SUMMARY Do animals know at a physiological level how much they weigh, and, if so, do they make homeostatic adjustments in response to changes in body weight? Skeletal muscle is a likely tissue for such plasticity, as weight-bearing muscles receive mechanical feedback regarding body weight and consume ATP in order to generate forces sufficient to counteract gravity. Using rats, we examined how variation in body weight affected alternative splicing of fast skeletal muscle troponin T (Tnnt3), a component of the thin filament that regulates the actin–myosin interaction during contraction and modulates force output. In response to normal growth and experimental body weight increases, alternative splicing of Tnnt3 in rat gastrocnemius muscle was adjusted in a quantitative fashion. The response depended on weight per se, as externally attached loads had the same effect as an equal change in actual body weight. Examining the association between Tnnt3 alternative splicing and ATP consumption rate, we found that the Tnnt3 splice form profile had a significant association with nocturnal energy expenditure, independently of effects of weight. For a subset of the Tnnt3 splice forms, obese Zucker rats failed to make the same adjustments; that is, they did not show the same relationship between body weight and the relative abundance of five Tnnt3 β splice forms (i.e. Tnnt3 β2–β5 and β8), four of which showed significant effects on nocturnal energy expenditure in Sprague–Dawley rats. Heavier obese Zucker rats displayed certain splice form relative abundances (e.g. Tnnt3 β3) characteristic of much lighter, lean animals, resulting in a mismatch between body weight and muscle molecular composition. Consequently, we suggest that body weight-inappropriate skeletal muscle Tnnt3 expression in obesity is a candidate mechanism for muscle weakness and reduced mobility. Weight-dependent quantitative variation in Tnnt3 alternative splicing appears to be an evolutionarily conserved feature of skeletal muscle and provides a quantitative molecular marker to track how an animal perceives and responds to body weight.

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