Ecophysiological analysis of woody species in contrasting temperate communities during wet and dry years

This study employed an intensive sampling regime in which leaf gas exchange and tissue-water relations were measured simultaneously on the same leaf at midday on 19 tree species from three distinct forest communities during wet (1990) and dry (1991) growing seasons. The study sites were located on a xeric barrens, a misic valley floor, and a wet-mesic floodplain in central Pennsylvania, United States. The xeric, mesic, and wetmesic sties had drought-related decreases in gravimetric soil moisture of 53, 34 and 27%, respectively. During the wet year, xeric and mesic communities had high seasonal mean photosynthetic rates (A) and stomatal conductance of water vapor (g _wv) and low midday leaf water potential (), whereas the wet-mesic community had low A and g _wv and high midday . The mesic and wet-mesic communities had dry year decreases in predawn , g _wv and A with the greatest drought effect occurring in the mesic community. Regression analysis indicated that species from each site that exhibited high wet-year A and g _wv tended to have low midday . This trend was reversed only in the mesic community in the drought year. Despite differences in midday , all three communities had similar midday leaf turgor pressure (_p) in the wet year attributable to lower osmotic potential at zero turgor ( ⁰ ) with increasing site droughtiness. Lower wet year ⁰ in the xeric community was due to low symplast volume rather than high solute content. Species with the lowest ⁰ in the wet year often did not have the lowest ¹⁰⁰ possibly related to differences in tissue elasticity. Moreover, increased elasticity during drought may have masked osmotic adjustment in ¹⁰⁰ but not in ⁰ , via dilution of solutes at full hydration in some species. Despite the sampling regime used, there were no relationships between gas exchange and osmotic and elastic parameters that were consistently significant among communities or years. This result questions the universal, direct effect of osmotic and elastic adjustments in the maintenance of photosynthesis during drought. By including a large number of species, this study provided new insight to the ecophysiology of contrasting forest communities, and the community-wide impact of drought on contrasting sites.     

The effect of pinealectomy on entrainment of the locomotor activity rhythm in sparrows maintained on various short days

Summary Pinealectomy of house sparrows on 3L:21D (3 h light per 24 h) resulted in a significant increase in the time between the onset of perch-hopping activity and lights on (on) as well as the time between the offset of activity and lights of (off). The daily variance in on and off was also increased following the removal of the pineal gland. On longer light cycles (i.e., 5L:19D; 7L:17D), neither on or off, nor the variance of on or off was different between sham-pinealectomized and pinealectomized sparrows. Upon returning the birds to an ultrashort light cycle, 1L:23D, off, as well as the variance in on and off were again found to be significantly larger in the pinealectomized birds when compared to sham-operated controls. These results indicate that the effects of pinealectomy on the entrained rhythm of locomotor activity are most pronounced when birds are exposed to a weak entraining agent, such as an ultrashort light: dark cycle. In view of the observation that pinealectomy can alter the phase relationship between activity onset and offset, it is suggested that the pineal gland may be involved in the coupling of the oscillators that regulate activity onset and offset.     

Estimation of Membrane Potential Δψ in Reconstituted Plasma Membrane Vesicles Using a Numerical Model of Oxonol VI Distribution

A model of membrane potential-dependent distribution of oxonol VI to estimate the electrical potential difference across Schizosaccharomyces pombe plasma membrane vesicles (PMV) has been developed. was generated by the H⁺-ATPase reconstituted in the PMV. The model treatment was necessary since the usual calibration of the dye fluorescence changes by diffusion potentials (K⁺ + valinomycin) failed. The model allows for fitting of fluorescence changes at different vesicle and dye concentrations, yielding in ATP-energized PMV of 80 mV. The described model treatment to estimate may be applicable for other reconstituted membrane systems.     

Measurements of electrical potential differences across yeast plasma membranes with microelectrodes are consistent with values from steady-state distribution of tetraphenylphosphonium inPichia humboldtii

Summary Electrical potential differences across the plasma membrane () of the yeastPichia humboldtii were measured with microelectrodes (filled with 0.1m KCl) inserted into cells immobilized in microfunnels. The registered signals were reproducible and stable for several minutes. On attainment of stable reading for the specific membrane resistanceR _sp was determined by applying square-current pulses to the preparation. Both andR _sp were pH dependent and displayed equal but opposite deflection, reaching its maximal value of –88±9 mV (n=13) andR _sp its minimal value of 10 k·cm² (maximal conductance) at pH 6.5. Uncouplers and the polyene antibiotic nystatin depolarized the cells, decreasing to –21±15 mV (n=10) with concomitant decrease ofR _sp. Comparison of values from microelectrode measurements with those calculated from the steady-state distribution of tetraphenylphosphonium ions agreed within 10 mV under all physiological conditions tested, except at pH values above 7.0. During microelectrode insertion transient voltage signals (a few msec long) were detected by means of an oscilloscope. These voltage signals were superimposed on the stable recordings described above. These short voltage signals disappeared in uncoupled cells. The closely related values obtained by two independent methods (direct measurements with microelectrodes and calculation from steady-state distribution of a lipophilic cation) provide evidence that these values reffect the true membrane potential of intact cells.     

Phlorizin binding to isolated enterocytes: Membrane potential and sodium dependence

Summary Phlorizin binding is studied in isolated intestinal epithelial cells of the chick. Cells are ATP depleted to allow extensive manipulation of ionic gradients and membrane potential (). Phlorizin binding is assayed at steady state. Carrier specific phlorizin binding is defined asd-glucose (90 mM) inhibitable binding. Specific binding displays simple Michaelian kinetics as a function of phlorizin. indicating the presence of a single homogeneous binding site. Sodium concentrations and modify the apparent binding affinity but not the maximum number of binding sites. In contrast, the activation curve as a function of sodium concentrations is sigmoid and the apparent maximum number of binding sites at saturating sodium is phlorizin dependent. The rate of phlorizin association is both and sodium-concentration dependent. Dissociation is sodium-concentration dependent but not dependent. Theoretical analysis indicates binding order of substrates is random. In addition, data suggests that the phlorizin/sodium stoichiometry is 2:1. The dependence can be explained by two models: either translocation is the -dependent step and the free carrier is anionic, or sodium binding is the -dependent step.     

Preparations of Ψ-peptide bond and peptide-aldehyde inhibitors of atrial granule serine proteinase, a candidate processing enzyme of pro-atrial natriuretic factor

Pseudo-peptide bond inhibitors (-bond inhibitors) and peptide-aldehyde inhibitors of atrial granule serine proteinase, the candidate processing enzyme of pro-atrial natrieuretic factor, are prepared in high yield and purity by novel synthetic routes. The -bond compounds retain essential residues for enzyme binding, but place the enzyme inhibition site in the midst of the peptide sequence. Thus, Bz-APR--LR and Bz-APR--SLRR can be considered readthrough inhibitors of atrial granule serine proteinase. The most potent -peptide, Bz-APR--SLRR (IC₅₀=250 M), is about fivefold less potent than the best peptide-aldehyde inhibitor (EACA-APR-CHO), and both the -bond and peptide-aldehyde compounds are competitive, reversible inhibitors of the enzyme. The -bond peptides containing two C-terminal Arg residues are three-to tenfold more potent than the analogous compounds containing only one C-terminal Arg residue, confirming the importance of both Arg residues in the enzyme processing recognition site. As expected, because of their moderate potencies, the -peptides are not useful affinity ligands for purification of atrial granule serine proteinase, but both peptide aldehydes are effective affinity ligands [Damodaran and Harris (1995),J. Protein Chem., this issue].Abbreviations AGSP atrial granule serine proteinase - ANF atrial natriuretic factor - Bz benzoyl - DIEA diisopropylethylamine - DIPCDI diisopropylcarbodiimide - DMF dimethylformamide - DMSO dimethylsulfoxide - EACA 6(e)-aminocaproic acid - EtOAc ethyl acetate - HEPES N-2-hydroxyethylpiperazine-N-propanesulfonic acid - HOBt N-hydroxybenzotriazole - HPLC high-performance liquid chrornatography - NMR nuclear magnetic resonance - PEG polyethylene glycol-3350 - PyBOP benzotriazole-1-yl-oxy-trispyrrolidino-phosphonium-hexafluorophospate - TEA triethylamine - TFA trifluoroacetic acid - THF tetrahydrofuran - TLC thin-layer chromatography - UV ultraviolet - pseudo-peptide bond -CH₂-NH-. Single-letter abbreviations are used to denote amino acids     

Effects of shoot and root application of thiamin on salt-stressed sunflower plants

Plants of sunflower (Helianthus annuus L. cv Giza2) were salt-stressed with a combination of NaCl and CaCl₂ inconcentrations having different osmotic potentials (s from 0 to –1.0MPa) and were treated with 5 and 10mg L^–1 of thiamin either sprayed on the shoot orapplied to the root. The membranes of leaf discs from salt-stressed plantsappeared to be less stable (more injured) under heat(51°C) and drought (40% polyethylene glycol6000) stresses than control plants. Salinity slowed the rate of growth (lengthand dry mass production), lowered leaf relative water content (RWC) and leafandroot water potential (w), decreased the contents of chlorophyll (Chl),soluble sugars (SS) and the K⁺/Na⁺ ratio butenhanced total free amino acids (TAA), Na⁺,Ca²⁺and Cl^– accumulation in the shoot and root system. Root orshoot application of thiamin reduced membrane injury by either heat ordehydration stress, lowered leaf w, improved uptake of K⁺,and increased leaf RWC, Chl, SS, TAA contents and dry mass production. Theeffects of salinity (s), thiamin (Thi.) and their interaction(s×Thi) on the parameters tested were significant.Salinity was dominant (as indicated by ² values) in affectingthe contents of Ca²⁺, Cl^–, TAA and membranestability to heat and leaf w. The role of thiamin was dominant forNa⁺, K⁺ and SS contents and the contribution ofinteraction was dominant for growth parameters, Chl. and root w.     

The carotenoids of Flavobacterium strain R1560

The main carotenoid of Flavobacterium strain R1560 has been identified as (3R,3R)-zeaxanthin. Also present were small amounts of 15-cis-phytoene, phytofluene, -carotene (7,8,7,8-tetrahydro-, -carotene plus 7,8,11,12-tetrahydro-, -carotene), neurosporene, lycopene, -zeacarotene, -carotene, -carotene, -cryptoxanthin, rubixanthin, 3-hydroxy--zeacarotene and several apo-carotenals. Zeaxanthin production was inhibited by nicotine (10 mM), and lycopene and rubixanthin accumulated. The biosynthesis of zeaxanthin is discussed in terms of pathways and also of half-molecule reaction sequences. The presence of zeaxanthin may be a characteristic of a group of Flavobacterium species, and may thus be useful in the taxonomic classification of these organisms.     

Secondary structural effects on protein NMR chemical shifts

For an amino acid in protein, its chemical shift, (, )_s, is expressed as a function of its backbone torsion angles ( and ) and secondary state (s): (, )_{s=, )_coil+(, )_s}, where (, )_coil represents its chemical shift at coil state (s=coil); (, )_s (s=sheet or helix) is herein defined as secondary structural effect correction factor, which are quantitatively determined from Residue-specific Secondary Structure Shielding Surface (RSS) for ¹³CO, ¹³C, ¹³C,¹H, ¹⁵N, and ¹HN nuclei. The secondary structural effect correction factors defined in this study differ from those in earlier investigations by separating out the backbone conformational effects. As a consequence, their magnitudes are significantly smaller than those earlier reported. The present (, )_sheet and (, )_helix were found varying little with backbone conformation and the 20 amino acids, specifically for ¹³CO, ¹³C, and ¹H nuclei. This study also carries out some useful investigations on other chemical shift prediction approaches – the traditional shielding surfaces, SHIFTS, SHIFTX, PROSHIFT, and identifies some unexpected shortcomings with these methods. It provides some useful insights into understanding protein chemical shifts and suggests a new route to improving chemical shifts prediction. The RSS surfaces were incorporated into the program PRSI [Wang and Jardetzky, J. Biomol. NMR, 28: 327–340 (2004)], which is available for academic users at http://www.pronmr.com or by sending email to the author (yunjunwang@yahoo.com).     

Water relations and growth of three grasses during wet and drought years in a tallgrass prairie

Summary The water relations and growth of three tallgrass prairie species Panicum virgatum, Andropogon gerardii and A. scoparius were examined in irrigated and unwatered prairie in eastern Kansas (USA). Measurements of the osmotic potential at full turgor, ¹⁰⁰ , at zero turgor, ⁰, and growth of vegetative and reproductive tillers were made in a year with above-normal precipitation and a drought year to evaluate: 1) the ability of these grasses to osmotically adjust in response to water stress and 2) the effect of drought or supplemental water on growth of these species. Although these grasses adjusted osmotically even in the wet year, the degree of adjustment of ¹⁰⁰ and ⁰ in the drought year was relatively large (0.60–0.78 MPa and 0.88–1.34 MPa, respectively) compared to reports for other species. Seasonal minimum values of ¹⁰⁰ and ⁰ for these grasses in the drought year were lower than in most mesic species and seasonal fluctuations in ¹⁰⁰ and ⁰ were greater than reported for most mesic or xeric species. The relatively frequent occurrence of drought in sub-humid tallgrass prairies may partially explain the greater than expected magnitude of osmotic adjustment in these grasses.Irrigation in the wet year increased reproductive biomass in the mesic grass P. virgatum, but had no effect on A. gerardii or the more xeric grass A. scoparius. However, irrigation in the drought year increased maximum shoot biomass in all three grasses significantly with the largest increase in P. virgatum. Reproduction in P. virgatum was also increased more by irrigation in the drought year compared to the other grasses. Irrigation did not increase season's end production of A. gerardii in the wet year, but in the drought year production was 28% greater in irrigated than unwatered prairie. The combination of these water relations and growth responses of the three grasses to wetter than normal and drought years supports their reported distribution along a moisture gradient in tallgrass prairies.     

Kinetics of Δψ-Dependent Sucrose Transport in Plasma Membrane Vesicles from Higher Plant Cells

The inside-out vesicles of plasma membranes were isolated from pumpkin stem cells, and the kinetics of sucrose efflux induced by the K⁺-diffusion potential (_D) was studied by measuring light transmission. Two phases differing in their rates and duration were identified in _D-dependent changes of light transmission. The increase in _Delevated the rate and magnitude of the fast phase in light transmission changes but did not markedly affect the rate of the slow phase. These two phases also differed in their sensitivity to inhibitors and to changes in sucrose concentration in the external medium. Measurements of _Dduring sucrose transport by means of the fluorescence probe dis-C₃-(5) revealed differences in the magnitude of _Dand its stability in vesicles loaded with sucrose and mannitol, as well as under the action of inhibitors. The two-phase dependence of sucrose efflux from vesicles on the applied diffusion potential is discussed in the context of modern concepts on the functioning of sucrose carriers in the membranes.     

Protocol-dependence of equivalent circuit parameters of toad urinary bladder

Summary Determinations of current-voltage relationships are widely employed in the characterization of epithelial sodium transport. In order to determine the protocol dependence of transport parameters in the toad urinary bladder, studies were carried out in the presence and absence of amiloride, an inhibitor of active sodium transport. With symmetric positive and negative perturbations of the transepithelial electrical potential difference (0±100 mV) for 30 sec, the amiloride-sensitive current-voltage (i ^a -) relationship was near linear over the range –75+100 mV, indicating constancy of the conductance ^a and the apparent electromotive force E _Na, lumped parameters of the standard electrical equivalent circuit model of the active transport system. With a reverse protocol (±1000 mV) or 15 min perturbations thei ^a - relationships were highly nonlinear. Nonlinearity reflected voltage dependence of parameters: perturbations that increased active transport decreased E _Na and increased ^a, as evaluated from 10 sec perturbations of ; slowing of active transport produced the converse changes. These effects are usefully analyzed in both quasi-steady states and true steady states by means of a detailed equivalent circuit incorporating the significant ionic currents across each plasma membrane. Precise understanding of the significance of ^a and E _Na will require characterization of the partial ionic conductances on perturbation of .     

Leaf osmotic potentials of 104 plant species in relation to habitats and plant functional types in Hunshandak Sandland,Inner Mongolia,China

Leaf osmotic potentials ( _s) of 104 plant species from different habitats, i.e., fixed sand dunes, lowland and wetlands in Hunshandak Sandland, Inner Mongolia, China, were investigated. The values of _s were strongly species-specific, and varied from –6.54 MPa ( Caragana microphylla), to –0.44 MPa ( Digitaria ischaemum); 75% of plants investigated had _s from –1.01 to –3.0 MPa. Shrubs were found to have the lowest _s, with an average value of –3.19 MPa, while grasses showed the highest _s. The order of plant _s is shrubs<trees<grasses. The result may relate to anatomical features of shrubs. C₄ photosynthetic pathway plants showed lower _s values. The _s values of 104 species were negatively correlated with their rooting depths ( r ²=0.42; P <0.001). High hydraulic pressure resulting from the deep roots may well explain this trend. The value of _s increased as the environment became wetter, ranging from –0.79 MPa in wetlands to –2.09 MPa in fixed sand dunes. Although soil salt content was higher in wetlands, we did not find any effect on _s.An erratum to this article can be found at     

Leaf water potential,component potentials and relative water content in a xeric grass,Agropyron dasystachyum (Hook.) Scribn.

Summary Leaf water potential ( _l ), osmotic potential ( _s ), pressure potential ( _p , turgor pressure), relative water content (R) and their interrelationships were determined for a xeric grass (Agropyron dasystachyum) found in the grasslands of Canada. Thermocouple psychrometers were used to measure _l and _s ; _p was obtained by subtraction. _l dropped from near 0 bars to about-28 bars as R went from 90% to 75%. R greater than 90% was not observed, perhaps because of a systematic error in determination of turgid water content. R remained relatively high in A. dasystachyum, even at low _l . The slope of the _l -R relationship was similar to other species which are generally considered to be drought tolerant. _p as high as 14 bars was observed. Most of the decrease in _l was accounted for by a decline in _p . The ability of A. dasystachyum to adjust to fluctuating water stress over the growing season is probably as much related to changes in tissue structure and turgor relationships as to simple changes in osmotic potential.     

Chloroplast volume: cell water potential relationships and acclimation of photosynthesis to leaf water deficits

Studies were undertaken to determine if there is an association between nonstomatally-mediated acclimation of photosynthesis to low water potential (w) and the maintenance of chloroplast volume during water stress. Spinach plants either kept well watered throughout their growth (non-acclimated), or subjected to water stress such that leaf w dropped to -1.5 megapascals (MPa) and then were rewatered (acclimated) were subjected to drought episodes. During these stress periods, photosynthesis was maintained to a greater extent in acclimated plants as compared to non-acclimated plants at w below -1 MPa.Estimates of internal leaf [CO₂] suggested that photosynthetic acclimation to low w was not primarily due to altered stomatal response. As w dropped from initial values, a decline in steady state levels of ribulose 1,5-bisphosphate (RuBP) occurred in both non-acclimated and acclimated plants. RuBP decline was less severe in acclimated plants.Low w effects on chloroplast volume in non-acclimated and acclimated plants were estimated by measuring the volume of intact chloroplasts isolated from plants in solutions which were made isotonic to declining leaf osmotic potential during the drought episodes. Chloroplast volume was maintained to a greater extent at low w in acclimated, as compared with non-acclimated plants. Although substantial osmotic adjustment occurred in both non-acclimated and acclimated plants, the extent of osmotic adjustment was the same. These data were interpreted as supporting the hypothesis that cellular-level acclimation to low w is associated with chloroplast volume maintenance, and this physiological acclimation is correlated with enhanced photosynthetic capacity of the leaf at low w.Abbreviations [CO₂]_i internal leaf CO₂ concentration - s osmotic potential - RWC relative water content - RuBP ribulose 1,5-bisphosphate - w water potential     

Derivation of 13C chemical shift surfaces for the anomeric carbons of oligosaccharides and glycopeptides using ab initio methodology

The dependence between the anomeric carbon chemical shift and the glycosidic bond , dihedral angles in oligosaccharide and glycopeptide model compounds was studied by Gauge-Including Atomic Orbital (GIAO) ab initio calculations. Complete chemical shift surfaces versus and for d-Glcp-d-Glcp disaccharides with (11), (12), (13), and (14) linkages in both - and -configurations were computed using a 3-21G basis set, and scaled to reference results from calculations at the 6-311G^** level of theory. Similar surfaces were obtained for GlcNAcThr and GlcNAcSer model glycopeptides in - and -configurations, using in this case different conformations for the peptide moiety. The results obtained for both families of model compounds are discussed. We also present the determination of empirical formulas of the form ¹³C=f(,) obtained by fitting the raw ab initio data to trigonometric series expansions suitable for use in molecular mechanics and dynamics simulations. Our investigations are consistent with experimental observations and earlier calculations performed on smaller glycosidic bond models, and show the applicability of chemical shift surfaces in the study of the conformational behavior of oligosaccharides and glycopeptides.     

Chloroplast acclimation to low osmotic potential

Photosynthetic potential of isolated chloroplasts was investigated during in situ water deficits. An eight day stress cycle imposed on spinach plants reduced leaf _w by 0.57MPa, and leaf by 0.50MPa, resulting in partial turgor maintenance during the stress cycle. Pressure/volume curves confirmed the occurrence of osmotic adjustment. Leaf depression was associated with an altered response of chloroplasts to low in vitro. Optimum reaction medium for photosynthesis shifted from –1.04 to –1.57MPa, and low was not as inhibitory to photosynthesis of plastids pre-exposed to stress in situ. These data indicate that chloroplasts acclimate to low external in response to leaf water deficits. This response was still evident four days after a stress cycle ended, but was nearly reversed eight days after stress. Repeated stress cycles in situ did not increase the degree of chloroplast acclimation to low in vitro. Fast dehydration of leaves did not induce this apparent chloroplast acclimation.Abbreviations osmotic potential - _w water potential - PEG polyethylene glycol 8000 - MPa megapascals     

Structural and Functional Rearrangements of Mesophyll as a Probable Basis for the Cytokinin-Dependent Assimilate Translocation in Detached Leaves

The effects of benzyladenine (BA) on the mesophyll functioning, such as osmotic potential (), the effect of the inhibitors of ⁺-ATPase on the influx of ¹⁴C-sucrose, the direction of carbon metabolism, and the rate of dark respiration, were followed in the detached leaves of pumpkin (Cucurbita pepo L.) and broad beans (Vicia faba L.). BA elevated and established a gradient of (_p) between the treated and untreated leaf regions. The inhibitors of H⁺-ATPase did not affect the BA-induced influx of ¹⁴C-sucrose. The changes were accompanied with the elevated synthesis of starch and other polymeric compounds and the diminished synthesis of the substances of relatively low molecular weight. The stimulation of dark respiration was short and inconsiderable. The author concludes that the BA-induced transport was a passive process related to a increase. Leaf expansion accompanied by the synthesis of high-molecular-weight substances essential for cell growth and by starch synthesis apparently increased the sink capacity of the BA-treated detached leaves. The diminished efflux from the leaf blade was probably related to a lowered level of the transportable carbon compounds restricting their entry into the phloem. The influx induction could result from the activation of growth and metabolic processes, the decline in the number of organic molecules per cell volume unit, and the development of _p between the source and sink leaf regions.     

Cell-wall tension of the inner tissues of the maize coleoptile and its potential contribution to auxin-mediated organ growth

Plant organs such as maize (Zea mays L.) coleoptiles are characterized by longitudinal tissue tension, i.e. bulk turgor pressure produces unequal amounts of cell-wall tension in the epidermis (essentially the outer epidermal wall) and in the inner tissues. The fractional amount of turgor borne by the epidermal wall of turgid maize coleoptile segments was indirectly estimated by determining the water potential ^* of an external medium which is needed to replace quantitatively the compressive force of the epidermal wall on the inner tissues. The fractional amount of turgor borne by the walls of the inner tissues was estimated from the difference between -^* and the osmotic pressure of the cell sap (_i) which was assumed to represent the turgor of the fully turgid tissue. In segments incubated in water for 1 h, -^* was 6.1–6.5 bar at a _i of 6.7 bar. Both -^* and _i decreased during auxin-induced growth because of water uptake, but did not deviate significantly from each other. It is concluded that the turgor fraction utilized for the elastic extension of the inner tissue walls is less than 1 bar, i.e. less than 15% of bulk turgor, and that more than 85% of bulk turgor is utilized for counteracting the high compressive force of the outer epidermal wall which, in this way, is enabled to mechanically control elongation growth of the organ. This situation is maintained during auxin-induced growth.Abbreviations and Symbols _i osmotic pressure of the tissue - ₀ external water potential - ^* water potential at which segment length does not change - IAA indole-3-acetic acid - ITW longitudinal inner tissue walls - OEW outer epidermal wall - P turgor Supported by Deutsche Forschungsgemeinschaft (SFB 206).     

The mitochondrial membrane potential (deltapsi(m)) in apoptosis; an update

Mitochondrial dysfunction has been shown to participate in the induction of apoptosis and has even been suggested to be central to the apoptotic pathway. Indeed, opening of the mitochondrial permeability transition pore has been demonstrated to induce depolarization of the transmembrane potential (_m), release of apoptogenic factors and loss of oxidative phosphorylation. In some apoptotic systems, loss of _m may be an early event in the apoptotic process. However, there are emerging data suggesting that, depending on the model of apoptosis, the loss of _m may not be an early requirement for apoptosis, but on the contrary may be a consequence of the apoptotic-signaling pathway. Furthermore, to add to these conflicting data, loss of _m has been demonstrated to not be required for cytochrome c release, whereas release of apoptosis inducing factor AIF is dependent upon disruption of _m early in the apoptotic pathway. Together, the existing literature suggests that depending on the cell system under investigation and the apoptotic stimuli used, dissipation of _m may or may not be an early event in the apoptotic pathway. Discrepancies in this area of apoptosis research may be attributed to the fluorochromes used to detect _m. Differential degrees of sensitivity of these fluorochromes exist, and there are also important factors that contribute to their ability to accurately discriminate changes in _m.