Calcium

Calcium is used in many different signaling pathways, i.e. through <a href="G-proteins">G proteins</a>, <a href="Ion channels">ion channel</a>.

Every person starts out as a calcium wave. The wave triggered during fertilization it stops multiple <a href="Sperm">sperm</a> from fertilizing the <a href="Ovum">ovum</a>. This is called the the fertilisation envelope. Ca²⁺ signalling within a cell can be very complicated and hard to understand what is going on as it is involved in a number of pathways. This brings about the complication of cross talk between messengers. The fertlization process and the role that calcium plays is one of the few that is understood. Another example includes T-cell activation. The calcium signal will only be released once a mature oocyte is created and not earlier. This starts off the developmental cycle. When an ovum becomes fertlized, the machinery required for calcium signalling changed which enables the signal to be released.

Calcium is used as an intracellular messenger as it is kept at low levels in resting cells; cells have many sensitive mechanisms that detect rises and falls in calcium levels. These include a number of "OFF" and "ON" mechanisms.

"OFF" mechanisms are important in the recovery of intracellular calcium levels, particularly in <a href="Muscle">muscle</a> cells, as the cell cannot contract again until calcium levels fall after the rise associated with <a href="Contraction">contraction</a>. "OFF" mechanisms include buffer proteins in the <a href="Sarcoplasmic reticulum">sarcoplasmic reticulum</a> (<a href="Sarcoplasmic reticulum">SR</a>) (e.g. <a href="Calsequestrin">calsequestrin</a>). These <a href="Proteins">proteins</a> have low affinity and high capacity for calcium, so that it is still easily released for signalling. Another "OFF" mechanism is <a href="Na+/Ca2+ exchanger pump">Na⁺/Ca²⁺ exchanger pump</a> in the <a href="Plasma membrane">plasma membrane</a>, which operates at higher levels of Ca^2+. The differences in affinity of the "OFF" mechanisms for calcium allow the cell to respond effectively to different concentrations. <a href="PMCA">PMCA</a> and <a href="SERCA">SERCA</a> also keep cytosolic Ca²⁺ levels low by utilising <a href="ATP">ATP</a> to pump Ca²⁺ out of the <a href="Cytosol">cytosol</a> into the <a href="Endoplasmic reticulum">Endoplasmic reticulum</a> (<a href="Endoplasmic reticulum">ER</a>), <a href="Sarcoplasmic reticulum">SR</a>, or outside the cell.

"ON" mechanisms respond to extracellular signals indicating to increase the levels of cytosolic Ca²⁺. These include <a href="Ip3">InsP</a>_{<a href="Ip3">3</a>} and <a href="Ryanodine receptor">Ryanodine receptors</a>, both which are activated by the binding of <a href="ATP">ATP</a>, however, <a href="InsP3 receptors">InsP₃ receptors</a> also require binding of the coagonist <a href="InsP3">InsP₃</a> to activate them .

The major pathway in calcium signalling is the <a href="Phosphoinositide pathway">Phosphoinositide pathway</a> (PI). This uses the <a href="Second messenger">second messenger</a> IP₃  to bind to the <a href="IP3 receptor">IP₃ receptor</a> and release calcium from the <a href="Endoplasmic reticulum">endoplasmic reticulum</a>. The calcium release can cause more calcium release known as <a href="Calcium induced calcium release">calcium induced calcium release</a> (CICR) wich creates the calcium wave.

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