The EC50 values for concentration-dependent stimulation of cAMP accumulation by CRF (1.3 nM) and urocortin (1.0 nM) were equivalent in human retinoblastoma Y79 cells. The time course and magnitude of CRF-and urocortin-induced CRF, receptor desensitization were similar. A significant 3-fold increase in GRK3, but not GRK2, mRNA levels accompanied the emergence of CRF1 receptor desensitization in Y79 cells exposed to CRF. In preliminary experiments, retinoblastoma GRK3 protein expression became upregulated during a 48-h CRF expo sure. Neither GRK3 nor GRK2 expression increased in Y79 cells exposed to urocortin for 10 min to 48 It. We hypothesize that GRK3 upregulation may be a cellular negative feedback process directed at maximizing CRF1 receptor desensitization by heightening GRK3 phosphorylating capacity during prolonged exposure to high CRF. Regulation of GRK expression associated with urocortin- and CRF-induced CRF1 receptor desensitization appears to differ, despite a similar level of signaling via the cAMP-protein kinase A pathway. (C) 2002 Elsevier Science (USA). All rights reserved.