Circadian and seasonal variations of some metabolic parameters in the serum of the albino laboratory rat (Rattus norvegicus var. alba).

Total protein (TP), glucose (GL), esterified fatty acids (EFA) and total cholesterol (TC) were determined at 4-hour intervals during the 24-hour cycle in the serum of 270 female Wistar rats, in February, May and July. The results can be summed up under the following points: 1) The differences found between the levels of the given parameters in February, May and July were statistically highly significant. The greatest differences were observed in the TC level, which was lowest in July (0.82 mmol/l) and highest in February (1.87 mmol/l). The highest TP and GL values were found in May (62.5 g/l; 10.69 mmol/l) and the lowest in February (40.1 g/l; 7.8 mmol/l). The smallest differences were observed in the EFA level, which was highest in July (8.09 mEq/l) and lowest in February (6.39 mEq/l). The minimum 24-hour variations in the levels of most of the given parameters during the 24-hour cycle were observed in May (compared with February and July). 2) In a study of TP and GL concentration, the time of day was found to be a very important factor. The maximum TP level during the 24-hour cycle was found most often in the early morning or forenoon, while the blood glucose level was highest at about midday and in some cases in the early morning. The blood cholesterol level was relatively stable during the 24-hour cycle and it is impossible to draw definite conclusions from its fluctuations. Changes in the EFA level during the 24-hour cycle were likewise irregular. 3) Except for cholesterol, interindividual differences in the levels of the given parameters in the various months seem to be statistically non-significant.