Hemodynamic actions of endothelin in conscious and anesthetized dogs

Endothelin infusion reduces hypoxic pulmonary hypertension in pigs in vivo

Previous work has shown that the plasma levels of the potent vasoactive peptide endothelin (ET) are increased in pathophysiological conditions with increased pulmonary vascular resistance and it has been speculated that ET may play some part in hypoxic pulmonary hypertension. We have therefore evaluated the effects of ET-infusion in the porcine pulmonary circulation after hypoxia-induced hypertension. Pits under general anaesthesia were artificially ventilated through an endotracheal tube and hypoxia was induced by decreasing the fraction inhaled O2 from 0.21 to 0.10. Haemodynamic parameters were continuously recorded using a Swan-Ganz catheter in combination with thermodilution for cardiac output measurements. ET-1 or ET-3 was given as an i.v. infusion through the Swan-Ganz catheter in the right ventricle. Hypoxia induced a reproducible increase in pulmonary vascular resistance (PVR), mean pulmonary artery pressure (MPAP) and right ventricular stroke work (RVSW) while the systemic vascular resistance (SVR) slightly decreased. Cumulative infusion of ET-1 (10, 25 and 50 ng kg-1 min-1) dose-dependently decreased MPAP and PVR; at a higher dose (100 ng kg-1 min-1), the PVR returned to the level observed at hypoxia. ET-infusions at 50 and 100 ng kg-1 min-1 evoked an increase in SVR and a decrease in cardiac output (CO) and stroke volume (SV). RVSW also gradually decreased during ET-1 infusion. Infusion of ET-3 evoked effects similar to those of ET-1 infusions, although the response to ET-3 was not that rapid in onset. In a second series of animals, repeated 15 min periods of hypoxia evoked a stable, reproducible response with a consistent increase in PVR, MPAP and RVSW which returned to baseline values during normoxia. Infusion of ET-1 (25 ng kg-1 min-1) evoked a rapidly developing decrease in PVR and MPAP which was quickly normalized upon cessation of the ET-infusion. ET-1 infusion at this concentration did not per se influence the haemodynamic parameters during normoxia. It is concluded that in the pig, short-term ET-infusion reduces the pulmonary hypertension associated with acute hypoxia.

Endothelin as a neuropeptide. Cardiovascular effects in the brainstem of normotensive rats

The relevance of endothelin in central cardiovascular function was studied in urethane-anesthetized Sprague-Dawley rats. Blood pressure (BP) was monitored intra-arterially, and cerebrospinal fluid (CSF) was collected through an intracisternal catheter for radioimmunoassay of endothelin-1 (ET-1). Endothelin levels in the CSF were significantly higher (39 +/- 3 pg/ml) than in plasma (10 +/- 3 pg/ml, n = 11). ET-1 in CSF or plasma was not affected by systemic infusion of saline, but its levels significantly decreased when a sustained increase in BP was elicited with phenylephrine (14 +/- 7 pg/ml in the CSF and 6 +/- 4 pg/ml in plasma, n = 5). In sinoaortic-denervated animals, phenylephrine failed to reduce CSF endothelin levels. In different experiments, intracisternal administration of ET-1 (10 pmol) evoked an initial decrease in BP and heart rate (HR), followed by pronounced hypertension, bradycardia, and, in 70% of the animals, death from cardiorespiratory failure. Intracisternal administration of endothelin-3 (ET-3, 80 pmol, n = 11) evoked only a modest hypotensive and bradycardic response without cardiorespiratory impairment. Microinjection of ET-1 (0.5, 1, 2, 4, and 6 pmol/60 nl) into the nucleus of the solitary tract or area postrema produced a decrease in BP and HR. On the other hand, injection of low concentrations of ET-3 into the nucleus of the solitary tract increased BP and HR (at 2 pmol, 17 +/- 3 mm Hg, 14 +/- 6 beats per minute, n = 7), whereas ET-3 in the area postrema produced a prominent dose-related decrease in BP and HR. In the rostroventrolateral medulla, the lowest doses of ET-1 first modestly increased BP and renal sympathetic nerve activity. These effects were followed by hypotension, bradycardia, increase in respiratory frequency, and further enhancement of sympathetic nerve traffic. In 29% of the animals, these effects were followed by cardiorespiratory arrest. The specificity of the cardiovascular response to endothelin was demonstrated by the inhibitory effects of the receptor antagonist BQ-123. These results demonstrate that endothelin has specific cardiovascular effects in the brainstem of the rat and support a role for endothelin in cardiovascular regulation.

Interactions between endothelin-1 and other chronotropic agents in rat isolated atria

In isolated spontaneously beating right and left atria and in electrically driven left atrium from rat, endothelin-1 increased the rate and force of contraction, but significantly decreased the positive chronotropic and inotropic responses to sympathetic nerve stimulation. The decrease may be partly dependent on the positive cronotropic and inotropic effects of endothelin-1, since other agents with chronotropic activity (noradrenaline, isoprenaline, serotonin and Bay k 8644) also decreased stimulation-induced chronotropic responses. Endothelin-1 caused a significant rightward shift of the linear portion of the log concentration-response curve for the chronotropic actions of noradrenaline and isoprenaline. The changes in the log concentration-response curve were not a consequence of the direct chronotropic effect of endothelin-1, since they were still evident when the chronotropic action of endothelin-1 was offset by carbachol. Furthermore, the chronotropic agent, Bay k 8644, did not shift the linear portion of the log concentration-response curves for noradrenaline and isoprenaline. The mechanism of the effects of endothelin-1 in rat atria is not known, but they were not changed by blockade of alpha-adrenoceptors or of L-type voltage-sensitive Ca2+ channels.

No major role for atrial natriuretic peptide in the vasodilator response to endothelin-1 in the spontaneously hypertensive rat

Porcine endothelin-1 (endothelin) is a powerful releaser of atrial natriuretic peptide (ANP) from rat atrial myocytes in vitro. The fact that release is greater when atria are taken from spontaneously hypertensive rats (SHR) than normotensive control animals led to the suggestion that ANP release could be the basis of the prominent vasodilator responses seen in SHR in vivo. The present experiments were carried out in the anaesthetized, ganglion-blocked SHR to test this hypothesis. Bolus injections (1-4 nmol/kg) of ANP (atriopeptin III-rat) gave slowly developing (1-5 min), small (less than 10 mm Hg) falls in blood pressure associated with weak but consistent vasoconstrictor responses in the renal, mesenteric, carotid and hindquarters vascular beds. In contrast, i.v. injections of endothelin, 1 nmol/kg, induced rapid (less than 15 s), substantial (greater than 30 mm Hg) falls in blood pressure associated with vasodilation in the carotid and hindquarters vascular beds. In animals rendered essentially unresponsive to ANP following repeated i.v. injections of high doses (4-10 nmol/kg) of the peptide, endothelin still induced prominent vasodilator responses. Injections of endothelin given into the aortic arch in order to minimize contact with the atria, gave vasodepressor/vasodilator responses which were qualitatively similar and quantitatively somewhat greater than those following intra-jugular venous injection. Taken together these data suggest ANP release is not a major factor in the vasodilator effects of endothelin in the rat.

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.

Endothelin produces systemic vasodilation independent of the state of consciousness

The effects of endothelin, ET-1, on pulmonary and systemic hemodynamics were studied in the open chest dog and changes in systemic arterial pressure in dogs under conscious and anesthetized states were compared. Rapid intravenous (IV) bolus injections of ET-1, 100-1,000 nanograms/kg, significantly decreased systemic arterial pressure, and significantly decreased systemic vascular resistance whereas left atrial pressure and pulmonary vascular resistance were not altered. Reductions in systemic arterial pressure in response to bolus injection of ET-1, 100 and 300 nanograms/kg IV, during conscious state and during anesthesia were similar, respectively. The present data suggest that ET-1 dilates the systemic vascular bed independent of the animal's state of consciousness. The present data also suggest that when compared to the systemic vascular bed, the pulmonary vascular bed is less responsive to bolus administration of ET-1.

Interactions between circulating peptides and the central nervous system in hemodynamic regulation

Enkephalins and endothelins are endogenous peptides, which, at least at pharmacologic doses, produce complex hemodynamic responses after intravenous administration. The enkephalins, when injected into conscious animal models and humans, increase blood pressure, heart rate and minute ventilation. This response occurs by activation of specific opiate receptors located outside the bloodbrain barrier; the actual mechanism involves an increase in adrenergic autonomic nervous system tone and a decrease in cholinergic tone. These opiate receptors may activate afferent fibers, perhaps nicotinic cholinoceptors; in many ways their properties are suggestive of chemoreceptors. Furthermore, enkephalin responses appear to be modulated by gamma-aminobutyric acid complexes, in that the reversal of the excitatory hemodynamic responses seen in the conscious state to vasodepressor responses after barbiturate anesthesia may result from alteration of the state of activation of the gamma-aminobutyric acid complex. The enkephalin receptors are localized to the vertebral artery vascular distribution; the specific site may be the area postrema, a blood-brain barrier-deficient circum-ventricular organ demonstrated to modulate heart rate and blood pressure and to represent a target site for circulating angiotensin II. Endothelin increases heart rate and blood pressure when infused slowly into conscious or anesthetized dogs, although barbiturates do blunt the increase in heart rate. The mechanism appears to involve modification of autonomic tone, but also some element of direct vasoconstrictor activity. Interestingly, rapid bolus doses of endothelin produce only vasodepressor responses, suggesting that the rate and concentration at which circulating endothelin reaches afferent receptors or vasoconstrictor sites on vascular smooth muscle may determine the net hemodynamic response observed.(ABSTRACT TRUNCATED AT 250 WORDS)

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.