Desensitization of the alpha-2 adrenergic receptor in HT29 and opossum kidney cell lines

Preincubation of HT29 cells with an alpha-2 adrenergic agonist resulted in a parallel rightward shift in the subsequent dose-response curve to 5-bromo-6-[2-imidazoline-2-yl-amino] quinoxaline (an alpha-2 adrenergic agonist) in inhibiting vasoactive intestinal peptide-stimulated cyclic AMP production. This rightward shift in the dose-response curve, which was concentration and time dependent, was interpreted as desensitization of the alpha-2 adrenergic receptor-mediated inhibition of cyclic AMP production. The fact that no decrease in efficacy was observed appears to result from a receptor reserve. Agonist preincubation effects on subsequent p-aminoclonidine (an alpha-2 adrenergic partial agonist) inhibition and partial irreversible inactivation of receptors confirmed the presence of an alpha-2 adrenergic receptor reserve in HT29 cells. Desensitization appeared to have a heterologous component since inhibition of vasoactive intestinal peptide-stimulated cyclic AMP production by somatostatin was also attenuated. We also assessed the effect of alpha-2 agonist preincubation on subsequent 5-bromo-6-[2-imidazoline-2-yl-amino] quinoxaline inhibition of parathyroid hormone-stimulated cyclic AMP production in OK cells. As with HT29 cells, agonist preincubation resulted in a concentration- and time-dependent shift in the dose-response curve. In both cell lines, long-term preincubation with an alpha-2 adrenergic agonist resulted in a 40% decrease in subsequent [3H]yohimbine binding, indicating down-regulation of the alpha-2 adrenergic receptor.