Hormone Interactions – Anatomy, Types, Functions

Types of Hormone

Different types of hormones have varying chemical structures. While some are peptides or proteins composed of amino acids, others are steroid? hormones derived from lipids. Some peptide hormones include a covalently attached oligosaccharide, in which case they are termed glycoproteins. The different chemical structures of varying types of hormones mean they all have receptors with a specific shape, size, and function.

Water-soluble hormones

Most water-soluble hormones such as glycoproteins and peptides combine with a receptor present on the plasma membrane because they are not lipid-soluble and cannot move through the phospholipid cell membrane. As this hormone binds to its receptor, a cascade of reactions is triggered within the cytoplasm of the cell.

The integral membrane receptors usually function through the activation of secondary messengers inside the plasma membrane which carry out intracellular signal transduction to convey the effects of the hormones that cannot enter the cells.

Lipid soluble receptors

Receptors for lipid-soluble hormones such as the steroid? hormones estrogen and thyroxine lie within the cytoplasm. These hormones need to enter the cell by crossing the cell membrane. Once bound, the hormone-receptor complex moves into the nucleus where it binds to specific DNA sequences and acts as a transcription factor, either increasing or suppressing the expression of certain genes.

Hormone Interactions Types and Effects

Interactions of Hormones at Target Cells

Hormones that act to return body conditions to within acceptable limits from opposite extremes are called antagonistic hormones.

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.

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.

Glossary

adenylyl cyclase

membrane-bound enzyme that converts ATP to cyclic AMP, creating cAMP, as a result of G-protein activation

cyclic adenosine monophosphate (cAMP)

second messenger that, in response to adenylyl cyclase activation, triggers a phosphorylation cascade

diacylglycerol (DAG)

molecule that, like cAMP, activates protein kinases, thereby initiating a phosphorylation cascade

downregulation

decrease in the number of hormone receptors, typically in response to chronically excessive levels of a hormone

first messenger

hormone that binds to a cell membrane hormone receptor and triggers activation of a second messenger system

G protein

protein associated with a cell membrane hormone receptor that initiates the next step in a second messenger system upon activation by hormone–receptor binding

hormone receptor

protein within a cell or on the cell membrane that binds a hormone, initiating the target cell response

inositol triphosphate (IP₃)

molecule that initiates the release of calcium ions from intracellular stores

phosphodiesterase (PDE)

cytosolic enzyme that deactivates and degrades cAMP

phosphorylation cascade

signaling event in which multiple protein kinases phosphorylate the next protein substrate by transferring a phosphate group from ATP to the protein

protein kinase

enzyme that initiates a phosphorylation cascade upon activation

second messenger

molecule that initiates a signaling cascade in response to hormone binding on a cell membrane receptor and activation of a G protein

upregulation

increase in the number of hormone receptors, typically in response to chronically reduced levels of a hormone

References