Insulin-like growth factor-I binding in injury-induced intimal hyperplasia of rabbit aorta

The proliferation of arterial-wall smooth muscle cells is an important step in the formation of intimal hyperplasia. Insulin-like growth factor-I (IGF-I) is a mitogen that exerts its effects through specific 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-induced intimal hyperplasia in a rabbit model.

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, segments of abdominal aortas were harvested from two control and six study rabbits, and 20-micrometer-thick frozen sections were obtained. Hematoxylin and eosin-stained sections confirmed the presence of intimal hyperplasia in the hyperplastic aortas. Adjacent sections were incubated in a buffer solution containing 125I-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 developed at 3 days and analyzed by comparison with the hematoxylin and eosin-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 inhibition studies were performed and the dissociation constant (K d) and maximum binding capacity (B max) were obtained from Scatchard analysis.

Six hyperplastic aortas for each time interval and a total of nine control aortas were evaluated. The K d of the hyperplastic aortas (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-affinity IGF-I binding sites in normal and hyperplastic arteries. The results 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 variance (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-month interval.

Increased IGF-I binding in the hyperplastic aortas suggests that IGF-I plays an important role in the proliferation of arterial wall cellular components during the hyperplastic process.