Introduction to Endocrinology

Introduction to Endocrinology

• The multiple activities of cells, tissues, and organs in the body coordinate using chemical messenger systems

Chemical Messengers

• Neurotransmitters and neuromodulators release from neuron axon terminals into synaptic junctions.
• They act locally to control nerve cell functions and target organs.
• Cells can communicate via gap junctions or chemical messengers directly.

Types of Signaling

• Direct communication occurs through gap junctions between adjacent cells.
• Autocrine signaling involves a chemical messenger diffusing a short distance through extracellular fluid, binding to a receptor on the same or nearby cell.
• Paracrine signalling is when a chemical messenger diffuses a short distance through extracellular fluid, binding to a receptor on a nearby cell .
• Nervous signaling involves rapid transmission of action potentials and neurotransmitter release at a synapse.
• Endocrine signaling involves hormone release into the bloodstream and binding to specific target cell receptors.

Hormones

• Hormones facilitate cell communication.
• The glands or cells produce hormones.
• Hormones transport via blood.
• Hormones act on distant or local target tissue receptors.
• Hormones activate physiological responses in the target cells.

Hormone Action

• Some endocrine hormones affect many cell types, like growth hormone.
• Other hormones affect specific target tissues due to receptors, like ACTH stimulating the adrenal cortex.

Pheromones

• Pheromones facilitate communication between organisms.
• Chemicals act outside the secreting individual's body, affecting the receiver's behavior.
• Alarm, food trail, and sex pheromones affect behavior or physiology.
• In humans, axillary steroids are produced by the testes, ovaries, and adrenal glands, becoming biologically active after puberty.

Chemical Structures of Hormones

• Proteins and polypeptides are a class of hormones.
• Most hormones in the body are proteins and polypeptides.
• They range in size from small peptides(3 amino acids, e.g. thyrotropin-releasing hormone) to large proteins (200 amino acids, e.g., growth hormone and prolactin).
• Amine Hormones is a class of hormones.
• Steroid Hormones is a class of hormones.

Protein and Polypeptide Hormones

• Polypeptide and protein hormones store in secretory vesicles within the cell until needed.
• Peptide hormones are water-soluble for easy transport in the circulatory system to target tissues.
• Receptors are on the membrane of target cells.

Amine Hormones

• Amine hormones derive from the amino acid tyrosine.
• Thyroid hormones and catecholamines are amine hormones.
• They are stored until secreted.
• Receptors are on the surface or within cells.

Steroid Hormones

• Steroid hormones are usually synthesized from cholesterol and not stored.
• Much of the cholesterol in steroid-producing cells comes from plasma, with some synthesis occurring in the cells.
• Steroids lipid-soluble, they diffuse across the cell membrane into the interstitial fluid and then the blood after synthesis.

Steroid Hormone Action

Hormone Transport in Blood

• Water-soluble hormones (peptides and catecholamines) dissolve in plasma.
• They are transported from synthesis sites to target tissues, diffusing out of capillaries into interstitial fluid.
• Steroid and thyroid hormones primarily circulate bound to plasma proteins.
• Usually, less than 1% exist freely in solution.

Steroid Hormones Bound to Proteins

• The large amounts of steroid hormones bound to protein is a reservoir.
• This replenishes free hormones when target receptors bind or are lost from circulation.
• Plasma protein binding slows hormone clearance.

Clearance of Hormones

• Clearance of Hormones occurs through metabolic destruction by tissues.
• Clearance of Hormones occurs through binding with tissues.
• Clearance of Hormones occurs through excretion by the liver into the bile.
• Clearance of Hormones occurs through excretion by the kidneys into the urine.

Hormone Release

• Negative feedback prevents overactivity of hormone systems.
• Surges of hormones can occur with positive feedback.
• Cyclical variations occur in hormone release.
• Seasonal changes, development stages, aging, diurnal cycles, and sleep influence hormone release.

Action of Hormones

• Membrane receptors on the cell surface are specific to protein, peptide, and catecholamine hormones.
• Cytoplasm contains primary receptors for steroid hormones.
• Nucleus contains receptors for thyroid hormones.

Intracellular Hormone Receptors

• Adrenal/gonadal steroids, thyroid, retinoid hormones & Vit D bind to intracellular protein receptors.
• As lipid-soluble, these easily cross the cell membrane and interact with receptors in the cytoplasm or nucleus.

Protein, Polypeptide, Amine Hormone Receptors

• These hormones bind on the membranes of target cells at surface receptors.
• This results in hormone/receptor activation, opening or closing ion channels or enzyme activation/inhibition.
• Most of the hormones opening/closing ion channels couple with G protein-linked or enzyme-linked receptors.

G Proteins

• Some hormones couple to inhibitory G proteins (Gi), while others couple to stimulatory G proteins (Gs).
• The coupling of a hormone receptor to a G protein either increases or decreases the activity of intracellular enzymes, and opens/closes ion channels.

Guanylate Cyclase and cGMP

• ANP and NO use Guanylate Cyclase and cGMP