Hormones circulating in the bloodstream bind to receptors located either on the surface of or inside target cells. Therefore, it is the presence or absence of a receptor for a given hormone that determines which cells respond and which ones do not. For example, when the hormone oxytocin is released into the bloodstream of mammals, it affects only cells that express a receptor on the cell surface capable of binding oxytocin as it flows by in the bloodstream. These cells are uterine muscle cells and secretory cells in breast tissue. When oxytocin binds these cell-surface receptors, uterine muscle cells are stimulated to contract, and secretory cells in breast tissue release milk during breastfeeding. The cells of other organs do not express the receptor, so the hormone can exert its effect only on these specific tissues and no others.

The binding of a hormone to its receptor triggers changes in the target cell, resulting in a cellular response (Chapter 9). The specific action of a hormone depends on the kind of response that it triggers in the target cell. For example, the binding of a hormone can alter ion flow across the cell membrane, activate an intracellular signal transduction cascade that leads to changes in the biochemical activity of a target cell, or initiate more substantial changes in gene and protein expression.

Quick Check 2 What general features make a chemical compound a hormone, and how do hormones achieve specificity for certain kinds of target cell?