Effect of electrical stimulation on postmortem titin, nebulin, desmin, and troponin-T degradation and ultrastructural changes in bovine longissimus muscle.

Electrical stimulation (ES) of bovine carcasses is usually done to increase tenderness and has been hypothesized to increase the activity of proteolytic enzymes that may degrade structural proteins in muscle cells and cause fractures and breaks in muscle fibers, thus enhancing meat tenderness. Our objective was to compare postmortem (PM) changes in the muscle proteins, titin, nebulin, alpha-actinin, desmin, and troponin-T and in myofibrillar structure in nonstimulated (NS) and ES bovine skeletal muscle. One side of eight beef carcasses was stimulated within 1 h of death, and the other side was the NS control. Myofibrils for SDS-PAGE and samples for transmission electron microscopy were prepared from the longissimus muscle at 0, 1, 3, 7, 14, and 28 d PM. In SDS-PAGE, titin migrated as three bands in both NS and ES 0-d samples. The slowest migrating band, T1 (intact titin), decreased slightly faster in ES samples from five animals. The fastest migrating band, T2 (degraded titin), increased in amount through 3 d and was still present at 28 d. A titin monoclonal antibody (mAb) identified a large family of degradation products that migrated faster than myosin heavy chains and that was more heavily labeled in Western blots of ES samples than in NS samples. In SDS-PAGE of NS samples, intact nebulin disappeared by 3 d in three animals, by 7 d in four animals, and by 14 d in one animal, but in ES samples the nebulin band was absent by 3 d in three animals and by 7 d in five animals. SDS-PAGE showed that the amount of intact desmin decreased slightly sooner in two ES samples and was absent earlier in one ES sample than in the corresponding NS control samples. Blots labeled with a polyclonal antibody to desmin showed that a more heavily labeled 38-kDa desmin degradation product was present in ES than in NS samples. Postmortem degradation of alpha-actinin was not detected. Contraction node (CN) formation, stretching of conjoined sarcomeres adjacent to the nodes, increased frequency of I-band fractures and accelerated appearance of wide I-band fractures adjacent to the Z-line, and, in some animals, slightly accelerated degradation of titin, nebulin, and troponin-T were characteristics of ES muscle.