A daily rhythm in glucose tolerance - A role for the suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN), the biological clock, is responsible for a 24-h rhythm in plasma glucose concentrations, with the highest concentra tions toward the beginning of the activity period. To investigate whether t he SCN is also responsible for daily fluctuations in glucose uptake and to examine how these fluctuations relate to the rhythm in plasma glucose conce ntrations, SCN-intact rats and SCN-lesioned rats were injected intravenousl y with a glucose bolus at different time points. We found an increase in gl ucose uptake toward the beginning of the activity period, followed by a gra dual reduction in glucose uptake toward the end of the activity period. The daily variation in glucose tolerance seemed not to be caused by fluctuatio ns in insulin responses of the pancreas but by a daily variation in insulin sensitivity. Lesioning the SCN resulted in the disappearance of the daily fluctuation in glucose uptake and insulin sensitivity. Interestingly, SCN-l esioned rats showed an enhancement in glucose tolerance that could not be e xplained by higher insulin responses or enhanced insulin sensitivity. There fore, these findings suggest a role for the SCN in insulin-independent gluc ose uptake. The present results further show that the daily rhythm in gluco se tolerance follows the same pattern as the daily rhythm in plasma glucose concentrations. We hypothesized that the biological clock prepares the ind ividual for the upcoming activity period by two separate mechanisms: increa sing plasma glucose concentrations and making tissue more tolerant to gluco se.