INHIBITION OF ENDOTHELIUM-DEPENDENT RELAXATION BY HEMOGLOBIN IN RABBIT AORTIC STRIPS - COMPARISON BETWEEN ACELLULAR HEMOGLOBIN DERIVATIVES AND CELLULAR HEMOGLOBINS

Hemoglobin (Hb)-based artificial oxygen carriers are supposed to induc e vasoconstriction through the inactivation of endothelium-derived rel axing factor (EDRF). We examined the vasoconstrictive activity of acel lular Hb and cellular Hb solutions in rabbit aortic strips. Unmodified Hb, pyridoxalated Hb, bovine unmodified Hb, haptoglobin-Hb complex (H p-Hb), and polyoxyethylene glycol-conjugated Hb (PEG-Hb) were used as acellular Hbs having different molecular masses. Cellular Hbs included liposome-encapsulated Hb and red blood cells (RBC). In the first expe riment, Hb (10 ng/ml to 1 mg/ml) was cumulatively added to the tissues in which steady-state relaxation was evoked by acetylcholine (ACh) af ter precontraction induced by phenylephrine. Although all Hb solutions induced a dose-dependent reversal of ACh-induced relaxation, the most potent vasoconstrictive effect was noted with acellular Hbs, and thei r contractile activities were almost the same independent of molecular mass. On the other hand, liposome-Hb and RBC showed reduced potencies in this order. These results indicate the importance of cellularity a s the major factor determining Hb-related EDRF inactivation. In anothe r experiment, the tissues were exposed to Hb at 0.01, 0.1, or 1 mg/ml for 30 min and ACh-induced relaxation was recorded after the complete removal of Hb in an organ bath chamber. Exposure to unmodified Hb at > 0.1-mg/ml concentrations significantly reduced the ACh-induced relaxat ion, whereas the relaxation was not affected by PEG-Hb, Hp-Hb, liposom e-Hb, or RBC. These results suggest that unmodified Hb might be persis tently associated with tissues and thereby inhibit ACh-induced relaxat ion. From these findings, we propose two attributes of Hb-related inhi bition of endothelium-dependent relaxation: Acellular Hbs inhibit EDRF more efficiently in the luminal space than cellular Hbs, and unmodifi ed Hb can also inhibit it adluminally and/or adventitially.