Age dependent changes in plasma creatine kinase activity in broiler chickens

Plasma activity of the intracellular, muscle enzyme creatine kinase increases between 2 and 6 weeks of age in the modern broiler fowl. The corresponding activities in the circulation of the genetic predecessors of these birds in whom selection has been relaxed were lower at each age examined and the interline difference increased with time. In relaxed birds grown to the same final body weight as the selected animals enzyme activity was still significantly less. These trends were paralleled by plasma lactate dehydrogenase but not aspartate aminotransferase activities. The differences in plasma creatine kinase were not attributable to the tissue enzyme contents but are probably the result of efflux of the protein from muscle as a consequence of altered plasmalemmal integrity or permeability. The mechanism mediating this process awaits elucidation but increased membrane lipid oxidation attributable to different thyroid status in the two lines is unlikely as circulating thyroid hormone concentrations and chronological changes are similar in selected and relaxed birds.