DISSOCIATION CHARACTERISTICS OF ENDOTHELIN ET(A) RECEPTOR AGONISTS AND ANTAGONISTS

Endothelin (ET) binding to receptors has been shown to be almost irrev ersible. We studied the dissociation characteristics of ET(A) receptor agonists and antagonists using membranes prepared from rat pituitary cells (MMQ). Consistent with our previous report, competition studies comparing ET-I, ET-3, BQ123, FR139317, PD142893, and Ro46-2005 show th at MMQ cells contained predominantly the ET, receptor. [I-125]ET-1 bou nd to the receptor was difficult to dissociate. In contrast, bound BQ1 23, FR139317, and Ro46-2005 were easier to dissociate, suggesting that antagonist binding was more reversible. Although BQ123 (5 nM), FR1393 17 (1 nM), and Ro46-2005 (0.5 mu M) inhibited 0.1 nM [I-125]ET-1 bindi ng by greater than 80% after 15 min of incubation, the inhibition decr eased to less than 20% after 24 h of incubation. This decrease in bind ing inhibition was not the result of antagonist degradation. These res ults suggest that, similar to our previous observation made with the E T(B) receptor in porcine cerebellum, the ability of antagonists to inh ibit [I-125]ET-1 binding to the ET(A) receptor is critically dependent on the incubation time because of the difference in the dissociation characteristics between antagonists and ET-1.