Changes in the Somatotrophic Axis Associated with the Initiation of Lactation

Abstract

The demands of early lactation in dairy cattle are met through a coordinated physiological response that causes partitioning of nutrients for milk production. Among the hormones involved, growth hormone (GH or somatotropin) assumes a pivotal role in lipid and carbohydrate metabolism. Growth hormone's actions are transduced by the cell-surface GH receptor (GHR). The molecular regulation of the GHR is complex because at least three promoters control GHR transcription. Transcription of GHR 1A mRNA is controlled by GHR promoter 1 (P1). The GHR 1A mRNA is found in liver of adult animals where it represents the bulk of the GHR mRNA. Binding of hepatocyte nuclear factor-4 (HNF-4) to GHR P1 may cause liver-specific expression of GHR 1A. Transcription of GHR 1B and 1C mRNA is controlled by GHR P2 and P3. The activity of GHR P2 and P3 depends on an enhancer element that binds Sp1 as well as an unknown transcription factor. The liver GHR 1A is decreased at calving and then subsequently increases during the early postpartum period. The decrease in liver GHR 1A mRNA coincides with a period of liver refractoriness to GH when GH-dependent IGF-I synthesis and secretion are decreased. We hypothesize that the decrease in liver GHR 1A at calving leads to a decrease in the concentration of GHR in liver and a decrease in GHR second messenger signaling in early postpartum cows. Failure to regain GHR 1A expression during early lactation may compromise GH-dependent functions in liver and affect total milk yield.