Chloroethylclonidine discriminates between alpha 1A- and alpha 1B-adrenoceptors in the presence of guanosine 5'-triphosphate in rabbit thoracic aorta.

Studies on the displacement of [3H]prazosin binding by the alpha 1-agonist phenylephrine revealed the presence of at least high- and low-affinity binding sites in membrane preparations prepared from rabbit thoracic aorta. Although the low-affinity site was reduced by the pretreatment of tissues with chloroethylclonidine, this site was unaffected by the same pretreatment of membrane preparations that did not contain the GTP analog. However, in membrane preparations with the metabolically stable GTP analog GTP gamma-S (10(-5) M) and single cell preparations, the low-affinity site was completely eliminated by the chloroethylclonidine pretreatment. Displacement studies with the alpha 1-antagonist WB4101 also revealed high- and low-affinity binding sites labeled by [3H]prazosin. Displacement curves of WB4101 obtained from membrane preparations in the presence of GTP gamma-S (10(-5) M) did not differ from those in the absence of GTP gamma-S. These results suggest that the low affinity phenylephrine binding site labeled by [3H]prazosin was selectively bound by the chloroethylclonidine used to pretreat the tissues, membrane preparation containing GTP gamma-S and single cells, and that chloroethylclonidine is able to recognize these two distinct subtypes of alpha 1-adrenoceptors only when GTP gamma-S is present.