HUMAN NEUTROPHILS AND HL-60 CELLS DO NOT POSSESS ALPHA(2)-ADRENOCEPTORS

Human neutrophils have been reported to possess both alpha(2)- and beta(2)-adrenoceptors. While activation of beta(2)-adrenoceptors is known to inhibit N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP)-induced superoxide anion (O-2(-),) production, the functional role of alpha(2)-adrenoceptors is not known. We studied the effects of a range of structurally unrelated alpha(2)-adrenoceptor agonists on fMLP-induced O-2(-) production and UTP-induced increases in cytosolic free calcium concentration ([Ca2+](i)) in human neutrophils. No effect of alpha(2)-adrenoceptor agonists was seen on either fMLP-induced O-2(-) production or UTP-induced increases in [Ca2+](i). alpha(2)-Adrenoceptor agonists by themselves had no effect on either O-2(-) production or [Ca2+](i). We then studied a model for neutrophils, differentiated HL-60 cells and human erythroleukaemia (HEL) cells, a cell line known to possess alpha(2)-adrenoceptors. While the alpha(2)-adrenoceptor agonists 5-bromo-6-(2-imidazolin-2-ylamino)-quinoxaline (UK 14304) and 5-allyl-2-amino-5,6,7,8- tetrahydro-4H-thiazolo-[4,5-d]azepin-dihydrochloride increased the [Ca2+](i) in HEL cells, they had no effect by themselves on either [Ca2+](i) or UTP-induced increases in [Ca2+](i) in differentiated HL-60 cells. Activation of high-affinity GTPase by UK 14304 was seen in membranes from HEL cells but not in membranes from differentiated HL-60 cells. Similarly, a selective alpha(2)-adrenoceptor antagonist, [H-3]2-(2-methoxy-1,4-benzodioxan-2yl)-2 imidazoline, bound specifically and saturably to membranes from HEL cells, but not to membranes from HL-60 promyelocytes or differentiated HL-60 cells. Taken together, these data suggest that neither HL-60 promyelocytes nor differentiated HL-60 cells possess alpha(2)-adrenoceptors, and that the lack of functional responses to cu,-adrenoceptor agonists in human neutrophils is due to the absence of alpha(2)-adrenoceptors.