Effects of photoperiod and feeding on plasma corticosteroid concentrations and maximum corticosteroid-binding capacity in pigs.

The effects of time of feeding and photoperiod on the circadian rhythm of total and free plasma corticosteroid concentrations and the maximum corticosteroid-binding capacity (MCBC) were measured in pigs. There were four photoperiod treatments: natural long and short daylengths (light:dark, 14:10; 101/2:131/2, respectively), artificial short daylength and reverse artificial short daylength (light:dark, 9:15). Photoperiod had a significant effect on corticosteroid concentrations. The corticosteroid concentrations on long daylength showed a major increase in the morning (from an average of 8 ng ml-1 at 0700 h to a peak level of 53.5 ng ml-1 at 0910 h). The amplitude of the morning peak was lower in the other treatments. Apart from this increase, corticosteroid levels at other times of day for pigs in all treatments were generally low. The MCBC varied greatly between animals (less than 25 - greater than 50 ng ml-1) but showed no circadian rhythm. There was no change in corticosteroid levels directly attributable to feeding. It was concluded that, in experiments where the effect of acute stressors on corticosteroid levels is examined, blood sampling should be performed in the afternoon-evening period because the between-animal variation in hormone levels is less and the hormonal levels are more stable, and also that time of feeding did not affect the rhythm of corticosteroids. A blood-sampling regime to define the circadian rhythm in corticosteroid concentration and to determine the MCBC is described.