GLUT3 is primarily located in which tissues?

GLUT3 is a glucose transporter primarily found in high-affinity tissues that require a consistent supply of glucose for energy. This transporter is predominantly expressed in the brain, neurons, the placenta, and the testes. Each of these tissues relies on glucose as a critical energy source, particularly neurons, which have a high demand for glucose due to their metabolic activity and the need for neural function. The presence of GLUT3 facilitates the efficient uptake of glucose in these areas, even in low glucose concentrations.

In contrast, other transporters are responsible for glucose uptake in different tissues. Skeletal and cardiac muscle mainly express GLUT4, which is insulin-regulated and is important during states of increased metabolic activity. Hepatocytes and kidney cells primarily utilize GLUT2, which handles glucose in relation to blood glucose levels and is involved in gluconeogenesis. Pancreatic beta cells also use GLUT2 to sense glucose levels for insulin secretion. Thus, the specific function of GLUT3 in tissues where glucose is critically needed supports why the correct answer is C, highlighting its predominant presence in neurons, the placenta, and the testes.