Calcium homeostasis: transport and storage
The myocardial cell carefully controls the concentration of calcium ions ($Ca^{2+}$), which is essential for the alternation of contraction and relaxation.
- Extrusion from the cell: Occurs via the sodium-calcium exchanger (exchange mechanism), where three sodium ions ($Na^+$) enter in exchange for the extrusion of one calcium ion.
- Cytoplasmic binding: Free calcium is able to form complexes with the protein calmodulin.
- Storage in the sarcoplasmic reticulum (SR): Active transport pumps calcium into the SR using an ATP-dependent transporter. Inside the store, calcium binds to the protein calsequestrin, which allows for the storage of significant amounts of the ion.
- Release mechanism: Desequestration is triggered via ryanodine receptors (calcium channels of the SR membrane) under the action of externally derived calcium (calcium-induced calcium release).
Physiological mechanism of contraction
The contractile act in a cardiomyocyte represents a precise sequence of events translating an electrical impulse into mechanical work:
- Initiation (Depolarization): Sodium channels open, and $Na^+$ ions rush into the cell, depolarizing the membrane.
- Trigger calcium influx: Depolarization activates voltage-gated calcium channels, providing an influx of $Ca^{2+}$ from the extracellular space.
- Massive release: Extracellular calcium opens SR ryanodine receptors, releasing the main stores of $Ca^{2+}$ into the cytoplasm.
- Protein interaction: Calcium ions bind to Troponin C (a component of the troponin-tropomyosin complex). This alters the conformation of the complex and removes the inhibitory effect on actin and myosin.
- Result: The association of actin and myosin is enhanced, leading to cardiomyocyte contraction.
Neural regulation and cardiac reflexes
Myocardial function and cardiac output are influenced by central mechanisms and reflex arcs:
- Central influences: Cardiac glycosides can cross the blood-brain barrier and exert a direct stimulating effect on the vagus nerve nuclei in the medulla oblongata.
- Cardio-cardiac reflex: Stimulation of vagal sensory endings directly in the myocardium increases afferent impulse transmission to the brain, reflexively increasing the tone of the vagus nerve nuclei.
- Baroreceptor reflex: An increase in cardiac output stimulates baroreceptors of the aortic arch and carotid sinuses. The signal enters the nucleus tractus solitarii, which secondarily increases vagal tone.
Briefly: answers to topical questions
What is the difference between the sarcoplasmic reticulum calcium pump and the sodium-calcium exchanger?
The sarcoplasmic reticulum calcium pump and the sodium-calcium exchanger differ in localization, operating mechanism, and the direction of calcium ion transport.
| Characteristic | SR calcium pump | Sodium-calcium exchanger |
|---|---|---|
| Localization | Sarcoplasmic reticulum membrane | Sarcolemma |
| Direction of transport | From the cytoplasm into the lumen of the reticulum | From the cell into the extracellular fluid |
| Mechanism | Active transport (ATP-dependent) | Secondary active transport |
| Features | Transport against the concentration gradient | 3 $Na^+$ ions enter in exchange for 1 $Ca^{2+}$ ion |
Calcium cascade: "Trigger entry from outside $\rightarrow$ Ryanodine receptor opened SR $\rightarrow$ Troponin C bound calcium $\rightarrow$ Actin and myosin contracted".
Class and exam questions
What is the main mechanism for calcium extrusion from the cardiomyocyte at rest?
Elimination is carried out via the sodium-calcium exchanger, where three sodium ions enter the cell in exchange for the extrusion of one calcium ion.
Which protein is responsible for calcium storage inside the sarcoplasmic reticulum?
Inside the sarcoplasmic reticulum, calcium binds (is sequestered) to the protein calsequestrin, which allows large reserves to be stored.
Which specific protein does calcium bind to in order to trigger contraction?
Calcium ions bind to Troponin C, which is part of the troponin-tropomyosin complex of myofilaments.
What else to study on this topic
- Stoichiometry of the sodium-calcium exchange mechanism
- The role of calsequestrin in calcium storage
- Structure and function of sarcoplasmic reticulum ryanodine receptors
- Features of the cardiocardiac reflex
- Mechanisms of the baroreceptor pressor-depressor reflex
- Pharmacological effects on vagus nerve nuclei
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