Comprehensive Study Notes: The Endocrine System and Prostaglandin Synthesis

Overview of the Endocrine System

• The endocrine system is composed of diverse organs scattered throughout the human body, functioning collectively to maintain homeostasis.

• This system is characterized by a lack of physical continuity between its constituent organs, distinguishing it from other organ systems.

• Control of homeostatic functions is achieved through the utilization of chemical messengers known as hormones.

Biochemistry of Hormones

• Hormones are defined as chemical substances secreted by endocrine cells into extracellular fluids to regulate the metabolic activity of other cells.

• Nearly all hormones are classified into two chemical categories:

  • Amino acid–based molecules: This category includes proteins, peptides, and amines.

  • Steroids: These are lipid-soluble hormones synthesized from cholesterol. They include sex hormones produced by the gonads (ovaries and testes) and hormones produced by the adrenal cortex.

• All hormones not derived from cholesterol are classified as nonsteroidal amino acid derivatives.

• Hormones circulate through the entire body via blood, but they only affect specific tissues or organs known as target cells or target organs.

• Hormonal response requires the presence of a specific protein structure on the target cell called a receptor.

General Action of Hormones

• Hormones elicit cellular changes through several mechanisms:

  • Altering plasma membrane permeability or membrane potential (electrical state) by opening or closing ion channels.

  • Activating or inactivating enzymes.

  • Stimulating or inhibiting cell division.

  • Promoting or inhibiting the secretion of specific products.

  • Turning on or turning off the transcription of specific genes, such as those encoding architectural proteins or regulatory molecules.

Mechanisms of Hormonal Signaling

Direct Gene Activation

• This mechanism is utilized by steroid-based and lipid-soluble hormones.

• Process steps:

  1. The hormone diffuses directly through the plasma membrane into the cell.

  2. The hormone enters the nucleus and binds to a specific receptor.

  3. The resulting hormone-receptor complex binds to specific regions of the $DNA$.

  4. Gene activation occurs, leading to the transcription of $mRNA$.

  5. The $mRNA$ migrates to the cytoplasm where translation occurs, resulting in the production of new proteins.

Second-Messenger Systems

Adenylyl Cyclase Mechanism ($cAMP$)

• Used by polypeptide or protein-based hormones.

• Process steps:

  1. The hormone (first messenger) binds to a receptor on the plasma membrane.

  2. The hormone-receptor complex activates the enzyme Adenylyl Cyclase.

  3. Adenylyl Cyclase converts $ATP$ to Cyclic $AMP$ ($cAMP$), which functions as the second messenger.

  4. $cAMP$ activates Protein Kinase A, which then activates other enzymes via phosphorylation to produce the cellular response.

• Hormones utilizing $cAMP$: Corticotrophin Releasing Hormone ($CRH$), Antidiuretic Hormone ($ADH$), Calcitonin, Parathyroid Hormone ($PTH$), Adrenocorticotrophic Hormone ($ACTH$), Human Chorionic Gonadotropin ($hCG$), Luteinizing Hormone ($LH$), Thyroid Stimulating Hormone ($TSH$), Glucagon, Lipotropin, Melanocyte Stimulating Hormone, Follicle Stimulating Hormone ($FSH$), and Somatostatin.

Guanylyl Cyclase Mechanism ($cGMP$)

• Used by polypeptide or protein-based hormones.

• Process steps:

  1. The hormone binds to a plasma membrane receptor.

  2. The hormone-receptor complex activates Guanylyl Cyclase.

  3. Guanylyl Cyclase converts $GTP$ (implied based on enzyme type and pathway) into Cyclic $GMP$ ($cGMP$) acting as the second messenger.

  4. $cGMP$ activates enzymes via phosphorylation to illicit a response.

• Hormones utilizing $cGMP$: Atrial Natriuretic Factor ($ANP$) and Nitric Oxide ($NO$).

Phosphatidyl Inositide Mechanism ($PIP_2$)

• Used by polypeptide or protein-based hormones.

• Process steps:

  1. The hormone binds to the plasma membrane receptor.

  2. The hormone-receptor complex activates a G protein, which subsequently activates Phospholipase C.

  3. Phospholipase C breaks down an intracellular compound called Phosphoinositol Bisphosphate ($PIP_2$) into Inositol Triphosphate ($IP_3$) and Diacyl Glycerol ($DAG$).

  4. $IP_3$ Effect: Increases intracellular calcium ($Ca^{2+}$), which acts as a second messenger to activate compounds for a response.

  5. $DAG$ Effect: Activates Protein Kinase C, which phosphorylates intracellular compounds to trigger a response.

• Hormones utilizing Phosphatidyl Inositides: Thyrotropin Releasing Hormone ($TRH$), Gonadotropin Releasing Hormone ($GnRH$), and Oxytocin.

Protein Kinase C and Phosphatase Mechanism

• Hormones utilizing this system: Growth Hormone ($GH$), Erythropoietin, Chorionic Somatomammotropin, Insulin, and Prolactin.

Stimuli for Hormone Release

• Hormonal Stimulus: The most common stimulus; endocrine organs are prodded into action by other hormones. For example, the hypothalamus secretes hormones that stimulate the anterior pituitary gland, which in turn stimulates other glands (Thyroid, Adrenal cortex, Gonads).

• Humoral Stimulus: Changing blood levels of certain ions and nutrients stimulate hormone release. For example, low concentration of $Ca^{2+}$ in capillary blood stimulates the parathyroid glands to secrete Parathyroid Hormone ($PTH$).

• Neural Stimulus: Nerve fibers stimulate hormone release. For example, preganglionic sympathetic fibers stimulate the adrenal medulla to secrete catecholamines (epinephrine and norepinephrine).

The Pituitary Gland and Hypothalamic Control

• The pituitary gland is approximately the size of a pea, hanging by a stalk from the inferior surface of the hypothalamus, nestled in the sella turcica of the sphenoid bone.

• Functional Lobes:

  • Adenohypophysis (Anterior Pituitary): Glandular tissue.

  • Neurohypophysis (Posterior Pituitary): Nervous tissue.

Hypothalamic Secretions

Adenohypophysis (Anterior Pituitary) Hormones

• The adenohypophysis secretes six hormones. All are proteins/peptides, act via second-messenger systems, and are regulated by hormonal stimuli and negative feedback.

• Two hormones, $GH$ and Prolactin ($PRL$), affect non-endocrine targets. The other four are trophic hormones.

Growth Hormone and IGF-1 Physiology

• Growth Hormone ($GH$) induces sodium retention and decreased insulin sensitivity.

• $GH$ stimulates the production of Insulin-like Growth Factor 1 ($IGF-1$).

• $GH$ Specific Actions: Lipolysis, protein synthesis, and epiphysial growth.

• $IGF-1$ Specific Actions: Anti-lipolytic activity, protein synthesis, and epiphysial growth.

Neurohypophysis (Posterior Pituitary)

• Acts as a storage site for hormones produced by hypothalamic neurosecretory cells.

The Thyroid Gland

• Located inferior to the Adam’s apple; consists of two lobes joined by an isthmus.

• Composed of follicles containing sticky colloid material.

• Hormone Synthesis Steps:

  1. Iodide Uptake & Trapping.

  2. Oxidation (mediated by thyroid peroxidase).

  3. Organification (mediated by thyroid peroxidase).

  4. Coupling & Synthesis (mediated by thyroid peroxidase).

  5. Storage.

  6. Release.

Thyroid Secretions

• Thyroxine ($T_4$) and Triiodothyronine ($T_3$):

  • Control glucose oxidation rate (ATP production).

  • Crucial for normal tissue growth/development (especially nervous and reproductive systems).

  • Associated Pathologies: Hyperthyroidism, Hypothyroidism, Grave's Disease, Hashimoto's Thyroiditis, Cretinism.

• Calcitonin:

  • Produced by parafollicular cells.

  • Lowers blood calcium levels by suppressing osteoclast activity and increasing urinary calcium excretion.

  • Associated Pathology: Hypocalcemia.

Systemic Effects of Thyroid Hormones

• CNS: Increased cerebration, synaptic transmission, and nerve myelinization.

• Cardiovascular: Increased heart rate ($SA$ node activity), increased myocardial contractility (stroke volume), and increased cardiac output ($CO$). Increased systolic BP, but decreased diastolic BP due to vasodilation (via heat production).

• GI Tract: Increased motility, secretion, and absorption.

• Hematopoiesis: Increased oxygen consumption decreases $PO_2$, stimulating erythropoiesis.

• Respiratory: Increased intracellular activity leads to high $pCO_2$ and low $PO_2$, triggering increased respiratory activity.

• Metabolism: Increased lipolysis and lipogenesis. Normal doses are anabolic for protein metabolism.

• Body Weight: Increased levels lead to increased appetite but decreased body weight.

• Vitamins: Increases conversion of carotene to Vitamin A.

• Adrenal Medulla: Increases secretion of Epinephrine and Norepinephrine.

• Thermoregulation: Increased hormone leads to heat intolerance; decreased leads to cold intolerance.

Parathyroid and Pineal Glands

• Parathyroid Glands: Tiny masses on the posterior thyroid. Secrete Parathyroid Hormone ($PTH$) to regulate $Ca^{2+}$. Low blood calcium triggers $PTH$, which stimulates osteoclasts to break down bone matrix and release calcium.

• Pineal Gland: Located in the roof of the third ventricle. Secretes Melatonin, a "sleep trigger" regulating the sleep-wake cycle.

The Adrenal Glands

Adrenal Cortex Structure and Layers

1. Zona Glomerulosa (Outer): Synthesis of mineralocorticoids (e.g., Aldosterone).

2. Zona Fasciculata (Middle): Synthesis of glucocorticoids (e.g., Cortisol).

3. Zona Reticularis (Inner): Secretes some sex hormones (Androgens/Estrogens).

Steroidogenesis Pathway Enzymes

• Cholesterol converted by Desmolase to Pregnenolone.

• Pregnenolone pathway involves $17α$-hydroxylase, $3β$-hydroxysteroid dehydrogenase, and $17,20$ lyase to produce DHEA, Progesterone, and Androstenedione.

• Aldosterone production involves $21$-hydroxylase, $11β$-hydroxylase, and Aldosterone synthase.

• Cortisol production involves $21$-hydroxylase and $11β$-hydroxylase.

• Sex hormones involve Aromatase (for Estradiol/Estrone) and $17$-ketoreductase (for Testosterone).

Adrenal Secretions

Renin-Angiotensin System

1. Drop in blood pressure/fluid volume causes kidney to release Renin.

2. Renin acts on Angiotensinogen (from liver) to form Angiotensin I.

3. $ACE$ (from lungs) converts Angiotensin I to Angiotensin II.

4. Angiotensin II stimulates systemic vasoconstriction and the adrenal release of Aldosterone.

5. Aldosterone causes $NaCl$ and $H_2O$ reabsorption in kidneys.

Pancreatic Islets

• Insulin ($β$ cells): Increases glucose import into cells; hypoglycemic effect; leads to glucose oxidation or glycogen/fat storage. (Pathology: Type I and II Diabetes Mellitus).

• Glucagon ($α$ cells): Stimulates liver to break down glycogen to glucose; hyperglycemic effect. (Pathology: Glucagonoma).

Gonadal and Other Endocrine Organs

• Ovaries: Produce Estrogen (secondary sex characteristics, endometrial proliferation) and Progesterone (quiets uterine muscles during pregnancy, prepares corpus luteum). (Pathology: Sterility).

• Testes: Produce Testosterone (male reproductive maturation, sperm production). (Pathology: Sterility).

• Thymus: Located in upper thorax; decreases in size with age; produces Thymosin, essential for T lymphocyte development.

• Placenta: Temporary organ producing $hCG$, estrogen, and progesterone to maintain pregnancy. Produces Human Placental Lactogen ($hPL$) for lactation prep and Relaxin to relax pelvic ligaments.

Prostaglandin Synthesis Pathway

1. Precursors: Diacylglycerol or Phospholipids.

2. Transition: Phospholipase C or Phospholipase $A_2$ releases Arachidonic acid.

3. Cyclooxygenase Pathway: Arachidonic acid is converted by $PGH_2$ synthase ($COX-1$ or $-2$ and peroxidase) into Prostaglandin $H_2$ ($PGH_2$).

4. End Products of $PGH_2$:

   • $PGE_2$ (via $PGE$ synthase).

   • $PGD_2$ (via $PGD$ synthase).

   • $PGF_{2α}$.

   • Prostacyclin ($PGI_2$) in endothelium (via Prostacylcin synthase).

   • Thromboxane ($TXA_2$) in platelets (via Thromboxane synthase).

5. Lipoxygenase Pathway: Arachidonic acid converted by Lipooxygenase to $HPETE$, then Leukotriene $A_4$, and eventually Leukotrienes $B_4$, $C_4$, $D_4$, and $E_4$.