Effects of endothelin on regional hemodynamics in conscious rats

Enhanced endothelin-1-mediated leg vascular tone in healthy older subjects

Advanced age is associated with a decreased leg blood flow and reduced physical activity. Endothelin (ET-1), a powerful vasoconstrictor, may play a role in the increased leg vascular tone in older men. objectives: to assess the ET-1-mediated vascular tone in the legs of healthy sedentary older men, both before and after 8 wk of exercise training. methods: in 8 younger subjects (19-50 yr) and 8 older men (67-76 yr), bilateral leg blood flow was measured using venous occlusion plethysmography before and after antagonizing ET-1 (using selective ET(A/B)-receptor antagonists). In older men, reversibility of the observations was assessed after 8 wk of cycling. results: ET-receptor inhibition increased leg blood flow significantly more in older men compared with younger individuals (29 +/- 9% and 10 +/- 4%, respectively, P < 0.05). Eight-week cycling training increased baseline blood flow in older men. The blood flow response to ET-receptor inhibition in older men was not affected by the training program (25 +/- 8%, P > 0.05 for comparison with pretraining). The flow ratio (blood flows infused leg/noninfused leg) decreased significantly by training from 26 +/- 8% to 7+3% (P < 0.05).

CONCLUSION

the increased baseline vascular tone in aging is at least in part mediated by the endothelin. Eight-weeks cycling training in older sedentary men decreased leg vascular tone and seems to partly decrease the ET-1-mediated vascular tone.

A Causal Role for Endothelin-1 in the Vascular Adaptation to Skeletal Muscle Deconditioning in Spinal Cord injury

OBJECTIVE

Endothelin-1 (ET-1) contributes to the increased peripheral resistance in heart failure and hypertension. Physical inactivity is associated with cardiovascular disease and characterized by increased vascular tone. In this study, we assess the contribution of ET-1 to the increased vascular tone in the extremely deconditioned legs of spinal cord-injured (SCI) individuals before and after exercise training.

METHODS AND RESULTS

In 8 controls and 8 SCI individuals, bilateral thigh blood flow was measured by plethysmography before and during the administration of an ET(A)/ET(B)-receptor blocker into the femoral artery. In SCI, this procedure was repeated after 6 weeks of electro-stimulated training. In a subset of SCI (n=4), selective ET(A)-receptor blockade was performed to determine the role of the ET(A)-receptors. In controls, dual ET-receptor blockade increased leg blood flow at the infused side (10%, P<0.05), indicating a small contribution of ET-1 to leg vascular tone. In SCI, baseline blood flow was lower compared with controls (P=0.05). In SCI, dual ET-receptor blockade increased blood flow (41%, P<0.001). This vasodilator response was significantly larger in SCI compared with controls (P<0.001). The response to selective ET(A)-receptor blockade was similar to the effect of dual blockade. Electro-stimulated training normalized baseline blood flow in SCI and reduced the response to dual ET-receptor blockade in the infused leg (29%, P=0.04).

CONCLUSIONS

ET-1 mediates the increased vascular tone of extremely inactive legs of SCI individuals by increased activation of ET(A)-receptors. Physical training reverses the ET-1-pathway, which normalizes basal leg vascular tone.

Endogenous endothelin attenuates the pressor response to acute environmental stress via the ETA receptor

Clinical studies have documented an abrupt rise in plasma endothelin-1 (ET-1) coincident with an increase in mean arterial pressure (MAP) during the response to acute stress. We therefore examined the ET(A) and ET(B) receptor-dependent effects of ET-1 on the pressor response to acute environmental stress in ET-1-dependent hypertension. Stress was induced by administration of air jet pulses (3 min) in ET(B) receptor-deficient (ET(B) sl/sl) rats fed normal salt (NS; 0.8% NaCl), high salt (HS; 8% NaCl), and HS plus the ET(A) receptor antagonist ABT-627 (5 mg.kg(-1).day(-1)) on successive weeks. MAP was chronically monitored by telemetry. Total pressor response (area under the curve) was significantly reduced in ET(B) sl/sl rats maintained on a HS vs. NS diet [-6.8 mmHg (SD 18.7) vs. 29.3 mmHg (SD 8.1) x 3 min, P < 0.05]. Conversely, the total pressor response was augmented in both wild-type [34.2 mmHg (SD 29.2) x 3 min, P < 0.05 vs. NS] and ET(B) sl/sl rats [49.1 mmHg (SD 11.8) x 3 min, P < 0.05 vs. NS] by ABT-627. Blockade of ET(B) receptors in Sprague-Dawley rats caused an increase in basal MAP that was enhanced by HS and lowered by mixed ET(A)/ET(B) receptor antagonism; none of these treatments, however, had any effect on the pressor response. These data demonstrate that increasing endogenous ET-1 suppresses the pressor response to acute stress through ET(A) receptor activation in a genetic model of ET-1-dependent hypertension. These results are consistent with reports that ET-1 can attenuate sympathetically mediated responses.

Endothelin as a regulator of cardiovascular function in health and disease

The endothelins are a family of endothelium-derived peptides that possess characteristically sustained vasoconstrictor properties. Endothelin-1 appears to be the predominant member of the family generated by vascular endothelial cells. In addition to its direct vascular effects, endothelin-1 has inotropic and mitogenic properties, influences homeostasis of salt and water, alters central and peripheral sympathetic activity and stimulates the renin-angiotensin-aldosterone system. Studies with endothelin receptor antagonists have indicated that endothelin-1 probably has complex opposing vascular effects mediated through vascular smooth muscle and endothelial ET(A) and ET(B)receptors. Endogenous generation of endothelin-1 appears to contribute to maintenance of basal vascular tone and blood pressure through activation of vascular smooth muscle ET(A)receptors. At the same time, endogenous endothelin-1 acts through endothelial ET(B) receptors to stimulate formation of nitric oxide tonically and to oppose vasoconstriction. In view of the multiple cardiovascular actions of endothelin-1, there has been much interest in its contribution to the pathophysiology of hypertension. Results of most studies suggest that generation of, or sensitivity to, endothelin-1 is no greater in hypertensive than it is in normotensive subjects. Nonetheless, the deleterious vascular effects of endogenous endothelin-1 may be accentuated by reduced generation of nitric oxide caused by hypertensive endothelial dysfunction. It also appears likely that endothelin participates in the adverse cardiac and vascular remodelling of hypertension, as well as in hypertensive renal damage. Irrespective of whether vascular endothelin activity is increased in hypertension, anti-endothelin agents do produce vasodilatation and lower blood pressure in hypertensive humans. There is more persuasive evidence for increased endothelin-1 activity in secondary forms of hypertension, including pre-eclampsia and renal hypertension. Endothelin-1 also appears to play an important role in pulmonary hypertension, both primary and secondary to diseases such as chronic heart failure. The hypotensive effects of endothelin converting enzyme inhibitors and endothelin receptor antagonists should be useful in the treatment of hypertension and related diseases. Development of such agents will increase knowledge of the physiological and pathological roles of the endothelins, and should generate drugs with novel benefits.

Endothelin and myocardial ischemia

Endothelin is a potent vasoconstrictor with a wide range of effects on the heart. Changes in myocardial and circulating levels of endothelin have been described in various experimental models of myocardial ischemia, and in humans with acute myocardial infarction and different forms of angina pectoris. The role played by endothelin in the different states of myocardial ischemia is unclear. However, myocardial damage has been shown to be reduced in several experimental models of myocardial infarction by administering agents that block the action of endothelin. The aim of this review article is to present the current literature concerning the interaction between endothelin and the various forms of myocardial ischemia, and to explore the significance of such interactions.

Modulation of the effect of endothelin-3 on blood pressure by atrial natriuretic peptide in conscious spontaneously hypertensive (SHR) and normotensive (WKY) rats

Endothelin (ET) exerts direct vasoconstrictory effects and stimulates release of vasoactive substances. It has been demonstrated that ET stimulates the release of atrial natriuretic peptide (ANP) both under in vitro and in vivo conditions. The present study aimed at elucidating whether the cardiovascular effects of endothelin-3 (ET-3) in normotensive (WKY) and spontaneously hypertensive (SHR) rats are modulated by ANP. The experiments were performed on 17 conscious WKY and 17 SHR rats. The effects of i.v. administration of 1 microgram of ET-3 on blood pressure (BP) and heart rate (HR) were investigated under control conditions and during ANP infusion (0.3 microgram/kg/min). In both strains ET-3 elicited a transient significant hypotensive effect followed by an increase in BP. BP fall was significantly greater and pressor effect significantly smaller in SHR than in WKY. In WKY, but not in SHR rats, both hypotensive and pressor phases were significantly attenuated during ANP administration. The results are evidence of differential involvement of endogenous blood pressure regulating factors in the cardiovascular effects of ET-3 in WKY and SHR rats during ANP infusion.

Recombinant human erythropoietin (rHuEPO) increases endothelin-1 release by endothelial cells

Hypertension is a major complication of rHuEPO therapy in hemodialysis (HD) patients. We have previously reported that patients receiving rHuEPO intravenously (i.v.) had higher mean arterial pressure (MAP) and plasma endothelin-1 (ET-1) levels than those in which the hormone was administered subcutaneously (s.c.). To test whether the increased serum ET-1 levels associated with i.v. rHuEPO administration are the result of a direct effect of the hormone on ET-1 release by the endothelial cells (EC), we examined the effects of rHuEPO in vitro. Bovine pulmonary artery endothelial cells (BPAEC) were exposed to doses of rHuEPO of 0.8; 1.6; 3.3 and 6.6 U/ml. A 24 hour-time course showed maximal ET-1 production at 12 hours for all the doses tested. A significant increase in cell proliferation over controls was observed at 24 hours, for all rHuEPO doses, and no correlation was found between ET-1 values and cell proliferation. Inhibition of protein synthesis by cycloheximide (10 micrograms/ml) abolished the stimulation of ET-1 release by rHuEPO. Thrombin (4 U/ml) and angiotensin II (10(-7) M), two potent stimulators of ET-1 release, had additive effects to those of rHuEPO. Specific thrombin and angiotensin II antagonists blocked these additive effects, reducing ET-1 release to the level of rHuEPO stimulation alone. In summary, rHuEPO stimulates vascular EC in culture to increase ET-1 release through an increase in synthesis and in a time dependent fashion. The routes of stimulation seem to differ from other known ET-1 secretogoges. Our data also confirm a significant mitogenic effect of rHuEPO on the endothelial cell.

Discrimination between ETA- and ETB-receptor-mediated effects of endothelin-1 and [Ala1,3,11,15]endothelin-1 by BQ-123 in the anaesthetized rat

1. The influence of BQ-123 (a selective ETA-receptor antagonist) on the haemodynamic response elicited by endothelin-1 (ET-1) and [Ala1,3,11,15]ET-1 (a selective ETB-receptor agonist) was studied in anaesthetized rats instrumented with ultrasonic Doppler flow probes on the carotid, coeliac, mesenteric, renal and iliac arteries. 2. BQ-123 alone (1.6 mumol kg-1, i.v.) induced a decrease in femoral mean arterial pressure (AP), accompanied by a systemic vasodilatation. The response was maximal after 3 min and then returned slowly to baseline. None of these effects was observed after a 0.016 mumol kg-1 dose of BQ-123. 3. ET-1 (1 nmol kg-1, i.v.) induced a biphasic response characterized by a transient initial decrease in AP accompanied by regional vasodilatation (mainly in the carotid and iliac beds) and by immediate mesenteric and renal vasoconstrictions. This was followed, within 1 min, by a marked and prolonged increase in AP accompanied by systemic vasoconstriction. Pretreatment with BQ-123 (1.6 mumol kg-1, i.v., 8 min before ET-1) increased and prolonged the vasodilator effect of ET-1 (mainly in the carotid, coeliac, mesenteric and iliac beds) and reduced its systemic vasoconstrictor effects with marked regional differences (the coeliac, mesenteric and renal beds being poorly affected). 4. [Ala1,3,11,15]ET-1 (3 nmol kg-1, i.v.) induced an initial and marked decrease in AP accompanied by regional vasodilatation (mainly in the carotid, coeliac and iliac beds) and by mesenteric and renal vasoconstrictions. This was followed, within 5 min, by a small increase in AP and systemic vasoconstriction. All these effects were dose-dependent. Pretreatment with BQ-123 (1.6 tmol kg'; 8 min before ET-1) did not modify the early effect of [Ala'3""5]ET-l, but abolished its secondary vasoconstrictor effect except in the mesenteric bed.5. This study demonstrates that pretreatment with BQ-123 not only reduced a large part of the sustained vasoconstrictor activity of ET-1, suggesting the involvement of ETA-receptors, but also enhanced the early vasodilator activity of ET-1 revealing a functional antagonism between the two effects. The vasodilator effect of [Ala1"3""l '5]ET-1 was not affected by BQ-123 and ET-1 induced a similar vasodilatation, that was potentiated by BQ-123, suggesting the involvement of ETB-receptors in this vasodilator response. Marked regional differences were however observed which might be partly related to different levels of functional antagonism between ETB- and ETA-mediated effects, but differences in receptor types, or subtypes, cannot be excluded, mainly in the mesenteric and renals beds.

Regional effects and clearance of endothelin-1 across pulmonary and splanchnic circulation

To determine the impact of i.v. endothelin-1 on systemic, pulmonary and splanchnic circulation, as well as the peptide's regional clearance, hepatic venous and right heart catheterization was performed in healthy volunteers. During the peptide's continuous i.v. administration (0.4 pmol x kg-1 x min-1, 60 min) its plasma concentration rose from 2.1 +/- 0.5 to 9.5 +/- 5.3 pmol/l (pulmonary artery), from 2.1 +/- 0.9 to 5.0 +/- 1.6 pmol/l (femoral artery), and from 1.5 +/- 0.6 to 2.9 +/- 1.2 pmol/l (hepatic vein). This was accompanied by an increase in mean systolic arterial pressure from 127 +/- 14 to 131 +/- 12 mmHg (P less than 0.05). Concomitantly, cardiac output and heart rate decreased from 7.0 +/- 1.1 to 5.8 +/- 1.0 l/min and from 63 +/- 6 to 56 +/- 5 beats/min, respectively, while total vascular resistance increased from 964 +/- 273 to 1204 +/- 338 dyn x cm x s-5 (P less than 0.01). No major changes in pulmonary circulation were observed, while splanchnic vascular resistance increased from 4472 +/- 1056 to 5361 +/- 1420 dyn x cm x s-5 (P less than 0.01) and estimated hepatic blood flow decreased from 1403 +/- 218 to 1218 +/- 219 ml min-1 (P less than 0.01). During endothelin-1 infusion the pulmonary vascular bed accounted for approximately 53% of the peptide's overall disposal.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for antagonistic activity of endothelin for clonidine induced hypotension and bradycardia

Effect of endothelin (ET) on clonidine induced cardiovascular effects was studied in male Sprague-Dawley rats. Clonidine (75 micrograms/kg, iv) produced significant decrease in blood pressure and heart rate. ET-1 (50 ng/kg, iv) pretreatment completely antagonized the hypotension and bradycardia induced by clonidine. ET-2 (50 ng/kg, iv) and ET-3 (50 ng/kg, iv) had similar antagonistic effect on clonidine induced hypotension and bradycardia. The antagonistic effect of ET lasted for several hours, however, 4 hours after ET pretreatment only partial blockade of clonidine induced hypotension and bradycardia was observed. This indicated that the antagonistic effect of ET was reversible. Initial hypertensive response induced by high dose of clonidine (750 micrograms/kg, iv) could not be antagonized by ET-1, ET-2 or ET-3, while phenoxybenzamine, an alpha adrenoceptor antagonist, blocked the hypertensive response of clonidine. Thus, ET has no antagonistic effect on the initial hypertensive response but antagonizes the hypotensive and bradycardic effect induced by clonidine. Clonidine induced hypotension and bradycardia are mediated through central alpha 2 adrenoceptors while hypertension is mediated through peripheral alpha 2 adrenoceptors. It is concluded that central alpha 2 adrenoceptors are different from peripheral alpha 2 adrenoceptors and ET antagonizes the effect of clonidine only on central alpha 2 adrenoceptors but has no antagonistic activity on peripheral alpha 2 adrenoceptors.

Potent vasoconstrictor effects and clearance of endothelin in the human forearm

The vascular effects of endothelin-1 in humans were investigated by infusion into the brachial artery of healthy volunteers. Endothelin-1 (5-500 pmol min-1) evoked potent and long lasting increase in forearm vascular resistance (FVR) and reduction in venous compliance, suggesting constriction of both resistance and capacitance vessels. The threshold for effect on forearm vascular resistance was at a calculated plasma concentration of 614 pmol 1-1. Endothelin-1 was on a molar basis 10-20 times more potent than noradrenaline as constrictor of both resistance and capacitance vessels. The increase in forearm vascular resistance induced by endothelin-1 lasted more than 30 min and that by noradrenaline less than 3 minutes. The endothelin-1-like immunoreactivity collected in the venous effluent during the infusion was 10-26% of the calculated arterial plasma concentration, indicating local removal of endothelin. After the infusion of endothelin-1 the urine concentration of prostacyclin metabolite increased significantly, indicating release of prostacyclin, whereas the concentration of thromboxane metabolite did not increase. It is concluded that endothelin-1 is a highly potent constrictor of human resistance and capacitance vessels, that it causes release of prostacyclin and that circulating endothelin-1 is efficiently removed by the forearm in vivo.

Dual action of endothelin-1 on the Ca2(+)-activated K+ channel in smooth muscle cells of porcine coronary artery

The effects of endothelin-1 (ET-1) on the activity of the large Ca2(+)-activated K+ channel (BK channel) in enzymatically dissociated smooth muscle cells of porcine coronary artery were studied with the cell-attached patch-clamp technique. ET-1 at concentrations between 0.1 and 10 nM potentiated the BK channel activity. This effect was maximal at 1 nM ET-1, resulting in an average of 4.2-fold increase in channel open-state probability as compared with control. ET-1 at concentrations higher than 10 nM produced an irreversible inhibition of the BK channel activity, primarily due to a marked decrease in the channel mean open-time. The activation by lower doses of ET-1, but not the inhibition by higher doses of ET-1, of the BK channel was blocked by 0.1 microM PN 200-110, a Ca2+ channel blocker. The modulation of the BK channel activity in smooth muscle cell membrane may be a possible mechanism for ET-induced vasodilator and vasoconstrictor actions.

Endothelin-1-induced vasoconstriction in humans. Reversal by calcium channel blockade but not by nitrovasodilators or endothelium-derived relaxing factor

The vascular effects of endothelin-1 (ET) in humans were investigated by brachial artery infusions of ET into 25 healthy volunteers. Forearm blood flow increased from a mean +/- SD value of 2.3 +/- 1.5 to 2.5 +/- 1.5 ml/min/100 ml forearm tissue (n = 25, p less than 0.05) in response to low dose (0.5 ng/min/100 ml forearm tissue) ET infusion and decreased to 1.78 +/- 1.3 and 1.1 +/- 0.9 ml/min/100 ml forearm tissue (p less than 0.001) during higher dosages (25 and 50 ng/min/100 ml forearm tissue). Sodium nitroprusside (0.6 micrograms/min/100 ml forearm tissue, n = 6), acetylcholine (16 micrograms/min/100 ml forearm tissue, n = 7), nifedipine (6 micrograms/min/100 ml forearm tissue, n = 6), and verapamil (80 micrograms/min/100 ml forearm tissue, n = 6) were infused alone and in combination with ET to evaluate the interactions between ET-induced vasoconstriction and stimulation of vascular muscle cyclic GMP levels by sodium nitroprusside, release of endothelium-derived relaxing factor by acetylcholine, and blockade of voltage-operated calcium channels by nifedipine and verapamil. Neither the vasodilator nor the vasoconstrictor response to ET was influenced by sodium nitroprusside or acetylcholine. In contrast, both calcium antagonists converted ET-induced vasoconstriction (e.g., delta forearm vascular resistance to ET 50 ng/min/100 ml forearm tissue, 151 +/- 100% and 164 +/- 92% in verapamil and nifedipine groups, respectively) to vasodilation (-35 +/- 12% and -21 +/- 16%, p less than 0.05). Our results demonstrate both ET-induced vasodilation (at low dosages) and vasoconstriction (at high dosages) in resistance vessels of normal humans. Blockade of voltage-operated calcium channels prevented ET-induced vasoconstriction and unmasked the vasodilator effect of high ET dosages. In human resistance vessels, blockade of voltage-operated Ca2+ channels but not cyclic GMP-dependent vasodilation may be an effective tool to inhibit ET-induced vasoconstriction.

Vascular activities of the endothelins

The endothelins are a family of novel 21 amino-acid peptides and are the most potent vasoconstrictor substances yet discovered. The endothelins not only produce prolonged pressor responses in intact animals but they also constrict large and small arterial and venous vessels studied as isolated vascular preparations, influence autonomic transmission, exert positive inotropic effects on the heart and have been shown to be capable of releasing EDRF, prostanoids and atrial natriuretic factor. Release of endothelins occurs after de novo synthesis which may be stimulated by various agonists, fluid-flow and possibly hypoxia. The endothelins have been implicated in the pathophysiology of a variety of cardiovascular disorders but their precise role remains to be elucidated.

Cerebrospinal fluid endothelin-1 and endothelin-3 levels in normal and neurosurgical patients: a clinical study and literature review

Endothelins are a family of structurally related, potent, long-lasting vasoconstrictor peptides. There are no established normal human levels of endothelin-1 or endothelin-3 in the cerebrospinal fluid. We measured cerebrospinal fluid endothelin-1 and endothelin-3 levels in five groups of patients: normal controls, patients with subarachnoid hemorrhage and cerebral vasospasm, patients with severe head injuries, patients undergoing temporal lobectomy for intractable epilepsy, and a patient with a gunshot injury to the thoracic spine. Endothelin-3 levels were significantly elevated in patients with subarachnoid hemorrhage and may participate in cerebral vasospasm and subsequent neurologic deterioration.

Structure-activity studies on endothelin (16-21), the C-terminal hexapeptide of the endothelins, in the guinea-pig bronchus

1. We describe the results of a structure-activity study in which the C-terminal hexapeptide of the endothelins, endothelin (16-21), is compared with shorter fragments; hexapeptides bearing amino-acid substitutions and the corresponding C-terminal fragments of sarafotoxins. The guinea-pig bronchus was used in this study since it is the most sensitive preparation for endothelin (16-21) thus far developed. 2. The biological results obtained with endothelin (16-21) and analogues demonstrate that the contractile activity of the C-terminal hexapeptide of endothelin on the guinea-pig bronchus depends on quite close structural requirements, strongly suggestive of a receptor interaction. The following features appear to be essential for the biological activity: (a) the C-terminal free carboxylic function; (b) the L-configuration of Trp-21; (c) the beta-carboxylic function of Asp-18; (d) the presence of Leu-17 and (e) the imidazole moiety of His-16. 3. The hexapeptide corresponding to the C-terminal portion of sarafotoxin, sarafotoxin (16-21), was devoid of biological activity. This behaviour might be related to the proposed existence of more than one receptor for the endothelin/sarafotoxin family in the guinea-pig bronchus.

Uptake and metabolism of endothelin in the isolated perfused rat lung

The uptake and metabolism of endothelin was studied in the isolated perfused rat lung. 125I-Labeled endothelin (4 x 10(-12) M) was rapidly removed from the lung perfusate. More than 90% of the label was removed by 5 min, and only 4-5% remained in the perfusate after 30 min. In the presence of 10(-9) or 5 x 10(-9) M unlabeled endothelin added 6 min before 125I-labeled endothelin, the radiolabel was more slowly removed, so that after 30 min 19 or 73% of the label, respectively, remained in the perfusate. Although a high level of unlabeled endothelin in the perfusate reduced the uptake of 125I-labeled endothelin, it did not displace radiolabel previously taken up by the lung. Analysis of radiolabel composition showed that endothelin metabolism was less than 15% in both the lung and the perfusate after 30 min of lung perfusion. Autoradiography of the lung indicated that the radiolabel was located primarily within the alveolar wall. These results suggest that circulating endothelin is readily but finitely taken up in the pulmonary microvasculature where it is avidly bound but slowly metabolized.

Chronic hypertension produced by infusion of endothelin in rats

Endothelin, a potent vasoconstrictor peptide synthesized by the vascular smooth muscle endothelium, was chronically infused into male Sprague-Dawley rats to determine whether a long-term increase in circulating endothelin levels would cause a sustained elevation in mean arterial pressure. Rats were catheterized, housed in metabolic cages, and maintained on a fixed 6 meq/day sodium intake throughout the experiment with daily measurements including mean arterial pressure, heart rate, water intake, urine output, urinary sodium excretion, urinary potassium excretion, cardiac output, total peripheral resistance, and stroke volume. Infusion of endothelin-1 (ET-1) at rates of 3, 5, or 7.5 pmol/kg/min for 7 days was associated with significant, sustained, and dose-dependent increases in mean arterial pressure and smaller less consistent elevations in total peripheral resistance. Other parameters were unaffected. Similar results were observed in rats receiving endothelin-3 (ET-3), except that a higher dose of ET-3 was required. These results indicate that elevated blood levels of endothelin could produce a maintained hypertension without sodium or water retention and that the hemodynamic basis for the increased mean arterial pressure is similar to that seen in most other forms of experimental and clinical hypertension.

Differences in regional vascular sensitivity to endothelin-1 between spontaneously hypertensive and normotensive Wistar-Kyoto rats

1. The systemic and regional haemodynamic effects of porcine endothelin-1 (endothelin) have been measured in anaesthetized spontaneously hypertensive (SH) rats rendered areflexic by ganglion blockade; comparisons were made with age-matched Wistar-Kyoto (WKY) control animals. 2. In both SH and WKY rats endothelin (0.1-1 nmol kg-1 i.v.) elicited an initial, short-lived (less than 2 min), fall in blood pressure which was associated with substantial increases in hindquarter and carotid vascular conductances. Both the blood pressure falls and the peripheral vasodilator responses were greater in SH than in WKY rats. 3. The initial depressor effects of endothelin were followed by marked and sustained increases in blood pressure associated with constriction in carotid, hindquarter, renal and mesenteric vascular beds. Vasoconstrictor responses were quantitatively similar in the two rat strains. 4. Pretreatment with indomethacin (5 mg kg-1 i.p. or i.v.) did not alter the responses to endothelin, 1 nmol kg-1, in SH rats. 5. The regional haemodynamic effects of intravenously administered acetylcholine (0.01-1 microgram kg-1), nitroprusside (0.3-10 micrograms kg-1) and angiotensin II (0.01-0.1 microgram kg-1) were similar in SH and WKY rats. 6. Endothelin (10(-10)-3 x 10(-8) M) contracted aortic rings from both SH rats and WKY control animals. Removal of the endothelium enhanced significantly the sensitivity of tissues from both WKY and SH rats to endothelin; the increase in sensitivity was greater in tissues from SH than WKY rats. 7. The results demonstrate qualitative similarities in the complex haemodynamic effects of endothelin in SH rats and WKY control animals. However, the SH rats display substantially greater vasodilator responses to endothelin than WKY. Eicosanoid generation is not the mechanism of the vasodilator action of endothelin in SH rats under the conditions of our experiments.

Interaction between endothelin-1 and endothelium-derived relaxing factor in human arteries and veins

Endothelin-1 is a 21-amino acid endothelial vasoconstrictor peptide that may be the physiological antagonist of endothelium-derived relaxing factor (EDRF). Endothelin-1 (10(-11)-3 x 10(-7) M) evoked potent contractions of isolated internal mammary arteries, internal mammary veins, and saphenous veins, which were enhanced in internal mammary veins as compared with internal mammary arteries (concentration shift, 6.3-fold; p less than 0.05) but not in the saphenous veins. Endothelial removal augmented the response to the peptide (at 3 x 10(-7) M) in internal mammary arteries (p less than 0.05) but not in veins. In the artery, EDRF released by acetylcholine or bradykinin reversed endothelin-1-induced contractions; in saphenous veins, both agonists were much less effective compared with the artery and veins contracted with norepinephrine (p less than 0.005-0.01). This inhibition of endothelium-dependent relaxations in veins occurred at half-maximal contractions but was most prominent at maximal contractions to the peptide. Nitric oxide similarly inhibited contractions to endothelin-1 and norepinephrine in internal mammary arteries, whereas in veins that were contracted with endothelin-1 but not with norepinephrine, the relaxations were blunted (p less than 0.005). The nitric oxide donor SIN-1 and sodium nitroprusside induced complete relaxations of internal mammary arteries but were less effective in veins contracted with endothelin-1 (p less than 0.005). Thus, in normal human arteries, EDRF inhibits endothelin-1-induced contractions, whereas the peptide specifically attenuates the effects of EDRF and nitrovasodilators in veins. This may be important in pathological conditions associated with increased levels of endothelin-1 and in veins used as coronary bypass grafts.

Endothelin causes contraction of canine and bovine arterial smooth muscle in vitro and in vivo

We have studied the effect of endothelin, an endothelium-derived peptide, on isolated canine and bovine cerebral arteries in vitro and on canine vertebral blood flow (VBF) in vivo. Endothelin produced a dose-dependent contraction of canine and bovine arterial smooth muscle with ED50 values ranging from 4 to 8 nM. The response to endothelin developed slowly and persisted as a sustained contraction. Maximal contraction by endothelin required the presence of extracellular calcium and was independent of the presence of endothelium. The maximal contraction produced by endothelin was approximately 2-3 times greater than that produced by neuropeptide Y or angiotensin II. The injection of endothelin into the vertebral artery decreased vertebral blood flow (VBF) dose-dependently without affecting systemic blood pressure or heart rate. The decrease in VBF produced by endothelin was long-lasting, like that produced by neuropeptide Y, but more potent. The present data, together with our previous study demonstrating that the intracisternal injection of endothelin induces an unusually long-lasting decrease in the basilar artery diameter angiographically, suggests that endothelin may act as a long-lasting vasoconstrictor in cerebral vascular disease.

Endothelium-derived constricting factor(s): the last novelty--endothelin

The vascular endothelium is not merely a passive physical barrier between the blood and the tissue surrounding the blood vessel, but may actively participate in key processes of metabolic, secretory, and vasoregulatory character. In addition, the endothelium plays an important role in the control of platelet activation. Under certain conditions endothelial cells have been shown to produce powerful vasodilators, like endothelium-derived relaxing factor (EDRF) and prostacyclin (GPI2), and vasoconstrictors like endothelium-derived constricting factor (EDCF) and endothelin (ET) (Griffith et al., 1988; Vanhoutte & Katusic, 1988). In contrast to the extensive studies performed to characterize the actions and nature of EDRF, recently identified chemically as nitric oxide (Moncada et al., 1988), relatively little is known about EDCF(s). This paper reviews recent data on EDCF, with special emphasis on the newly discovered vasoconstrictor peptide, endothelin (ET).

Haemodynamic effects of long-term endothelin infusion in conscious sheep

1. Synthetic human endothelin-1 was infused intravenously at 15 micrograms/h for 24 h to examine its cardiovascular actions in five conscious sheep. 2. Endothelin produced a maximum increase in mean arterial pressure (MAP) of +8 mmHg at 8 h, with an increase in calculated total peripheral resistance (CTPR) of +2.6 mmHg/L per min, whilst cardiac output (CO) was unchanged. At 24 h MAP was not significantly elevated, however CTPR had increased by +2.8 mmHg/L per min and CO had decreased by 0.9 L/min. 3. This study shows that long-term administration of endothelin produces sustained arterial vasoconstriction in sheep.

Endothelial control of vascular tone in large and small coronary arteries

The endothelium modulates coronary vascular tone by the release of endothelium-derived relaxing or contracting substances. The endothelium-derived relaxing factor has been identified as nitric oxide synthesized in endothelial cells from L-arginine. The endothelium can release other relaxing substances such as prostacyclin and a hyperpolarizing factor. Endothelin-1 is a potent vasoconstrictor peptide formed by endothelial cells, and is likely to be the physiologic antagonist of endothelium-derived relaxing factor. Other putative contracting factors include superoxide anions and products of arachidonic acid metabolism. Endothelium-derived relaxing factor is released spontaneously and in response to flow, platelet-derived products (that is, serotonin, thrombin and adenosine diphosphate) and certain autacoids (that is, acetylcholine, bradykinin, histamine, substance P, vasopressin, alpha-adrenergic agonists). A considerable heterogeneity of responses exists among vessels of different size from different anatomic origin and different species. Hypercholesterolemia, atherosclerosis, hypertension and myocardial ischemia or reperfusion, or both, impair endothelium-dependent relaxation. Under normal conditions, endothelium-derived relaxing factor appears to dominate the control of vascular tone of large and small coronary vessels, whereas in disease states, endothelium-derived contracting factors are released. Impairments of endothelial function may be important in the development of various forms of cardiovascular disease.

Endothelin and sarafotoxin S6b have similar vasoconstrictor effects and postsynaptically mediated mechanisms

In the isolated perfused mesenteric vascular bed, porcine endothelin (ET) and sarafotoxin S6b produced direct vasoconstriction and potentiated nerve stimulation-induced vasoconstriction. ET also enhanced the vasoconstrictor response to exogenous norepinephrine (NE). Basal or stimulated endogenous NE release was not affected either by ET or sarafotoxin S6b. Qualitatively similar responses to ET and sarafotoxin S6b were always observed, although, in many cases, the response to ET was greater and longer lasting than to sarafotoxin S6b. These results indicate that vasoconstrictor responses to ET or sarafotoxin S6b are mediated primarily by postsynaptic mechanisms. No initial vasodilator response to ET or sarafotoxin S6b was observed in this mesenteric vascular preparation.

Effect of neuropeptide Y on cardiac output, its distribution, regional blood flow and organ vascular resistances in the pithed rat

1. The effects of neuropeptide Y on cardiac output, its distribution and organ vascular resistances were determined with tracer microspheres in pithed rats. 2. Neuropeptide Y increased blood pressure by increasing both cardiac output and total peripheral resistance. The increase in cardiac output was due to an increase in stroke volume as heart rate was not changed. Increased vascular resistance in the splenic, renal, testicular, epididymal, skeletal muscle, large intestinal and mesenteric vascular beds contributed to the increase in total peripheral resistance. Vasoconstriction was most pronounced in the mesenteric bed. 3. This study indicates that neuropeptide Y increases blood pressure by increasing cardiac output and total peripheral resistance. The increased cardiac output is possibly due to an increase in venous return, whilst the increased total peripheral resistance was due to regional vasoconstriction, particularly in the mesenteric bed.

Further studies on the response of the guinea-pig isolated bronchus to endothelins and sarafotoxin S6b

Endothelin-1 (ET-1), endothelin-3 (ET-3) and sarafotoxin S6b (SRFTX) produced a concentration-dependent contraction of the guinea-pig isolated bronchus. In untreated preparations, SRFTX was about 10 times more potent than ET-1 or ET-3 and produced a greater maximal effect. Mechanical removal of the bronchial epithelium (rubbing) or the addition of indomethacin (5 microM) increased the potency of all peptides, ET-3 being enhanced more than ET-1 or SRFTX. Further, the activity of ET-3 was enhanced by a mixture of peptidase inhibitors (thiorphan, captopril, bestatin, 1 microM each). When the three potentiating factors were combined (rubbed bronchi in presence of indomethacin and peptidase inhibitors), the following order of potency was found, ET-3 congruent to SRTFX greater than ET-1. For comparison, the activity of the three peptides was also studied in the rubbed rat isolated aorta in the presence of indomethacin and peptidase inhibitors and the following rank order of potency was found: ET-1 greater than SRFTX greater than ET-3. When the activity of the peptides was compared in these two preparations, ET-1 was 4 times more potent in the aorta than in the bronchus, while SRFTX and ET-3 were 8.7 and 133 times more potent in the bronchus than the aorta, respectively. These findings indicate that, in the guinea-pig bronchus, the contractile activity of these peptides is attenuated by the concomitant production of epithelium-derived relaxant factor(s), prostanoid production via the cyclooxygenase pathway and, possibly, enzymatic inactivation of added peptides. Further, the existence of multiple receptors mediating the contractile response to these peptides is suggested.

Neuropeptides in hypertension: role of neuropeptide Y and calcitonin gene related peptide

1. The effect of neuropeptide Y (NPY) on cardiovascular function at three levels of the noradrenergic axis where the peptide is known to co-exist with noradrenaline (NA) and or adrenaline (A) was studied in normotensive Sprague-Dawley (SD), Wistar-Kyoto (WKY) or spontaneously hypertensive rats (SHR). 2. In the perfused mesenteric arterial bed, NPY and the structurally similar peptide intestinal polypeptide (PYY) decreased the periarterial nerve stimulation induced release of NA and potentiated the increase in perfusion pressure to nerve stimulation or exogenously applied agonists (e.g. angiotensin, vasopressin, phenylephrine). In contrast to NPY and PYY, C-terminal NPY fragments inhibited NA release and produced a parallel decrease in perfusion pressure thus supporting the concept of Y1 (post) and Y2 (pre) NPY receptors. 3. In the mesenteric artery of SHR the prejunctional inhibitory effect of NPY was attenuated while the postjunctional response was enhanced. 4. Following intrathecal (Int) injection of NPY, there was a decrease in blood pressure, total peripheral resistance (predominantly by a decrease in mesenteric vascular resistance) and renal nerve activity. The depressor effect of Int NPY was attenuated in the SHR. 5. Unilateral injections of NPY into the posterior hypothalamic nucleus increased blood pressure, hindquarter and renal vascular resistance and renal nerve activity. The pressor effect was enhanced in the SHR. 6. Periarterial nerve stimulation of the perfused mesenteric artery produced a frequency dependent vasodilation in beds pretreated with guanethidine and precontracted with methoxamine.(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelium-derived contractile factors

1. Vascular endothelium releases different substances (endothelium-derived contractile factors, EDCFs), which mediate vasoconstrictor responses induced by several agents. 2. Clear differences have been reported in endothelium-dependent contractions, which suggest at least three distinct EDCFs, named EDCF1, EDCF2 and EDCF3, respectively. 3. EDCF1 is a cyclooxygenase metabolite(s) of arachidonic acid. EDCF2 is a polypeptide released from cultured endothelial cells. It has been isolated and identified as a 21-amino acid peptide called endothelin, which is described as the most potent vasoconstrictor agent known to date. EDCF3 is an unidentified contractile factor(s), which is neither EDCF1 nor EDCF2. 4. The physiological role of these endothelial contractile factors is not yet clear. However, they have been implicated in the local mechanisms involved in blood flow regulation, as well as in some pathological conditions, such as hypertension or cerebral vasospasm.

Cardiac and vascular actions of sarafotoxin S6b and endothelin-1

Snake venom-derived sarafotoxin S6B (SRT) and porcine endothelium-derived endothelin-1 (ET) have striking structural similarities. In conscious, freely-moving rats, ET (0.67 nmol/kg) produced a transient tachycardia and fall in arterial blood pressure which was followed by a long-lasting increase in arterial pressure, bradycardia, decrease in cardiac output (CO) and marked increase in total peripheral resistance. In contrast, SRT (0.67 nmol/kg) produced only the sustained cardiovascular responses. The sustained cardiovascular effects of SRT or ET were similarly attenuated by nifedipine. SRT and ET (30 nM) produced vasoconstriction in the isolated perfused mesenteric vascular bed without initial vasodilation. SRT and ET had potent positive inotropic and negative chronotropic effects on isolated perfused hearts and induced toxic reactions including coronary vasospasm, arrhythmias, A-V block and ventricular fibrillation. In addition to SRT lacking the initial depressor response in vivo, several differences in the activities of the peptides were also observed. ET produced greater and longer-lasting actions than SRT in producing pressor and vasoconstrictor responses in all 3 preparations, and in its ability to induce toxic effects on the heart.

The activity of peptides of the endothelin family in various mammalian smooth muscle preparations

The activity of natural endothelins such as ET-1, ET-2, ET-3 and of sarafotoxin S6b (SRFTX) as compared to that of the C-terminal hexapeptide ET-(16-21) was investigated in several smooth muscle preparations in the presence of indomethacin, captopril, bestatin and thiorphan. In some tissues (rat thoracic aorta, guinea-pig ileum, human urinary bladder, renal pelvis or renal artery), ET-(16-21) had little if any agonist activity. In other preparations (guinea-pig bronchus, rat vas deferens, rabbit pulmonary artery) ET-(16-21) was a full agonist. The amidated form of ET-(16-21) was either inactive or less potent than ET-(16-21). These findings provide evidence that at least two receptors exist for peptides of the ET family; these receptors were termed ETA and ETB. ET-(16-21) is a full agonist at ETB receptors while being inactive or weakly active at ETA receptors. The free acid of tryptophan in position 21 plays a key role in the activity of these peptides at ETB receptors.

Hemodynamic effects of endothelin after systemic and central nervous system administration in the conscious rat

The effects of endothelin-1 (ET-1) administered i.v. or intracerebroventricularly (i.c.v.) on arterial pressure (MAP), heart rate and cardiac output were studied in the conscious rat. Systemic injection of ET-1 (0.1 to 1 nmol/kg i.v.) increased dose-dependently MAP and decreased heart rate. The doses of 0.3 and 1 nmol/kg produced initially transient hypotension and tachycardia which were accompanied by a decrease in total peripheral resistance index (TPRI). Cardiac output was significantly reduced and TPRI increased during the pressor response to 0.3 and 1 nmol/kg ET-1. ET-1 i.c.v. (30 pmol/kg) produced a profound pressor and vasoconstrictor response which was followed by cardiovascular collapse and death within 20 min after i.c.v. injection. Low doses (1 to 10 pmol/kg) of ET-1 i.c.v. had no effect on the cardiovascular system. The present data are in accordance with the studies demonstrating biphasic blood pressure and heart rate responses to i.v. ET-1 in the rat. Profound cardiac depressor and peripheral vasoconstrictor responses were found to accompany the pressor phase. Our results also showed for the first time direct central pressor and vasoconstrictor effects of ET-1 insinuating that endothelin might be a neuropeptide participating in the central cardiovascular control.

Effects of moderate hypoxia, hypercapnia and acidosis on haemodynamic changes induced by endothelin-1 in the pithed rat

1. Pithed rats were respired at a fixed rate of 54 cycles min-1 and with a ventilation volume of either 20 (control) or 10 ml kg-1. In these two preparations, the dose-response relationships for the systemic blood pressure responses to endothelin-1, administered i.v., were examined. Also, cardiac output, its distribution, tissue blood flows and vascular resistances were determined at both respiratory volumes in pithed rats given saline or during pressor responses to endothelin-1 (750 ng, i.v.). Finally, a comparison was made of the pressor responses to endothelin-1 in the blood perfused superior mesenteric arterial bed of pithed rats respired at 10 or 20 ml kg-1. 2. In control rats the systemic blood pressure responses to i.v. endothelin-1 were biphasic with an initial, transient (30 s) decrease in blood pressure followed by a well sustained pressor response. These responses were dose-dependent (the ED50 for the pressor response being 0.27 +/- 0.04 micrograms). The pressor effect of endothelin-1 was due to an increase in total peripheral resistance with no change in heart rate or cardiac output. This increased total peripheral resistance was due to vasoconstriction of the spleen, stomach, large intestine, small intestine and the pancreas/mesentery (in which it was most severe). Endothelin-1 also increased blood flow through the heart, lungs, liver, epididimides, fat and skin through redistribution of cardiac output to these vascular beds. 3. At the lower ventilation volume there was moderate acidosis, hypoxia and hypercapnia relative to those rats respired at 20 ml kg-1. With respiration at 10 ml kg-1, the pressor response to endothelin-1 was not sustained and, after oscillations in both blood pressure and heart rate, death occurred 15-20 min after administration. The pressor effect resulted from increases in cardiac output (due to increased stroke volume) and total peripheral resistance: the latter was caused by vasoconstriction in the stomach, small intestine, large intestine and pancreas/mesentery. Endothelin-1 increased blood flow through the heart, lungs, liver, kidneys, testes, fat and skin due to either an increase in cardiac output, redistribution of cardiac output or both. 4. Endothelin-1 induced dose-dependent pressor responses in the mesenteric bed in situ. At the lower ventilation volume the potency of endothelin-1 in this vascular bed was increased approximately two fold with the ED50 being 68 +/- 7 pmol compared to 113 +/- 15 pmol in the rats respired at 20 ml kg-1. 5. This study indicates that, in normoxic control pithed rats, the pressor response to endothelin-1 was due largely to vasoconstriction of the splanchnic vascular bed. In rats with moderate hypoxia, hypercapnia and acidosis, the pressor response was due to vasoconstriction of the gastrointestinal tract as well as an increase in cardiac output. Endothelin-1 induced profound vasoconstriction in the mesenteric bed of the pithed rat both in vivo and in situ. The potency of endothelin-1 on this bed in situ was doubled by lowering the ventilation volume. An increase in cardiac contractility and severe gastrointestinal vasoconstriction may be the initial events leading to the eventual toxic effect of endothelin-1 in the hypoxic pithed rat.

Effect of endothelin on systemic and regional hemodynamics in rats

In this study we analyzed the changes in systemic haemodynamics induced by endothelin, 1 nmol/kg, given as an i.v. bolus to anaesthetized rats. Endothelin induced a dramatic decrease in cardiac output whereas the total peripheral resistance increased more than two times. In addition, renal blood flow decreased while renal vascular resistance increased to a similar extent. A marked decrease in blood flow to the splanchnic organs was also observed. In conclusion, endothelin seems to modulate peripheral vasomotor tone, at least under certain conditions.

The effect of endothelin on noradrenergic transmission in rat and guinea-pig atria

Pre-and postjunctional effects of 10 nM endothelin have been studied in rat and guinea-pig spontaneously beating isolated atria. Endothelin increased the rate and force of contraction of atria from both species, but decreased the positive inotropic and chronotropic responses to stimulation of intramural sympathetic nerves by approximately 60% in rat and by 10-15% in guinea-pig atria. The stimulation-induced release of noradrenaline from either rat or guinea-pig atria was not significantly changed by endothelin.

Cardiovascular effects of endothelin in dogs: positive inotropic action in vivo

Endothelin, administered i.v. to anesthetized dogs, dose dependently increased the cardiac output, left ventricular systolic pressure (LVSP), and maximum upstroke velocity (max dp/dt) of the LVSP for about 10 min without changing the heart rate. Thereafter the cardiac output decreased to below the control level but max dp/dt decreased to the control level. The arterial pressure and total peripheral resistance showed an initial, transient decrease followed by a sustained increase. These results suggest that endothelin has positive inotropic and long-lasting vasoconstrictive effects preceded by transient vasodilatation in vivo.

Dilator actions of endothelin in coronary resistance vessels and the abdominal aorta of the guinea pig

Endothelin has been characterized as a potent constricting factor. The purpose of this study was to investigate possible dilator effects of this peptide and to examine whether dilator responses occur through an endothelium-mediated mechanism in guinea pig coronary resistance vessels and isolated aortic rings. Changes in perfusion pressure after bolus injections of endothelin were measured using a constant-flow modified Langendorff preparation with a transducer between the flow pump and the heart. An immediate fall in perfusion pressure, averaging 6 mmHg, was observed after injection of endothelin (10(-14)-10(-12) moles). This effect was maximal at 1 minute and tended to return toward baseline levels within 4 minutes. In response to endothelin (10(-9) M), isolated aortic rings relaxed 35% after being contracted with prostaglandin F2 alpha (10(-7) M). In both preparations, dilation was converted to constriction after endothelium damage by oxygen radicals or endothelium removal (mechanical rubbing). Dilator responses to endothelin were blocked by pretreatment for 30 minutes with indomethacin (14 microM) in the presence of an intact endothelium in coronary resistance vessels, whereas in the abdominal aorta they were not. We conclude that endothelin has significant dilator properties and that this effect is opposed by its constrictor action at higher doses. In addition, dilator responses to endothelin require an intact endothelium in both coronary vessels and abdominal aorta. Finally, endothelin-induced dilation in coronary resistance vessels appears to occur through a cyclooxygenase product-mediated mechanism.

Endothelin--a new family of endothelium-derived peptides with widespread biological properties

Endothelin (ET) is a novel family of three isopeptides (ET-1, ET-2, ET-3) each containing twenty-one amino acids and two disulfide bonds. Initially isolated from the supernatant of cultured porcine aortic endothelial cells, ET is stored as a preproform and released through an unusual proteolytic cleavage. In general, ET-1, ET-2, ET-3 differ quantitatively but not qualitatively in their biologic activity. ET have potent contractile activity in a variety of isolated tissues including arteries veins, trachea, duodenum urinary bladder and uterus. In vivo, ET possesses potent vasodilator and vasoconstrictor properties. Although the mechanisms mediating the hemodynamic effects of ET are not entirely clarified, recent evidence indicates a role for endothelium-derived relaxant factor (EDRF), protein kinase C and extracellular calcium. Moreover, ET appears to produce inflammation and bronchoconstriction through the formation of arachidonic acid metabolites via the cyclooxygenase pathway. The presence of ET binding sites in blood vessels and in several organ systems suggests ET may have important regulatory functions, which remain to be determined.

Endothelin stimulates release of atrial natriuretic peptides in vitro and in vivo

The effect of endothelin (END) on the release of atrial natriuretic peptides (ANP) was studied in isolated rat atria and in conscious rats. END stimulates the ANP release in vitro in a dose-dependent manner. An increase in ANP plasma levels and cyclic GMP plasma levels was also observed in conscious rats after injection of END. When a monoclonal antibody directed against ANP was injected together with END the increase in cyclic GMP was completely blocked. From this study it is concluded that END is a potent secretagogue for ANP both in vitro and in vivo.