Circadian rhythms in blood glucose and the effect of different lighting schedules, hypophysectomy, adrenal medullectomy and starvation

Abstract

Abstract With other environmental factors rigidly standardized, normal Sprague‐Dawley rats were maintained under the following lighting schedules: (1) LD 12:12, artificial light from 6:00 A.M. to 6:00 P.M. alternating with 12 hours of darkness; (2) DL 12:12, reversal of the first schedule; (3) DD, constant darkness; and (4) LL. constant illumination. During each lighting regimen, blood glucose levels were determined on separate subgroups of 11 to 18 animals at bi‐hourly intervals during a 24‐hour period. Significant circadian fluctuations or rhythms under all lighting conditions were found when the mean blood glucose values for each bi‐hourly group were plotted as a function of time. Rats maintained under LD 12:12 conditions showed approximately a 25% difference between the maximum and minimum values. When the phasing of the peaks and troughs of blood glucose rhythm in Sprague‐Dawley rats was compared with that of a separate colony of Wistar rats, it was noted that they were similar. Starvation of the LD rats for 66 hours did not abolish the characteristic rhythm, although a lower level of blood glucose resulted. The LD 12:12 blood glucose pattern was reversed by reversing the LD cycle 180 degrees. Under LL and DD the troughs and crests seen in the glucose curves were out of phase with those of LD and with each other; this suggested that under LL or DD the rhythm had become free‐running because of the absence of the light‐dark synchronizer. Hypophysectomy modified the phasing of the normal LD pattern but did not abolish its fluctuating nature; this suggested that the LD cycle of light no longer was an effective entraining agent. Adrenal medullectomy caused an overall 13% decrease in the 24‐hour mean but did not alter the phasing of the LD pattern. The significance of periodicity analysis in relation to bioassay is discussed.