Certain glucose transporters that are expressed predominantly on skeletal muscle cells and adipocytes are unique compared to those transporters found on other cell types within the body. Without directly affecting glucose transport in other cell types, which of the following would be most likely to selectively increase glucose uptake in skeletal muscle cells and adipocytes?
Q132
An investigator is studying obesity in mice. Over the course of 2 weeks, mice in the experimental group receive a daily injection with a synthetic analog of an endogenous hormone. Compared to the control group, the hormone-injected mice eat more and gain significantly more weight. Which of the following is the most likely explanation for the observed weight gain in the experimental group?
Q133
A healthy 22-year-old male participates in a research study you are leading to compare the properties of skeletal and cardiac muscle. You conduct a 3-phased experiment with the participant. In the first phase, you get him to lift up a 2.3 kg (5 lb) weight off a table with his left hand. In the second phase, you get him to do 20 burpees, taking his heart rate to 150/min. In the third phase, you electrically stimulate his gastrocnemius with a frequency of 50 Hz. You are interested in the tension and electrical activity of specific muscles as follows: Biceps in phase 1, cardiac muscle in phase 2, and gastrocnemius in phase 3. What would you expect to be happening in the phases and the respective muscles of interest?
Q134
An investigator is studying the function of the lateral nucleus of the hypothalamus in an experimental animal. Using a viral vector, the genes encoding chloride-conducting channelrhodopsins are injected into this nucleus. Photostimulation of the channels causes complete inhibition of action potential generation. Persistent photostimulation is most likely to result in which of the following abnormalities in these animals?
Cardiovascular US Medical PG Practice Questions and MCQs
Question 131: Certain glucose transporters that are expressed predominantly on skeletal muscle cells and adipocytes are unique compared to those transporters found on other cell types within the body. Without directly affecting glucose transport in other cell types, which of the following would be most likely to selectively increase glucose uptake in skeletal muscle cells and adipocytes?
A. Increased plasma glucose concentration
B. It is physiologically impossible to selectively increase glucose uptake in specific cells
C. Increased levels of circulating insulin (Correct Answer)
D. Decreased plasma glucose concentration
E. Decreased levels of circulating insulin
Explanation: ***Increased levels of circulating insulin***
- Insulin stimulates the translocation of **GLUT4 transporters** from intracellular vesicles to the cell membrane in **skeletal muscle** and **adipocytes**, thereby increasing glucose uptake.
- This mechanism is **selective** because other cell types (e.g., brain, liver) primarily use insulin-independent glucose transporters (e.g., GLUT1, GLUT2, GLUT3) that are constitutively active or respond to different signals.
*Increased plasma glucose concentration*
- While increased glucose concentration would drive glucose uptake in many cells, it is not **selective** for skeletal muscle and adipocytes since other cells also take up glucose.
- Insulin-independent tissues would also increase glucose uptake, making this a non-specific effect.
*It is physiologically impossible to selectively increase glucose uptake in specific cells*
- This statement is incorrect because the body has mechanisms, such as **insulin-mediated GLUT4 translocation**, that specifically regulate glucose uptake in certain cell types like skeletal muscle and adipocytes.
- This regulatory specificity is crucial for maintaining **glucose homeostasis**.
*Decreased plasma glucose concentration*
- A decrease in plasma glucose would generally **reduce** glucose uptake across all cell types, including skeletal muscle and adipocytes.
- It would not selectively increase uptake in any specific cell population.
*Decreased levels of circulating insulin*
- Decreased insulin levels would lead to **reduced** glucose uptake in insulin-sensitive tissues like skeletal muscle and adipocytes, as GLUT4 transporters would remain sequestered intracellularly.
- This would result in higher circulating glucose levels rather than increased uptake.
Question 132: An investigator is studying obesity in mice. Over the course of 2 weeks, mice in the experimental group receive a daily injection with a synthetic analog of an endogenous hormone. Compared to the control group, the hormone-injected mice eat more and gain significantly more weight. Which of the following is the most likely explanation for the observed weight gain in the experimental group?
A. Leptin stimulation of the ventromedial hypothalamus
B. Somatostatin inhibition of the anterior pituitary
C. Cholecystokinin stimulation of the nucleus tractus solitarius
D. Ghrelin stimulation of the lateral hypothalamus (Correct Answer)
E. Glucagon stimulation of hepatocytes
Explanation: ***Ghrelin stimulation of the lateral hypothalamus***
- **Ghrelin** is a hormone that stimulates **hunger** and promotes **weight gain**. Stimulation of the **lateral hypothalamus** specifically leads to increased appetite and food intake.
- The experimental mice are exhibiting increased eating and weight gain consistent with the effects of an analogue that mimics or enhances ghrelin's action.
*Leptin stimulation of the ventromedial hypothalamus*
- **Leptin** is a hormone that suppresses appetite and induces satiety, promoting **weight loss**.
- Stimulation of the **ventromedial hypothalamus** also leads to satiety and reduced food intake, which is contrary to the observed weight gain.
*Somatostatin inhibition of the anterior pituitary*
- **Somatostatin** primarily inhibits the release of various hormones, including **growth hormone** and **thyroid-stimulating hormone**, from the anterior pituitary.
- It does not directly regulate appetite or mediate acute changes in food intake and weight in the manner described.
*Cholecystokinin stimulation of the nucleus tractus solitarius*
- **Cholecystokinin (CCK)** is a gut hormone involved in digestion and induces **satiety** and reduces food intake.
- Its action on the **nucleus tractus solitarius** contributes to feelings of fullness, which would lead to decreased eating and weight, not increased.
*Glucagon stimulation of hepatocytes*
- **Glucagon** primarily functions to raise blood glucose levels by stimulating **glycogenolysis** and **gluconeogenesis** in hepatocytes.
- While it plays a role in metabolism, it does not directly drive increased food intake and weight gain as the primary mechanism for the observed effect.
Question 133: A healthy 22-year-old male participates in a research study you are leading to compare the properties of skeletal and cardiac muscle. You conduct a 3-phased experiment with the participant. In the first phase, you get him to lift up a 2.3 kg (5 lb) weight off a table with his left hand. In the second phase, you get him to do 20 burpees, taking his heart rate to 150/min. In the third phase, you electrically stimulate his gastrocnemius with a frequency of 50 Hz. You are interested in the tension and electrical activity of specific muscles as follows: Biceps in phase 1, cardiac muscle in phase 2, and gastrocnemius in phase 3. What would you expect to be happening in the phases and the respective muscles of interest?
A. Increase of tension in experiments 2 and 3, with the same underlying mechanism
B. Increase of tension in all phases (Correct Answer)
C. Recruitment of large motor units followed by small motor units in experiment 1
D. Fused tetanic contraction at the end of all three experiments
E. Recruitment of small motor units at the start of experiments 1 and 2
Explanation: ***Increase of tension in all phases***
- In **phase 1**, lifting a 2.3 kg weight requires the **biceps** to contract, generating sufficient force (**tension**) to overcome gravity.
- In **phase 2**, the **cardiac muscle** increases its contractile force (**tension**) to meet the metabolic demands of **exercise**, leading to a heart rate of 150/min.
- In **phase 3**, electrical stimulation of the **gastrocnemius** at 50 Hz triggers muscle contraction, leading to an increase in **tension**.
*Increase of tension in experiments 2 and 3, with the same underlying mechanism*
- While tension increases in phases 2 and 3, the **underlying mechanisms differ**: cardiac muscle tension increases due to increased sympathetic stimulation and preload, while skeletal muscle tension increases due to unfused or fused tetanus from electrical stimulation.
- Cardiac muscle contraction is regulated by **calcium-induced calcium release**, while skeletal muscle involves direct coupling of DHP receptor and ryanodine receptor.
*Recruitment of large motor units followed by small motor units in experiment 1*
- **Motor unit recruitment** follows the **size principle**, meaning smaller, more easily excitable motor units are activated first, followed by larger ones as more force is needed.
- Therefore, in phase 1, **small motor units** would be recruited first, not large ones.
*Fused tetanic contraction at the end of all three experiments*
- **Fused tetanic contraction** occurs in **skeletal muscle** when stimulation frequency is high enough that individual twitches summate completely, leading to sustained contraction.
- This phenomenon is **not possible in cardiac muscle** due to its long **refractory period**, which prevents sustained contraction and allows for adequate filling time.
*Recruitment of small motor units at the start of experiments 1 and 2*
- **Motor unit recruitment** applies to **skeletal muscle** (phase 1) and involves recruiting small motor units first for fine or gentle movements.
- **Cardiac muscle** (phase 2) does not have motor units; instead, it relies on the **Frank-Starling mechanism** and hormonal/nervous regulation to adjust its contractile force as a syncytium.
Question 134: An investigator is studying the function of the lateral nucleus of the hypothalamus in an experimental animal. Using a viral vector, the genes encoding chloride-conducting channelrhodopsins are injected into this nucleus. Photostimulation of the channels causes complete inhibition of action potential generation. Persistent photostimulation is most likely to result in which of the following abnormalities in these animals?
A. Hyperthermia
B. Anorexia (Correct Answer)
C. Nocturnal hyperactivity
D. Polydipsia
E. Hypothermia
Explanation: ***Anorexia***
- The **lateral nucleus of the hypothalamus** is primarily associated with **hunger** and **feeding behavior**; its destruction or inhibition leads to aphagia and reduced food intake.
- Inhibiting action potential generation in this region would mimic a lesion, preventing the animal from feeling hunger and consequently leading to anorexia.
*Hyperthermia*
- The **anterior hypothalamus** is more involved in **heat dissipation** (cooling), while the posterior hypothalamus handles heat conservation.
- Disrupting the lateral hypothalamus would not directly lead to hyperthermia.
*Nocturnal hyperactivity*
- The **suprachiasmatic nucleus** and other areas of the hypothalamus regulate circadian rhythms, but the lateral hypothalamus is not primarily associated with activity levels or sleep-wake cycles in this manner.
- Activation or inhibition of the lateral hypothalamus typically affects feeding, not general activity levels in a nocturnal pattern.
*Polydipsia*
- **Thirst regulation** is primarily attributed to the **supraoptic** and **paraventricular nuclei** and the **organum vasculosum of the lamina terminalis (OVLT)**.
- The lateral hypothalamus also plays a role in **drinking behavior**, and its inhibition would cause **adipsia** (decreased drinking), not polydipsia (increased drinking).
*Hypothermia*
- While the hypothalamus regulates body temperature, direct inhibition of the lateral **"hunger center"** would not cause systemic hypothermia.
- **Heat conservation** and **production** are more closely linked to the **posterior hypothalamus**.
