Desensitization and resensitization of beta-adrenergic receptors in a smooth muscle cell line

Exposure to the beta-adrenergic agonist, metaproterenol, elicits extensive receptor loss and desensitization of adenylate cyclase activity in the hamster DDT1, MF-2 cell line. The reappearance of beta-adrenergic receptors and restoration of adenylate cyclase activity were investigated. Receptor reappearance was investigated under conditions in which lost receptors were not detectable either on the cell surface or within the cell. Exposure to metaproterenol resulted in a 3-5-fold decrease in beta-adrenergic receptor affinity for agonist, an 85% reduction in beta-adrenergic receptor number per cell, and a 65% reduction in isoproterenol-stimulated adenylate cyclase activity without any change in NaF-stimulated enzyme activity. The rate of reappearance of the lost receptors was proportional to the concentration of metaproterenol to which the cells were initially exposed. Metaproterenol, at a concentration of 250 microM, induced long-term receptor loss which required 16 days in fresh media devoid of metaproterenol before the full complement of receptors reappeared. This prolonged receptor loss may be due to residual metaproterenol; however, the resensitization of isoproterenol-stimulated adenylate cyclase activity was restored 2 days after removal of metaproterenol. The lag period for the reappearance of receptors was shortened by incubation with either the beta-adrenergic antagonist, nadalol, or the glucocorticoid, methylprednisolone. Both pharmaceuticals reversed receptor down-regulation and up-regulated receptor number in control cells, although the extent and time course of restoration were different. These data suggest that the process of resensitization in DDT1 cells involves rapid restoration of adenylate cyclase activity and a slower reappearance of receptors over a time period of six population doublings.