Hormones

Hormone comes from the Greek word hormao, which means “a substance which starts, urges on, initiates, irritates, stimulates or excites.” It was in 1656 that Thomas Wharton gave the first thorough account of the glands of the human body. It was only in 1905, however, that the word hormone, on the suggestion of Sir William Bate Hardy, was first used by Ernest Henry Starling, in one of his Croonian lectures, to name the internal secretions. This established the basis for the subsequent development of the discipline of endocrinology.

Virtually all multicellular organisms (plants and animals) produce hormones. Generally, they are described as chemical messengers that control metabolism, growth, development, and even reproduction. Through these chemical messengers, the cells are able to communicate and interact, maintaining a steady state which we call homeostasis. In this entry, we are going to concentrate on the hormones secreted by the human body, their functions and regulation.

Many aspects of the human body, including appetite, are controlled by a number of hormones.

In principle, hormones function gradually. More often than not, their effects are not immediate. They also do not act directly on behavior; rather, they change its probability and intensity. They are influenced by factors that are both endogenous (to do with development and maturation) and exogenous (to do with the environment). Hormones often have multiple target sites and are produced in pulsatile [Page 367]secretions; others follow a circadian rhythm regulated by other hormones or exogenous triggers. In action, they are target specific; they can only affect cells with receptor sites for a particular hormone. Last, the effects of hormones are interactive and long term, because they influence the buildup and breakdown of carbohydrates, lipids, and proteins, together with other metabolic changes.

There are three kinds of chemical signaling:

• Autocrine—the cells signal themselves through a chemical that synthesizes and then responds. Autocrine signaling can occur
  • -solely within the cytoplasm of the cell, or
  • -by a secreted chemical interaction with receptors of the same cell.
• Paracrine—chemical signals can diffuse into an area and interact with receptors on nearby cells. Examples are
  • -the release of cytokines which cause an inflammatory response in the area,
  • -the release of neurotransmitters at synapses in the nervous system.
• Endocrine—the chemicals can be secreted into the blood and carried by blood and tissue fluids to the cells upon which they act.

There are five known chemical classifications of hormones found in the human body. The first group is amine-derived hormones. Amine-derived hormones originate from the amino acids tyrosine (e.g., dopamine) and tryptophan (e.g., melatonin). The second is peptide hormones. Peptide hormones are made up of chains of amino acids (e.g., hormones of the stomach). The third is steroid hormones. Steroid hormones are derived from cholesterol (e.g., the androgens). The fourth is sterol hormones; they are derived from vitamin D (e.g., calciferol, calcitriol). The fifth group contains lipid and phospholipid hormones. Lipid hormones (e.g., prostaglandin) are derived from linoleic acid; phospholipid hormones are derived from arachidonic acid (e.g., leukotrienes).

In terms of cellular action, protein and amine hormones bind to the extracellular portion of a receptor site; it releases a second messenger within the cell; and its effects are relatively fast (seconds to minutes). In contrast, steroid hormones pass through cells; they bind to receptor genes within cells (receptor-gene complexes), which in turn bind to deoxyribonucleic acid (DNA). The action of the DNA may interact with other factors, which determine the cell's receptivity to the hormone.

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