15.4A: Interactions of Hormones at Target Cells
Hormones that act to return body conditions to within acceptable limits from opposite extremes are called antagonistic hormones.
- Differentiate among the interactions (permissiveness, antagonism, and synergy) of hormones at target cells
Key Points
- Permissiveness is the situation in which a hormone cannot exert its full effects without the presence of another hormone.
- Synergism occurs when two or more hormones produce the same effects in a target cell and their results are amplified.
- Antagonism occurs when a hormone opposes or reverses the effect of another hormone.
Key Terms
- antagonism : When a substance binds to the same site an agonist would bind to without causing activation of the receptor.
- synergism : Two or more things functioning together to produce a result not independently obtainable.
- permissiveness : A certain relationship between hormones and the target cell when the presence of one hormone, at a certain concentration, is required in order to allow a second hormone to fully affect the target cell.
Permissiveness
In biology, permissiveness is a certain relationship between hormones and the target cell. It can be used to describe situations in which the presence of one hormone, at a certain concentration, is required to allow a second hormone to fully affect the target cell.
For example, thyroid hormones increase the number of receptors available for epinephrine at the latter’s target cell, thereby increasing epinephrine’s effect at that cell. Without the thyroid hormones, epinephrine would have only a weak effect. Another example is cortisol, which exerts a permissive effect on growth hormones.
Antagonism
Maintaining homeostasis often requires conditions to be limited to a narrow range. When conditions exceed the upper limit of homeostasis, a specific action—usually the production of a hormone—is triggered. When conditions return to normal, hormone production is discontinued.
If conditions exceed the lower limits of homeostasis, a different action, usually the production of a second hormone, is triggered. Hormones that act to return body conditions to within acceptable limits from opposite extremes are called antagonistic hormones. The two glands most responsible for homeostasis are the thyroid and the parathyroid.
The regulation of blood glucose concentration (through negative feedback ) illustrates how the endocrine system maintains homeostasis by the action of antagonistic hormones. Bundles of cells in the pancreas, called the islets of Langerhans, contain two kinds of cells: alpha cells and beta cells. These cells control blood glucose concentration by producing the antagonistic hormones insulin and glucagon.
Beta cells secrete insulin. When the concentration of blood glucose rises, such as after eating, beta cells secrete insulin into the blood. Insulin stimulates the liver and most other body cells to absorb glucose.
Liver and muscle cells convert glucose to glycogen, for short-term storage, and adipose cells convert glucose to fat. In response, glucose concentration decreases in the blood, and insulin secretion discontinues through negative feedback from the declining levels of glucose.
Alpha cells secrete glucagon. When the concentration of blood glucose drops, such as during exercise, alpha cells secrete glucagon into the blood. Glucagon stimulates the liver to release glucose.
The glucose in the liver originates from the breakdown of glycogen. Glucagon also stimulates the production of ketone bodies from amino acids and fatty acids. Ketone bodies are an alternative energy source to glucose for some tissues. When blood glucose levels return to normal, glucagon secretion discontinues through negative feedback.
The glucagon receptor structure : Glucagon is a pancreatic peptide hormone that, as a counter-regulatory hormone for insulin, stimulates glucose release by the liver and maintains glucose homeostasis.
Synergy
Synergism occurs when two or more hormones combine to produce effects greater than the sum of their individual effects. For example, testosterone and follicle – stimulating hormones are required for normal sperm production.
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