Metabolic Adaptations During Exercise — MCQs
Glucagon acts on liver to cause:
All of the following are features of exercise EXCEPT:
Reducing equivalents produced in glycolysis are transported from cytosol to mitochondria by ?
Which of the following processes primarily utilizes lactate produced anaerobically?
During heavy exercise the cardiac output (CO) increases up to five fold while pulmonary arterial pressure rises very little. This physiological ability of the pulmonary circulation is best explained by
Growth hormone secretion is decreased by
Final common pathway of metabolism of carbohydrate, lipids, and protein metabolism is?
During a 100 m sprint which of the following is used by the muscle for meeting energy demands?
Which of the following processes is inhibited by glucagon?
During moderate exercise, the respiratory rate increases in response to which of the following?
Metabolic Adaptations During Exercise Indian Medical PG Practice Questions and MCQs
Question 1: Glucagon acts on liver to cause:
- A. Kreb's cycle
- B. Glycolysis
- C. Gluconeogenesis
- D. Glycogenolysis (Correct Answer)
Explanation: ***Glycogenolysis*** * **Glucagon's primary role** is to increase blood glucose levels during fasting states * Glucagon binds to **glucagon receptors on hepatocytes** (liver cells) * This activates **adenylyl cyclase → cAMP → protein kinase A** cascade * PKA phosphorylates and activates **phosphorylase kinase**, which activates **glycogen phosphorylase** * Result: **Glycogen breakdown to glucose-1-phosphate → glucose-6-phosphate → free glucose** (via glucose-6-phosphatase in liver) * The free glucose is released into bloodstream to maintain blood glucose levels *Kreb's cycle* * The **Kreb's cycle (citric acid cycle)** oxidizes acetyl-CoA to produce ATP, NADH, and FADH2 * It is not directly stimulated by glucagon; its activity is regulated by **substrate availability** and **energy demands** (ATP/ADP ratio, NADH/NAD+ ratio) * Glucagon's effects are primarily on glucose homeostasis, not directly on oxidative metabolism *Glycolysis* * **Glycolysis** breaks down glucose into pyruvate, generating ATP * Glucagon **inhibits glycolysis** in the liver by decreasing fructose-2,6-bisphosphate levels * This inhibition makes sense: glucagon's role is to **increase** blood glucose, not consume it * Glucagon activates phosphodiesterase which reduces cAMP levels needed for PFK-2 activity *Gluconeogenesis* * While glucagon does **stimulate gluconeogenesis** in the liver (glucose synthesis from non-carbohydrate sources) * The question asks about the direct action, and **glycogenolysis is the primary and immediate response** to glucagon * Gluconeogenesis is a slower process that becomes more important during prolonged fasting
Question 2: All of the following are features of exercise EXCEPT:
- A. Increased blood supply to muscles
- B. Increased O₂ extraction
- C. Increased stroke volume
- D. Left shift of Hb-O₂ dissociation curve (Correct Answer)
Explanation: ***Left shift of Hb-O₂ dissociation curve*** - During exercise, **tissue metabolism** increases, leading to higher levels of **CO₂, H⁺, and 2,3-BPG**, and higher temperature which all cause a **right shift** of the Hb-O₂ dissociation curve. - A **right shift** signifies decreased hemoglobin affinity for oxygen, facilitating **oxygen unloading** to metabolically active tissues. *Increased blood supply to muscles* - Exercise drastically increases the **metabolic demands** of skeletal muscles, requiring a greater supply of **oxygen and nutrients**. - This is achieved through **vasodilation** in the active muscles and redistribution of blood flow. *Increased O₂ extraction* - As muscles work harder during exercise, their demand for oxygen increases, leading to a higher **arteriovenous oxygen difference**. - This means that a greater percentage of the oxygen delivered to the muscle is **extracted and utilized** by the tissues. *Increased stroke volume* - The **heart pumps more blood** with each beat to meet the increased circulatory demands of exercise. - This is a key mechanism for increasing **cardiac output** during physical activity.
Question 3: Reducing equivalents produced in glycolysis are transported from cytosol to mitochondria by ?
- A. Carnitine
- B. Creatine
- C. Malate-aspartate shuttle (Correct Answer)
- D. Glutamate shuttle
Explanation: ***Malate shuttle*** - The **malate-aspartate shuttle** is a primary mechanism for transporting **NADH reducing equivalents** from the cytosol to the mitochondrial matrix for **oxidative phosphorylation**. - It involves a series of **enzymes and transporters** that indirectly move electrons from NADH by converting **oxaloacetate to malate** in the cytosol, which then enters the mitochondria. *Carnitine* - **Carnitine** is primarily involved in the transport of **long-chain fatty acids** into the mitochondrial matrix for **beta-oxidation**. - It is not directly involved in the shuttle of NADH reducing equivalents generated during glycolysis. *Creatine* - **Creatine** and its phosphorylated form, **phosphocreatine**, are crucial for **energy buffering and transport** in tissues with high and fluctuating energy demands, like muscle and brain. - The creatine-phosphocreatine shuttle facilitates the rapid regeneration of ATP, but it is not involved in transporting glycolytic reducing equivalents. *Glutamate shuttle* - While glutamate and aspartate are components of the **malate-aspartate shuttle**, there isn't a standalone "glutamate shuttle" for transporting glycolytic reducing equivalents. - The **glutamate-aspartate transaminase** is an enzyme within the malate-aspartate shuttle, converting oxaloacetate to aspartate and alpha-ketoglutarate to glutamate from the matrix to the cytosol.
Question 4: Which of the following processes primarily utilizes lactate produced anaerobically?
- A. Cori cycle (Correct Answer)
- B. Gluconeogenesis
- C. TCA cycle
- D. Glycolysis
Explanation: ***Cori cycle*** - The **Cori cycle** (lactic acid cycle) involves the transport of **lactate** produced during anaerobic metabolism in muscles to the liver. - In the **liver**, this lactate is then converted back to **glucose** via gluconeogenesis, which can be returned to the muscles. *Gluconeogenesis* - **Gluconeogenesis** is the synthesis of glucose from non-carbohydrate precursors, one of which is lactate. - While it uses lactate, it is only one component of the broader **Cori cycle**, which describes the inter-organ cooperation. *Glycolysis* - **Glycolysis** is the metabolic pathway that breaks down glucose into pyruvate, which can then be converted to lactate under anaerobic conditions. - This process *produces* lactate but does not *utilize* it, acting upstream of lactate production. *TCA cycle* - The **TCA cycle** (Krebs cycle) is a central part of aerobic respiration that oxidizes acetyl-CoA to produce ATP, NADH, and FADH2. - It does not directly utilize lactate; instead, lactate is typically converted to pyruvate before potentially entering the TCA cycle under aerobic conditions.
Question 5: During heavy exercise the cardiac output (CO) increases up to five fold while pulmonary arterial pressure rises very little. This physiological ability of the pulmonary circulation is best explained by
- A. Large amount of smooth muscle in pulmonary arterioles
- B. Increase in the number of open capillaries (Correct Answer)
- C. Sympathetically mediated greater distensibility of pulmonary vessels
- D. Smaller surface area of pulmonary circulation
Explanation: ***Increase in the number of open capillaries*** - During heavy exercise, the significant increase in cardiac output is accommodated by the **recruitment of previously closed pulmonary capillaries**. - This recruitment, along with **distension of existing capillaries**, reduces overall pulmonary vascular resistance, allowing blood flow to increase without a substantial rise in pulmonary arterial pressure. *Large amount of smooth muscle in pulmonary arterioles* - While pulmonary arterioles do contain smooth muscle, their primary role is in **regulating regional blood flow** and response to hypoxia, not facilitating large increases in overall blood flow during exercise. - The pulmonary circulation is characterized by **low resistance** and high capacitance compared to the systemic circulation, meaning it has less smooth muscle tone at baseline. *Sympathetically mediated greater distensibility of pulmonary vessels* - The pulmonary vasculature has **limited sympathetic innervation** compared to systemic vessels, and sympathetic activity plays a minor role in its distensibility during exercise. - Changes in pulmonary vascular resistance during exercise are primarily due to **mechanical factors** (recruitment and distension) rather than neurogenic control. *Smaller surface area of pulmonary circulation* - The pulmonary circulation actually has a **vast capillary surface area** crucial for efficient gas exchange. - A smaller surface area would lead to **higher resistance** and a greater pressure increase for a given flow, which contradicts the observation during exercise.
Question 6: Growth hormone secretion is decreased by
- A. Exercise
- B. Stress
- C. Glucagon
- D. Glucose (Correct Answer)
Explanation: ***Glucose*** - **High blood glucose levels** act as a negative feedback signal to the **hypothalamus** and **anterior pituitary**, inhibiting the release of Growth Hormone-Releasing Hormone (GHRH) and Growth Hormone (GH), respectively. - This physiological response helps to prevent excessive growth and metabolism when energy substrates are readily available. *Exercise* - **Physical exercise**, particularly intense activity, is a powerful stimulus for **GH secretion**, likely due to increased sympathetic activity and changes in circulating metabolites. - This GH surge during exercise contributes to **fat mobilization** and **muscle protein synthesis**. *Stress* - Both **physical and psychological stress** can significantly stimulate **GH release**, mediated by increased activity of the **hypothalamic-pituitary-adrenal (HPA) axis** and adrenaline. - GH's role here is to mobilize energy resources to cope with the stressful situation. *Glucagon* - **Glucagon**, a hormone primarily known for raising blood glucose, also has a direct stimulatory effect on **GH secretion**, especially during periods of low glucose or fasting. - This effect is utilized in certain diagnostic tests for GH deficiency.
Question 7: Final common pathway of metabolism of carbohydrate, lipids, and protein metabolism is?
- A. Gluconeogenesis
- B. TCA (Correct Answer)
- C. HMP pathway
- D. Glycolysis
Explanation: ***TCA (Tricarboxylic Acid Cycle)*** - The **TCA cycle** (also called Krebs cycle or citric acid cycle) is the **final common oxidative pathway** where all three macronutrients converge - **Carbohydrates** → Pyruvate → **Acetyl-CoA** (via pyruvate dehydrogenase) - **Lipids** → Fatty acids → **Acetyl-CoA** (via beta-oxidation) - **Proteins** → Amino acids → **Acetyl-CoA or TCA intermediates** (via deamination/transamination) - Complete oxidation of acetyl-CoA occurs in the TCA cycle, producing **NADH, FADH2, and GTP** for energy production *Gluconeogenesis* - This is a **biosynthetic pathway** that synthesizes glucose from non-carbohydrate precursors (lactate, glycerol, amino acids) - It is an **anabolic process**, not the catabolic final common pathway for energy production from all macronutrients *Glycolysis* - **Carbohydrate-specific pathway** that converts glucose to pyruvate - It is only the initial breakdown pathway for carbohydrates, not the common pathway where lipids and proteins also converge - Pyruvate from glycolysis must enter TCA cycle for complete oxidation *HMP pathway (Pentose Phosphate Pathway)* - Parallel pathway to glycolysis that generates **NADPH** (for biosynthesis and antioxidant defense) and **ribose-5-phosphate** (for nucleotide synthesis) - Processes only **glucose-6-phosphate** from carbohydrate metabolism - Not involved in lipid or protein metabolism integration
Question 8: During a 100 m sprint which of the following is used by the muscle for meeting energy demands?
- A. Phosphofructokinase
- B. Phosphocreatine (Correct Answer)
- C. Glucose 1 - phosphate
- D. Creatine phosphokinase
Explanation: ***Phosphocreatine*** - **Phosphocreatine (PCr)** is the primary energy source for a **100m sprint** (lasting 10-20 seconds). - The **ATP-PC (phosphagen) system** provides **immediate energy** by rapidly regenerating **ATP** from ADP through the transfer of a high-energy phosphate group. - This system is crucial for **short bursts of maximal intensity exercise** where energy demand exceeds the capacity of glycolysis and oxidative phosphorylation to respond quickly enough. - Phosphocreatine stores can fuel maximum effort for approximately **10-15 seconds**, making it ideal for sprint activities. *Phosphofructokinase* - **Phosphofructokinase (PFK)** is a key regulatory enzyme in **glycolysis**, not an energy substrate. - While PFK-catalyzed glycolysis contributes ATP during intense exercise, it cannot provide energy as rapidly as the phosphocreatine system. - Glycolysis becomes more prominent after the first 10-15 seconds of maximal effort. *Glucose 1-phosphate* - **Glucose 1-phosphate** is an intermediate in **glycogenolysis** (breakdown of glycogen to glucose-6-phosphate). - It is part of the pathway leading to glucose availability for glycolysis, but is not a **direct, immediate energy source** for muscle contraction. - Unlike phosphocreatine, it cannot directly regenerate ATP. *Creatine phosphokinase* - **Creatine phosphokinase (CPK)**, also known as **creatine kinase (CK)**, is the **enzyme** that catalyzes the reversible transfer of phosphate from phosphocreatine to ADP. - It facilitates the energy transfer reaction but is **not an energy substrate** itself. - The enzyme enables the phosphocreatine system to function, but the actual energy comes from phosphocreatine.
Question 9: Which of the following processes is inhibited by glucagon?
- A. Gluconeogenesis
- B. Lipolysis
- C. Glycogenolysis
- D. Glycogenesis (Correct Answer)
Explanation: ***Glycogenesis*** - **Glucagon** is a hormone that counteracts the effects of insulin, primarily to raise **blood glucose levels**. Therefore, it inhibits processes that store glucose, such as **glycogenesis** (the synthesis of glycogen from glucose). - High glucagon levels signal a need for glucose release, thus stopping processes that would remove glucose from the bloodstream. *Glycogenolysis* - **Glycogenolysis** is the breakdown of **glycogen** into **glucose**, which increases blood glucose levels. - **Glucagon** actually **stimulates**, rather than inhibits, glycogenolysis to release stored glucose from the liver. *Gluconeogenesis* - **Gluconeogenesis** is the synthesis of **glucose** from non-carbohydrate precursors (e.g., amino acids, glycerol). - **Glucagon** is a potent **stimulator** of gluconeogenesis, particularly during fasting states, to maintain blood glucose. *Lipolysis* - **Lipolysis** is the breakdown of **triglycerides** into **fatty acids** and **glycerol**, which can be used for energy or as substrates for gluconeogenesis. - **Glucagon** **stimulates** lipolysis in **adipose tissue** to provide alternative fuel sources and precursors for glucose production.
Question 10: During moderate exercise, the respiratory rate increases in response to which of the following?
- A. Increased PCO2 in arterial blood (Correct Answer)
- B. Proprioceptive feedback from muscle spindles
- C. Decreased PO2 in arterial blood
- D. Stimulation of J-receptors
Explanation: ***Increased PCO2 in arterial blood*** - This is the **marked correct answer**, though it requires clarification: during **moderate exercise**, **arterial PCO2** typically remains **stable** (~40 mmHg) because ventilation increases proportionally to CO2 production. - However, **central chemoreceptors** respond to even small oscillations in PCO2 and pH, and there is increased CO2 delivery to the respiratory center from **mixed venous blood**. - The **chemical stimulus** becomes more prominent during **intense exercise** when metabolic acidosis develops and arterial PCO2 may actually rise. - Note: The primary drivers during moderate exercise are **multifactorial**, including neural mechanisms (central command, proprioceptive feedback) and chemical factors working together. *Proprioceptive feedback from muscle spindles* - **Proprioceptors** from muscles and joints provide important **neurogenic drive** that contributes significantly to increased ventilation during moderate exercise. - This mechanism works alongside **central command** (feedforward signals from motor cortex) to initiate and sustain the ventilatory response. - While this is a major contributor, the question likely seeks the **chemical stimulus** as the "classical" answer, though modern physiology recognizes the integrated nature of exercise hyperpnea. *Decreased PO2 in arterial blood* - **Arterial PO2** typically remains **stable or increases slightly** during **moderate exercise** due to improved ventilation-perfusion matching and increased ventilation. - Significant hypoxemia triggering **peripheral chemoreceptors** occurs only during **strenuous exercise** (especially in untrained individuals), at high altitude, or in patients with cardiopulmonary disease. *Stimulation of J-receptors* - **J-receptors** (juxtacapillary receptors) in alveolar walls are stimulated by increased **pulmonary interstitial fluid**, such as in pulmonary edema or heart failure. - They cause **rapid, shallow breathing** and are not involved in the normal ventilatory response to moderate exercise.
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