Mechanism of the contractile effect of diaspirin-cross-linked hemoglobin in rat isolated aorta strip denuded of endothelium as revealed using an oil-immersion procedure

Diaspirin-cross-linked hemoglobin (DCLHb(TM)) is a chemically modified hemoglobin (Hb) (i.e., alpha-subunits are cross-linked by a covalent bond) currently being tested as a potential oxygen-carrying blood substitute. It was examined for possible vasoactive properties, using the rat isolated aorta strip denuded of endothelium. In this experimental model, DCLHb (1.6-155 mu M) was found to be inactive as a vasoconstrictor when added to the Krebs medium but to elicit contractile responses once the Krebs medium containing DCLHb was replaced by mineral oil, a procedure that favors the sequestration of a fixed amount of DCLHb within a substantially reduced volume of extracellular fluid. The contractile activity of DCLHb in our experimental model (i.e., prior exposure of tissues to drugs in the Krebs medium followed by replacement of the Krebs medium by mineral oil) was mimicked by methemoglobin and metmyoglobin, but not by cytochrome c, albumin, hemin, hematin, Fe2+, and a variety of hemorphins. It was abolished by indomethacin, SQ-29548 (prostaglandin H-2-thromboxane A(2) receptor antagonist), thiourea, or N-2-mercaptopropionylglycine (MCPG), reduced partially by verapamil, but not affected by dazmegrel, MK-886 (leukotriene biosynthesis inhibitor), dimethylsulfoxide, vitamin C or E, deferoxamine, N-G-nitro-L-arginine, naloxone, and a variety of other drug receptor antagonists (e.g., prazosin) and protease inhibitors (e.g., pepstatin). Rat aorta strips denuded of endothelium exhibited contractile responses to arachidonic acid added in the Krebs medium (i.e., with no mineral oil added afterwards). Such contractile activity was reduced by SQ-29548, thiourea, or MCPG. Addition of U-46619 (prostaglandin H-2-thromboxane A(2) mimetic) to the Krebs medium also elicited contractile responses in rat aorta strips denuded of endothelium. Such contractile activity was reduced by SQ-29548, thiourea, or verapamil but not by MCPG. Within the limitations of our experimental approach, these results suggest that (1) the contractile activity of DCLHb in rat aorta strips denuded of endothelium following replacement of the Krebs medium by mineral oil involves the participation of a secondary mediator, which could be a vasoconstrictor metabolite of arachidonic acid; (2) the participation of reactive oxygen species, potential degradation products of DCLHb (e.g., heme, Fe2+, hemorphins), or other mediators in the contractile activity of DCLHb is unlikely; and (3) Ca2+ entry into target cells might be involved in the process by which DCLHb, elicits its contractile activity in our experimental model.