Is inhibition of oxygen radical production of neutrophils by sympathomimetics mediated via beta-2 adrenoceptors?

Despite their beneficial effects on cardiovascular derangements in patients with severe sepsis, high doses of sympathomimetics might contribute to an impaired neutrophil function. This study was conducted to examine whether various sympathomimetics [(-)-epinephrine (EPI), dopamine (DA) and dobutamine (DOB)] differ in their potency to suppress the formation of oxygen radicals by neutrophils and whether this potency correlates with their affinity to or intrinsic activity for beta-2 adrenoceptors (beta-2 AR). Oxygen radical production of human neutrophils was induced by N-formyl-methionyl-leucyl-phenyl-alanine and detected by chemiluminescence measurements. Dose-response curves for the inhibition of chemiluminescence by sympathomimetics were measured in the absence and presence of 0.1 microM CGP 20,712 A (1-[2(3-carbamoyl-4-hydroxy phenoxy)-ethylamino]-3-[4-(1-methyl-4-trifluoromethyl-2-imidazolyl) phenoxy]-2-propanol methanesulfonate) and 0.1 microM ICI 118,551 (erythro-(+/-)-1-(7-methylindan-4-yloxy)-3 isopropylaminobutan-2-ol hydrochloride) to selectively antagonize beta-1 AR and beta-2 AR, respectively. Inhibition of chemiluminescence of neutrophils by EPI was approximately 100-fold more potent than that by DA and DOB. Only the inhibition curve by EPI exhibited two components, one at nanomolar and one at micromolar concentrations. The nanomolar component was sensitive against beta-2 AR blockade, whereas the micromolar one was insensitive against both beta AR antagonists. Dose-response curves for DA and DOB exhibited a simple hyperbolic shape at micromolar concentrations and were insensitive against both beta AR antagonists. Maximum inhibition by DA and DOB was equipotent to that by EPI. However, the EC50 for DA was much lower than its dissociation constants, KD, assayed in membrane preparations by radioligand binding, whereas the EC50 of DOB matched KD. 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 DOB (34%). Therefore, sympathomimetics were also investigated in a cell-free system, in which chemiluminescence was generated by horseradish peroxidase with hydrogen peroxide as substrate. Surprisingly, all of the sympathomimetics suppressed chemiluminescence with micromolar concentrations. We conclude that sympathomimetics with high affinity and high intrinsic activity (EPI) inhibit neutrophil function via occupation of beta-2 AR, whereas sympathomimetics with low affinity (DA) or low intrinsic activity (DOB) may act by direct scavenging of oxygen radicals.