Abstract

Aims: Shift workers have an increased risk to develop type 2 diabetes. Recently, a human study showed that an acute 12h phase shift has acute negative effects on muscle insulin sensitivity at the onset of the active period. To disentangle the underlying neuroendocrinological and metabolic mechanisms we subjected rats to acute circadian desynchronization.

Methods: First, male Wistar rats were placed in metabolic cages for 3 days in a 12h light: 12h dark (LD) cycle with standard chow available ad libitum. Then a complete LD reversal (12h: 12h DL cycle) was done from day4 in three different feeding time groups. Food and water intake, locomotor activity and RER were measured continuously. In a second experiment with food available ad libitum, jugular vein cannulation surgery was performed in rats, and 7-10 days later an intravenous glucose tolerance test (ivGTT) was performed at Zeitgeber Time 2 (ZT2) and ZT14. After 7-10 days rats underwent a complete 12h phase LD reversal, and after 3 days the ivGTTs were repeated.

Results: The inverted LD cycle leads to an expected gradual adaptation of the daily rhythms in rats. Rats adapt their rhythm of locomotor activity faster to the inverted LD cycle compared to their rhythms of food and water intake. Food-restriction to the light period reduces the adaptation speed of the daily rhythm in locomotor activity. Nonetheless, the glucose tolerance test showed that the daily rhythm of glucose tolerance had returned to normal in three days after the LD reversal.

Conclusion: Our results indicate that locomotor activity in itself is not a sufficient marker of internal time after a phase shift, since the rhythms of food intake and energy expenditure take longer to adapt to the new schedule. In contrast, the daily rhythm of glucose tolerance has adapted within three days of inversed light exposure.