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Comparison of agonist-induced changes in beta- and alpha 1-adrenergic receptors of DDT1 MF-2 cells.

Agonist-induced changes in beta- and alpha 1-adrenergic receptors (BARs and AARs) were compared in the DDT1 MF-2 smooth muscle cell line. During equilibrium competition binding assays with intact cells at 37 degrees, agonists induced conversion of both BARs and AARs from a native form with high affinity for agonists to a form with much lower affinity for agonists. The native high affinity form of both receptors could be detected either in short-time competition binding assays at 37 degrees or in equilibrium competition binding assays on ice. Conversion to the low affinity form was nearly complete for BARs, but only about half of the AARs were converted to the low affinity form. For BARs, the high affinity form of the receptor observed in short-time assays with intact cells was similar to that observed in membrane preparations, whereas for AARs this form exhibited much higher affinity than was seen in membrane assays. None of these changes were observed during competition binding assays with antagonists. Both short-time competition binding assays with hydrophilic competing ligands and sucrose density gradient centrifugation assays were consistent with the occurrence of agonist-induced internalization of BARs. These same assays for AARs were consistent with the presence of some AARs in an intracellular compartment in the native state, but no agonist-induced increases in intracellular AARs were detected. During more prolonged exposure (13 hr) to agonists, about 80% down-regulation of BARs occurred, whereas only about 20% down-regulation of AARs was detected. These results may indicate that internalization and down-regulation are not involved in conversion of these receptors to the low affinity form observed in intact cell binding assays.