Kryptopterus Vitreolus Enables Real-Time In Vivo Sarcomere Length Tracking of Pharmacological and Age-Related Skeletal Muscle Atrophy

Resting sarcomere length (SL) is a fundamental readout of muscle architectural state, but longitudinal in vivo measurement requires invasive or terminal sampling in mammalian models. We tested whether the transparent epaxial myotome of the glass catfish Kryptopterus vitreolus permits repeated, non-invasive SL measurement by laser diffraction as a standardized operating-length readout of resting sarcomere architecture during experimentally induced and naturally occurring muscle remodeling. Adult fish (n = 8/group) were exposed to dexamethasone (DEX, 50–100 μM bath) for up to 14 days, with or without α-ketoisocaproate (KIC, 5 mM), with SL measured on Days 0, 7, and 14 under standardized body posture and myotome operating length; a separate aging cohort (n = 30) was tracked from 1 to 11 months. SL was treated as the primary methodological readout, with terminal cross-sectional area (CSA), fiber typing, qPCR, and Western blot for phospho-Akt(Ser473) and phospho-mTOR(Ser2448) providing phenotypic context for each perturbation rather than serving as central discoveries. DEX produced dose- and time-dependent reductions in SL (from 2.01±0.09 μm to 1.54±0.11 μm at 100 μM, p < 0.001), with concordant CSA loss and 3- to 4-fold E3 ubiquitin ligase upregulation; KIC attenuated these changes. Laser diffraction agreed with TEM (bias 0.02 μm, r = 0.78), and natural aging produced a comparable, slower decline. DEX and aging are presented as distinct perturbation contexts—not equivalent disease models. This transparent teleost provides a non-terminal, standardized operating-length readout of resting sarcomere architecture, advancing the 3Rs principle.