Twenty-four-hour secretory pattern of cortisol in the bull: evidence of episodic secretion and circadian rhythm.

A single antibody RIA method for measurement of plasma cortisol concentrations in the bull is described. Antisera were obtained from rabbits immunized against cortisol-3-oxime-bovine serum albumin. By this technique, peripheral cortisol levels were determined in seven adult bulls (one blind) during 24- and 48-h periods, with blood collections every 30 min. Statistical evaluation of the 24-h profiles using time series analysis revealed that cortisol is secreted episodically throughout the day-night cycle (range, 0.4-9.7 ng/ml). Despite individual variability in both frequency and amplitude of secretory episodes, a distinct circadian secretion pattern was recognized. After dividing the 24 h into three 8-h time periods (I, 0900-1700 h; II, 1700-0100 h; III, 0100-0900 h), a depressed secretory activity with small episodic bursts not exceeding 3.5 ng/ml plasma consistently occurred during time period II. Increased cortisol secretion with high fluctuating levels was evident during time periods I and III. Maximum cortisol concentrations greater than 8 ng/ml were noticed in the morning at the onset of daylight, whereas lowest values were recorded in the evening when darkness began. Results from this study indicate that there is a temporal correlation between the rhythm of cortisol secretion and the light-dark cycle in the bovine species.