Development of static, low-merit control line (CL) and contemporary, high-merit select line (SL) Holstein cows was initiated in 1964 at the University of Minnesota. Currently the difference in milk yield between the 2 lines is more than 4000 kg/305 d lactation. Studies described in this thesis used young growing heifers from the 2 lines to determine the effects of bovine somatotropin (bST) administration and nutrient supply on components of the somatotropic axis, relative expression of isoforms of some of these components, the effects of nutrient supply on ST response to growth hormone releasing hormone (GHRH) administration, and the effects of genetic potential for milk yield on the decrease in plasma ST concentration after bST administration. Cows and heifers from these 2 lines were also used to determine the distribution frequencies of leptin single nucleotide polymorphisms (SNP) and if the SNP alleles or genotypes were associated with indices of productive merit. Furthermore, to ensure the accuracy and reliability of hepatic gene expression measurements, an internal control probe was developed for ribonuclease protection assays (RPA) and an internal control selection procedure was developed for real-time RT-PCR.

Results of these studies indicate that: selection for milk yield increased plasma IGF-1 and ST concentrations, hepatic IGF-1 expression and the ratio of class 2 to total IGF-1. Administration of bST increased plasma IGF-1, IGFBP3, and IGFBP5, but had no effect on plasma NEFA or insulin. Feed restriction decreased plasma IGF-1 and hepatic expression of GHR-IA, insulin receptor, IGF-1, IGFBP3, and IGFBP4. However, increased potential for milk yield did not alter response of the somatotropic axis to bST administration or negative energy balance. The pre-GHRH injection concentration of ST was greater in SL than in CL heifers, but selection for increased milk yield had no effect on ST response to GHRH (peak ST height and AUC60). Feed restriction (negative energy balance) reduced basal ST concentration and ST response to GHRH. Parameters that described the post-peak decrease in plasma ST concentration after bST administration were not affected by selection for milk yield. Differences between the lines in plasma IGF-1, hepatic IGF-1 expression, and ST response to GHRH during the transition from negative to positive energy balance suggest the potential for differences in the ability of heifers from the lines to adapt to change in nutrient supply and warrants further investigation.

Reduced frequency of the T allele of Lept1 SNP (suggested to be associated with increased feed intake) and increased frequency of the T allele of Lept2 SNP (suggested to be associated with increased milk yield in SL cows indicates presence of these alleles was not associated with increased milk yield. However, these 2 leptin SNPs might interact with other factors to improve animal performance.

A bovine specific β-actin antisense internal control template for RPA was developed successfully. Among candidate genes evaluated as possible internal control genes for real-time RT-PCR, HPRT was selected for its relative consistency and abundance of expression. Results indicate potential internal control genes can be expressed consistently under some, but not all, conditions.