Histamine receptors on leukocytes are expressed differently in vitro and ex vivo

The effect of histamine and histamine antagonists on the respiratory burst activity of leukocytes was studied. The activity was measured as zymosan-induced luminol-dependent chemiluminescence (CL) of isolated leukocytes and 1:2,500 diluted whole blood (WB). Histamine caused a dose-dependent inhibition of CL. For separated leukocytes the ID50 was 8 x 10(-5) M and for WB it was 1.5 x 10(-3) M. Diphenhydramine, an H1-antagonist increased the inhibitory effect of histamine while H2-antagonist cimetidine blocked the inhibition of CL of separated leukocytes. Cimetidine was not capable of reversing the effect of histamine on WB-CL. These data suggest that the histamine receptors on leukocytes are associated with Fc and/or complement receptors and the expression and function of histamine receptors can be studied by measuring the respiratory burst activity. The results also provide evidence of differences in histamine action in vitro and ex vivo conditions in WB.