Facilitation and inhibition by endothelin-1 of adrenal catecholamine secretion in anesthetized dogs

Regulation of blood pressure and salt homeostasis by endothelin

Endothelin (ET) peptides and their receptors are intimately involved in the physiological control of systemic blood pressure and body Na homeostasis, exerting these effects through alterations in a host of circulating and local factors. Hormonal systems affected by ET include natriuretic peptides, aldosterone, catecholamines, and angiotensin. ET also directly regulates cardiac output, central and peripheral nervous system activity, renal Na and water excretion, systemic vascular resistance, and venous capacitance. ET regulation of these systems is often complex, sometimes involving opposing actions depending on which receptor isoform is activated, which cells are affected, and what other prevailing factors exist. A detailed understanding of this system is important; disordered regulation of the ET system is strongly associated with hypertension and dysregulated extracellular fluid volume homeostasis. In addition, ET receptor antagonists are being increasingly used for the treatment of a variety of diseases; while demonstrating benefit, these agents also have adverse effects on fluid retention that may substantially limit their clinical utility. This review provides a detailed analysis of how the ET system is involved in the control of blood pressure and Na homeostasis, focusing primarily on physiological regulation with some discussion of the role of the ET system in hypertension.

Possible autocrine regulation of chromaffin cell activity in adrenal glands of the frog by endothelin-1-induced serotonin release

The aim of the present study was the demonstration of mechanisms of regulation of activity of chromaffin cells in the adrenal gland of Rana ridibunda (Anura-Amphibia). Previous studies have shown that endothelin-1 is an important factor for the maintenance of adrenal gland function. On the basis of these findings, frogs were injected with [Ala(1,3,11,15)]-endothelin-1 (0.025 mg/0.2 ml), which is a selective agonist of the endothelin B receptor, whereas control animals were injected with Ringer solution (0.2 ml). The adrenal glands were removed at 5, 20, and 60 min after injection and fixed, embedded in paraffin wax and stained by histological and immunohistochemical means, applied on adjacent 4-microm-thick sections. Sections were stained with hematoxylin and eosin for overall tissue analysis and, in parallel, serotonin was localized using the streptavidin-biotin complex technique while dopamine beta-hydroxylase and serotonin 2A receptors were shown by the peroxidase-antiperoxidase (PAP)-3,3'-diaminobenzidine tetrachloride (DAB) method. After injection of [Ala(1,3,11,15)]-endothelin-1, chromaffin cells secreted serotonin and synthesized dopamine beta-hydroxylase. In conclusion, these findings suggest that [Ala(1,3,11,15)]-endothelin-1 stimulates chromaffin cell activity in frog adrenal glands. Moreover, the presence of serotonin 2A receptors in chromaffin cells indicates that these cells are also targets for serotonin and that there is an autocrine signaling pathway in chromaffin cells. This is the first report providing data on the effects of endothelin-1 on chromaffin cells in frog adrenal glands.

The effects of candesartan and ramipril on adrenal catecholamine release in anaesthetized dogs

We have investigated the effects of the angiotensin II type 1 receptor antagonist candesartan, and the angiotensin II converting enzyme inhibitor ramipril, on catecholamine release from the anaesthetized dog's adrenal gland. These drugs were given systemically in low and high doses. The gland was stimulated electrically (0.5-12 Hz) and by angiotensin II infusion (40 ng/kg/min). Electrical stimulation resulted in frequency-dependent increases in catecholamine release. Candesartan (0.8, 4.0 mg/kg) and ramipril (0.125, 0.625 mg/kg) increased basal catecholamine release along with decreases in blood pressure. Both drugs diminished direct nerve stimulation-induced catecholamine release. When both drugs were combined, their inhibitory effect was slightly enhanced. Candesartan blocked catecholamine release induced by angiotensin II. Ramipril was not tested in this respect. The percentage of noradrenaline released during electrical stimulation of the gland remained constant and ranged from 14% to 22%. Both drugs appear to act by blocking local modulation of catecholamine release by the chromaffin cells.

Effect of angiotensin II and endothelin-1 receptor blockade on the haemodynamic and hormonal changes after acute blood loss and after retransfusion in conscious dogs

AIM

This study investigates angiotensin II and endothelin-1 mediated mechanisms involved in the haemodynamic, hormonal, and renal response towards acute hypotensive haemorrhage.

METHODS

Conscious dogs were pre-treated with angiotensin II type 1 (AT1) and/or endothelin-A (ETA) receptor blockers or not. Protocol 1: After a 60-min baseline period, 25% of the dog's blood was rapidly withdrawn. The blood was retransfused 60 min later and data recorded for another hour. Protocol 2: Likewise, but preceded by AT1 blockade with i.v. Losartan. Protocol 3: Likewise, but preceded by ETA blockade with i.v. ABT-627. Protocol 4: Likewise, but with combined AT1 plus ETA blockade.

RESULTS

In controls, haemorrhage decreased mean arterial pressure (MAP) by approximately 25%, cardiac output by approximately 40%, and urine volume by approximately 60%, increased angiotensin II (3.1-fold), endothelin-1 (1.13-fold), vasopressin (116-fold), and adrenaline concentrations (3.2-fold). Glomerular filtration rate and noradrenaline concentrations remained unchanged. During AT1 blockade, the MAP decrease was exaggerated (-40%) and glomerular filtration rate fell. During ETA blockade, noradrenaline increased after haemorrhage instead of adrenaline, and the MAP recovery after retransfusion was blunted. The decrease in cardiac output was similar in all protocols.

CONCLUSIONS

Angiotensin II is more important than endothelin-1 for the short-term regulation of MAP and glomerular filtration rate after haemorrhage, whereas endothelin-1 seems necessary for complete MAP recovery after retransfusion. After haemorrhage, endothelin-1 seems to facilitate adrenaline release and to blunt noradrenaline release. Haemorrhage-induced compensatory mechanisms maintain blood flow more effectively than blood pressure, as the decrease in cardiac output--but not MAP--was similar in all protocols.

Elevation of plasmatic endothelin in patients with heart failure

BACKGROUND

Endothelin 1 is an autocrine and paracrine factor with vasoconstrictive, mitogenic, and inotropic activities in vascular and cardiac muscles. Its elevation has been reported in patients with chronic heart failure and its production may be conditioned by activation of other neurohumoral factors that are stimulated by the disease.

METHODS

The objective of this study was to correlate level of endothelin (ET) with echocardiographic, clinical, and biochemical markers and to determine its role as an independent marker of severity. We included patients with congestive heart failure in whom echocardiographic evaluation had been done and serum markers measured. Serum endothelin 1 levels were determined by radioimmunoassay (RIA). Correlation between endothelin concentration, echocardiographic parameters, potentially confounding factors, and severity of heart failure was made.

RESULTS

Patients with symptomatic heart failure and longer time of evolution had higher levels of endothelin unrelated to levels of troponin T, tumor necrosis factor, and atrial natriuretic peptide. There were no differences in levels of endothelin, independently of the etiology of cardiac failure.

CONCLUSIONS

These results support the idea that endothelin plays an important independent role in the physiopathology of heart failure. It may be a severity marker and an attractive therapeutic target.

Changes of brain endothelin levels and peripheral endothelin receptors by chronic cigarette smoke in spontaneously hypertensive rats

The present study was conducted to evaluate the contribution of endothelin (ET) to the pharmacodynamic response to chronic cigarette smoke in spontaneously hypertensive rats (SHR). The contribution of ET was studied consequent to the hemodynamic response following 8 weeks of cigarette smoke by determining the changes in tissue ET-1 content and ET receptors. The blood pressure (BP) at the early phase of smoking and the heart rate (HR) 24 h later were apparently reduced in SHR, while the HR at the early phase was transiently elevated in normotensive Wistar Kyoto (WKY) rats. Tissue ET-1 levels in the hypothalamus, striatum, and cortex of SHR were higher than those in WKY rats, and these higher levels in SHR were reduced by exposure to chronic cigarette smoke. The ET-1 contents in the medulla oblongata and midbrain of both strains were clearly increased by smoke exposure, although the levels of SHR and WKY rats were not different. In addition, the immunoreactivity of the ET type A receptor in the adrenal glands and type B receptor in the kidneys of SHR showed a different response to smoke exposure as compared to WKY rats. Our present findings suggest that the changes of ETs may relate to the pharmacodynamic effects of chronic cigarette smoke.

Adrenergic and endothelin B receptor-dependent hypertension in dopamine receptor type-2 knockout mice

Polymorphism of the dopamine receptor type-2 (D(2)) gene is associated with essential hypertension. To assess whether D(2) receptors participate in regulation of blood pressure (BP), we studied mice in which the D(2) receptor was disrupted. In anesthetized mice, systolic and diastolic BPs (in millimeters of mercury) were higher in D(2) homozygous and heterozygous mutant mice than in D(2)+/+ littermates. BP after alpha-adrenergic blockade decreased to a greater extent in D(2)-/- mice than in D(2)+/+ mice. Epinephrine excretion was greater in D(2)-/- mice than in D(2)+/+ mice, and acute adrenalectomy decreased BP to a similar level in D(2)-/- and D(2)+/+ mice. An endothelin B (ET[B]) receptor blocker for both ET(B1) and ET(B2) receptors decreased, whereas a selective ET(B1) blocker increased, BP in D(2)-/- mice but not D(2)+/+ mice. ET(B) receptor expression was greater in D(2)-/- mice than in D(2)+/+ mice. In contrast, blockade of ET(A) and V(1) vasopressin receptors had no effect on BP in either D(2)-/- or D(2)+/+ mice. The hypotensive effect of an AT(1) antagonist was also similar in D(2)-/- and D(2)+/+ mice. Basal Na(+),K(+)-ATPase activities in renal cortex and medulla were higher in D(2)+/+ mice than in D(2)-/- mice. Urine flow and sodium excretion were higher in D(2)-/- mice than in D(2)+/+ mice before and after acute saline loading. Thus, complete loss of the D(2) receptor results in hypertension that is not due to impairment of sodium excretion. Instead, enhanced vascular reactivity in the D(2) mutant mice may be caused by increased sympathetic and ET(B) receptor activities.

Role of endogenous endothelins in catecholamine secretion in the rat adrenal gland

We investigated the role of endogenous endothelins in catecholamine secretion in response to transmural electrical stimulation in the retrogradely perfused rat adrenal gland. (R)2-[(R)-2-[(S)-2-[[1-(hexahydro-1H-azepinyl)]carbonyl]amino-4-++ +methy l-pentanoyl]amino-3-[3-(1-methyl-1H-indoyl)]propionyl]amino-3-(2-+ ++pyridyl) propionic acid (FR139317; 0.03-3 microM), an endothelin ET(A) receptor antagonist, inhibited the electrical stimulation-induced epinephrine and norepinephrine output. Neither N-cis-2, 6-dimethylpiperidinocarbonyl-L-gamma-methylleucyl-D-1- methoxycarbonyl tryptophanyl-D-norleucine (BQ-788; 0.03-3 microM), an endothelin ET(B) receptor antagonist, nor phosphoramidon (1-100 mM), an endothelin-converting enzyme inhibitor, affected the catecholamine output responses. However, the inhibition by FR139317 of the catecholamine output responses was abolished by pretreatment with phosphoramidon (100 mM) or BQ-788 (3 microM). These results indicate that activation of endothelin ET(B) receptors by endogenous endothelins inhibits the catecholamine output responses under the condition in which endothelin ET(A) receptors are blocked. Exogenous endothelin-1 (1-100 nM) did not affect the catecholamine output responses, but it inhibited the responses under treatment with phosphoramidon and FR139317. Activation of endothelin ET(A) receptors may interfere with the endothelin ET(B) receptor-mediated inhibitory action on the neuronally evoked secretion of adrenal catecholamines.