The influence of the PRKAG3 mutation on glycogen, enzyme activities and fibre types in different skeletal muscles of exercise trained pigs.

BACKGROUND:AMP-activated protein kinase (AMPK) plays an important role in the regulation of glucose and lipid metabolism in skeletal muscle. Many pigs of Hampshire origin have a naturally occurring dominant mutation in the AMPK γ3 subunit. Pigs carrying this PRKAG3 (R225Q) mutation have, compared to non-carriers, higher muscle glycogen levels and increased oxidative capacity in m. longissimus dorsi, containing mainly type II glycolytic fibres. These metabolic changes resemble those seen when muscles adapt to an increased physical activity level. The aim was to stimulate AMPK by exercise training and study the influence of the PRKAG3 mutation on metabolic and fibre characteristics not only in m. longissimus dorsi, but also in other muscles with different functions. METHODS:Eight pigs, with the PRKAG3 mutation, and eight pigs without the mutation were exercise trained on a treadmill. One week after the training period muscle samples were obtained after euthanisation from m. biceps femoris, m. longissimus dorsi, m. masseter and m. semitendinosus. Glycogen content was analysed in all these muscles. Enzyme activities were analysed on m. biceps femoris, m. longissimus dorsi, and m. semitendinosus to evaluate the capacity for phosphorylation of glucose and the oxidative and glycolytic capacity. Fibre types were identified with the myosin ATPase method and in m. biceps femoris and m. longissimus dorsi, immunohistochemical methods were also used. RESULTS:The carriers of the PRKAG3 mutation had compared to the non-carriers higher muscle glycogen content, increased capacity for phosphorylation of glucose, increased oxidative and decreased glycolytic capacity in m. longissimus dorsi and increased phosphorylase activity in m. biceps femoris and m. longissimus dorsi. No differences between genotypes were seen when fibre type composition was evaluated with the myosin ATPase method. Immunohistochemical methods showed that the carriers compared to the non-carriers had a higher percentage of type II fibres stained with the antibody identifying type IIA and IIX fibres in m. longissimus dorsi and a lower percentage of type IIB fibres in both m. biceps femoris and m. longissimus dorsi. In these muscles the relative area of type IIB fibres was lower in carriers than in non-carriers. CONCLUSIONS:In exercise-trained pigs, the PRKAG3 mutation influences muscle characteristics and promotes an oxidative phenotype to a varying degree among muscles with different functions.

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