Differential effects of heat stress on fibre capillarisation in tenotomised soleus and plantaris muscles

Purpose: Capillary regression is commonly observed in response to disuse muscle atrophy. Heat stress is known to alleviate muscle atrophy, while effect of heat exposure on capillary adaptation following disuse atrophy is not defined. Here, we examined the effect of heat treatment on capillarisation and the associated signalling in slow-oxidative soleus and fast-glycolytic plantaris muscles following Achilles tendon ablation (tenotomy).

Materials and methods: Male Wistar rats were assigned into control (CON), control with heat stress (CON?+?HEAT), tenotomy (TEN) and tenotomy with heat stress (TEN?+?HEAT) groups. Tenotomy was induced for 8?days in TEN and TEN?+?HEAT groups. Heat stress was maintained at 40.5–41.5?°C, 30?min for 7?days.

Results: Tenotomy resulted in reduction of capillary-to-fibre ratio, decreased VEGFR-2 and increased TSP-1 in soleus muscle, whereas VEGF protein expression remained unaffected. Tenotomy had no effect on capillary distribution and angiogenic signalling in plantaris muscle. These results were concomitant with larger reduction of cross-sectional area (CSA) in MHC type I and II myofibres of soleus compared to plantaris muscles. Interestingly, heat stress increased VEGFR-2 and attenuated TSP-1 protein expression in tenotomised soleus, but not plantaris muscles. Additionally, CSA of both type I and type II myofibres was greater in tenotomised soleus than plantaris muscles after heat treatment.

Conclusions: Heat stress mitigated effect of tenotomy-induced capillary regression in a fibre-type-specific response, in part, by shifting the balance between angiogenic and angiostatic regulators. These results suggest beneficial effect of heat treatment for maintaining microcirculation in disuse muscle atrophy.