Histomorphometric and biochemical correlates of arterial procollagen gene expression during vascular repair after experimental angioplasty

Background: To determine the transcriptional, biochemical, and histomorphometric correlates of neointimal procollagen accumulation during arterial repair after balloon angioplasty of atherogenic vessels, rabbit iliac artery collagen content and the induction of α₁(I) and α₁(III) procollagen mRNA were assessed in normal vessels and at 2, 7, and 30 days after angioplasty. Methods and Results: Quantitative iliac artery histomorphometric neointimal collagen analysis was performed using a specific picrosirius red stain under polarized light. Arterial cross-sectional area reduction, total cellularity, and vascular smooth muscle cell density (per 10⁴ μ² of neointima) were quantified in routine and immunohistochemically stained sections (α-actin and RAM-11), from which biochemical concentrations of tissue protein, RNA, and DNA were also measured. Collagen comprised 0.23±0.1 mg/mg of total protein in the normal vessel wall and did not increase in vessels studied 2 and 7 days after angioplasty (0.26±0.06, 0.28±0.05 mg/mg of protein, P=NS). By 30 days after angioplasty, >50% of the protein concentration was collagen (0.55±0.11 mg/mg of protein, P=.02). Collagen-positive histological staining also increased significantly from 17±2% of the neointima at day 2 to 32±5% by day 30 (P=.01). The transcript regulatory signal for α₁(I) procollagen mRNA was induced 2 days after angioplasty, peaking at 7 days for both α₁(I) and α₁(III), and returning to control levels 30 days after angioplasty. A significant luminal cross- sectional area reduction of the arterial wall was confirmed both by angiography and histomorphometry (P=.01). This was not associated with a significant change in α-actin (+) vascular smooth muscle cell density (38±7 nuclei per 10⁴μ² at day 2 and at day 30) or tissue DNA concentration (P=NS). Conclusions: We conclude that procollagen genes are transcriptionally activated early (2 to 7 days) after angioplasty vessel injury and that collagen subsequently constitutes a major biochemical and histological component of the proliferative neointima by 30 days after angioplasty. Alterations in pathways regulating procollagen metabolism may also contribute to the accumulation of extracellular matrix and growth of the neointima in the late repair phase after vessel wall injury.