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Milk synthesis is known to be modulated by peptide hormones such as prolactin (PRL), growth hormone (GH), and insulin-like growth factor I (IGF-I). Previous studies suggested that PRL and IGF-I acted directly on mammary epithelial cells and were involved in lactation. Meanwhile, GH is thought to be indirectly involved in lactation by stimulating the secretion of IGF-I. It is controversial as growth hormone receptors (GHR) is expressed in the mammary epithelial cells. In order to clarify whether GH acted directly on mammary gland tissue, we investigated the prolactin receptors (PRLR), IGF-I receptors (IGF-IR), and GHR as well as the gene expression levels of the downstream signaling molecule for each receptor in the mammary gland tissue of Holstein cows during different stages of lactation. The results revealed that the mRNA expressions of PRLR and IGF-IR were highest during early lactation, and the mRNA expression of the GHR was highest during mid-lactation. We also found that the expression profiling of the signal transducer and activator of transcription 5 (STAT5) genes was similar to that of the GHR gene. On the other hand, the expression profiling of the PRLR gene was similar to that of the SHP2 gene. These results suggest that GH acts on the mammary glands directly, milk synthesis and secretion are chiefly stimulated in mid-lactation, and the timing of the action is different for PRL and IGF-I.

Comparative Expression Profiling of Lactogenic Hormone Receptor and It’s Signaling Molecules of Bovine Mammary Glands during lactation