Long-Term Somatotropic and Galactopoietic Effects of a (1-30) Ethyl Amide Analog of Growth Hormone-Releasing Factor

Thirty-five lactating Holstein cows (165±8 DIM) received the following treatments for 96 d: uninfused controls; i.v. infusion of .9, 2.7, or 8.1 mg/d of Ile2, Ser8,28, Ala15, Leu27, Hse30-bovine growth hormone-releasing factor (1–30) ethyl amide; or i.v. infusion of 12 mg/d of recombinant Leu27, Hse45-bovine growth hormone-releasing factor (1–45) lactone. Concentrations of somatotropin in serum and SCM yield of cows infused with 1–30 releasing factor increased quadratically as the dose increased. Responses for somatotropin and SCM yield were quantitatively similar for cows infused with 2.7 and 8.1mg of 1–30 or 12mg of 1–45. While concurrently infused with the 1–30 or 1–45 as described, cows retained their ability to release somatotropin following an acute i.v. injection of 10 nmo1/100kg of BW of either growth hormone-releasing factor 1–30 or 1–45. Mean concentrations of IGF-I in serum increased similarly in magnitude for cows infused at all doses of 1–30 or 1–45. The 1–30 releasing factor generally increased IGF-binding protein-3, but had little effect on IGF-binding protein-2. The 1–45 releasing factor did not significantly affect either binding protein. Yield of SCM was correlated with serum concentrations of somatotropin, but not with IGF-I. Concentrations of NEFA in serum were elevated through 36 and 50 d in response to the highest doses of 1–30 and 1–45, respectively. Treatment did not affect DMI, BW, or body condition score. In conclusion, continuous i.v. infusion of 2.7 and 8.1 mg/d of 1–30 and 12 mg/d of 1–45 similarly increased yield of SCM, somatotropin, and IGF-I without inducing refractoriness in the ability of the anterior pituitary gland to release somatotropin in response to a concurrent, acute challenge with 1–30 or 1–45.