HOMOLOGOUS DESENSITIZATION OF THE BETA-ADRENERGIC-RECEPTOR - FUNCTIONAL INTEGRITY OF THE DESENSITIZED RECEPTOR FROM MAMMALIAN LUNG

Abstract

Previous work has demonstrated that injection of rats with isoproterenol is rapidly (10 min) followed by the development of a homologous form of desensitization of the .beta.-agonist-coupled adenylate cyclase in lung membranes. Half the receptor pool becomes sequestered in a light membrane fraction while the other half remains in the plasma membranes but becomes functionally uncoupled. In the present work we sought to assess whether "local sequestration" of the functionally intact receptor away from the effector adenylate cyclase in the plasma membrane contributes to the uncoupling of the .beta.-adrenergic receptor observed in the plasma membranes. We tested the functionality of the desensitized .beta.-adrenergic receptor in three different ways. We reconstituted the affinity chromatography purified control and "desensitized" receptors with pure Ns from human erythrocytes and assessed the ability to induce GTPase activity in Ns. Both control and desensitized .beta.-adrenergic receptors stimulate similar levels of GTPase activity in Ns 852 .+-. 38 versus 738 .+-. 49 fmol of Pi released/30 min (p > 0.05, n = 4). To further assess the relative ability of control and desensitized .beta.-adrenergic receptors to couple to another source of Ns we fused reconstituted .beta.-adrenergic receptors to Xenopus laevis erythrocytes, which contain Ns and adenylate cyclase but essentially no .beta.-adrenergic receptors. The functional interactions of control and desensitized .beta.-adrenergic receptor with the adenylate cyclase system of the acceptor cells was assessed by measuring the .beta.-agonist-stimulated adenylate cyclase activity and the agonist-induced formation of the high affinity state of the .beta.-adrenergic receptor (RH). Again both control and desensitized .beta.-adrenergic receptors appeared to interact with Ns to the same extent. To test if a local sequestration of the .beta.-adrenergic receptor away from Ns within the plasma membrane might contribute to the uncoupling of the .beta.-adrenergic receptors during desensitization, plasma membranes from control and desensitized lungs were treated with the fusogen polyethylene glycol to disrupt any compartmentalization of protein components within the plasma membrane. After polyethylene glycol treatment the previously uncoupled .beta.-adrenergic receptors could be recoupled to Ns as assessed by the formation of RH in agonist competition curves. These data suggest that in marked contrast to the heterologous type of desensitization, homologous desensitization may involve a local sequestration of a functionally intact .beta.-adrenergic receptor away from the adenylate cyclase effector system.