IGF-I Receptor Localization and Constant Infusion of a Supraphysiologic Dose of IGF-I in the Sprague-Dawley Rat

Abstract

Previous studies have shown an increased growth of the tibial growth plate in rats infused with supraphysiologic doses of IGF-I. However, no one has demonstrated this effect on the TMJ in vivo. To determine the effect of a constant infusion of IGF-1 on the TMJ, 20 Sprague-Dawley rats were divided into three groups: (1) control, (2) surgical control, and (3) IGF-1 and placebo infused. IGF-I was delivered at a rate of 1 μg/day over the TMJ via osmotic minipumps. lntravital bone labels were administered at two-week intervals to monitor growth rate. Following sacrifice, seven mandibular (Mn) dimensions were measured anthropometrically. The mandibles were then imbedded in acrylic and stained with tetrachrome to visualize the Mn cartilage. Fluorescence microscopy was utilized to measure the Mn growth between bone labels and calculate growth rates. In addition, the hypertrophic cartilage layer thickness was measured photomicrographically. ANOVA showed no significant difference (P<0.05) in growth rates or cartilage thicknesses between the groups. However, multiple t tests did show an increase in several Mn dimensions (increase in length from gonion to the mental foramen; increase in length from condylion to the mental foramen; and increase in condylar head anterior-posterior length) in the experimental animals comparing the IGF-I infused side with the placebo infused control side. Therefore, it was concluded that IGF-I, when infused at a constant supraphysiologic dose, may increase mandibular growth in certain directions. The present study is not able to definitively demonstrate that these increases are due to direct effects on Mn cartilage growth.