THE UPTAKE OF GLUCOSE INTO CELLS AND THE ROLE OF INSULIN IN GLUCOSE TRANSPORT

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Abstract
This presentation has been restricted to the role of insulin in glucose transport in muscle cells and deals mainly with experiments using the perfused rat heart. The several possible means for glucose transfer into cells, diffusion, pores, pinocytosis, carriers, and dimerization, have been discussed; and arguments in favor of the carrier theory, namely, specificity, kinetics, inhibition, competition, and counterflow, have been elaborated. Glucose uptake has been considered to consist of three sequential steps: (1) passage of glucose from within the capillary to the cell surface, (2) transport across the cell membrane, and (3) metabolism of glucose within the cell. The first is considered to take place by diffusion and not to be significantly limiting under normal conditions, nor to be influenced by insulin. Transport across the cell membrane is thought to be mainly under the control of insulin and is the major rate-limiting step in glucose uptake when the extracellular glucose levels are in the normal range. Metabolism of glucose within the cell is the major rate-limiting step in glucose uptake when intracellular glucose concentration is so high that its phosphorylation is near saturation.