The Endocrine System

These flashcards cover essential concepts related to the Endocrine System and hormonal functions.

Endocrine System

A system composed of unicellular and multicellular glands that secrete hormones into the bloodstream.

Hormones

Signaling molecules synthesized by specific cells, tissues, or glands, carried in the blood to target cells.

Negative Feedback Loop

A regulatory mechanism where an increase in a product or response inhibits further hormone release or synthesis.

Amines

Small molecules derived from amino acids, including catecholamines such as epinephrine and norepinephrine.

Peptide and Protein Hormones

Large, complex molecules derived from peptide subunits, such as adrenocorticotropic hormone and antidiuretic hormone.

Steroid Hormones

Hormones derived from cholesterol characterized by cyclic hydrocarbon structures, including testosterone and estrogen.

Eicosanoids

Biologically active lipids synthesized from arachidonic acid; may not be classified as true hormones by some, but rather, signaling molecules

Lipid-Soluble Hormones

• Hormones that can cross cell membranes and bind to cytoplasmic receptors, affecting gene transcription.

• Will be bound to carrier proteins while in circulation that ensure the hormone gets to the target cell

Lipid-Insoluble Hormones

• Hormones that bind to surface receptors, generating secondary messengers that mediate quick cellular responses.

• Three groups:

  • Cyclic Nucleotide Monophosphates (cNMP)

    • cAMP (adenosine 3’,5’-cyclic monophosphate)

    • cGMP (guanine 3’,5’-cyclic monophosphate)

  • Inositol Phospholipids

    • 1,4,5-triphosphate (IP3)

    • 1,2-diacyglycerol (DAG)

  • Ca+2

cAMP hormonal pathway

1. A lipid-insoluble hormone binds to a receptor protein on the target cell membrane

2. The binding of the receptor and hormone causes GTP to bind to the G protein

3. The activated G protein will activate adenylate cyclase, which will catalyze the conversion of ATP to cAMP

4. cAMP will then bind to Protein Kinase A

5. The catalytic subunit is now free to phosphorylate effector proteins using ATP for the high-energy phosphate group

6. In the case of hormones, the phosphorylated proteins will display an increase in activity

G proteins

Molecular switches that transmit signals from receptors to other intracellular proteins, involved in cAMP pathways.

Inositol Phospholipid Signaling Pathway

1. A lipid-insoluble hormone binds to a receptor protein on the target cell membrane

2. The binding of the hormones to the receptor-Gp protein activates Gp which, in turn, induces the activation of Phosphoinositide-Specific Phospholipase C (PLC)

3. PLC hydrolyzes Phosphatidyl Inositol Biphosphate (PIP2) into two major secondary messengers

4. When IP3 is the secondary messenger: IP3 detaches from the cell membrane and diffuses into the cytoplasm. IP3 then acts on calcium stores within the cell, especially in the ER

5. When DAG is the secondary messenger, it has two signaling roles:

   1. It can be cleaved to release arachidonic acid

   2. DAG can activate Protein Kinase C (PKC)

Growth Hormone (GH)

A peptide hormone secreted by acidophils that stimulates growth and metabolism in various tissues.

Oxytocin

Milk ejection, uterine contraction, arteriole vasoconstriction

Antidiuretic Hormone

Increases tubular reabsorption and water conservation

Somatotropin

Promotes growth

Prolactin (PR)

A hormone produced by acidophils that stimulates milk production and various metabolic processes in mammals.

Adrenocorticotropic Hormone (ACTH)

A polypeptide hormone that stimulates the adrenal glands to secrete glucocorticoids and other hormones. (mineralocorticoids, gonadocorticoids)

Thyroid Stimulating Hormone (TSH)

A tropic hormone that stimulates the thyroid gland to produce thyroid hormones like Thyroxine and Triiodothyronine

Melanocyte Stimulating Hormone

Promotes melanogenesis

Luteinizing Hormone

Promotes sex hormone production

Follicle Stimulating Hormone

Promotes the development of the reproductive system and gametogenesis

Calcitonin

A polypeptide hormone produced by parafollicular cells that decreases blood calcium levels by inhibiting osteoclast activity.

Parathyroid Hormone (PTH)

A hormone that increases blood calcium levels by stimulating bone resorption and renal reabsorption of calcium.

Catecholamines

Hormones produced by the adrenal medulla that include epinephrine and norepinephrine, affecting body metabolism and stress responses.

Insulin

A peptide hormone secreted by the pancreas that reduces blood glucose levels by promoting glucose uptake and storage.

Glucagon

A peptide hormone produced by the pancreas that raises blood glucose levels by stimulating glucogenolysis.

Follicular Phase

Begins with the release of FSH and stimulates the activation of several primordical follicles to enter into follicular maturation

Luteal Phase

The phase begins with ovulation, and the remaining cells from the follicle become the corpus luteum

Parturition

Oxytocin will stimulate uterine contractions to expel the offspring and placenta

Anestrus

A quiescent period when the animal is not receptive to breeding

Proestrus

The beginning stage when follicles are stimulated to go into the maturation cycle, the uterus swells, and the vagina

Estrus

Period of sexual receptivity of the female

Metestrus

• If fertilization occurs: the corpus leteum will continue the thickening of the placenta and the embryos will go through gestation

• If fertilization does not occur: the uterine lining will not be maintained and will be reabsorbed

Buriscon

Promotes cuticle development, induces tanning of cuticle

Ecdysone

Promoted the secretion of a new cuticle

Eclosion Hormone

Induces the emergence of the adult form from the chrysalis

Juvenile Hormone

Promotes synthesis of larval structures and inhibits metamorphosis in the larva

Prothoracicotropin

Stimulates the release of ecdysone

Corticotropin Release Factor

a peptide that stimulates ACTH release (Adrenocorticotropic hormone)

Gonadotropin Release Factor

A peptide that stimulates the release of FSH and LH

TSH Release Factor

A peptide that stimulates the release of TSH and PR

Somatostatin Inhibiting Factor

A peptide that inhibits the release of primarily GH and secondarily other hormones, including TSH, insulin, and glucagon

Neurosecretory cells in the hypothalamus

• Located in two hypothalamic nuclei:

  • paraventricular

  • supraoptic nuclei

• Stored in Hering Bodies

Neurohormones

• Antidiuretic Hormone (ADH)/Vasopressin: a peptide that increases renal tubular reabsorption for increased water conservation

• Oxytocin:

  • Causes vasoconstriction in arterioles in many animals

  • In mammals, oxytocin causes ejection of milk from the mammaries

  • Oxytocin may play a role in the orgasm and seems to promote pair bonding in many different species

  • Oxytocin causes uterine constrictions in mammals

Hormones of the Adenohypophysis

• Acidophils:

  • GH (aka somatotropin)

    • targets many tissues including the liver and adipose tissue

    • promotes growth and lipid and carbohydrate metabolism

    • antagonist to insulin, stimulates gluconeogenesis

  • PR

    • targets the ovaries

    • promotes secretion of estrogen and progesterone

• Basophils: ACTH, TSH, MSH, LH and FSH

  • ACTH

    • Targets the steroidogenic tissues associated with the adrenal gland

    • Stimulates the secretion of glucocorticoids, mineralocorticoids, and gonadocorticoids

  • TSH

    • Targets the thyroid gland

    • Two subunits: alpha and beta

      • Alpha is considered to be the effector region responsible for stimulating adenylate cyclase

      • Beta is an amino acid sequence UNIQUE TO TSH

  • MSH

    • Targets the pigment cells

  • LH

    • Glycoprotein that targets the gonads and promotes sex hormone production

  • FSH

    • Glycoproteine that targets the gonads and promotes development of hte reproductive system and gametogenesis

Antidiuretic Hormone (ADH), or Vasopressin

• Regulation of water, glucose, and salts in the blood

  • Retains water, constricts blood vessels

Effect of ADH on the Mammalian Kidney

• Raises water permeability in distal tubule and collecting duct cells, mainly through inserting Aquaporin-2 water channels into their membranes via V2 receptors

• The G-protein coupled receptors activate adenylyl cyclase, converting ATP to cAMP, which triggers aquaporin-2 exocytosis and gene transcription, thus enhancing water absorption and urine concentration.

  • Boosts calcium by releasing Ca+2 from stores and increases urea reabsorption through urea transporter regulation, further concentrating urea.

Effect of ADH on the Blood Vessels

• Increases resistance in the peripheral vasculature by stimulating the smooth muscles of the arterial walls to constrict, utilizing inositol phospholipids as the secondary messenger

• Increases arterial blood pressure

Vasotosin

• Similar to oxytocin and ADH - peptide hormone

• Stimulates the recovery of water from kidneys

• Plays a role in sexual behavior

Thyronines

• Cells that produce the thyroid hormones

• Arranged into hollow spheres called follicles (Follicular Cells)

  • Stimulated by TSH to produce the hormones THyroxine (T4) and 3,5,3’-Triiodothyronine (T3)

    • Iodine is required

• Lipid-soluble, can cross the cell membrane and bind to recpetors within the cytoplasm

Parafollicular Cells (C cells)

• Arranged into small clusters between thyroid follicle

• Produce the hormone Calcitonin (Thyrocalcitonin)

  • Calcium and phosphorous metabolism

    • Nonmammalian vertebrates produce calcitonin via the Ultumiobranchial Gland

Parathyroid

• Parathyroid Hormone (PTH)

  • Polypeptide hormone that acts as an antagonist to calcitonin

  • Enhances the release of calcium

    • By increasing production of activated vitamin D

Adrenal Gland

• Adrenal medulla (aminogenic tissue): produces catecholamines, derived from ectoderm

• Adrenal cortex (steroidogenic tissue): produces steroid hormones, derived from mesoderm

Vertebrate Variation

• Mammals: distinct adrenal gland with cortex and medulla

  • Fishes: aminogenic and steroidogenic tissues are scattered cell clusters

Adrenal Medulla Function

• Considered similar to a sympathetic ganglion

  • Innervated by sympathetic preganglionic neurons

    • Releases hormones into blood instead of synapses: acts like modified postganglionic neurons

Chromaffin Cells

Acetylcholine → Increased Ca+2 permability → increased intracellular Ca+2 → exocytosis

Catecholamines

Produced by the adrenal medulla: epinephrine and norepinephrine, causes fight-or-flight response

Catecholamine Synthetsis

Tyrosine → DOPA → Dopamine - Norepinephrine → Epinephrine

Adrenoreceptors

• Alpha receptors:

  • IP3 → increased Ca+2 → smooth muscle contraction

• Beta receptors:

  • cAMP → cardiac contraction, bronchodilation, lipolysis

Adrenal Cortex Layers

• Zona glomerulosa: mineralocorticoids (aldosterone)

• Zona fasciculata: glucocorticoids (cortisol)

• Zona reticularis: androgens

Aldosterone Function

• primary regulator of blood pressure, fluid balance, and electrolytes

• increases sodium reabsorption and water retention while promoting potassium excretion, thus raising blood volume and pressure.

  • LONG TERM BLOOD PRESSURE REGULATION

Cortisol Function

• Increased glucose (gluconeogenesis)

• Increased fat and protein breakdown

• Anti-inflammatory

• Stress response hormone

What hormones are produced by the Zona Reticularis?

Androgens (testosterone, DHT, androstenedione, DHEA)

How to steroid hormones like cortisol and aldosterone act?

They cross cell memrbanes, bind to intracellular receptors, and affect gene transcription

DHEA

Steroid hormone that serves as a metabolic intermediate for the production of other androgens and estorgenF

Follicular phase

• Begins with the release of FSH by the adenohypophysis

  • During follicular maturation, primordial follicles will go through developmental changes, resulting in a mature follicle called a Graafian Follicle

• LH will also be released from the adenohypophysis

  • FSH will stimualte the productionof the enzyme aramotase which will convert androgens into estrogens

• Surge in estrogen levels stimualtes the hypothalamus, triggers the adenohypophysis to increase FSH and LH

Insect Development

• Holometabolous: complete metamorphosis

  • Hemimetabolous: incomplete metamorphosis

PTTH (Prothoracicotropic Hormone)

• Produced in brain neurosecretory cells

• Stored/released via corpora allata

• Stimulates prothoracic gland, releasing ecdysone

Ecdysone

• Steroid hormone

• alpha-ecdysone: prohormone

• beta-ecdysone: active form (molting and new cuticle formation)

Juvenile Hormone (JH)

Maintains juvenile characteristics

How do ecdysone and juvenile hormones work together?

Ecdysone triggers molting, juvenile hormone determine what stage comes next

Bursicon

Protein hormone, causes hardening and darkening (tanning) of new cuticle

Corporal allata

Stores/releases hormoensPr

Prothoracic gland

Produces ecdysone