What is the voltage that causes the sarcoplasmic reticulum to release calcium ions into the cell? Which integral membrane protein is responsible?
The correct answer and explanation is:
The voltage that triggers the sarcoplasmic reticulum (SR) to release calcium ions into the muscle cell is typically around -40 to -50 millivolts. This is the threshold potential at which depolarization of the muscle cell membrane occurs, initiating the process of excitation-contraction coupling.
The integral membrane protein responsible for the release of calcium ions from the SR is the ryanodine receptor (RyR). The RyR is a calcium channel located in the membrane of the sarcoplasmic reticulum. It plays a critical role in muscle contraction by facilitating the release of calcium ions into the cytoplasm once the muscle cell membrane is depolarized.
Here’s how the process works:
- Action Potential: The process begins when an action potential travels along the muscle cell membrane (sarcolemma) and reaches the T-tubules (transverse tubules).
- Depolarization: The depolarization of the T-tubules causes a conformational change in the dihydropyridine receptor (DHPR), which is another voltage-gated integral membrane protein located in the T-tubule membrane.
- Conformational Coupling: The DHPR is physically connected to the ryanodine receptor on the SR membrane. When the DHPR undergoes a conformational change due to the depolarization, this change is transmitted to the ryanodine receptor.
- Calcium Release: The activation of the ryanodine receptor opens calcium channels, allowing calcium ions to flow from the SR into the cytoplasm of the muscle cell.
- Muscle Contraction: The increase in intracellular calcium concentration triggers the interaction of actin and myosin, leading to muscle contraction.
This calcium release mechanism is crucial for muscle contraction. The process ends when calcium is actively pumped back into the SR by SERCA (sarcoplasmic/endoplasmic reticulum calcium ATPase), which is a pump that helps relax the muscle by lowering the calcium concentration in the cytoplasm.
In summary, the ryanodine receptor is the integral membrane protein that responds to voltage changes, enabling calcium release from the SR, which is essential for muscle contraction.