In the context of diabetes, what happens when insulin is not present?

When insulin is not present in the body, one significant effect is that blood glucose levels can rise due to reduced activity of glucose transporters (GLUTs). Normally, insulin facilitates the uptake of glucose into cells by promoting the translocation of specific glucose transporter proteins to the cell membrane, particularly in muscle and adipose tissues. This process helps maintain normal blood glucose levels.

In the absence of insulin, this mechanism is disrupted. The GLUT4 transporter, which is primarily responsible for glucose uptake in insulin-sensitive tissues, is not effectively mobilized to the cell membrane. As a result, glucose cannot enter these cells as efficiently, leading to decreased cellular utilization of glucose. Consequently, while glucose production by the liver continues (as it is typically not directly inhibited by insulin), the overall effect is an increase in circulating blood glucose levels, which is a hallmark of uncontrolled diabetes.

This concept is critical for understanding the metabolic derangements that occur in insulin deficiency and their contribution to the clinical manifestations of diabetes. The increase in blood glucose levels due to reduced GLUT activity illustrates the essential role that insulin plays not only in facilitating glucose uptake but also in the overall regulation of glucose homeostasis.