Ca2koncentration

Ca2+ concentration refers to the amount of calcium ions (Ca2+) present in a given cellular compartment or solution. In biological systems, concentrations are described for specific locations, such as the cytosol, extracellular space, or organelle lumens. Extracellular Ca2+ is typically around 1–2 mM in mammalian plasma, while resting cytosolic free Ca2+ is about 50–100 nM. Inside organelles like the endoplasmic reticulum, luminal Ca2+ can reach much higher levels, and transient cytosolic elevations during signaling can reach micromolar concentrations.

Ca2+ functions as a universal intracellular second messenger. It translates various stimuli into cellular responses, including muscle contraction, neurotransmitter release, enzyme activity, and changes in gene expression. The specificity of the response depends on the spatial and temporal pattern of Ca2+ signals, with local microdomains near Ca2+-permeable channels reaching high concentrations, contrasted with tightly buffered, global cytosolic Ca2+ levels.

Maintenance of Ca2+ homeostasis involves a range of transporters, buffers, and storage systems. Pumps such as plasma membrane Ca2+-ATPases (PMCA) and sarco/endoplasmic reticulum Ca2+-ATPases (SERCA) remove Ca2+ from the cytosol, while exchangers like the sodium-calcium exchanger (NCX) contribute to fluxes across membranes. Mitochondria and the endoplasmic reticulum act as major intracellular reservoirs, connected to the cytosol by channels and the mitochondrial calcium uniporter. Calcium-binding proteins and buffers shape signaling dynamics, with techniques like Fura-2, Fluo-4, and genetically encoded indicators (GCaMP) used to measure Ca2+ levels in research.

Disruptions in Ca2+ concentration are linked to various diseases, including cardiac arrhythmias, neurodegenerative disorders, and muscle dysfunction, highlighting its central role in physiology and pathology.