External ca2+ concentrations associated with membrane alkalinization in mitochondria

Application of Ca2+-selective electrode for determination of Ca2+ efflux from smooth muscles into Ca2+-free physiological solution

The Ca2+ efflux from smooth muscle into a Ca2+-free physiological solution was continuously monitored by using a Radiometer Ca2+-selective electrode and examined for its availability for determination of Ca2+ effluxed from the smooth muscle. The results show that the Ca2+ electrode used here is sufficiently sensitive to investigate Ca2+ efflux from the smooth muscle into the Ca2+-free solution. Further, this method is suitable for the continuous recording of free ionized Ca2+ effluxed from the smooth muscle into the Ca2+-free physiological solution.

The role of Pi in glycolytic inhibition of calcium ion uptake by ELD ascites tumor cells

In contrast to previous investigations at 25 degrees C, glucose was shown to support 45Ca2+ uptake at 37 degrees C in intact ELD ascites tumor cells. Intact ascites tumor cells in vitro accumulated up to 5.0 micromol of 45Ca2+ per g cells dry wt. within 20 min. In the presence of 10.0 mM glucose, intracellular P(i) levels fell from 40.0 micromol x g(-1) cells dry wt. to 20.0 micromol x g(-1) cells dry wt. in 5 min. Intracellular P(i) levels were maintained by 20.0 mM extracellular Tris-P(i). 45Ca2+ uptake was inhibited in P(i)-depleted cells, even though the metabolic rate (as measured by Q(lactate)) and energy state (as measured by ATP levels) were at acceptable levels. Evidence has been presented suggesting that previous reports of glucose inhibition of calcium uptake can be attributed to a competition for available intracellular P(i) between glycolytic processes and the mitochondrial calcium uptake mechanism.

The inhibition of mitochondrial calcium transport by lanthanides and ruthenium red

An EGTA (ethanedioxybis(ethylamine)tetra-acetic acid)-quench technique was developed for measuring initial rates of (45)Ca(2+) transport by rat liver mitochondria. This method was used in conjunction with studies of Ca(2+)-stimulated respiration to examine the mechanisms of inhibition of Ca(2+) transport by the lanthanides and Ruthenium Red. Ruthenium Red inhibits Ca(2+) transport non-competitively with K(i) 3x10(-8)m; there are 0.08nmol of carrier-specific binding sites/mg of protein. The inhibition by La(3+) is competitive (K(i)=2x10(-8)m); the concentration of lanthanide-sensitive sites is less than 0.001nmol/mg of protein. A further difference between their modes of action is that lanthanide inhibition diminishes with time whereas that by Ruthenium Red does not. Binding studies showed that both classes of inhibitor bind to a relatively large number of external sites (probably identical with the ;low-affinity' Ca(2+)-binding sites). La(3+) competes with Ruthenium Red for most of these sites, but a small fraction of the bound Ruthenium Red (less than 2nmol/mg of protein) is not displaced by La(3+). The results are discussed briefly in relation to possible models for a Ca(2+) carrier.

Ca2+ transport by mitochondria from L1210 mouse ascites tumor cells

Mitochondria isolated from the ascites form of L1210 mouse leukemia cells readily accumulate Ca(2+) from the suspending medium and eject H(+) during oxidation of succinate in the presence of phosphate and Mg(2+), with normal stoichiometry between Ca(2+) uptake and electron transport. Ca(2+) loads up to 1600 ng-atoms per mg of protein are attained. As is the case in mitochondria from normal tissues, Ca(2+) uptake takes precedence over oxidative phosphorylation. However, Ca(2+) transport by the L-1210 mitochondria is unusual in other respects, which may possibly have general significance in tumor cells. The apparent affinity of the L1210 mitochondria for Ca(2+) in stimulation of oxygen uptake is about 3-fold greater than in normal liver mitochondria; moreover, the maximal rate of Ca(2+) transport is also considerably higher. Furthermore, when Ca(2+) pulses are added to L1210 mitochondria in the absence of phosphate or other permeant anions, much larger amounts of Ca(2+) are bound and H(+) ejected per atom of oxygen consumed than in the presence of phosphate; up to 7 Ca(2+) ions are bound per pair of electrons passing each energy-conserving site of the electron-transport chain. Such "superstoichiometry" of Ca(2+) uptake can be accounted for by two distinct types of respiration-dependent interaction of Ca(2+) with the L1210 mitochondria. One is the stimulation of oxygen consumption, which is achieved by relatively low concentrations of Ca(2+) (K(m) congruent with 8 muM) and is accompanied by binding of Ca(2+) up to 40 ng-atoms per mg of protein. The second process, also dependent on electron transport, is the binding of further Ca(2+) from the medium in exchange with previously stored membrane-bound protons, in which the affinity for Ca(2+) is much lower (K(m) congruent with 120 muM).