Effect of transport stress and hydrocortisone on the oestrogen-induced luteinizing hormone surge in pigs

Pre-pubertal gilts were ovariectomized at 5–6 months age and 1 month later were challenged with oestradiol benzoate (OB; 20 μg kg−1 i.m.). In Experiment, 1 gilts were not transported (Treatment 1; n=6), or were transported for 24 h (Treatment 2; n=11) or 48 h (Treatment 3; n=6) after OB for a distance of 30 km. In Experiment 2, animals received saline (control; n=6) or an injection of hydrocortisone (HC; 500 mg per animal i.m.) at 24 h (HC 24; n=5) or 48 h (HC 48; n=5) after OB. In five of 11 gilts transported 24 h after OB (TR 24A), luteinizing hormone (LH) secretion was higher (P<0.05) during the negative feedback (6–48 h after OB) than in the remaining gilts (TR 24B) of the same treatment and the controls. LH concentrations during positive feedback (54–96 h after OB) did not differ between TR 24A and controls, but two of five gilts did not show LH surges. 24A gilts had a higher rise in cortisol immediately after transportation than animals in the TR 24B group (P<0.05). During the positive feedback there was a significant increase of LH concentrations (ng ml−1) in gilts transported 48 h after OB when compared with the control and TR 24A groups (1.70±0.40 vs. 0.85±0.30 and 0.87±0.25, respectively; P<0.05). The LH surge parameters (times of emergence and greatest surge and amplitude) did not differ between the control, TR 24A, TR 24B and TR 48 groups. eriment 2, administration of HC caused significant elevation of cortisol in the blood of treated groups. An increase of LH level during ‘late’ negative feedback occurred in only one of five gilts given HC 24 h after OB. The positive feedback parameters were not affected in the two groups given HC with the exception of the earlier onset (h) of the LH surge in HC 24 group when compared with the control and HC 48 groups (52.8±1.2 vs. 62.0±2.0 and 61.2±2.2, respectively; P<0.05). These results demonstrate that transport can modify the profile of negative and positive feedback of LH secretion in ovariectomized pigs in different ways depending on the time of the gilts transportation after OB injection. Deviations of the positive and negative phases of feedback are not caused by cortisol.