Skeletal muscle cation contents and effects of genetic selection for production traits: a relationship with spontaneous myopathy?

Recent studies have indicated that genetic selection for useful production traits in meat type poultry is associated with an increased incidence and degree of idiopathic (spontaneous) and stress-induced myopathies. The mechanisms mediating these pathologies await full elucidation but alterations in intracellular calcium homeostasis have been strongly implicated. Whilst a direct role for altered function in calcium regulatory systems is well recognized, disruptions in the balance of the intracellular active concentrations of other cations may also contribute to these conditions. The present study examined the effects of genetic selection on muscle cation contents and related these to myopathic status. Pectoral muscle samples were prepared from 4 birds (49d) from each of 34 chicken pure-lines from three categories; broiler (B), traditional (T) and layers (L) selected for the relevant production traits. The contents of sodium (Na), potassium (K), calcium (Ca) and magnesium (Mg) were determined by atomic absorption spectrophotometry in muscle homogenates and expressed per unit tissue dry and ash weights. Plasma cation contents were also measured in each bird and the degree of spontaneous myopathy was assessed by plasma creatine kinase (CK) activity. Plasma Ca was 40% (p<0.001) higher in B lines whilst Mg was lower in traditional breeds. Plasma Na and K were higher in B lines than in the other categories (p<0.05). Muscle Ca content did not differ between categories. Muscle Mg content was higher in B lines than in the other lines. Dry and ash weight elemental analysis, however, revealed a 30% (p<0.01) elevation muscle Na content in B lines and a corresponding 14% increase in muscle K. These observations are consistent with myopathic and dystrophic conditions in other species. It is proposed that selection associated alterations in cation (other than Ca) regulation mechanisms play a significant role in the aetiology of spontaneous myopathy in modern poultry.