Module 1: Hormones
In this module, we introduce hormones*. Working with the nervous system, hormones play an important role in coordination and control of various activities in the body.
This module contains only one section:
Section 1: Hormones
After reading this section you should be able to:
- Define the term hormones.
- Differentiate between local and general hormones
- Compare the chemical make-up and half-life of the three classes of general hormones.
- List two internal feedback and three external controls which regulate hormones.
- Describe the biosynthesis, transportation and metabolism of hormones.
- List three mechanisms of action of hormones.
Definition of Hormones
Hormones are chemical messengers of the endocrine system that are produced at one site and are transported by the circulation to target tissues.
Hormones regulate activities of the target tissues. The target tissues may have receptors to which the hormones attach. Hormones may act on only certain cell types, specific organs, or on the entire body.
Hormones help to control many physiological processes including development and maturation, reproduction, energy supply to body tissues, and the control of fluid and electrolyte balance.
Local versus General Hormones
Hormones are grouped as local or general hormones. Local hormones act on a nearby specific target tissue, while general hormones are transported by the blood and other routes to distant target tissues. Some general hormones, such as growth hormone* and thyroid hormones*, affect all tissues in the body. Other general hormones act on specific target tissues. General hormones are secreted by the endocrine glands*. See Table 1.1-1
Classes of General Hormones
Chemically, the general hormones are divided into three classes:
- steroid hormones*
- peptide hormones*
- amine hormones*
1) Steroid hormones
Steroid hormones have a characteristic steroid ring in their basic chemical structure. The side chains attached to the steroid ring determine the specific biological action. Steroid hormones have longer half-lives than peptide or amine hormones and can exist in the bloodstream for up to 120 minutes.
2) Peptide hormones
Peptide hormones are made up of amino acid* chains. These chains can have as few as three or up to hundreds of amino acids. The half-life of a peptide hormone ranges from 5 to 30 minutes.
3) Amine hormones
Amine hormones are all derivatives of the amino acid tyrosine. The half-life of an amine hormone is usually less than 5 minutes.
Regulation of Hormones
1) Internal feedback control
The concentration of a hormone in the body is controlled and regulated through the gland responsible for its production. The hormone level rises or falls in response to suppression or stimulation of the gland. For example, a rise in the circulating concentration of a hormone causes the gland to reduce secretion of the hormone, allowing the circulating concentration to fall to normal. On the other hand, if the hormone concentration falls below normal, secretion of the hormone is increased to raise the concentration to normal. This self-regulating activity is called negative feedback*. See Figure 1.1-1.
Negative feedback control can be linked between hormones in a process called signalling*. Consider that two glands, A and B, are inter-related. See Figure 1.1-2. Gland A secretes Hormone A which inhibits the ability of Gland B to release its product, Hormone B. Hormone B acts to stimulate Gland A to secrete Hormone A. As the concentration of Hormone A rises, the release of Hormone B is inhibited. Because Hormone B is required for release of Hormone A, as the concentration of Hormone B drops, the secretion of Hormone A is inhibited, resulting in return of Hormone A levels towards normal.
2) External controls
In addition to the internal feedback control of hormone secretion, many of the body’s endocrine glands are affected by external controls.
Some endocrine glands may be stimulated by sympathetic* or parasympathetic* neurons, to secrete their hormones. Also, the hormones of the hypothalamus* and its extension, the posterior pituitary*, are under the direct control of neurons in the brain itself.
The secretion of some hormones is controlled by the plasma concentrations of specific minerals or organic nutrients. Through a negative feedback mechanism, the hormone regulates the plasma concentration of the mineral or nutrient controlling its secretion. For example, low calcium levels stimulate release of parathyroid hormone (PTH)* which acts to increase calcium levels. As calcium levels increase, the secretion of PTH decreases.
Hormones can also be affected by circadian rhythm*. For example, the concentration of cortisol*, released from the cortex of the adrenal gland*, varies depending on the time of day. The female menstrual cycle in which hormone concentrations have approximately a 28 to 30 day cycle of fluctuation is another example.
Biosynthesis of hormones generally involves conversion of precursors to more active forms. The precursors of hormones are called prohormones and the precursors of prohormones are called pre-prohormones.
Hormones are generally stored in the gland and released from a secretory cell into the extracellular fluid (usually blood).
Peptide hormones are water-soluble and most are carried in the plasma simply dissolved in the plasma water.
In contrast, thyroid hormones and most steroid hormones circulate in the bloodstream partly bound to serum proteins. These proteins act as a reservoir for the hormone, since only the unbound (or “free”) hormone is available to act on the target tissues. As the free hormone is used up at the target site, bound hormone separates from the carrier proteins to act on the target tissues.
The liver and kidneys are the most important organs that metabolize hormones. Hormones are sometimes metabolized by the cells upon which they act. The peptide hormones and catecholamine* hormones (such as epinephrine* and norepinephrine*) are easily attacked by enzymes in the blood or tissues, and therefore tend to remain in the bloodstream for only brief periods. Protein-bound hormones are less vulnerable to metabolism by enzymes, and thus the elimination of these hormones can take many hours or even several days.
Metabolism of a hormone can sometimes activate rather than deactivate, the hormone. For example, circulating thyroxine* is converted to the more active triiodothyronine*.
Mechanisms of Action
Hormones act at the cellular level in very minute concentrations.
Hormones combine with specific receptor sites either on the cell membrane or inside the nucleus of the target cells.
Receptors have a high specificity and affinity for their hormone. They generally bind to only one type of hormone and are reluctant to release it once bound.
There are three common mechanisms of hormone action. Hormones may:
- control the passage of chemicals across membranes
- affect the activity of enzymes within the cell
- alter gene function in the cell nucleus
Hormones can influence the ability of target cells to respond to them by regulating hormone receptors. An increase in the number of a hormone’s receptors on a target cell resulting from prolonged exposure to a low concentration of the hormone is known as up-regulation. A decrease in the number of receptors resulting from prolonged exposure to high concentrations is known as down-regulation.
1) Membrane transport
Hormones can control the movement of chemicals into and out of cells. They can influence both passive transport* and active transport*. This action is usually rapid, occurring in seconds to minutes. An example of this action is the ability of insulin* to increase the rate of glucose* entry into cells. Insulin, glucagon*, growth hormone, and thyroid hormones can all accelerate amino acid transport into cells.
2) Enzyme activation
Some hormones can alter the activity of pre-existing enzymes to affect metabolic processes inside the cell. These effects occur in minutes to hours.
Hormones may act directly on the enzyme or, alternatively, hormones may act through a second messenger, such as cyclic AMP (cAMP)*.
3) Alteration of gene function
Steroid and thyroid hormones cross through the cell membrane and bind to receptors inside the cell nucleus. They act in the nucleus of the cell to regulate protein synthesis. Inside the nucleus, the hormone-receptor complex combines with the genetic material (chromatin*) of the cell. This signals DNA to synthesize new messenger RNA (mRNA)*. Messenger RNA causes the ribosomes* of the cell to synthesize proteins and produce the desired metabolic and physiologic effects. See Figure 1.1-3.
Hormone activity involving the cell’s genetic material is much slower than the other two mechanisms. It may take one to two days to reach maximum effect.
Summary — Section 1: Hormones
Cells in the body primarily communicate through the nervous and endocrine systems. Hormones are the chemical messengers of the endocrine system.
Endocrine glands secrete hormones which control certain metabolic functions of the body, such as rates of chemical reactions, cellular growth, and development.
Local hormones act on nearby specific target tissues. General hormones secreted by endocrine glands are transported to distant target tissues.
The three major types of hormones are:
- steroid hormones
- peptide hormones
- amine hormones
The concentration of hormones in the body is regulated by:
- negative feedback mechanisms
- signalling between hormones
- sympathetic or parasympathetic neurons
- plasma concentrations of minerals or organic nutrients
- circadian rhythms
Hormones are stored in endocrine glands and then secreted into the bloodstream as needed. Peptide hormones circulate freely in the bloodstream, while thyroid and most steroid hormones are partly bound to serum proteins.
Hormones bind to specific receptor sites either on the cell membrane or inside the cell nucleus.
Hormones exert their effects by:
- controlling the passage of chemicals across the membrane
- affecting the activity of enzymes within the cell
- altering gene function in the cell nucleus
Progress Check — Section 1: Hormones
Define the term hormone
Hormones which act on a nearby specific target tissue are called ______________________ hormones. Hormones which are transported by the blood to distant target tissues are called ______________________ hormones.
Compare the three classes of general hormones by filling in the following table.
List two internal feedback controls which regulate hormone secretion.
List three external controls which help to regulate hormone secretion.
Indicate whether each of the following statements is True or False.
The precursors of hormones are called prohormones. True False
Most peptide hormones are partly bound to serum proteins. True False
Metabolism of a hormone always results in a less active product. True False
List the three mechanisms of hormone action.
Progress Check Answers — Section 1: Hormones
Hormones are chemical messengers of the endocrine system. They regulate activities of the target tissues and help to control many physiological processes.
1) negative feedback mechanisms
2) signalling between hormones
2) plasma concentrations of minerals or organic nutrients
3) circadian rhythm
Most peptide hormones are carried in the plasma simply dissolved in the plasma water.
Metabolism of a hormone can sometimes activate rather than deactivate the hormone.
Three mechanisms of hormone action:
1) control the passage of chemicals across membranes
2) affect the activity of enzymes within the cell
3) affect gene function in the cell nucleus
Module 1 Test: Hormones
Hormones are chemical messengers of the:
a) endocrine system
b) exocrine system
c) nervous system
d) endocrine and nervous system
Which of the following types of hormones has the shortest half-life?
a) steroid hormones
b) peptide hormones
c) amine hormones
d) reproductive hormones
Most hormones are transported in the body bound : to
a) blood cells
b) serum proteins
Negative feedback control that is linked between hormones is known as:
a) circadian rhythm
b) direct neuronal control
Which of the following is an amine hormone?
Most hormones are metabolized in the:
a) liver and kidneys
b) cells upon which they act
d) gastrointestinal tract
By causing the production of cAMP, some hormones are able to:
a) alter protein synthesis in the nucleus
b) control active transport of chemicals into cells
c) alter metabolic processes inside the cell
d) increase the rate of glucose entry into cells
Module 1 Test Answers: Hormones
a) endocrine system
c) amine hormones
b) serum proteins
a) liver and kidneys
c) alter metabolic processes inside the cell