Growth hormone secretion in anestrous ewes subjected to prolonged stressful stimuli

Abstract

The study was performed on 6 three-year-old Polish Merino ewes to examine the effects of prolonged intermittent footshock stimuli on the secretion of growth hormone (GH) in anestrous ewes. The state of stress was induced by electrical footshocks of enduring and repetitive character (20 min/h, 9 h daily during 3 consecutive days, current intensity 3 mA). The blood samples were taken at 2 h intervals on the day prior to stimulation (controls), during 3 days of footshock application, and on the post-stress day. The control levels of plasma GH were quite low (from 1 to 6 ng/ml) and in some samples they were at, or near,the limit of detection. The mean 24 h plasma GH level ranged from 0.51 to 3.70 ng/ml. The mean concentration of GH in all ewes prior to the exposure to footshock stress was 2.12 + 0.55 (SE) ng/ml. Footshock led to marked increase in GH concentration; the mean plasma level of GH in animals reached about 200% of pre-stress values. There were no significant differences between GH responses in the first, second and third day of exposure to footshock stimuli. The mean level of plasma GH on the post-stress day did not differ significantly from the control concentration. However, in 4 out of 6 ewes the mean 24 h plasma GH level was lower as compared to the value found in controls. In conclusion, these results indicate that prolonged intermittent footshock stimulation increases GH secretion in anestrous ewes, after which the concentration of this hormone in the blood plasma in most animals on the post-stress day returns to normal, pre-stress values. It is suggested that changes in GH concentrations, among others, may be to some extent a valuable indicator of the biological cost of environmental challenges.