The effect of cell-free hemoglobin on intravascular clearance and cellular, plasma, and organ distribution of bacterial endotoxin in rabbits.

Cell-free hemoglobin (Hb) and bacterial endotoxin (LPS) synergistically produce toxicity. To elucidate possible mechanisms, three groups of rabbits received LPS alone, LPS plus human serum albumin (HSA), or LPS plus Hb (Hb group). The intravascular retention of injected iodine 125-labeled LPS during the 30-minute period analyzed was significantly longer in the Hb group than in the LPS alone or HSA groups (p = 0.0007 and p = 0.03, respectively), especially during the initial 10 minutes. The intravascular half-life of LPS in the LPS control, HSA, and Hb groups was 2.8, 4.0, and 4.9 minutes, respectively; the area under the curve was 1369 +/- 483, 1594 +/- 360, and 1731 +/- 481, respectively (ng/ml x minutes, mean +/- SD); and the total body clearance was 24.7 +/- 9.2, 20.1 +/- 5.4, and 18.9 +/- 6.0 (ml/min, mean +/- SD), respectively. The proportion of LPS associated with blood cells was very small at the initial 1-minute time period, and this decreased even further during the 30-minute period analyzed (p = 0.0001). Over 96% of injected LPS was associated with the cell-free plasma, with 51% to 54% of LPS in the apoprotein fraction at the initial time point and 35% to 37% in the HDL fraction. The proportion of LPS increased significantly in the HDL fraction and decreased significantly in apoproteins during the 30-minute period analyzed (p = 0.0006 and p = 0.002, respectively). However, there were no differences between the three groups. The liver was the main distribution site (74%) of injected LPS among the six organs evaluated. In the Hb group the accumulation of 125I-labeled LPS in the spleen was significantly lower than that in the HSA group (p = 0.05). The synergism of the in vivo toxicity reported for LPS and Hb may be due, in part, to the decreased rate of intravascular clearance of endotoxin.