A preliminary study of oscillating electromagnetic field effects on human spermatozoon motility

Some effects of extremely low frequency electromagnetic fields (ELF-EMFs) on human spermatozoa are reported. Significant increases in the values of the motility and of the other kinematic parameters have been observed when spermatozoa were exposed to an ELF-EMF with a square waveform of 5 mT amplitude and frequency of 50 Hz. By contrast, a 5 mT sine wave (50 Hz) and a 2.5 mT square wave (50 Hz) exposure did not produce any significant effect on sperm motility. The effects induced by ELF-EMF (50 Hz; 5 mT) during the first 3 h of exposure persisted for 21 h after the end of the treatment. These results indicate that ELF-EMF exposure can improve spermatozoa motility and that this effect depends on the field characteristics.

Antioxidant enzymatic defences in human follicular fluid: characterization and age-dependent changes

The aim of this work was to study the antioxidant enzymatic defences in human follicular fluid and investigate their possible changes during reproductive ageing. To this end, we tested the specific activities and protein expression of enzymes involved in reactive oxygen species (ROS) scavenging and in detoxification of ROS byproducts in follicular fluid from young (range 27-32 years, n = 12) and older (range 39-45 years, n = 12) women participating in an IVF programme. Results show that all the tested enzymes [superoxide dismutase (SOD), catalase, glutathione peroxidase, glutathione transferase, glutathione reductase] were significantly expressed in human follicular fluid. However, when the two age groups were compared, we found that follicular fluid from older women exhibited a reduced level of glutathione transferase and catalase activities and a higher level of SOD activity. Immunoblot analysis revealed that ageing was associated with decreased protein expression of GST Pi isoform and did not affect SOD and catalase protein expression. Taken together, these findings indicate that reproductive ageing is accompanied by a change in the antioxidant enzymatic pattern that could impair ROS scavenging efficiency in the follicular environment.

Gp273, the ligand molecule for sperm-egg interaction in the bivalve mollusk, Unio elongatulus, binds to and induces acrosome reaction in human spermatozoa through a protein kinase C-dependent pathway

In a previous article, we suggested that gp273, the ligand molecule for sperm-egg interaction in the bivalve mollusk Unio elongatulus has functional carbohydrate epitopes in common with a human zona pellucida glycoprotein, probably ZP3. We demonstrated that: 1) anti-gp273-purified immunoglobulin G (IgG), which recognizes a carbohydrate gp273 epitope including a Lewisa-like structure, interacts with a zona pellucida protein; 2) human sperm specifically bind to gp273; and 3) binding is reversed by anti-gp273 IgG. In the present study, we confirm this suggestion by demonstrating that heat-solubilized zonae pellucidae reverse gp273-human sperm binding, that gp273-binding sites are restricted to the acrosomal region, and that gp273 induces the acrosome reaction in human sperm. We also demonstrated that gp273-binding sites on human sperm function as signaling receptors because exposure of spermatozoa to this glycoprotein results in significant stimulation of protein kinase C (PKC) activity. Because the PKC inhibitor, bisindolylmaleimide I, reverses both PKC activation and the acrosome reaction, this kinase is a key component of the signal transduction pathway activated by gp273 and leading to the exocytotic event.

Possible role for Ca(2+) calmodulin-dependent protein kinase II as an effector of the fertilization Ca(2+) signal in mouse oocyte activation

The present study shows that Ca(2+) calmodulin-dependent protein kinase II (CaM kinase II) is physiologically activated in fertilized mouse oocytes and is involved in the Ca(2+) response pathways that link the fertilization Ca(2+) signal to meiosis resumption and cortical granule (CG) exocytosis. After 10 min of insemination, CaM kinase II activity increased transiently, then peaked at 1 h and remained elevated 30 min later when most of the oocytes had completed the emission of the second polar body. In contrast, in ethanol-activated oocytes the early transient activation of CaM kinase II in response to a monotonic Ca(2+) rise was not followed by any subsequent increase. Inhibition of CaM kinase II by 20 micromol/l myristoylated-AIP (autocamtide-2-related inhibitory peptide) negatively affected MPF (maturing promoting factor) inactivation, cell cycle resumption and CG exocytosis in both fertilized and ethanol-activated oocytes. These results indicate that the activation of CaM kinase II in mouse oocytes is differently modulated by a monotonic or repetitive Ca(2+) rise and that it is essential for triggering regular oocyte activation.

Mitochondria and cell death. Mechanistic aspects and methodological issues

Mitochondria are involved in cell death for reasons that go beyond ATP supply. A recent advance has been the discovery that mitochondria contain and release proteins that are involved in the apoptotic cascade, like cytochrome c and apoptosis inducing factor. The involvement of mitochondria in cell death, and its being cause or consequence, remain issues that are extremely complex to address in situ. The response of mitochondria may critically depend on the type of stimulus, on its intensity, and on the specific mitochondrial function that has been primarily perturbed. On the other hand, the outcome also depends on the integration of mitochondrial responses that cannot be dissected easily. Here, we try to identify the mechanistic aspects of mitochondrial involvement in cell death as can be derived from our current understanding of mitochondrial physiology, with special emphasis on the permeability transition and its consequences (like onset of swelling, cytochrome c release and respiratory inhibition); and to critically evaluate methods that are widely used to monitor mitochondrial function in situ.

Chloromethyltetramethylrosamine (Mitotracker Orange) induces the mitochondrial permeability transition and inhibits respiratory complex I. Implications for the mechanism of cytochrome c release

We have investigated the interactions with isolated mitochondria and intact cells of chloromethyltetramethylrosamine (CMTMRos), a probe (Mitotracker Orange) that is increasingly used to monitor the mitochondrial membrane potential (Deltapsi(m)) in situ. CMTMRos binds to isolated mitochondria and undergoes a large fluorescence quenching. Most of the binding is energy-independent and can be substantially reduced by sulfhydryl reagents. A smaller fraction of the probe is able to redistribute across the inner membrane in response to a membrane potential, with further fluorescence quenching. Within minutes, however, this energy-dependent fluorescence quenching spontaneously reverts to the same level obtained by treating mitochondria with the uncoupler carbonylcyanide-p-trifluoromethoxyphenyl hydrazone. We show that this event depends on inhibition of the mitochondrial respiratory chain at complex I and on induction of the permeability transition pore by CMTMRos, with concomitant depolarization, swelling, and release of cytochrome c. After staining cells with CMTMRos, depolarization of mitochondria in situ with protonophores is accompanied by changes of CMTMRos fluorescence that range between small and undetectable, depending on the probe concentration. A lasting decrease of cellular CMTMRos fluorescence associated with mitochondria only results from treatment with thiol reagents, suggesting that CMTMRos binding to mitochondria in living cells largely occurs at SH groups via the probe chloromethyl moiety irrespective of the magnitude of Deltapsi(m). Induction of the permeability transition precludes the use of CMTMRos as a reliable probe of Deltapsi(m) in situ and demands a reassessment of the conclusion that cytochrome c release can occur without membrane depolarization and/or onset of the permeability transition.

Transient and long-lasting openings of the mitochondrial permeability transition pore can be monitored directly in intact cells by changes in mitochondrial calcein fluorescence

The occurrence and the mode of opening of the mitochondrial permeability transition pore (MTP) were investigated directly in intact cells by monitoring the fluorescence of mitochondrial entrapped calcein. When MH1C1 cells and hepatocytes were loaded with calcein AM, calcein was also present within mitochondria, because (i) its mitochondrial signal was quenched by the addition of tetramethylrhodamine methyl ester and (ii) calcein-loaded mitochondria could be visualized after digitonin permeabilization. Under the latter condition, the addition of Ca2+ induced a prompt and massive release of the accumulated calcein, which was prevented by CsA, indicating that calcein release could, in principle, probe MTP opening in intact cells as well. To study this process, we developed a procedure by which the cytosolic calcein signal was quenched by Co2+. In hepatocytes and MH1C1 cells coloaded with Co2+ and calcein AM, treatment with MTP inducers caused a rapid, though limited, decrease in mitochondrial calcein fluorescence, which was significantly reduced by CsA. We also observed a constant and spontaneous decrease in mitochondrial calcein fluorescence, which was completely prevented by CsA. Thus MTP likely fluctuates rapidly between open and closed states in intact cells.

Biochemical and biological effects of KN-93, an inhibitor of calmodulin-dependent protein kinase II, on the initial events of mouse egg activation induced by ethanol

Calmodulin-dependent protein kinase II (CaMKII) is transiently activated in mouse eggs by the increase in calcium that occurs upon activation with ethanol. This study investigated the biological and biochemical effects of KN-93, a reported selective inhibitor of CaMKII, to explore the potential role of this kinase in the initial events of egg activation. Mouse eggs were incubated for 30 min in the presence of different concentrations of KN-93 and induced to activate by 7% ethanol. KN-93 elicited a dose-dependent inhibition of polar body emission that resulted from the failure of the eggs to undergo meiosis resumption and inactivation of maturation-promoting factor (MPF). Furthermore, 15 mumol KN-93 l-1 produced a marked reduction in ethanol-induced loss of cortical granules. In vivo biochemical analysis revealed that 15 mumol KN-93 l-1 was responsible for significant inhibition of ethanol-stimulated CaMKII. The activity of the enzyme remained at a resting value, in spite of the presence of a calcium signal similar to that measured in control activated eggs. The inhibitory effects of KN-93 on the parameters tested in this study could not be mimicked by the inactive analogue KN-92. These results show that in mouse eggs, when ethanol-induced CaMKII activation was prevented, cortical granule exocytosis and meiosis resumption were inhibited. This suggests that CaMKII acts as a switch in the transduction of the calcium signal triggering mammalian egg activation.

Antral follicle development influences plasma membrane organization but not cortical granule distribution in mouse oocytes

In the present study, we evaluated the contributions of antral follicle development and antral granulosa cell-released factor(s) to the acquisition of a mature mouse oocyte plasma membrane organization and cortical granule distribution. This has been performed by comparing in-vitro matured oocytes derived from early antral follicles (here referred to as denuded oocytes) or from pre-ovulatory follicles, and cultured either as cumulus-intact or cumulus-free oocytes, with in-vivo ovulated eggs. By using scanning and transmission electron microscopy, the denuded oocyte surface appears to be characterized by the presence of long microvilli, while that of pre-ovulatory oocytes and of ovulated eggs by shorter microvilli. However, denuded oocytes can acquire a pre-ovulatory-like plasma membrane configuration when matured in vitro in the presence of early antral granulosa or cumulus cells, but not of NIH-3T3 fibroblasts. On the contrary, fluorescence and confocal microscopy analyses after labelling with fluorescent Lens culinaris agglutinin show that all the oocyte classes analysed are characterized by similar cortical granule distribution and density. Thus, complete antral follicle development plays an important role in the process of oocyte surface differentiation, probably through the action of antral granulosa cell-released factor(s), but it does not affect oocyte capacity to normally distribute cortical granules.

Induction of the mitochondrial permeability transition by N-ethylmaleimide depends on secondary oxidation of critical thiol groups. Potentiation by copper-ortho-phenanthroline without dimerization of the adenine nucleotide translocase

Addition to energized rat liver mitochondria of low micromolar concentrations of the thiol oxidant, copper-o-phenanthroline [Cu(OP)2], causes opening of the permeability transition pore, a cyclosporin A-sensitive channel. The effects of Cu(OP)2 can be reversed by reduction with dithiothreitol (DTT), suggesting that a dithiol-disulfide interconversion is involved. However, at variance with all pore inducers known to act through dithiol oxidation, the effects of Cu(OP)2 are not prevented by treatment of mitochondria with low (10-20 microM) concentrations of N-ethylmaleimide (NEM). Rather, these concentrations of NEM potentiate the inducing effects of Cu(OP)2. We show that this enhancing effect of NEM is blocked by the subsequent addition of DTT, indicating that potentiation by NEM is mediated by an oxidative event rather than by substitution as such. We find that also pore induction by high (0.5-1.0 mM) concentrations of NEM in the absence of oxidants is completely blocked by reduction with DTT or beta-mercaptoethanol. These results underscore the unexpected importance of oxidative events in pore opening by substituting agents. Since we find that pore opening by Cu(OP)2 or by high concentrations of NEM is not accompanied by dimerization of the adenine nucleotide translocase, we conclude that the translocase itself is not the target of the pore-inducing oxidative events triggered by Cu(OP)2 and NEM.

An evaluation of propofol toxicity on mouse oocytes and preimplantation embryos

Mouse biological assays were used to investigate potential adverse effects of propofol on the oocyte's competence to fuse with spermatozoa and on the embryo's ability to develop to the blastocyst stage. Cumulus-enclosed metaphase II oocytes were exposed for 1 h to 0.01, 0.1, 0.4, 1 and 10 microg/ml propofol (Diprivan) and subjected to a sperm-oocyte fusion test based on the dye (Hoechst 33342) transfer technique. Oocytes exposed to 0.4, 1 and 10 microg/ml propofol showed a significant reduction in the rate of sperm fusion and underwent pronuclei formation at a rate similar to that of sperm fusion. In a second trial, mouse 1-cell and 2-cell embryos were exposed to varying propofol concentrations for 14h and then checked for subsequent development. Although adverse effects were not observed in 2-cell embryos, treatment of 1-cell embryos with propofol concentrations ranging from 0.01 to 10 microg/ml resulted in the inhibition of cleavage to blastocyst stage. We conclude that propofol can negatively influence fertilization in the mouse by impairing the oocyte's ability to fuse with spermatozoa, without interfering with the sperm-induced activation of the cell cycle. Moreover, we document the peculiar sensitivity to propofol of mouse 1-cell embryos as compared with 2-cell embryos.

The mitochondrial permeability transition

This review summarizes recent work on the regulation of the permeability transition pore, a cyclosporin A-sensitive mitochondrial channel that may play a role in intracellular calcium homeostasis and in a variety of forms of cell death. The basic bioenergetics aspects of pore modulation are discussed, with some emphasis on the links between oxidative stress and pore dysregulation as a potential cause of mitochondrial dysfunction that may be relevant to cell injury.

Imaging the mitochondrial permeability transition pore in intact cells

The involvement of mitochondrial permeability transition pore (MTP) in cellular processes is generally investigated by indirect means, such as changes in mitochondrial membrane potential or pharmacological inhibition. However, such effects could not be related univocally to MTP. In addition, source of errors could be represented by the increased retention of membrane potential probes induced by cyclosporin A (CsA) and the interactions between fluorescent probes. We developed a direct technique for monitoring MTP. Cells were co-loaded with calcein-AM and CoCl2, resulting in the quenching of the cytosolic signal without affecting the mitochondrial fluorescence. MTP inducers caused a rapid decrease in mitochondrial calcein fluorescence which, however, was not completely prevented by CsA. Besides the large and rapid efflux of calcein induced by MTP agonists, we also observed a constant and spontaneous decrease of mitochondrial calcein which was completely prevented by CsA. Thus, MTP likely fluctuates between open and closed states in intact cells.

Protein kinase C is required for the disappearance of MPF upon artificial activation in mouse eggs

The aim of the present study was to investigate the implication of protein kinase C (PKC) in the mouse egg activation process. We used OAG (1-oleoyl-2-acetyl-sn-glycerol) as a PKC activator, calphostin C as a specific PKC inhibitor, and the calcium ionophore A23187 as a standard parthenogenetic agent. The exposure of zona-free eggs to 150 microM or 50 microM OAG for 10 min resulted in meiosis II completion in approximately 80% of instances. By contrast, at a lower concentration (25 microM), the PKC stimulator was ineffective as parthenogenetic agent. Shortly after the application of 150 microM OAG, the cytosolic Ca2+ concentration ([Ca2+]i) increased transiently in all the eggs examined, whereas after the addition of 50 microM OAG, [Ca2+]i remained unchanged for at least 20 min. During this period, the activity of M-phase promoting factor (MPF) dramatically decreased and most of the eggs entered anaphase except when the PKC was inhibited by calphostin C. Similarly, MPF inactivation and meiosis resumption were prevented in calphostin C-loaded eggs following treatment with A23187, even though the ionophore-induced Ca2+ signalling was not affected. Taken together, our results indicate that stimulation of PKC is a sufficient and necessary event to induce meiosis resumption in mouse eggs and strongly suggest that, in this species, the mechanism by which a transient calcium burst triggers MPF inactivation involves a PKC-dependent pathway.

Influence of granulosa cells and of different somatic cell types on mammalian oocyte development in vitro

In mammals the ability of an oocyte to become fertilised is the result of a complex process occurring within the ovarian follicle which depends on the stagespecific expression of oocyte genes and the presence of granulosa cells (for a review see Buccioneet al., 1990a). The coordinated development of germinal and somatic components of the follicle is regulated by two principal systems of interaction, based on the presence of gap junctions and on the production of paracrine factors. Gap junctions link granulosa cells to each other and to the oocyte (Anderson & Albertini, 1976), and represent a major route for the transfer of small molecules involved in oocyte metabolism (for a review see Mangiaet al., 1992) and regulation of the arrest and resumption of meiosis (for a review see Eppig, 1993). The production of paracrine factors by granulosa cells has been suggested by the findings that these cells express the production of theSteellocus, the Steel factor (SLF) or kit ligand (KL; Motroet al., 1991; Manovaet al., 1993), and that this factor promotes oocyte growthin vitrowhen used at high concentrations (Packeret al., 1994). Since KL is too large to be transmitted through gap junctions, it must necessarily be released in the extracellular environment before binding to the c-kitreceptor present on oocyte membrane (Manovaet al., 1990; Horieet al., 1991).

Mammalian oocyte growth in vitro is stimulated by soluble factor(s) produced by preantral granulosa cells and by Sertoli cells

We have evaluated the possibility that mouse oocyte growth in vitro could be achieved under the influence of soluble compound(s) released by different somatic cell types. For this purpose, zona-free denuded oocytes from 12-day-old mice were cultured on monolayers of NIH-3T3 fibroblasts, which are able to establish gap junctional communications with them, in the presence or absence of media conditioned by preantral granulosa cells or by Sertoli cells, plated at increasing concentrations from 0.3-1 x 10(6) ml-1 cells. After 3 days, no increase in vitellus diameter was recorded from fibroblast-coupled oocytes maintained in culture medium or in the presence of media conditioned by 0.3 x 10(6) ml-1 Sertoli cells. By contrast, increasing proportions of coupled oocytes grew, provided the continuous presence of media conditioned by 0.5 or 1 x 10(5) ml-1 Sertoli cells, or by 0.3, 0.5, and 1 x 10(5) ml-1 preantral granulosa cells. Since the ligand of c-kit, the growth factor KL, promotes the growth in vitro of oocytes cultured in follicles from 8-day-old mice, an antibody against mouse KL was used to evaluate whether in our culture conditions KL might also be responsible for the growth of oocytes from 12-day-old mice. No inhibition of growth was evident in oocytes cultured directly on preantral granulosa or Sertoli-cell monolayers. Furthermore, the growth of fibroblast-coupled oocytes cultured in media conditioned by preantral granulosa cells was not significantly affected by the presence of this antibody during culture. By contrast, a high percentage of oocytes cultured on fibroblasts in the presence of media conditioned by Sertoli cells showed a significant inhibition of growth and no metabolic cooperativity. It was concluded that, besides KL, other bioactive factor(s) released by either preantral granulosa or Sertoli cells can induce a significant stimulation of mouse oocyte growth in vitro.

Role of antral follicle development and cumulus cells on in vitro fertilization of mouse oocytes

The role exerted by antral follicle development and by companion granulosa cells on the ability of mouse oocytes at metaphase II to fuse with spermatozoa (as estimated by the kinetics of sperm-egg fusion), and to support normal fertilization (estimated by the formation of a male pronucleus) was investigated. After 17 h of culture in vitro, nuclear maturation occurred in 82% of oocytes derived from early antral follicles devoid of granulosa cells (denuded oocytes) and in 95% of oocytes derived from preovulatory follicles and cultured cumulus-intact or cumulus-free. Among the matured oocytes, 95% of cumulus-intact oocytes were arrested at metaphase II, while 61% of denuded and 50% of cumulus-free oocytes were arrested at metaphase I. The competence of denuded and cumulus-free oocytes to override the metaphase I stage was not affected by the addition of preantral granulosa, early antral granulosa or cumulus cells to the maturation medium. The kinetics of sperm-oocyte fusion were comparable in cumulus-intact and cumulus-free oocytes, and were more rapid than those of the early antral oocytes for insemination periods up to 60 min. However, when denuded oocytes were matured in medium containing early antral granulosa or cumulus cells, the kinetics of sperm-egg fusion was greatly accelerated to values equivalent to those of preovulatory oocytes. The ability of denuded oocytes to fuse with spermatozoa was unaffected by the addition of preantral granulosa cells to the maturation medium. Sixty minutes after insemination, the fertilization rates were 39% for denuded oocytes, 50% for cumulus-free oocytes and 73% for cumulus-intact oocytes. The fertilization rate of denuded and cumulus-free oocytes was significantly improved by the addition to the maturation medium of cumulus cells, but not by the addition of preantral or early antral granulosa cells. Taken together, the present results show that in mice, the maturation of both oolemma and ooplasm, required for successful fertilization, is acquired together with the development of antral follicles and is positively affected by cumulus cells during meiotic progression.

Lipid interaction of the 37-kDa and 58-kDa fragments of the Helicobacter pylori cytotoxin

Helicobacter pylori cytotoxin vacA (95 kDa) causes a vacuolar degeneration of epithelial cells. There is evidence that this protein toxin acts inside cells, and hence has to cross a cell membrane. This cytotoxin is frequently obtained as two fragments of 58 kDa (p58) and 37 kDa (p37) and it is available only in minute amounts. Here, its membrane interaction was studied with the two fragments, produced in Escherichia coli. Light scattering and energy transfer experiments show that p37 and p58 cause aggregation and fusion of small unilamellar lipid vesicles; only a reversible aggregation is induced at neutral pH, whereas at acid pH fusion also takes place. p58, but not p37, causes potassium efflux from liposomes and this occurs only at acid pH. Hydrophobic photolabelling with photoactivatable phosphatidylcholines inserted into liposomes shows that both fragments are labelled at neutral pH. The amount of labelling of the two fragments is much higher at acid pH, consistent with a further penetration into the hydrophobic core of the lipid bilayer. Tryptophan fluorescence measurements indicate that the two fragments undergo a pH-driven conformational change. These data are consistent with cytotoxin entry in the cell cytosol via an intracellular acidic compartment.

Mitochondrial membrane potential in single living adult rat cardiac myocytes exposed to anoxia or metabolic inhibition

1. The relation between mitochondrial membrane potential (delta psi m) and cell function was investigated in single adult rat cardiac myocytes during anoxia and reoxygenation. delta psi m was studied by loading myocytes with JC-1 (5,5',6,6'-tetrachloro-1,1',3,3'- tetra-ethylbenzimidazolylcarbocyanine iodide), a fluorescent probe characterized by two emission peaks (539 and 597 nm with excitation at 490 nm) corresponding to monomer and aggregate forms of the dye. 2. De-energizing conditions applied to mitochondria, cell suspensions or single cells decreased the aggregate emission and increased the monomer emission. This latter result cannot be explained by changes of JC-1 concentration in the aqueous mitochondrial matrix phase indicating that hydrophobic interaction of the probe with membranes has to be taken into account to explain JC-1 fluorescence properties in isolated mitochondria or intact cells. 3. A different sensitivity of the two JC-1 forms to delta psi m changes was shown in isolated mitochondria by the effects of ADP and FCCP and the calibration with K+ diffusion potentials. The monomer emission was responsive to values of delta psi m below 140 mV, which hardly modified the aggregate emission. Thus JC-1 represents a unique double sensor which can provide semi-quantitative information in both low and high potential ranges. 4. At the onset of glucose-free anoxia the epifluorescence of individual myocytes studied in the single excitation (490 nm)-double emission (530 and 590 nm) mode showed a gradual decline of the aggregate emission, which reached a plateau while electrically stimulated (0.2 Hz) contraction was still retained. The subsequent failure of contraction was followed by the rise of the emission at 530 nm, corresponding to the monomer form of the dye, concomitantly with the development of rigor contracture. 5. The onset of the rigor was preceded by the increase in intracellular Mg2+ concentration ([Mg2+]i) monitored by mag-indo-1 epifluorescence. Since under these experimental conditions intracellular [Ca2+] and pH are fairly stable, the increase in [Mg2+]i was likely to be produced by a decrease in ATP content. 6. The inhibition of mitochondrial ATPase induced by oligomycin during anoxia was associated with a rapid and simultaneous change of both the components of JC-1 fluorescence, suggesting that delta psi m, instead of producing ATP, is generated by glycolytic ATP during anoxia. 7. The readmission of oxygen induced a rapid decrease of the monomer emission and a slower increase of the aggregate emission. These fluorescence changes were not necessarily associated with the recovery of mechanical function.(ABSTRACT TRUNCATED AT 400 WORDS)

On the effects of paraquat on isolated mitochondria. Evidence that paraquat causes opening of the cyclosporin A-sensitive permeability transition pore synergistically with nitric oxide

This paper reports an investigation on the effects of the bipyridylium herbicide, paraquat, on rat liver mitochondria in vitro. We show that paraquat induces a Ca(2+)-dependent permeability increase of the inner mitochondrial membrane leading to membrane depolarization, uncoupling and matrix swelling. The permeability increase is not observed in the absence of Ca2+ accumulation, and is not due to a direct effect of paraquat on the membrane energy level, as assessed by measurements of membrane potential, respiration and mitochondrial permeability to solutes at high concentrations of paraquat in the presence of excess ethylene-bis(oxoethylenenitrilo)tetraacetic acid (EGTA), a Ca2+ chelator. The Ca(2+)-dependent permeability increase is due to inappropriate opening of the endogenous permeability transition pore (MTP), a regulated, voltage-dependent channel of the inner mitochondrial membrane. The pore is primarily affected by paraquat through a shift of the gating potential to more negative values, allowing pore opening at physiological membrane potential. This effect apparently involves oxidation of a critical dithiol in the pore voltage sensor, while other regulatory aspects of the MTP (matrix pH and Ca2+) are unaffected by paraquat, which is not transported inside the mitochondrial matrix. The effects of paraquat on MTP opening depend on inhibition of electron transfer at Site I by rotenone, or by respiratory chain inhibition by nitric oxide, one of the proposed endogenous mediators of paraquat toxicity to the lung (Berisha, H.I., Hedayatollah, P., Absood, A., and Said, S.I. (1994) Proc. Natl. Acad. Sci. USA 91, 7445-7449). Taken together, these data provide an additional biochemical mechanism by which paraquat may affect cell function, and support the idea that mitochondrial damage is an important determinant in paraquat toxicity (Hirai, K.-I., Ikeda, K., and Wang, G.-Y. (1992) Toxicology 72, 1-16).

Regulation of the permeability transition pore, a voltage-dependent mitochondrial channel inhibited by cyclosporin A

Mitochondria from a variety of sources possess a regulated inner membrane channel, the permeability transition pore (MTP), which is responsible for the 'permeability transition', a sudden permeability increase to solutes with molecular masses < or = 1500 Da, most easily observed after Ca2+ accumulation. The MTP is a voltage-dependent channel blocked by cyclosporin A with Ki in the nanomolar range. The MTP open probability is regulated by both the membrane potential and matrix pH. The probability of pore opening increases as the membrane is depolarized, while it decreases as matrix pH is decreased below 7.3 through reversible protonation of histidine residues. Many physiological and pathological effectors, including Ca2+ and ADP, modulate MTP operation directly through changes of the gating potential rather than indirectly through changes of the membrane potential (Petronilli, V., Cola, C., Massari, S., Colonna, R. and Bernardi, P. (1993) J. Biol. Chem. 268, 21939-21945). Here we present recent work from our laboratory indicating that (i) the voltage sensor comprises at least two vicinal thiols whose oxidation-reduction state affects the MTP gating potential; as the couple becomes more oxidized the gating potential increases; conversely, as it becomes more reduced the gating potential decreases; (ii) that MTP opening is fully reversible, as mitochondria maintain volume homeostasis through several cycles of pore opening/closure; and (iii) that the mechanism of MTP inhibition by cyclosporin A presumably involves a mitochondrial cyclophilin but does not utilize a calcineurin-dependent pathway.

Follicle-stimulating hormone-induced phospholipase A2 activity and eicosanoid generation in rat Sertoli cells

The possibility that FSH stimulates the phospholipase A2 (PLA2) pathway was studied in cultured immature Sertoli cells. FSH induced [3H]-arachidonic acid (AA) release from prelabeled cells in a time- and concentration-dependent fashion (ED50 = 21.8 +/- 1.9 ng/ml). This response could be fully prevented by pretreatment of cells with the PLA2 inhibitor, mepacrine. That PLA2 was the main enzyme responsible for cleavage of AA from membrane phospholipids was directly shown by PLA2 activity assay using vesicles of radiolabeled phosphatidylcholine (PC) as substrate. Furthermore, FSH stimulated eicosanoid generation in a time-dependent manner through the cyclooxygenase but not the lipoxygenase pathway. In fact, higher levels of prostaglandin (PG) E2, F2 alpha, and the stable products of PGI2 and thromboxane A2 (6-keto PGF1 alpha and thromboxane B2, respectively) were generated by the gonadotropin-treated cells as compared to control cells. The effect was inhibited by mepacrine, further supporting the pivotal role of PLA2 in the release of the eicosanoid precursor, AA. Finally, the effect of the main product of FSH-induced AA metabolism, i.e., PGE2, was studied. Intracellular cAMP accumulation in Sertoli cells was stimulated by the prostanoid in a dose-dependent manner (ED50 = 2.3 +/- 0.37 nM). PGE2 also significantly stimulated aromatase activity, a specific marker of Sertoli cell functions, measured as 17 beta-estradiol production (ED50 = 4.7 +/- 0.8 nM). Similar results were obtained with PGF2 alpha. Our findings show that FSH, through the activation of PLA2, leads to AA release with consequent metabolism by the cyclooxygenase pathway.(ABSTRACT TRUNCATED AT 250 WORDS)

The voltage sensor of the mitochondrial permeability transition pore is tuned by the oxidation-reduction state of vicinal thiols. Increase of the gating potential by oxidants and its reversal by reducing agents

Reaction of isolated mitochondria with a variety of agents that lead to oxidation or cross-linking of sulfhydryl groups leads to an increased "open" probability of the permeability transition pore, a cyclosporin A-sensitive channel. We have investigated the mechanism by which the pore is induced by menadione, diamide, arsenite, and tert-butylhydroperoxide. We find that these inducers increase the probability of pore opening by shifting its gating potential to higher values. Furthermore, the induced shift was prevented by treatment with N-ethylmaleimide or dithiothreitol. At moderate levels of depolarization an apparent I50 for N-ethylmaleimide of bout 5 microM can be defined, while the N-ethylmaleimide or dithiothreitol effects are overcome by maximal depolarization. We conclude that the oxidation-reduction state of vicinal thiols in cysteinyl residues plays a critical role in tuning the voltage sensor of the transition pore, with an increase of gating potential (i.e. an increase in the probability of pore opening despite a high transmembrane potential difference) as the couple is poised to a more oxidized state. These findings may have implications for the mechanism of cell damage under oxidative stress.

Plasma membrane block to sperm entry occurs in mouse eggs upon parthenogenetic activation

The ability of parthenogenetically activated mouse eggs to establish a plasma membrane (PM) block to sperm penetration was studied. Zona-free eggs preloaded with Hoechst 33342 were activated by exposure to ethanol or OAG (1-oleoyl-2-acetyl-sn-glycerol) and inseminated after different periods. Eggs challenged with sperm at 30- or 60-min postactivation displayed a fertilization frequency significantly lower than that of control eggs. Conversely, when insemination was carried out at 120-min postactivation, the proportion of fertilized eggs was equivalent to that observed in the control group. Moreover, we report that when the eggs were induced to resume meiosis without any notable loss of CGs (egg exposure to OAG at 100 microM external Ca2+ or to heat shock), a normal ability to be penetrated was recorded at 30-min postactivation. Similar behaviour was exhibited by eggs that underwent a CG exocytosis close to that triggered by sperm in absence of nuclear activation (microinjection of inositol 1,4,5-trisphosphate into the egg at 1 microM cytosolic concentration). Present data support the conclusion that parthenogenetically activated mouse eggs are capable of a transitory PM block response that requires both CG exocytosis and meiosis resumption to occur.

Physiological effectors modify voltage sensing by the cyclosporin A-sensitive permeability transition pore of mitochondria

This paper reports an investigation on the modulation of the mitochondrial permeability transition pore (MTP) by the membrane potential. Energized rat liver mitochondria loaded with a small Ca2+ pulse in sucrose medium supplemented with phosphate favor a high MTP "closed" probability because of the high membrane potential and therefore maintain a low permeability to sucrose. Upon depolarization by the addition of fully uncoupling concentrations of carbonyl cyanide p-trifluoromethoxyphenylhydrazone (FCCP) mitochondria favor a high MTP "open" probability and rapidly undergo a process of osmotic swelling following sucrose diffusion toward the matrix. A titration with FCCP reveals that discrete subpopulations of mitochondria with different gating potentials for MTP opening may exist, since increasing concentrations of FCCP increase the fraction of mitochondria undergoing osmotic swelling. We show that physiological effectors (Ca2+, Mg2+, ADP, palmitate) modify pore opening in a mitochondrial population by shifting the fraction of mitochondria with a functionally open pore at any given membrane potential. Many inducers and inhibitors may therefore affect the pore directly through an effect on the MTP voltage sensing rather than indirectly through an effect on the membrane potential. Thus, many effectors may induce pore opening by shifting the MTP gating potential to higher levels, whereas many inhibitors may induce pore closure by shifting the MTP gating potential to lower levels.

Protein kinase C-dependent and independent events in mouse egg activation

The involvement of calcium- or protein kinase C (PKC)-dependent pathways in cortical granule exocytosis (CGE) and pronucleus formation was examined in mouse eggs using the specific PKC stimulator OAG (1-oleyl-2-acetyl-sn-glycerol) at different external calcium concentrations ([Ca2+]e) ranging from 1.7 mM to 0.1 microM. A 10 min exposure of eggs to 150 microM OAG in the presence of 1.7 mM [Ca2+]e caused a large calcium influx, cortical granule release and 82% activation. The increased permeability of the egg membrane to Ca2+ ions after OAG treatment lasted 20 min. At [Ca2+]e lower than 1.7 mM, both OAG-induced calcium influx and CGE decreased, reaching a non-detectable level at 0.1 microM and 100 microM [Ca2+]e, respectively. Resumption of meiosis was not affected by [Ca2+]e above 200 microM but it was reduced at any lower [Ca2+]e, with a minimum activation frequency of 46% at 0.1 microM [Ca2+]e. Loading of eggs with > or = 3 microM of the calcium chelator BAPTA AM (1,2-bis(o-aminophenoxy)ethane-N',N',N',N'-tetraacetic acid-acetoxymethyl ester) prior to OAG treatment caused a reduction in meiosis resumption with 50% of eggs forming pronuclei. Potent inhibitors of PKC, such as acridine orange and sphingosine, did not interfere with OAG-induced CGE. Conversely, these compounds prevented OAG-induced pronucleus formation in a dose-dependent manner with an IC50 (inhibiting concentration, 50%) of 5 microM and 30 microM for acridine orange and sphingosine, respectively. Microinjection of inositol 1,4,5-trisphosphate into eggs at 0.1 microM elicited Ca2+ release from intracellular stores and the cortical reaction, but failed to stimulate pronucleus formation. These results indicate that, in mouse eggs, CGE is a PKC-independent event, and that the transition from M-phase to interphase may require PKC activity for stimulation.

Modulation of the mitochondrial permeability transition pore. Effect of protons and divalent cations

We have studied the induction of the mitochondrial cyclosporin A-sensitive permeability transition pore (PTP) by the bifunctional SH group reagent phenylarsine oxide (PhAsO). Addition of nanomolar concentrations of the electroneutral H(+)-K+ ionophore nigericin to nonrespiring mitochondria in sucrose medium determines a dramatic increase of the time required for PTP induction by PhAsO, while no effect of nigericin is apparent in KCl medium. Using mitochondria loaded with the internal pH indicator 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein, we show that the effect of nigericin is mediated by the ionophore-induced acidification of matrix pH. Indeed, experimental manipulation of pHi by a number of treatments indicates that PTP induction is directly related to matrix pH, in that the PTP induction process becomes slower as pHi decreases at constant pHo. PTP induction by PhAsO in respiration-inhibited mitochondria is stimulated by Ca2+ and inhibited by a series of divalent cations. Since PhAsO induces the PTP even in the presence of excess EGTA and in the absence of respiration (Lenartowicz, E., Bernardi, P., and Azzone, G.F. (1991) J. Bioenerg. Biomembr. 23, 679-688), we have been able to study the Ca2+ dependence of the induction process. We show that the apparent Km for Ca2+ activation is about 10(-5) M and that Ca2+, cyclosporin A, and inhibitory Me2+ ions behave as if they were competing for the same binding site(s) on the pore. Since similar results are obtained from patch-clamp experiments on the mitochondrial megachannel (Szabó, I., Bernardi, P., and Zoratti, M. (1992) J. Biol. Chem. 267, 2940-2946), we suggest that (i) the PTP and the mitochondrial megachannel are the same molecular structures and (ii) the same factors affect both the process of pore induction and its open-closed orientation.

pH-dependent lipid packing, membrane permeability and fusion in phosphatidylcholine vesicles

We have studied the rate of membrane fusion, the lipid dynamics and order and the membrane permeability of phosphatidylcholine vesicles as a function of pH. Acidification induced very different effects depending on the state of the bilayer. In liquid-crystalline bilayers, acidification decreased the rate of membrane fusion, the acyl chain motion and disorder and the rate of K+ release, whereas in solid bilayers acidification increased the rate of membrane fusion, the lipid acyl chain disorder and the rate of K+ release. These pH-dependent modifications are interpreted in terms of conformational and/or packing changes of the phosphatidylcholine head group in the membrane. In solid bilayers, these changes are not easily accommodated by the rigid structure, and the resulting stress leads to an unstable bilayer.

Granulosa cell-oocyte interactions: the phosphorylation of specific proteins in mouse oocytes at the germinal vesicle stage is dependent upon the differentiative state of companion somatic cells

The role of granulosa cells in the regulation of mouse ovarian oocyte metabolism was investigated. Fully grown antral oocytes, isolated from surrounding cumulus cells, were cultured on monolayers of preantral granulosa cells in the presence of dbcAMP to prevent the resumption of meiosis. Under these conditions metabolic cooperativity was established between the two cell types as early as 1 hr after seeding. Moreover, cocultured oocytes phosphorylated two polypeptides of 74 and 21 kDa which are normally phosphorylated in follicle-enclosed growing oocytes but not in cumulus cell-enclosed fully grown oocytes at the germinal vesicle stage. When cocultured oocytes were allowed to resume meiosis, the 74 and 21 kDa proteins were synthesized but no longer phosphorylated even though intercellular coupling between the two cell types was maintained during radiolabeling. It appears therefore: a) that the different protein kinase activity of growing and fully grown germinal vesicle-stage mouse oocytes is related to the differentiative state of granulosa cells, and b) that the regulation of oocyte protein phosphorylation activity by granulosa cells is dependent on the meiotic stage of the oocyte.

Effects of protein kinase C stimulation and free Ca2+ rise in mammalian egg activation

Protein phosphorylation activity, chromosome segregation, and cortical granule exocytosis (CGE) have been studied in mouse eggs activated parthenogenetically by specific PKC stimulators such as 4 beta-phorbol 12-myristate 13-acetate (PMA) and 1-oleyl-2-acetylglycerol (OAG), or by agents inducing an immediate increase in cytosolic calcium concentration ([Ca2+]i) such as ethanol and Ca-ionophore A23187. When protein phosphorylation activity of mouse eggs was analyzed 10 min after different activation treatments, the phosphorylation of a 32 kDa polypeptide was a feature common to all different parthenogenetic agents used. The appearance of such labeling was independent of an increasing [Ca2+]i, as indicated by direct measurements of 1) cytosolic Ca2+ concentration with fura-2 and 2) exogenous Ca2+ entrance into activated eggs. Emission of the second polar body was blocked in PMA-elicited parthenogenones, whereas it was apparently normal in OAG-treated eggs, unless the eggs were continuously exposed to OAG. CGE was almost immediate in ethanol-activated eggs, but in PMA-treated cells, it occurred significantly later, with a timing corresponding to that found for the appearance of sustained Ca2+ oscillations in this system. Here, we propose that in mammalian eggs 1) PKC stimulation represents an early regulatory step in egg activation; 2) this kinase activity is turned off before the second meiotic cleavage; and 3) cytosolic free Ca2+ rise is essential for CGE occurrence.

Somatic cell-oocyte interactions in mouse oogenesis: stage-specific regulation of mouse oocyte protein phosphorylation by granulosa cells

The relative rate of synthesis of a number of proteins and the protein phosphorylation pattern of growing and fully grown oocytes were influenced by the presence of granulosa cells. In particular, a 74-kDa phosphorylated protein was detected only in granulosa cell-enclosed growing mouse oocytes. When reaggregated with granulosa cells, the growing oocyte displayed the phosphorylated form of the 74-kDa protein but when oocytes were cultured on Sertoli cell monolayers or in granulosa cell-conditioned medium the 74-kDa protein was not phosphorylated. We propose that (1) granulosa cells regulate protein phosphorylation in mouse oocytes; (2) a 74-kDa protein is phosphorylated only in growing oocytes when surrounded by granulosa cells; and (3) granulosa cells, but not Sertoli cells, are competent to send the appropriate "signal" to the growing oocyte.

Actin synthesis is not regulated by granulosa cells in mouse growing and preovulatory oocytes

The synthesis and intracellular distribution of actin were studied in isolated dictyate and metaphase II mouse oocytes by 1) sodium dodecyl sulfate-polyacrylamide gel electrophoresis of newly synthetized oocyte protein and 2) cytochemical F-actin labeling by fluorescent phalloidin. Unpermeabilized, fully grown oocytes bound phalloidin intensely at the level of the zona pellucida (ZP), such ZP-associated actin representing a significant portion of total actin found in these cells. In contrast, phalloidin binding to ZP was very low in growing oocytes and was undetectable in ovulated, metaphase II eggs. When ZP-associated actin of fully grown oocytes was removed by prolongedly exposing oocytes to alpha-chymotrypsin, the amount of newly synthesized actin displayed by cumulus-enclosed oocytes was reduced to a level comparable to that shown by oocytes isolated from granulosa cells. We demonstrate that ZP-associated actin belongs to granulosa cell processes that remain within the ZP as a consequence of oocyte isolation procedures. We conclude that actin synthesis of mouse oocytes is not regulated by granulosa cells.

Interaction of the fluorescent probe N-(lissamine Rhodamine B sulfonyl)dipalmitoylphosphatidylethanolamine with phosphatidylcholine bilayers

The surface density of the fluorescent probe N-(lissamine Rhodamine B sulfonyl)dipalmitoylphosphatidylcholine is the same in the two lipid leaflets of phosphatidylcholine bilayers containing the probe. In the liquid-crystalline state, the probe molecules aggregate above a threshold amount, approximately 0.2 mol/mol phospholipids. Above this threshold value, the surface density of the free probe molecules is constant, and all probe molecules added are incorporated in the aggregated form. The aggregation of the probe increases by approximately 20% when the medium pH is lowered to 4. In the gel state, the probe aggregation is higher than that in the liquid-crystalline state, and the free probe molecules distribute unevenly in the bilayer surface. Even though the results obtained in our model system cannot be directly extrapolated to all model systems, we point out that care is to be taken in the use of the probe. In fact, only in membranes in the liquid-crystalline state in which the amount of probe molecules to phospholipid molecules is lower than 1:7 the fluorescence response of the probe is independent of the pH changes and of the molecular aggregation.

Lipid interaction of diphtheria toxin and mutants with altered fragment B. 1. Liposome aggregation and fusion

The interaction of diphtheria toxin and its cross-reacting mutants crm 45,228 and 1001 with small unilamellar vesicles has been followed by a turbidity assay, electron microscopy, fluorescence energy transfer and membrane permeability. All toxins at pH lower than 6 induce the aggregation and fusion of liposomes containing negatively charged phospholipids; crm 45 and crm 1001 are less potent than diphtheria toxin. Isolated diphtheria toxin fragment B is very effective while isolated fragment A is ineffective. Liposome fusion induced by the toxins at low pH occurs without release of the internal content implying that fusion does not involve vesicle breakage and resealing. The pH dependence of the membrane interaction of diphtheria toxin monitored by turbidity is in close agreement with that monitored by fluorescence energy transfer. It shows that diphtheria toxin can alter the lipid bilayer structure in the pH interval 5-6. This pH range occurs in endosomes and suggests that histidyl and carboxyl residues are likely to be involved in the conformational change of diphtheria toxin triggered by acidic pH.

Follicle cell regulation of mammalian oocyte growth

To investigate mechanisms of follicle cell control on mammalian oocyte growth, preantral mouse oocytes free from surrounding follicle cells were individually cocultured with monolayers of different somatic cells competent to form gap junctions, and the rate of in vitro oocyte growth was directly correlated with the level of metabolic coupling on the same cells. The results indicate that 1) at a similar extent of metabolic coupling, mouse oocytes grew on follicle cells but not on 3T3 and Sertoli cell monolayers, and 2) the growth rate of oocytes cultured on follicle cells was dependent on the extent of metabolic coupling. It was concluded that gap-junction-mediated nutrition of ovarian mouse oocytes exerted by somatic cells is necessary but not sufficient to maintain oocyte growth. A specific regulatory role of follicle cells on mammalian oocyte growth is proposed.

Diphtheria toxin and its mutant crm 197 differ in their interaction with lipids

The interaction of diphtheria toxin and its enzymatically deficient mutants crm 176 and crm 197 with liposomes has been studied by turbidity measurement and hydrophobic photolabelling with photoactivatable phosphatidylcholines. Diphtheria toxin and crm 176 at neutral pH bind to the surface of lipid bilayers while crm 197 also appears to interact with the fatty acid chains of phospholipids. All proteins undergo a change in conformation over the same range of acidic pH and become able to insert in the lipid bilayer. The tighter lipid interaction of crm 197 may account for its higher cell association constant. The possibility is discussed that the binding of diphtheria toxin to cells is mediated by both a protein receptor and an interaction with the head group of phospholipids.

Lipid miscibility and size increase of vesicles composed of two phosphatidylcholines

The size increase of small unilamellar vesicles composed of binary mixtures either of saturated fatty acid phosphatidylcholines with different chain lengths or of saturated and unsaturated phosphatidylcholines was found to depend on the miscibility properties of the lipid components. No size increase was detected in vesicles formed by two miscible phosphatidylcholines. In vesicles composed of two lipids which are partially immiscible in the gel state, a size increase was observed at temperatures which mainly overlapped the range of temperatures of the lipid phase transition. The rate of size increase of vesicles composed of two lipids which are immiscible in the gel state was faster than that of vesicles composed of two partially immiscible phosphatidylcholines, and the process occurred not only at the temperature ranges of the lipid phase transition, but also when both lipids were in the gel state. The vesicle size increase process occurred without the mixing of the internal content of the vesicles. A model is proposed in which the presence of 'fractures' between membrane regions of different fluidity and/or lipid composition controls the rate of this process.

Gramicidin induced aggregation and size increase of phosphatidylcholine vesicles

To investigate the role of membrane proteins in the fusion process, linear hydrophobic polypeptide gramicidin was used as fusogenic agent in small unilamellar vesicles (SUV) constituted of saturated lecithins. It was found that gramicidin, externally added to a suspension of vesicles, induces a reversible vesicles aggregation. When incorporated into the bilayer, gramicidin induces increase in vesicle size. The vesicle size increase was monitored by column chromatography and transmission electron microscopy. The process of vesicle size increase occurs only when the lipid membrane is in the gel state. A maximum is observed in the kinetics at a temperature of approx. 25 degrees C lower than the phase transition temperature of lipids. Higher rates of vesicle size increase are obtained as the lipid chain length increases. The process is accompanied by a release of internal vesicle content and by membrane lipid mixing.

Stage-dependent modifications of amino acid uptake by antral and metaphase II mouse oocytes

Modifications of leucine transport system of mouse oocytes have been studied throughout Graafian follicle development and oocyte maturation. In contrast to sheep oocytes (Moor and Smith, 1979), in the mouse kinetic constants and efflux rate of leucine transport system did not vary in diestrus, proestrus, and metaphase II (met II) oocytes. However, kinetics of leucine equilibration in proestrus and met II oocytes was significantly slower than that found in diestrus cells, and this may reflect a decreased availability of internal amino acids for exchange.

Amino acid transport systems in growing mouse oocytes

Amino acid transport systems have been studied in isolated mouse oocytes throughout oogenesis. While these cells were lacking the A-transport system of Ehrlich cell (Christensen, 1975), they were found to be provided with the L- and ASC-systems of Ehrlich cell (Christensen, 1975). Since, in contrast to the A-system, the L- and ASC-systems mainly work by exchanging internal and external amino acids, we propose that their main function is to actively maintain the appropriate internal balance between different amino acids within the growing oocyte.

Voltage oscillations in mammalian metaphase II oocytes

The membrane potential has been measured in ovulated mouse oocytes using conventional electrophysiological techniques. Temporal oscillations in membrane voltage have been observed in the oocytes, with periods of about 6 h. This oscillatory pattern, peculiar to oocytes in metaphase II, might explain the differences in membrane potential values reported in several studies on mammalian oocytes.

Mechanisms of amino acid uptake in cumulus-enclosed mouse oocytes

The nutritional role of mouse granulosa cells on antral dictyate mouse oocytes has been studied by measuring the transfer of different amino acids through gap junctional channels between somatic and germ cells. When present in the incubation medium at concentrations resembling in vivo conditions, glycine, alanine, proline, serine, tyrosine, glutamic acid and lysine entered cumulus-enclosed oocytes cooperatively, while valine, leucine and phenylalanine did not. However, cooperative uptake of leucine and phenylalanine was observed at higher external precursor concentrations. We conclude that in vivo antral mouse oocytes depend on surrounding granulosa cells for amino acid uptake, with the exception of amino acids carried by the leucine exchange transport system, and propose that amino acid transfer between granulosa cells and oocytes is dependent on precursor concentrations in the coupled cells.