Lipoxins A4 and B4: comparison of icosanoids having bronchoconstrictor and vasodilator actions but lacking platelet aggregatory activity.

Lipoxins A4 (LxA4) and B4 (LxB4), two lipoxygenase-generated icosanoids of arachidonic acid metabolism, were found to have a distinct biological profile. Both LxA4 and LxB4 slowly contracted pulmonary parenchymal strips isolated from guinea pigs, rabbits, and rats in a concentration-dependent manner over the range 0.1-1 microM. This bronchoconstrictor effect was not associated with release of peptide leukotrienes or thromboxane A2, nor was it blocked by lipoxygenase inhibitors or thromboxane receptor antagonists, suggesting it is a direct effect of lipoxins. However, the leukotriene D4 (LTD4) receptor antagonist LY-171883 reduced the LxA4 response, indicating that LTD4 and LxA4 may share the same receptor. LxA4 and LxB4 also exerted an endothelium-dependent vasorelaxation in guinea pig, rat, and, to a lesser extent, rabbit aortic vascular smooth muscle. In contrast to other vasoactive icosanoids, LxA4 and LxB4 failed to aggregate rat, rabbit, or guinea pig platelets or to inhibit ADP-induced aggregation. LxA4 also enhanced the release of liver lysosomal hydrolases in a liver large granule fraction, indicating a lysosomal labilizing action of LxA4. LxA4 and LxB4 share a similar biological profile. It is not clear yet whether the lipoxins could be mediators of circulatory or pulmonary disease states.