Many processes in the body vary in a 24-hour rhythm called the circadian rhythm. These rhythms are controlled by molecular clocks, in organs such as the liver, in tissues such as skeletal muscle, and in the hypothalamus, a part of the brain. The clock in the hypothalamus is the central controller and keeps all the peripheral clocks in synch.
Exercise can reset circadian rhythms. Most scientists thought this process was mediated purely by inputs to the hypothalamus, which can alter the expression of genes in the central clock. Now researchers, from the Gladstone Institute of Cardiovascular Disease, the University of California, and Northwestern University, have found that exercise can also alter the expression of clock genes in the muscles themselves.
The research team, headed by Bruce Conklin, analysed the expression of a wide range of genes in human skeletal muscle biopsies. These biopsies were taken from both legs of four male volunteers, 6 and 18 hours after they had exercised only their right leg for half an hour.
To find out if the expression of different genes in skeletal muscle varied over time, the researchers assessed which genes were 'switched on or off' at different times of the day in the unexercised legs. They discovered hundreds of genes that increased or decreased their expression over time. By studying previously published results from similar experiments in mice, they found 44 genes that are regulated in a circadian rhythm in one or more mouse tissues and human skeletal muscle. These included some genes involved in the molecular clock, namely Per1, Per2 and Clock.