Voltage activation of heart inner mitochondrial membrane channels

The patch clamp records obtained from mitoplast membranes prepared in the presence of a calcium chelator generally lack channel activity. However, multiconductance channel (MCC) activity can be induced by membrane potentials above +/- 60 mV [Kinnally et al., Biochem. Biophys. Res. Commun. 176, 1183-1188 (1991)]. Once activated, the MCC activity persists at all voltages. The present report characterizes the activation by voltage of multiconductance channels of rat heart inner mitochondrial membranes using patch-clamping. In some membrane patches, the size of single current transitions progressively increases with time upon application of voltage. The inhibitor cyclosporin has also been found to decrease channel conductance in steps. The results suggest that voltage-induced effects which are inhibited by cyclosporin A are likely to involve either an increase in effective pore diameter or the assembly of low-conductance units. In activated patches, we have found at high membrane potentials (e.g., 130 mV) changes in conductance as high as 5 nS occurring in large steps (up to 2.7 nS). These were generally preceded by a smaller transition. Similar results were obtained less frequently at lower voltages. These results can be explained on the assumption that once assembled the channels may act in unison.

Identification of anion and cation pathways in the inner mitochondrial membrane by patch clamping of mouse liver mitoplasts

Alkalinization of the matrix side of the mitochondrial inner membrane by pH shifts from 6.8 to 8.3 caused a reversible increase in current of 3.2 +/- 0.2 pA (mean +/- SE, n = 21) at +/- 40 mV measured using patch-clamp techniques. The current increase was reversed in a graded fashion by the addition of Mg2+ as well as a reduction in pH. Detection of single-channel events was done at 0.5, 1 and 2 M KCl. The single-channel amplitude in 0.15 M KCl corresponds to approximately 15 pS. Reversal potentials derived from whole patch currents indicated that the inner mitochondrial membrane was primarily cation selective at pH 6.8 with a PK/PCl = 32 (n = 6). Treatment with alkaline pH (8.3) increased the current and anion permeability (PK/PCl = 16, n = 6). The membrane becomes completely cation selective when low concentrations (12 microM) of the drug propranolol are added. The amphiphilic drugs amiodarone (4 microM), propranolol (70 microM) and quinine (0.6 mM) blocked almost all of the current. The pH-dependent current was also inhibited by tributyltin. These results are consistent with the presence of two pathways in the inner mitochondrial membrane. One is cation selective and generally open and the other is anion selective and induced by alkaline pH. The alkaline pH-activated channel likely corresponds to the inner membrane anion channel postulated by others from suspension studies.

Selective effect of inhibitors on inner mitochondrial membrane channels

The effect of amphiphilic cationic drugs on the channel activity of the mitochondrial inner membrane was examined with patch-clamp techniques. The therapeutic drugs amiodarone, propranolol and quinine reduced the probability of being open for the multiconductance channel (MCC) activity (levels from 30 pS to over 1 nS). While amiodarone decreased the probability of being open for the voltage dependent approximately 100 pS channel, it increased the conductance 42 +/- 20% (mean +/- SD, n = 6) with no significant change in mean open time. Similar results were obtained with propranolol. These data indicate that the approximately 100 pS channel is distinct from MCC activity.

Anatomic examination of a case of open trigeminal nucleotomy (nucleus caudalis dorsal root entry zone lesions) for facial pain

Nucleus caudalis dorsal root entry zone lesions (open trigeminal nucleotomy) are a surgical procedure which can achieve pain control without major complications in the difficult clinical setting of deafferentation-type facial pain. Two patients are reported, who had relief of pain, but also experienced neurological complications. One patient succumbed to pulmonary complications, which provided the opportunity for anatomic analysis of the lesioned area, which is discussed in detail. Potential modifications of the surgical technique are suggested.

Properties of channels in the mitochondrial outer membrane

Patch-clamping studies with native outer mitochondrial membranes show a complex behavior. In the range of potentials in which the polarity of the pipette is positive, the behavior resembles that of VDAC incorporated into bilayers. Accordingly, there is a decrease in conductance with voltage. An involvement of VDAC is also supported by responses of the patches to the presence of polyanion or treatment with succinic anhydride, both of which affect VDAC. In contrast, in the negative range of potential, the conductance of the patches generally increases with the magnitude of the voltage. The increase in conductance shows a biphasic time course which is consistent with a model in which channels are first activated (first phase) and then assembled into larger high-conductance channels (second phase). A variety of experiments support the notion that an assembly takes place. The time course of the conductance increase is consistent with formation of the second-phase channels from 6 +/- 1 subunits.

Mitochondrial channel activity studied by patch-clamping mitoplasts

Patch-clamping mitoplasts, we have observed a complex pattern of conductance transitions. This report discusses primarily the 45, 120-150, 350, and 1,000 pS transitions.

Kinetics of voltage-induced conductance increases in the outer mitochondrial membrane

The kinetics of the increase in conductance in the outer mitochondrial membrane induced by patch-clamping at various negative potentials (pipette inside negative) are reported. The changes are biphasic, a rapid increase is followed by a slowly developing larger change. The results can be predicted by a model in which an initial activation of channels is followed by their assembly into highly conducting channels. The model suggests that five to seven activated subunits form each high-conductance channel.

Evidence for a novel voltage-activated channel in the outer mitochondrial membrane

Patch-clamp studies of the outer mitochondrial membrane indicate a voltage-dependent increase in conductance for potentials positive relative to the exterior of the mitochondrion. The time course of the conductance changes is consistent with an activation of channels. Voltage pulse experiments suggest that the activation phenomenon corresponds to assembly of the channels from subunits with disassembly occurring after recovery of the original conductance. Effects of temperature and concanavalin A on the voltage-induced conductances are also consistent with a channel assembly model.

Channels in the mitochondrial outer membrane: evidence from patch clamp studies

Patch clamp techniques were applied to outer mitochondrial membranes of giant mitochondria from mice kept on a cuprizone diet or to vesicles produced by fusing membranes derived from the outer membrane of Neurospora mitochondria. In the negative range of potentials the conductances decreased with increases in the magnitude of voltage, suggesting the closing of channels. Experiments in which mitochondria were treated with the polyanion polymethacrylate maleate styrene (1:2:3) or succinic anhydride suggest that the channels correspond to VDAC. Although sometimes conductance also decreased with increasing potential over a narrow range of positive potentials, more commonly the conductances increased. Although this phenomenon may represent a detachment of the patch, the changes in conductance are reversible, suggesting that they correspond to the formation or the opening of channels.

Importance of the mitochondrial outer membrane channel as a model biological channel

The channels of the mitochondrial outer membrane represent a useful model for studies into the mechanisms underlying phenomena of voltage-dependent gating and ion selectivity.

Patch clamping the outer mitochondrial membrane

Intact giant mitochondria isolated from the liver of mice fed a diet containing cuprizone were studied using patch microelectrodes. The current-voltage curves were nonlinear, suggesting the presence of voltage-sensitive channels. In the negative range of voltage, the channels appear to close with increasing magnitude of the voltage. The dependence of the conductance on voltage is similar to that of the outer membrane channels (VDAC) studied in planar bilayers. Occasionally, over a narrow range of positive potentials, the conductance also decreases as in the bilayer studies. However, more frequently the conductance increases sharply in a completely reversible manner at potentials greater than 10 to 20 mV. The increase in conductance with voltage may be interpreted as a major rearrangement of membrane components. Qualitatively comparable results were obtained using fused outer membranes isolated from Neurospora mitochondria. The behavior of VDAC is affected by treatment with succinic anhydride or the polyanion, polymethacrylate, maleate, styrene (1:2:3). We have found similar effects in the negative range of potentials in patches from giant mitochondria treated in the same fashion.

Protonmotive force and photophosphorylation in single swollen thylakoid vesicles

Swollen vesicles generally 40 micron in diameter were prepared from spinach chloroplasts. These vesicles appear to originate from thylakoids. The present study reports results obtained with individual vesicles using micromanipulative procedures. The electric potential across the membrane was measured with microelectrodes and the pH of the internal space was calculated from the fluorescence of the pH indicator pyranine. The individual vesicles photophosphorylate as measured with luciferin-luciferase. Impalement with microelectrodes did not affect the ability of individual vesicles to photophosphorylate. However, there was no significant membrane potential either with continuous illumination or light flashes. In contrast, we found a delta pH of 3.7 under photophosphorylative conditions and the incubation with the appropriate buffers blocked photophosphorylation presumably by preventing formation of a pH gradient. We propose that, in these vesicles, the membrane potential plays no role in photophosphorylation, whereas a pH gradient is obligatory.

The internal concentration of K+ in isolated rat liver mitochondria

A simple osmotic method has been developed to determine the internal K+ concentration of mitochondria by determining the concentration of external K+ at constant osmotic pressure at which metabolically inhibited mitochondria neither shrink nor swell. This concentration has been found to correspond to approx. 80-85 mM in freshly isolated mitochondria and considerably lower after additional centrifugation procedures. Since mitochondria are in osmotic equilibrium with the suspending medium (in this case, 0.32 osmolal), and K+ is the primary exchangeable internal ion, a significant proportion of the internal osmotic pressure must be exerted by the sucrose. Results for experiments determining internal K+ after centrifuging mitochondria at various G values confirm the reports of Sitaramam et al. (Sitaraman, V. and Sarma, M.K.J. (1981) Proc. Natl. Acad. Sci. USA 78, 3441-3445 and Sambasivarao, D. and Sitaramam, V. (1983) Biochim. Biophys. Acta 722, 256-270) that centrifugation induces the entry of sucrose in mitochondria isolated in a sucrose medium.

Permeability properties of mammalian cell nuclei in living cells and in vitro

The present study evaluates the role of the nuclear envelope in mammalian cells by applying two different approaches using either intact cells of mouse liver or isolated nuclei. In one approach the nuclei were studied with microelectrodes. The transmembrane voltage drop produced by passing current through an impaling microelectrode was measured with a second impaling microelectrode. In the second approach, the permeability of the nuclear envelope was studied by injection of a series of fluorescent probes. Lucifer Yellow CH and a variety of exogenous proteins labelled by conjugation with Lucifer Yellow VS were delivered into either the cytoplasm or the nucleus in situ. The fluorescence of the probe was followed either with a video camera or photographically. The results agree with the idea that the mammalian nuclear envelope is permeable to rather large molecules. Molecules with estimated radii below 2.4 nm seem to exchange rapidly, whereas molecules with estimated radii of 2.8 nm or above are excluded. The low electrical resistance of the envelope yields an estimate of pore radius, in the range of 3.4-6.5 nm.

Assay of ATP synthesis using single giant mitochondria

Two assays of the capacity of single giant mitochondria or mitoplasts to phosphorylate ATP from Pi and ADP have been developed. One depends on the placement of a single mitochondrion next to a glycerinated myofibril, by micromanipulation. With the appropriate controls, contraction of the myofibril serves as an indication of ATP synthesis. The other assay similarly requires the isolation of one single mitochondrion but the assay of ATP synthesized uses the luciferin-luciferase reaction with a conventional photometric system. With the latter, we found that either impalement with microelectrodes or electrophoretic microinjection of two dyes, Lucifer Yellow CH or pyranine , into the inner space have no effect on the phosphorylative capacity of mitochondria or mitoplasts. The electric potential across the mitochondrial membrane was monitored during the assay and found to be small and generally positive inside.

Protonmotive force in giant mitochondria

The accompanying communication [Eur. J. Biochem. 141 (1984) 1-4] indicates that the microinjection of the pH fluorescent indicator pyranine (8-hydroxy-1,2,6- pyrenesulfonate ) into giant mitochondria or mitoplasts does not affect their ability to carry out oxidative phosphorylation. The dye can therefore be used as a quantitative indicator of internal mitochondrial pH. We found that activation of metabolism in rotenone-inhibited giant mitochondria by the addition of succinate produces an internal pH change corresponding to a pH shift of 0.3 to the alkaline range, approximately the same value found previously for conventional rat liver mitochondria.

Kinetics of Lucifer yellow CH efflux in giant mitochondria

The fluorescent dye Lucifer yellow CH was microinjected electrophoretically into giant mitochondria isolated from mice maintained on a diet containing cuprizone. The dye was retained by the mitochondria, indicating that it was contained in a space bounded by a selectively permeable membrane. The labelling was reversible by reversing the polarity of the current. A study of the disappearance of the fluorescence indicates that the permeability of the mitochondrial membrane to the dye (probably the lithium and/or the potassium salts) ranges from 10(-7) to 10(-8) cm/s.

Adenosine triphosphate synthesis coupled to K+ influx in mitochondria

The influx of K+ into swollen mitochondria in the presence of valinomycin results in the synthesis of adenosine triphosphate in which approximately one H+ disappears per adenosine triphosphate synthesized. The synthesis is blocked by atractyloside but is insensitive to oligomycin and relatively insensitive to uncouplers.

The transport of cations in mitochondria

The present paper has reviewed several factors related to ion transport and examined the properties of cation transport in mitochondria. The analysis suggests that: (1) The concept that a metabolically dependent electrical potential across the mitochondrial membrane plays a role in determining ion fluxes and steady-state concentrations is not justified and the data indicate that such exchanges are generally electroneutral. (2) Generally, the influx and efflux of an ion proceed by the same mechanism with at least one exception. (3) There are indications that some of the steps in transport are common to several cations. (4) The idea that carrier or ionophoric molecules are involved in cation transport has been examined in some detail together with the possible involvement of some known mitochondrial components. In particular, a model has been introduced in which local charge imbalances produced by H+ fluxes serve as the driving force of transport. The molecules of the complex are arranged in series in a tripartite arrangement including a filter or gate, a nonselective channel and an H+-transferring portion linked to either electron transport or the ATPase. Parts of this model have been introduced by other investigators. Models in which different portions of channels have differing functions have been proposed previously for other transport systems.

Electrophoretic injection of a fluorescent dye into giant mitochondria and mitoplasts

Studies of the electrical properties of giant mitochondria and mitoplasts with microelectrodes have indicated that there are no significant metabolically dependent membrane potentials. The internal location of the microelectrode has been confirmed by electrophoretically microinjecting the water-soluble dye Lucifer yellow CH into giant mitochondria or mitoplasts.

Use of dyes to estimate the electrical potential of the mitochondrial membrane

A number of cationic or anionic fluorescent dyes were investigated as possible monitors of the membrane potential of rat liver mitochondria, and giant mitochondria isolated from the liver of mice maintained on a diet containing cuprizone. The fluorescence of four dyes (8-anilino-1-naphthalenesulfonic acid, merocyanine 540, 3,3'-dipropyl-thiocarbocyanine, and bis[1,3-dibutylbarbituric acid-(5)]-pentamethine oxonol) was found to respond appropriately to changes in an apparent K+ diffusion potential. Generally, valinomycin-induced K+ diffusion potentials as calculated using the Nernst equation were used to calibrate the dependence of the fluorescence on the membrane potential. The appropriateness of this approach was verified for two dyes using microelectrodes in giant mitochondria. The apparent membrane potential change induced by the addition of succinate was variable but was very low and generally less than 60 mV in magnitude. The results are consistent with the notion that a large membrane potential is not established upon the initiation of metabolism and that the membrane potential does not play a significant role in the observed ADP phosphorylation.