Abstract
In experiments on neonatal and adult rat ventricular myocytes, endothelin (ET) binding and the effects of ET on transmembrane signaling are quasi-irreversible. The ETA receptor antagonist BQ123 competes for binding and biochemical effects if added simultaneously with ET; when added after ET, the antagonist prevents neither binding nor activation of the Gi and Gqpathways. At 4°C, at which internalization of the ligand should be minimized, the interaction of [125I]ET is still irreversible. After binding of radio-labeled ligand at either 4°C or 37°C, only 50% of ligand is removed by acid washing. Permeabilization of the cells with Triton X-100 fails to release irreversibly bound ligand. Binding experiments in cell membranes mimic this irreversible binding. At 37°C, the addition of mercaptoethanol or dithiothreitol inhibits concurrent ET binding but does not cause the dissociation of previously bound ligand or the reversal of previously activated signaling. We conclude that ET binds irreversibly to myocytes, that this irreversibility is reflected in the biochemical responses of the cells to ET and that the irreversibility is more complex than the formation of S—S bonds between surface receptors and ET or internalization of bound ET. We interpret these findings and others in the literature in light of a testable model of ETA receptor/G protein/effector interaction in which quasi-irreversible binding of ET to the ETA receptor occurs before the interaction of the ligand/receptor complex with G protein and in which irreversible binding contributes to the prolonged effects of ET and is a prelude to refractoriness and to the slow regeneration of free ETA receptor.
Footnotes
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Send reprint requests to: Laurence L. Brunton, Ph.D., Department of Pharmacology 0636, UCSD School of Medicine, La Jolla, CA 92093-0636.
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↵1 This work was supported by National Institutes of Health Grant HL41307; American Heart Association Grant 91015560 (L.L.B.); a fellowship from the American Heart Association, California Affiliate (R.H.-D.); and National Institutes of Health Training Grants HL07444 and GM07752. A.G. is a Syntex Pharmacology Fellow.
- Abbreviations:
- TCA
- trichloroacetic acid
- PI
- phosphatidylinositol
- IP
- inositol phosphate
- ET
- endothelin
- PTX
- pertussis toxin
- DTT
- dithiothreitol
- GRK
- G protein-linked receptor kinase
- ETAR
- ET type A receptor
- ETBR
- ET type B receptor
- Received July 16, 1996.
- Accepted December 5, 1996.
- The American Society for Pharmacology and Experimental Therapeutics
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