Dissociation characteristics of endothelin ETA receptor agonists and antagonists

Endothelin (ET) binding to receptors has been shown to be almost irreversible. We studied the dissociation characteristics of ETA receptor agonists and antagonists using membranes prepared from rat pituitary cells (MMQ). Consistent with our previous report, competition studies comparing ET-1, ET-3, BQ123, FR139317, PD142893, and Ro46-2005 show that MMQ cells contained predominantly the ETA receptor. [125I]ET-1 bound to the receptor was difficult to dissociate. In contrast, bound BQ123, FR139317, and Ro46-2005 were easier to dissociate, suggesting that antagonist binding was more reversible. Although BQ123 (5 nM), FR139317 (1 nM), and Ro46-2005 (0.5 microM) inhibited 0.1 nM [125I]ET-1 binding by greater than 80% after 15 min of incubation, the inhibition decreased to less than 20% after 24 h of incubation. This decrease in binding inhibition was not the result of antagonist degradation. These results suggest that, similar to our previous observation made with the ETB receptor in porcine cerebellum, the ability of antagonists to inhibit [125I]ET-1 binding to the ETA receptor is critically dependent on the incubation time because of the difference in the dissociation characteristics between antagonists and ET-1.