INSULIN-LIKE GROWTH-FACTOR-I BINDING IN INJURY-INDUCED INTIMAL HYPERPLASIA OF RABBIT AORTA

Purpose: The proliferation of arterial-wall smooth muscle cells is an important step in the formation of intimal hyperplasia. Insulin-like g rowth factor-I (IGF-I) is a mitogen that exerts its effects through sp ecific receptors located on the cell membrane. IGF-I has been found to promote the multiplication of vascular smooth muscle cells in culture . This study aimed to evaluate the status of IGF-I binding in injury-i nduced intimal hyperplasia in a rabbit model. Methods: We used binding techniques to study IGF-I binding of control and hyperplastic aortas of adult White New Zealand rabbits. Hyperplasia was induced by balloon -catheter injury. At 2 weeks and 1, 2, 4, and 7 months after injury, s egments of abdominal aortas were harvested from two control and six st udy rabbits, and 20-mu m-thick frozen sections were obtained. Hematoxy lin and eosin-stained sections confirmed the presence of intimal hyper plasia in the hyperplastic aortas. Adjacent sections were incubated in a buffer solution containing I-125-IGF-I in the presence and absence of an excess of unlabeled IGF-I. Autoradiograms were then obtained by apposing the treated sections to autoradiography film, which was devel oped at 3 days and analyzed by comparison with the hematoxylin and eos in-stained sections under light microscopy. A marked increase in IGF-I binding grain density was observed in the areas corresponding to the hyperplastic lesions. To characterize these binding sites, binding inh ibition studies were performed and the dissociation constant (K-d) and maximum binding capacity (B-max) were obtained from Scatchard analysi s. Results: Six hyperplastic aortas for each time interval and a total of nine control aortas were evaluated. The K-d of the hyperplastic ao rtas (1.5 +/- 0.2 nmol/L) was not significantly different from that of control aortas (1.3 +/- 0.2 nmol/L), which indicated similar high-aff inity IGF-I binding sites in normal and hyperplastic arteries. The res ults of B-max were 6.9 +/- 1.2, 8.5 +/- 2.1, 12.4 +/- 2.1, 20.4 +/- 5. 9, 20.6 +/- 3.2, and 8.1 +/- 1.3 pmol/L for control, 2 weeks, 1 month, 2 months, 4 months, and 7 months, respectively. With analysis of vari ance (p < 0.05), B-max values at 1, 2, and 4 months were significantly higher than those of control aortas. B-max values returned to levels not significantly different from those of control aortas at the 7-mont h interval. Conclusion: Increased IGF-I binding in the hyperplastic ao rtas suggests that IGF-I plays an important role in the proliferation of arterial wall cellular components during the hyperplastic process.