Adrenal Medulla, Epinephrine, and Exam Review

Exam Review and Logistics

• The upcoming exam will be held next Wednesday.

• A review session for the exam will take place on Monday, after the completion of the adrenal medulla presentation.

• Concept guides covering all lectures will be provided on Monday to aid in studying.

• Students are encouraged to study over the weekend and prepare questions for the Monday review session.

• The exam format will be comprehensive, including a mix of multiple-choice questions, short answer questions, and fill-in-diagrams.

• For short answer questions, the expectation is concise, direct answers, often one word or a brief phrase (e.g., "SON and PVN" instead of a full sentence).

• Some short answer questions, like those involving complex cellular mechanisms (e.g., how the cell prevents improper cortisol activation through $11 \beta HSD1$ in kidneys and $11 \beta HSD2$ in target cells), might require a more detailed answer of $2-3$ sentences.

• Concerning the pituitary's blood supply, the focus should be on understanding the portal system and its significance for hormone transport, rather than memorizing specific arteries or veins.

• For pituitary embryology, students should refer to their notes for specific transcription factors and signals discussed.

Thyroid Hormone Transporters and Muscle Fiber Type (Based on Figure 2)

• The study investigated the impact of thyroid hormone transporters on $T3$ and $T4$ profiles and downstream activity within muscle tissue.

• Muscle Selection: Soleus (predominantly slow-twitch muscle, shown by more slow MHC) and Plantaris (predominantly fast-twitch muscle, shown by more fast MHC) were chosen in mice, as they represent these distinct muscle fiber types.

• Wild Type (Control) Observations:

  • Soleus muscle exhibited approximately <6\% fast MHC and about $40\%$ slow MHC.

  • Plantaris muscle showed primarily fast MHC, with levels around $95\%$ or higher.

• Effect of Oatp1c1 Knockout (Thyroid Hormone Transporter): In the soleus muscle, there was a decrease in fast MHC expression and an increase in slow MHC expression.

• Effect of Double Knockout (MCT8 and Oatp1c1): An overall increase in fast MHC expression was observed in both muscle types.

• Conclusion: The thyroid hormone transporter MCT8 appears to play a more significant role in determining the expression of specific MHC types. Generally, knocking out these thyroid hormone transporters leads to an increase in fast MHC expression over slow MHC.

PAX7 and Stem Cells (Based on Figure 4)

• PAX7: Serves as a universal marker for identifying stem cells, not exclusively within muscle tissue.

• Observation: Muscles that lack thyroid hormone transporters demonstrate an increase in PAX7 positive cells, indicating a higher number of stem cells, suggesting a role for thyroid hormone in muscle activation.

• Vehicle Control:

  • A vehicle (e.g., water, saline) is the solvent in which a drug is dissolved for injection or administration.

  • Vehicle controls are crucial in research to ensure that observed effects are due to the active compound and not the vehicle itself or the stress associated with the injection/treatment.

  • Example: When injecting a drug dissolved in saline, a control group receiving only saline injection accounts for potential effects of the saline or the physical injection process.

  • DMSO (Dimethyl sulfoxide): Often used for hydrophobic drugs, DMSO is known to be biologically active in cell culture systems and thus absolutely requires a vehicle control group to distinguish its effects from the drug's effects.

• Sham Surgery:

  • A sham surgery is a control procedure in which an animal undergoes all aspects of a surgical intervention (anesthesia, incision, tissue handling, suturing) except the actual experimental manipulation (e.g., organ removal).

  • This control accounts for the trauma, stress, and recovery associated with surgery, allowing for a more accurate comparison with experimental groups.

Muscle Damage and Regeneration (Based on Figure 6)

• The study examined PAX7 (stem cells) and MyoG (myogenin, a marker for stem cell activation) in double knockout mice, both uninjured and following injury.

• Baseline Damage in Knockouts:

  • Uninjured double knockout mice showed an elevated presence of PAX7 positive stem cells at baseline.

  • Increased TUNEL staining (a marker for cell apoptosis) was observed across all knockout groups.

  • Interpretation: These findings suggest that muscle damage already exists at baseline in knockout mice, prior to any induced injury.

• Post-Injury Response:

  • At $10$ and $21$ days post-CTX (cardiotoxin) induced injury, there was no significant difference in PAX7 levels.

  • However, MyoG expression, indicative of stem cell activation, was decreased in both MCT8 knockout and double knockout mice compared to controls. This implies that while stem cells are present, their activation is impaired or significantly slower in the absence of these transporters.

• Regenerative Process: Muscle regeneration typically involves critical initial changes, particularly within the first $3$ days, followed by a time-dependent, but not constantly activated, process.

Conditional Knockouts and Fiber Repair (Based on Figure 7)

• This figure used a conditional knockout approach for MCT8 and Oatp1c1, allowing for gene deletion in adult mice. This method helps eliminate potential confounding factors related to developmental effects.

• Observations on Fiber Repair:

  • In uninjured muscles, fiber growth was comparable between control and knockout groups.

  • Following injury, a significant difference in fiber repair was observed, with knockouts showing impaired repair compared to controls. This indicates that the knocked-out thyroid hormone transporters are crucial for effective muscle fiber repair.

• Key Takeaways from the Paper:

  • MCT8 and Oatp1c1: These are critical thyroid hormone transporters. Thyroid hormone is not hydrophobic and requires specific transporters to cross cell membranes (e.g., when exiting the thyroid gland or entering target tissues).

  • Thyroid Hormone's Broader Role: Beyond its well-known systemic metabolic effects, thyroid hormone plays a vital role in local processes such as muscle function and regeneration.

Introduction to the Adrenal Medulla

• Anatomy: The adrenal gland is situated on top of the kidney (hence "ad-renal").

• Functional Divisions of the Adrenal Gland: It is functionally divided into four distinct sections:

  • Adrenal Cortex (Outer layers): Comprises three regions, each producing different steroid hormones:

    • Zona Glomerulosa: Produces aldosterone.

    • Zona Fasciculata: Produces cortisol.

    • Zona Reticularis: Produces DHEA (Dehydroepiandrosterone).

  • Adrenal Medulla (Inner region): Structurally distinct from the cortex, it is the primary site of epinephrine production.

• Significance of Blood Supply: Arteries feed the cortex from the outside, and veins drain from the center. Blood flows sequentially through all cortical regions, collecting various steroids, before reaching the medulla to pick up epinephrine, and finally moving into the venous system. This specific blood flow is critical for epinephrine synthesis.

Adrenal Medulla and the Sympathetic Nervous System

• Modified Sympathetic Tissue: The adrenal medulla is essentially a modified component of the sympathetic nervous system.

• Autonomic Nervous System Pathway: Typically, the autonomic nervous system involves a preganglionic neuron synapsing with a postganglionic neuron, which then innervates a target tissue (e.g., norepinephrine release onto blood vessels or the heart).

• Adrenal Medulla's Unique Pathway: In the case of the adrenal medulla, a preganglionic sympathetic fiber directly innervates the chromaffin cells within the medulla, bypassing the postganglionic neuron. This means that activation of the sympathetic nervous system directly triggers the adrenal medulla.

• Fight or Flight Response: During a fight or flight response, the sympathetic nervous system concurrently releases norepinephrine to various organs (e.g., increasing heart rate, vasoconstriction) and activates the adrenal medulla, which then releases epinephrine (and some norepinephrine) into the bloodstream.

• Hormonal Redundancy: This dual mechanism provides redundancy; the nervous system directly influences organs, while circulating epinephrine acts as a hormone to reinforce and sustain these fight or flight responses throughout the body.

Epinephrine Synthesis

• Precursor: The synthesis of epinephrine begins with the amino acid tyrosine.

• Key Enzyme: Phenylethanolamine N-methyltransferase (PNMT) is the enzyme responsible for the final conversion step.

• Cortisol's Role: The adrenal medulla is exposed to high levels of cortisol because blood flows through the adrenal cortex first. Cortisol induces the expression of PNMT, which is essential for epinephrine synthesis.

• Synthesis Pathway in Chromaffin Cells:

  1. Cytoplasm: Tyrosine is converted to L-DOPA, then to dopamine.

  2. Granule Entry: Dopamine is transported into chromaffin granules via a countercurrent exchange mechanism with hydrogen ions.

  3. Within Granule: Dopamine is converted into norepinephrine inside the granule.

  4. Cytoplasm Re-entry: Norepinephrine is shuttled back out into the cytoplasm.

  5. Cytoplasm: PNMT converts norepinephrine into epinephrine in the cytoplasm.

  6. Granule Storage: Epinephrine then re-enters the granule for storage and subsequent release.

• Storage and Release: Epinephrine is stored in chromaffin granules. Its release via exocytosis is triggered by an increase in intracellular calcium levels.

• Stimulus for Calcium Increase: Acetylcholine, released from the preganglionic sympathetic neuron, binds to the chromaffin cell, leading to increased calcium and exocytosis.

• Hormones Released: Upon stimulation, the adrenal medulla primarily releases epinephrine (approximately $85\%$ of granule content), with a smaller amount of norepinephrine (approximately $15\%$) and potentially trace amounts of dopamine.

• Nomenclature: Historically, these hormones were called adrenaline and noradrenaline (due to their origin in the adrenal gland). The current preferred terms are epinephrine and norepinephrine. However, their receptors are still referred to as adrenergic receptors.

Epinephrine Action and Receptor Regulation

• Short Half-Life: Epinephrine has a relatively short half-life, which is consistent with the transient nature of immediate fight or flight responses.

• Context-Dependent Effects: The effects of epinephrine are highly dependent on the specific receptor type it binds to and the target tissue where the receptor is located.

• Example (Smooth Muscle):

  • Vascular Smooth Muscle: Epinephrine binds to specific adrenergic receptors, causing muscle contraction (vasoconstriction), which increases total peripheral resistance (TPR) and elevates blood pressure (part of the fight or flight response).

  • Intestinal Smooth Muscle: Epinephrine binds to different adrenergic receptors, causing muscle relaxation (decreasing intestinal motility), diverting resources during fight or flight.

• Second Messenger Pathways: Different adrenergic receptor subtypes are coupled to distinct second messenger pathways:

  • Receptors causing contraction might be linked to pathways that increase intracellular calcium.

  • Receptors causing relaxation might be linked to pathways that inhibit or reduce calcium.

  • Alpha-2 receptors, for example, typically inhibit cyclic AMP ($cAMP$).

  • Alpha-1 receptors are often linked to the $IP3$ (inositol triphosphate) / diacylglycerol pathway.

• Receptor Regulation: Adrenergic receptors undergo regulation:

  • Desensitization / Downregulation: Chronic and persistent exposure to high levels of epinephrine (e.g., in sustained high blood pressure conditions) can lead to a decrease in receptor number or sensitivity. This is considered a protective mechanism to prevent prolonged overstimulation of cells.

  • Upregulation: Receptors can also increase in number or sensitivity. This can be a permissive action, where other hormones like thyroid hormone or glucocorticoids (cortisol) can upregulate the expression of adrenergic receptors, making tissues more responsive to epinephrine.

Overall Effects of Epinephrine (Fight or Flight Response)

• Primary Goals: The main physiological outcomes of epinephrine release during a fight or flight response are to increase cardiovascular activity and elevate energy availability.

• Specific Functional Outcomes:

  • Lungs: Enhanced oxygen intake and gas exchange to provide more oxygen to the body.

  • Cardiovascular System: Significantly increased cardiac output, meaning more blood is pumped faster throughout the body.

  • Metabolic Tissues (Liver, Fat, Adipose tissue): Increased mobilization of energy sources, such as glucose and ketones, into the bloodstream.

• Integrated Response: These actions collectively drive an increased supply of nutrients and oxygen to critical tissues (like muscles and the brain) to sustain the fight or flight response.

Pathological Condition

• Pheochromocytoma: This is a primary disease of the adrenal medulla characterized by a tumor that leads to abnormally high levels of epinephrine production, resulting in an exaggerated and persistent fight or flight response throughout the body.