Hormone Transport

Each hormone acts on target organs and may bind to a receptor on the target cell membrane, which sets off a second messenger system within the cytoplasm, or may diffuse into the cell and bind to a receptor.

Hormones are specific in the organs they target. While hormones travel throughout the body and can reach virtually every organ, each hormone has a particular target. Each of these targets is called a target cell because the cell has customized receptors on its surface that bind with a particular hormone. Once the hormone is bound, it alters the activity of the cell, either by increasing or decreasing the target activity. In terms of altering the activity, a hormone can do the following:

1. Change the permeability of the cell membrane, allowing for the passage of more or fewer ions.

2. Cause additional enzymes and proteins to be produced within the cell.

3. Induce the parameters needed for mitosis to occur.

4. Turn enzyme production on or off.

5. Cause the cell to begin secreting materials such as chemical messengers for other cells.

Hydrophilic hormones bind to receptors on the surface of the cell membrane, while lipophilic hormones bind to receptors within the cell itself.

Activation of Peptide Hormones

Peptide hormones bind with receptors on the target cell membrane. When this happens, enzymes initiate the release of secondary messengers, which activate other enzymes in the cell. This changes the shapes and functions of the proteins in the cytoplasm.
When hydrophilic hormones bind to the receptors on the surface of the cell, they initiate a secondary signaling mechanism. This means that the hormones cause another messenger to be secreted inside the cell in order to produce the desired response. Peptide hormones bind to membrane receptors. The receptor then activates a protein called a G-protein, an intermediate molecule that, when activated during the binding of a receptor, turns on a secondary messenger system. When activated, the G-protein moves along the membrane to change its integrated proteins in order to prepare for the binding of the secondary messenger. One such messenger is calcium. Adenylyl cyclase becomes activated and converts ATP to cAMP, which acts as the secondary messenger in the process, in turn activating protein kinases. A protein called calmodulin binds the calcium to the receptor, forming a complex that will activate other enzymes to begin the signaling process.

Once the hormone has attached, it causes the receptor to change shape. This new shape allows the receptor to bind with other proteins on the surface of the cell. These other proteins release specific enzymes that convert adenosine triphosphate (ATP), the energy molecule of the cell, into another enzyme, adenosine diphosphate (ADP). Finally, this new enzyme is again reshaped, and its effect on the target protein gets magnified. The number of product molecules increases with each step. In fact, a single hormone molecule attached to a receptor can cause the release of millions of final enzyme molecules.

Some hydrophilic hormones can work without a secondary messenger. Insulin has certain receptors on its surface that accept proteins called relay proteins, which activate the hormone and begin the cell's response.

Lipophilic hormones act as messengers to produce their signaling response. These hormones diffuse directly through the cell membrane of their target cells to the appropriate receptors. The bound hormone/receptor molecule navigates to the cell nucleus, where it locates the specific sequence of DNA needed to produce the response. It activates the specific gene that produces the response, which starts the processes of transcription and translation. Specific proteins are produced, including enzymes that increase reactions within the body.

Activation of Lipophilic Hormone Receptors

Lipophilic hormones do not need a secondary messenger in order to produce a desired response. Instead, they diffuse directly into the target cell and induce the process of protein synthesis. This creates new enzymes that influence the increase in the desired metabolic activity.