Free energy change g may be written nca2 1 ca 2 0 g

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free-energy change ~G may be written (~nCa2+ = - 1) [Ca 2 +]0 ~G = - RT· In C 2+ - 2F ~'\(f. [a ]i (3.1l)
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IV. THE TRANSPORT AND REGULATION OF Ca 2 + IONS IN HIGHER ORGANISMS 131 From this analysis it may be inferred that the limiting Ca2+ concentration (or activity) ratio that can be achieved by this electrogenic pump (i.e., /)"G = 0) IS e -2F!!.P/RT (3.12) With /)" 'l' = 150 mV, this ratio is calculated to be 8.4 X 10 - 6 at 25°C. It is evident that, as long as the Ca 2+ influx would not lower the membrane potential difference, the Ca2+ uniporter has a very high pumping potential. Measured values of the pumping rate, V max, are indeed high (> 10 nmol/mg protein 59) and probably limited only by the rate of electron transport and H + extrusion in the mitochondria. Mitochondria may accumulate large quantities of Ca 2 +, probably to main- tain electroneutrality. To prevent the buildup of high concentrations of free Ca 2+ (and of osmotic pressure), phosphate ions are also transported into the inner matrix, where an amorphous calcium phosphate--or possibly a phosphocitrate 60 - is formed. The equilibrium concentration of free Ca 2+ in the mitochondrial matrix may as a result be comparatively low, on the order of 1 p,M. The molecular nature of the mitochondrial Ca2+ uniporter continues to be elusive, and needs to be studied further. 4. Mitochondrial Ca 2+ transport: efflux Mitochondria, as well as SR, release Ca 2+ ions by mechanisms other than "back leakage" through the pumps. In mitochondria from excitable cells, the efflux occurs mainly through an antiport, where 2 Na + ions are transported inward for every Ca 2+ ion departing for the cytosolic compartment.6l In other cells there is evidence for the dominance of a 2H + - Ca2+ antiport. 59 In all likelihood the Ca 2+ efflux is regulated, possibly by the redox state of pyridine nucleotides in the mitochondria. As with the Ca 2+ uniporter, few details on the molecular nature of the antiporters are presently available. 5. Ca 2+ efflux from non-mitochondrial stores Release of Ca 2 + from ER and SR presently appears to be the prime effect of the new intracellular messenger 1,4,5-triphosphoinositol (l,4,5-IP3) released into the cytoplasm as a result of an external hormonal stimulus (see Section IV.C). It seems that receptors for 1,4,5-IP 3 have been established on ER, and that the binding of 1,4,5-IP 3 causes a release of Ca 2+ stored in this orga- nelle. 62 ,63,170,J71 In addition to the receptor-controlled Ca 2 + efflux, there may be other pathways for Ca 2 + release, and Ca 2 + mobilization may be regulated by other intracellular entities, the Ca 2 + ions themselves included.
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132 3 / CALCIUM IN BIOLOGICAL SYSTEMS 6. Other voltage-gated or receptor-activated Ca 2 + channels In addition to the transport pathways already discussed, some cells seem to have Ca 2 + channels in the plasma membrane that can be opened by the action of an agonist on a receptor or that are gated in response to changes in membrane potential.
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