Elastic energy-storing tendons including the equine superficial digital flexor tendon (SDFT) and human Achilles tendon significantly increase locomotor efficiency, but suffer high injury rates and experience increased core temperatures during exercise. Tenocytes are linked by gap junctions (GJ) comprised of connexin (Cx) proteins that mediate intercellular communication and are necessary for strain-induced collagen synthesis. The effects of hyperthermia on gap junction intercellular communication (GJIC) are unknown. We investigated the hypothesis that there is a down regulation in GJIC and Cx protein by tenocytes in response to hyperthermic episodes similar to those experienced in the SDFT of galloping horses.

Monolayers of equine SDFT-derived cells were heated to 45°C for 10 minutes. GJIC, Cx43 and Cx32 protein expression and cell viability were measured by fluorescence recovery after photobleaching (FRAP) and immunofluorescent labelling respectively.

There was a marked reduction in GJIC (p=< 0.0001) compared with controls (37oC) at 30min and 1h post-heating, with significant recovery by 2h (p=< 0.0001). The number of Cx43 plaques/cell also decreased significantly at 30mins (p=< 0.05), 1h (p=< 0.0001) and 2h (p=< 0.0001). There was however a 3-fold increase in the number of Cx32 plaques/cell at 1h (p=< 0.0001) that returned to normal by 2h. There was little change in cell viability up to 2h, however by 24h post-heating there was an 80% decrease in cell number indicating significant levels of cell death (p=< 0.0001).

Reductions in GJIC following exercise-induced hyperthermia may inhibit tenocyte collagen synthesis. Connexin isotypes may differentially modulate tenocyte collagen synthesis, therefore the dissimilar alterations in Cx43 and Cx32 following heating could have functional importance. The return of GJIC a few hours post-heating might facilitate spread of apoptotic death signals, killing neighbouring cells which would have otherwise escaped death. Understanding the responses of GJ to increased temperature, and the effects of this on viability and collagen synthetic capacity is likely to increase our knowledge of how exercise-induced SDFT core degeneration accumulates.