Adrenal Glands, Pineal Gland, and Pancreas Flashcards

Anatomy and Organization of the Adrenal Cortex

• The adrenal gland is divided into an outer cortex and an inner medulla. While the medulla primarily releases epinephrine and some norepinephrine (likely via the hemolysis pathway), the cortex is categorized into three distinct layers based on the arrangement of endocrine cells and their structure.
• The Capsule: A layer of connective tissue that encases the organ, binding it to surrounding fatty tissue on the body wall.
• Zona Glomerulosa: The outermost layer located just beneath the capsule. The term "glomerulus" means "tangle," referring to the jumbled arrangement of endocrine cells in this region. This layer produces mineralocorticoids.
• Zona Fasciculata: The middle layer where endocrine cells are organized into long columns or rows called fascicles (similar to bundles found in muscles and nerves). This layer produces glucocorticoids.
• Zona Reticularis: The innermost layer bordering the medulla. The term "reticulum" means "network," indicating a more organized structure than the glomerulosa. This layer produces sex hormones, particularly androgens like testosterone equivalents.

Mineralocorticoids: Aldosterone and the RAAS Pathway

• Mineralocorticoids: A group of hormones that regulate ions (minerals), primarily $Na^+$ (sodium) and $K^+$ (potassium). The primary mineralocorticoid is aldosterone, synthesized in the zona glomerulosa.
• Stimuli for Release: Aldosterone secretion is triggered by low blood pressure, low blood volume, hyponatremia (low blood sodium), or hyperkalemia (high blood potassium).
• The Renin-Angiotensin-Aldosterone System (RAAS):
  • Renin: An enzyme (protease) produced by granular cells in the kidney in response to low blood pressure. It does not bind to receptors but acts on a precursor protein.
  • Angiotensinogen: A precursor protein produced constitutively by the liver and released into the plasma.
  • Angiotensin I: Renin converts angiotensinogen into angiotensin I.
  • Angiotensin Converting Enzyme (ACE): Found on the luminal membranes of endothelial cells, especially high concentrations in the lungs. ACE converts angiotensin I into angiotensin II.
  • Angiotensin II Functions:
    1. Acts as a potent vasoconstrictor to increase blood pressure.
    2. Stimulates cardiovascular centers in the medulla oblongata to increase sympathetic input, heart rate, and contractility.
    3. Stimulates the hypothalamus to release antidiuretic hormone ($ADH$) and trigger thirst.
    4. Stimulates the adrenal cortex to release aldosterone.
• Mechanism in the Kidney:
  • Aldosterone targets the distal convoluted tubule (the section before the collecting duct).
  • It binds to cytoplasmic receptors, translocates to the nucleus, and initiates transcription of $Na^+$ channels and $Na^+/K^+$ ATPase pumps.
  • $Na^+$ Channels: Inserted into the luminal surface to increase sodium absorption from the urine.
  • $Na^+/K^+$ ATPase: Inserted into the basilar surface. It pumps $3$ sodium ions out of the cell for every $2$ potassium ions pumped in. This process is electrogenic, helping maintain negative membrane potential.
  • Water follows the sodium reabsorption, leading to a small increase in blood volume and pressure, which provides negative feedback to stop renin release.
• Clinical Notes: ACE inhibitors (drugs ending in "-pril" like captopril or lisinopril) are used for high blood pressure. A common side effect is a dry cough due to the abundance of ACE in the lungs. If RAAS is blocked, patients must be cautious with potassium intake (e.g., bananas) to avoid hyperkalemia.

Glucocorticoids: Stress and Metabolism

• Glucocorticoids: Named for their effect on glucose metabolism and their origin in the adrenal cortex. Examples include cortisol, corticosteroid, and cortisone.
• HPA Axis Regulation:
  1. Hypothalamus: Releases Corticotropin Releasing Hormone ($CRH$), also called adrenochorocortropic releasing hormone. It has a circadian rhythm with a peak in the morning to prepare for "waking up stress."
  2. Anterior Pituitary: $CRH$ travels through the portal system to stimulate the release of Adrenocorticotropic Hormone ($ACTH$).
  3. Adrenal Cortex: $ACTH$ targets the zona fasciculata to stimulate glucocorticoid release.
• Cellular Synthesis: Glucocorticoids are synthesized from cholesterol (a steroid ring structure).
  • $ACTH$ activates a $G_s$-protein coupled receptor, increasing cyclic $AMP$ ($cAMP$) and Protein Kinase A ($PKA$).
  • This increases lipoprotein uptake to provide cholesterol.
  • Cholesterol is modified by enzymes in the mitochondria and the smooth endoplasmic reticulum ($SER$). Cells in the zona fasciculata are enriched with these organelles.
• Transport and Receptors:
  • Being hydrophobic, glucocorticoids require binding proteins: $25\%$ is bound to serum albumin (abundant plasma protein also found in synovial joints/egg whites), and the rest is carried by transcortin (an alpha globulin).
  • Receptors are cytoplasmic. Once bound, they translocate to the nucleus to bind to Glucocorticoid Response Elements ($GRE$) on DNA.
• Physiological Effects:
  • Metabolism: Increases gluconeogenesis (creating glucose from amino acids and glycerol). Decreases glucose uptake in fat and muscle cells to preserve it for the brain. Increases lipolysis.
  • Growth: Decreases growth hormone release; chronic stress in childhood can stunt height.
  • Immune System: Depresses immune function, decreases white blood cell migration, decreases inflammation, reduces cytokines/histamine, and decreases antibody secretion.
  • Brain: Crosses the blood-brain barrier. Effects on memory are variable; it can enhance or fade memories of traumatic events.
  • Fat Distribution: Chronic high stress promotes the formation of belly fat.

The Pineal Gland and Melatonin

• The pineal gland is located posterior to the hypothalamus at the end of the epithalamus, sitting over the superior colliculus.
• Pathway of Activation: Darkness inhibits retinal ganglion cells, which communicate via the retinal hypophthalamic tract to the suprachiasmatic nucleus ($SCN$) (located above the optic chiasm). Output goes to the paraventricular nucleus ($PVN$) and then to the sympathetic nervous system to trigger the pineal gland.
• Melatonin Properties:
  • Derived from the amino acid tryptophan (pathway: Tryptophan → Serotonin → Melatonin).
  • Described as esoporific (helps with sleep onset).
  • Acts as an antioxidant and DNA repair stimulant.
  • Cancer Link: Melatonin facilitates DNA repair of damage caused by reactive oxygen species. Shift workers (who have low melatonin when active at night) have higher risks of breast, cervical (females), and prostate cancer (males).
• Circadian Cycle: Levels rise late at night (around $11:00\,PM$), peaking around the time of waking. The speaker notes the curve's peak suggests it may be more related to repair than sleep onset.

The Pancreas: Endocrine Function and Glucose Regulation

• The pancreas is a mixed organ (exocrine and endocrine). It consists of a head (near the duodenum/duodenum) and a tail.
• Anatomy: Exocrine acini produce digestive enzymes. Endocrine cells are clustered in pancreatic islets (islands).
• Hormones and Cell Types:
  • Beta Cells: Produce Insulin. Insulin is a small protein ($51$ amino acids, $2$ polypeptide chains, internal disulfide bonds).
  • Alpha Cells: Produce Glucagon. Glucagon is a single chain of $2$ amino acids with no stable tertiary structure or post-translational modifications.
• Insulin Mechanism:
  • Released when blood glucose is high (Target: $100\,mg/100\,\mu L/dL$).
  • Uses a Tyrosine Kinase Receptor: Insulin binds, causing the receptor to dimerize and autophosphorylate. This activates a signaling cascade that inserts glucose transporters into the cell membrane.
  • Promotes glycogen synthesis in the liver and glucose uptake in cells. Decreases blood fatty acids and amino acids.
• Glucagon Mechanism: Released when blood glucose drops. Stimulates glycogenolysis (glycogen breakdown) to release glucose into the blood.
• Regulation:
  • Humoral: Simple negative feedback loops based on blood glucose levels.
  • Neural: Parasympathetic stimulation (rest and digest) increases insulin in anticipation of a glucose spike. Sympathetic stimulation (epinephrine) inhibits insulin to preserve glucose for the brain.
• High Protein Meal Interaction: A meal high in protein but low in carbohydrates increases blood amino acids. This stimulates both alpha and beta cells. The simultaneous release of insulin and glucagon causes a net increase in protein synthesis while canceling out effects on blood glucose, maintaining homeostasis.

Questions & Discussion

• Melatonin Efficacy: A student asked about melatonin's effectiveness for sleep. Some students reported it doesn't work for them. The speaker expressed interest in why it works for some and not others and noted its role in DNA repair might be more significant than sleep onset.
• Dietary Restrictions and Hyperkalemia: Discussion on why patients taking RAAS-targeting drugs (like ARBs) should avoid eating too many bananas. Since these drugs block aldosterone, potassium secretion is reduced, leading to a risk of hyperkalemia ($high\,K^+$), which is dangerous for heart and skeletal muscle function.