Inner mitochondrial membrane anion channel is present in brown adipocytes but is not identical with the uncoupling protein

Patch-clamp technique to study mitochondrial membrane biophysics

Mitochondria are double-membrane organelles crucial for oxidative phosphorylation, enabling efficient ATP synthesis by eukaryotic cells. Both of the membranes, the highly selective inner mitochondrial membrane (IMM) and a relatively porous outer membrane (OMM), harbor a number of integral membrane proteins that help in the transport of biological molecules. These transporters are especially enriched in the IMM, where they help maintain transmembrane gradients for H+, K+, Ca2+, PO43-, and metabolites like ADP/ATP, citrate, etc. Impaired activity of these transporters can affect the efficiency of energy-transducing processes and can alter cellular redox state, leading to activation of cell-death pathways or metabolic syndromes in vivo. Although several methodologies are available to study ion flux through membrane proteins, the patch-clamp technique remains the gold standard for quantitatively analyzing electrogenic ion exchange across membranes. Direct patch-clamp recordings of mitoplasts (mitochondria devoid of outer membrane) in different modes, such as whole-mitoplast or excised-patch mode, allow researchers the opportunity to study the biophysics of mitochondrial transporters in the native membrane, in real time, in isolation from other fluxes or confounding factors due to changes in ion gradients, pH, or mitochondrial potential (ΔΨ). Here, we summarize the use of patch clamp to investigate several membrane proteins of mitochondria. We demonstrate how this technique can be reliably applied to record whole-mitoplast Ca2+ currents mediated via mitochondrial calcium uniporter or H+ currents mediated by uncoupling protein 1 and discuss critical considerations while recording currents from these small vesicles of the IMM (mitoplast diameter = 2-5 µm).

Patch-Clamp Analysis of the Mitochondrial H+ Leak in Brown and Beige Fat

Mitochondria convert the chemical energy of metabolic substrates into adenosine triphosphate (ATP) and heat. Although ATP production has become a focal point of research in bioenergetics, mitochondrial thermogenesis is also crucial for energy metabolism. Mitochondria generate heat due to H+ leak across the inner mitochondrial membrane (IMM) which is mediated by mitochondrial uncoupling proteins. The mitochondrial H+ leak was first identified, and studied for many decades, using mitochondrial respiration technique. Unfortunately, this method measures H+ leak indirectly, and its precision is insufficient for the rigorous insight into the mitochondrial function at the molecular level. Direct patch-clamp recording of H+ leak would have a significantly higher amplitude and time resolution, but application of the patch-clamp technique to a small subcellular organelle such as mitochondria has been challenging. We developed a method that facilitates patch-clamp recording from the whole IMM, enabling the direct measurement of small H+ leak currents via uncoupling proteins and thus, providing a rigorous understanding of the molecular mechanisms involved. In this paper we cover the methodology of measuring the H+ leak in mitochondria of specialized thermogenic tissues brown and beige fat.

Electro-pharmacological profiles of two brain mitoplast anion channels: Inferences from single channel recording

We have characterized the conduction and blocking properties of two different chloride channels from brain mitochondrial inner membranes after incorporation into planar lipid bilayers. Our experiments revealed the existence of channels with a mean conductance of 158 ± 7 and 301 ± 8 pS in asymmetrical 200 mM cis/50 mM trans KCl solutions. We determined that the channels were ten times more permeable for Cl⁻ than for K⁺, calculated from the reversal potential using the Goldman-Hodgkin-Katz equation. The channels were bell-shaped voltage dependent, with maximum open probability 0.9 at ± 20 mV. Two mitochondrial chloride channels were blocked after the addition of 10 µM DIDS. In addition, 158 pS chloride channel was blocked by 300 nM NPPB, acidic pH and 2.5 mM ATP, whereas the 301 pS chloride channel was blocked by 600 µM NPPB but not by acidic pH or ATP. Gating and conducting behaviors of these channels were unaffected by Ca²⁺. These results demonstrate that the 158 pS anion channel present in brain mitochondrial inner membrane, is probably identical to IMAC and 301 pS Cl channel displays different properties than those classically described for mitochondrial anion channels.

Anion Channels of Mitochondria

Mitochondria are the "power house" of a cell continuously generating ATP to ensure its proper functioning. The constant production of ATP via oxidative phosphorylation demands a large electrochemical force that drives protons across the highly selective and low-permeable mitochondrial inner membrane. Besides the conventional role of generating ATP, mitochondria also play an active role in calcium signaling, generation of reactive oxygen species (ROS), stress responses, and regulation of cell-death pathways. Deficiencies in these functions result in several pathological disorders like aging, cancer, diabetes, neurodegenerative and cardiovascular diseases. A plethora of ion channels and transporters are present in the mitochondrial inner and outer membranes which work in concert to preserve the ionic equilibrium of a cell for the maintenance of cell integrity, in physiological as well as pathophysiological conditions. For, e.g., mitochondrial cation channels KATP and BKCa play a significant role in cardioprotection from ischemia-reperfusion injury. In addition to the cation channels, mitochondrial anion channels are equally essential, as they aid in maintaining electro-neutrality by regulating the cell volume and pH. This chapter focusses on the information on molecular identity, structure, function, and physiological relevance of mitochondrial chloride channels such as voltage dependent anion channels (VDACs), uncharacterized mitochondrial inner membrane anion channels (IMACs), chloride intracellular channels (CLIC) and the aspects of forthcoming chloride channels.

Role of adipocytes in hypertension

Although it has known for some time that obesity is associated with salt sensitivity and hypertension, recent data suggests that the adipocyte may actually be the proximate cause of this physiological changes. In the following review, the data demonstrating this association as well as the potentially operative pathophysiological mechanisms are reviewed and discussed.

Mitochondrial channels: ion fluxes and more

The field of mitochondrial ion channels has recently seen substantial progress, including the molecular identification of some of the channels. An integrative approach using genetics, electrophysiology, pharmacology, and cell biology to clarify the roles of these channels has thus become possible. It is by now clear that many of these channels are important for energy supply by the mitochondria and have a major impact on the fate of the entire cell as well. The purpose of this review is to provide an up-to-date overview of the electrophysiological properties, molecular identity, and pathophysiological functions of the mitochondrial ion channels studied so far and to highlight possible therapeutic perspectives based on current information.

Mitochondrial chloride channels: electrophysiological characterization and pH induction of channel pore dilation

Physiological and pathological functions of mitochondria are highly dependent on the properties and regulation of mitochondrial ion channels. There is still no clear understanding of the molecular identity, regulation, and properties of anion mitochondrial channels. The inner membrane anion channel (IMAC) was assumed to be equivalent to mitochondrial centum picosiemens (mCS). However, the different properties of IMAC and mCS channels challenges this opinion. In our study, we characterized the single-channel anion selectivity and pH regulation of chloride channels from purified cardiac mitochondria. We observed that channel conductance decreased in the order: Cl⁻ > Br⁻ > I⁻ > chlorate ≈ formate > acetate, and that gluconate did not permeate under control conditions. The selectivity sequence was Br⁻ ≥ chlorate ≥ I⁻ ≥ Cl⁻ ≥ formate ≈ acetate. Measurement of the concentration dependence of chloride conductance revealed altered channel gating kinetics, which was demonstrated by prolonged mean open time value with increasing chloride concentration. The observed mitochondrial chloride channels were in many respects similar to those of mCS, but not those of IMAC. Surprisingly, we observed that acidic pH increased channel conductance and that an increase of pH from 7.4 to 8.5 reduced it. The gluconate current appeared and gradually increased when pH decreased from pH 7.0 to 5.6. Our results indicate that pH regulates the channel pore diameter in such a way that dilation increases with more acidic pH. We assume this newly observed pH-dependent anion channel property may be involved in pH regulation of anion distribution in different mitochondrial compartments.

Modulation of the hepatocyte rough endoplasmic reticulum single chloride channel by nucleotide-Mg2+ interaction

The effect of nucleotides on single chloride channels derived from rat hepatocyte rough endoplasmic reticulum vesicles incorporated into bilayer lipid membrane was investigated. The single chloride channel currents were measured in 200/50 mmol/l KCl cis/trans solutions. Adding 2.5 mM adenosine triphosphate (ATP) and adenosine diphosphate (ADP) did not influence channel activity. However, MgATP addition inhibited the chloride channels by decreasing the channel open probability (Po) and current amplitude, whereas mixture of Mg(2+) and ADP activated the chloride channel by increasing the Po and unitary current amplitude. According to the results, there is a novel regulation mechanism for rough endoplasmic reticulum (RER) Cl(-) channel activity by intracellular MgATP and mixture of Mg(2+) and ADP that would result in significant inhibition by MgATP and activation by mixture of Mg(2+) and ADP. These modulatory effects of nucleotide-Mg(2+) complexes on chloride channels may be dependent on their chemical structure configuration. It seems that Mg-nucleotide-ion channel interactions are involved to produce a regulatory response for RER chloride channels.

Mitochondrial ion channels as therapeutic targets

The study of mitochondrial ion channels changed our perception of these double-wrapped organelles from being just the power house of a cell to the guardian of a cell's fate. Mitochondria communicate with the cell through these special channels. Most of the time, the message is encoded by ion flow across the mitochondrial outer and inner membranes. Potassium, sodium, calcium, protons, nucleotides, and proteins traverse the mitochondrial membranes in an exquisitely regulated manner to control a myriad of processes, from respiration and mitochondrial morphology to cell proliferation and cell death. This review is an update on both well established and putative mitochondrial channels regarding their composition, function, regulation, and therapeutic potential.

Modulation of intracellular chloride channels by ATP and Mg2+

We report the effects of ATP and Mg2+ on the activity of intracellular chloride channels. Mitochondrial and lysosomal membrane vesicles isolated from rat hearts were incorporated into bilayer lipid membranes, and single chloride channel currents were measured. The observed chloride channels (n=112) possessed a wide variation in single channel parameters and sensitivities to ATP. ATP (0.5-2 mmol/l) modulated and/or inhibited the chloride channel activities (n=38/112) in a concentration-dependent manner. The inhibition effect was irreversible (n=5/93) or reversible (n=15/93). The non-hydrolysable ATP analogue AMP-PNP had a similar inhibition effect as ATP, indicating that phosphorylation did not play a role in the ATP inhibition effect. ATP modulated the gating properties of the channels (n=6/93), decreased the channels' open dwell times and increased the gating transition rates. ATP (0.5-2 mmol/l) without the presence of Mg2+ decreased the chloride channel current (n=12/14), whereas Mg2+ significantly reversed the effect (n=4/4). We suggest that ATP-intracellular chloride channel interactions and Mg2+ modulation of these interactions may regulate different physiological and pathological processes.

Inhibition of anion channels derived from mitochondrial membranes of the rat heart by stilbene disulfonate--DIDS

The objective of this work was to characterize in more detail the inhibition effect of diisothiocyanatostilbene-2',2-disulfonic acid (DIDS) on anion channels isolated from the rat heart mitochondria. The channels reconstituted into a planar lipid membrane displayed limited powers of discrimination between anions and cations and the ion conductance measured under asymmetric (250/50 mM KCl, cis/trans) and symmetric (150 mM KCl) conditions was approximately 100 pS. DIDS caused a dramatic decrease in the channel activity (IC(50) = 11.7 +/- 3.1 microM) only when it was added to the cis side of a planar lipid membrane. The inhibition was accompanied by the significant prolongation of closings and the shortening of openings within the burst as well as gaps between bursts were prolonged and durations of bursts were reduced. The blockade was complete and irreversible when concentration of DIDS was increased up to 200 microM. Our data indicate that DIDS is an allosteric blocker of mitochondrial anion channels and this specific effect could be used as a tool for reliable identification of anion channels on the functional level.

Stilbene derivatives inhibit the activity of the inner mitochondrial membrane chloride channels

Ion channels selective for chloride ions are present in all biological membranes, where they regulate the cell volume or membrane potential. Various chloride channels from mitochondrial membranes have been described in recent years. The aim of our study was to characterize the effect of stilbene derivatives on single-chloride channel activity in the inner mitochondrial membrane. The measurements were performed after the reconstitution into a planar lipid bilayer of the inner mitochondrial membranes from rat skeletal muscle (SMM), rat brain (BM) and heart (HM) mitochondria. After incorporation in a symmetric 450/450 mM KCl solution (cis/trans), the chloride channels were recorded with a mean conductance of 155 ± 5 pS (rat skeletal muscle) and 120 ± 16 pS (rat brain). The conductances of the chloride channels from the rat heart mitochondria in 250/50 mM KCl (cis/trans) gradient solutions were within the 70–130 pS range. The chloride channels were inhibited by these two stilbene derivatives: 4,4′-diisothiocyanostilbene-2,2′-disulfonic acid (DIDS) and 4-acetamido-4′-isothiocyanostilbene-2,2′-disulfonic acid (SITS). The skeletal muscle mitochondrial chloride channel was blocked after the addition of 1 mM DIDS or SITS, whereas the brain mitochondrial channel was blocked by 300 μM DIDS or SITS. The chloride channel from the rat heart mitochondria was inhibited by 50–100 μM DIDS. The inhibitory effect of DIDS was irreversible. Our results confirm the presence of chloride channels sensitive to stilbene derivatives in the inner mitochondrial membrane from rat skeletal muscle, brain and heart cells.

Evidence for a large conductance voltage gated cationic channel in rough endoplasmic reticulum of rat hepatocytes

In this work, we report the single channel characterization of a voltage gated cationic channel from rough endoplasmic reticulum (RER) membranes of rat hepatocytes incorporated into a planar lipid bilayer. The channel was found to be cation selective with a main conductance of 598+/-20 pS in 200 mM KCl cis/50 mM KCl trans. The channel open probability appeared voltage dependent with a voltage for half activation (V(1/2)) of 38 mV and an effective gating charge z of -6.66. Adding either 4-AP (5 mM) or ATP (2.5 mM) to the side corresponding to the cell internal medium caused a strong inhibition of the channel activity. This channel is likely to be involved in maintaining proper cation homeostasis in the RER of hepatocytes.

Bongkrekic acid and atractyloside inhibits chloride channels from mitochondrial membranes of rat heart

The aim of this work was to characterize the effect of bongkrekic acid (BKA), atractyloside (ATR) and carboxyatractyloside (CAT) on single channel properties of chloride channels from mitochondria. Mitochondrial membranes isolated from a rat heart muscle were incorporated into a bilayer lipid membrane (BLM) and single chloride channel currents were measured in 250/50 mM KCl cis/trans solutions. BKA (1-100 microM), ATR and CAT (5-100 microM) inhibited the chloride channels in dose-dependent manner. The inhibitory effect of the BKA, ATR and CAT was pronounced from the trans side of a BLM and it increased with time and at negative voltages (trans-cis). These compounds did not influence the single channel amplitude, but decreased open dwell time of channels. The inhibitory effect of BKA, ATR and CAT on the mitochondrial chloride channel may help to explain some of their cellular and/or subcellular effects.

Ca2+-activated K channel of the BK-type in the inner mitochondrial membrane of a human glioma cell line

A single channel current was recorded from mitoplasts (i.e., inner mitochondrial membrane) of the human glioma cell line LN229 using patch-clamp techniques in the mitoplast-attached mode. We frequently found a 295 +/- 18 pS channel that showed a straight i-E relation in the range +/-60 mV in 150 mM KCl solutions on either side of the mitoplast. If KCl in the bath was exchanged against NaCl, outward currents were undetectable, indicating potassium selectivity. Channel activity determined as open probability increased with increasing Ca2+ concentrations (EC50 = 0.9 microM at 60 mV). Open probability was voltage dependent. An e-fold increase of time spent in the open state was induced by a depolarization of 10.5 mV. Open probability was decreased by charybdotoxin concentration and voltage dependently (EC50 = 1.4 nM). In conclusion, we show for the first time that the inner mitochondrial membrane in human glioma cells contains a calcium-dependent K channel of the BK-type.

Patch clamp investigation into the phosphate carrier from Saccharomyces cerevisiae mitochondria

After heterologous expression in E. coli, functionally active phosphate carrier (PIC) from Saccharomyces cerevisiae mitochondria was purified and reconstituted into giant liposomes and used for patch clamp experiments. Single channel currents across excised patches revealed an anion channel function of the PIC protein. Besides the three transport modes known to date, namely phosphate/phosphate exchange, phosphate/OH exchange and mercurial-induced unidirectional transport, this channel activity represents the fourth transport mode of the PIC. The PIC channel activity was sensitive towards phosphate as its physiological substrate. Phosphate (10 mM) blocked in a specific but reversible manner the PIC channel, suggesting a phosphate-dependent conformational change of the protein into the carrier mode. Furthermore, the current through the channel and its gating activity were affected by divalent cations. In the presence of Ca2+ and Mg2+, the channel displayed a mean conductance of 25 +/- 5 pS whereas 40 +/- 10 pS was observed in the absence of divalent cations. Also, the dwell times in either the open or closed state of the PIC channel appeared to be prolonged in the presence of Ca2+ and Mg2+. The observed PIC channel characteristics are discussed with respect to previously reported electrophysiological in situ measurements on anion channels of the inner mitochondrial membrane. Similarities of the PIC channel to the inner mitochondrial anion channel (IMAC) have been found.

Chloride channel properties of the uncoupling protein from brown adipose tissue mitochondria: a patch-clamp study

The uncoupling protein (UCP) from brown adipose tissue mitochondria possesses H+ and Cl- transport activities [reviewed in Klingenberg, M. (1990) Trends Biochem. Sci. 15, 108-112]. Being a member of a mitochondrial carrier family, the transport of H+ and Cl- is carrier-like, i.e., much slower as compared to channels. Here we report that UCP reconstituted into giant liposomes displays stable chloride channel properties under patch-clamp conditions. The transport inhibitors (GTP, GDP, ATP, and ADP) also inhibit this channel in a reversible way, showing that the channel activity is associated with UCP. The slightly inward-rectifying chloride channel has a unit conductance of approximately 75 pS in symmetrical 100 mM KCl and closes at high positive potentials on the matrix side of UCP. Channel gatings switch from slow open-closure transitions to fast flickerings as the holding potential increases over +60 mV. Substitution experiments reveal a strong discrimination against cations [P(Cl-)/P(K+) approximately 17] and a permeability ratio order of Cl- > Br- > F- > SCN- > I- > NO3- > SO4(2-) > HPO4(2-) > gluconate. Nucleotide inhibition studies indicate that 70% UCP molecules had its matrix side oriented outside in the giant liposomes. Fatty acids, pH, divalent cations (Ca2+ and Mg2+), and mersalyl do not influence these Cl- currents. The Cl- channel can be blocked by 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (DIDS) from the matrix side of UCP. The data are consistent with a dimer consisting of two monomeric 75-pS Cl- channels or with a monomeric 150-pS channel having a 50% subconductance state. The channel current increases with Cl- concentration showing a typical saturation curve with Km approximately 63 mM and gmax approximately 120 pS (100 mM KCl in the pipet). The Cl- conductance measured under these conditions is 6 orders of magnitude higher than the Cl- transport activity reported earlier, suggesting that the UCP has the potential of behaving as an anion channel.

Temperature dependence of the mitochondrial inner membrane anion channel. The relationship between temperature and inhibition by protons

In this paper, we investigate the temperature and pH dependence of the mitochondrial inner membrane anion channel (IMAC) that is believed to be involved in mitochondrial volume homeostasis. At pH 7. 4, the flux of malonate is highly temperature-dependent with rates increasing from 1 nmol/min mg at 5 degrees C to 1900 nmol/min mg at 45 degrees C. The Arrhenius plot is nonlinear with the activation energy increasing from 21 kJ/mol (Q10 = 1.3) to 193 kJ/mol (Q10 = 13) as the temperature is decreased. This temperature dependence is unusual and not seen with solutes that are transported through the bilayer such as NH4OAc, malonamide, and KSCN (plus valinomycin) or even for cytochrome c oxidase-dependent uptake of potassium (plus valinomycin). The temperature dependence of IMAC is closely related to the inhibition of IMAC by protons. Thus, we find that the pIC50 for protons decreases from 9.3 (Hill coefficient = 1.0) at 5 degrees C to 7.1 (Hill coefficient = 2.5) at 45 degrees C. This behavior is explained on the basis of a new kinetic model for IMAC in which the net open probability is not only modulated by the binding of three protons but also by temperature via effects on the open probability of the unprotonated channel and the pK of one of the inhibitory protonation sites.

Anion channels of the inner membrane of mammalian and yeast mitochondria

The inner membrane of yeast and mammalian mitochondria has been studied in situ with a patch clamp electrode. Anion channels were found in both cases, although their behavior and regulation are different. In mammalian mitochondria, the principal channel is of around 100 pS conductance and opens mainly under depolarized membrane potentials. As no physiological compound able to alter its peculiar voltage dependence has yet been found, it is proposed that this channel may serve as a safeguard mechanism for recharging the mitochondrial membrane potential. Two other anion channels, each with a distinct conductance (one of approx. 45 pS, the second of at least a tenfold higher value) and kinetics are harbored in the yeast inner membrane. Matrix ATP was found to interact with both, but with a different mechanism. It is proposed that the 45 pS channel may be involved in the homeostatic mechanism of mitochondrial volume.

Electrophysiology of the inner mitochondrial membrane

The application of electrophysiological techniques to mitochondrial membranes has allowed the observation and partial characterization of several ion channels, including an ATP-sensitive K(+)-selective one, a high-conductance "megachannel", a 107 pS anionic channel and three others studied at alkaline pH's. A reliable correlation with the results of non-electrophysiological studies has been obtained so far only for the first two cases. Activities presumed to be associated with the Ca2+ uniporter and with the adenine nucleotide translocator, as well as the presence of various other conductances have also been reported. The review summarizes the main properties of these pores and their possible relationship to permeation pathways identified in biochemical studies.

Single anion channels reconstituted from cardiac mitoplasts

Ion channels from sheep cardiac mitoplast (inverted inner mitochondrial membrane vesicle) preparations were incorporated into voltage-clamped planar lipid bilayers. The appearance of anion rather than cation channels could be promoted by exposing the bilayers to osmotic gradients formed by Cl- salts of large, relatively impermeant, cations at a pH of 8.8. Two distinct activities were identified. These comprised a multisubstate anion channel of intermediate conductance (approximately 60 pS in 300 vs. 50 mM choline Cl, approximately 100 pS in symmetric 150 mM KCl), and a lower-conductance anion channel (approximately 25 or approximately 50 pS in similar conditions), which only displayed two well-defined substates, at approximately 25 and approximately 50% of the fully open state. The larger channels were not simple multiples of the lower-conductance channels, but both discriminated poorly, and to a similar extent, between anions and cations (PCl-/Pcholine+ approximately 12, PCl-/PK+ approximately 8). The lower-conductance channel was only minimally selective between different anions (PNO3-(1.0) = PCl- > PBr- > PI- > PSCN-(0.8)), and its conductance failed to saturate even in high (> 1.0 M) activities of KCl. The channels were not obviously voltage dependent, and they were unaffected by 0.5 mM SITS, H2O2, propranolol, quinine or amitriptyline, or by 2 mM ATP, or by variations in pH (5.5-8.8). Ca2+ and Mg2+ did not alter single channel activity, but did modify single current amplitudes in the lower-conductance channel. This effect, together with voltage-dependent substate behavior, is described in the following paper.

Channels in mitochondrial membranes: knowns, unknowns, and prospects for the future

Rapid diffusion of hydrophilic molecules across the outer membrane of mitochondria has been related to the presence of a protein of 29 to 37 kDa, called voltage-dependent anion channel (VDAC), able to generate large aqueous pores when integrated in planar lipid bilayers. Functional properties of VDAC from different origins appear highly conserved in artificial membranes: at low transmembrane potentials, the channel is in a highly conducting state, but a raise of the potential (both positive and negative) reduces drastically the current and changes the ionic selectivity from slightly anionic to cationic. It has thus been suggested that VDAC is not a mere molecular sieve but that it may control mitochondrial physiology by restricting the access of metabolites of different valence in response to voltage and/or by interacting with a soluble protein of the intermembrane space. The latest application of the patch clamp and tip-dip techniques, however, has indicated both a different electric behavior of the outer membrane and that other proteins may play a role in the permeation of molecules. Biochemical studies, use of site-directed mutants, and electron microscopy of two-dimensional crystal arrays of VDAC have contributed to propose a monomeric beta barrel as the structural model of the channel. An important insight into the physiology of the inner membrane of mammalian mitochondria has come from the direct observation of the membrane with the patch clamp. A slightly anionic, voltage-dependent conductance of 107 pS and one of 9.7 pS, K(+)-selective and ATP-sensitive, are the best characterized at the single channel level. Under certain conditions, however, the inner membrane can also show unselective nS peak transitions, possibly arising from a cooperative assembly of multiple substrates.

The 'ins' and 'outs' of mitochondrial membrane channels

The outer membrane of the mitochondrion contains thousands of copies of a pore-forming protein called VDAC or porin. Considerable progress has been made towards elucidating the molecular structure of this channel. Moreover, mounting evidence that the permeability of VDAC may be regulated is challenging the textbook notion of the outer membrane as a simple sieve. Numerous other channel activities have been detected by electrophysiol approaches in both the outer and inner mitochondrial membranes. The inner-membrane channels do not appear to be open under normal physiological conditions and so should not dissipate energy-transducing ion gradients. The biological functions of the different classes of mitochondrial channels are uncertain, but several possibilities (including protein translocation) are being explored.

Modulation of inner mitochondrial membrane channel activity

Three classes of inner mitochondrial membrane (IMM) channel activities have been defined by direct measurement of conductance levels in membranes with patch clamp techniques in 150 mM KCl. The "107 pS activity" is slightly anion selective and voltage dependent (open with matrix positive potentials). "Multiple conductance channel" (MCC) activity includes several levels from about 40 to over 1000 pS and can be activated by voltage or Ca2+. MCC may be responsible for the Ca(2+)-induced permeability transition observed with mitochondrial suspensions. A "low conductance channel" (LCC) is activated by alkaline pH and inhibited by Mg2+. LCC has a unit conductance of about 15 pS and may correspond to the inner membrane anion channel, IMAC, which was proposed from the results obtained from suspension studies. All of the IMM channels defined thus far appear to be highly regulated and have a low open probability under physiological conditions. A summary of what is known about IMM channel regulation and pharmacology is presented and possible physiological roles of these channels are discussed.