Increase of plasma eCG binding rate after administration of repeated high dose of eCG to cows

Equine chorionic gonadotrophin (eCG) is still used to promote follicular gr owth in cattle and, more recently with an increased frequency of administra tion, in ovum pick-up protocols. The aim of this experiment was to verify t he possible effect of high frequency of administration on the immune respon se to eCG. The profiles of eCG binding rate, in the blood of two groups (A, B) of 4 primiparous cross breed beef cows (3-3.5 years old) submitted week ly for 5 to 10 weeks to repeated high doses (1000-2000 IU) of equine chorio nic gonadotrophin, are presented in this paper. A sensitive radiometric met hod was used to detect antibodies in plasma. The profiles clearly indicated a marked increase of eCG binding rate after 3 to 5 injections of the exoge nous hormone to the females. The statistical analysis of the results establ ished that treatments induced a significant increase (P<0.01) in binding ra tes after 6 and 3 injections in group A and B respectively. These binding r ates remained elevated for at least 1 week following the last injection and decreased afterwards. The values of plasma binding rates following repeate d eCG administration differed significantly between groups (0.90<plus/minus >1.04 and 1.04 +/-0.11 for groups A and B before treatment versus 11.77 +/- 0.92, 6.70 +/-0.85 for groups A and B after treatment, P<0.01) and from one cow to another (P<0.01) with some cows presenting no significant immune re sponse while others were more reactive against the hormone (at least 3 inje ctions).