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Cardiopulmonary bypass (CPB) has been implicated in the development of organ injury associated with cardiac surgery. At the molecular level, CPB is accompanied by a pronounced proinflammatory response including an increase in plasma interleukin (IL)-6. The IL-6 has been shown to be increased in rheumatoid arthritis, a chronic inflammatory disease, where it has been implicated in decreasing G protein–coupled receptor kinases (GRKs) in peripheral blood mononuclear cells. Since IL-6 is substantially increased after CPB, the study tested whether the increase of IL-6 during CPB leads to a decrease of GRKs in mononuclear cells. This is important because GRKs regulate the function of G protein–coupled receptors involved in inflammation.Fifteen patients had blood withdrawn before CPB, 2 h after CPB, and on postoperative day one (POD1). Plasma IL-6 concentrations were determined by enzyme-linked immunosorbent assay. The GRK protein expression and activity were determined by Western blot and phosphorylation of rhodopsin using [γ-32P] adenosine triphosphate, respectively.Plasma IL-6 increased over 20-fold after CPB and remained increased on POD1. Cytosolic GRK activity in mononuclear cells decreased by 39 ± 29%; cytosolic GRK2 and membrane-bound GRK6 decreased by 90 ± 15 and 65 ± 43%, respectively. The GRK activity and expression of GRK2/GRK6 on POD1 returned to basal levels in many but not all patients.The CPB causes a profound decrease in mononuclear cell GRKs, and the recovery of these kinases on POD1 is quite variable. The significance of the variable recovery of GRKs after CPB and their potential role as a marker of clinical outcome deserves further investigation.