Background and context
Insulin is a type of secreted peptide hormone and is stored within vesicles that produce it. It is released when the body receives signals such as high sugar levels in the blood. Hormones bring about changes in target cells by binding to specific receptors. A cell may have many receptors for the same hormone and different receptors for different hormones. A cellular response depends upon the number of receptors that bind to a hormone and this varies from time to time.
If the hormone levels increase in the body, the number of receptors also increases making the cell more sensitive to the hormone, and the cellular activity increases. This is called up-regulation. If an increase in hormone level causes a decline in the number of receptors, making the cell less sensitive to the hormone, the cellular activity also decreases. This is called down-regulation.