pt 1: Endocrine Basics + Cell Signaling + Hormone Mechanisms

Endocrine system definition

The body's second major control system using hormones to regulate functions

Functions of the endocrine system

Regulates development, growth, metabolism, blood volume/composition, digestion, and reproduction

Chemical messenger (ligand)

A molecule released by a cell that binds a receptor on another cell to cause a change

Autocrine signals

The cell releases a chemical that acts on itself; not considered hormones

Paracrine signals

Local chemical messengers that act on nearby cells; not transported in blood

Endocrine signals

Hormones released into the bloodstream to act on distant target cells

Hormone definition

Chemical substance that regulates the activity of specific target cells

Target cell

A cell that has receptors for a specific hormone and responds to it

Signal transduction

The process of converting a hormone signal into a cellular response

Second messenger system

When a hormone binds a membrane receptor and triggers internal messengers like cAMP

Direct gene activation

Lipid-soluble hormones enter cell, bind receptors, and alter gene transcription

G protein-coupled receptors (GPCRs)

Cell membrane receptors that activate G proteins when a hormone binds

Amino-acid based hormones

Water-soluble hormones that bind surface receptors and use second messengers

Steroid hormones

Lipid-soluble hormones that pass through membranes and bind intracellular receptors

Steps in cAMP pathway

Hormone binds receptor → G protein activates → adenylate cyclase → cAMP → protein kinases activated

PIP2-IP3-DAG pathway

Hormone activates G protein → phospholipase C → PIP2 splits into IP3 & DAG → Ca2+ release → PKC activation

Role of Ca2+ in signaling

Acts as a third messenger to trigger cellular responses

Role of GTP in G-proteins

GTP activates G-protein; GDP is inactive

Steroid hormone mechanism

Steroid enters cell → binds receptor → goes to nucleus → changes gene transcription

Factors affecting target cell activation

Hormone concentration, receptor number, and receptor affinity

Hormone blood concentration influenced by

Rate of release and rate of removal

Hormone removal methods

Enzymatic breakdown, liver metabolism, kidney excretion

Affinity

defines how well a hormone binds its receptor

Avidity

how long a hormone stays bound to its receptor

Permissive effect

One hormone needs another to work (ex: thyroid hormone + reproductive hormones)

Synergistic effect

Two hormones produce amplified effect when combined

Antagonistic effect

One hormone opposes the action of another

Half-life of hormones

Time needed for half the hormone to be removed from blood

Pharmacokinetics

What the body does to the hormone (absorb, distribute, metabolize, excrete)

Pharmacodynamics

What the hormone does to the body (effects, actions)

Three major hormone types

Amino-acid based, steroids, eicosanoids

Eicosanoids

Locally active lipid messengers (prostaglandins & leukotrienes)

Actions of hormones

Change membrane permeability, activate enzymes, stimulate protein synthesis, cause secretion, stimulate mitosis

Humoral stimulus

Changing blood levels of ions/nutrients cause hormone release

Neural stimulus

Nerves stimulate hormone release (ex: adrenal medulla)

Hormonal stimulus

One hormone causes release of another (ex: TRH → TSH)

Negative feedback loop

Hormone levels regulate themselves by shutting off further release

Positive feedback loop

Hormone stimulates more of itself (ex: oxytocin during labor)

Bound vs unbound hormones

Bound hormones attached to proteins; only unbound hormones are active

Up-regulation

Cells increase receptor number → more sensitive

Down-regulation

Cells reduce receptor number → less sensitive

Glycogenolysis

Breakdown of glycogen into glucose

Gluconeogenesis

Creation of glucose from fats/proteins