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Bovine somatotropin (bST) and insulin-like growth factor-1 (IGF-1) are peptide hormones that are involved in the regulation of milk production in dairy cows. Because these hormones are present at extremely low concentration in fresh and processed bovine milk, a highly sensitive and specific electrochemiluminescent immunoassay (ECLIA) has been developed to better estimate the concentration of these hormones in milk. The assay employs an imager, a capture antibody bound to a carbon electrode, and a detection antibody coupled to a ruthenium label. In the presence of tripropylamine and an electric pulse, ruthenium generates light proportional to the amount of antigen bound, and the light is captured as signal by a charge-coupled device (CCD) camera. Using bovine milk as the starting matrix, 99.69% of bST and 104.79% of IGF-1 were recoverable. The limit of detection (LOD) was <5 pg/mL for bST and <1 pg/mL for IGF-1. The limit of quantification (LOQ) was <14 pg/mL for bST in milk and <2 pg/mL of IGF-1. The assay is highly specific and shows <0.2% cross-reactivity with other peptide hormones found in bovine milk such as insulin and IGF-2. These data indicate this new, ECLIA is highly sensitive and specific for estimating the concentration of bST or IGF-1 in milk.

Six Holstein cows were used in a complete block design to examine effects of period of lactation and somatotropin (bST) administration on concentrations of insulin, insulin-like growth factors (IGF-I, IGF-II), and IGF-binding protein 2 (IGFBP-2). During late lactation, the dry period and the subsequent early lactation, cows received injections of NaHCO3 buffer for 5 d and bST for 7 d. Cows were in positive energy and protein balances during late lactation and the dry period and in slight negative balances during early lactation. Basal insulin concentrations were highest in late lactation (170 pmol/L), whereas bST concentrations were higher in early lactation (0.6 micrograms/L). Insulin was increased by bST in the dry period (255 pmol/L) and late lactation (149 pmol/L) but not in early lactation (14 pmol/L), probably because of greater availability of glucose during positive nutrient balance. Basal IGF-I was lowest in early lactation (63.6 micrograms/L) but was increased by bST during all periods. The IGF-I response to bST administration was lower during early lactation (74.1 micrograms/L) compared with late lactation (123.6 micrograms/L) and dry period (146.0 micrograms/L). The IGF-II concentrations were not affected by period of lactation of bST administration but IGF-II tended to be higher during bST administration when cows were dry. Concentration of IGFBP-2 was higher during early lactation when cows were in negative nutrient balance (479.5 micrograms/L) than during the dry period (289.2 micrograms/L) and was decreased with bST. These data support a role of insulin and IGF in regulation of milk production. Availability of nutrients may be involved in regulating these hormones, particularly during bST treatment.

To establish the naturally occurring range of insulin-like growth factor-I concentrations in bovine milk, samples from individual cows (n = 409) managed on five Missouri dairy herds were assayed. Parity, stage of lactation, and farm affected milk insulin-like growth factor-I concentration. Milk insulin-like growth factor-I concentration was higher in early lactation than mid and late lactation with concentrations in multiparous cows exceeding those in primiparous cows. Insulin-like growth factor-I concentration was negatively correlated to milk production the day of sample collection (r = -.15) and not correlated to predicted 305-d milk yields. Unprocessed bulk tank milk samples (n = 100) from a commercial processing plant had a mean concentration of insulin-like growth factor-I in milk of 4.32 ng/ml with a range of 1.27 to 8.10 ng/ml. This distribution was similar to the range detected in samples from individual cows, but values were lower than those reported for human milk. Concentration of insulin-like growth factor-I in milk was not altered by pasteurization (at 79 degrees C for 45 s). However, insulin-like growth factor-I was undetectable in milk heated to temperatures (121 degrees C for 5 min) required for infant formula preparation or in commercially available infant formula. These data indicated that insulin-like growth factor-I is a normal but quantitatively variable component of bovine milk that is not destroyed by pasteurization but is undetectable in infant formula. Concentration of insulin-like growth factor-I in bovine milk is lower than concentrations reported for human milk yet similar to those reported for human saliva.(ABSTRACT TRUNCATED AT 250 WORDS)