Characterization of [125I]-endothelin-1 and [125I]-BQ3020 binding to rat cerebellar endothelin receptors

Endothelin-1 regulates oligodendrocyte development

In the postnatal brain, oligodendrocyte progenitor cells (OPCs) arise from the subventricular zone (SVZ) and migrate into the developing white matter, where they differentiate into oligodendrocytes and myelinate axons. The mechanisms regulating OPC migration and differentiation are not fully defined. The present study demonstrates that endothelin-1 (ET-1) is an astrocyte-derived signal that regulates OPC migration and differentiation. OPCs in vivo and in culture express functional ET(A) and ET(B) receptors, which mediate ET-1-induced ERK (extracellular signal-regulated kinase) and CREB (cAMP response element-binding protein) phosphorylation. ET-1 exerts both chemotactic and chemokinetic effects on OPCs to enhance cell migration; it also prevents lineage progression from the O4(+) to the O1(+) stage without affecting cell proliferation. Astrocyte-conditioned medium stimulates OPC migration in culture through ET receptor activation, whereas multiphoton time-lapse imaging shows that selective ET receptor antagonists or anti-ET-1 antibodies inhibit OPC migration from the SVZ. Inhibition of ET receptor activity also derepresses OPC differentiation in the corpus callosum in slice cultures. Our findings indicate that ET-1 is a soluble astrocyte-derived signal that regulates OPC migration and differentiation during development.

Metabolic regulation of coronary vascular tone: role of endothelin-1

Coronary tone is determined by a balance between endogenously produced endothelin and metabolic dilators. We hypothesized that coronary vasodilation during augmented metabolism is the net result of decreased endothelin production and increased production of vasodilators. Isolated rat myocytes were stimulated at 0, 200, and 400 beats/min to modify metabolism. Supernatant from these preparations was added to isolated coronary arterioles with and without blocking vasoactive pathways (adenosine, bradykinin, and endothelin). Chronically instrumented swine were studied while resting and running on a treadmill before and after endothelin type A (ET(A)) receptor blockade. The vasodilatory properties of the supernatant increased with increased stimulation frequencies. Combined blockade of adenosine and bradykinin receptors abolished vasodilation in response to supernatant of stimulated myocytes. ET(A) blockade increased vasodilation to supernatant of unstimulated myocytes but did not affect dilation to supernatant of myocytes stimulated at 400 beats/min. In vivo, ET(A) blockade resulted in coronary vasodilation at rest, which waned during exercise. Thus endothelin has a tonic constrictor influence through the ET(A) receptor at low myocardial metabolic demand but its influence decreased during increased metabolism.

Function of the endothelinB receptor in cardiovascular physiology and pathophysiology

One of the two receptors by which the potent vasoactive effects of endothelin (ET)-1 are mediated is the ET(B) receptor (ET(BR)), which is found in several tissues, but, more importantly from a cardiovascular point of view, on the endothelial cell. The endothelial cell also has the unique capability of releasing ET-1, as well as other factors, such as the endothelial-derived relaxing factors and prostacyclin, which counteract the myotropic effects of the peptide. The secretory and contractile responses to ET-1 rely on G-protein-coupled ET(BR)s, as well as ET(A)-G-protein-coupled receptor-like proteins. The mitogenic properties of ET-1 via ET(A) receptors (ET(AR)s) coupled to mitogen-activated protein kinases and tyrosine kinases on the vascular smooth muscle may occur in conjunction with the anti-apoptotic characteristics of the endothelial ET(BR)s. Interestingly, most of the relevant antagonists and agonists for both ET(AR)s and ET(BR)s have been developed by the pharmaceutical industry. This highlights the therapeutical potential of compounds that act on ET receptors. In normal as well as in physiopathological conditions, the ET(BR) plays an important role in the control of vascular tone, and must be taken into account when using ET receptor antagonists for the treatment of cardiovascular diseases. For the management of congestive heart failure, renal failure and primary pulmonary hypertension, the most recent literature supports the use of selective ET(AR) antagonists rather than mixed antagonists of ET(AR)s and ET(BR)s. Nonetheless, validation of this view will have to await the first clinical trials comparing the actions of ET(A) to mixed ET(A)/ET(B) receptor antagonists.

ET-1 modulates KCa-channel activity and arterial tension in normoxic and hypoxic human pulmonary vasculature

The molecular mechanisms by which endothelin (ET)-1 induces pulmonary hypertension are poorly understood. We investigated the effects of ET-1 on outward K+ currents of normoxic and chronically hypoxic human pulmonary arterial (PA) smooth muscle cells (HPSMCs). In normoxic HPSMCs, ET-1 has dual effects. In intact cells, 5 nM ET-1 activates the large-conductance and Ca2+-activated K+ (KCa)-channel current [IK(Ca)] by increasing intracellular Ca2+ concentration, whereas it directly inhibits IK(Ca) in isolated membrane patches. At a higher concentration (10 nM), ET-1-induced IK(Ca) inhibition predominates. In hypoxic HPSMCs, ET-1 at 5 nM significantly reduces IK(Ca). The ETA-receptor antagonist BQ-123 reverses the ET-1-induced decrease in IK(Ca). Chronic BQ-123 treatment also prevents the hypoxia-induced decrease in IK(Ca). In PA rings obtained from human organ donors, ET-1 causes a concentration-dependent increase in tension. The ET-1-mediated increase in tension is reversed by a KCa-channel agonist. The increase in tension at the highest concentration studied (9 nM) was more pronounced in PA rings obtained from patients with chronic obstructive pulmonary disease. These results imply that an ET-1-induced decrease in IK(Ca) contributes to chronic hypoxia-induced pulmonary hypertension.

Endothelin B receptor-mediated increase of cerebral blood flow in experimental pneumococcal meningitis

Study investigates the role of endothelin (ET) receptors in mediating early changes in cerebral blood flow--as measured by laser Doppler flowmetry (CBFLDF)--during experimental pneumococcal meningitis. Meningitis was induced with heat-killed pneumococci and confirmed by a significant increase in CBFLDF (baseline 100%; 225.3 +/- 21.8% after 6 hours; mean +/- SD), intracranial pressure (ICP), brain water content, and white blood cell count in the CSF. Intravenous administration of the selective endothelin B (ETB) receptor antagonist BQ-788 immediately before pneumococcal challenge (but not 4 hours afterward) significantly attenuated these pathophysiologic alterations (e.g., CBFLDF 6 hours after pneumococcal challenge: 116.7 +/- 17.4%). Pretreatment with BQ-123, a selective endothelin A receptor antagonist, had no significant effect on ICP and brain water content, but augmented the increase in CBFLDF and CSF white blood cell count. Since ET is known to trigger the release of nitric oxide (NO) by ETB receptor activation, we examined specific ET-NO interactions in primary rat cerebromicrovascular endothelial cells after stimulation with heat-killed pneumococci. Pneumococci induced a significant increase in both ET and NO concentrations in endothelial cell culture medium. Treatment with phosphoramidon, an inhibitor of the endothelin-converting enzyme, prevented the production of endothelin and markedly reduced NO generation. Our data provide evidence that ET is involved as a mediator in early pneumococcal meningitis in the rat and contributes to the increase in CBFLDF, ICP, brain water content, and CSF pleocytosis, presumably through ETB receptor-mediated NO production.

L-2-chloropropionic acid-induced neurotoxicity in the rat: a valuable model for studying selective neuronal cell death in vivo

1. L-2-Chloropropionic acid (L-CPA) is neurotoxic when administered orally as a neutral sodium salt in high doses to rats, resulting in a selective destruction of cerebellar granule cells with the result that animals develop marked difficulties in maintaining normal locomotion. 2. Cerebellar granule cell destruction is accompanied by a reduction in cerebellar glutamate and aspartate concentrations, reductions in the density of glutamate receptors located in the cerebellar granule cell layer and development of cerebellar oedema. No other cell type in the brain, nor other organ, is affected by L-CPA. 3. The neuronal cell death is necrotic in type, involving the activation of N-methyl-D-aspartate (NMDA) type glutamate receptors and there is some evidence for a partial role of nitric oxide in the development of the neurotoxicity. 4. Contrary to work performed on NMDA mediated cell death using neuronal cell culture approaches, L-CPA-induced granule cell death does not appear to arise from the production of excess quantities of cytotoxic free radicals, but may involve selective calcium-activated proteases, such as the calpains. 5. Tentative evidence suggests that L-CPA may interfere with voltage-dependent calcium channels in the cerebellum leading to activation of the cell death and resulting in the destruction of the granule cells. 6. In conclusion, L-CPA-induced neurotoxicity may provide valuable information on the neurochemical pathways involved in neuronal cell death that is associated with many neurological diseases.