Calcium in biology
They play an important role in signal transduction pathways,[2][3] where they act as a second messenger, in neurotransmitter release from neurons, in contraction of all muscle cell types, and in fertilization.
Extracellular calcium is also important for maintaining the potential difference across excitable cell membranes, as well as proper bone formation.
Plasma calcium levels in mammals are tightly regulated,[2][3] with bone acting as the major mineral storage site.
Calcitonin secreted from the parafollicular cells of the thyroid gland also affects calcium levels by opposing parathyroid hormone; however, its physiological significance in humans is dubious.
Intracellular calcium is stored in organelles which repetitively release and then reaccumulate Ca2+ ions in response to specific cellular events: storage sites include mitochondria and the endoplasmic reticulum.
"[20] The EFSA rejected a claim that a cause-and-effect relationship existed between the dietary intake of calcium and potassium and maintenance of normal acid-base balance.
[21] The EFSA also rejected claims for calcium and nails, hair, blood lipids, premenstrual syndrome and body weight maintenance.
[22] The United States Department of Agriculture (USDA) web site has a very complete searchable table of calcium content (in milligrams) in foods, per common measures such as per 100 grams or per a normal serving.
To measure Ca2+ concentration or spatial distribution within the cell cytoplasm in vivo or in vitro, a range of fluorescent reporters may be used.
Within a typical cell, the intracellular concentration of ionized calcium is roughly 100 nM, but is subject to increases of 10- to 100-fold during various cellular functions.
This gradient is maintained through various plasma membrane calcium pumps that utilize ATP for energy, as well as a sizable storage within intracellular compartments.
In electrically excitable cells, such as skeletal and cardiac muscles and neurons, membrane depolarization leads to a Ca2+ transient with cytosolic Ca2+ concentration reaching around 1 μM.
It has been estimated that mitochondrial matrix free calcium concentration rises to the tens of micromolar levels in situ during neuronal activity.
In addition, the plasma membrane Ca2+ ATPase (PMCA) obtains energy to pump calcium out of the cell by hydrolysing adenosine triphosphate (ATP).
Both nitric oxide and hyperpolarization cause the smooth muscle to relax in order to regulate the amount of tone in blood vessels.
An example a protein with calcium coordination is von Willebrand factor (vWF) which has an essential role in blood clot formation process.
It was discovered using single molecule optical tweezers measurement that calcium-bound vWF acts as a shear force sensor in the blood.
Shear force leads to unfolding of the A2 domain of vWF whose refolding rate is dramatically enhanced in the presence of calcium.
[46] Substantial decreases in extracellular Ca2+ ion concentrations may result in a condition known as hypocalcemic tetany, which is marked by spontaneous motor neuron discharge.
Ca2+ ions can damage cells if they enter in excessive numbers (for example, in the case of excitotoxicity, or over-excitation of neural circuits, which can occur in neurodegenerative diseases, or after insults such as brain trauma or stroke).
Some invertebrates use calcium compounds for building their exoskeleton (shells and carapaces) or endoskeleton (echinoderm plates and poriferan calcareous spicules).
Ca2+ ions are usually kept at nanomolar levels in the cytosol of plant cells, and act in a number of signal transduction pathways as second messengers.
