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

Abstract

With other environmental factors rigidly standardized, normal SpragueDawley rats were maintained under the following lighting schedules: (1) LD 12:12, artificial light from 6:OO A.M. to 6:OO 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 wgre 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.

Circadian rhythm in blood glucose levels in normal and hepatectomized mice

“…A clear circadian variation is seen in the blood glucose level in normal mice. The peak position in our experiment is found 4 hrs later than the peak position found in rats (Pauly et al, 1967). This différence can be attributed to species différences or, alternatively, to différences in the method of standardization which, probably, produces différent feeding patterns in animais caged in groups of two per cage, as is the case in thèse experiments and in the single caged animais of the présent experiments (Vilchez et al, 1971).…”

“…The existence of circadian variations in blood glucose has been repeatedly reported (for review see Pauly et al, 1967), but very few observations hâve been made in mice ) and we hâve not found références to circadian variations in blood glucose in hepactomized animais.…”

Ghrelin secretion is more closely aligned to energy balance than with feeding behaviour in the grower pig

“…Studies in humans and rodents have shown an alteration in both plasma glucose and insulin concentrations and variation in sensitivity of tissues to insulin over a 24-h light/darkness period. For example, humans have a higher glucose output and insulin requirement in the early morning hours (Carroll & Nestel 1973, Lee et al 1992, whereas rodents have higher concentrations of glucose and insulin in the dark period compared with the light period (Pauly & Scheving 1967), which may be associated with the initiation of physical activity (La Fleur 2003): it is important to recognise that rodents are nocturnally active and circulating insulin and glucose concentrations are higher during the dark phase (Pauly & Scheving 1967). Similarly, diurnal fluctuations in the actions of the D 2 dopamine receptor agonist bromocryptine on insulin sensitivity have been observed, with increased sensitivity in the morning and suppression in hepatic glucose production in the afternoon (Luo et al 1999).…”

Glucose Turnover and Disposal in Maturity-Onset Diabetes

“…The alteration in hepatic glucose formation is consistent in phase with the serum cortisol cycle, but is said to survive, at least in part, adrenalectomy and hypophysectomy (63). It may be light-mediated (71). It may reflect multifactorial variation in hepatic gluconeogenetic enzymes (72)(73)(74) coordinated with respect to time in some unknown way by the hypothalamus (74).…”