Study Questions 11 and 12

Study Questions for NPB 130, Autumn 2025

HPA Axis Overview

1. Categories of Steroids Produced by the Adrenal Gland
  • The three main categories of steroids are:
    • Glucocorticoids: A class of steroid hormones involved in the metabolism of carbohydrates, proteins, and fats, as well as the immune response.
    • Mineralocorticoids: Primarily involved in the regulation of electrolyte and water balance.
    • Androgens: Sex hormones that influence reproductive and sexual functions.

HPA Axis Components and Functionality

2. Anatomical Components and Signaling of the HPA Axis
  • **Major Components: **
    • Hypothalamus: Produces corticotropin-releasing hormone (CRH).
    • Signaling Component: Releases CRH in response to stress.
    • Main Target: Anterior pituitary gland.
    • Anterior Pituitary: Produces adrenocorticotropic hormone (ACTH).
    • Signaling Component: Secretes ACTH upon receiving CRH.
    • Main Target: Adrenal cortex.
    • Adrenal Cortex: Releases glucocorticoids, primarily cortisol.
    • Signaling Component: Responds to ACTH, leading to cortisol secretion.
    • Main Targets: Various tissues including liver, muscle, and immune cells.
3. Circadian Rhythm of Glucocorticoid Secretion
  • Glucocorticoid Secretion Pattern: Not steady; shows surges.
  • Functional Significance of Surges: Surges in cortisol, particularly in the early morning, correspond to the anticipation of daily stresses, preparing the body to mobilize energy and manage stress effectively.
  • Explanation of Rises: Synchronized with the circadian rhythm; cortisol levels peak shortly before awakening, aiding in alertness and metabolic readiness.
4. Glucocorticoids and Stress Responses
  • Statement Analysis: Glucocorticoids do not only rise during stress; they also exhibit a circadian surge.
  • Agreement with the Statement: Circadian rises serve essential roles in regulating metabolism and energy balance. The physiological role during stress is more complex than mere secretion during stress episodes.

Effects of Adrenal Gland Removal and Hormone Regulation

5. Effects of Adrenalectomy (ADX) on Glucocorticoids
  • Consequences of ADX: Leads to a significant drop in circulating glucocorticoids (i.e., cortisol levels).
  • Reason for Drop: Absence of adrenal production of glucocorticoids post-surgery.
  • Administration of Dexamethasone (DEX): After ADX, administering synthetic glucocorticoids like DEX achieves hormonal replacement but may inhibit the hypothalamus-pituitary-adrenal (HPA) feedback loop, impacting corticotropin-releasing hormone (CRH) and ACTH levels.
6. ACTH Levels in Brattleboro Rats Post Insulin Injection
  • Graph of ACTH Levels: The ACTH levels in Brattleboro rats will differ from wild-type rats after insulin treatment.
  • Significance: This difference indicates the dependence of ACTH release on vasopressin (AVP) during hypothalamic signaling; hence, ACTH release in Brattleboro rats is affected by their deficiency in AVP.
7. Secretagogs and ACTH Response to Metabolic Stress
  • Figure Analysis: Trends in CRH and AVP levels indicate key secretagogs for ACTH response, showing reduced levels in insulin-treated groups.
  • Key Observations: Lowered levels of CRH and AVP post-insulin injection suggest a regulatory mechanism where the metabolic state alters the release of ACTH secretagogs.

Structure and Function of the Adrenal Cortex

8. Zones of the Adrenal Cortex
  • Three Zones:
    • Zona Glomerulosa: Outermost layer; produces mineralocorticoids (like aldosterone).
    • Zona Fasciculata: Middle layer; primarily produces glucocorticoids (like cortisol).
    • Zona Reticularis: Innermost layer; produces androgens.
  • Relative Locations:
    • Zona glomerulosa is the outermost, followed by zona fasciculata and zona reticularis, with the adrenal medulla located centrally.
9. Determinants of End-Product Hormones in Adrenal Zones
  • Main Determinant: The specific availability and expression of enzymes within each zone are crucial for which end-product hormones are synthesized in each cortical layer.
10. ACTH Receptor Characteristics
  • Main Adrenal Receptor for ACTH: Called the melanocortin 2 receptor (MC2R).
  • Location: Primarily in the zona fasciculata of the adrenal cortex.
  • Receptor Type: This is a G protein-coupled receptor (GPCR).

Signaling Mechanisms in Glucocorticoid Production

11. Cholesterol Overview
  • Cholesterol Definition: A sterol and a vital component of cell membranes and precursor for steroid hormones.
  • Source: Derived from both dietary intake and endogenous synthesis.
  • Roles Beyond Steroid Biosynthesis: Critical for membrane structure, its derivatives help in producing bile acids, vitamin D, and hormones.
12. Importance of Genes in Steroid Biosynthesis
  • Response to Argument Against Genes: Many essential enzymes for steroidogenesis, like those modifying cholesterol, are encoded by genes; thus, genes play a significant role in steroid biosynthesis.
13. Enzymatic Modifications in Cholesterol Processing
  • Initial Modification: Removal of fatty acid chains attached to cholesterol.
  • Enzymes Involved: Specific enzymes responsible include cholesterol side-chain cleavage enzyme (P450scc) for the side chain and fatty acyl-CoA thioesterases for the fatty acid chain.
  • Location: Most of these enzymes are located in the mitochondria, acting at early stages of steroid synthesis.
14. Role of LDL in Steroid Biosynthesis
  • Definition of LDL (Low-Density Lipoprotein): A type of lipoprotein responsible for transporting cholesterol in the bloodstream.
  • Function in Steroid Biosynthesis: LDL facilitates delivery of cholesterol to steroidogenic cells where it is utilized in hormone biosynthesis.
15. LDL Receptor (LDLR) Function
  • Definition of LDLR: The receptor that binds LDL, facilitating the uptake of cholesterol into cells.
  • Role in Steroid Biosynthesis: Crucial for providing cholesterol required for steroid hormone production within adrenal cells.
16. StAR Protein Characteristics
  • Definition: Steroidogenic Acute Regulatory (StAR) protein is critical for transporting cholesterol into mitochondria in steroid-producing cells.
  • Activation: Triggered by ACTH and other signaling molecules.
  • Location: Mitochondria, enabling the initiation of steroid hormone production.
17. CHO Esterase Overview
  • Function: Cholesterol esterase (CHO esterase) hydrolyzes cholesterol esters, freeing cholesterol for steroidogenesis.
  • Location: Cytosol of steroidogenic cells where it plays a key regulatory role in hormone biosynthesis.
18. Cellular Locations of Glucocorticoid Conversion
  • Process Location: Conversion of cholesterol to glucocorticoids primarily occurs in the mitochondria and endoplasmic reticulum (ER).
  • First Step Enzyme: The initial reaction is catalyzed by P450scc (cholesterol side-chain cleavage enzyme), located in the mitochondria.
  • Precursor Produced: The precursor molecule produced is pregnenolone, which is essential for generating cortisol and other glucocorticoids.

Specifics on Adrenal Hormones

19. Aldosterone Production
  • Zone of Production: Aldosterone is produced in the zona glomerulosa.
  • Type of Steroid: Aldosterone is classified as a mineralocorticoid.
  • Dependency on Corticosterone and Cortisol: The production of aldosterone does not directly require prior production of corticosterone or cortisol; however, the steroidogenesis pathways are interconnected.
20. Cortisol Production Zones
  • Cortisol Production Zone: Cortisol is synthesized in the zona fasciculata.
  • Type of Steroid: Cortisol is classified as a glucocorticoid.
  • Direct Production from Corticosterone: Cortisol cannot be produced directly from corticosterone; it requires several enzymatic conversions involving different intermediates.
21. Androgens Production in Adrenal Cortex
  • Conversion to Androgens: Conversion occurs through enzymatic modifications that reduce the carbon number from the steroid precursors to produce androgens, which typically have 19 carbons.
  • Common Androgens Produced: Include testosterone and androstenedione.
22. Similarities and Differences Between Steroid Hormones
  • Main Similarities: All are derived from cholesterol and share a steroid structure.
  • Key Differences: Varying functional groups and structures which dictate their specific biological activities and receptors they bind with, influencing their actions in target cells.
23. Localization of Corticosteroid Biosynthesis
  • Mitochondrial vs SER Components: Mitochondria are involved in initial steroid conversion steps while steroid hormone modifications and final synthesis steps unfold in the smooth endoplasmic reticulum (SER).
24. Specificity of Receptor Activation by Glucocorticoids and Aldosterone
  • Receptor Binding Affinity: While glucocorticoids and aldosterone bind to mineralocorticoid receptors, the specificity can be maintained through tissue-specific expression of 11β-hydroxysteroid dehydrogenase enzymes, which convert glucocorticoids to inactive forms in certain tissues, preventing overactivation of mineralocorticoid receptors by glucocorticoids.