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Changes in muscle fiber populations and muscle enzyme activities in the primiparous lactating sow.

An experiment involving 12 primiparous Large White sows was conducted to investigate changes in contractile and metabolic characteristics of skeletal muscle during the first 3 weeks of lactation. The sows lost 19.7 +/- 6.6 kg of body weight. No change in DNA concentration was observed in the longissimus dorsi (LD), a fast-twitch glycolytic muscle, and the trapezius (T), a mainly slow-twitch oxidative muscle during lactation. The percentage of type I fibers increased (P less than 0.05) in LD, but not in T. The muscle fiber cross sectional area (CSA) of IIB fibers, which represents about 78% of the total number of LD fibers, decreased by 18% (P less than 0.01) by lactation; the CSAs of I and IIA fibers were not significantly affected. Marker enzyme activities for oxidative and glycolytic metabolisms decreased in both muscles during lactation. The decrease in oxidative enzyme activities was particularly dramatic in T (P less than 0.001). No significant relationship was observed between sow weight loss and changes in muscle fiber CSA or enzyme activities. The extent to which the results could be related to a negative nutritional balance or to changes in hormonal status is discussed.