An important source of metabolic inefficiency is a futile cycle of protons across the mitochondrial inner membrane, a process called proton leak. In brown fat, proton leak is catalysed by a protein called uncoupling protein 1(UCP1), and two homologues of this protein have been discovered called UCP2 and UCP3. UCP2 is expressed in most tissues, while UCP3 is expressed mostly in skeletal muscle. Many studies have reported rapid and drastic increases in the level of UCP3 mRNA in skeletal muscle after a single bout of exercise, whilst others demonstrated decreases in UCP3 mRNA during longterm training. It was assumed that these changes had functional significance and that an increase in the level of UCP3 expression could protect muscle cells against elevated substrate supply following exercise by uncoupling substrate oxidation from ATP production, whereas a decrease during long-term training could augment metabolic efficiency. These data suggest changes in the composition of mitochondria in response to exercise such that they display markedly different UCP3 levels, relative to other mitochondrial proteins. Since exercise leads to an increase in the content of mitochondria in muscle fibers, and the expression of UCPs is most probably regulated in a manner similar to that of other mitochondrial proteins, Jones and collaborators hypothesized that UCP3 protein level in skeletal muscle increases during training as part of the increase in the content of mitochondria and that each mitochondrion preserves similar levels of UCP3 proteins. Additionally, they tested whether mitochondria rich type I muscle fibers have higher levels of UCP3 expression than type IIa or IIb fibers that have a lower mitochondrial content.
To test their main hypothesis, the authors used rats accustomed to swimming. The exercise protocol consisted of two 3 hour swimming sessions separated by a resting period. The first group of rats performed the protocol for one day, the second for three days and the last group for ten days. The team collected muscle samples after each group's last training session and measured the muscles' UCP3 transcript and protein levels. The transcript level of UCP3 increased very rapidly after a single bout of exercise, while UCP3 protein level displayed a significant increase only after several hours. The level of UCP3 protein increased steadily over 10 days of swimming. Importantly, the increase in UCP3 protein level paralleled that of other mitochondrial proteins, indicating that exercise leads to a rise in the number of mitochondria in the muscle, rather than to a modification in the UCP3 protein content of pre-existing mitochondria.
In a separate experiment using sedentary rats, the protein level of UCP3 was compared with that of other mitochondrial proteins in type I, IIa and IIb muscle fibers. The content of UCP3 was higher in type I fibers than in type IIa and IIb fibers, and this pattern is similar to that of other mitochondrial proteins. So the UCP3 protein content of each fiber type parallels their number of mitochondria.
Overall, the results presented in this paper support the original hypothesis of the authors and suggest that modification in the expression level of UCPs under certain conditions could reflect changes in the content of mitochondria in cells, possibly due to alteration in cellular energy status.