THE PHYSIOLOGICAL-EFFECTS OF NATURAL VARIATION IN GROWTH-HORMONE GENECOPY NUMBER IN RAM LAMBS

The effects of natural variation in the number of copies of the growth hormone (GH) gene on growth parameters, plasma GH profiles, and the r esponse to GHRH challenge were compared in Coopworth ram lambs from se lection lines differing in body composition and GH levels. Different g enotypes at the GH locus carried two, three, or four copies of the GH gene and GH secretion was studied under ad libitum feeding conditions and in the fasted state. There were no significant effects of GH genot ype on any parameters of growth or body composition. Basal serum GH co ncentration, GH pulse frequency, and GH pulse amplitude differed signi ficantly with selection line and fasting, but did not differ significa ntly between the GH genotypes. Significant differences of subtle natur e were found between the GH genotypes in their responsiveness to GHRH. For the ad libitum-fed Lean selection line animals, the first GHRH ch allenge resulted in a higher mean maximum response for GH1/GH1 than GH 2/GH2 (P < 0.05). Between the first and the second challenges there wa s a decrease in maximum response for the GH1/GH1 genotype and an incre ase for the GH2/GH2 genotype (P < 0.05 for GH genotype main effect). T he differences between GH genotypes in response to GHRH challenge sugg est that polymorphism in the number of GH gene copies in sheep may hav e physiological implications for the function of the GH axis, which ma y be manifested in growing lambs only under specific genotype-environm ent combinations. (C) Elsevier Science Inc. 1998.