The Role of the Kidneys in the Excretion of Chemically Modified Hemoglobins

Stroma-free hemoglobin (SFHb) can be chemically modified to prolong the intravascular retention (prevent renal filtration), and to improve oxygen delivering capability for use as a red cell substitute. Hb derivatives radioactively tagged with tritium [3H] or 14C were used to study their metabolic fate following clearance from the circulation. Fully conscious, chronically cannulated rats were treated by exchange transfusion (ET). Hb solutions tested were: PLPHb (Hb monovalently reacted with pyridoxal 5′-phosphate); HbXL (Hb crosslinked β-β with 2-nor-2-formylpyridoxal 5′-phosphate, or with bis-pyridoxal tetraphosphate); ααHb (Hb cross-linked between the α-chains using bis-3,5-dibromosalicyl fumarate); and polyHb (polymerized with glutaraldehyde or o-raffinose). Plasma retention (T1/2) was significantly affected by dose and the degree of cross-Unking. Urine flow rates all increased transiently above normal. In rats treated with any 64 kDa interdimerically cross-linked Hb, mild hemoglobinuria was evident and kidney tissue had the highest label concentration at all time points (1, 5, 10, 24, 48 hr, 7 d, and 14 days post-ET). For polymerized Hb derivatives, the amount of radioactivity in urine and kidneys was inversely related to the MW of the polyHb molecules. In all rats, regardless of the Hb derivative tested, the majority of radioactivity (dpm's) was excreted in urine. About 75% of all renal excretion of radioactivity occurred from 12-60 hours post-ET. This provided evidence that catabolism of cross-linked Hb's began early, and that the kidneys are primarily responsible for excreting smaller degradation fragments.