Biochemical and site-specific effects of insulin-like growth factor I on intrinsic tenocyte activity in equine flexor tendons.

OBJECTIVE

To examine the site-specific and dose-dependent effects of insulin-like growth factor I (IGF-I) on normal equine tendon in vitro.

SAMPLES

Superficial digital flexor tendon explants derived from a euthanatized 3-year-old horse.

PROCEDURE

Explants in culture were treated with 0, 100, 250, or 500 ng of IGF-I/ml for 14 days with an end-stage radiolabel of 20 microCi of [3H]proline/ml or 5 microCi of [3H]thymidine/ml. The tendon tissues were then analyzed biochemically for hydroxyproline content by reverse-phase high-performance liquid chromatography, DNA content by fluorometry, and glycosaminoglycan content by the dimethylmethylene blue dye-binding assay. In addition, morphologic analysis of the explants comprised histologic examination, autoradiography, and immunohistochemistry.

RESULTS

Hydroxyproline content was significantly increased in explants treated with 100 and 250 ng of IGF-I/ml. Additionally, the collagen synthetic rate, measured by incorporation of [3H]proline into hydroxyproline, was significantly increased for all treatment groups. On the basis of autoradiograms, fibroblast proliferation and collagen synthesis were predominantly confined to the endcap and adjacent endotenon of the explants. Enhanced immunoreactivity for type-I collagen, compared with type-III collagen, was evident in the treated explants, an observation supported by positive staining for type-I collagen with picrosirius red. Histologically, treated explants contained greater numbers of larger and more metabolically active fibroblasts, compared with untreated controls.

CONCLUSION

IGF-I enhances collagen synthesis in normal equine flexor tendon in a dose-dependent manner. IGF-I also exerts its primary effect on cell proliferation and collagen synthesis in the epitenon and adjacent endotenon and accompanying perivascular connective tissues, consistent with enhancement of intrinsic tendon metabolism.

CLINICAL RELEVANCE

IGF-I may have a potential role in the treatment of tendinitis in horses.