Hormones Basics
By: Derek Charlebois

Hormones are communicating molecules produced by a variety of glands, organs, and other tissues throughout the body. They relay their messages by binding to receptors located on their target cells. The message is then transduced (transformed) into a biological effect.

Types of Hormones

Types of hormones include endocrine hormones, neurotransmitters, and
neurohormones.

Endocrine hormones

Endocrine hormones are produced by glands or other organs and secreted into the blood or interstitium. An example of an endocrine hormone is insulin released by the pancreas. Insulin can act on specific receptors throughout the body to stimulate nutrient uptake.

Insulin

Neurotransmitters

Neurotransmitters are produced by neurons and released into a synapse to activate receptors on adjacent neurons. An example of a neurotransmitter is acetylcholine (ACh). ACh is released from the axon terminal of a motor neuron to bind to ACh receptors on muscle fibers. The binding of ACh opens ion channels, which allows sodium ions (Na+) to depolarize the muscle fiber and cause contraction of the fiber.

Acetylcholine

When large amounts of neurotransmitters are produced, such as norepinepherine produced by sympathetic postganglionic cells, they can “spillover” into the blood and act on receptors throughout the body.

Neurohormones

Neurohormones are hormones made by nerve cells and released into the blood. An example of a neurohormone is corticotropin-releasing hormone (CRH) produced by the hypothalamus. CRH is released into the circulation during exercise or other forms of stress as a signal for the release of cortisol.

Autocrine and Paracrine Messengers

Autocrine and Paracrine messengers are two types biologically messengers that can be considered subcategories of the three other types.
An autocrine messenger is a biological messenger that acts on the same cell that released it. An example is insulinlike growth factor-1 (IGF-1), which is released by muscle cells in response to damage to act on the same muscle cell that released it.
A paracrine messenger is a biological messenger is released into the intersitium to act on different nearby cells. Neurotransmitters are an example of paracrine messengers.

Chemical Structure of Hormones

Hormones can be distinguished by their chemical structure as amines, peptides, proteins, or lipid derived hormones.

Amines

Amine hormones are formed from aromatic amino acids acquired from food. The catecholamines dopamine (DA), epinephrine (E), and norepinepherine (NE), which are synthesized from the amino acid tyrosine, are examples of amine hormones. Amine hormones are synthesized in the rough endoplasmic reticulum (RER).

General Structure of an Amine

Peptides and Proteins
Peptide and protein hormones are chains of amino acids. Peptide and protein hormones differ in the length of their amino acid chains with peptide being shorter. These hormones can range from three to over 400 amino acids. Growth hormone (GH) is an example of a protein hormone. Like amine hormones, peptide and protein hormones are synthesized in the RER.

Lipid Derivative Hormones

Hormones derived from lipids include steroids and eicosanoids. The lipid precursor cholesterol is used to synthesize steroid hormones, androgens and estrogens, in the adrenal cortex and gonads (testis in men and ovaries in women). Cholesterol is also used to synthesize vitamin D3.
Eicosanoid messengers are hormones derived from the polyunsaturated arachidonic fatty acid. Examples of eicosanoids are thromboxane and prostacyclin.

Receptors

Hormones transmit their messages by binding to specific receptors. Because hormones travel throughout the body, they reach multiple different receptors that display specificity for only certain hormones. If not for this specificity, any hormone could activate any receptor. This would cause chaos throughout the body. Hormone-receptor binding is governed by four principles: specificity, affinity, saturability, and reversibility.

Specificity

Specificity refers to the selectivity of a receptor to bind to only one type of hormone or molecule.

Affinity

Affinity refers to the strength a hormone binds with its receptor. Affinity is determined by the concentration at which the binding or a hormone to its receptor is half of its total capacity. A hormone that binds half maximally to its receptors at a lower concentration has a higher affinity than a hormone that needs a higher concentration to bind half maximally.

Saturability

Saturation of a receptor occurs when all receptors are bound to a molecule. When receptors become saturated, no more hormones can bind to the receptors even if more hormones are added.

Reversibility

The principle reversibility simple means a hormone can unbind from its receptor after originally binding to it.

To make application of these principles, we will consider E’s and NE’s binding characteristics. Both E and NE bind to adrenergic receptors, which are divided into alpha (further subdivided into α1 and α2 receptors) and beta (further subdivided into β1, β2, and β3 receptors) receptors. E has a greater affinity for β receptors then it does for α receptors. This means E binds more strongly to β receptors and needs a lower concentration to reach half-maximal binding. NE on the other hand has a greater affinity for α receptors than it does for β receptors. This means NE binds more strongly to α receptors and needs a lower concentration to reach half-maximal binding. Additional, E has a greater affinity than NE for β receptors and NE has a greater affinity than E for α receptors. These differences in affinity are important because each of the five adrenergic receptors have different functions. One should note that despite these differences in affinity, both E and NE still bind to both α and β adrenergic receptors.

Conclusion

This article is just an introduction into hormones. Obtaining an understanding of these basic hormone properties is important in understanding more specific information and then applying what you learn.