Tissue Specificity of Endothelin Binding Sites

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Abstract
A measurement was made of the binding of 125I-labeled endothelin (125I-ET) to crude membrane fractions prepared from rat aorta, atrium, ventricle, portal vein, trachea, lung parenchyma, vas deferens, ileum, bladder, and guinea-pig taenia coli and lung parenchyma. Scatchard analysis of 125I-ET binding in all tissues indicated binding to a single class of saturable sites. The affinity and density of 125I-ET binding sites varied between tissues. The Kd of 125I-ET binding was .apprx. 0.5 nM for rat aorta, trachea, lung parenchyma, ventricle, bladder, and vas deferens, and guinea-pig taenia coli and lung parenchyma, 1.8 nM for rat portal vein and atrium, and 3.3 nM for ileum. The Bmax of 125I-ET binding had the following rank order of density in rat tissues: trachea > lung parenchyma = vas deferens .mchgt. aorta = portal vein = atrium > bladder > ventricle = ileum. The properties of 125I-ET endothelin binding were characterized in rat ventricular membranes. 125I-ET binding was time dependent, reaching a maximum within 45-60 min at 25.degree.C. The calculated microassociation constant was 9.67 .+-. 105 s-1 M-1. Only 15-20% of 125I-ET dissociated from its binding site even when dissociation was studied as long as 3 h. Preincubation of ventricular membranes with ET prevented binding of 125I-ET. 125I-ET binding was destroyed by boiling of ventricular membranes and was temperature, pH, and cation (Ca2+, Mg2+, and Na+) dependent. F125I-ET binding was not affected by specific calcium channel ligands, N-methyl-D-asparate (NMDA) receptor-gated ionophore antagonists, peripheral benzodiazepines, opiates, atrial natriuretic factor, angiotensin II, and sodium nitroprusside. There was no correlation between the density of ET binding sites and the contractile efficacy of ET in rat and guinea pig tissues. Exposures of rat atrium tissues to ET (10-7 M) in vitro resulted in a significant (25%) inhibition of ET binding to atrium membranes isolated from these tissues. These results indicate that specific high-affinity 125I-ET binding sites with varying properties exist in a number of tissues. ET binding does not display simple reversible biomolecular kinetics but instead demonstrates kinetics characteristic of irreversible binding.

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