Atrial natriuretic factor release by angiotensin II in the conscious rat

Phospholipase C signaling tonically represses basal atrial natriuretic factor secretion from the atria of the heart

The cardiac hormone atrial natriuretic factor (ANF or ANP) plays significant, well-established roles in a large number of physiological and pathophysiological processes, including water and electrolyte balance, blood pressure regulation, and cardiovascular growth. Understanding the regulation of its production and secretion by atrial cardiomyocytes is incomplete. We have previously established a significant role of G(i/o) protein signaling in modulating ANF secretion as promoted by stretch of the atrial myocardium. In the present study, we investigated the role of G(q) protein signaling and its relationship to G(i/o) protein signaling using pharmacological manipulation of proximal effectors of G(αq) in an ex vivo model of spontaneously beating rat atria. Phospholipase C (PLC) and protein kinase C (PKC) inhibitors dramatically increased basal secretion of ANF. Furthermore, although atrial wall stretch is a potent stimulus for secretion, stretch unexpectedly reduced ANF secretion to basal levels under PLC and PKC inhibitory conditions. Inhibition of the inositol triphosphate receptor did not appear to affect basal secretion but dose-dependently blocked stretch-secretion coupling. The results obtained demonstrate that the PLC and PKC signaling cascades play important albeit unexpected roles in the regulation of basal and stimulated ANF secretion and suggest interplay between the G(q) and G(i/o) protein signaling pathways.

Endogenous angiotensin II suppresses stretch-induced ANP secretion via AT1 receptor pathway

Angiotensin II (Ang II) is released by stretch of cardiac myocytes and has paracrine and autocrine effects on cardiac myocytes and fibroblasts. However, the direct effect of Ang II on the secretion of atrial natriuretic peptide (ANP) is unclear. The aim of the present study is to test whether Ang II affects stretch-induced ANP secretion. The isolated perfused beating atria were used from control and two-kidney one-clip hypertensive (2K1C) rats. The volume load was achieved by elevating the height of outflow catheter connected with isolated atria from 5cmH(2)O to 7.5cmH(2)O. Atrial stretch by volume load caused increases in atrial contractility by 60% and in ANP secretion by 100%. Ang II suppressed stretch-induced ANP secretion and tended to increase atrial contractility whereas losartan stimulated stretch-induced ANP secretion. Neither PD123319 nor A779 had direct effect on stretch-induced ANP secretion. The suppressive effect of Ang II on stretch-induced ANP secretion was blocked by the pretreatment of losartan but not by the pretreatment of PD123319 or A779. In hypertrophied atria from 2K1C rats, stretch-induced ANP concentration attenuated and atrial contractility augmented. The response of stretch-induced ANP secretion to Ang II and losartan augmented. The expression of AT1 receptor protein and mRNA increased but AT2 and Mas receptor mRNA did not change in 2K1C rat atria. Therefore, we suggest that Ang II generated endogenously by atrial stretch suppresses stretch-induced ANP secretion through the AT1 receptor and alteration of Ang II effect in 2K1C rat may be due to upregulation of AT1 receptor.

Alpha-adrenergic agonists inhibit the dipsogenic effect of angiotensin II by their stimulation of atrial natriuretic peptide release

Angiotensin II (ANG-II) and atrial natriuretic peptide (ANP) have opposing actions on water and salt intake and excretion. Within the brain ANP inhibits drinking induced by ANG-II and blocks dehydration-induced drinking known to be caused by release of ANG-II. Alpha-adrenergic agonists are known to release ANP and antagonize ANG II-induced drinking. We examined the hypothesis that alpha agonists block ANG-II-induced drinking by stimulating the release of ANP from ANP-secreting neurons (ANPergic neurons) within the brain that inhibit the effector neurons stimulated by ANG-II to induce drinking. Injection of ANG-II (12.5 ng) into the anteroventral region of the third ventricle (AV3V) at the effective dose to increase water intake increased plasma ANP concentrations (P<0.01) within 5 min. As described before, previous injection of phenylephrine (an alpha(1)-adrenergic agonist) or clonidine (an alpha(2)-adrenergic agonist) into the AV3V region significantly reduced ANG-II-induced water intake. Their injection also induced a significant increase in plasma ANP concentration and in ANP content in the olfactory bulb (OB), AV3V, medial basal hypothalamus (MBH) and median eminence (ME). These results suggest that the inhibitory effect of both alpha-adrenergic agonists on ANG-II-induced water intake can be explained, at least in part, by the increase in ANP content and presumed release from these neural structures. The increased release of ANP from the axons of neurons terminating on the effector neurons of the drinking response by stimulation of ANP receptors would inhibit the stimulatory response evoked by the action of ANG-II on its receptors on these same effector neurons.

Development of hypertension induced by subpressor infusion of angiotensin II: role of sensory nerves

Long-term administration of a subpressor dose of angiotensin II (Ang II) leads to pressor hyperresponsiveness and slow development of hypertension. Our preliminary data show that mRNA expression for calcitonin-gene related peptide in dorsal root ganglia was significantly increased by subpressor infusion of Ang II. To determine the role of sensory nerves in the development of hypertension induced by subpressor infusion of Ang II, newborn Wistar rats were given 50 mg/kg SC capsaicin on the 1st and 2nd days of life. After the weaning period, male rats were divided into 4 groups and subjected to the following treatments for 2 weeks: capsaicin+Ang II (150 ng. kg(-1). min(-1) SC by osmotic pumps, CAP-AII), capsaicin+vehicle (CAP), control+Ang II (CON-AII), and control+vehicle (CON). The results show that mean arterial pressure was significantly elevated in both Ang II-infused rats compared with non-Ang II-treated rats (P<0.05), and it was higher in CAP-AII than in CON-AII rats (P<0.05). The 24-hour urinary and sodium excretions were lower in CAP-AII than in CON-AII, CAP, and CON rats (P<0.05). These data demonstrated that sensory denervation exacerbates the development of hypertension and impairs renal excretory function when a subpressor dose of Ang II is given. These results indicate that activation of sensory nerves, either by Ang II or by other hormonal or hemodynamic factors, plays a compensatory role in promoting urine and sodium excretion and attenuating elevated blood pressure initiated by Ang II.

Regulation of natriuretic peptide secretion by the heart

Secreted by the heart, more specifically by atrial cardiomyocytes under normal conditions but also by ventricular myocytes during cardiac hypertrophy, natriuretic peptides are now considered important hormones in the control of blood pressure and salt and water excretion. Studies on natriuretic peptide secretagogues and their mechanisms of action have been complicated by hemodynamic changes and contractions to which the atria are constantly subjected. It now appears that atrial stretch through mechano-sensitive ion channels, adrenergic stimulation via alpha 1A-adrenergic receptors, and endothelin via its ETA receptor subtype are major triggering agents of natriuretic peptide release. With several other stimuli, such as angiotensin II and beta-adrenergic agents, modulation of natriuretic peptide release appears to be linked to local generation of prostaglandins. In all cases, intracellular calcium homeostasis, controlled by several ion channels, is considered a key element in the regulation of natriuretic peptide secretion.

Mechanisms contributing to angiotensin II regulation of body weight

Previous studies in our laboratory have implicated adipose tissue as a potential site for local angiotensin II (ANG II) synthesis. However, functions of ANG II in adipose tissue and the impact of ANG II on body weight regulation are not well defined. To study the effect of ANG II on body weight, a chronic ANG II infusion model was used. In study 1, a low dose of ANG II (175 ng.kg-1.min-1) was infused into rats for 14 days. Plasma ANG II levels were not elevated after 14 days of infusion. ANG II-infused rats did not gain weight over the 14-day protocol and exhibited a lower body weight than controls on day 8. Food intake was not altered, but water intake was increased in ANG II-infused rats. Blood pressure gradually increased to significantly elevated levels by day 14. Thermal infrared imaging demonstrated an increase in abdominal surface temperature. Measurement of organ mass demonstrated site-specific reductions in white adipose tissue mass after ANG II infusion. In study 2, the dose-response relationship for ANG II infusion (200, 350, and 500 ng.kg-1.min-1) was determined. Body weight (decrease), blood pressure (increase), white adipose mass (decrease), plasma ANG II levels (increase), and plasma leptin levels (decrease) were altered in a dose-related manner after ANG II infusion. In study 3, the effect of ANG II infusion (350 ng.kg-1.min-1) was examined in rats treated with the vasodilator hydralazine. Hydralazine treatment normalized blood pressure in ANG II-infused rats. The effect of ANG II to decrease body weight was augmented in hydralazine-treated rats. These results demonstrate that low levels of ANG II infusion regulate body weight through mechanisms related to increased peripheral metabolism and independent of elevations in blood pressure.

Angiotensin II-induced phosphoinositide production and atrial natriuretic peptide release in rat atrial tissue

The effect of angiotensin II (Ang II) on inositol phosphate (IP) production and atrial natriuretic peptide (ANP) release was studied in sliced rat atrial tissue. The ability of Ang II (10(-7) M) to stimulate IP accumulation was detected after 1 min of incubation, and the maximal increase was observed at 5 min. In (2-3H) inositol-labeled atrial tissue, Ang II induced the formation of (2-3H) inositol monophosphate (IP1) in a dose-dependent manner. The effect of Ang II (10(-7) M) on IP1 was prevented by losartan (10(-7) M) but was not affected by PD123319 (10(-7) M). Similar effects were observed on Ang II-induced ANP release in the presence of these antagonists. The mechanism of ANP liberation induced by this peptide was independent of cyclic adenosine monophosphate (cAMP) and regulated by nitric oxide (NO). The role of Ca2+ in the effect of Ang II was tested by 1,2-bis (o-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid tetra (acetoxymethyl) ester (BAPTA-AM; 10(-5) M), a chelator of intracellular Ca2+ that prevented the release of ANP by Ang II stimulation. We concluded that Ang II induced IP production and ANP release through AT1 receptors. Stimulation of ANP release by Ang II was dependent on intracellular Ca2+.

Combined inhibition of endothelin and angiotensin II receptors blocks volume load-induced cardiac hormone release

Volume expansion has been shown to increase plasma atrial natriuretic peptide (ANP) levels, but the precise role of paracrine and autocrine factors in stretch-induced cardiac hormone release is not clear. In the present study, we report the effects of endothelin (ET) and angiotensin receptor (AT receptor) antagonists on baseline and atrial stretch-induced immunoreactive ANP (IR-ANP) and immunoreactive N-terminal ANP (IR-NT-ANP) release in vivo by using BQ-123 (ETA receptor antagonist), bosentan (ETA and ETB receptor antagonist), and losartan (AT1 receptor antagonist). Intravenous administration of BQ-123 had no significant effect on baseline hemodynamics in conscious rats, whereas bosentan (10 mg/kg) and losartan (10 mg/kg) decreased slightly (4 to 7 mm Hg, P < .05 to .001) the mean arterial pressure. Both the ETA receptor antagonist BQ-123 and ETA/ETB receptor antagonist bosentan decreased plasma ANP and NT-ANP responses to volume load (P < .05 to .001), whereas the AT1 receptor antagonist losartan had no significant effect on this response. The relative increase in plasma IR-ANP corresponding to a 3 mm Hg increase in right atrial pressure was 2.7-fold in the vehicle-treated group. BQ-123 (0.3 and 1.0 mg/kg) decreased this response 2.5- and 2.1-fold (P < .05); bosentan (3 and 10 mg/kg), 1.7-fold (P < .001) and 1.9-fold (P < .05); and bosentan (10 mg/kg)+losartan (10 mg/kg), 1.6-fold (P < .001). The responses in plasma IR-NT-ANP decreased simultaneously. These results indicate that combined inhibition of ETA/B and AT1 receptors almost completely blocks ANP response to acute volume load. Therefore, our study shows that endogenous paracrine and/or autocrine factors liberated in response to atrial wall stretch rather than myocyte stretch itself are responsible for the activation of ANP peptide secretion in response to acute volume load. Our results also show that ETA receptors are more important in the regulation of mechanical stretch-induced changes in cardiac hormone secretion than AT1 receptors.

Cellular mechanism of angiotensin II-induced atrial natriuretic peptide release in rat right atrial tissue

This study presents an investigation of the mechanism of angiotensin II (Ang II)-induced atrial natriuretic peptide (ANP) release in superfused sliced right atria of rats. Ang II (0.1 microM) enhanced ANP release by 49%. This phenomenon was significantly blocked by (Sara1-Ileu 8) Ang II (1 microM) and losartan (0.1 microM). The use of neomycin (100 microM), a phospholipase-C inhibitor completely suppressed the effect of Ang II on ANP increase. To elucidate the intracellular mechanism of ANP released by Ang II, the role of protein kinase C (PKC) was determined by 1-(5-isoquinolinesulfonyl)-2-methylpiperazine (H-7) and phorbol ester : 4-beta-phorbol 12-myristate-13-acetate (PMA). We observed that PMA (0.1 microM) stimulated ANP release whereas H-7 (10 microM), an inhibitor of PKC in the presence of Ang II, prevented ANP increase. The role of calcium was also evaluated with 8-(N-N-diethylamino)-ocytyl-3,4,5, trimethoxy-benzoate (TMB-8) (10 microM) and N-(6-aminohexyl)-5-chloro-1-naphtalene sulfonamide (W-7) (10 microM), which completely inhibited ANP release by Ang II. Pre-treatment with diltiazem (10 microM), an antagonist of the Ca++ channel, did not prevent ANP increase due to Ang II, but A23187 (5 microM) enhanced ANP release by Ang II. These results suggest that PKC and intracellular calcium play an important role in ANP release under the influence of Ang II in rat atrial tissue.

Twenty-four-hour pattern of atrial natriuretic peptide in heart transplantation: evidence for lack of circadian rhythm. Temporal inter-relationships with plasma renin activity, aldosterone and cortisol

We have investigated the circadian rhythm of plasma atrial natriuretic peptide in 13 stable output heart transplanted patients, all without evidence of histological rejection and cardiac impairment, following antirejection therapy with Cyclosporine, Azathioprine and Prednisone. The 24-h pattern of plasma renin activity, plasma aldosterone and plasma cortisol has been studied as well. All the investigated variables were assayed six times over the 24-h span. The circadian time-qualified data were analyzed by ANOVA and Cosinor method. The 24-h mean levels of atrial natriuretic peptide, plasma renin activity and plasma aldosterone are significantly increased, while the concentrations of plasma cortisol are reduced in the heart transplanted recipients. ANOVA detected a significant within-day variability of all these humoral variables only in healthy subjects. A statistically significant circadian rhythm was validated by Cosinor procedure for all the investigated molecules in healthy subjects but not in heart transplanted patients. In our opinion, the increase of atrial natriuretic peptide is a counterregulatory mechanism aimed to compensate the cyclosporine-mediated activation of the renin-angiotensin-aldosterone system. The disappearance of the plasma renin activity, aldosterone and atrial natriuretic peptide circadian rhythm can be ascribed to the constant activation of the renin-angiotensin-aldosterone system. The hypocortisolism is due, in our opinion, both to glucocorticoid therapy and increase of plasma ANP concentration.

Release of atrial natriuretic factor

The cardiac atria synthesize and store a hormone termed atrial natriuretic factor (ANF). ANF is released into the systemic circulation, and the circulating 28 amino acid peptide can be measured by radioimmunoassay. The hormone participates in body fluid homeostasis through its effect on renal sodium excretion and by inducing a shift of circulating fluid to the interstitial space. Release of ANF is mainly regulated by mechanical changes in the left and right atrial wall. It has been demonstrated that ANF release is related to changes in atrial wall tension occurring during each atrial cycle, and therefore, release of ANF will increase with increasing heart rate. Not only the increase in wall tension during passive atrial distension (v wave), but also the increase in tension during atrial systole (a wave) are determinants of ANF release. The mechanochemical transducer is most likely located in the atrial myocytes, but its nature is unknown. There is no evidence to suggest that efferent cardiac nerves are essential in the regulation of ANF release. Humoral factors have been suggested as regulators of ANF release, particularly catecholamines and angiotensin II. A receptor-mediated direct stimulatory effect of alpha-adrenergic stimulation and an inhibitory effect of beta-adrenergic stimulation have been demonstrated, but these direct effects are small compared to the effect of changes in atrial wall tension. Circulating catecholamines and angiotensin II stimulate ANF release mainly through their haemodynamic effects.

Inhibition of the aldosterone-suppressant activity of atrial natriuretic factor by progesterone and pregnancy in rats

Angiotensin II (AII) caused concentration-dependent increase in aldosterone secretion by dispersed zona glomerulosa cells from non-ovariectomized (non-OVX) and ovariectomized (OVX) rats treated with the vehicle (peanut oil), beta-estradiol (0.1 mg/kg/d x 3) or progesterone (2 mg/kg/d x 3); this effect of AII was greater on cells from progesterone- than from estrogen-treated animals. In contrast, atrial natriuretic factor (ANF) was 100- to 1,000-fold less effective in suppressing AII-stimulated aldosterone production by cells from progesterone-treated (both non-OVX and OVX) and pregnant (17-20 day) rats than by cells from nonpregnant controls and estrogen-treated animals. To our knowledge, this is the first demonstration of an inhibition of an important action of ANF by another hormone and our data suggest that increased circulating levels of progesterone during pregnancy produce a relative refractoriness to the aldosterone-suppressant activity of ANF, which favors fluid/salt expansion.

Angiotensin II directly stimulates release of atrial natriuretic factor in isolated rabbit hearts

BACKGROUND

Previous studies have shown that infusion of angiotensin II (Ang II) increases plasma concentrations of atrial natriuretic factor (ANF) in vivo. This phenomenon has been considered secondary to the effects of Ang II on cardiac and systemic hemodynamics. The present study was designed to assess whether Ang II may exert a direct stimulatory effect on ANF release from the heart independent of changes in hemodynamics.

METHODS AND RESULTS

Isolated rabbit hearts were perfused in the Langendorff mode. Heart rate, coronary flow, and atrial and left ventricular (LV) volumes were kept constant. After stabilization, Ang II was infused intracoronary at increasing doses (10(-11) to 10(-8) M) in nine hearts and at a single dose of 10(-10) M in 10 hearts. Each infusion lasted for 5 minutes and was followed by a 10-minute washout period. Four hearts received vehicle alone for 80 minutes. Ang II induced a dose-dependent increase in coronary perfusion pressure and in LV developed pressure. ANF release, measured by radioimmunoassay on the extracts of the cardiac effluent, also increased during Ang II infusion and returned to the basal values during the 10-minute washout period. In the control group, coronary perfusion pressure, LV developed pressure, and LV end-diastolic pressure did not change appreciably over the observation period, whereas ANF release progressively decreased during perfusion.

CONCLUSIONS

Ang II can directly stimulate cardiac release of ANF in isolated rabbit hearts independently of changes in hemodynamics.

Effects of arginine vasopressin, angiotensin II and endothelin-1 on the release of brain natriuretic peptide in vivo and in vitro

1. The stimulatory effects of the vasoactive peptides arginine vasopressin (AVP), angiotensin II (AII) and endothelin-1 (ET-1) on the release of brain natriuretic peptide (BNP) were investigated in anaesthetized rats and in cultured rat atrial and ventricular cardiocytes. 2. A bolus injection of AVP induced a dose-dependent increase in plasma immunoreactive (ir)-BNP concentration in rats. AII induced a rapid and transient elevation in the ir-BNP level, while the increase produced by ET-1 was long-lasting. The elevation of the plasma ir-BNP concentration after stimulation by these three vasoconstrictors appeared to be paralleled by the elevation in mean blood pressure. 3. In the in vitro study, the rat atrial and ventricular cardiocytes both secreted ir-BNP into the medium in a time-dependent manner. ET-1 clearly stimulated the secretion of ir-BNP in both atrial and ventricular cardiocytes. In contrast, AVP and AII had no stimulatory effect in vitro. 4. Reverse-phase high performance liquid chromatography of the rat plasma and culture medium revealed a single major ir-BNP component that corresponded to synthetic rat BNP-45. 5. These observations indicate that AVP, AII and ET-1 stimulate the release of ir-BNP (probably rat BNP-45) through a change in blood pressure. In addition, ET-1 may also induce ir-BNP release through direct stimulation. As a cardiac hormone secreted from ventricles as well as atria, rat BNP may play a role in the regulation of blood pressure against the pressor effects of AVP, AII and ET-1.

Atrial natriuretic factor release during volume expansion in the spontaneously hypertensive rat--effect of long-term hydralazine treatment

We tested the ability of spontaneously hypertensive rats (SHR) to release atrial natriuretic factor (ANF) during acute volume loading and its relationship to right and left atrial pressures. The effect of decreasing cardiac afterload by hypotensive treatment was also investigated. Fourteen to 15-week-old SHR and Wistar-Kyoto (WKY) rats were treated with hydralazine (12 mg/kg.day p.o.) for a period of 4 weeks. Untreated rats served as controls. The systolic blood pressure (BP) of SHR decreased to normotensive levels and cardiac hypertrophy was also reduced. Isotonic, iso-oncotic volume expansion (VE) was performed 3 times as 10% increments and at 15-min intervals. Despite greater changes in left-ventricular end-diastolic pressures (LVEDP) and to a lesser extent in central venous pressure (CVP) in SHR controls, the increases in plasma ANF (N-terminal concentrations) induced by VE were of a similar magnitude in both SHR and WKY control rats. The LVEDP and ANF C-terminal elevations were of a lower magnitude in treated SHR. Auricular ANF concentrations, measured at the end of VE, were lower in the left and higher in the right atrium in SHR versus WKY. It is concluded that despite a lower distensibility of their left atrium, ANF release is not impaired in SHR upon a VE. This adequate ANF release is obtained through higher increases in atrial pressures.

Atrial natriuretic factor is unlikely to be involved in the reduced aldosterone production in the Brattleboro rat

We have previously reported that basal and stimulated aldosterone production in Brattleboro rat (DI) lacking hypothalamic arginine vasopressin is lower than that observed in control Long-Evans rat (LE). In the present study, we investigated the secretion under various experimental conditions, adrenal binding sites, and the aldosterone-inhibiting effect of atrial natriuretic factor (ANF). In the conscious resting state, the plasma ANF concentration was similar between LE and DI rats. Pentobarbital anaesthesia (5 mg/100 g body wt.) reduced the plasma ANF concentration equally in both groups, with or without captopril pretreatment. Morphine (10 mg/100 g body wt.) increased ANF secretion dramatically and equally in the two groups of pentobarbital anaesthetized (2 mg/100 g body wt.) rats. In dexamethasone pretreated-pentobarbital anaesthetized rats, a concurrent i.v. ANF infusion (50 ng/min) did not change significantly the corticosterone response to ACTH (1-24) (1 mI.U./100 g body wt.) but steeply depressed ACTH-induced aldosterone production to a similar extent between DI and LE rats. A single class of adrenal ANF receptor sites was found with a similarity in high affinity and maximum binding capacity between the two groups of rats. Taken together, these results suggest that the reduced aldosterone production by Brattleboro rat adrenals is unlikely to be related to the inhibitory effect of ANF.

Angiotensin II-induced atrial natriuretic factor release in dogs is not related to hemodynamic responses

Angiotensin II (Ang II) and atrial natriuretic factor (ANF) appear to act as functional antagonists in the regulation of fluid and electrolyte homeostasis and blood pressure. To further define the relations between these hormones in vivo, we investigated the effect of low doses of Ang II (1-10 ng/kg/min) on plasma ANF levels. We also evaluated the influence of ANF release on the renal and hormonal responses to ANG II. Studies were performed in anesthetized and conscious instrumented dogs during sustained saline load and converting enzyme inhibition. In the anesthetized dogs, Ang II significantly increased plasma ANF levels and ANF arteriovenous difference without changing either atrial pressures or hematocrit. In both conscious and anesthetized dogs, ANF increases were not correlated with blood pressure responses to Ang II and did not occur in control groups when Ang II was replaced by vehicle. Ang II-induced sodium retention and stimulation of aldosterone production were attenuated, and renin suppression was enhanced in dogs having the largest changes in plasma ANF in response to converting enzyme inhibition or Ang II. These results demonstrate that in volume-replete dogs Ang II can promote ANF release independently of changes in atrial pressures or systemic hemodynamics, suggesting that Ang II may exert a significant modulatory effect on ANF secretion. The results also show significant relations between ANF and renal and adrenal responses to Ang II, which may suggest that, in turn, endogenous ANF modulates the effects of Ang II.

Endothelin-induced vasoconstriction and release of atrial natriuretic peptides in the rat

Endothelin-1 (ET-1) was given to male Sprague-Dawley rats in i.v. bolus injections to evaluate its effects on blood pressure and the release of atrial natriuretic peptides (ANP). In awake rats ET-1 (0.3, 1 and 3 nmol kg-1 body wt) transiently reduced mean arterial pressure (MAP) and increased heart rate (HR), followed by a prolonged increase in MAP. The magnitude of these changes and the duration of the increase in MAP were dose-related. The increase in MAP was completely blocked by verapamil, reversed by sodium nitroprusside, slightly reduced by rat atrial natriuretic factor (103-126) and unaffected by saralasin. The initial fall in MAP was also unaltered by these agents. In all groups HR changes were mirror-images of MAP. In anaesthetized rats ET-1 (1 nmol kg-1 body wt) induced a sustained release of ANP. Right atrial pressure increased transiently and then fell below baseline. When the increase in MAP was blocked with sodium nitroprusside, ET-1 still produced an increase in ANP. In conclusion we find that repeated i.v. administration of ET-1 induces immediate vasodilatation, without signs of tachyphylaxis, followed by long-lasting severe vasoconstriction. Baroreceptor function seems to be unchanged. ET-1 appears to induce ANP release by a direct action on atrial myocytes, independent of right atrial and systemic arterial pressure. We hypothesize that endothelin may be a mediator of stretch-induced release of ANP.

Levels of atrial natriuretic peptide are not always consistent with atrial pressure: is there alternative regulation as evidenced in Gordon's and Bartter's syndromes?

1. In Bartter's syndrome, atrial pressures were low, consistent with volume contraction, while atrial natriuretic peptide (ANP) levels were unexpectedly elevated. Infusion of normal saline increased both right atrial pressure (RAP) and ANP levels, while administration of prostaglandin inhibitors raised RAP, probably due to volume expansion, but ANP levels fell paradoxically. 2. In Gordon's syndrome, atrial pressures were unexpectedly low or normal despite volume expansion, while ANP levels were normal. Pressor infusions of angiotensin II either raised right and left atrial pressures (LAP) without increasing ANP, or increased ANP without increasing atrial pressures. 3. In these two syndromes, atrial pressures and ANP levels were poorly correlated, leading to the proposal that other regulators of ANP may be important.