Human migration inhibition factor (MIF), at concentrations that inhibit guinea pig macrophage migration, do not alter the cytoplasmic levels of cAMP. However, modest elevations of intracellular cAMP, achieved either with isoproterenol or by the addition of db cAMP, markedly interfere with the action of MIF. This effect is, however, very time dependent. Elevation of macrophage cAMP 1 hr after the addition of MIF has no effect on MIF action. The addition of isoproterenol or db cAMP, caused a brief rise of macrophage cAMP levels, and this was followed by a period of unresponsiveness to MIF that lasts for at least 1 hr. In addition, we find that nonphysiologic increases in cAMP (caused by the addition of 1 mM db cAMP) can also interfere with macrophage migration.
We have also studied the dependence of MIF action and binding on divalent cations. The calcium ionophore A23187 (which causes a 3-fold increase in the uptake of calcium by the macrophages) resembles MIF in preventing macrophage migration. However, MIF itself does not increase calcium uptake. Calcium and magnesium do appear necessary, however, for binding MIF to the macrophage cell surface and possibly for the continuing action of MIF.
Finally, examination of the macrophage surface with the scanning electron microscope reveals that the inhibition of migration by MIF, cAMP, or A23187 are all accompanied by different morphologic arrangements of the macrophage cell membrane. We conclude that the inhibition of macrophage migration produced by these three agents depends on different underlying mechanisms.