Heat stress activates the Akt/mTOR signalling pathway in rat skeletal muscle

It is well known that various stimuli, such as mechanical stress and nutrients, induce muscle hypertrophy thorough the Akt/mTOR signalling pathway, which is a key mediator of protein synthesis and hypertrophy in skeletal muscle. It was recently reported that heat stress also induces an increase in muscle weight and muscle protein content. In addition, heat stress enhances Akt/mTOR signalling after one bout of resistance exercise. However, it remains unclear whether increased temperature itself stimulates the Akt/mTOR signalling pathway.

Forty-two male Wistar rats (279.5 ± 1.2 g) were divided into a control group (CON) or one of five thermal stress groups at 37, 38, 39, 40 or 41 °C (n = 7 each group). After overnight fasting, both legs were immersed in different temperatures of hot water for 30 min under sodium pentobarbital anaesthesia. The soleus and plantaris muscles were immediately removed from both legs after the thermal stress.

The phosphorylation of mTOR or 4E-BP1 and heat shock protein (HSP) expression levels were similar among groups in both the soleus and plantaris muscles. However, Akt and p70S6K phosphorylation significantly increased at 41 °C in the soleus and plantaris muscles. Moreover, we observed a temperature-dependent increase in Akt and p70S6K phosphorylation in both muscles.

Our data indicate that the altered temperature increased phosphorylation in a temperature-dependent manner in rat skeletal muscle and may itself be a key stimulator of Akt/mTOR signalling.