IS INHIBITION OF OXYGEN RADICAL PRODUCTION OF NEUTROPHILS BY SYMPATHOMIMETICS MEDIATED VIA BETA-2 ADRENOCEPTORS

Despite their beneficial effects on cardiovascular derangements in pat ients with severe sepsis, high doses of sympathomimetics might contrib ute to an impaired neutrophil function. This study was conducted to ex amine whether various sympathomimetics [(-)-epinephrine (EPI), dopamin e (DA) and dobutamine (DOB)] differ in their potency to suppress the f ormation of oxygen radicals by neutrophils and whether this potency co rrelates with their affinity to or intrinsic activity for beta-2 adren oceptors (beta-2 AR). Oxygen radical production of human neutrophils w as induced by N-formyl-methionyl-leucyl-phenylalanine and detected by chemiluminescence measurements. Dose-response curves for the inhibitio n of chemiluminescence by sympathomimetics were measured in the absenc e and presence of 0.1 mu M CGP 20,712 A (1-[2(3-carbamoyl-4-hydroxy -4 -trifluoromethyi-2-imidazolyl)phenoxy-2-propanol methanesulfonate) and 0.1 mu M ICI 118,551 (erythro-(+/-)-1-(7-methylindan-4-yloxy)-3 isopr opylaminobutan-2-ol hydrochloride) to selectively antagonize beta-1 AR and beta-2 AR, respectively. Inhibition of chemiluminescence of neutr ophils by EPI was approximately 100-fold more potent than that by DA a nd DOE. Only the inhibition curve by EPI exhibited two components, one at nanomolar and one at micromolar concentrations. The nanomolar comp onent was sensitive against beta-2 AR blockade, whereas the micromolar one was insensitive against both beta AR antagonists. Dose-response c urves for DA and DOE exhibited a simple hyperbolic shape at micromolar concentrations and were insensitive against both beta AR antagonists. Maximum inhibition by DA and DOE was equipotent to that by EPI. Howev er, the EC(50) for DA was much lower than its dissociation constants, K-D, assayed in membrane preparations by radioligand binding, whereas the EC(50) of DOE matched K-D. This difference could not be explained by a different efficiency of signal transduction, which was determined in receptor-coupled adenylate cyclase activity and which only showed a slightly higher efficiency of DA (51%) than of DOE (34%). Therefore, sympathomimetics were also investigated in a cell-free system, in whi ch chemiluminescence was generated by horseradish peroxidase with hydr ogen peroxide as substrate. Surprisingly, all of the sympathomimetics suppressed chemiluminescence with micromolar concentrations. We conclu de that sympathomimetics with high affinity and high intrinsic activit y (EPI) inhibit neutrophil function via occupation of beta-2 AR, where as sympathomimetics with low affinity (DA) or low intrinsic activity ( DOE) may act by direct scavenging of oxygen radicals.