| The purpose of the study was to determine the unique contribution of mechanically strained skeletal muscle cells to neutrophil chemotaxis. Human myotubes were exposed to 5, 10, 20, or 30% cyclic strain (0.25Hz) or remained unmampulated (CT) for 2 hours. Myotube conditioned media were collected 5 hours after the initiation of the protocol. Conditioned media from scrape-injured myotubes served as a positive control for neutrophil chemotaxis and injury. Neutrophils were isolated from healthy volunteers (n = 8) and chemotaxis induced by the conditioned media was assayed in a Boyden chamber and expressed as a chemotaxis index (CI). Lactate dehydrogenase (LDH) release into the conditioned media was used as an index of myotube injury. Transmission electron microscopy using lanthanum as an extracellular tracer was also used to evaluate myotube injury following strain. The CI following 10% (11.5 ± 2.3; mean ± SEM), 20% (14.7 ± 3.2), 30% strain (18.2 ± 1.9), and scrape injury (14.4 ± 1.6) was significantly increased (p < 0.05) relative to 5% strain (1.3 ± 1.2) and CT (0.5 ± 0.6). CI was not significantly different among, 10. 20, and 30% strain or scrape injury. No neutrophil chemotaxis was induced following 5% strain or CT conditions. Significant myotube LDH release (17 ± 3%) was detected following 30% strain, whereas the greatest LDH release (44 ± 5%) was induced following scrape injury. Electron microscopy observations revealed signs of injury only on 30% strained myotubes as indicated by membrane rupture and membrane bleb formation. In conclusion, human myotubes are capable of eliciting neutrophil chemotaxis in a manner that is dependent on a threshold of strain (>5%) and independent of myotube injury. |
