AMP-activated protein kinase and muscle metabolism

AMP-activated protein kinase (AMPK) is a major regulator of skeletal muscle metabolism with relevance to agriculture and human health. During the conversion of muscle to meat, the rate and extent of postmortem metabolism and pH decline largely determine pork quality development. Pigs with the AMPKγ3 R200Q mutation generate pork with low ultimate pH (pHu); this is attributed to high glycogen content, and greater "potential" to produce lactate and H+. We hypothesized that decreasing muscle phosphocreatine and creatine would decrease ATP buffering capacity, resulting in earlier termination of glycolysis and pH decline. Dietary supplementation with the creatine analogue, β-GPA, decreased muscle total creatine but negatively affected performance. Another experiment was conducted using control or β-GPA diet and wild type and AMPKγ3R200Q pigs in a 2Ã 2 factorial design. The loss of muscle total creatine was important in maintenance of ATP levels in AMPKγ3R200Q muscle early postmortem. Moreover, elevated glycogen did not affect pHu, supporting that energetic modifications induced by feed restriction and β-GPA supplementation influence extent of pH decline. Next, we utilized a line of pigs selected for differences in pHu. Another AMPKγ3 mutation (V199I), which is associated with higher pHu and lower glycolytic potential, was prevalent. The 199II genotype increased pHu in castrated males only. The wild type VV genotype increased glycolytic potential, but neither glycolytic potential nor lactate predicted pHu.

In humans, AMPK activation is at least partly responsible for the beneficial effects of exercise on glucose transport and increased oxidative capacity in skeletal muscle. An inverse relationship exists between skeletal muscle fiber cross-sectional area and oxidative capacity, which suggests muscle fibers hypertrophy at the expense of oxidative capacity. Therefore, we utilized pigs possessing mutations associated with increased oxidative capacity (AMP-activated protein kinase, AMPKγ3R200Q) or fiber hypertrophy (ryanodine receptor 1, RyR1R615C) to determine if these events occur in parallel. RyR1R615C increased muscle fiber size; AMPKγ3R200Q increased oxidative capacity, evidenced by enhanced enzyme activity, mitochondrial function, and expression of mitochondrial proteins. Thus, pigs with both AMPKγ3R200Q and RyR1R615C possess increased fiber size and oxidative capacity, suggesting hypertrophy and oxidative capacity can occur simultaneously in skeletal muscle.