Mitochondrial membrane potentials measured with microelectrodes: probable ionic basis

The membrane potentials of isolated Drosophila mitochondria have been measured under a variety of conditions by microelectrodes driven by a piezoelectric device. The results support the interpretation that the potentials are the result of a distribution of ions imposed by a Donnan effect.

Arteriovenous malformations of the basal ganglia region: rationale for surgical management

Surgical indications for arteriovenous malformations (AVMs) arising at the region of the basal ganglia are questionable and the majority of cases are considered inoperable. Albeit not free from risks of morbidity and mortality, the treatment of choice is usually radiosurgery for small lesions, and embolization plus radiosurgery for larger lesions. Nevertheless, some lesions may be amenable for surgical resection due to their favorable location. Eighteen cases of such AVMs were selected for a direct approach in our series. Seventeen cases were classified as Spetzler and Martin [22] grade III, and one case was a grade V. Ten patients were males and eight females. The mean age was 28.3 years (ranged from 2 to 43 years). Sixteen patients had had previous hemorrhagic events prior to hospital admission. Eleven patients had pre-operative well established neurological deficits, and seven patients although symptomatic had a normal neurological exam at admission. Fifteen patients had their AVMs completely resected. Among the eleven patients with previous neurological deficits nine had no change in their pre-operative condition and two experienced postoperative neurological worsening. In the long-term follow-up six patients had a complete recovery and five recovered only partially. Among the seven patients whose neurological examination was normal pre-operatively five remained unchanged, one had a transient motor deficit, and one died due to a thalamic venous infarction and massive bleeding into the thalamus. The anatomical knowledge and the precise localization of the arteriovenous malformation through the aid of neuroimage studies has provided the means to classify these AVMs and plan operative strategies for some small selected cases with relatively low morbidity.

Management strategies for posterior cerebral artery aneurysms: a proposed new surgical classification

In a period of 10 years fifteen patients bearing sixteen aneurysms arising at the posterior cerebral artery were operated at our institution. Based on the approaches selected for each location a division of the posterior cerebral artery into three surgical segments is proposed. The first segment (S1), or anterior extends from the basilar artery bifurcation to the point where the artery reaches the level of the most lateral edge of the cerebral peduncle, the second segment (S2), or middle extends from the posterior limit of S1 to a point located just before the most medial extent of the artery in the quadrigeminal cistern (collicular point), and the third segment (S3), or posterior corresponds to the collicular point and to the portions of the posterior cerebral artery distal to it. Utilizing the concept of surgical segments all aneurysms in our series were satisfactorily exposed. Those arising at the S1 segment (8 cases) were operated either through a pterional or a pretemporal approaches; those from the S2 segment (6 cases) were operated either via the subtemporal or the subtemporal transventricular routes; and that arising from the S3 segment (1 case) was managed through the occipital interhemispheric approach. Among the aneurysms eleven were small, one was large, and four were large or giant. Ten of these aneurysms were surgically clipped, two coagulated, three treated by trapping and in one case the aneurysm was resected and the posterior cerebral artery was reconstructed by a termino-terminal anastomosis. The surgical results were considered good in all cases but one, where the patient died due to clinical complications three months after surgery. It is our belief that the use of this classification can provide the means to best select the most appropriate surgical approach to treat aneurysms arising at the posterior cerebral artery.

Persistent trigeminal artery: a unique anatomic specimen--analysis and therapeutic implications

OBJECTIVE

Persistent primitive carotid-basilar artery anastomoses are uncommon, and are usually seen on an angiogram or discovered at autopsy. The most frequent type of anastomosis is the persistent trigeminal artery.

METHODS

A single case of a medial variation of the persistent trigeminal artery, as seen in a well-preserved human adult anatomic specimen injected with red latex, is presented, and the anatomy of this uncommon anastomosis is discussed.

RESULTS

This specimen is unique in its clear preservation of the artery, which arises from the medial portion of the right intracavernous carotid artery and gives rise to two branches, the inferior hypophyseal artery and the dorsal meningeal artery to the clivus.

CONCLUSION

A new variation of the persistent trigeminal artery is described, which is important to support the possibility that more than one variety of carotid-basilar anastomosis exists in this region.

The pretemporal transcavernous approach to the interpeduncular and prepontine cisterns: microsurgical anatomy and technique application

OBJECTIVE

To illustrate in a stepwise fashion the microsurgical anatomy of the transcavernous approach to the interpeduncular and prepontine cisterns and to discuss our initial results with 15 basilar tip aneurysms managed through that approach.

METHODS

Using 10 embalmed cadaveric heads perfused with colored silicon, we performed bilateral stepwise dissections of the transcavernous approach via an orbitozygomatic pretemporal craniotomy. Measurements of the exposure of the basilar artery obtained along the dorsum sellae and upper clivus were taken. Our clinical data were derived from a series of 15 patients with large basilar tip aneurysms treated surgically via the transcavernous approach between 1997 and 1999. Indications for surgery were based on the size of the aneurysm (all were large) and its position in relation to the dorsum sellae (eight were more than 5 mm below the level of the dorsum sellae).

RESULTS

Good exposure of the neurovascular structures of the interpeduncular and prepontine cisterns (namely, the basilar artery) was obtained in all cases as compared with other well-established approaches to the area. All patients in our surgical series did well except that all incurred an expected third nerve palsy, caused by surgical manipulation, which resolved over the course of 2 weeks to 3 months.

CONCLUSION

Although technically difficult, the transcavernous approach provides better exposure of the interpeduncular and prepontine cisterns relative to that afforded by other, more conventional approaches. The satisfactory results obtained in our preliminary series of patients greatly support the use of this approach for complex basilar tip aneurysms.

Microsurgical anatomy of the temporal lobe: part 1: mesial temporal lobe anatomy and its vascular relationships as applied to amygdalohippocampectomy

OBJECTIVE

We review the anatomy of the mesial temporal lobe region, establishing the relationships among the intraventricular, extraventricular, and surrounding vascular structures and their angiographic characterization. We also demonstrate the clinical application of these anatomic landmarks in an anatomic temporal lobectomy plus amygdalohippocampectomy.

METHODS

Fifty-two adult cadaveric hemispheres and 12 adult cadaveric heads were studied, using a magnification ranging from 3x to 40x, after perfusion of the arteries and veins with colored latex.

RESULTS

The intraventricular elements are the hippocampus, fimbria, amygdala, and choroidal fissure; the extraventricular elements are the uncus and parahippocampal and dentate gyri. The uncus has an anterior segment, an apex, and a posterior segment that has an inferior and a posteromedial surface; the uncus is related medially to cisternal elements and laterally to intraventricular elements. The anterior segment is related to the proximal sylvian fissure, internal carotid artery, proximal M1 segment of the middle cerebral artery, proximal cisternal anterior choroidal artery, and amygdala. The apex is related to the oculomotor nerve, uncal recess, and amygdala; the posteromedial surface is related to the P2A segment of the posterior cerebral artery inferiorly, to the distal cisternal anterior choroidal artery superiorly, and to the head of the hippocampus and amygdala intraventricularly. The choroidal fissure is located between the thalamus and fimbria; it begins at the inferior choroidal point behind the head of the hippocampus and constitutes the medial wall of the posterior two-thirds of the temporal horn.

CONCLUSION

Not only is the knowledge of these relations useful to angiographically characterize the mesial temporal region, but it has also proven to be of extreme value during microsurgeries involving this region.

Multidisciplinary approach to arteriovenous malformations

The treatment of arteriovenous malformations (AVMs) depends on the efforts of a multidisciplinary team whose ultimate goal is to achieve better results when compared to the natural history of the pathology. The role of adjuvant treatment modalities such as radiosurgery and endovascular embolization is discussed. Treatment strategies and surgical results from a personal series of 344 patients operated in a 10-year period are reviewed. The Spetzler and Martin classification was modified to include subgroups IIIA (large size grade III AVMs) and IIIB (small grade III AVMs in eloquent areas) to assist the surgical resection criteria. The treatment strategy followed was surgery for grades I and II, embolization plus surgery for grade IIIA, radiosurgery for grade IIIB, and conservative for grades IV and V. According to the new proposed classification 45 (13%) patients were grade I, 96 (28%) were grade II, 44 (13%) grade IIIA, 97 (28%) grade IIIB, 45 (13%) grade IV, and 17 (5%) were grade V. As for surgical results 85.8% of the patients had a good outcome (no additional neurological deficit), 12.5% had a fair outcome (minor neurological deficit), 0.6% had a bad outcome (major neurological deficit), and 1.2% died. These figures indicate that the treatment of AVMs can achieve better results compared to the natural history if managed by a well trained group of specialists led by an experienced neurosurgeon.

Comprehensive management of arteriovenous malformations

The treatment of arteriovenous malformations depends on the efforts of a multidisciplinary team whose ultimate goal is to achieve better results when compared to the natural history of the pathology. The role of adjuvant treatment modalities such as radiosurgery and endovascular embolization is discussed. Treatment strategies and surgical results from a personal series of 344 patients operated in a ten-year period are reviewed. The Spetzler and Martin classification was modified to include subgroups IIIA (large size grade III AVMs) and IIIB (small grade III AVMs in eloquent areas) to assist the surgical resection criteria. The treatment strategy followed was surgery for grades I and II, embolization plus surgery for grades IIIA, radiosurgery for grades IIIB, and conservative for grades IV and V. According to the new proposed classification 45 (13%) patients were grade I, 96 (28%) were grade II, 44 (13%) grade IIIA, 97 (28%) grade IIIB, 45 (13%) grade IV, and 17 (5%) were grade V. As for surgical results 85.8% of the patients had a good outcome (no additional neurological deficit), 12.5% had a fair outcome (minor neurological deficit), 0.6% had a bad outcome (major neurological deficit), and 1.2% died. These figures indicate that the treatment of arteriovenous malformations can achieve better results compared to the natural history if managed by a well trained group of specialists led by an experienced neurosurgeon.

Effects of carbonyl cyanide phenylhydrazones on two mitochondrial ion channel activities

The respiratory uncouplers carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) and carbonyl cyanide m-chlorophenylhydrazone (CCCP) affect the activities of two mitochondrial ion channels from mouse liver. At micromolar concentrations, the phenylhydrazones block the voltage-dependent 100-pS channel, mCS, and induce the multiple-conductance-level channel, MCC. The binding site(s) involved in perturbation of channel activities are probably distinct from the sites involved in uncoupling of oxidative phosphorylation which occurs at nanomolar concentrations of the phenylhydrazones. The effects of FCCP and CCCP on the mitochondrial ion channels could be partially reversed by washing with fresh media and were always reversed by perfusion with dithiothreitol. These results indicate that the effects of the phenylhydrazones on mitochondrial ion channels may be related to the ability of these compounds to act as sulfhydryl reagents and not to their protonophoric and uncoupling activity.

Sinus skeletonization: a treatment for dural arteriovenous malformations of the tentorial apex. Report of two cases

Two cases of dural arteriovenous malformation of the tentorial apex are presented. Both were treated surgically by means of a sinus skeletonization technique. The operative technique included a combined bioccipital and median suboccipital craniotomy in which the posterior third of the superior sagittal and the straight and bilateral transverse sinuses were skeletonized by incising the falx and the tentorium along the sinuses. Endovascular embolization was used prior to the surgical approach in one case. Clinical and angiographic cure was achieved in both patients, with a follow up of 4 years in the first case and 1 year in the second one. The surgical technique is described in detail.

Anatomical and technical aspects of the contralateral approach for multiple aneurysms

Microsurgery of multiple aneurysms is still a controversial subject. In order to avoid the risk of rebleeding and the consequent increase in morbidity in such cases all aneurysms or at least as many aneurysms as possible should be treated in the first operative procedure. To reach that goal aneurysms located on the contralateral side should also be considered for clipping during the first operation. Between 1984 and 1994 a series of 51 patients harboring multiple aneurysms of which 55 aneurysms were located on the contralateral side of the craniotomy were operated at our institution. No mortality or morbidity could be directly ascribed to the aneurysm that was clipped contralaterally. Based on that series we have described the anatomical features, technical aspects and surgical difficulties of approaching bilateral aneurysms through the same craniotomy.

The pretemporal approach to the interpeduncular and petroclival regions

A pretemporal approach to the interpeduncular and petroclival regions is described. Through a frontotemporal craniotomy based very low in the middle fossa the temporal lobe is completely exposed. The Sylvian, carotid, chiasmatic, and lamina terminalis cisterns are widely opened. The arachnoid fibers between the uncus and the frontal lobe, as well as those binding the temporal lobe to the tentorial edge and to the oculomotor nerve are also separated. The bridging veins from the temporal pole to the spheno-parietal sinus are usually coagulated and sacrificed allowing for posterior displacement of the temporal lobe. The approach combines the advantages of both the classical pterional and subtemporal approaches providing unhindered exposure of the anterior portion of the tentorial incisura in dealing with vascular and tumoural lesions arising at the sellar, parasellar, and interpeduncular regions, and at the superior aspect of the petroclival region.

Single-channel activity induced in mitoplasts by alkaline pH

Exposure of patch-clamped mitoplasts to alkaline pH induces a reversible conductance increase (Antonenko, Yu. N., Kinnally, K.W. and Tedeschi, H. (1991) J. Membr. Biol. 124, 151-158) which is due to an increase in open probability of a channel activity of 15 pS and larger transitions. The present study defines in more detail some of the characteristics of the channel activity involved in this conductance increase. The results suggest the presence of two channels one slightly cation-selective of approx. 15 pS (referred to here as alkaline-induced cation-selective activity, ACA) and another slightly anion selective of approx. 45 pS (referred to as alkaline-induced anion-selective activity, AAA). The possible implication of these results in relation to other channels and the permeability transitions reported by others using mitochondrial suspensions is discussed.

Lateral approaches to the petroclival region

The microsurgical anatomy that provides the basis for dealing with lesions arising in the petroclival region was reviewed in 15 adult cadaver heads and 25 dry skulls. The eight surgical approaches studied were the retrosigmoid, extreme lateral transcondylar, translabyrinthine, transcochlear, combined supra and infratentorial presigmoid, subtemporal anterior transpetrosal, subtemporal preauricular infratemporal, and the postauricular transtemporal approach. Considerations important in the selection of these approaches are discussed. Special attention was directed to the course of the facial nerve and internal carotid artery in the temporal bone and the major venous pathways draining the region.

Mitochondrial benzodiazepine receptor linked to inner membrane ion channels by nanomolar actions of ligands

The mitochrondrial benzodiazepine receptor (mBzR) binds a subset of benzodiazepines and isoquinoline carboxamides with nanomolar affinity and consists of the voltage-dependent anion channel, the adenine nucleotide translocator, and an 18-kDa protein. The effect of ligands of the mBzR on two inner mitochondrial membrane channel activities was determined with patch-clamp techniques. The relative inhibitory potencies of the drugs resemble their binding affinities for the mBzR. Ro5-4864 and protoporphyrin IX inhibit activity of the multiple conductance channel (MCC) and the mitochondrial centum-picosiemen (mCtS) channel activities at nanomolar concentrations. PK11195 inhibits mCtS activity at similar levels. Higher concentrations of protoporphyrin IX induce MCC but possibly not mCtS activity. Clonazepam, which has low affinity for mBzR, is at least 500 times less potent at both channel activities. Ro15-1788, which also has a low mBzR affinity, inhibits MCC at very high concentrations (16 microM). The findings indicate an association of these two channel activities with the proteins forming the mBzR complex and are consistent with an interaction of inner and outer membrane channels.

Modified supraorbital craniotomy: technical note

The authors present a surgical approach that incorporates the frontal sinus and extends a supraorbital craniotomy to include the lateral orbital rim and zygoma. The craniotomy provides wide exposure of the anterior fossa, orbit, ipsilateral middle fossa, and cavernous sinus. The procedure can be performed easily, and the bone flaps can be secured rapidly back into the anatomical position at the time of closure. This modified supraorbital craniotomy is ideal for large benign lesions originating along the sphenoid wing or orbit that expand into the anterior fossa.

The effect of antimycin A on mouse liver inner mitochondrial membrane channel activity

In a patch-clamp study, we found antimycin A in low (1-2) microM concentrations decreased the open probability of the multiple conductance channel activity and the approximately 110 picosiemens channel of the inner mitochondrial membrane (for a review of mitochondrial channels see Kinnally, K. W., Antonenko, Yu. N., and Zorov, D. B. (1992) J. Bioenerg. Biomembr. 24, 99-110). Higher antimycin A concentrations (e.g. 10 microM) facilitated multiple conductance channel opening. These effects were reversible, and the binding site(s) are probably distinct from those responsible for the inhibition of the electron transport chain, since the latter are virtually irreversible. A model with two closed and two open states is presented for the approximately 110-picosiemens activity.

Multiple conductance levels in rat heart inner mitochondrial membranes studied by patch clamping

The behavior of the mitochondrial inner membrane multiple conductance channel (MCC) which has a peak conductance of 1-1.5 nS has been examined in rat heart mitochondria. MCC can display several unique characteristics: (a) prolonged open and closed times on the order of seconds to minutes, (b) a voltage dependence in which MCC opens (negative potential) or closes (positive potential) generally in steps, (c) a response to inhibitors such as amiodarone in steps corresponding at least approximately to those in (b), (d) a 'free-running mode' in which the current level rapidly fluctuates between a minimum of nine conductance levels but with a preferred occupation of the 0.5-0.7 nS levels, and (e) very large transitions (1-1.5 nS) resolved at 4 kHz bandwidth as single events with variable mean open time.

Microsurgical anatomy of acoustic neuroma

Because acoustic neuromas most frequently arise in the posteriorly placed vestibular nerves, they usually displace the facial and cochlear nerves anteriorly (Figs. 11, 12, and 13). The facial nerve is stretched around the anterior half of the tumor capsule. Variability in the direction of growth of the tumor arising from the vestibular nerves may result in the facial nerve being displaced, not only directly anteriorly, but also anterior-superiorly or anterior-inferiorly. The nerve is infrequently found on the posterior surface of the tumor. Because the facial nerve always enters the facial canal at the anterior-superior quadrant of the lateral margin of the meatus, it is usually easiest to locate it here, rather than at a more medial location where the degree of displacement of the nerve is more variable. The cochlear nerve also lies anterior to the vestibular nerve and is most frequently stretched around the anterior half of the tumor. The strokes of the fine dissecting instruments used in removing the tumor should be directed along the vestibulocochlear nerve from medial to lateral rather than from lateral to medial because traction medially may tear the tiny filaments of the cochlear nerve at the site where these filaments penetrate the lateral end of the meatus to enter the cochlea. The landmarks that are helpful in identifying the facial and vestibulocochlear nerves at the brain stem on the medial side of the tumor have been reviewed. These nerves, although distorted by tumor, can usually be identified on the brain stem side of the tumor at the lateral end of the pontomedullary sulcus, just rostral to the glossopharyngeal nerve and just anterior-superior to the foramen of Luschka, flocculus, and choroid plexus protruding from the foramen of Luschka. After the facial and vestibulocochlear nerves are identified on the medial and lateral sides of the tumor, the final remnants of the tumor are separated from the intervening segment of the nerves. In the three approaches to the meatus and cerebellopontine angle--retrosigmoid, translabyrinthine, and middle fossa--a communication may be established between the subarachnoid space and the mastoid air cells that requires careful closure to prevent a cerebrospinal fluid leak.

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.