Circadian rhythm of the lactate and pyruvate concentration in rat liver and blood.

Diurnal changes in the blood and liver lactate and pyruvate concentration and in the blood glucose, serum insulin and liver glycogen level were studied in the spring, at 3-hour intervals, in adult ad libitum fed male Wistar rats adapted to standard laboratory conditions and a 12:12 h light and dark regimen (7-19 and 19-7 h). The marked liver and blood lactate oscillations attained maximum values during the dark period, together with the serum insulin oscillations. The pyruvate concentration oscillations culminated in the light part of the day in the liver and in the dark part in the blood; they were not very pronounced, probably owing to rapid metabolization. The curve for the liver lactate/pyruvate index resembled the lactate curve and attained the maximum in the dark period; the blood curve for this index was different, because of the different times at which the two substrates pass from the liver and other tissues into the blood stream. The blood glucose circadian oscillations culminated in the second half of the light period, parallel with the first serum insulin maximum. The oscillations of the liver glycogen values were typical, with the maximum at the end of the dark period and the beginning of the light period. The circadian oscillations of the given substances furnish information on the time sequence of period metabolic processes dependent on the illumination or alimentary regimen, on the waking and sleep cycle and on other synchronizers.