Regulation of glucose-transporter function.

Glucose transport in mammals is mediated by a multigene family whose expression can be highly tissue specific. All cells express at least one transporter isoform in a constitutive fashion, because a certain level of glucose uptake is an absolute necessity, regardless of influences by various regulatory factors. The level of the constitutive transporter, usually the erythroid glucose-transporter isoform, can be regulated by environmental factors, e.g., nutrition and transformation. Certain cells express unique transporter isoforms, the quantitatively most important of which is the muscle-adipocyte glucose-transporter isoform that functions in response to insulin to clear most of the blood glucose after a meal. The available data suggest that the major insulin target tissues are uniquely able to produce this transporter isoform, sequester it in a unique organelle, and bring it to the cell surface in response to insulin. This insulin response is dramatically different from that seen in various fibroblastic cells, quantitatively and qualitatively, and suggests the expression in adipose tissue and muscle of a multigene program that defines the insulin-stimulated glucose transport of relevance to organismal glucose homeostasis.