Exertional Heat Stroke Causes Long‐Term Satellite Cell Dysfunction and Delayed Muscle Repair

Abstract

Exertional heat stroke (EHS) is an acute life-threatening event that appears to lead to long-term health problems such as cardiovascular disease and immunosuppression. However, how skeletal muscle (SKM) is affected in the long term by EHS is not well understood. One well-known acute effect of EHS on SKM is rhabdomyolysis (RM), caused by SKM damage. To regain muscle function, this damage requires an effective muscle repair response, which is likely to include a robust contribution of progenitor satellite cell (SCs). We hypothesized that SCs display EHS-induced phenotype changes that contribute to a delay in the muscle repair process. Purpose To determine whether EHS exposure alters SC phenotype after 30 d of recovery in mice and to evaluate whether this impairs muscle repair. Methods Thirty-two female mice were either subjected to a standardized EHS protocol using a forced running wheel (environmental temp: 37.5°C, 40% humidity) or a matched exercise control trial (EXC)(22.5°C). The EHS mice achieved peak core temps of ~42.2°C accompanied by transient loss of consciousness. Mice were sacrificed at 30 d of recovery. In one cohort (N=8/group), SKM was collected to isolate SCs for quantification of proliferation patterns every 24 h over 96 h and to determine their myogenic ability. Of the eight EHS SC isolations, three were unable to proliferate at all following isolation, a response not seen in any control SC populations. Since a significant portion of the samples did not respond, we used a nominal stratification classification for statistical analysis. In a second cohort, SKM was collected to sequence mRNA and perform Western protein analyses. Results EHS SCs were found to proliferate significantly less than EXC SCs at the 72 h & 96 h time points (P=0.0273); however, the ability to differentiate into myotubes was not different. mRNA analysis of EHS gastroc elucidated a robust, ongoing oxidative muscle transcriptional process at 30 d that included upregulation of such genes as Myh6, Myh7, Atp2a2, Casq2, Tnnc1, Tnnt1, and Tnni1. This response was not seen in isoforms associated with fast glycolytic muscle genes. The mRNA encoding embryonic myosin, Myh3, was found upregulated, an indicator of muscle regeneration. The protein encoded by adult oxidative myosin (Myh7) was found to be significantly less in EHS muscle (P<0.01), whereas Myh4 protein was not different and Myh3 was undetectable in either group. Conclusions SC proliferation following 30 d of recovery from a single incidence of EHS is significantly attenuated in female mice. This phenomenon has the potential to hinder the renewal of the SC pool, following depletion, and leading to delayed muscle repair. This delay may be apparent in EHS muscle through significantly lower Myh7 protein in EHS muscle and an ongoing oxidative gene transcription program.