Hepoxilin A3 is the endogenous lipid mediator opposing hypotonic swelling of intact human platelets.

When human blood platelets are exposed to hypotonic medium they swell first but, shortly thereafter, revert toward their original volume in a process termed regulatory volume decrease (RVD). RVD is the result of an enhanced efflux of K+ and Cl- ions and associated water. Platelet RVD is controlled by a short-lived lipoxygenase-derived product (LP). By using a combination of high-performance liquid chromatography, gas chromatography-mass spectrometry, and RVD reconstitution bioassay, we show that LP is identical with hepoxilin A3. In addition we demonstrate that authentic hepoxilin A3 possesses the same biological properties on RVD reconstitution as LP and that the activity of both compounds is amplified through epoxide hydrolase inhibition with 3,3,3-trichloropropene-1,2-oxide. Therefore, we report here that volume expansion causes the formation and release of hepoxilin A3 from intact human platelets and that this hepoxilin plays a major role in volume regulation.