A research team is investigating the effects of a novel drug on cardiac muscle contraction. They observe that the drug significantly increases the force of contraction without altering the resting membrane potential or action potential duration. Further analysis reveals that the drug directly enhances the sensitivity of the contractile proteins to calcium ions. This drug's mechanism of action is primarily affecting which phase of excitation-contraction coupling?
- AReuptake of calcium into the sarcoplasmic reticulum.
- BGeneration of action potential in cardiomyocytes.
- CRelease of calcium from the sarcoplasmic reticulum.
- DBinding of calcium to troponin.
Show answer & explanationAnswer & explanation
Correct answer: D. Binding of calcium to troponin.
The question specifies that the drug enhances the sensitivity of contractile proteins to calcium without affecting action potential or calcium release/reuptake. This points directly to the interaction between calcium and the contractile machinery. In cardiac muscle, calcium binds to troponin C, initiating the conformational changes that allow actin-myosin cross-bridge cycling. Enhancing this sensitivity means that for a given amount of calcium, a stronger contractile response is achieved, increasing the force of contraction.
Why the other options are wrong
- A. Calcium reuptake is also not altered, as the effect is on sensitivity to existing calcium, not its removal.
- B. The question states resting membrane potential and action potential duration are not altered, ruling out effects on action potential generation.
- C. Calcium release from the sarcoplasmic reticulum is not altered, as the drug enhances sensitivity, not the amount of calcium released.
Calcium-Troponin Interaction
In muscle contraction, calcium ions released from the sarcoplasmic reticulum bind to troponin C. This binding causes a conformational change in the troponin-tropomyosin complex, exposing the myosin-binding sites on actin filaments and allowing for cross-bridge formation and muscle contraction.
- Calcium binds to troponin C.
- Causes conformational shift in troponin-tropomyosin complex.
- Exposes actin-myosin binding sites.
- Initiates cross-bridge cycling and muscle contraction.
Memory trick: ECC: 'Excitement' leads to 'Calcium' binding, 'Contraction' follows.