Hormones are compounds secreted by specific tissues, called endocrine glands, directly into the bloodstream, rather than being excreted through ducts or stored in bladders. The response to a hormonal signal comes as a direct and rapid result of its secretion. Figure 23.6 shows the locations of the major endocrine organs in the human body.

Hormones usually stimulate metabolic activities in tissues remote from the secretory organ. They are active at exceedingly low concentrations, too. Furthermore, most hormones are metabolized rapidly, so their effects are often short-lived, allowing rapid adaptations to metabolic changes.

A special class of hormones includes the eicosanoids-prostaglandins, thromboxanes, and leukotrienes (see here). These mediators act like hormones but are distinctive in their extreme metabolic lability, their synthesis in many cell types instead of just one endocrine gland, and their actions primarily on cells close to those that secreted them.

Hormones differ from other intercellular mediators, such as the following:

1. Pheromones, which are transmitted between cells of different individuals;

2. Neurotransmitters, which act immediately across a synaptic junction from their sites of release (see here); and

3. Growth factors, whose growth-stimulating activities are continuous, rather than being short-lived in response to a burst of secretion.

Distinctions among these classes of regulators are somewhat indefinite. Catecholamines, such as epinephrine and norepinephrine, function both as neurotransmitters and as hormones, depending upon their sites of synthesis and release.

The effect of epinephrine in stimulating glycogen mobilization illustrates a typical mechanism of hormone action. Figures 12.13 and 13.18 show that epinephrine binds to a macromolecular receptor at the cell surface and stimulates the formation of cyclic AMP, which acts as a second messenger inside the cell and in turn stimulates the phosphorylation of target enzymes. The hormone itself is the first messenger. All hormones so far investigated act through binding to specific receptors, whether the receptors are located inside the target cell or on the cell surface. The presence of specific receptors on the specific cell types determines how hormones, secreted into the bloodstream, affect only certain tissues. Second messengers are often used to transmit the message to the target metabolic pathway, though not all hormone actions involve a second messenger.

Chemically, the hormones in vertebrate metabolism include the following:

1. Peptides or polypeptides, such as insulin or glucagon;

2. Steroids, including glucocortiocoids and the sex hormones (androgens and estrogens); and

3. Amino acid derivatives, including the catecholamines and thyroxine.