Genetic variation in stimulated GH release and in IGF-I of young dairy cattle and their associations with the leucine/valine polymorphism in the GH gene

Genetic variations in plasma GH concentrations before and following thyrotr opin-releasing hormone (TRH) stimulation and in IGF-I concentrations were s tudied in 11-mo-old Polish Friesian cattle (104 heifers and 110 bulls). A p ossible association between stimulated GH release, IGF-I, and the polymorph ism in the GH gene causing substitution of leucine-Leu to valine-Val at ami no acid position 127 of the protein was also investigated. The GH concentra tions were determined in serial plasma samples collected every 15 min from 15 min before to 135 min after intravenous administration of 0.15 mug TRH/k g live weight. The analysis was performed on three variables: baseline (mea n of samples at -15 and 0 min), peak (sample at 15 min after injection) and rate (peak minus sample at 60 min, divided by 45 min). The IGF-I concentra tions were measured in plasma samples taken before the TRH stimulation. Add itionally, first lactation records from the 75 cows earlier tested for GH r elease and IGF-I were used to study a possible association of milk producti on traits with GH genotypes. The data were analyzed by multivariate mixed l inear models. The heritability of IGF-I reached a higher value (0.35) than variables baseline, peak, and rate (0.02, 0.14, and 0.14, respectively). Th e GH variables were positively genetically correlated with each other (0.22 to 0.93), whereas they had negative genetic correlations with IGF-I (-0.26 ). The Val/Val genotypes reached the highest peak value compared with other GH genotypes (P greater than or equal to 0.01), whereas the Leu/leu genoty pes had the highest IGF-I concentrations (P less than or equal to 0.05). Mo reover, the Leu Veil heterozygotes were superior to others in milk and prot ein yields, whereas the Leu/Leu homozygotes reached the highest fat yield ( P greater than or equal to 0.01). We conclude that GH peak, GH rate, and IG F-I are heritable traits in young dairy cattle and are affected by the Leu/ Leu polymorphism in the GH gene.