The Endocrine System

• The endocrine system is one of the body’s two major control systems

• Functions:

  • Regulation (control) of major body processes of:

    • Reproduction

    • Growth/development

    • Water

    • Electrolyte and nutrient balance

    • Regulation of cellular metabolism/energy balance and mobilization of body defenses

  • Compare and contrast the endocrine & nervous systems, as used to regulate the activity of body cells

    • Nervous system: physically connected neurons that deliver electrochemical impulses very rapidly (milliseconds)

    • Endocrine system: acts over minutes, hours, and days, influencing metabolic cellular activity by using chemicals called hormones

      • Endocrine glands

        • Small masses of secreting epithelial cells, ductless, release hormones

        • Rich vascular supply/lymphatic drainage to distribute hormones throughout the body

        • Differs from exocrine glands [non-hormone secretions to a membrane surface or to the outside of the body (e.g., salivary glands)]

        • Exocrine = releases secretions onto a surface

        • Endocrine = releases secretions into the bloodstream

        • Just outside the cell is called the interstitial fluid

• Hormones

  • Chemically produced by one set of cells that travel through the body to change the metabolic activity (either increase or decrease) of a different set of cells (“target cells”)

  • Discoveries lead to exceptions to the “target cell” concept

    • Autocrine: secreted by a cell and affects that same cell’s metabolic activity

    • Paracine: secreted by a cell and affects nearby, but different, cell populations

• The chemical structure of a hormone determines how it acts

• Amino acid-based: derived from the amino acids themselves, a short chain of amino acids called “peptides” or a long chain of specifically arranged amino acids called “protein”

  • Water-soluble, generally faster acting (travels freely in fluid compartments)

  • Binds to a receptor on the cell's surface (exception is thyroxin [TH] → uses internal receptor)

• Steroid-based (lipids): assembled using a cholesterol component (lipid can penetrate cell membranes)

  • Not water-soluble (bound to protein when in fluid compartments)

  • Slower acting, harder to shut off

  • Binds to internal cell receptors, activating genes (specific regions of DNA… think recipes)

• Hormones act through second messengers (amino acid-based hormones) or by activating genes (lipid-based hormones)

• Target cells: while hormones will circulate throughout the body, only cells with the appropriate internal or external receptor for that specific hormone will be “targeted” for effect

  • Tissue response (a group of target cells) is dependent upon the tissue type (e.g., muscle cells would increase or decrease contraction, while epithelial cells may increase or decrease secretion of a product

  • List of common target cell responses to specific hormones

    • Opens/closes ion channels which may increase or decrease plasma membrane potential or permeability (nerve or muscle cells could act quickly or slowly, etc.)

    • Stimulates the synthesis or activity of regulatory molecules like enzymes within a cell

    • Stimulates mitosis, leading to the repair, replacement, and growth of tissues

• How many amino acid hormones work: plasma membrane receptors and second-messenger systems

  • Summary of action: hormones with quick cellular responses work this way:

    • Hormone (the first messenger) binds to the external receptor (stays outside)

    • Engaged external receptor activates internal G protein structures, which trigger secondary messengers, causing quick intracellular changes in metabolic activities

      • Advantage: quick on and off. Internal ingredients premade, ready to go if 1st messenger (hormone) lands

      • Disadvantage: repeated receptor stimulation is needed to prolong a response

      • Note: Ca⁺⁺ is often used inside of cells as a second messenger, triggering body cells to action. Blood levels of Ca⁺⁺ are low to prevent accidental diffusion into the cell

    • Some amino acid hormones work on the cell membrane alone (such as insulin, which enables a glucose molecule to enter the cell where it can be used for ATP production. This is the battleground for treating type 2 diabetes, as cells become less responsive to insulin, leaving blood glucose high.)

• How steroid hormones work: intracellular receptors and direct gene activation

  • Summary of receptor activation: being lipid soluble, steroid hormones diffuse through the plasma membrane and bind to internal receptors (so does the amino acid-based thyroxine hormone, which is an exception to the rule)

  • The receptor-hormone complex then enters the nucleus and activates “recipes” stored on the DNA molecules as “genes”

  • These synthesized products may turn on (enhance) or turn off (inhibit) cellular/tissue responses in the body, but gene activation is generally slower to start and stop than surface receptor hormone action (days, weeks rather than seconds or minutes)

• Three types of stimuli cause hormone release

  • The control of hormone release is regulated

    • Homeostasis is desired (the right amount of something your body needs at a given time)

  • Negative feedback is the most common endocrine control

    • Too little of something produces a response to increase synthesis or cell activity to make more

    • Too much of something produces a response to decrease synthesis or cell activity to make less

  • Rare positive feedback when climactic occurrence is desired (e.g., sexual response, childbirth)

• Cells respond to a hormone if they have a receptor for that hormone (called “target cells”)

  • Target cell specificity

    • What receptors a cell has determines whether or not a hormone will bind to & activate it

    • Some receptors are expressed on the membrane by most body cells (e.g., thyroxine), but receptors for other hormones are found in fewer, more specific body locations (e.g., adrenocorticotropic hormone receptors found only on certain cells within the adrenal cortex)

    • Target cell interaction with a hormone is still dependent on 3 things:

      • Circulating blood levels of a hormone (we often draw blood and check to see if the patient has too much or too little of a hormone available)

      • Number of receptors on or in target cells available for binding

        • Upregulation: a response by a cell to produce more receptors for a certain hormone, so its response increases due to more binding sites for the hormone

        • Downregulation: a response by a cell to decrease the amount of receptors for a certain hormone, leading to a decrease as fewer binding sites are available

      • Affinity: the strength of the binding between a hormone and the receptor (some artificial hormone compounds (medicines) do not bind as well with your cells as the real stuff your body makes).

• Half-life, onset, and duration of hormone activity

  • Hormones circulating in the blood are either:

    • Free form: quickly removed from circulation by the kidneys if not used

    • Bound to a carrier protein (now a hormone protein complex; larger size is harder for the kidney to take out), which increases the time available in the body before removal by the liver (may take weeks)

  • Half-life: the time it takes to remove half of the amount of something like a hormone from your blood (bound hormones, usually lipid-based, increase half-life) (e.g., urine vs. blood test)

  • Exact hormone levels are determined by blood tests, but urine is used to screen for “yes” or “no”

• Types of interaction of two or more hormones on the same target cell

  • Permissiveness: one hormone must be present to allow another hormone to work well.

    • Ex. mom’s TH gives “permission” to fetus’s GH to work properly

    • Clinical note: Check mom’s thyroxine level to prevent growth problems with the baby.

  • Synergism: more response from 2 hormones together than both separately

    • Ex. glucagon & epinephrine increase blood glucose more when released together

  • Antagonism: 2 hormones work to achieve opposite goals from one another

    • Ex. glucagon increases blood glucose levels, while insulin decreases blood glucose levels