Biochemical and morphological assessments of bleomycin pulmonary toxicity in rats

Abstract

Low doses (5 units) or high doses (15 units) of bleomycin were administered ip twice weekly to separate groups of male rats. A progressive dose-dependent reduction in body weight gain with a concomitant increase in wet lung weight to body weight ratio was observed over 6 weeks of treatments. Light microscopic confirmation of perivascular edema suggested that the increase in wet lung weight resulted from fluid accumulation. Although histological evidence of mild fibrosis was also present following low-dose treatment, only a slight increase in collagen content was observed biochemically. In vivo elevation of prolyl hydroxylase activity was observed in lungs of animals treated with low-dose bleomycin for 6 weeks. High-dose bleomycin regimen did not accelerate fibrosis, but rather inhibited the prolyl hydroxylase activity. In vitro, this enzyme activity was also stimulated at 1-10 μM concentration, but was inhibited at 0.1 mM. Lung angiotensin-converting enzyme activity was examined as an indication of damage to endothelial cells. This activity was unchanged after low-dose bleomycin treatments until 6 weeks, at which time a 59% decrease in activity was observed. High-dose bleomycin treatment produced a biphasic response in angiotensin-converting enzyme activity, with an initial 32% stimulation after 1.5 weeks of treatment followed by a 41% inhibition after 3 weeks. Kinetic analysis of the in vitro interaction of purified porcine plasma angiotensin-converting enzyme with bleomycin indicated that bleomycin acts as a competitive inhibitor with a K(i) of 3.7 μM. Stimulation of angiotensin-converting enzyme activity following acute high-dose bleomycin administration could be due to the initial endothelial cell repair, while the inhibition at later stages may indicate extensive destruction of endothelial surface upon repeated bleomycin administration. Direct inhibition of the enzyme by the chelating effect of bleomycin may also be a possibility.