Super-relaxed myosin in rodents with diet-induced adiposity
Authors: Roloff Zachery , Weyrauch Luke , McMillin Shawna , Sullivan Brian , Woods Philip , Barok Rebecca , Phung Lien , Zhang Naixin , O'Connell Timothy , Dougherty Brendan , Miller Mark , Lowe Dawn 
Published: May 2023

Objective: There are two distinct states of myosin in relaxed (non-contracting) muscle that have different ATP hydrolysis rates: disordered relaxed (DRX) and super-relaxed (SRX). Myosin in DRX is weakly bound to actin. In SRX, myosin heads fold back on the thick filament and inhibit ATP turnover up to 5-fold compared to DRX. Another factor that influences ATP hydrolysis is muscle fiber type defined by myosin heavy chain (MHC) isoform expression. If DRX and SRX fraction or lifetimes are different with intramuscular fat, this could explain changes in resting energy expenditure (REE). To gain insights on the complex interplay among the factors affecting REE, this study aimed to measure relaxed states of myosin in fibers from rodents with increased adiposity. We hypothesized that rodents fed a high-fat or Western diet would have shorter SRX lifetimes than those fed normal chow. Methods: Male C57BL/6 mice were fed a high-fat diet or normal chow (n=4 each) for 30 wk, and male Sprague-Dawley rats were fed a Western diet or normal chow for 12 wk (n=12 each). Single skeletal muscle fibers (n=8-10 per soleus) were isolated, their ATP turnover measured by fluorescent decay, and DRX and SRX fractions and lifetimes calculated. Mouse fibers were identified by myosin expression via SDS-PAGE as MHC isoforms have different relaxed properties. Adult rat soleus fibers are almost 100% MHC I, so fiber type was not quantified. Triglycerides were measured in the contralateral soleus to show effect of diet. Data are mean±SE. Results: Muscle triglycerides were 62% greater in HFD mice than controls (p=0.012) and trended higher in rats fed a Western diet than controls (p=0.059). In mouse fibers, the fraction of myosin heads in SRX were 0.23±0.01 and 0.27±0.02 for control and HFD groups (p=0.109). DRX lifetimes did not differ between control and HFD mice (28.6±1.9 and 24.9±2.9 s; p=0.321), nor did SRX lifetimes (210.0±18.7 and 260.1±54.1 s; p=0.403). When mouse fibers were differentiated by type, SRX lifetimes did not differ between HFD and control mice in MHC I (p=0.311) or MHC IIA fibers (p=0.114). SRX fractions were also not different between HFD and control mice in MHC I (p=0.849) or MHC IIA fibers (p=0.634). Rat soleus fibers did not differ between diet groups in SRX fractions or lifetimes (p≥0.729) or DRX lifetimes (p=0.853). Conclusions: While the diets meant to increase adiposity in rodents led to increased intramuscular fat, parameters of SRX and DRX measured in fibers did not differ between diet groups in mice or rats. These results suggest that adiposity does not affect isolated ATP turnover in relaxed skeletal muscle. Our preliminary results indicate that differences in relaxed myosin populations do not appear to contribute to changes in REE between normal weight and obese rodents. NIH R01AG031743 and T32AR00761200 This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.