Ca2i

Ca2i denotes the intracellular calcium ion concentration, typically represented as [Ca2+]i. Calcium ions act as a universal second messenger in nearly all cell types, translating electrical, chemical, and mechanical signals into diverse cellular responses.

Resting and dynamic levels: In resting cells, [Ca2+]i is very low, around 50 to 100 nanomolar. Upon stimulation, [Ca2+]i can rise to micromolar levels, with rapid, localized spikes and waves that encode information through amplitude, duration, and frequency. These dynamics enable precise control of processes such as contraction, secretion, metabolism, and gene expression.

Sources and handling: Increases in [Ca2+]i originate from two main sources: influx across the plasma membrane through voltage-gated, receptor-operated, or store-operated calcium channels, and release from intracellular stores in the endoplasmic reticulum or sarcoplasmic reticulum via IP3 receptors and ryanodine receptors. Cells restore low [Ca2+]i through pumps and exchangers, including SERCA pumps that resequester calcium into stores, plasma membrane Ca2+-ATPases, and Na+/Ca2+ exchangers. Intracellular buffers and calcium-binding proteins shape the signals.

Measurement and tools: [Ca2+]i is commonly measured with fluorescent indicators such as Fura-2, Fluo-4, and genetically encoded calcium indicators like GCaMP. These tools allow visualization of Ca2+ dynamics in space and time, though calibration can be complex.

Physiological and clinical relevance: [Ca2+]i signaling governs muscle contraction, neurotransmitter release, enzyme activity, and transcriptional programs. Dysregulation of Ca2+ signaling is linked to cardiac arrhythmias, neurodegeneration, apoptosis, and various metabolic disorders.

Terminology: Ca2i is synonymous with intracellular calcium concentration, often denoted [Ca2+]i in the literature.