Circadian fluctuations in plasma corticosterone, corticosterone-binding activity and total protein in male rats: possible disruption by serial blood sampling.

Male rats were bled serially every 6 h for 48 consecutive hours. Plasma so obtained was assayed for corticosterone (B), corticosterone-binding activity (CBA) and total plasma protein (TPP). Although the averaged data indicated a significant circadian rhythm in plasma B, inspection of the circadian changes in individual animals showed that 40% had irregular or aberrant patterns of plasma B. Additionally, circadian fluctuations in plasma CBA were approximately 3-fold lower in magnitude than previously reported, and there was no significant circadian change in TPP. Running wheel activity during the 48 h blood sampling period decreased by over 50% indicating that the serial sampling procedure caused disruption of the activity cycle. However, the majority of wheel-running activity occurred during the dark portion of the day-night cycle. Because circadian fluctuations in plasma proteins, including corticosteroid binding globulin (CBG), may be directly related to changes in locomotor activity, it is suggested that the attenuation of circadian changes in CBA and TPP were due to disruption of the daily cycle of locomotor activity. The abnormal circadian patterns in plasma B observed in some animals also may have been caused by disruption of the activity cycle in spite of the fact that the rats did not appear to be stressed as evidenced by concentrations of plasma B that were well within the normal range.