Regulation of carbohydrate metabolism in skeletal muscle during and after contraction

Abstract

It is well known that exercise increases glucose transport into skeletal muscles. The regulation of this transport, however, is poorly understood. An increased understanding of the mechanisms underlying glucose transport and glycogen metabolism during exercise will lead to new strategies for treating or preventing increasingly prevalent diseases like type 2 diabetes. The aim of this thesis was to study the regulation of carbohydrate metabolism in skeletal muscle during and after contraction.

Three main areas were studied: (A) glucose transport, (B) glycogen synthesis, and (C) glycogen breakdown (A) The role of endogenously produced reactive oxygen species (ROS) in contraction-mediated glucose transport was investigated in mouse skeletal muscle. An antioxidant (Nacetylcysteine; NAC), added to block the accumulation of ROS during exercise significantly reduced contraction-mediated glucose transport. Furthermore, it was found that the addition of NAC to contracting muscles also inhibited AMPK activity, a key enzyme in contraction-mediated glucose transport. We pharmacologically inhibited cross-bridge force production to assess cross-bridge ATP consumption during contraction. Inhibition of the crossbridges decreased the initial force by ~95% in fast twitch skeletal muscle. We found that the cross-bridges only account for ~20% of the total ATP production during submaximal contraction and the contraction-mediated glucose transport was only slightly decreased.

(B) Glycogen supercompensation (i.e. an increased glycogen concentration above basal) is an established phenomenon but the underlying mechanisms are still unknown. We investigated the insulin independent glycogen supercompensation in skeletal muscle. Muscles were stimulated to deplete glycogen. Glycogen levels were about ~35% greater than basal levels after 6 h of recovery. Glycogen transport was slightly increased whereas glycogen synthase activity was unaffected at the time of supercompensation. Furthermore, glycogen phosphorylase (the rate limiting enzyme of glycogen breakdown) was decreased after stimulation and was still low at the time of supercompensation.

(C) We investigated the mechanism behind the increased glycogen breakdown that creatine kinase deficient mice (CK-/- ) exhibit during contraction. Glycogen phosphorylase, which catalyzes glycogenolysis, is regulated by substrate availability (Pi), phosphorylation/dephosphorylation and allosterically by AMP. The results show that phosphorylase from CK-/- muscles has an increased affinity for AMP.

Conclusion: (A) ROS stimulate glucose transport during contractionas well as increasing AMPK activity. Removal of ROS decreases contraction-mediated glucose transport and it is therefore questionable if healthy individuals will benefit from intake of antioxidants. Furthermore, cross-bridges only account for a small part of the total ATP turnover during submaximal contraction and mechanical load does not play a major part in contraction-mediated glucose transport.

(B) Insulin-independent glycogen super-compensation is a result of a decreased glycogen breakdown and increased or constant glycogen synthesis. (C) CK-/- mice have an increased glycogen breakdown during contraction compared to wild type mice despite the fact that they exhibit low increase in Pi and have a lower phosphorylation of glycogen phosphorylase. These data therefore suggest that allosteric activation of glycogen phosphorylase by AMP could be an important regulatory mechanism for glycogen breakdown.