E4: Intro to ENDO

What is the definition of a hormone?

A chemical substance secreted into body fluids (especially blood) by one cell or group of cells that has a physiologic control effect on other cells via a specific receptor.

Why are hormones effective in such tiny concentrations?

Because of amplification — each step of the signaling cascade recruits more and more molecules, so one tiny hormone molecule triggers a massive cellular response.

What are the three biologic functions that hormones regulate?

They regulate reproduction, growth, and metabolism (including blood glucose level and other homeostatic processes).

What does 'receptor specificity' mean for hormones?

Each hormone works only through receptors specific to its class. For example, estradiol binds estrogen receptors, not insulin receptors. Insulin binds insulin receptors, etc.

How does Dr. Carney Anderson classify hormones (versus the textbook)?

She compresses the three textbook categories (steroids, peptides, amino acid derivatives) into two groups: lipid-soluble (steroids + thyroid hormone) and water-soluble (proteins/peptides + catecholamines), based on shared mechanisms.

What is the difference between an exocrine and an endocrine gland?

Exocrine glands secrete through a duct onto an epithelial surface (e.g., salivary glands, pancreatic acini). Endocrine glands secrete directly into the blood.

What is the chemical origin of steroid hormones?

All steroid hormones are cholesterol derivatives — enzymes modify cholesterol's side chains to produce hormones like cortisol, aldosterone, testosterone, estradiol, progesterone, and calcitriol.

Why can't steroid hormones be stored in secretory vesicles?

Because steroids are lipid-soluble — they would diffuse right through the vesicle membrane and not stay inside. Instead, synthesis and secretion happen simultaneously ('on demand').

How are steroid hormones transported in the blood, and why does this matter?

They bind to carrier proteins (specific ones like sex-binding globulin, or general ones like albumin). Only the free form is biologically active and can enter the liver for degradation — so carrier binding extends half-life.

What is the intracellular mechanism of action for steroid hormones?

Free steroid diffuses into the cell, binds an intracellular receptor (cytoplasm or nucleus), and the steroid-receptor complex acts as a transcription factor → mRNA → new protein → slow, long-lasting effect.

What is the chemical nature of protein/peptide hormones, and how are they made?

Large water-soluble molecules made of amino acid chains. Synthesized as pre-prohormones → prohormones → active hormone, then stored in secretory vesicles and released by exocytosis (e.g., insulin + C-peptide).

How do protein hormones reach their receptor, and where is the receptor located?

Protein hormones are water-soluble, so they dissolve freely in blood. Their receptors are on the plasma membrane — they cannot cross the lipid bilayer (unlike steroids).

Walk through the 2nd messenger cascade for a protein hormone (cAMP pathway).

Hormone (1st messenger) → binds membrane receptor → activates G-protein → adenylyl cyclase → ATP → cAMP (2nd messenger) → activates PKA → phosphorylates enzymes → fast, amplified cellular response.

Why are protein hormones considered 'fast-acting' compared to steroids?

They work through pre-existing enzymes via second messengers (non-genomic effect) — no need to wait for new protein synthesis. One hormone molecule triggers amplification of 1,000,000+ product molecules.

Compare the half-lives of steroid vs. protein hormones and explain why they differ.

Steroids: long half-life (estradiol ~13 hrs) — protected by carrier protein from liver degradation. Proteins: short half-life (GH 20–30 min, epinephrine 2 min) — freely circulating, cleaved by plasma enzymes or degraded by endocytosis.

What is the key take-home about steroid metabolic pathways?

Steroids share biosynthetic pathways (all from cholesterol). A deficit or excess of one steroid — or a missing enzyme — will shift flux and affect the levels of other steroids (law of mass action / Le Chatelier's principle).

What are the two ways to increase a tissue's response to a hormone?

(1) Secrete more hormone, or (2) change the number of receptors on target cells (up-regulation or down-regulation) — receptor affinity does not change, just the count.

What is up-regulation and what is its functional consequence?

Up-regulation = increased receptor number → increased tissue sensitivity. Example: T3 (thyroid hormone) up-regulates beta-adrenoreceptors in the heart, making it more sensitive to catecholamines → higher HR and contractility.

What is down-regulation and give a clinically relevant example.

Down-regulation = decreased receptor number → decreased tissue sensitivity. Example: chronic stress causes down-regulation of GH receptors → reduced sensitivity to growth hormone → shorter stature and less muscle mass in chronically stressed children.

Define permissive effects and give two examples from lecture.

Hormone A must be present for hormone B to exert its full effect — A enables B. Examples: Insulin is permissive for GH to cause growth. Cortisol is permissive for glucagon to raise blood glucose. Thyroid hormone is permissive for catecholamines (epinephrine) to cause lipolysis and raise HR — by up-regulating beta receptors.