Down-regulation is an adaptation of the cell to a situation of overstimulation and subsequent too large effect in the cell.The upper part of the graphic shows 3 consecutive steps of down-regulation. This process occurs for example when an agonist is added to a normal situation (1). The cell is overly stimulated by the agonist for a prolonged period of time (2). By decreasing the expression of the receptor the cell protects itself against continuous stimulation
The lower part shows the different mechanisms by which the down-regulation can be performed. Signal transduction from activated receptors and signals from a high rate of internalisation result in decreased gene expression and corresponding receptor synthesis. Also more receptors are degraded and less receptors are recycled back to the membrane.
An example: receptor down-regulation occurs in diabetes type II patients using insulin.