Agonist-induced changes in beta- and alpha-1 adrenergic receptors of DDT/sub 1/ ME-2 smooth muscle cells

Agonist-induced changes in beta- and alpha-1 adrenergic receptors (BAR and AAR) of DDT/sub 1/ MF-2 smooth muscle cells were compared. Agonist competition for /sup 125/I-iodopindolol binding BAR and for /sup 3/H-prazosin binding to AAR was measured with intact cells in monolayer culture. In short-time assays (30 sec or 1 min), agonists exhibited predominantly high affinity for both BAR and AAR. In equilibrium assays (60 min), predominantly low affinity interaction was observed at both BAR and AAR. Thus, both of these receptors undergo conversion from high to low affinity for agonists during equilibrium competition binding assays. The possible occurrence of agonist-induced sequestration of BAR and AAR was assessed by sucrose density gradient centrifugation and by short-time competition binding assays with hydrophilic competing ligands. Following agonist pretreatment, both assays revealed rapid conversion of about half of cellular BAR, but not AAR, to a form having properties consistent with sequestration away from the cell surface. Thus, the apparently similar conversion of BAR and AAR to a low affinity form during equilibrium competition binding assays occurs together with receptor sequestration for BAR but in the apparent absence of receptor sequestration in the case AAR.