Eccentric contraction-induced injury of mouse soleus muscle: Effect of varying [Ca²⁺](o)

The objective of this study was to determine the effect of varying extracellular Ca²⁺ concentration ([Ca²⁺](o)) on eccentric contraction- induced muscle injury. Isolated mouse soleus muscles (n = 64) performed either 20 eccentric or 20 isometric contractions over a 40-min period in a Krebs buffer containing 0.5, 1.25, or 5.0 mM Ca²⁺. Measurements of contractile function and lactate dehydrogenase accumulation in the buffer were then made every 15 min for 2 h. Prostaglandin E₂, leukotriene B₄, and tyrosine accumulation in the incubation medium and total muscle [Ca²⁺] were measured at the end of the experiment. Reductions in maximal isometric tetanic force for muscles immediately after performance of 20 eccentric and 20 isometric contractions were 21.1 ± 1.4 and 1.2 ± 0.7%, respectively. Total muscle [Ca²⁺] was 28-37% higher in muscles that performed eccentric contractions than in those that performed isometric contractions. However, estimates made with a confocal laser scanning microscope and fluo 3 do not indicate that there was a difference in free cytosolic [Ca²⁺] between fibers from injured and control muscles. Also, leukotriene B₄, prostaglandin E₂, and tyrosine accumulation in the buffer from muscles that performed eccentric contractions was not elevated over that from muscles that performed isometric contractions. Furthermore, lactate dehydrogenase accumulation and reductions of contractile function over the 2-h incubation period were not enhanced by higher [Ca²⁺](o) or influenced by the type of contraction. These findings suggest that muscles that were injured by eccentric contractions were able to buffer the increased influx of extracellular Ca²⁺, maintain a normal free cytosolic [Ca²⁺], and avoid activation of Ca²⁺-sensitive degradative pathways.