Monocrotaline pyrrole (
MCTP) is a highly reactive pneumotoxic metabolite of the
pyrrolizidine alkaloid plant toxin
monocrotaline. When administered to rats, it causes a delayed and progressive
lung injury,
vascular remodeling, and
pulmonary hypertension. Structural remodeling consists of endothelial cell swelling followed by increased thickness of the vascular media in small pulmonary arteries and muscularization of normally nonmuscular arteries. Experiments were performed to characterize
DNA synthesis and cell proliferation in vascular smooth muscle cells (VSMCs) after
MCTP and to determine their relationship to changes in the thickness of the arterial medial layer of pulmonary resistance vessels. Male Sprague-Dawley rats were treated with
MCTP (3.5 mg/kg, intravenously) or its vehicle (
dimethylformamide). To label cells actively synthesizing
DNA, rats were given the
thymidine analog,
bromodeoxyuridine (
BrdU), 3 times by
intraperitoneal injection during the 24 hr preceding
euthanasia. Using immunohistochemistry,
BrdU incorporation was quantified as a ratio of labeled nuclei to total nuclei. Within 5 days after
MCTP administration, the thickness of the medial smooth muscle layer in arteries 60-250 microm in diameter was increased, prior to evidence of right
heart hypertrophy.
BrdU incorporation by VSMCs in pulmonary arteries was not different in vehicle- and
MCTP-treated rats for the first 48 hr
after treatment. However,
MCTP caused a significant increase in
DNA synthesis in VSMC on days 3-8 in arteries up to 250 microm in diameter. Although increased
DNA synthesis precedes cell proliferation, the relative number of medial VSMCs did not increase over 8 days, suggesting that
hypertrophy alone was responsible for the increased thickness of the arterial media. These results demonstrate that
MCTP causes thickening of the media of pulmonary vessels through VSMC
hypertrophy and that the prolonged
DNA synthesis that accompanies VSMC
hypertrophy is not followed by proliferation.