DIURNAL FLUCTATION OF PLASMA CORTISOL LEVELS IN THE GUINEA PIG

Abstract

ABSTRACT Diurnal changes in the basal levels of plasma cortisol were studied in the female guinea pig. Plasma cortisol levels were determined at 8 bleeding times during the entrained 24 h photoperiod (14 h light/10 h dark: lights on 06.00 h). Plasma cortisol levels remained low during the dark phase of the cycle ranging between 6.4 and 9.0 μg/100 ml. However, at 4 h prior to the onset of the light phase of the photoperiod, a dramatic rise in the plasma cortisol level was measured, which peaked between 04.00 and 08.00 h at 12.3 μg/100 ml. A subsequent decline in plasma cortisol levels was measured throughout the light phase of the cycle, reaching basal levels before the onset of the dark phase. These data indicate that a diurnal fluctation in plasma cortisol occurs in the guinea pig which is very reminiscent of that seen in the human and in contrast with that observed in the rat.