The role of intracellular sodium in skeletal muscle damage: effects on muscles from two broiler genotypes.

The incidence of idiopathic myopathy is increasing in poultry lines selected for high growth rates and muscle yields. The condition has been linked with impaired muscle intracellular calcium (Ca²⁺_i) homeostasis. Elevations in Ca²⁺_i are known to be associated with the development of myopathy. These increases may be mediated by initial changes in other ions, such as sodium (Na⁺). This study examines the role of Na⁺_i on Ca²⁺_I and muscle damage and compares its effects on these parameters in muscles from two broiler genotypes. The study employed an isolated in vitro chicken skeletal muscle preparation. Alterations in Na⁺_i were induced using the Na⁺ ionophore monensin and Na⁺/K⁺ exchange inhibitor ouabain. In a parallel experiment, muscles from fast (FG) and slow (SG) growing broiler lines were similarly treated. Muscle damage was determined from creatine kinase (CK) loss into the incubation medium. Muscle calcium accumulation was estimated by ⁴⁵Ca²⁺ uptake. Monensin and ouabain treatments both caused similar elevations in ⁴⁵Ca²⁺ uptake (p<0.05) and were associated with increases in CK loss (p<0.001). Effects of simultaneous incubation with both agents were additive. In studies on muscles from FG and SG lines, ⁴⁵Ca²⁺ uptakes and CK losses were not different under control conditions. Treatment with monensin produced differential increases in ⁴⁵Ca²⁺ uptake (p<0.01) and CK loss (p<0.001) in muscles from the two lines. Absolute uptakes of ⁴⁵Ca²⁺ (p<0.05) and CK losses (p<0.001) were greater in muscles from the FG line as were their relative increases (p<0.05). The results suggest initial increases in Na⁺_i can produce elevations in Ca²⁺_i and muscle injury. FG and SG lines exhibit differences in the extent of Na⁺-mediated alterations in muscle Ca²⁺_i and CK loss. It is proposed that subjecting birds to situations which may cause changes in muscle Na⁺_i (e.g. raised muscle activity and stress) should be avoided, especially in current broiler lines, in which sensitivity to Na⁺-induced changes in myopathology appears to be increased.