Ca2+ is important for cellular signalling, for once it enters the cytosol of the cytoplasm it exerts allosteric regulatory effects on many enzymes and proteins.
[1][2] To maintain this low concentration, Ca2+ is actively pumped from the cytosol to the extracellular space, the endoplasmic reticulum (ER), and sometimes into the mitochondria.
[3] Specific signals can trigger a sudden increase in the cytoplasmic Ca2+ levels to 500–1,000 nM by opening channels in the ER or the plasma membrane.
Recent studies have cited the phospholipase A2 beta,[6] nicotinic acid adenine dinucleotide phosphate (NAADP),[7] and the protein STIM 1[8] as possible mediators of ICRAC.
[2] These include muscle contraction, neuronal transmission (as in an excitatory synapse), cellular motility (including the movement of flagella and cilia), fertilization, cell growth (proliferation), neurogenesis, learning and memory as with synaptic plasticity, and secretion of saliva.
Once depolarized the sarcoplasmic reticulum (SR) releases Ca2+ into the myoplasm where it will bind to a number of calcium sensitive buffers.
When a Ca2+ influx occurs, cross bridges form between myosin and actin leading to the contraction of the muscle fibers.
The second is a rapid time dependent change in the membrane potential which leads to a very quick and uniform increase of Ca2+.
This type of release increases the activation of protein kinase, and is seen in cardiac muscle where it causes excitation-concentration coupling.
These influxes are often spontaneous and localized as seen in the colon and portal vein, but may lead to a global Ca2+ wave as observed in many vascular tissues.
These Ca2+ signals integrate extracellular and intracellular fluxes, and have been implicated to play roles in synaptic plasticity, memory, neurotransmitter release, neuronal excitability, and long term changes at the gene transcription level.
These transmitters allow communication between neurons and are triggered by calcium levels increasing around astrocytes from inside stores.
[23][24] Once the Ca2+ is released from the ER the egg starts the process of forming a fused pronucleus and the restart of the mitotic cell cycle.