Administration Of Endothelin-1 Research Articles

Endothelins induce ET B receptor-mediated mechanical hypernociception in rat hindpaw: roles of cAMP and protein kinase C

The present study assesses the capacity of endothelins to induce mechanical hypernociception, and characterises the receptors involved and the contribution of cAMP and protein kinases A (PKA) and C (PKC) to this effect. Intraplantar administration of endothelin-1, endothelin-2 or endothelin-3 (3–30 pmol) induced dose- and time-dependent mechanical hypernociception, which was inhibited by BQ-788 ( N-cys-2,6-dimethylpiperidinocarbonyl- l-γ-methylleucyl- d-1-methoxycarboyl- d-norleucine; endothelin ET B receptor antagonist), but not BQ-123 (cyclo[ d-Trp- d-Asp-Pro- d-Val-Leu]; endothelin ET A receptor antagonist; each at 30 pmol). The selective endothelin ET B receptor agonist BQ-3020 ( N-Ac-Ala 11,15-endothelin-1 (6–21)) fully mimicked the hypernociceptive effects of the natural endothelins. Treatments with indomethacin, atenolol or dexamethasone did not inhibit endothelin-1-evoked mechanical hypernociception. However, endothelin-1-induced mechanical hypernociception was potentiated by the cAMP phosphodiesterase inhibitor rolipram (4-[3-(cyclopentyloxy)-4-methoxyphenyl]-2-pyrrolidinone) and inhibited by the PKC inhibitors staurosporine and calphostin C, but was unaffected by the PKA inhibitor H89 ( N-[2-(( p-bromocinnamyl)amino)ethyl]-5-isoquinolinesulfonamide). Thus, endothelins, acting through endothelin ET B receptors, induce mechanical hypernociception in the rat hindpaw via cAMP formation and activation of the PKC-dependent phosphorylation cascade.

Estradiol’s effect on portal response to endothelin-1 after trauma-hemorrhage

Background The fine balance between vasoconstrictors and vasodilators maintains portal circulation. Studies have shown that portal response to endothelin-1 (ET-1), a potent vasoconstrictor, is enhanced following hemorrhagic-shock, which subsequently leads to the impaired hepatic circulation and hepatic damage. Although protective effects of 17β-estradiol (E 2) against hepatic damage following trauma-hemorrhage have been observed, it remains unknown whether E 2 directly improves hepatic circulation. We hypothesized that the salutary effects of E 2 are mediated, at least in part, by the attenuation of portal response to ET-1 following trauma-hemorrhage. Materials and methods Male adult Sprague-Dawley rats were randomly assigned to sham operation or trauma-hemorrhage with or without in vivo E 2 treatment. Trauma-hemorrhage included midline laparotomy and ∼90 min of hemorrhagic shock (35 mmHg), then resuscitation with four times the shed blood volume with Ringer’s lactate solution over 60 min. For the E 2 treatment group, 1 mg/kg of E 2 was added to the Ringer’s lactate solution. At 5 h after the end of resuscitation, the liver was isolated and perfused in vitro to measure portal pressure responses to exogenous ET-1 (60 pmol in 150 ml perfusate, bolus) with or without E 2 (1,500 pg/ml). Results Peak portal pressure after the administration of ET-1 was significantly higher in vehicle-treated trauma-hemorrhage group compared with the sham group. This effect was significantly attenuated in the E 2 treatment group. Furthermore, E 2 treatment restored bile production and prevented hepatic damage following trauma-hemorrhage. Conclusions The beneficial effects of estradiol observed following trauma-hemorrhage, at least partly, are caused by the attenuation of portal pressure response to increased ET-1.

Sensory fibers resistant to the actions of tetrodotoxin mediate nocifensive responses to local administration of endothelin-1 in rats

Endothelin-1 (ET-1) applied to the sciatic nerve or injected into the plantar hindpaw of rats induces pain behavior (ipsilateral hindpaw flinching) and selective excitation of nociceptors by activation of endothelin-A (ET A) receptors. To determine the pharmacological profile of the sensory fibers that mediate this pain behavior, we administered lidocaine (LID, a non-selective conduction blocker) or tetrodotoxin (TTX) prior to ET-1. LID (1 or 2%, 0.1 ml) was injected percutaneously into the sciatic notch, or TTX (10 μM, 4 μl) was injected into the sciatic nerve prior to the more distal application of ET-1 (400 μM, 40 μl) onto the sciatic nerve or subcutaneously into the plantar hindpaw (400 μM, 10 μl). LID inhibited ET-1-induced flinching in a dose-dependent manner; the mean total number of flinches was reduced by 39% for 1% LID and by 87% for 2% LID. In contrast, TTX failed to inhibit flinching behavior induced by sciatic nerve application of ET-1 despite a similar magnitude of motor and sensory blockade as that observed with 2% LID. Partial blockade of flinching behavior by intraneural TTX (mean total flinches were reduced by 51%) was observed after subcutaneous injection of ET-1. Unexpectedly, ET-1 prolonged the actions of 1% LID and, even when applied alone, produced clear signs of motor and sensory conduction block. These results are evidence that ET-1-induced pain is transmitted to the central nervous system via sensory fibers using tetrodotoxin-resistant sodium channels, and that ET-1 has analgesic actions that exist despite the activation of local pain pathways.

Functional and structural analysis of the visual system in the rhesus monkey model of optic nerve head ischemia.

A redistribution of neurochemicals has been identified in the visual cortex of monkeys with laser-induced glaucoma. Examined were functional, structural, and neurochemical changes to the retina, optic nerve, and central visual system in a nonhuman primate model of optic nerve head (ONH) ischemia caused by sustained unilateral administration of endothelin (ET)-1 to the optic nerve. ET-1 or sham control solution was delivered by osmotic minipump to the retrolaminar region of one optic nerve of rhesus monkeys (Macaca mulatta) for 1.5 years. ONH topography and blood flow velocity were serially studied with scanning laser tomography and laser Doppler flowmetry, respectively. Retinal and cortical electrophysiologic measurements from pattern-derived stimuli were obtained quarterly. Immunohistochemistry was used to identify the distribution of calbindin (CB) and c-Fos labeled neurons in the visual cortex areas V1 and V2, and lateral geniculate nucleus (LGN). Retinal ganglion cell counts and optic nerve axon density were determined by light microscopy. No significant changes in retinal and ONH morphology, ONH blood flow velocity, and retinal and cortical pattern-derived functional activity were detected. Measurement of CB-positive cell density in V1 and V2 showed a significant decrease in CB labeling to the contralateral side of the ET-1-treated eye (P < 0.04). CB-positive cells were present in the magnocellular layers of the LGN with no differences noticed between the ET-1- and sham-treated eyes. c-Fos-labeled neurons were found in striate area V1 and extrastriateV2 of both groups. No c-Fos labeling was observed in the LGN. Administering ET-1 to the orbital optic nerve alters neuronal metabolic activity in the visual cortex in rhesus monkeys. Metabolic activity reductions in the visual cortex precede the ability to detect functional and structural alterations in the retina, ONH, and visual cortex in this animal model.

Effect of Iloprost ® on endothelin-1-induced free radical activation in rabbit brain stem

Iloprost, a stable analogue of prostacyclin, was used to reverse the early period of vasoconstriction provoked by Endothelin-1 by administering into the rabbit basilar artery. We observed if this produced an effect on the central nervous system parenchyma mediated by free radical system. The red neurons were counted in brain stem sections stained with haematoxylin and eosin, while superoxide dismutase and malondialdehyde levels were measured in brain stem tissue samples as a marker of reactive oxygen metabolites; both 30 and 90 min after administration of either Endothelin-1 (0.25 ng) alone or Endothelin-1 followed by Iloprost (0.5 μg/kg) into the basilar artery. Endothelin-1 significantly increased the number of red neurons, while Iloprost significantly reduced them after 30 and 90 min. However, regarding the reactive oxygen metabolites; a similar reversing effect of Iloprost was not observed although superoxide dismutase levels were significantly decreased after Endothelin-1 infusion.

Intracerebroventricular administration of an endothelin ETB receptor agonist increases expressions of GDNF and BDNF in rat brain

Endothelins (ETs) are suggested to be involved in functional alterations of astrocytes after brain injury, including proliferation, hypertrophy and production of neurotrophic factors. In this study, effects of Ala1,3,11,15-endothelin-1 (Ala1,3,11,15-ET-1), an ETB receptor selective agonist, on neurotrophic factor production were examined in rat brain. A continuous intracerebroventricular administration of Ala1,3,11,15-ET-1 (500 pmol/day for 7 days) increased the numbers of GFAP- and vimentin-positive astrocytes in the hippocampus, caudate putamen and cerebrum. Ala1,3,11,15-ET-1 did not induce neuronal degeneration and activation of microglia/macrophage in these brain regions. The intracerebroventricular administration of Ala1,3,11,15-ET-1 for 7 days caused two- to three-fold increases in glial cell line-derived neurotrophic factors (GDNF) mRNA in the hippocampus and cerebrum. The mRNA levels of brain-derived neurotrophic factors (BDNF) in caudate putamen were increased by Ala1,3,11,15-ET-1. Expressions of nerve growth factor (NGF) and basic fibroblast growth factor (bFGF) mRNA in these regions were not largely affected by Ala1,3,11,15-ET-1, except cerebral NGF mRNA level was increased. The Ala1,3,11,15-ET-1-induced increases in GDNF and BDNF mRNA levels were accompanied by increases in immunoreactive GDNF and BDNF. Immunohistochemical observations showed that GFAP-positive astrocytes expressed GDNF and BDNF in the brain regions of Ala1,3,11,15-ET-1-infused rats. In cultured rat astrocytes, Ala1,3,11,15-ET-1 (100 nm) increased mRNA levels of GDNF and BDNF. These results suggest that activation of brain ETB receptors induced GDNF and BDNF expression in astrocytes.

Chronic endothelin-1 treatment leads to insulin resistance in vivo.

We determined whether chronic endothelin-1 (ET-1) treatment could lead to in vivo insulin resistance. Like insulin, ET-1 acutely stimulated glucose transport in isolated soleus muscle strips of WKY rats. ET-1 pretreatment (1 h) decreased insulin-stimulated glucose transport in muscle strips (-23%). Both ET-1-mediated effects were generated through ET(A) receptors, because a specific ET(A) receptor antagonist (BQ610) blocked these effects of ET-1. Osmotic minipumps were used to treat normal rats with ET-1 for 5 days. Subsequent hyperinsulinemic-euglycemic clamps showed that ET-1 treatment led to an approximately 30% decrease in insulin-stimulated glucose disposal rates in male and female rats. In addition, ex vivo study of soleus muscle strips showed decreased glucose transport into muscle from ET-1-treated animals. With respect to insulin signaling, chronic in vivo ET-1 treatment led to a 30-40% decrease in IRS-I protein content, IRS-I-associated p110(alpha), and AKT activation. In summary, 1) in vitro ET-1 pretreatment leads to decreased insulin-stimulated glucose transport in skeletal muscle strips; 2) chronic ET-1 administration in vivo leads to whole-body insulin resistance, with decreased skeletal muscle glucose transport and impaired insulin signaling; and 3) elevated ET-1 levels may be a cause of insulin resistance in certain pathophysiologic states.

Endothelin is involved in pancreatic and intestinal ischemia during severe acute pancreatitis

Introduction: Severe acute pancreatitis is occasionally associated with pancreatic and intestinal necrosis. Vasoconstriction of the abdominal artery is one of the most probable pathogeneses of these complications. Hence, we investigated the involvement of endothelin-1 (ET-1), a potent vasoconstrictor, in the pathogenesis of acute pancreatitis. Materials and methods: (1) Plasma samples from patients with acute pancreatitis were analyzed. (2) Production of ET-1 mRNA in rat pancreas and ET-1 protein by human umbilical vein endothelial cells (HUVECs) in vitro was analyzed. (3) After the administration of ET-1, pancreatic and intestinal blood flow was analyzed in rat. Results: (1) Plasma ET-1 concentrations were extremely high in patients with pancreatic and diffuse intestinal necrosis. (2) ET-1 mRNA was demonstrated in rat pancreas after the induction of pancreatitis, and the production of ET-1 protein by HUVECs was enhanced by TNF-α, thrombin, and trypsin receptor-activating peptide. (3) Administration of ET-1 induced mesenteric arterial spasm and resultant decreased splanchnic blood flow. Conclusions: These results suggest that (1) ET-1 is produced in and around the pancreas, mainly by endothelial cells, in severe acute pancreatitis; (2) in the inflammatory setting, cytokines, thrombin, and trypsin may stimulate ET-1 production through paracrine fashion; (3) produced ET-1 may exaggerate the splanchnic microcirculation; and (4) progressive ischemia may lead to necrosis of the pancreas and intestine.

Role of endothelin in regulation of resistance, fluid-exchange, and capacitance functions of the systemic circulation.

This article reviews data at the in vivo whole animal and human level. The importance of both flow and pressure recordings and of the methods used to record these variables is emphasized. Exogenous administration of endothelin-1 evokes a transient depressor response mediated by endothelial endothelinB receptors, but the predominate effect of endothelin-1 is a sustained increase in blood pressure resulting from increases in total peripheral resistance. Resistance in the superior mesenteric, renal, and hindquarter vascular beds of animals and forearm resistance in humans is increased. Both endothelinA and, to a lesser extent, endothelinB receptors on vascular smooth muscle mediate the increases in resistance. Endothelin-1 evokes decreases in the precapillary/postcapillary resistance ratio, resulting in increased capillary pressure and net transcapillary filtration. Endothelin-1 evokes increases in mean circulatory filling pressure in animals and in constriction of the human dorsal hand vein. This venoconstrictor activity is mediated primarily through endothelinA and to a lesser extent endothelinB receptors. Endogenously generated endothelin contributes to the hemodynamic effects of angiotensin and vasopressin in certain animal models of hypertension. Antagonists of endothelin evoke modest hemodynamic changes in healthy humans and in some healthy animals, and they decrease vascular resistance dramatically in several salt-sensitive rat models of hypertension and also in some hypertensive human subjects. Thus, endogenously generated ET appears to play a modest role in the healthy organism, but it likely plays a major role in many pathophysiological states as described in companion articles in this issue.

Effects of endothelin-1 on mitochondrial function during the protection against myocardial cell apoptosis

Endothelin-1 is a potent survival factor against myocardial cell apoptosis. While apoptotic stimuli often perturb mitochondrial function by decreasing the membrane potential as well as oxygen consumption, it is unknown whether ET-1 can rescue such perturbation by apoptotic stimuli. Administration of endothelin-1 inhibited the H 2O 2-induced release of cytochrome c from mitochondria to the cytosol in cardiac myocytes, indicating the involvement of the mitochondria-dependent pathway in the anti-apoptotic effect of endothelin-1. We showed here by cytofluorimetric analysis that endothelin-1 prevented the H 2O 2-induced decrease of membrane potential. However, endothelin-1 was unable to reverse the H 2O 2-mediated decrease in oxygen consumption and electron transport in the mitochondria of cardiac myocytes. Endothelin-1 was unable to rescue cardiac myocytes from apoptosis when administered after the decrease in mitochondrial membrane potential. These data suggest that endothelin-1 does not target the mitochondrial respiratory chain, but rather stabilizes the mitochondrial membrane during the protection against apoptosis.

Further evidence for endothelin as an important mediator of pancreatic and intestinal ischemia in severe acute pancreatitis.

Severe acute pancreatitis is occasionally associated with pancreatic and intestinal necrosis. Mesenteric vasoconstriction is one of the most probable types of pathogenesis of these complications. To investigate the involvement of endothelin-1 (ET-1), a potent vasoconstrictor. Plasma ET-1 concentrations were extremely high in patients with pancreatic and/or diffuse intestinal necrosis. ET-1 mRNA was demonstrated in the rat pancreas, and the production of ET-1 protein by human umbilical vein endothelial cells was enhanced by tumor necrosis factor-alpha, thrombin, and protease-activated receptor-2-activating peptide. Administration of ET-1 in vivo induced mesenteric arterial spasm and decreased pancreatic and intestinal blood flow. These results suggest the following: ET-1 is produced in and around the pancreas, mainly by endothelial cells, in severe acute pancreatitis; in the inflammatory setting, cytokines, activated thrombin and trypsin, may stimulate ET-1 production in a paracrine fashion; produced ET-1 may exaggerate the splanchnic microcirculation; and progressive ischemia may lead to necrosis of the pancreas and intestine.

Regulation of human retinal blood flow by endothelin-1

There is evidence from in vitro and animal studies that endothelin is a major regulator of retinal blood flow. We set out to characterize the role of the endothelin-system in the blood flow control of the human retina. Two studies in healthy subjects were performed. The study design was randomized, placebo-controlled, double-masked, balanced, two-way crossover in protocol A and three way–way crossover in protocol B. In protocol A 18 healthy male subjects received intravenous endothelin-1 (ET-1) in a dose of 2·5 ng kg −1 min −1 for 30 min or placebo on two different study days and retinal vessel diameters were measured. In protocol B 12 healthy male subjects received ET-1 in stepwise increasing doses of 0, 1·25, 2·5 and 5 ng kg −1 min −1 (each infusion step over 20 min) in co-infusion with the specific ET A-receptor antagonist BQ123 (60 μg min −1) or placebo or BQ123 alone investigating retinal vessel diameters, retinal blood velocity and retinal blood flow. Measurements of retinal vessel size were done with the Zeiss retinal vessel analyzer. Measurements of blood velocities were done with bi-directional laser Doppler velocimetry. From these measurements retinal blood flow was calculated. In protocol A exogenous ET-1 tended to decrease retinal arterial diameter, but this effect was not significant versus placebo. No effect on retinal venous diameter was seen. In protocol B retinal venous blood velocity and retinal blood flow was significantly reduced after administration of exogenous ET-1. These effects were significantly blunted when BQ-123 was co-administered. By contrast, BQ-123 alone had no effect on retinal hemodynamic parameters. Concluding, BQ123 antagonizes the effects of exogenously administered ET-1 on retinal blood flow in healthy subjects. In addition, the results of the present study are compatible with the hypothesis that ET-1 exerts its vasoconstrictor effects in the retina mainly on the microvessels.

Intracerebroventricular administration of endothelin-1 impairs the habituation of rats to a novel environment in conjunction with brain serotonergic activation

The effects of i.c.v. administration of endothelin-1, at a low dose that does not produce abnormal behaviors such as barrel-rolling, on the emotional state of rats exposed to a novel environment were examined. Changes in the emotional state of rats with a novel environment were evaluated in terms of changes in exploratory activity in the hole-board apparatus, i.e., locomotor activity as well as the number and duration of rearing and head-dipping behaviors. Rats treated with i.c.v. saline showed marked exploratory behaviors immediately after exposure to the hole-board apparatus, but these exploratory behaviors decreased rapidly with time. On the other hand, the habituation of rats to a novel environment was prolonged by the i.c.v. administration of endothelin-1 (0.3 and 1 pmol). Furthermore, we also found that i.c.v. administration of endothelin-1 (1 pmol) significantly increased the serotonin (5-hydroxytryptamine) turnover in some brain regions, i.e., the cerebral cortex, hippocampus and midbrain, and the inhibition of brain 5-hydroxytryptamine synthesis by treatment with p-chlorophenylalanine (200 mg/kg/day, s.c.) for 2 days suppressed the behavioral effects of endothelin-1 (1 pmol, i.c.v.). In addition, i.c.v. administration of endothelin-1 (1 pmol) did not affect the spontaneous motor activity of rats. The present study demonstrated that i.c.v. administration of low doses of endothelin-1 impairs the habituation of rats to a novel environment in conjunction with brain 5-hydroxytryptaminergic activation. These results suggest that the central endothelin system may play a significant role in mediating emotionality.

Effect of endothelin on vasomotor and respiratory neurons in the rostral ventrolateral medulla in rats.

1. We have previously shown that intracisternal administration of endothelin-1 (ET-1) elicited cardiorespiratory responses acting on the ventral surface of the medulla oblongata (VSM) subjacent to the rostral ventrolateral medulla (RVLM). In this study, we examined whether vasomotor and respiratory neurons in RVLM participate in above-mentioned responses and whether those neurons respond to direct iontophoretic application of ET-1 and/or an ET-A receptor antagonist, FR139317. 2. Unit activity of vasomotor, respiratory, or nociceptive neurons in RVLM was recorded together with arterial blood pressure (AP) and heart rate (HR) in urethaneanesthetized Sprague-Dawley rats. 3. Intracisternal administration or topical application of ET-1 (0.1-1 pmol) to VSM caused excitation of the majority of vasomotor neurons (15/18) and respiratory neurons (10/11) but not in nociceptive neurons (0/7). Changes in neuronal activity were in similar time course with corresponding changes in AP and HR. Iontophoretic application of ET-1 to the vicinity of recording neuron caused excitation in 19 of 21 vasomotor neurons without affecting AP nor HR. Remaining two neurons were insensitive to ET-1. FR139317 did not affect basal activity of the vasomotor neurons but inhibited ET-1-evoked excitation. Twenty-four of 40 respiratory neurons were excited and 13 were inhibited by iontophoretic application of ET-1. Five of ET-1-excited respiratory neurons were inhibited by FR139317 alone while six of ET-1-inhibited neurons were not affected by FR139317 alone. In both cases, FR139317 inhibited the effect of simultaneously applied ET-1. Iontophoretic application of ET-1 excited only one out of 10 nociceptive neurons so far tested. 4. These results support the view that intracisternally administered ET-1 alters activity of vasomotor and respiratory neurons in the RVLM, at least in part by acting directly on neurons themselves and hence causes systemic cardiorespiratory changes. Majority of vasomotor and respiratory neurons should express ET-A receptors and some respiratory neurons are under tonic excitatory control by ET-1.

Highly enhanced permeability of blood-brain barrier induced by repeated administration of endothelin-1 in dogs and rats.

The effects of intracisternal administration of endothelin-1 on blood-brain barrier permeability were examined in dogs and rats. Single doses of endothelin-1 elevated blood-brain barrier permeability about two times compared with control groups in both species. However, repeated dosing with endothelin-1 at a 24 hr intervals caused a highly enhanced disruption of blood-brain barrier permeability in dogs, but not in rats, whereas the repeated administration with a 48 hr interval markedly increased the blood-brain barrier permeability in both species of animals (dogs: 923%, rats: more than 661%). Moreover, this abnormally enhanced permeability of blood-brain barrier in dogs was completely blocked by pretreatment with the endothelin ET-A receptor selective antagonist, S-0139, administered prior to either the first or second dosing with endothelin-1. From these results, we conclude that a repeated attack of endothelin-1 from the adventitial site of brain blood vessels produces a severe disruption of blood-brain barrier permeability through an endothelin ET-A receptor-mediated process.

Nociceptive reaction and thermal hyperalgesia induced by local ET-1 in mice: a behavioral and Fos study.

The peptide endothelin-1 (ET-1) has been involved in nociception independently of its vasoconstrictor effects. We have studied the direct nociceptive behavior produced by this peptide as well as its ability to induce thermal sensitisation (as measured by the unilateral hot plate method, UHP) when intraplantarly (i.pl.) administered in mice. These behavioural measures were complemented by the quantification of Fos-protein immunoreactivity in the superficial laminae of the dorsal horn spinal neurons located ipsilateral to the injected paw.ET-1 induces licking (60-600 pmol, i.pl.) and thermal hyperalgesia (20-200 pmol, i.pl.) in the injected paw, both effects being inhibited by the coadministration of ET-1 with endothelin type A (ET(A)) receptor antagonist, BQ-123 (0.3-10 nmol), but not with endothelin type B (ET(B)) receptor antagonist, BQ-788 (10 nmol). Moreover, the licking behavior induced by ET-1 was dose-dependently inhibited by the prototypical micro -opioid agonist, morphine. The prior i.pl. administration of ET-1 (200 pmol) to mice subjected to thermal heat stimulus (55+/-1 degrees C, 10 s) increases the number of Fos-immunoreactive dorsal horn spinal neurons compared with the application of noxious heat alone. This effect is inhibited by BQ-123 (10 nmol) but not by BQ-788 (10 nmol).Thus, local ET-1 induces nocifensive behavior and thermal hyperalgesia acting through ET(A) receptors. These same receptors seem to be also involved in the amplification of Fos immunoreactivity induced by ET-1 under heat stimulus in the dorsal horn neurons. These results could help to characterize the role of ET-1 in nociceptive processing, a topic of special interest due to the pathophysiological involvement of this peptide in painful states such as cancer.

Endothelin-1 produces no renal vasoconstriction in conscious newborn lambs.

Experiments were carried out to investigate the effects of endothelin-1 (ET-1) on renal vascular tone during development under physiological conditions in conscious lambs. Renal blood flow (RBF), renal vascular resistance (RVR), mean arterial pressure (MAP), and heart rate (HR) were measured in conscious, chronically instrumented lambs aged approximately 1 week and 6 weeks before and after intra-arterial (i.a.) injection of 0, 100, 200, and 400 ng/kg body weight of ET-1. In addition, plasma levels of ET-1 were measured in 39 sheep aged 5-85 days. In 6-week-old lambs, i.a. injection of ET-1 was associated with a rapid dose-dependent decrease in RBF that resulted from a dose-dependent increase in RVR. In 1-week-old lambs, there was no renal vasoconstriction observed after ET-1 administration, even at the highest dose tested. In response to i.a. injection of ET-1 to 1-week-old and 6-week-old lambs, MAP increased and there was a concomitant decrease in HR; these effects were dose dependent but not age dependent. Plasma levels of ET-1 were 10.7+/-4.2 pg/ml at 5-10 days, and remained constant throughout the first 3 months of life in conscious sheep. We conclude that ET-1 is not a renal vasoconstrictor agent in the immediate newborn period, and that the effects of ET-1 on renal vascular tone appear to be developmentally regulated.

Platelet-activating factor enhanced the pressor response of endothelin-1.

Both endothelin-1 (ET-1) and platelet-activating factor (PAF) have been suggested to play a role in the regulation of the cardiovascular system. In view of the limited data regarding the interaction between ET-1 and PAF, the hemodynamic effects of ET-1 and PAF, either alone or in combination, were investigated in the current study. Anesthetized male Sprague Dawley rats received bolus intravenous injections of ET-1 (1 and 2 nmol/kg) and/or PAF (0.075, 0.15 and 0.3 nmol/kg). In some experiments, the ET receptor antagonist, FR-139317 (2.5 or 5 mg/kg), were injected 5 min before the administration of ET-1 or PAF. ET-1 caused a biphasic response consisting of an initial depressor followed by a delayed but sustained pressor response. Injection of PAF to anesthetized rats resulted only in a decrease in arterial blood pressure. Interestingly, the pressor effect of ET-1 was significantly enhanced in the concomitant presence of PAF. Pretreatment with FR-139317 inhibited the magnitude of ET-1-induced hypertension and increased the duration of the depressor action of ET-1. The time-course of PAF-induced decrease of arterial blood pressure was also prolonged in rats pretreated with FR-139317. These data therefore suggested that ET receptors were activated, either directly or indirectly, by PAF, possibly to facilitate the return of blood pressure to resting level following a depressor response. Thus the activation of ET receptors by PAF might result in the enhancement of the pressor response of ET-1 observed in the current study.

Endothelin-1-induced elevations in purine metabolite concentrations -- autoregulatory protection in the canine pericardium?

Pericardial fluid accumulates the cardioprotective purine metabolites, as well as the endogenous vasoconstrictor agent endothelin-1 (ET-1). The aim of the present study was to characterize the pericardial concentrations of the purine metabolites adenosine, inosine and hypoxanthine before and after intrapericardial administration of ET-1 to the in situ heart. The closed pericardial sac of anaesthetized dogs (n=9) was cannulated for ET-1 administration and for obtaining native pericardial fluid and control pericardial infusate samples (C1 and C2), as well as consecutive pericardial infusate samples (samples I, II and III) obtained 15 min after intrapericardial administration of 150 pmol/kg ET-1. In an additional five dogs, using the same protocol, ventricular epicardial and endocardial monophasic action potential recordings were performed to assess local ischaemic electrophysiological changes. Significant elevations of pericardial purine metabolite concentrations (measured by HPLC) were found in sample II compared with sample C2: adenosine, 4.5+/-1.7 compared with 0.5+/-0.1 microM (P<0.05); inosine, 18.3+/-2.8 compared with 0.9+/-0.2 microM (P<0.001); hypoxanthine, 38.1+/-8.0 compared with 13.4+/-2.6 microM (P<0.01). Systemic blood pressure, left ventricular pressure and contractility, and systemic plasma levels of the purine metabolites remained unchanged during the ET-1 effect, while significant ECG ST elevations (ST(max) 0.68+/-0.01 mV; P<0.001) were observed. In the five dogs analysed electrophysiologically, the left ventricular epicardial monophasic action potential duration and upstroke velocity decreased significantly at time point II compared with C2, while the endocardial monophasic action potential duration and upstroke velocity did not show ischaemia-related changes. The results suggest that intrapericardial administration of ET-1 induces subepicardial ischaemia, with parallel activation of coronary metabolic adaptive and cardiac self-protective mechanisms in the epimyocardial layer of the heart.

Expressed monophasic action potential alternans before the onset of ventricular arrhythmias induced by intracoronary bolus administration of endothelin-1 in dogs.

We showed previously a direct arrhythmogenic effect of the intracoronary infusion of endothelin-1 (ET-1). We aimed to examine the electrophysiological effects of intracoronary bolus administration of ET-1 using monophasic action potential (MAP) recordings. Eight mongrel dogs received boli of ET-1 (1 and 2 nmol) into the left anterior descending coronary artery. These intracoronary ET-1 boli rapidly caused a marked decrease in coronary blood flow (1 nmol, 78+/-7%; 2 nmol, 89+/-7%). Ischaemic changes of MAP morphology, a decrease in upstroke velocity (baseline, 1.78+/-0.2 V/s; 1 nmol, 0.95+/-0.18 V/s; 2 nmol, 0.45+/-0.21 V/s; P<0.01) and a decrease in MAP duration at 90% repolarization (MAPD(90)) [1 nmol, from 191+/-3 to 176+/-5 ms (P<0.05); 2 nmol, from 212+/-4 to 180+/-8 ms (P<0.05)] occurred after ET-1 bolus administration. However, at 7-10 min after the 1 nmol bolus, a significant increase in MAPD(90) was observed (10 min, in the left ventricular anterior epicardial region: from 191+/-3 to 206+/-6 ms; P<0.05). The incidence of ventricular arrhythmias was as follows: after the 1 nmol ET-1 bolus: ventricular tachycardia, 3/8 animals; ventricular fibrillation, 1/8; after the 2 nmol ET-1 bolus: ventricular tachycardia, 5/7; ventricular fibrillation, 5/7. MAP alternans was present in each animal (1 nmol, 18.2+/-5.8%; 2 nmol, 10.8+/-2.5%). Thus electrophysiological and coronary blood flow changes indicate the predominance of an ischaemic arrhythmogenic effect of the bolus administration of ET-1 (shortening of action potential duration; appearance of MAP alternans), whereas the observed delayed prolongation of MAPD(90) suggests a direct arrhythmogenic effect of ET-1. The expressed MAP alternans could have a pathogenic role in the onset of ventricular arrhythmias induced by an intracoronary bolus of ET-1.