Mitochondria calcium transport

Considerable evidence now supports the hypothesis that the inner membrane of a rat liver mitochondrion contains a specific permease or carrier for Ca, which makes possible the inward transport of this cation from regions of low concentration in response to electrochemical gradients generated by electron transport. The carrier has a very hig affinity for Ca + and it also has the ability to transport Sr + and Mn + but not Mg +. Lehninger has discussed mitochondrial transport of calcium he has also shown " that La + is a specific inhibitor of the Ca + carrier and is therefore a potential probe for calcium transport. The dye Ruthenium Red also inhibits calcium transport and, since this dye reacts specifically with mucopolysaccharides, it has been concluded that the latter (in the form of mucoproteins or muco- or glyco-lipids) are at the active centre of the sites of mediation of mitochondrial Ca + transport. Fluorescence enhancement of 8-anilino-l-naphthalene has also been used as a probe for calcium transport. 

It has been known for many years that the mitochondrion shows a respiration-linked transport of a number of ions. Of these, calcium has attracted the most attention since it depends on a specific transport system with high-affinity binding sites. The uptake of calcium usually also involves a permeant anion, but in the absence of this, protons are ejected as the electron transfer system operates. The result is either the accumulation of calcium salts in the mitochondrial matrix or an alkalinization of the interior of the mitochondrion. The transfer of calcium inwards stimulates oxygen utilization but provides an alternative to the oxidative phosphorylation of ADP618 ... 

The transport of calcium into the mitochondrion can lower external Ca2+ to levels of 1 to 0.1 jumole/1. This has, therefore, been interpreted as a basic mechanism in maintaining intracellular calcium at these levels. Only about 3 % of the calcium which passively diffuses into the cell is expelled by a calcium pump into the plasma membrane, whereas the remaining 97 % is sequestered into the mitochondria. This occurs because both processes have similar rate constants, but the total mitochondrial surface is some 30 times larger than that of the plasma membrane. This argument presupposes, however, that the calcium which enters the cell is equally available to both sets of membranes. 

Calcium levels are believed to be controlled in part at least by the uptake and release of Ca2+ from mitochondria.172"174 The capacity of mitochondria for Ca2+ seems to be more than sufficient to allow the buffering of Ca2+ at low cytosol levels. Mitochondria take up Ca2+ by an energy-dependent process either by respiration or ATP hydrolysis. It is now agreed that Ca2+ enters in response to the negative-inside membrane potential developed across the inner membrane of the mitochondrion during respiration. The uptake of Ca2+ is compensated for by extrusion of two H+ from the matrix, and is mediated by a transport protein. Previous suggestions for a Ca2+-phosphate symport are now discounted. The possible alkalization of the mitochondrial matrix is normally prevented by the influx of H+ coupled to the influx of phosphate on the H - PCV symporter (Figure 10). This explains why uptake of Ca2+ is stimulated by phosphate. Other cations can also be taken up by the same mechanism. 

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