Physiological significance of altered insulin metabolism in the conscious rat during lactation

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Abstract
Uptake of radioactively labeled insulin by the mammary gland of the rat increased 12-fold in lactation compared with non-lactating controls. This uptake was decreased by the presence of unlabeled insulin, indicating that it occurred via insulin receptors. The plasma half-life of insulin is decreased in lactation from 9.4 min to 4.8 min, and the metabolic clearance rate for insulin increased from 7.26 to 13.03 ml/kg body wt per min. The basal insulin and glucose concentrations in the plasma were decreased in lactation. Infusion of insulin, at a dose which led to a small physiological rise in plasma insulin concentration, increased lipogenic rates in the mammary gland by 100% without causing marked hypoglycemia. The lactating mammary gland is a highly insulin-sensitive tissue, and the lower plasma insulin during lactation occurs primarily as a result of this sensitivity increasing extraction of glucose by the gland and thus, producing a decrease in the plasma glucose concentration. Apparently, a secondary result of the fall in plasma insulin concentration is the preferential direction of substrates (glucose and non-esterified fatty acids) towards the lactating mammary gland and away from adipose tissue and the liver.