Type 2 diabetes adversely affects the properties of native connective tissue. The underlying mechanisms, however, by which diabetes alters connective tissue metabolism, especially tendon, are poorly defined. The aim of this study was to determine the effect of type 2 diabetes on the mechanical, histological, and molecular properties of the intact and healing Achilles tendon. The right Achilles tendon was transected in 11 male diabetic Goto-Kakizaki (GK) and 10 age- and sex-matched Wistar control rats, while the left Achilles tendon was left intact. At 2 wk postinjury the intact and injured tendons were assessed by biomechanical testing and histology. The gene expression of collagen I and III, biglycan, versican, MMP-13, and MMP-3 was measured by quantitative RT-PCR, and their protein distribution was studied by immunohistochemistry. Intact tendons exhibited only small differences between the groups. In injured tendons, however, a significantly smaller transverse area and lower stiffness was found in diabetic GK compared with Wistar control rats. This correlated with impaired structural organization of collagen fibers and a reduced expression of collagen I and III in the injured tendons of the diabetic GK compared with Wistar control. Moreover, MMP-3 gene expression was downregulated in the injured diabetic GK tendons compared with injured Wistar controls. Our results indicate that in a rat model of diabetes tendon healing is impaired mainly due to altered expression of collagen and MMPs reflecting decreased degradation of matrix proteins and impaired tissue remodeling. Further our data suggest that therapeutic modulation of collagens or MMPs might be targets for new regenerative approaches in operated, injured, or maybe also degenerative tendon diseases in diabetes.

Elevated levels of the neurotransmitter glutamate and the presence of its receptor, N-methyld-aspartate receptor type 1 (NMDAR1) have been established in patients with tendinopathy, i.e. chronic tendon pain and degeneration. However, whether NMDAR1 is up-or downregulated in tendinopathy and co-localized with glutamate is still unexplored. We hypothesize that an alteration in tissue expression and in co-existence of NMDAR1 and glutamate occurs in tendinopathy and might play a role in nociception and possibly also progression of tendon degeneration (tendinosis).We therefore examined tissue distribution and levels of NMDAR1 and glutamate in biopsies from patients with patellar tendinopathy (n=10) and from controls (n=8). The biopsies were single-and double-stained immunohistochemically for glutamate and NMDAR1 and assessed subjectively and semi-quantitatively.The chronic painful tendons exhibited a significant elevation of NMDAR1 (9-fold), which was independent from the observed increase in glutamate (10-fold). This up-regulation of NMDAR1 and glutamate was found to be co-localized on nerve fibers as well as on morphologically altered tenocytes and blood vessels. None of the controls exhibited neuronal co-existence of glutamate and NMDAR1. The neuronal co-existence of glutamate and NMDAR1, observed in painful tendinosis but not in controls, suggests a regulatory role in intensified pain signalling.2

Tendon healing is characterized mostly by slow rehabilitation and, as in tendinopathy, aberrant, protracted sensory nerve ingrowth. This study investigated whether administration of the sensory neuropeptide substance P (SP) could enhance healing and modulate sensory nerve plasticity after Achilles tendon rupture. Fifty-four male Sprague-Dawley rats were allocated to three groups, all receiving six daily injections post-rupture of; (1) SP (10(-6) mol/kg body weight)+endopeptidase inhibitors captopril and thiorphan, (2) captopril/thiorphan only and (3) saline control. At 1, 3 and 6 weeks post-rupture tendon healing was evaluated by assessments of fibroblast proliferation, collagen III-LI (like) occurrence, diameter of newly organized collagen and sensory nerve fiber ingrowth. At 1 week, the SP-treated group exhibited increased occurrence of collagen III-LI (P=0.03) and of organized collagen (P=0.04) compared with control. At 3 weeks, the SP group notably displayed reduced SP-nerve fiber ingrowth (P=0.02), and higher fibroblast density (P=0.004). Both the SP and captopril/thiorphan groups demonstrated increase in collagen fiber organization compared with control (P=0.02 and 0.004, respectively). At 6 weeks, no significant differences were observed between the groups. SP supply in tendon repair promotes early tissue proliferation and regulation of endogenous sensory nerve ingrowth, suggesting implications for novel treatment in tendinopathy.

Tendinopathy, pain, and degeneration, may be related to the up-regulation of substance P (SP) and its activation of glutamate receptors. We hypothesized that the pathogenesis of tendinopathy involves N-methyl-D-aspartate receptor type 1 (NMDAR1) activation (phosphorylated NMDAR1; phospho-NMDAR1) co-existing with SP. Moreover, we examined the presence of metabotropic receptors that increase (mGluR1 and mGluR5) or decrease (mGluR6 and mGluR7) NMDAR1 excitability. Biopsies from patients with patellar tendinopathy (n ¼ 10) and from controls (n ¼ 8) were immunohistochemically analyzed according to the occurrence and tissue distribution of NMDAR1, phosho-NMDAR1, mGluR (1, 5-7), and SP. The biopsies were immunohistochemically single-and doublestained and semi-quantitatively assessed. Tendinopathic biopsies exhibited increased occurrence of NMDAR1, phospho-NMDAR1, SP, and mGluR5, while mGluR6-7 were not increased and mGluR1 was not found. The occurrence of NMDAR1 and SP correlated in tendinopathy (r 2 ¼ 0.54, p ¼ 0.03), but not in the controls (r 2 ¼ 0.11, p ¼ 0.4). Co-localization of SP and phospho-NMDAR1 within the tendon proper was only found in tendinopathy, localized on hypertrophic tenocytes, blood vessels, and penetrating free-nerve fibres. Up-regulation and activation of the glutamate receptor, phospho-NMDAR1, suggests a role in the pathophysiology of tendinopathy. Increased NMDAR1 excitability may be related to increased SP and mGluR5. The unique co-existence of SP and phospho-NMDAR1 in tendinopathy presumably reflects a tissue proliferative and nociceptive role. ß