Effects of Diet Induced Obesity on Rat Skeletal Muscle Ca2+ Handling and Cellular Adaptations

Obesity and other metabolic conditions are known to have an effect on a host of physiological mechanisms, in particular skeletal muscle thermoregulation. Heat generation in skeletal muscle during rest and contraction is driven primarily by the reuptake of Ca2+ via the sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) pump. ATP consumption during this process contributes to a large amount of energy expenditure. The upregulation of this Ca2+ cycling process has been described in obesity and is thought to be an adaptive mechanism in response to high energy stores, however it is not well understood. Therefore, we aimed to assess cellular adaptations that may favour resting thermogenesis by quantifying ryanodine receptor (RyR1) Ca2+ leak and the associated basal Ca2+ handling properties of rat muscle under the stress of obesity. Wistar rats were fed a high fat diet to induce a metabolically altered model. Plasma free fatty acid, plasma triglyceride, plasma insulin and liver triglycerides were quantified and confirmed the obesity phenotype we targeted. Mechanically skinned extensor digitorum longus (EDL) muscle fibres were used to determine RyR Ca2+ leak. Compared with control fed rats, muscles in obese rats exhibited significant RyR leak and increased [Ca2+]t-system when presented with increasing concentrations of cytoplasmic Ca2+. Excessively leaky RyRs were present in long term obese rats and displayed spontaneous store operated calcium entry (SOCE) activation, which was inhibited by tetracaine. The introduction of caffeine following tetracaine induced a slowly depleting [Ca2+]t-system indicative of a well-buffered SR for Ca2+, suggesting an upregulation of calsequestrin (CSQ). Our results identified altered skeletal muscle Ca2+ handling properties that may be adaptations to enhance resting thermogenesis and subsequent energy expenditure in obese rats.