PHYSIOLOGICAL SIGNIFICANCE OF THE CONCENTRATION OF HUMAN-MILK GLUCOSE

The concentration of glucose in human milk, [Glucose]milk, was directly related to the volume of milk secreted not only during lactogenesis and weaning but also during full lactation. To investigate the mechanism for this observation we first established that glucose equilibrates across the apical membrane of the mammary alveolar cell, using infusion of stable isotopically labelled glucose into lactating women. Our results indicate that [Glucose]milk can be used to measure the glucose concentration in the mammary alveolar cell, [Glucose]cell. We then investigated the regulation of glucose transport into the mammary alveolar cell using glucose clamp methodology in fully lactating and weaning women. Maintenance of high plasma insulin concentrations for four hours under euglycemic conditions had no effect on [Glucose]milk, demonstrating that insulin does not regulate glucose transport into the mammary gland. On the other hand, maintaining the [Glucose]plasma at twice the fasting level resulted in a 3-fold increase in the steady state [Glucose]milk in fully lactating women and a 5-fold increase in [Glucose]milk in weaning women. Kinetic analysis of the data showed that the V(max) for glucose transport into the mammary alveolar cell across the basolateral membrane is regulated by the level of synthetic activity in the mammary alveolar cell.