Fig. 2. General scheme of intracellular Ca²⁺ cycling. Ca²⁺ can enter the cell via Ca²⁺ channels and, in some situations, via the Na⁺—Ca²⁺ exchanger (NCX). Ca²⁺ can be released from sarcoplasmic/endoplasmic reticulum (SR/ER) via Ca²⁺-release channels, including ryanodine receptors and inositol (1,4,5)-trisphosphate receptors. Ca²⁺ is removed from the cytosol by SR/ER Ca²⁺-ATPase (SERCA), cytolemmal Ca²⁺-ATPase, Na⁺—Ca²⁺ exchange and the mitochondrial uniportor.