Hypoxia-induced chemiluminescence in human leukocytes: the role of Ca2+

Circumstantial evidence suggests that in vivo hypoxia can activate human leukocytes to release a host of mediators including oxygen free radicals. Endothelial cells have always been presumed to play an important role in hypoxia-induced leukocyte activation. The primary aim of this study was to determine whether acute exposure of whole blood to a hypoxic mixture could stimulate the production of oxygen free radicals in the absence of endothelial cells, and to determine the role of Ca2+ in this process. Freshly drawn whole blood was obtained by venepuncture from 30 healthy volunteers. Exposure of blood to 6% O2 in a purpose built environmental chamber for periods of 8, 10, 15, 30, and 45 min resulted in a significant increase in the production of oxygen free radicals. This response was significantly increased after a 10-min incubation with the Ca2+ ionophore (A23187), supporting the idea that the influx of extracellular Ca2+ is an important step in the process. Incubation of human leukocytes with verapamil (10 min) and subsequent exposure to hypoxia resulted in the significantly reduction in hypoxia-induced production of oxygen free radicals at 15 min and a paradoxical increase at 30- and 45-min exposures. The significance of the results is discussed.