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1.
Mitochondria isolated from corn ( Zea mays L.) coleoptiles by an improved procedure which yields functionally intact preparations are much more active in respiration-coupled Ca 2+ accumulation than those employed in most earlier studies. Ca 2+ uptake by these mitochondria is phosphate-dependent and is accompanied by decrease in Δψ, H + extrusion and increase in the rate of respiration. A sigmoidal plot with a Hill coefficient of 2.22 was obtained when the rates of Ca 2+ uptake were plotted as a function of free Ca 2+ concentration. The K0.5 for Ca 2+ influx was about 31 μM and a Vmax of 140 nmol Ca 2+ per min per mg was attained at a free-Ca 2+ concentration of about 120 μM. Ca 2+ uptake is sensitive to inhibition by ruthenium red and Mg 2+. The external free-Ca 2+ concentration maintained at steady state was about 2 μM and was independent of the respiratory substrate and of external Na +, but was increased by exogenous Mg 2+. In addition, this preparation of corn mitochondria has shown a much higher ability for Ca 2+ retention in the presence of phosphate and NAD(P)H oxidants than liver mitochondria. 相似文献
2.
Relationships among several of the ion movements associated with the acrosome reaction of S. purpuratus were investigated. Egg jelly initiates 45Ca 2+ and 22Na + uptake, and K + and H + efflux. H + efflux and 22Na + uptake occur with approximately equivalent stoichiometries as rapidly as the appearance of acrosomal rods, perhaps reflecting a linked process. Most K + loss, as measured either by 42K + efflux or K +-ion-selective electrodes, occurs after the acrosome reaction is complete. Since an elevation of seawater K + (from 10 to 15 m M) or the addition of 0.5 m M tetraethylammonium (TEA), an inhibitor of K + channels, inhibits the acrosome reaction half-maximally, K + movements or alterations of K +-dependent membrane potentials may regulate the triggering by jelly. Most, but not all, of the 45Ca 2+ influx is inhibited with a mixture of 10 μ M FCCP, 1 m M CN ?, and 2 μg/ml oligomycin, suggesting that the mitochondria store most of the Ca 2+. The extracellular Na + concentration affects Ca 2+ fluxes: sperm placed into 5 m M Na + seawater have enhanced 45Ca 2+ uptake, but do not undergo the acrosome reaction, unless 30 m M Na + is also added. Low Na + concentrations lead to spontaneous triggering, by allowing for both Ca 2+ influx and Na +-dependent H + efflux. At least one early Ca 2+ requirement precedes the Na + and H + movements, as inferred from attempts at reversing the inhibitors of jelly induction of the acrosome reaction. When sperm are incubated with jelly in the absence of Ca 2+, then washed and incubated with jelly in the presence of Ca 2+, the acrosome reaction is triggered only upon the second incubation. However, when sperm are mixed with jelly in the presence of the other inhibitors (verapamil, TEA, 5 m M Na + seawater, low pH, or elevated K +), they are altered so that even upon subsequent washing, jelly-mediated triggering is no longer possible. This suggests the existence of an intermediate state in the reaction pathway, that follows an event for which Ca 2+ is required, but that precedes the Na + and H + movements, which are inhibited by all inhibitors of the acrosome reaction. These data are used to develop a partial sequence of ionic changes associated with the triggering mechanism. 相似文献
3.
The efflux 42K + from isolated beef heart mitochondria under conditions of near steadystate K + is increased by repiration and is sensitive to uncouplers and to exogenous Mg 2 The respiration-dependent efflux is strongly activated by inorganic phosphate in the presence of external K +, but not Na +, and is inhibited by oxidative phosphorylation. Low concentrations of mersalyl also activate respiration-dependent efflux of 42K + in the absence of net alteration in matrix K +. Acetate in the presence of mersalyl brings about net accumulation of K + with retention of internal 42K +. The results are consistent with a model in which nearly constant matrix K + is maintained by the regulated interplay between a K + uniport (which is responsive to membrane potential and which is the pathway for K + influx) and a exchanger (which responds to the transmembrane pH differential and which is the pathway for net K + efflux). 相似文献
4.
The effect of Sr 2+ on the set point for external Ca 2+ was studied in rat heart and liver mitochondria with the aid of a Ca 2+-sensitive electrode. In respiring mitochondria the set point is determined by the rates of Ca 2+ influx on the Ca 2+ uniporter and efflux by various mechanisms. We studied the Ca 2+-Na + exchange pathway in heart mitochondria and the Δψ-modulated efflux pathway in liver mitochondria. Prior accumulation of Sr 2+ was found to shift the set points towards lower external Ca 2+ both in heart mitochondria under conditions of Ca 2+-Na + exchange and in liver mitochondria under conditions that should promote opening of the Δψ-modulated pathway. The effect on the set point was found to be due to inhibition of Ca 2+ efflux by Sr 2+ taken up by the mitochondria, while Sr 2+ efflux was too slow to be measurable. 相似文献
5.
Na + and Ca 2+ regulation were compared in two euryhaline species, killifish (normally estuarine-resident) and rainbow trout (normally freshwater-resident)
during an incremental salinity increase. Whole-body unidirectional fluxes of Na + and Ca 2+, whole body Na + and Ca 2+, and plasma concentrations (trout only), were measured over 1-h periods throughout a total 6-h protocol of increasing salinity
meant to simulate a natural tidal flow. Killifish exhibited significant increases in both Na + influx and efflux rates, with efflux slightly lagging behind efflux up to 60% SW, but net Na + balance was restored by the time killifish reached 100% SW. Whole body Na + did not change, in agreement with the capacity of this species to tolerate daily salinity fluctuations in its natural habitat.
In contrast, rainbow trout experienced a dramatic increase in Na + influx (50-fold relative to FW values), but not Na + efflux between 40 and 60% SW, resulting in a large net loading of Na + at higher salinities (60–100% SW), and increases in plasma Na + and whole body Na + at 100% SW. Killifish were in negative Ca 2+ balance at all salinities, whereas trout were in positive Ca 2+ balance throughout. Ca 2+ influx rate increased two- to threefold in killifish at 80 and 100% SW, but there were no concomitant changes in Ca 2+ efflux. Ca 2+ flux rates were affected to a larger degree in trout, with twofold increases in Ca 2+ influx at 40% SW and sevenfold increases at 100% SW. Again, there was no change in Ca 2+ efflux with salinity, so plasma Ca 2+ concentration increased in 100% SW. As the killifish is regularly submitted to increased salinity in its natural environment,
it is able to rapidly activate changes in unidirectional fluxes in order to ensure ionic homeostasis, in contrast to the trout. 相似文献
6.
The dithiol-reactive reagent phenylarsine oxide causes a pH-dependent stimulation of unidirectional K + flux into respiring rat liver mitochondria. This stimulation is diminished by subsequent addition of either the dithiol 2,3-dimercaptopropanol or the monothiol 2-mercaptoethanol. In contrast, uncoupling by phenylarsine oxide is reversed by 2,3-dimercaptopropanol but not by 2-mercaptoethanol. The data suggest separate sites of interaction of phenylarsine oxide with mechanisms of K + entry and ATP synthesis. Stimulatory effects of mersalyl and phenylarsine oxide on K + influx are not additive. Thus PheASO and mersalyl may affect K + influx at a common site. Pretreatment of the mitochondria with DCCD, which inhibits K + influx, fails to alter sensitivity to PheAsO or mersalyl. Thus the DCCD binding site associated with the K + influx mechanism appears to be separate from and independent of the sulfhydryl group(s) which mediate stimulation of K + influx by PheAsO and mersalyl.PheAsO, like mersalyl, also increases the rate of unidirectional K + efflux from respiring mitochondria. The combined presence of PheAsO plus mersalyl causes a greater stimulation of K + efflux than is observed with either reagent alone.Abbreviations used: BAL, British AntilLewisite or 2,3-dimercaptopropanol; DCCD, dicyclohexylcarbodiimide; DBCT, dibutylchloromethyltin chloride; 2-ME, 2-mercaptoethanol; PheAsO, phenylarsine oxide. 相似文献
7.
We have investigated the effects of hyperpolarization and depolarization, and the presence of K + and/or Ca 2+, on 22Na + influx into corn ( Zea mays L.) root segments. In freshly excised root tissue which is injured, Na + influx is unaffected by hyperpolarization with fusicoccin, or depolarization with uncoupler (protonophore), or by addition of K +. However, added Ca 2+ suppresses Na + influx by 60%. In washed tissue which has recovered, Na + influx is doubled over that of freshly excised tissue, and the influx is increased by fusicoccin and suppressed by uncoupler. This energy-linked component of Na + influx is completely eliminated by low concentrations of K +, leaving the same level and kind of Na + influx seen in freshly excised roots. The K +-sensitive energy linkage appears to be by the carrier for active K + influx. Calcium is equally inhibitory to Na + influx in washed as in fresh tissue. Other divalent cations are only slightly less effective. Net Na + uptake was about 25% of 22Na + influx, but proportionately the response to K + and Ca 2+ was about the same. The constancy of K+-insensitive Na+ influx under conditions known to hyperpolarize and depolarize suggests that if Na+ transport is by means of a voltage-sensitive channel, the rise or fall of channel resistance must be proportional to the rise or fall in potential difference. The alternative is a passive electroneutral exchange of 22Na+ for endogenous Na+. The data suggest that an inwardly directed Na+ current is largely offset by an efflux current, giving both a small net uptake and isotopic exchange. 相似文献
8.
Parallel measurements of Ca 2+ uptake, oxygen consumption, endogenous Mg 2+ efflux, and swelling in rotenone-poisoned rat liver and rat heart mitochondria showed that heart mitochondria is much more resistant to uncoupling by Ca 2+ in the presence of phosphate than rat liver mitochondria. The extent of Mg 2+ efflux and swelling induced by Ca 2+ accumulation are much less pronounced in heart mitochondria. Uncoupling and swelling in liver mitochondria seem to result from the loss of membrane-bound Mg 2+ as a consequence of Ca 2+ recycling across the membrane as induced by phosphate. Exogenous Mg 2+ protects liver mitochondria against the deleterious effects of Ca 2+ by inhibiting a ruthenium red-insensitive Ca 2+ efflux induced by phosphate. Phosphate does not induce recycling of Ca 2+ in heart mitochondria. On the other hand, heart mitochondria respiring on NAD-linked substrates or with succinate in the absence of rotenone behave like liver mitochondria with respect to the alterations caused by Ca 2+ recycling. In heart mitochondria the recycling of Ca 2+ is related to the redox state of pyridine nucleotides, which suggests that the ruthenium red-insensitive efflux of Ca 2+ is subject to metabolic control. In addition it has been observed that Sr 2+does not undergo cyclic movements across the membrane. The data indicate that the efflux pathway is more specific for Ca 2+ than the ruthenium red-sensitive influx transporter. 相似文献
9.
Ca 2+ plays a central role in energy supply and demand matching in cardiomyocytes by transmitting changes in excitation-contraction coupling to mitochondrial oxidative phosphorylation. Matrix Ca 2+ is controlled primarily by the mitochondrial Ca 2+ uniporter and the mitochondrial Na +/Ca 2+ exchanger, influencing NADH production through Ca 2+-sensitive dehydrogenases in the Krebs cycle. In addition to the well-accepted role of the Ca 2+-triggered mitochondrial permeability transition pore in cell death, it has been proposed that the permeability transition pore might also contribute to physiological mitochondrial Ca 2+ release. Here we selectively measure Ca 2+ influx rate through the mitochondrial Ca 2+ uniporter and Ca 2+ efflux rates through Na +-dependent and Na +-independent pathways in isolated guinea pig heart mitochondria in the presence or absence of inhibitors of mitochondrial Na +/Ca 2+ exchanger (CGP 37157) or the permeability transition pore (cyclosporine A). cyclosporine A suppressed the negative bioenergetic consequences (ΔΨ m loss, Ca 2+ release, NADH oxidation, swelling) of high extramitochondrial Ca 2+ additions, allowing mitochondria to tolerate total mitochondrial Ca 2+ loads of > 400 nmol/mg protein. For Ca 2+ pulses up to 15 μM, Na +-independent Ca 2+ efflux through the permeability transition pore accounted for ~ 5% of the total Ca 2+ efflux rate compared to that mediated by the mitochondrial Na +/Ca 2+ exchanger (in 5 mM Na +). Unexpectedly, we also observed that cyclosporine A inhibited mitochondrial Na +/Ca 2+ exchanger-mediated Ca 2+ efflux at higher concentrations (IC 50 = 2 μM) than those required to inhibit the permeability transition pore, with a maximal inhibition of ~ 40% at 10 μM cyclosporine A, while having no effect on the mitochondrial Ca 2+ uniporter. The results suggest a possible alternative mechanism by which cyclosporine A could affect mitochondrial Ca 2+ load in cardiomyocytes, potentially explaining the paradoxical toxic effects of cyclosporine A at high concentrations. This article is part of a Special Issue entitled: Mitochondria and Cardioprotection. 相似文献
10.
The Na +-Ca 2+ exchanger (NCX) links transmembrane movements of Ca 2+ ions to the reciprocal movement of Na+ ions. It normally functions primarily as a Ca 2+ efflux mechanism in excitable tissues such as the heart, but it can also mediate Ca 2+ influx under certain conditions. Na + and Ca 2+ ions exert complex regulatory effects on NCX activity. Ca 2+ binds to two regulatory sites in the exchanger's central hydrophilic domain, and this interaction is normally essential for activation of exchange activity. High cytosolic Na + concentrations, however, can induce a constitutive activity that by-passes the need for allosteric Ca 2+ activation. Constitutive NCX activity can also be induced by high levels of phopshotidylinositol-4,5-bisphosphate (PIP 2) and by mutations affecting the regulatory calcium binding domains. In addition to promoting constitutive activity, high cytosolic Na + concentrations also induce an inactivated state of the exchanger (Na +-dependent inactivation) that becomes dominant when cytosolic pH and PIP 2 levels fall. Na+-dependent inactivation may provide a means of protecting cells from Ca 2+ overload due to NCX-mediated Ca 2+ influx during ischemia. 相似文献
11.
During the ascidian sperm reaction the single large cylindrical mitochondrion which lies next to the nucleus in the head swells, becomes spherical, and migrates along the tail to be lost when it reaches the end. This sequence is initiated by eggs, egg water, high pH, low Na +, or the ionophore X537A. Accompanying the sperm reaction induced by low Na + are H + efflux and Ca 2+ influx in a ratio of near 100:1 as determined by 45Ca 2+ and atomic absorption analysis. Simultaneous pH and Ca 2+ electrode measurements suggest that the movement of H + begins 10–13 sec before the movement of Ca 2+. Ca 2+ uptake can be inhibited by verapamil without affecting H + efflux or the sperm reaction. Acid release and Ca 2+ uptake are proportional to the initial pH of the medium when the reaction is triggered by high pH. Acid release initiated by low Na + is proportional to Ca 2+ concentrations above 2 m M. H + and Ca 2+ movements differ in magnitude, kinetics, and inhibition by verapamil, thus suggesting that H + is probably not exchanged for Ca 2+. Instead we propose that loss of H + triggers the uptake of Ca 2+, which initiates the sperm reaction. 相似文献
12.
The effect of taurine has been studied on 45Ca 2+ efflux from mitochondria obtained from the flight muscle and thoracic ganglia of the desert locust. Mitochondria from both tissues readily accumulated 45Ca 2+ and this uptake was stimulated by the presence of phosphate 45Ca 2+ accumulation was abolished by ruthenium red (5 μ M). Only in the presence of 10 mM Na + and either ruthenium red (5 μ M) or EGTA (500 μ M), was an efflux of 45Ca 2+ observed. Taurine (20 mM) abolished the Na +-dependent 45Ca 2+ efflux but had no effect in the absence of Na +. These results suggest that taurine may contribute to the control of the concentration of intracellular free calcium. 相似文献
13.
The effect of external calcium and sodium ion concentrations on the calcium fluxes on the Pelvetia fastigiata De Toni egg was measured. Decreasing external [Ca 2+] greatly increased the permeability of the eggs to Ca 2+; at 1 mM external Ca 2+ this permeability was 60 times as great as it was at the normal [Ca 2+] of 10 mM. Lowering the external [Na +] also increased Ca 2+ influx; at 2 mM Na +, the Ca 2+ influx was 2–3 times as great as it was at the normal [Na +] if choline was used as a Na + substitute. Lithium was less effective as a Na + substitute in increasing Ca 2+ influx. The extra Ca 2+ influx in low [Na +] seemed to be dependent on internal [Na +]. The Ca 2+ efflux increased transiently and then declined in low Na + media. 相似文献
14.
Ion-selective microelectrodes are a powerful tool in studying adaptive responses of plant cells and tissues to various abiotic stresses. However, application of this technique in Na + flux measurements was limited due to poor selectivity for Na + ions of commercially available Na + cocktails. Often, these cocktails cannot discriminate between Na + and other interfering ions such as K + and Ca 2+, leading to inaccurate measurements of Na + concentration and, consequently, inaccurate Na + flux calculations. To overcome this problem, three Na +-selective cocktail mixtures were prepared using tetramethoxyethyl ester derivative of p-t-butyl calix[4]arene. These cocktail mixtures were compared with commercially available ETH 227-based Na + cocktail for selectivity for Na + ions over other ions (particularly K + and Ca 2+). Among the three calixarene-based Na + cocktails tested, cocktail 2 [in % w/w: Na + ionophore (4- tert-butylcalix[4]arene-tetra acetic acid tetraethyl ester) 3.5, the plasticizer (2-nitrophenyl octyl ether) 95.9 and lipophilic anion (potassium tetrakis (4-chlorophenyl) borate) 0.6] showed the best selectivity for Na + ions over K + and Ca 2+ ions and was highly stable over time (up to 10 h). Na + flux measurements under a wide range of NaCl concentrations (25-150 mM) using Na + cocktail 2 established a clear dose-response relationship between severity of salt stress and magnitude of Na + influx at the distal elongation and mature zones of Arabidopsis thaliana roots. Furthermore, Na + cocktail 2 was compared with commercially available ETH 227-based Na + cocktail by measuring Na + fluxes at the two Arabidopsis root zones in response to 100 mM NaCl treatment. With calixarene-based Na + cocktail 2, a large decreasing Na + influx (0-15 min) followed by small Na + influx (15-45 min) was measured, whereas with ETH-based Na + cocktail Na + influx was short-lived (1-3 min) and was followed by Na + efflux (3-45 min) that might have been due to K + and Ca 2+ efflux measured together with Na + influx. In conclusion, Na +-selective calixarene-based microelectrodes have excellent potential to be used in real-time Na + flux measurements in plants. 相似文献
15.
Washing corn ( Zea mays L.) root tissue in water causes loss of about one-third of the exchangeable Ca 2+ over the first 10 to 15 minutes. Upon transfer to K +-containing solutions, the tissue shows a short period of rapid K + influx which subsequently declines. Addition of 0.1 millimolar Ca 2+ decreases the initial rapid K + influx, but increases the sustained rate of K + and Cl − uptake. It was confirmed (Elzam and Hodges 1967 Plant Physiol 42: 1483-1488) that 0.1 millimolar Ca 2+ is more effective than higher concentrations for the initial inhibition, and that Mg 2+ will substitute. The inhibition arises from a mild shock affect of restoring Ca2+. With 0.1 millimolar Ca2+ net H+ efflux is blocked for 10 to 15 minutes and the cells are depolarized by about 30 millivolts. However, 1 millimolar Ca2+ rapidly produces increased K+ influx and blocks net H+ efflux for only a few minutes; blockage is preceded by a brief net H+ influx which may restore and increase ion transport by reactivating the plasmalemma H+-ATPase. Stimulation of electrogenic H+-pumping with fusicoccin eliminates the shock responses and minimizes Ca2+ effects on K+ influx. Fusicoccin also strongly decreases Ca2+ influx, but has no effect on Ca2+ efflux. Ice temperatures and high pH decreased Ca2+ efflux, but uncoupler and chlorpromazine did not. It is suggested that the inhibitory and promotive actions of Ca2+ are manifested through decreases or increases in the protonmotive force. 相似文献
16.
Ca 2+ efflux from rat liver mitochondria in the presence of glutamate is stimulated by a decrease in pH from 7.3 to 6.8 and the rate is dependent on the phosphate concentration. During Ca 2+ (13 μm) uptake and release at low pH (+ phosphate), swelling is minimal, but a large oxidation of pyridine nucleotides and sustained membrane depolarization occurs. The depolarization (but not Ca 2+ efflux) is reversed by ruthenium red. An absolute requirement for phosphate to support Ca 2+ efflux is demonstrated by using acetate or lactate to support Ca 2+ uptake (efflux is depressed at pH 6.8). Preincubation with mersalyl, to block phosphate movements, with subsequent phosphate addition preceeding Ca 2+ uptake also inhibits efflux. β-Mercaptoethanol then stimulates efflux concomittent with membrane repolarization. Ca 2+ efflux is not a simple result of collapse of ΔpH since nigericin inhibits phosphate transport and Ca 2+ release. Following Ca 2+ uptake at pH 6.8, respiratory inhibition occurs, but oxygen consumption coupled to ATP synthesis can be stimulated by succinate (+ rotenone). Addition of succinate allows reuptake of Ca 2+, reduction of pyridine nucleotides, and repolarization of the membrane potential. Respiratory inhibition is also seen with nigericin, but no Ca 2+ efflux is observed. Coupled respiration with glutamate is seen at pH 6.8 following Ca 2+ uptake in the presence of lactate with subsequent addition of phosphate to promote Ca 2+ efflux. We conclude that Ca 2+ efflux is not a consequence of respiratory inhibition, but is mediated solely by phosphate movements. The inhibitory effect of Mg 2+ on Ca 2+ efflux is probably due to Mg 2+-dependent inhibition of the Ca 2+ diffusion potential so that the compensatory increase in ΔpH due to membrane depolarization does not occur and phosphate entry is slowed. 相似文献
17.
The characteristics of Ca 2+ transport across the excitable membrane of Paramecium aurelia were studied by measuring 45Ca 2+ influx and efflux. The intracellular concentration of free Ca 2+ in resting P. aurelia was at least ten times less than the extracellular concentration. Ca 2+ influx was easily measurable at 0°C, but not at 23°C. The influx of 45Ca 2+ was stimulated by the same conditions which cause membrane depolarization and ciliary reversal. Addition of Na + and K + (which stimulate ciliary reversal) resulted in a 10-fold increase in the rate of Ca 2+ influx. An externally applied, pulsed, electric field (1–2 mA/cm 2 of electrode surface), caused the rate of Ca 2+ influx to increase 3–5 times, with the extent of stimulation dependent on the current density and the pulse width Ca 2+ influx had the characteristics of a passive transport system and was associated with the chemically or electrically triggered Ca 2+ “gating” mechanism, which has been studied electrophysiologically. In contrast, Ca 2+ efflux appeared to be catalyzed by an active transport system. With cells previously loaded at 0°C with 45Ca 2+, Ca 2+ efflux was rapid at 23°C, but did not occur at 0°C. This active Ca 2+ efflux mechanism is probably responsible for maintaining the low internal Ca 2+ levels in unstimulated cells. 相似文献
18.
It is now well established that mitochondria contain three antiporters that transport monovalent cations. A latent, allosterically regulated K +/H + antiport appears to serve as a cation-extruding device that helps maintain mitochondrial volume homeostasis. An apparently unregulated Na +/H + antiport keeps matrix [Na +] low and the Na +-gradient equal to the H +-gradient. A Na +/Ca 2+ antiport provides a Ca 2+-extruding mechanism that permits the mitochondrion to regulate matrix [Ca 2+] by balancing Ca 2+ efflux against influx on the Ca 2+-uniport. All three antiports have well-defined physiological roles and their molecular properties and regulatory features are now being determined. Mitochondria also contain monovalent cation uniports, such as the recently described ATP- and glibenclamide-sensitive K + channel and ruthenium red-sensitive uniports for Na + and K +. A physiological role of such uniports has not been established and their properties are just beginning to be defined. 相似文献
19.
We have investigated the kinetics of mitochondrial Ca 2+ influx and efflux and their dependence on cytosolic [Ca 2+] and [Na +] using low-Ca 2+-affinity aequorin. The rate of Ca 2+ release from mitochondria increased linearly with mitochondrial [Ca 2+] ([Ca 2+] M). Na +-dependent Ca 2+ release was predominant al low [Ca 2+] M but saturated at [Ca 2+] M around 400 μM, while Na +-independent Ca 2+ release was very slow at [Ca 2+] M below 200 μM, and then increased at higher [Ca 2+] M, perhaps through the opening of a new pathway. Half-maximal activation of Na +-dependent Ca 2+ release occurred at 5-10 mM [Na +], within the physiological range of cytosolic [Na +]. Ca 2+ entry rates were comparable in size to Ca 2+ exit rates at cytosolic [Ca 2+] ([Ca 2+] c) below 7 μM, but the rate of uptake was dramatically accelerated at higher [Ca 2+] c. As a consequence, the presence of [Na +] considerably reduced the rate of [Ca 2+] M increase at [Ca 2+] c below 7 μM, but its effect was hardly appreciable at 10 μM [Ca 2+] c. Exit rates were more dependent on the temperature than uptake rates, thus making the [Ca 2+] M transients to be much more prolonged at lower temperature. Our kinetic data suggest that mitochondria have little high affinity Ca 2+ buffering, and comparison of our results with data on total mitochondrial Ca 2+ fluxes indicate that the mitochondrial Ca 2+ bound/Ca 2+ free ratio is around 10- to 100-fold for most of the observed [Ca 2+] M range and suggest that massive phosphate precipitation can only occur when [Ca 2+] M reaches the millimolar range. 相似文献
20.
It is well established that mitochondria are the main source of ATP production within cells. However, mitochondria have other remarkable functions, serving as important modulators of cellular Ca 2+ signaling, and it is now generally recognized that control over Ca 2+ homeostasis is intrinsically interwoven with mitochondrial abilities to adjust and tune ATP production. In this review, we describe the mechanisms that mitochondria use to balance Ca 2+ homeostasis maintenance and cell energy metabolism. In recent years, the knowledge on the molecular machinery mediating Ca 2+ influx/efflux has been improved and, albeit still open to further investigations, several lines of evidence converge on the hypothesis that plasma membrane Na +/Ca 2+ exchanger (NCX) isoforms are also expressed at the mitochondrial level, where they contribute to the Ca 2+ and Na + homeostasis maintenance. In particular, the connection between mitochondrial NCX activity and metabolic substrates utilization is further discussed here. We also briefly focus on the alterations of both mitochondrial Ca 2+ handling and cellular bioenergetics in neurodegenerative diseases, such as Parkinson’s and Alzheimer’s disease. 相似文献
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