The cell specific temporal expression of nitric oxide synthase isoforms during Achilles tendon healing

Objective and design: We have previously shown that nitric oxide synthase ( NOS) activity is upregulated following tendon injury, and that this activit y is important to Achilles tendon healing. The aim of this study was to ide ntify the cellular distribution of nitric oxide synthase isoforms during te ndon healing. Material or subjects: Surgical division of the right Achilles tendon was pe rformed in eighty-five male Sprague-Dawley rats. Healing Achilles tendons w ere harvested at 4, 7, 14 and 21 days following the surgery. The un-injured left Achilles tendons were used as controls. Using RNase protection assays , in situ hybridization and immunohistochemistry, mRNA and protein of NOS i soforms were evaluated. Results: Minimal NOS expression was found in un-injured tendon. A cell spec ific temporal pattern for the mRNA and protein for all three NOS isoforms w as found following injury to the Achilles tendon. iNOS was maximal on day 4 in macrophages and fibroblasts. eNOS was maximal on day 4 in endothelial c ells and fibroblasts. bNOS expression gradually increased up to day 21 and was found only in fibroblasts. Conclusions: These results suggest that all three nitric oxide synthase iso forms are expressed by fibroblasts in a coordinated temporal sequence durin g tendon healing. The sequential pattern of NOS expression in healing fibro blasts suggests that each NOS isoform may play a different role in the heal ing process and provides opportunities to modify tendon healing in the clin ical setting.