Five Myofibrillar Lesion Types in Eccentrically Challenged, Unloaded Rat Adductor Longus Muscle—A Test Model
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The Anatomical Record
Sarcomere disruptions are observed in the adductor longus (AL) muscles following voluntary reloading of spaceflown and hindlimb suspension unloaded (HSU) rat, which resemble lesions in eccentrically challenged muscle. We devised and tested an eccentric contraction (ECCON) test system for the 14-day HSU rat AL. Six to 7 hours following ECCON, ALs were fixed to allow immunostaining and electron microscopy (EM). Toluidine blue-stained histology semithin sections were screened for lesion density (#/mm2). Serial semithin sections from the ECCON group were characterized for myosin immunointensity of lesions. Five myofibrillar lesion types were identified in histological semithin sections: focal contractions; wide A-bands; opaque areas; missing A-bands; and hyperstretched sarcomeres. Lesion density by type was greater for ECCON than NonECCON ALs (P ≤ 0.05; focal contractions and opaque regions). Lesion density (#-of-all-five-types/mm2) was significantly different (ECCON: 23.91 ± 10.58 vs. NonECCON: 5.48 ± 1.28, P ≤ 0.05; ECCON vs. SHAM: 0.00 ± 0.00; P ≤ 0.025). PostECCON optimal tension decreased (Poi-drop, 17.84 ± 4.22%) and was correlated to lesion density (R2 = 0.596), but prestretch tension demonstrated the highest correlation with lesion density (R2 = 0.994). In lesions, the darkly staining A-band lost the normally organized thick filament alignment to differing degrees across the different lesion types. Ranking the five lesion types by a measure of lesion length deformation (hypercontracted to hyperstretched) at the light microscopy level, related to the severity of thick filament registry loss across the lesion types at the electron microscopic level. This ranking suggested that the five lesion types seen in semithin sections at the light level represented a lesion progression sequence and paralleled myosin immunostaining loss as the distorted A-band filaments spread across the hyperlengthening lesion types. Lesion ultrastructure indicated damage involved calcium homeostasis loss (focal contraction lesions) and “thick-filamentcentering” failure of titin (wide A-band lesions) in the early stages of lesion development.