Reproductive and metabolic endocrinology of Romney rams selected for high or low circulating IGF-I concentrations

The objectives of this study were to characterise reproductive and metabolic endocrinology parameters in Romney rams from lines selected since 1986, for high or low circulating IGF-1 concentrations. Seasonal changes were recorded in the patterns of IGF-1, insulin and the response of serum testosterone to hCG administration. Serum testosterone concentrations, adjusted for covariance with IGF-1 concentrations, were higher in both strains (P < 0.05) in the autumn (March), compared to other times of year. The response of serum testosterone to hCG increased (P < 0.01) between mid-winter (July) and the following autumn. Serum testosterone concentrations were higher (P < 0.05) in high, compared to low-IGF rams in July and September (spring), but not in March (autumn). The serum IGF-1 concentrations were higher (P < 0.01) throughout the entire experiment in the high- versus the low-IGF rams, although the concentrations in the high-IGF rams declined between July and March. Insulin concentrations were also higher in the high-IGF line. The difference between the two lines was greatest in July, but not significant in March. Samples of liver and testes tissue were collected from 4 animals in each line for the polymerase chain reaction (PCR) determinations of mRNA expression for IGF-1, Type 1 IGF receptor (IGF-1R) and IGF-binding proteins (IGFBP's). Genes for IGF-1, IGF-1R and the IGFBP-2, -3, -4, -5 and -6 were expressed in the testis. Expression of IGF-1R and IGFBP-3 in the testis and that of IGF-1 in the liver, was greater in the high- than the low-IGF line rams. These studies suggest circulating concentrations IGF-1 to be associated with basal, but not gonadotrophin-stimulated steroidogenesis. The differences in IGF-1 status between lines are modulated by the differential expression of mRNA for IGF-1R and the IGFBP's and by differences between lines in the negative feedback of IGF-1.