Expression of Growth Hormone Receptor 1A mRNA is Decreased in Dairy Cows but not in Beef Cows at Parturition

The promoter controlling expression of a major bovine growth hormone (GH) receptor (GHR) mRNA variant, GHR 1A, contains a common DNA element for transcription factors hepatocyte nuclear factor 4α (HNF-4α), hepatocyte nuclear factor 4γ (HNF-4γ), and chicken ovalbumin transcription factor II (COUP-TFII). Expression of GHR 1A mRNA is decreased in the liver of dairy cows at parturition. The objective of this study was to determine whether reduced expression of GHR 1A mRNA in dairy cows at parturition is associated with changed expression of HNF-4α, HNF-4γ, or COUP-TFII mRNA. Liver biopsy samples were taken from multiparous Holstein cows 7 to 23 d before parturition, within 24h after parturition (i.e., at parturition), and 8 to 18 d after parturition, and the relative amounts of GHR 1A, insulin-like growth factor-I (IGF-I), HNF-4α, HNF-4γ, and COUP-TFII mRNA in these samples were measured by ribonuclease protection assays. As expected, expression of GHR 1A, total GHR, and IGF-I mRNA was decreased at parturition, compared with that detected prepartum or during the postpartum period. Expression of HNF-4α and COUP-TFII mRNA was unchanged, but that of HNF-4γ mRNA was increased at parturition. The same study was also conducted in multiparous Angus cows 7 to 23 d before parturition, at parturition, and 8 to 18 d after parturition. Neither expression of GHR 1A, total GHR, or IGF-I mRNA, nor expression of HNF-4α, COUP-TFII, or HNF-4γ mRNA was changed in the liver of beef cows at parturition. These results together suggest that, at the molecular level, decreased expression of GHR 1A mRNA in the liver of dairy cows at parturition may involve increased expression of HNF-4γ mRNA and that, at the systemic level, decreased expression of GHR 1A mRNA is not a direct result of the end of pregnancy, parturition, or the initiation of lactation.