Review Question - QID 100308

Skeletal muscle cells and adipocytes are the primary cell types that express the GLUT-4 glucose transporter. GLUT-4 is the only glucose transporter that is responsive to circulating insulin levels. An increase in plasma glucose concentration results in increased expression of GLUT-4 and an ensuing increase in glucose transport into the cell.

Initially residing in cytoplasmic vesicles, the GLUT-4 receptor moves to the plasma membrane in response to an elevation in insulin concentration. This allows glucose to be transported into the cell down its concentration gradient. Therefore, in the complete absence of insulin, these cells have little to no glucose uptake. The four other types of glucose receptors (GLUT-1,2,3,&5) are distributed throughout various different cell types of the body and are not responsive to circulating insulin levels.

Incorrect Answers:
Answer 2: Decreased levels of insulin would lead to a downregulation of GLUT-4 transporters and a resulting decrease in glucose transport into muscle cells and adipocytes.
Answer 3: Increased plasma glucose levels would not result in a selective increase in glucose transport solely in muscle cells and adipocytes but would instead lead to an increase in glucose uptake across many cell types.
Answer 4: Decreased plasma glucose concentration would neither increase glucose uptake nor selectively affect glucose transport.
Answer 5: Skeletal muscle cells and adipocytes are unique compared to other cell types in that their expression of the GLUT-4 glucose transporter allows for insulin-responsive glucose uptake.