Effects of recombinant human growth hormone in juvenile Nile crocodiles

Abstract

The effects of exogenous recombinant human growth hormone (rhGH) were studied on Nile crocodiles 4 weeks after hatching (group A) and at 9 months of age (group B). They were kept under controlled laboratory conditions. During the treatment period the animals were injected (intramuscularly) with rhGH or saline (controls) twice a week. The group A animals were treated with doses of 3.25 Jlg rhGH/g b.wt., twice weekly for 5 weeks. After 8 weeks without treatment they were given doses of 1.30 Jlg/g for 3 weeks. The group B animals were treated with doses of 0.325 Jlg/g for four weeks and then sacrificed for blood and tissue sampling. Recombinant hGH stimulated growth, food uptake and food conversion efficiency of the group A animals during the first treatment period, however, within a week after the last injection the treated crocodiles stopped eating and the growth was arrested. When treatment was restarted, they all died within 7 weeks, probably due to a secondary immunological response. Although the 9 month crocodiles were treated with 1/10 of the dose given to the hatchlings, the growth stimulating effects of rhGH may have been more pronounced in the older animals. Crocodiles seem to recognise the rhGH as a foreign protein since antibodies to rhGH were present in plasma after 4 weeks of treatment with rhGH. It is concluded that rhGH increase growth in young crocodiles by stimulating appetite and mechanisms involved in protein synthesis. However, such therapy is not likely to be of any practical use since the crocodiles seem to develop hypersensitivity to the human growth hormone