure (AP) regulation in conscious dogs (n = 9) with ANG II-induced hypertension. Infusion of ANG II at a rate of 10 ng.kg-1.min-1 for 7 days increased AP from 85 +/- 3 to 133 +/- 4 mmHg. This increase in AP was associated with an increase in total peripheral resistance (TPR) and a decrease in cardiac output (CO). After 7 days of ANG II infusion, ANP103-126 was then infused simultaneously at a rate of 20 ng.kg-1.min-1 for 7 days. Plasma levels of ANP increased from 59 +/- 15 to 285 +/- 28 pg/ml. Increasing plasma ANP levels for 7 days had no significant long-term effect on AP (133 +/- 4 vs. 125 +/- 6 mmHg), TPR, or CO. There were also no significant changes in glomerular filtration rate or sodium excretion during the 7 days of ANP infusion. These data indicate that long-term increases in circulating levels of ANP have minimal chronic hypotensive effects in dogs with ANG II hypertension. In addition, the results from this study suggest that physiological increases in plasma ANP do not exhibit long-term antagonistic effects toward the vasoconstrictor actions of ANG II.

Atrial natriuretic factor plays a significant role in body fluid homeostasis.

Atrial natriuretic factor (ANF) is a hormone with the physiological characteristics of a regulator of body fluid volume. It is potent, has a short duration of action, and responds to a physiologically relevant stimulus in a negative feedback-controlled system. It can act directly or indirectly (via inhibition of aldosterone biosynthesis) on the kidney to alter sodium transport and may regulate fluid distribution within the extracellular space. The peptide circulates at low (nanomolar) levels, and recent studies with renal inner medullary cells document relevant receptor binding and second messenger activation in this concentration range. In vivo data support a direct action on the kidney to enhance natriuresis, and blockade of a primary catabolic pathway for ANF within the kidney results in augmented natriuresis at concurrent endogenous peptide concentrations. Long-term, low dose infusion directly into the renal artery of conscious dogs supports a physiological action of ANF to promote urinary sodium excretion. Nevertheless, under certain circumstances, natriuresis does not occur even at high circulating levels of ANF. Apparently other factors such as renal perfusion pressure, volume status, and renal nerve activity are important in determining the natriuretic response to a given level of peptide. We hypothesize that the role played by ANF in volume regulation is highly complex, and the kidney responds with increased sodium excretion only when a constellation of variables is appropriately arrayed. That is, ANF is a necessary, but not sufficient, condition to induce natriuresis.

Thiorphan, an inhibitor of endopeptidase 24.11, potentiates the natriuretic activity of atrial natriuretic peptide.

To evaluate the role of endopeptidase 24.11 in metabolism of atrial natriuretic peptide (ANP) in vivo, we examined the effect of thiorphan, an inhibitor of this enzyme, on plasma ANP concentrations and the cardiovascular and renal actions of ANP(99-126). Thiorphan alone produced a modest increase in urinary sodium excretion in anesthetized rats; however, urine flow, arterial pressure, and basal plasma ANP concentrations were unchanged. When administered during an infusion of ANP(99-126) (330 ng/kg/min i.v.), thiorphan increased the plasma concentration of ANP and enhanced the diuretic and natriuretic activity of this hormone. The effects on urine flow and urinary sodium excretion were most pronounced immediately after the inhibitor was administered and later diminished in magnitude. Thiorphan did not alter the depressor activity of exogenous ANP(99-126). These data suggest that endopeptidase 24.11 participates in metabolism of ANP(99-126) and that thiorphan potentiates the renal actions of this hormone by inhibiting its degradation.

Expression, purification, and in vivo activity of atrial natriuretic factor prohormone produced in Escherichia coli.

Atrial muscles of the heart are known to produce polypeptide hormones called atrial natriuretic factors (ANF) which have potent diuretic and hypotensive action. These hormones are synthesized as a larger protein precursor called pro atrial natriuretic factor or proANF which contains the biologically active ANF sequences at its C-terminus. Rat proANF (representing amino acids -1 to 128 of the coding sequence) was expressed in a soluble form in Escherichia coli. A simple purification procedure was developed which consists of boiling E. coli cell extracts in 1 M acetic acid and subjecting the supernatant to reversed-phase HPLC. The effect of intravenous administration of the purified recombinant proANF on mean arterial blood pressure was examined. The displacement dose-response curves obtained demonstrated that proANF exhibits similar, albeit less potent, physiological activity than ANF.

Role of atrial peptide in the natriuresis and diuresis that follows relief of obstruction in rat.

The present studies demonstrate that endogenous levels of atrial peptide are significantly elevated in the plasma of rats with bilateral ureteral obstruction (BUO) compared with control rats or rats with unilateral ureteral obstruction. The contribution of endogenous atrial peptide to the natriuresis and diuresis that follows release of BUO was examined by the intravenous infusion of heparin with or without the exogenous administration of atrial peptide. Infusion of heparin, which binds atrial peptide and interferes with its biological effect, decreased the natriuresis and diuresis observed after release of BUO. Heparin administration also markedly blunted the natriuresis and diuresis observed after exogenous administration of atrial peptide following release of BUO in rats. In contrast, heparin administration did not decrease the natriuresis and diuresis seen in the experimental kidney after relief of unilateral ureteral obstruction. The finding of increased plasma levels of atrial peptide in rats with bilateral ureteral obstruction together with the decrease in diuresis and natriuresis observed with the administration of heparin after release of BUO in these animals indicate that endogenous levels of atrial peptide contribute to the natriuresis and diuresis that occur after release of BUO in rats.

Interaction of non-guanylate cyclase-linked atriopeptin receptor ligand and endopeptidase inhibitor in conscious rats.

We examined the interaction of SC-46542 [des(Phe106, Gly107, Ala115, Gln116)-AP(103-126)], a non-guanylate cyclase-linked atriopeptin (AP) binding site ligand, with thiorphan, an inhibitor of endopeptidase 24.11, on mean arterial pressure, urine flow, urinary sodium excretion and plasma AP immunoreactivity in conscious rats. The coadministration of SC-46542 (16 micrograms/kg/min) and thiorphan (30 mg/kg i.v. bolus) produced a greater diuresis and natriuresis (but had no effect on mean arterial pressure) than administration of either compound alone; plasma APir increased 2-fold during coadministration of SC-46542 and thiorphan (from 325 +/- 46 to 676 +/- 86 pg/ml). Administration of SC-46542 or thiorphan alone had small or no effects on mean arterial pressure, urine flow, urinary sodium excretion or plasma APir. Converting enzyme inhibition did not contribute to the effects of thiorphan since coadministration of captopril plus SC-46542 produced effects similar to SC-46542 alone. When a near threshold infusion of AP(103-126) was combined with the coadministration of SC-46542 and thiorphan, there was a potentiation of the depressor, diuretic and natriuretic responses. Neither SC-46542 nor thiorphan alone had these effects. SC-46542 potentiated the depressor but not diuretic or natriuretic responses to low dose AP(103-126) infusion; thiorphan had little or no effect on the responses to low dose AP(103-126). We conclude that blockade of non-guanylate cyclase-linked AP binding sites with SC-46542 combined with inhibition of AP degradation by endopeptidase 24.11 with thiorphan increases diuresis and natriuresis more than inhibition of either system alone.(ABSTRACT TRUNCATED AT 250 WORDS)