sarcoplasmic reticulum calcium ion transport

Definition

Target type: biologicalprocess

The directed movement of calcium ions (Ca2+) into, out of or within the sarcoplasmic reticulum. [GOC:BHF, GOC:vk]

Sarcoplasmic reticulum (SR) calcium ion transport is a fundamental process in muscle contraction and relaxation. It involves the movement of calcium ions (Ca2+) across the SR membrane, a specialized intracellular organelle within muscle cells. Here's a detailed breakdown:

1. **Excitation-Contraction Coupling:** The process begins with a nerve impulse arriving at the neuromuscular junction, triggering the release of acetylcholine. Acetylcholine binds to receptors on the muscle fiber membrane, causing depolarization. This depolarization travels along the muscle fiber membrane and into the T-tubules, which are invaginations of the membrane that run deep into the muscle fiber.

2. **Ca2+ Release:** The depolarization of the T-tubules activates voltage-sensitive receptors called dihydropyridine receptors (DHPRs). DHPRs are physically linked to calcium release channels (ryanodine receptors, RyRs) on the SR membrane. Activation of the DHPRs triggers the opening of RyRs, allowing Ca2+ to flood out of the SR into the sarcoplasm, the cytoplasm of the muscle cell.

3. **Muscle Contraction:** The increased concentration of Ca2+ in the sarcoplasm binds to the troponin complex, a protein associated with the thin filament (actin) of the muscle fiber. This binding causes a conformational change in troponin, which moves tropomyosin, another protein associated with actin, away from the myosin-binding sites on actin. Now, the myosin heads, the motor proteins of the thick filament, can bind to actin and initiate the sliding filament mechanism. This results in muscle contraction.

4. **Ca2+ Reuptake:** After the muscle fiber is activated, Ca2+ must be removed from the sarcoplasm to allow relaxation. This is achieved by the SR Ca2+ ATPase (SERCA) pump, an active transporter located in the SR membrane. SERCA uses energy from ATP hydrolysis to pump Ca2+ back into the SR lumen, against its concentration gradient. This process reduces Ca2+ levels in the sarcoplasm, allowing troponin to return to its resting state, and tropomyosin to cover the myosin-binding sites on actin, leading to muscle relaxation.

5. **Regulation of Ca2+ Transport:** The activity of SERCA is regulated by various factors, including:
* **Phosphorylation:** Phosphorylation by protein kinases, such as calcium/calmodulin-dependent protein kinase (CaMK) and protein kinase A (PKA), can activate SERCA, enhancing its Ca2+ pumping capacity.
* **Ca2+ Concentration:** High Ca2+ levels in the sarcoplasm can actually inhibit SERCA, providing a feedback mechanism to fine-tune Ca2+ handling.
* **Other Factors:** Other proteins, like phospholamban (PLN), can also influence SERCA activity, acting as an inhibitor or activator depending on its phosphorylation state.

6. **Role of SR Ca2+ Transport in Muscle Function:** The efficient and regulated transport of Ca2+ by the SR is essential for muscle function. It enables rapid and precise control of muscle contraction and relaxation, allowing for a wide range of muscle activities, from sustained isometric contractions to rapid twitch movements.'
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Proteins (3)

Compounds (9)