The influence of nupercaine on Ca2+ transport by rat liver and Ehrlich ascites cell mitochondria

The Ca(2+)-induced permeability transition pore is involved in Ca(2+)-induced mitochondrial oscillations. A study on permeabilised Ehrlich ascites tumour cells

The Ca(2+)-induced permeability transition of the mitochondrial inner membrane was studied in digitonin-permeabilized Ehrlich ascites tumour cells respiring on succinate in an isotonic medium. Addition of a sufficient amount of Ca2+ to induce an efflux of accumulated Ca2+ from mitochondria produced an oscillatory state with periodically changing rates of respiration, transmembrane potential, delta pH and direction of Ca2+ fluxes. This contrasts with liver mitochondria in which only a Ca2+ efflux is induced under these conditions. Addition of traces of cyclosporin A (approximately 0.1 nM) damped the oscillations by inhibiting the phase in which Ca2+ efflux occurs and promoting the reestablishment of a higher transmembrane potential. Efflux was also prevented by addition of ATP or ADP, ATP being more potent. Efflux was also inhibited by low concentrations of spermine. It is concluded that Ca(2+)-induced oscillations involve the cyclosporin A-sensitive pore and that the Ehrlich ascites tumour cell mitochondria differ from liver mitochondria in being far more sensitive to cyclosporin A and ATP. The possible physiological role of the oscillatory state is discussed.

Mechanisms by which mitochondria transport calcium

It has been firmly established that the rapid uptake of Ca2+ by mitochondria from a wide range of sources is mediated by a uniporter which permits transport of the ion down its electrochemical gradient. Several mechanisms of Ca2+ efflux from mitochondria have also been extensively discussed in the literature. Energized mitochondria must expend a significant amount of energy to transport Ca2+ against its electrochemical gradient from the matrix space to the external space. Two separate mechanisms have been found to mediate this outward transport: a Ca2+/nNa+ exchanger and a Na(+)-independent efflux mechanism. These efflux mechanisms are considered from the perspective of available energy. In addition, a reversible Ca2(+)-induced increase in inner membrane permeability can also occur. The induction of this permeability transition is characterized by swelling of the mitochondria, leakiness to small ions such as K+, Mg2+, and Ca2+, and loss of the mitochondrial membrane potential. It has been suggested that the permeability transition and its reversal may also function as a mitochondrial Ca2+ efflux mechanism under some conditions. The characteristics of each of these mechanisms are discussed, as well as their possible physiological functions.

Ruthenium red-insensitive Ca2+ uptake and release by mitochondria

Calcium uptake by rat liver mitochondria driven by an artificial pH gradient is ruthenium red insensitive, electrically neutral, and inhibited by the local anesthetic, nupercaine. This pH-driven Ca2+ transport is also inhibited by NH3, Pi, and acetate. Direct measurements of Pi indicate it is not translocated with Ca2+ during pH-driven Ca2+ uptake. Calcium is therefore not transported by a Ca2+-Pi symport mechanism. Ruthenium red-insensitive Ca2+ efflux is similar in its inhibition by nupercaine and its kinetics, and is also electroneutral. This suggests that the Ca2+ uptake described here occurs via reversal of the principal pathway of mitochondrial Ca2+ release. From the available data, pH-driven Ca2+ uptake (and presumably Ca2+ efflux) is hypothesized to occur by Ca2+ symport with unidentified anions. Protons may move counter to Ca2+ or reversibly dissociate from cotransported anions, which therefore couples Ca2+ transport to the pH gradient.

Uncoupler-stimulated release of Ca2+ from Ehrlich ascites tumor cell mitochondria

Ruthenium red-insensitive, uncoupler-stimulated release of Ca2+ from Ehrlich ascites tumor cell mitochondria is much slower than from rat liver mitochondria under comparable conditions. In the presence of Pi and at moderate or high Ca2+ loads, ruthenium red-insensitive Ca2+ efflux elicited with uncoupler is approximately 20 times more rapid for rat liver than Ehrlich cell mitochondria. This is attributed to resistance of tumor mitochondria to damage by Ca2+ due to a high level of endogenous Mg2+ that also attenuates Ca2+ efflux. Calcium release from rat liver and tumor mitochondria is inhibited by exogenous Mg2+. This applies to ruthenium red-insensitive spontaneous Ca2+ efflux associated with Ca2+ uptake and uncoupling, and (b) ruthenium red-insensitive Ca2+ release stimulated by uncoupling agent. The endogenous Mg2+ level of Ehrlich tumor mitochondria is approximately three times that of rat liver mitochondria. Endogenous Ca2+ is also much greater (six fold) in Ehrlich tumor mitochondria compared to rat liver. Despite the quantitative difference in endogenous Mg2+, the properties of internal Mg2+ are much the same for rat liver and Ehrlich cell mitochondria. Ehrlich ascites tumor mitochondria exhibit slow, metabolically dependent Mg2+ release and rapid limited release of Mg2+ during Ca2+ uptake. Both have been observed with rat liver and other types of mitochondria. The proportions of apparently "bound" and "free" Mg2+ (inferred from release by the ionophore, A23187) do not differ significantly between tumor and liver mitochondria. Thus, the endogenous Mg2+ of tumor mitochondria has no unusual features but is simply elevated substantially. Ruthenium red-insensitive Ca2+ efflux, when expressed as a function of the intramitochondrial Ca2+/Mg2+ ratio, is quite similar for tumor and rat liver. It is proposed, therefore, that endogenous Mg2+ is a major regulatory factor responsible for differences in the sensitivity to damage by Ca2+ and Ca2+ release by Ehrlich ascites tumor mitochondria compared to mitochondria from normal tissues.

Ca2+ release from energetically coupled tumor mitochondria

A Na+/Ca2+ exchange activity for Ca2+ efflux has been identified in isolated Ehrlich ascites tumor mitochondria. Further, under conditions favoring cycling of Ca2+ across the mitochondrial inner membrane, extramitochondrial [Ca2+] also was shown to be Na+-dependent. The Na+/Ca2+ exchange showed sigmoidal kinetics with a mean (+/-SD) [Na+] required for half maximal stimulation of Ca2+ efflux of 8.4 +/- 3.8 mM and a Hill coefficient of 1.6. Na+/Ca2+ exchange was very sensitive to inhibition by the Ca2+ antagonist diltiazem (56% inhibition at 7.5 nmoles X mg protein-1) whereas a number of other compounds, including verapamil, nupercaine, and trifluoperazine were less effective in inhibiting Ca2+ efflux. These data demonstrate for the first time the presence of a pathway in tumor mitochondria for unidirectional Ca2+ efflux induced by Na+, and provide a mechanism for regulation of tumor intra- and extramitochondrial [Ca2+]. Results of the present study support the need for further study of intracellular Na+ and its role in regulation of Ca2+ homeostasis in tumor cells.