Although ATP-MgCl2 attenuates the release of inflammatory cytokines and restores the defective macrophage (Mφ) antigen presentation function after hemorrhage and resuscitation, it is not known whether administration of this agent after hemorrhage affects Mφ adenosine 3',5'-cyclic monophosphate (cAMP) levels and β-adrenergic receptors. To determine this, rats underwent a midline laparotomy (i.e., induction of trauma) and were then bled to and maintained at a mean arterial pressure of 40 mmHg until 40% of maximum bleedout volume was returned in the form of Ringer lactate (RL). Animals were resuscitated with four times the volume of shed blood with RL, during and after which ATP-MgCl2 (50 μmol/kg) or saline was administered over 95 min. At 1.5 h postresuscitation (i.e., 10 min after completion of ATP-MgCl2 infusion), peritoneal Mφ and Kupffer cells were isolated, and cAMP levels were measured by radioimmunoassay, β-Receptor binding characteristics were also determined in isolated Kupffer cells. The results indicate that cAMP levels increased significantly in both peritoneal Mφ and Kupffer cells after hemorrhage and resuscitation. Maximum binding capacity (B(max)) of β- receptors increased in Kupffer cells, suggesting that the elevated cAMP may be due to the increased β-receptor B(max) under such conditions. ATP-MgCl2 treatment, however, markedly decreased β-receptor B(max) in Kupffer cells and cAMP in both peritoneal Mφ and Kupffer cells, and the values were similar to shams. Thus normalization of Mφ cAMP levels and β-receptor binding capacity by ATP-MgCl2 may contribute to the immunoenhancing effects of this agent observed after trauma-hemorrhage and fluid resuscitation.