Reproductive and metabolic endocrinology in rams selected for high or low plasma IGF-1 concentrations : a thesis presented in partial fulfilment of the requirements for the degree of Master of Animal Science at Massey University
The objectives of this study were to define reproductive and metabolic endocrinology in Romney rams selected at the time of weaning for high or low peripheral IGF-1 concentrations with particular relevance to the annual changes in the relationships between GH, IGF-I and insulin and the relationship between that system and the activity of the reproductive endocrine axis. Parameters examined in detail included testis function, the response of IGF-I and insulin to GRF, and expression of IGF-I and its binding proteins in the testis. A preliminary experiment (December 2005 and March 2006) examined semen sample high, low and unsclccted IGF-I rams. In the main experiment, blood and semen samples were collected in July, September and November 2006 and March 2007 from yearling rams (high or low IGF-I concentrations). Scrotal circumference and inguinal skin were recorded. Blood samples were collected via jugular venipuncture. After a collection of the control samples, rams were given 50 ug hGRF and 1,000 IU hCG. Second blood samples were collected 40 min (testosterone) and 10 h later (insulin and 1GF-I). Seasonal differences from July to March were evident in the percentage of abnormal sperm, scrotal circumference and sperm motility, and concentrations of testosterone, IGF-I and insulin. IGF-I line differences were also found in scrotal circumference and IGF-I and insulin concentrations. Testosterone concentrations did not differ between lines, but, when actual IGF-I concentrations were taken into consideration, exhibited different secretion patterns between lines. In the experiment 3, samples of liver and testis tissue were collected from four animals for expression of mRNA for IGF-I, Type I IGF receptor and IGF binding proteins. It was found that genes foi IGF-I, Types I IGF receptor and IGF binding proteins -2, -3, -4, -5 and -6 were expressed in the testis. Significant high expression of Type I IGF receptor and IGFBP-3 in the testis was found in the low line, with different levels of high and low expression of IGF-I in the liver of the two lines. These studies suggested that circulating concentrations IGF-I are associated with gonadotropin-stimulated steroidogenesis that affects scrotal circumference, but differences in IGF-I status between lines are modulated by the different expression of mRNA for IGF-1R and IGFBPs and by negative feedback between lines.