588

PART 3 Integration and Control Systems

Hormone

Receptor beingremoved fromplasma membrane

Internalizedand degradedreceptors

Time

(a)

receptors

Time

(b) Up-regulation

FIGURE 17.11 Down-Regulation and Up-Regulation of Target Cell Receptors

( a ) Down-regulation occurs when the number of hormone receptors in a target cell decreases. Often, the target cells internalize the receptors and destroythem. ( b ) Up-regulation occurs when the number of receptors for a hormone in a target cell increases. Often, the hormone stimulates the synthesisof receptors in the target cells.

TABLE 17.3

Hormone Receptor Types and Mechanisms of Action

*Hormones often exhibit more than one mechanism of action. For simplicity, hormones are listed in only one category.