Introduction to Endocrinology

Fundamental Concepts of Intercellular Communication

Definitions of Signaling Molecules:
- Hormone: An extracellular signaling molecule that is released into the blood and acts at its receptors in distal (distant) tissues in order to elicit a physiological response.
- Neurotransmitter: A chemical messenger released by a neuron into a synaptic cleft to act on a local target cell over a very short distance.
- Neurohormone: A hormone produced by a neuron (secreting cell) that is released into the blood to act on distant target cells (e.g., Vasopressin and Oxytocin).
- Paracrine: A chemical messenger secreted by a cell that acts on local neighboring target cells without entering the bloodstream.
Methods of Cell-to-Cell Communication:
- Direct Intercellular Communication:
  - Gap Junctions: Allow small molecules and ions to pass directly between adjacent cells.
  - Transient Direct Linkup: Specific surface markers on the membranes of two cells connect to facilitate communication.
- Indirect Intercellular Communication:
  - Mediated via extracellular chemical messengers (paracrine, neurotransmitter, hormone, or neurohormone).

Comparison of the Nervous vs. Endocrine Systems

Systemic Integration: Together, the nervous and endocrine systems provide the means to regulate systemic physiology and maintain homeostasis.
Anatomic Arrangement:
- Nervous System: A "wired" system with specific structural arrangements between neurons and target cells, maintaining structural continuity.
- Endocrine System: A "wireless" system where endocrine glands are widely dispersed and not structurally related to each other or their targets.
Chemical Messengers:
- Nervous System: Neurotransmitters released into the synaptic cleft.
- Endocrine System: Hormones released into the blood.
Distance of Action:
- Nervous System: Very short distance (diffusion across the synaptic cleft).
- Endocrine System: Long distance (carried by the bloodstream).
Major Functions:
- Nervous System: Coordinates rapid, precise responses.
- Endocrine System: Controls activities that require long duration rather than speed.

Major Physiological Roles of the Endocrine System

Digestion and Absorption: Controls and integrates the processing of food (often working with the autonomic nervous system).
Metabolism: Regulates fuel metabolism and storage.
Homeostasis: Regulates $H_2O$ and electrolyte balance.
Adaptation: Induces changes to help the body cope with stressful situations.
Development: Promotes smooth, sequential growth and development.
Erythropoiesis: Regulates red blood cell production.
Reproduction: Controls all aspects of reproductive function.

Identification of Endocrine Glands and Their Hormones

Dedicated Endocrine Glands (Non-Pregnant Adults)

Endocrine Pancreas: Secretes Insulin, Glucagon, and Somatostatin.
Pituitary Gland:
- Anterior Pituitary: Adrenocorticotropic hormone (ACTH), Thyroid-stimulating hormone (TSH), Growth hormone (GH), Prolactin, Follicle-stimulating hormone (FSH), and Luteinizing hormone (LH).
- Posterior Pituitary (Secreted here, made in Hypothalamus): Vasopressin and Oxytocin.
Adrenal Gland: Secretes Epinephrine, Norepinephrine, Cortisol, Aldosterone, and Dehydroepiandosterone sulfate (DHEAS).
Thyroid Gland: Secretes Tetraiodothyronine ( $T_4$ or Thyroxine), Triiodothyronine ( $T_3$ ), and Calcitonin.
Parathyroid Gland: Secretes Parathyroid hormone (PTH).
Ovaries: Secrete $17 \beta$ Estradiol, Progesterone, and Inhibin.
Testes: Secrete Testosterone, Antimullerian hormone (AMH), and Inhibin.

Organs with Secondary Endocrine Functions

White Adipose Tissue: Secretes Leptin (also Adiponectin and Resistin).
Stomach: Secretes Gastrin and Somatostatin (also Ghrelin).
Intestines: Secretes Cholecystokinin (CCK), Secretin, GLP-1, and GIP.
Kidney: Secretes Erythropoietin.
Liver: Secretes Insulin-like growth factor-1 (IGF-1).
Brain (Hypothalamus): Secretes Corticotropin-releasing hormone (CRH), Thyrotropin-releasing hormone (TRH), Gonadotropin-releasing hormone (GnRH), and Growth hormone-releasing hormone (GHRH).
Brain (Pineal Gland): Secretes Melatonin.

Solubility Properties: Hydrophilic vs. Lipophilic Hormones

1. Hydrophilic Hormones (Water-Loving)

Properties: High water solubility, low lipid solubility.
Sub-Groups:
- Amino Acid Derivatives (Amines):
  - Tyrosine derivatives: Dopamine, Norepinephrine, Epinephrine.
  - Tryptophan derivatives: Melatonin.
- Peptide Hormones: Most hormones fit this category. They are chains ranging from $3 \text{ aa}$ to $200 \text{ aa}$ .
  - Examples: Pancreatic hormones (Insulin), Digestive tract hormones (Secretin, Gastrin), Hypothalamic releasing/inhibiting hormones (except Dopamine), Pituitary hormones, Angiotensin II, IGF-1, Erythropoietin, Atrial Natriuretic Peptide, and Calcitonin.

2. Lipophilic Hormones (Lipid-Loving)

Properties: High lipid solubility, low water solubility.
Sub-Groups:
- Thyroid Hormones: Derived from two tyrosines plus iodine. Includes Thyroxine ( $T_4$ ) and Triiodothyronine ( $T_3$ ). Although they have some water solubility, they function like steroids (highly lipid soluble).
- Steroid Hormones: All are derived from Cholesterol.
  - Estrogens: Major form is Estradiol (activates estrogen receptor).
  - Androgens: e.g., Testosterone (activates androgen receptor).
  - Progestins: e.g., Progesterone (activates progesterone receptor).
  - Glucocorticoids: e.g., Cortisol (activates glucocorticoid receptor).
  - Mineralocorticoids: e.g., Aldosterone (activates mineralocorticoid receptor).
  - Vitamin D: $1,25 \text{ dihydroxy Vitamin } D_3$ (activates Vitamin D receptor).

Hormone Synthesis and Storage

Amine Hormone Synthesis (Tyrosine Pathway)

L-Tyrosine is converted via Tyrosine hydroxylase to L-Dihydroxyphenylalanine (L-DOPA). This requires Tetrahydrobiopterin.
L-DOPA is converted via DOPA decarboxylase (Aromatic L-amino acid decarboxylase) to Dopamine, releasing $CO_2$ .
Dopamine is converted via Dopamine } \beta \text{-hydroxylase} to Norepinephrine. This requires Ascorbic acid.
Norepinephrine is converted via Phenylethanolamine N-methyltransferase (PNMT) to Epinephrine. This requires S-adenosyl-methionine.
Storage: Amines are stored in secretory vesicles until a stimulus triggers secretion.

Peptide Hormone Synthesis

Nucleus: DNA is transcribed into mRNA.
Endoplasmic Reticulum: mRNA is translated into a Preprohormone.
Processing: The Preprohormone is converted into a Prohormone.
Golgi Apparatus: The Prohormone is packaged and processed into the active Hormone.
Storage: Stored in secretory vesicles in the cytoplasm.
Secretion: Released via exocytosis into the blood capillary lumen upon stimulation.

Steroidogenesis

All steroids are produced from Cholesterol.
The specific steroid produced depends on the expression and activity of "steroidogenic enzymes" within the specific gland (e.g., Gonads vs. Adrenals).
Storage: Unlike hydrophilic hormones, lipophilic hormones cannot be stored; they diffuse out of the cell as soon as they are synthesized.

Hormone Transport, Metabolism, and Excretion

Transport in Blood:
- Hydrophilic hormones: Travel dissolved in the plasma.
- Lipophilic hormones: Travel (loosely) bound to carrier proteins. Only the unbound ("free") form can enter cells to activate receptors.
Metabolic Inactivation/Excretion:
- Peptide Hormones: Cleaved and inactivated by circulating general proteases.
- Lipophilic Hormones: Modified by liver enzymes to become more water-soluble, preventing cell entry and facilitating excretion in urine or feces.
Hormone Activation (Increasing Activity):
- Peptide Hormones: Cleaved by specific proteolytic enzymes. Example: Liver-derived Angiotensinogen is cleaved by kidney-derived Renin into Angiotensin I, which is then cleaved by lung-derived Angiotensin Converting Enzyme (ACE) into the active Angiotensin II.
- Lipophilic Hormones: Enzymes in target cells modify hormones into more active versions:
  - Thyroxine ( $T_4$ ) is converted to Triiodothyronine ( $T_3$ ) by Deiodinase.
  - Testosterone is converted to Dihydrotestosterone (DHT) by 5̑̑̒\alpha reductase.

Mode of Action at Target Cells

1. Membrane Receptors (Hydrophilic Hormones)

Hormones bind to receptors on the exterior of the cell membrane.
Mechanism: Activate receptor-enzyme complexes or recruit second-messenger systems (e.g., cAMP).
cAMP Pathway Steps:
1. Hormone binds to a G-protein coupled receptor (GPCR).
2. GPCR activates a G-protein; its $\alpha$ subunit activates Adenylyl cyclase.
3. Adenylyl cyclase converts $ATP$ to $cAMP$ (the second messenger).
4. $cAMP$ activates Protein Kinase A (PKA).
5. PKA phosphorylates designated proteins, altering their shape and function to produce the cellular response.
6. Targets of PKA: Ion channels, transporters, enzymes, and transcription factors.

2. Nuclear Receptors (Lipophilic Hormones)

Hormones cross the membrane and bind to intracellular receptors in the nucleus.
Mechanism: Act as transcription factors to induce gene transcription.
Examples:
- Estrogen: Increases transcription of genes regulating cell count in the uterus and mammary glands.
- Cortisol: Increases transcription of genes for gluconeogenesis in the liver.

Speed and Duration

Membrane Receptors: Generally faster acting.
Nuclear Receptors: Generally slower to initiate (gene transcription takes time) but produce longer-lasting effects.

Regulation of Endocrine Function

Negative Feedback Control:
- Direct Sensing: The gland senses changes in a variable (e.g., blood glucose) and adjusts secretion.
- Endocrine Axis Sensing: One hormone in a chain (e.g., Hypothalamic-Pituitary axis) provides feedback to stop the production of the preceding hormone.
Neuroendocrine Reflexes: Produce sudden increases in secretion in response to stimuli (e.g., Epinephrine release from medulla due to sympathetic output; Vasopressin release due to low blood pressure).
Circadian Rhythms: Repetitive 24-hour oscillations responsive to light/dark. This is a form of feed-forward control (e.g., Cortisol levels rising before waking to increase blood glucose).
Receptor Activity: Regulation of the number, availability, or responsiveness of receptors on target cells.
Interactions with Other Hormones:
- Permissiveness: One hormone must be present in a specific amount to allow another hormone to exert its full effect.
- Synergy: The combined action of multiple hormones is greater than the sum of their individual effects.
- Antagonism: One hormone causes the loss or downregulation of another hormone's receptors.

Endocrine Disorders

Hyposecretion (Too Little activity):
- Insufficient secretion by the gland (most common cause).
- Increased removal from blood.
- Abnormal tissue responsiveness (lack of receptors or essential enzymes).
Hypersecretion (Too Much activity):
- Excessive secretion by the gland (most common cause).
- Reduced plasma protein binding (too much "free" hormone).
- Decreased removal, inactivation, or excretion from the blood.

Questions and Discussion

Thought Question: Which signaling strategy affects cells fastest? Membrane receptors acting through second messengers are faster than nuclear receptors inducing gene transcription.
Thought Question: Which has longest lasting effects? Nuclear receptor-mediated changes in gene expression usually last longer.
Advantage of Feed-forward control in Circadian Rhythms: By increasing Cortisol (and thus blood glucose) before waking, the body prepares for the energy demands of the day before the metabolic need actually arises.
Lead-off Quiz:
- Most hormones are Peptide hormones and are therefore Hydrophilic.
- Consequently, most hormones act through receptors on the target cell membrane.
- Lipophilic hormones include Cortisol and Thyroid hormone (Epinephrine is a hydrophilic amine).

Signaling Molecules: Hormones are chemicals that can affect parts of the body far away; neurotransmitters work on nearby cells; neurohormones are hormones released by nerve cells, and paracrine signals only affect nearby cells.
Ways Cells Communicate: Cells can talk directly through tiny holes between them (gap junctions) or by connecting their surfaces together. They can also use chemicals released into the space around them.
Nervous vs. Endocrine Systems: The nervous system sends quick messages through wires (nerves) using neurotransmitters, while the endocrine system sends slower, longer-lasting messages through the blood with hormones.
Roles of the Endocrine System: The endocrine system helps control digestion, how we use energy, keeps our body balanced, helps us handle stress, supports growth, makes red blood cells, and manages reproduction.
Endocrine Glands: Key glands in the body include the pancreas (controls sugar), pituitary (controls many other hormones), adrenal (stress response), thyroid (metabolism), parathyroid (calcium balance), ovaries (female hormones), and testes (male hormones).
Other Organs with Hormones: Some organs like fat tissue, the stomach, intestines, kidneys, the liver, and the brain also release hormones even though that’s not their main job.
Hormone Types Based on Solubility: Some hormones like insulin are water-loving (hydrophilic) and work on cell surfaces, while others like testosterone are fat-loving (lipophilic) and work inside cells.
How Hormones are Made: Some hormones come from a building block called tyrosine, others are made from longer chains of amino acids, and steroids come from cholesterol.
Transport in Blood and Excretion: Water-loving hormones travel freely in the blood, while fat-loving hormones need transport proteins. They are broken down or excreted by the body in different ways.
How Hormones Work: Water-loving hormones attach to receptors outside cells and cause quick effects, while fat-loving hormones enter cells and change how genes work, which takes longer to show effects.
How the Body Controls Hormones: Hormone levels are controlled by feedback systems, quick response signals, daily rhythms, and by how sensitive cells are to hormones.
Problems with Hormones: Hormones can be too low (hyposecretion) or too high (hypersecretion), which can be caused by gland issues or how well cells respond.
Questions and Discussion: Key discussion points include which communication method is fastest, which lasts the longest, and how the body prepares for changes throughout the day.