Effect of insulin on transport of several hexoses and pentoses into cells of muscle and brain.

The effect of insulin on transport of d-glucose, d-mannose, d-fructose, d-xylose, l-arabinose and d-ribose through the cell membrane has been studied. Each sugar was infused into eviscerated rats with or without insulin for 1 or 2 hours. The accumulation of free sugar within the cell was determined in the diaphragm, heart, gastrocnemius and brain. Intracellular sugar was estimated by subtracting the amount of sugar in the extracellular fluid space, as determined with mannitol-1-C14, from the total tissue sugar. In muscle virtually no intracellular free sugar was detected in the absence of insulin whereas, in the presence of insulin, large amounts were found except in the case of d-ribose. Since these sugars do not arise by intracellular metabolism, it was concluded that insulin accelerated their transport through the cell membrane. These and other observations support the view that membrane transport in muscle is a site of insulin action. The transport process is distinct from phosphorylation and presumably involves combination of the sugar with a constituent of the cell membrane. In brain, no effect of insulin was observed. d-Mannitol, d-ribose and d-fructose did not pass the blood-brain barrier to a measurable extent. This explains the inefficacy of fructose in relieving the symptoms of hypoglycemia.