Glucose Metabolism

Some of the increase in hepatic gluconeogenesis in late-pregnant ruminants is due to increased voluntary feed intake.[1] However, intake is often constrained by physical factors such as diet quality and abdominal distension, as well as endocrine factors such as the surge in estrogen secretion in late pregnancy.[2] Under these or more controlled conditions of feed restriction, an increase in glucose production is sustained by increased peripheral mobilization and hepatic uptake of endogenous substrates such as amino acids from skeletal muscle and glycerol from the lipolysis of adipose triglycerides.1-1'3-1

Glucose utilization by muscle and adipose tissue tends to decrease during late pregnancy, especially if maternal energy intake is restricted or voluntarily declines. These tissues account for most of the approximately 20% of total glucose disposal that, in nonpregnant, nonlactating sheep, appears to be insulin-dependent. It is therefore likely that altered responses to insulin in these tissues largely account for the development of moderate insulin resistance in various parameters of whole-body glucose disposal in late-pregnant ewes.[1] This effect, which presumably mediates the so-called glucose-sparing effect of pregnancy, is exaggerated by moderate maternal undernutrition.

Mechanisms appear to involve reduced expression and, possibly, altered intracellular translocation, of the insulin-responsive glucose-transport protein, GLUT4, in muscle and adipose tissues.[4]

The increase in glucose availability created by the combination of increased hepatic production and reduced utilization in peripheral tissues is of advantage to the pregnant uterus, as well as to vital maternal tissues such as brain, because these tissues do not require insulin to facilitate the uptake and transport of glucose. Thus, a relatively normal supply of maternal glucose to the fetus can be sustained even if the dam is somewhat undernourished, especially if she has recourse to adequate lipid stores in adipose tissue.[5]

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