In every cell, the cytosolic concentration of free ionized calcium (called the \(\mathrm {Ca}^{2+}\) concentration or \([\mathrm {Ca}^{2+}]\) ) is critically important for a vast array of functions including excitation–contraction coupling (Chaps. 15 and 12 ), exocytosis (Chaps. 8 and 9 ), fluid transport in exocrine epithelia (Chap. 18 ), synaptic plasticity in both presynaptic and postsynaptic neurons (Chap. 8 ), gene regulation and differentiation in every cell type, cell movement , light adaptation in vertebrate photoreceptors (Chap. 19 ), and electrical bursting in neuroendocrine cells (Chap. 9 ). It is one of the most important and ubiquitous cellular second messengers .