Insulin-stimulated glucose uptake in rat diaphragm during postnatal development: lack of correlation with the number of insulin receptors and of intracellular glucose transporters.

The insulin responsiveness of the membrane transport system for glucose (2-deoxy-D-glucose) in diaphragm was measured during postnatal development of the rat. At birth, the basal rate of 2-deoxy-D-glucose transport is 3 nmol/min X g and it gradually decreases to 1 nmol/min X g over a period of 40 days. On the other hand, the insulin-stimulated rate of transport is 6 nmol/min X g at birth, it increases to 9 nmol/min X g in 16- to 20-day-old rats, and it decreases again to approximately 4 nmol/min X g in the 40-day-old rats. The stimulation of 2-deoxy-D-glucose transport by insulin is 2-fold at birth and increases to 4- to 5-fold 20 days after birth. The number of insulin receptors in the plasma membrane and the number of intracellular glucose transporters was also measured as a function of age to determine if there might be a correlation between these components of the insulin responsive system and the development of the increased insulin stimulation of 2-deoxy-D-glucose transport. The number of insulin receptors per g of wet weight decreased continuously with increasing age; the diaphragm of 40-day-old rats had about 50% of the receptors present in the diaphragm of the newborn rat. Similarly, the number of intracellular D-glucose transporters per g of wet weight decreased with increasing age; for adult rats, the number of transporters per g of diaphragm was 60% of that of newborn rats. The results indicate that the extent of insulin stimulation of glucose (2-deoxy-D-glucose) transport in the diaphragm during the first 20 days of life is not directly or simply related to the number of insulin receptors or the number of intracellular glucose transporters. The extent of the insulin response depends on some other factor that activates or is part of the machinery for translocation of the transporter.