Endothelin ET(B1) receptor-mediated relaxation of rabbit basilar artery

Methamphetamine induces the release of endothelin

Methamphetamine is a potent psychostimulant drug of abuse that increases release and blocks reuptake of dopamine, producing intense euphoria, factors that may contribute to its widespread abuse. It also produces severe neurotoxicity resulting from oxidative stress, DNA damage, blood-brain barrier disruption, microgliosis, and mitochondrial dysfunction. Intracerebral hemorrhagic and ischemic stroke have been reported after intravenous and oral abuse of methamphetamine. Several studies have shown that methamphetamine causes vasoconstriction of vessels. This study investigates the effect of methamphetamine on endothelin-1 (ET-1) release in mouse brain endothelial cells by ELISA. ET-1 transcription as well as endothelial nitric oxide synthase (eNOS) activation and transcription were measured following methamphetamine treatment. We also examine the effect of methamphetamine on isolated cerebral arteriolar vessels from C57BL/6 mice. Penetrating middle cerebral arterioles were cannulated at both ends with a micropipette system. Methamphetamine was applied extraluminally, and the vascular response was investigated. Methamphetamine treatment of mouse brain endothelial cells resulted in ET-1 release and a transient increase in ET-1 message. The activity and transcription of eNOS were only slightly enhanced after 24 hr of treatment with methamphetamine. In addition, methamphetamine caused significant vasoconstriction of isolated mouse intracerebral arterioles. The vasoconstrictive effect of methamphetamine was attenuated by coapplication of the endothelin receptor antagonist PD145065. These findings suggest that vasoconstriction induced by methamphetamine is mediated through the endothelin receptor and may involve an endothelin-dependent pathway.

Selective blockade of endothelin-B receptors exacerbates ischemic brain damage in the rat

BACKGROUND AND PURPOSE

Endothelins act through 2 receptors, namely, ET(A) and ET(B). In the cerebral circulation, ET(A) mediates marked and prolonged vasoconstriction, and its blockade increases cerebral blood flow (CBF) and reduces ischemic brain damage. However, the role of ET(B) receptors remains unclear. In this study we examined, in rats, the kinetics of expression of ET(B) and the effects of ET(B) blockade on changes in CBF and brain damage after focal cerebral ischemia and N-methyl-D-aspartate (NMDA)-induced excitotoxic injury.

METHODS

Rats were subjected to transient (60 minutes) focal cerebral ischemia or cortical injection of NMDA. The selective ET(B) antagonist BQ-788 was injected intracerebroventricularly 30 minutes before and 30 minutes after the onset of ischemia. Cortical perfusion was monitored by laser-Doppler flowmetry. The volume of infarction or NMDA-induced cortical lesion was assessed at 24 hours after the insult. The reverse transcription-polymerase chain reaction technique was used to assess ET(B) expression.

RESULTS

Cerebral ischemia failed to alter the expression of ET(B) mRNA in both acute and chronic stages. Administration of BQ-788 resulted in an increase in infarction volume (178%; P<0.05) accompanied by a decrease in residual CBF (-26.7% versus control; P<0.01). In these animals we found a positive correlation between the volume of infarction and the severity of the decrease in CBF. NMDA-induced cortical lesions were not affected by the administration of BQ-788.

CONCLUSIONS

Our results suggest that the ET(B) antagonist BQ-788 induces deleterious effects that are mediated by the reduction of residual blood flow after ischemia and argue that the optimal therapeutic strategy in stroke would be to target the use of selective ET(A) antagonists and not mixed ET(A)/ET(B) antagonists.

Endothelin A but not endothelin B receptor blockade reduces capillary permeability in severe experimental pancreatitis

INTRODUCTION

Microcirculatory disorders, in particular increased capillary permeability (CapPerm), contribute to the multiple organ dysfunction syndrome in severe acute pancreatitis (AP). Endothelin receptor antagonists (ET-RA) have been shown to stabilize capillary leakage and improve organ function in AP.

AIM

To find out which endothelin receptor subtype (ET-A or ET-B) mediates the changes in CapPerm.

METHODOLOGY

Severe AP was induced in rats by intraductal bile salt infusion and i.v. cerulein. Animals were randomized to receive (1) saline; (2) selective ET-A-RA (LU-135252; 30 mg/kg); (3) selective ET-B-RA (A-192621); (4) nonselective ET-RA (LU-135252; 120 mg/kg); or (5) combined ET-A/B-RA (30 mg/kg LU-135252 + A-192621). Capillary blood flow (CBF) and CapPerm in the pancreas and colon and leukocyte rolling in mesenteric venules were determined.

RESULTS

Selective ET-A-RA increased CBF and decreased CapPerm in the pancreas and colon by 90-147% and reduced leukocyte rolling in AP but had no effect in healthy controls. Selective ET-B-RA increased pancreatic CBF (2.3 +/- 0.03 versus 2.1 +/- 0.04 nL/min) and enhanced CapPerm in the pancreas and colon by 24-35% in healthy controls but had no effect in AP. Blockade of both receptors produced effects similar to but less pronounced than those of selective ET-A-RA.

CONCLUSIONS

Blockade of ET-A and ET-B receptors has different effects on CapPerm in healthy animals and those with AP. This may explain the inconclusive results reported with nonselective ET-RA. In severe AP, blockade of ET-A but not ET-B receptors reduces CapPerm.

Reversal of hypercapnia induces endothelin-dependent constriction of basilar artery in rabbits with acute metabolic alkalosis

We recently concluded that constriction of basilar artery due to respiration-induced hypocapnia in rabbits with acute metabolic alkalosis and accompanying compensatory hypercapnia was independent of NO and K(ATP) channels. Based on reports that endothelin-1-mediated hypocapnic constriction of the rabbit basilar artery in vitro, we further investigated whether the respiration-induced hypocapnic constriction was endothelin-1 mediated. Metabolic alkalosis was induced acutely following ketamine/xylazine injection. The ET(A) plus ET(B) receptor antagonist, PD145065 (1 microM), and the selective ET(A) receptor antagonist, BQ610 (3 microM), completely relaxed the hypocapnic constriction, as determined in a cranial window. Unexpectedly, the ET(B) receptor antagonists, BQ788 and RES-701-1 (3 microM), relaxed the constriction by 72.1+/-2.8% (4) and 77.2+/-8.7% (5), respectively (means+/-S.E. (n)). To investigate whether the large magnitudes of relaxation to both ET(A) and ET(B) receptor antagonists were due to nonselectivity of the antagonists, the effects of the antagonists on the constriction to exogenous endothelin-1 were evaluated. BQ610, BQ788, and RES-701-1 relaxed the 3-5 nM endothelin-1 constriction by only 64.3+/-7.6% (4), 43.5+/-8.5% (5), and 26.7+/-4.8% (3) (means+/-S.E. (n)), respectively, consistent with the selective blocking action of these antagonists. To investigate whether the greater magnitude of BQ610, BQ788, and RES-701-1 relaxation of hypocapnic constricted versus exogenous endothelin-1-constricted vessels was due to differences between constriction elicited by endogenous versus exogenous endothelin-1, the effects of the endothelin receptor antagonists on constriction to isocapnic alkaline suffusate were evaluated. PD145065 (1 microM) and 0.1 mM phosphoramidon, an endothelin-converting enzyme inhibitor, inhibited the constriction to isocapnic alkaline suffusate by 83.8+/-7.8% (6) and 74.3+/-9.7% (8) (means+/-S.E. (n)), respectively, consistent with the endothelin-1 dependency of the constriction. BQ610, BQ788, and RES-701-1 relaxed the isocapnic alkaline suffusate constriction by 74.9+/-6.7% (5), 65.5+/-6.4% (5), and 78.0+/-6.5% (4) (means+/-S.E. (n)), respectively. Thus, the relaxation profile to the selective endothelin receptor antagonists in isocapnic alkaline constricted vessels more closely approximated the relaxation profile observed in hypocapnic constricted as compared to endothelin-1-constricted vessels. Hypocapnia did not alter the 5 nM endothelin-1 constriction. These results suggest that, under conditions of acute metabolic alkalosis and accompanying compensatory hypercapnia, subsequent hypocapnic constriction is endothelin mediated. Both ET(A) and ET(B) receptor activation may mediate the hypocapnic constriction. The hypocapnic constriction is not due to enhanced endothelin-1 constriction and, thus, is due to the release of endothelin-1 and/or additional endothelins.

Characterization of an endothelin ET(B) receptor in the gill of the dogfish shark Squalus acanthias

Endothelins (ETs) are potent vasoconstrictive peptides that are secreted by the vascular endothelium and other tissues in vertebrates. Previous studies have demonstrated that ETs are expressed in a variety of fish tissues and contract various blood vessels. In order to determine if receptors for ET are expressed in fish gill tissue, we examined the binding kinetics of (125)I-labeled, human ET-1 to membrane fragments isolated from the gill of the dogfish shark, Squalus acanthias. (125)I-ET-1 bound at a single site, with a dissociation constant (K(d)) and binding site number (B(max)) very similar to those described in a variety of mammalian blood vessels. ET-1 and ET-3 competed equally with (125)I-ET-1, suggesting that the receptor was ET(B), which has been shown in mammalian systems to bind to both ligands equally. The ET(B)-specific agonists sarafotoxin S6c, IRL-1620, and BQ-3020 also competed against (125)I-ET-1 at a single site, supporting this hypothesis. We conclude that the shark gill expresses an ET(B) receptor with substantial homology to the mammalian receptor and that ET may play an important role in modulating such vital gill functions as gas exchange, ion regulation, acid-base balance, and excretion of nitrogen.

Does blockade of endothelinB1-receptor activation increase endothelinB2/endothelinA receptor-mediated constriction in the rabbit basilar artery?

The purpose of this study was to investigate whether endothelin (ET)-1 activation of ETB1 receptors influences the relative magnitude of ETA/ETB2 receptor-mediated ET-1 constriction in the rabbit basilar artery. Initial challenge of ET-1-constricted vessels with BQ610, an ETA-receptor antagonist, resulted in approximately 60% relaxation, and subsequent addition of BQ788, an ETB1/2-receptor antagonist, relaxed the remaining constriction. To test whether blockade of ETB1 receptors influenced the relative magnitude of ETA/ETB2 receptor-mediated constriction, ET-1-constricted vessels were exposed to RES-701-1, an ETB1-receptor antagonist, before challenge with BQ610 or BQ788. RES-701-1 enhanced the ET-1 constriction by approximately 60%, consistent with blockade of ETB1 receptor-mediated endothelium-dependent relaxation. In ET-1-constricted vessels treated with RES-701-1, BQ610 challenge resulted in complete relaxation, whereas BQ788 was without effect. However, when 10 nM acetylcholine was added to RES-701-1-treated ET-1-constricted vessels, (a) BQ610 challenge resulted in only approximately 30% relaxation, and subsequent BQ788 addition relaxed the remaining constriction; and (b) BQ788 challenge resulted in approximately 35% relaxation, and subsequent BQ610 addition relaxed the remaining constriction. Acetylcholine induced approximately 10% relaxation of RES-701-1-treated ET-1-constricted vessels. It is speculated that a dynamic relation exists between ETA and ETB2 receptor-mediated constriction, such that ET-1-induced ETB2 receptor-mediated constriction of the basilar artery is dependent on ETB1 receptor activation and, in the absence of this activation, the constriction reverts to completely ETA receptor mediated.