Short-term measurement of D-xylose uptake by isolated rat soleus muscle

The uptake of D-[U-14C]xylose by isolated rat soleus muscle was studied, using D-[1-3H]-sorbitol as an extracellular marker. Xylose uptake was limited by the diffusion of the sugar into and through the extracellular water. This could be overcome in part by allowing the test sugars to pre-equilibrate in the extracellular water at 0 degrees C, before measuring xylose uptake. It was not necessary to fill the extracellular water with the test sugars to obtain maximum rates of xylose uptake. From this it was concluded that the sugar carrier sites were located in a specific region on the plasma membrane, readily accessible to sugar carrier sites were located in a specific region on the plasma membrane, readily accessible to sugars entering the interstitial water. Pre-equilibration was more effective in the absence of insulin than in the presence of the hormone. This suggested that insulin may influence sugar uptake at some site prior to the cell membrane. Pre-incubation at 0 degrees C itself stimulated sugar transport. This effect of cooling was not influenced by insulin, nor did it appear to affect the stimulatory action of insulin on xylose transport.