Hormones

A hormone is a substance that is synthesized and secreted by specialized cells and carried via the circulation to target cells, where it elicits specific changes in the metabolic behavior of the cell by interacting with a hormone-specific receptor.

Chemically, the hormones in vertebrate metabolism include

> (1) peptides or polypeptides, such as insulin or glucagon

> (2) steroid, including glucocorticoids and the sex hormones

> (3) amino acid derivatives, including the catecholamines and thyroxine.

Hormonal mechanisms include

> (1) enzyme activation or inhibition via second messengers, as noted for epinephrine and glucagon

> (2) stimulation of the synthesis of particular proteins, through activation of specific genes

> (3) selective increases in the cellular uptake of certain metabolites.

Among this last category are some receptors that serve directly as ion channels, with hormone binding causing a conformational change that opens the channel, and other receptors that stimulate uptake by still-unknown mechanisms, such as the effects of insulin upon glucose uptake.

See also: Catecholamines, Hormone Action, Hormone Mechanisms of Action, Hormone Receptors, Hormone Hierarchy of Action, Peptide Hormone Synthesis, Hormonal Regulation of Fuel Metabolism, Steroid and Thyroid Hormones - Intracellular Receptors

INTERNET LINKS:

> 1. Steroid Hormone Metabolism

> 2. Androgen and Estrogen Metabolism