Introduction: Adaptation to heat (acclimation [ACC]) and exercise training (EX) require global changes at all levels of body organization to enhance muscle performance. In this investigation, we combined these stressors and examined physiological and genomic aspects of adaptation in skeletal muscle (soleus).
Methods: Rats were divided into four groups: C (controls), ACC-acclimated to heat only at 34°C, EX-aerobic exercise on a treadmill at 24°C, and EXAC-acclimated to combined heat and aerobic training. The ACC period was 30 d. Isometric force generation was measured using isolated muscle preparations stimulated at 1-100 Hz, allowing assessment of muscle endurance. Global genomic responses of homeostatic genes were detected using a complementary DNA (cDNA) Atlas array (Rat 1.2; Clontech Laboratory, Palo Alto, CA).
Results: Significantly elevated force generation (P < 0.05) was only found in the EXAC group along with a marked decrease in relaxation velocity. Both heat-treated groups (ACC and EXAC) demonstrated less of a drop in power at stimulation frequencies above the highest force generation (P < 0.05). Gene reprogramming was noted in all treatment groups with stressor-specific dynamic profiles. Improved force generation in the EXAC soleus coincided with significant up-regulation in expression levels of genes encoding sarcoplasmic Ca2+-transporting proteins (SERCA2 and inositol triphospate receptor), glycolysis rate-limiting enzyme (phosphofructokinase), mitochondrial lipid metabolism (CPTII), and stress proteins with antiapoptotic or apoptotic activity.
Conclusions: Our data suggest that EXAC-specific gene up-regulation and cross talk between genes assigned to their gene ontology categories (transport, metabolism, and stress) differ in abundance and/or expression level (compared with other treatment groups) and contributed to the physiological advantage demonstrated by the EXAC soleus.