GROWTH HORMONE (GH) GENE POLYMORPHISMS APPEAR TO DIFFERENTIALLY INFLUENCE PITUITARY RESPONSIVENESS TO GROWTH HORMONE RELEASING HORMONE (GHRH) AMONG Bos taurus AND Bos indicus-DERIVATIVE CATTLE

¹ Department of Animal and Range Sciences, New Mexico State University, Las Cruces, NM, 88005, USA

² Department of Animal Science, Colorado State University, Fort Collins, CO, 80523, USA

Relationships of GH gene polymorphisms to pituitary responsiveness to GHRH were evaluated in 8 month-old Bos taurus (Angus) and Bos indicus-derivative (Brahman, F₁-Brahman x Angus, and Brangus-3/8:5/8) bulls (n = 361). Polymorphisms were an intron 4 Msp-I RFLP (+/+, +/-, -/-) and an exon V L-leucine to V-valine SNP. Pituitary responsiveness was determined by serial blood sampling after intravenous injection of GHRH and calculating area under the response curve (AUC) using trapezoidal summation. F₁-Brahman x Angus bulls were hyper-responsive to GHRH relative to Angus and Brangus bulls with an attenuated response in Brahman (AUC units, 2961 > 1963 = 2063 < 972 ± 340, P < 0.05). Angus had high frequencies (> 0.68) of the homozygous +/+ and LL genotypes. Conversely, Brahman had a high frequency (0.63) of the -/- genotype for the Msp-I RFLP, but all were homozygous LL for the LV-SNP. F₁-Brahman x Angus and Brangus had relative equivalent genotypic frequencies for each polymorphism. Genotype within breed was a significant (P < 0.05) source of variation in prediction of GH-AUC with the interaction of the two polymorphisms accounting for 10.7 % of the variation. The heterozygous (+/-) Msp-I genotype yielded higher GH-AUC relative to homozygous genotypes in Angus whereas the -/- and VV genotype yielded the greatest GH-AUC in Brangus. The +/+ and LV genotypes yielded greatest levels of GH-AUC in F₁-Brahman x Angus. Genotypic frequencies of GH polymorphisms differ among Bos taurus (Angus) and Bos indicus-derivative cattle and these polymorphisms appear to differentially influence pituitary responsiveness to GHRH.