Endocrine Hormones Quiz

Endocrine System Overview

• Endocrinology is the study of hormones
• Hormones are chemical messengers secreted into the bloodstream by specialized cells
• Hormones control long-term, ongoing body functions, including growth, development, metabolism, internal environment regulation, and reproduction
• Hormones act on target cells in three ways: controlling enzymatic reactions, controlling ion or molecule transport across cell membranes, or controlling gene expression and protein synthesis
• A hormone is a chemical secreted by a cell or group of cells into the blood to transport it to a distant target, where it exerts its effect at very low concentrations

What Makes a Chemical a Hormone?

• Hormones are secreted by cells, not just classic endocrine glands
• Endocrine cells, neurons (neurohormones), and immune cells (cytokines) can secrete hormones
• Hormones are secreted into the bloodstream; however, the term ectohormone is given to signal molecules secreted into the external environment (ex. pheromones)
• Some chemical signals in the blood may not be considered hormones because they need to be present in high concentrations to have an effect
• Hormones act on target cells at very low concentrations (nanomolar to picomolar)

Hormone Action and Termination

• Hormones bind to target cell receptors to initiate biochemical responses
• Hormone effects can vary depending on the tissue and developmental stage, or a hormone may have no effect in a certain cell
• Hormones must be terminated for the body to respond appropriately to changes in the body
• Hormones are terminated by limiting secretion, removing/inactivating circulating hormones in the blood, or terminating hormone activity in target cells
• Hormones are degraded into inactive metabolites by enzymes located primarily in the liver and kidneys
• Hormones' half-lives indicate how long the hormone is active in the body

Classification of Hormones

• Hormones are classified into three main types: peptide/protein hormones, steroid hormones, and amino acid-derived (amine) hormones
• Peptide/protein hormones are chains of amino acids
• Steroid hormones are derived from cholesterol
• Amine hormones are modifications of tyrosine or tryptophan

Peptide/Protein Hormones

• Synthesis, storage, and release of peptide hormones into membrane-bound secretory vesicles is similar to other proteins
• The initial peptide is a large inactive protein called a preprohormone
• Preprohormones undergo post-translational modification in the Golgi apparatus to become active hormones and other fragments

Steroid Hormones

• Steroid hormones are derived from cholesterol and have a similar chemical structure
• Hormone production for steroid hormones is located in specific organs, unlike peptide hormones
• Steroid hormones are not very soluble in plasma and other body fluids
• Most steroid hormones bind to carrier proteins in the blood, extending their half-lives

Cellular Mechanism of Action of Steroid Hormones

• Steroid hormone receptors are found within the cell, in either the cytoplasm or nucleus
• Steroid receptors bind to hormones and act as transcription factors in the nucleus
• Steroid hormone effects generally take a longer time because they affect gene activity
• Steroid hormones do not mediate rapid reflex pathways

Additional Hormone Interactions

• Three main types of hormone interactions: synergism, permissiveness, antagonism
• Synergism: combined effect of multiple hormones exceeding the additive sum of individual effects
• Permissiveness: one hormone enabling another hormone's full effect
• Antagonism: two hormones having opposite effects

Homeostatic Control of Metabolism and the Role of Hormones

• The ratio of insulin and glucagon influences an individual's hour-to-hour metabolic state
• Insulin dominates during the fed state, promoting anabolism by storing glucose as glycogen or fat
• Glucagon dominates during the fasted state, promoting catabolism of glycogen and other storage sources of glucose
• Glucagon has more significant metabolic effects when blood glucose levels are low

Endocrine Response to Hypoglycemia/Plasma Glucose Hormones

• Glucagon, released when glucose is low, promotes glycogenolysis and gluconeogenesis to replenish plasma glucose
• The ratio of insulin to glucagon dictates the direction of metabolism; both play roles in maintaining blood glucose levels

Adrenal Glucocorticoids

• Adrenal glands sit atop kidneys, comprised of cortex and medulla
• Medulla secretes catecholamines (mostly norepinephrine) in response to stress
• Cortex secretes steroid hormones including aldosterone (mineralocorticoid), glucocorticoids, and sex hormones
• Cortisol is the major glucocorticoid, impacting glucose metabolism, immunology, and stress response; a prime example of a stress hormone

Thyroid Hormones

• Located in the neck, thyroid gland contains follicular and C cells
• Follicular cells secrete thyroid hormones, essential for growth and development (especially in children)
• Thyroid hormones are made from tyrosine and iodine
• Thyroid stimulating hormone (TSH) from the anterior pituitary, regulates thyroid hormone synthesis and secretion

Growth Hormone

• Growth in humans is continuous but not consistently so across different periods in life (ex. adolescence)
• Growth hormone (GH) promotes protein synthesis and bone and tissue growth
• Several factors influence growth hormone, including: hormones other than GH, diet, genetics, and lack of stress

Calcium Balance

• Most calcium is stored in bones, however, nonbone calcium, found in extracellular and intracellular fluids, is crucial to physiological functioning
• Several hormones regulate calcium levels within the body.

Hormones and Their Action Pathways

• Specific hormones have their own distinct pathways for signaling and eliciting a response; all hormones bind to specific receptors
• Hormones can affect many tissues and organs at once through complex interactions
• Different types of hormones signal through different pathways