HORMONES

1. Are all hormones transported by blood? How does endocrine transmission differ from exocrine transmission?

2. What is an amine hormone? What is a peptide hormone? What is a steroid hormone? Finally, what is the biochemical derivation of the prostaglandins?

3.

Hormones have been classically defined as chemical substances produced by specialized ductless glands that are released into the blood and carried to other parts of the body to produce specific regulatory effects. Because of this, many substances that seem to have hormone-like activity are considered to be hormones, but are done so with apprehension, because they do not conform to one or more of the criteria in this definition. For example, the prostaglandins are not produced in any one gland of the body, but are produced by most cells of the body. Furthermore, prostaglandins can be transmitted by diffusion in the interstitial fluid rather than by circulation in the blood.

Therefore, it seems best to consider the hormones as chemical regulators and to recognize that they can be produced by cells with a specific location in a particular gland or by cells diffusely located in many parts of the body.

Modes of Transmission

The concept of the restriction of hormone transmission to blood circulation only must be abandoned and recognition given to other means of transmission. These are classified as epicrine, neurocrine, paracrine, endocrine, and exocrine transmission.

Epicrine Transmission

In epicrine transmission, hormones pass through gap junctions of adjacent cells without entering extracellular fluid.

Neurocrine Transmission

In neurocrine transmission, hormones diffuse through synaptic clefts between neurons, as do neurotransmitters. Also, the hormone (such as oxytocin) can be synthesized in the neuron cell body, stored in axons (like neurotransmitters), but can be secreted into the blood.

Paracrine Transmission

In paracrine transmission, hormones diffuse through interstitial fluid, as do prostaglandins.

Endocrine Transmission

In endocrine transmission, hormones are transported through blood circulation. This is typical of most hormones.

Exocrine Transmission

In exocrine transmission, the regulatory agent (hormone) is secreted to the exterior of the body. The lumen of the intestine is considered to be exterior to the body, so hormones secreted into it can affect cell activity more distal to the point of secretion. Some hormones, such as somatostatin, can have exocrine transmission (secretion to intestinal lumen), and subsequently act as inhibitors of many gastrointestinal functions, including intestinal motility and intestinal absorption. Inasmuch as pheromones are chemical communicators, they might be considered to have exocrine transmission because they are received by other animals of the same species (through olfaction) after they have been excreted to the exterior of the body.

Biochemistry

Under the classic definition, hormones are biochemically categorized as amines, peptides, or steroids. The amine hormones include thyroid hormone and the adrenal catecholamines, epinephrine and norepinephrine. All of the amine hormones are derived from the amino acid tyrosine. The peptide hormones include peptides, polypeptides, and proteins. All of the hormones of the hypothalamus and pituitary, as well as insulin and glucagon from the pancreas, are included in the peptide class. The steroid hormones include the adrenocortical and reproductive gland hormones and the active metabolites of vitamin D. Cholesterol is the common precursor of steroid hormones. The prostaglandins (not classic hormones) are derived from arachidonic acid (a fatty acid). More structural detail is provided in this chapter when some hormones of specific endocrine glands or other tissues are discussed.

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