Cytosolic-free calcium increases to greater than 100 micromolar in ATP-depleted proximal tubules

1,2-Bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic Acid Acetoxymethyl Ester Loaded Reactive Oxygen Species Responsive Hyaluronic Acid-Bilirubin Nanoparticles for Acute Kidney Injury Therapy via Alleviating Calcium Overload Mediated Endoplasmic Reticulum Stress

Calcium overload is one of the early determinants of the core cellular events that contribute to the pathogenesis of acute kidney injury (AKI), which include oxidative stress, ATP depletion, calcium overload, and inflammatory response with self-amplifying and interactive feedback loops that ultimately lead to cellular injury and renal failure. Excluding adjuvant therapy, there are currently no approved pharmacotherapies for the treatment of AKI. Using an adipic dihydride linker, we modified the hyaluronic acid polymer chain with a potent antioxidant, bilirubin, to produce an amphiphilic conjugate. Subsequently, we developed a kidney-targeted and reactive oxygen species (ROS)-responsive drug delivery system based on the flash nanocomplexation method to deliver a well-known intracellular calcium chelator, 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester (BAPTA-AM, BA), with the goal of rescuing renal cell damage via rapidly scavenging of intracellularly overloaded Ca²⁺. In the ischemia-reperfusion (I/R) induced AKI rat model, a single dose of as-prepared formulation (BA 100 μg·kg^-1) 6 h post-reperfusion significantly reduced renal function indicators by more than 60% within 12 h, significantly alleviated tissular pathological changes, ameliorated tissular oxidative damage, significantly inhibited apoptosis of renal tubular cells and the expression of renal tubular marker kidney injury molecule 1, etc., thus greatly reducing the risk of kidney failure. Mechanistically, the treatment with BA-loaded NPs significantly inhibited the activation of the ER stress cascade response (IRE1-TRAF2-JNK, ATF4-CHOP, and ATF6 axis) and regulated the downstream apoptosis-related pathway while also reducing the inflammatory response. The BA-loaded NPs hold great promise as a potential therapy for I/R injury-related diseases.

Metabolic mechanisms of acute proximal tubular injury

Damage to the proximal tubule (PT) is the most frequent cause of acute kidney injury (AKI) in humans. Diagnostic and treatment options for AKI are currently limited, and a deeper understanding of pathogenic mechanisms at a cellular level is required to rectify this situation. Metabolism in the PT is complex and closely coupled to solute transport function. Recent studies have shown that major changes in PT metabolism occur during AKI and have highlighted some potential targets for intervention. However, translating these insights into effective new therapies still represents a substantial challenge. In this article, in addition to providing a brief overview of the current state of the field, we will highlight three emerging areas that we feel are worthy of greater attention. First, we will discuss the role of axial heterogeneity in cellular function along the PT in determining baseline susceptibility to different metabolic hits. Second, we will emphasize that elucidating insult specific pathogenic mechanisms will likely be critical in devising more personalized treatments for AKI. Finally, we will argue that uncovering links between tubular metabolism and whole-body homeostasis will identify new strategies to try to reduce the considerable morbidity and mortality associated with AKI. These concepts will be illustrated by examples of recent studies emanating from the authors' laboratories and performed under the auspices of the Swiss National Competence Center for Kidney Research (NCCR Kidney.ch).

Supramaximal calcium signaling triggers procoagulant platelet formation

Procoagulant platelets promote thrombin generation during thrombosis. Platelets become procoagulant in an all-or-nothing manner. We investigated how distinct Ca2+ signaling between platelet subpopulations commits some platelets to become procoagulant, using the high-affinity Ca2+ indicator Fluo-4, which may become saturated during platelet stimulation, or low-affinity Fluo-5N, which reports only very high cytosolic Ca2+ concentrations. All activated platelets had high Fluo-4 fluorescence. However, in Fluo-5N-loaded platelets, only the procoagulant platelets had high fluorescence, indicating very high cytosolic Ca2+. This finding indicates a novel, "supramaximal" Ca2+ signal in procoagulant platelets (ie, much higher than normally considered maximal). Supramaximal Ca2+ signaling and the percentage of procoagulant platelets were inhibited by cyclosporin A, a mitochondrial permeability transition pore blocker, and Ru360, an inhibitor of the mitochondrial Ca2+ uniporter, with no effect on Fluo-4 fluorescence. In contrast, Synta-66, an Orai1 blocker, reduced Fluo-4 fluorescence but did not directly inhibit generation of the supramaximal Ca2+ signal. Our findings show a distinct pattern of Ca2+ signaling in procoagulant platelets and provide a new framework to interpret the role of platelet signaling pathways in procoagulant platelets. This requires reassessment of the role of different Ca2+ channels and may provide new targets to prevent formation of procoagulant platelets and limit thrombosis.

Cytosolic and mitochondrial Ca2+ signaling in procoagulant platelets

Platelets are the major cellular contributor to arterial thrombosis. However, activated platelets form two distinct subpopulations during thrombosis. Pro-aggregatory platelets aggregate to form the main body of the thrombus. In contrast, procoagulant platelets expose phosphatidylserine on their outer surface and promote thrombin generation. This apparently all-or-nothing segregation into subpopulations indicates that, during activation, platelets commit to becoming procoagulant or pro-aggregatory. Although the signaling pathways that control this commitment are not understood, distinct cytosolic and mitochondrial Ca²⁺ signals in different subpopulations are likely to be central. In this review, we discuss how these Ca²⁺ signals control procoagulant platelet formation and whether this process can be targeted pharmacologically to prevent arterial thrombosis.

Quantitative intravital Ca2+ imaging maps single cell behavior to kidney tubular structure

Ca2+ is an important second messenger that translates extracellular stimuli into intracellular responses. Although there has been significant progress in understanding Ca2+ dynamics in organs such as the brain, the nature of Ca2+ signals in the kidney is still poorly understood. Here, we show that by using a genetically expressed highly sensitive reporter (GCaMP6s), it is possible to perform imaging of Ca2+ signals at high resolution in the mouse kidney in vivo. Moreover, by applying machine learning-based automated analysis using a Ca2+-independent signal, quantitative data can be extracted in an unbiased manner. By projecting the resulting data onto the structure of the kidney, we show that different tubular segments display highly distinct spatiotemporal patterns of Ca2+ signals. Furthermore, we provide evidence that Ca2+ activity in the proximal tubule decreases with increasing distance from the glomerulus. Finally, we demonstrate that substantial changes in intracellular Ca2+ can be detected in proximal tubules in a cisplatin model of acute kidney injury, which can be linked to alterations in cell structure and transport function. In summary, we describe a powerful new tool to investigate how single cell behavior is integrated with whole organ structure and function and how it is altered in disease states relevant to humans.

The Role of Stress-Induced O-GlcNAc Protein Modification in the Regulation of Membrane Transport

O-linked N-acetylglucosamine (O-GlcNAc) is a posttranslational modification that is increasingly recognized as a signal transduction mechanism. Unlike other glycans, O-GlcNAc is a highly dynamic and reversible process that involves the addition and removal of a single N-acetylglucosamine molecule to Ser/Thr residues of proteins. UDP-GlcNAc—the direct substrate for O-GlcNAc modification—is controlled by the rate of cellular metabolism, and thus O-GlcNAc is dependent on substrate availability. Serving as a feedback mechanism, O-GlcNAc influences the regulation of insulin signaling and glucose transport. Besides nutrient sensing, O-GlcNAc was also implicated in the regulation of various physiological and pathophysiological processes. Due to improvements of mass spectrometry techniques, more than one thousand proteins were detected to carry the O-GlcNAc moiety; many of them are known to participate in the regulation of metabolites, ions, or protein transport across biological membranes. Recent studies also indicated that O-GlcNAc is involved in stress adaptation; overwhelming evidences suggest that O-GlcNAc levels increase upon stress. O-GlcNAc elevation is generally considered to be beneficial during stress, although the exact nature of its protective effect is not understood. In this review, we summarize the current data regarding the oxidative stress-related changes of O-GlcNAc levels and discuss the implications related to membrane trafficking.

PPARγ-Coactivator-1α, Nicotinamide Adenine Dinucleotide and Renal Stress Resistance

With one of the highest mitochondrial densities in the body, the kidneys consume approximately 10% of total oxygen while constituting 0.5% of body mass. Renal respiration is linear to solute extraction, linking oxidative metabolism directly to tubular function. This fundamental role of mitochondria in renal health may become an "Achilles heel" under duress. Acute kidney injury (AKI) related to each major class of stressor - inflammation, ischemia, and toxins - exhibits early and prominent mitochondrial injury. The mitochondrial biogenesis regulator, PPARγ-coactivator-1α (PGC1α), may confer tubular protection against these stressors. Recent work proposes that renal PGC1α directly increases levels of nicotinamide adenine dinucleotide (NAD+), an essential co-factor for energy metabolism that has lately been proposed as an anti-aging factor. This mini-review summarizes recent studies on AKI, PGC1α, and NAD+ that identify a direct mechanism between the regulation of metabolic health and the ability to resist renal stressors.

The calcium-cancer signalling nexus

The calcium signal is a powerful and multifaceted tool by which cells can achieve specific outcomes. Cellular machinery important in tumour progression is often driven or influenced by changes in calcium ions; in some cases this regulation occurs within spatially defined regions. Over the past decade there has been a deeper understanding of how calcium signalling is remodelled in some cancers and the consequences of calcium signalling on key events such as proliferation, invasion and sensitivity to cell death. Specific calcium signalling pathways have also now been identified as playing important roles in the establishment and maintenance of multidrug resistance and the tumour microenvironment.

Highly selective tridentate fluorescent probes for visualizing intracellular Mg2+ dynamics without interference from Ca2+ fluctuation

Novel fluorescent probes based on a tridentate Mg²⁺-selective chelator enabled the detection of intracellular Mg²⁺ concentration change without any response to Ca²⁺.

The role of glycine in regulated cell death

The cytoprotective effects of glycine against cell death have been recognized for over 28 years. They are expressed in multiple cell types and injury settings that lead to necrosis, but are still not widely appreciated or considered in the conceptualization of cell death pathways. In this paper, we review the available data on the expression of this phenomenon, its relationship to major pathophysiologic pathways that lead to cell death and immunomodulatory effects, the hypothesis that it involves suppression by glycine of the development of a hydrophilic death channel of molecular dimensions in the plasma membrane, and evidence for its impact on disease processes in vivo.

A Novel Nicotinamide Adenine Dinucleotide Correction Method for Mitochondrial Ca(2+) Measurement with FURA-2-FF in Single Permeabilized Ventricular Myocytes of Rat

Fura-2 analogs are ratiometric fluoroprobes that are widely used for the quantitative measurement of [Ca(2+)]. However, the dye usage is intrinsically limited, as the dyes require ultraviolet (UV) excitation, which can also generate great interference, mainly from nicotinamide adenine dinucleotide (NADH) autofluorescence. Specifically, this limitation causes serious problems for the quantitative measurement of mitochondrial [Ca(2+)], as no available ratiometric dyes are excited in the visible range. Thus, NADH interference cannot be avoided during quantitative measurement of [Ca(2+)] because the majority of NADH is located in the mitochondria. The emission intensity ratio of two different excitation wavelengths must be constant when the fluorescent dye concentration is the same. In accordance with this principle, we developed a novel online method that corrected NADH and Fura-2-FF interference. We simultaneously measured multiple parameters, including NADH, [Ca(2+)], and pH/mitochondrial membrane potential; Fura-2-FF for mitochondrial [Ca(2+)] and TMRE for Ψm or carboxy-SNARF-1 for pH were used. With this novel method, we found that the resting mitochondrial [Ca(2+)] concentration was 1.03 µM. This 1 µM cytosolic Ca(2+) could theoretically increase to more than 100 mM in mitochondria. However, the mitochondrial [Ca(2+)] increase was limited to ~30 µM in the presence of 1 µM cytosolic Ca(2+). Our method solved the problem of NADH signal contamination during the use of Fura-2 analogs, and therefore the method may be useful when NADH interference is expected.

Lithium induces ER stress and N-glycan modification in galactose-grown Jurkat cells

We previously reported that lithium had a significant impact on Ca(2+) regulation and induced unfolded protein response (UPR) in yeast cells grown on galactose due to inhibition of phosphoglucomutase (PGM), however the exact mechanism has not been established yet. In this study, we analysed lithium's effect in galactose-fed cells to clarify whether these ER-related changes are the result of a relative hypoglycemic state. Furthermore, we investigated whether the alterations in galactose metabolism impact protein post-translational modifications. Thus, Jurkat cells were incubated in glucose or galactose containing media with or without lithium treatment. We found that galactose-fed and lithium treated cells showed better survivability than fasting cells. We also found higher UDP-Hexose and glycogen levels in these cells compared to fasting cells. On the other hand, the UPR (X-box binding protein 1 mRNA levels) of galactose-fed and lithium treated cells was even greater than in fasting cells. We also found increased amount of proteins that contained N-linked N-acetyl-glucosamine, similar to what was reported in fasting cells by a recent study. Our results demonstrate that lithium treatment of galactose-fed cells can induce stress responses similar to hypoglycemia, however cell survival is still secured by alternative pathways. We propose that clarifying this process might be an important addition toward the better understanding of the molecular mechanisms that regulate ER-associated stress response.

Effects of glycyl-glutamine dipeptide supplementation on myocardial damage and cardiac function in rats after severe burn injury

Glutamine decreases myocardial damage in ischemia/reperfusion injury. However, the cardioprotective effect of glutamine after burn injury remains unclear. Present study was to explore the protective effect of glycyl-glutamine dipeptide on myocardial damage in severe burn rats. Seventy-two Wistar rats were randomly divided into three groups: normal control (C), burned control (B) and glycyl-glutamine dipeptide-treated (GG) groups. B and GG groups were inflicted with 30% total body surface area of full thickness burn. The GG group was given 1.5 g/kg glycyl-glutamine dipeptide per day and the B group was given the same dose of alanine via intraperitoneal injection for 3 days. The serum CK, LDH, AST, and, blood lactic acid levels, as well as the myocardium ATP and GSH contents, were measured. The indices of cardiac contractile function and histopathological change were analyzed at 12, 24, 48, and 72 post-burn hours (PBH). The serum CK, LDH, AST and blood lactic acid levels increased, and the myocardium ATP and GSH content decreased in both burned groups. Compared with B group, the CK, LDH, AST and blood lactic acid levels reduced, myocardium ATP and GSH content increased in GG group. Moreover, the inhibition of cardiac contractile function and myocardial histopathological damage were reduced significantly in GG group. We conclude that myocardial histological structure and function were damaged significantly after burn injury, glycyl-glutamine dipeptide supplementation is beneficial to myocardial preservation by improving cardiocyte energy metabolism, increasing ATP and glutathione synthesis.

Fluorescence imaging of intracellular calcium signals in intact kidney tissue

BACKGROUND

Intracellular calcium (Ca(2+)) plays an important role in normal renal physiology and in the pathogenesis of various kidney diseases; however, the study of Ca(2+) signals in intact tissue has been limited by technical difficulties, including achieving adequate loading of Ca(2+)-sensitive fluorescent dyes. The kidney slice preparation represents a model whereby three-dimensional tissue architecture is preserved and structures in both the cortex and medulla can be imaged using confocal or multiphoton microscopy.

METHODS

Ca(2+)-sensitive dyes Rhod-2, Fura-red and Fluo-4 were loaded into tubular and vascular cells in rat kidney slices using a re-circulating perfusion system and real-time imaging of Ca(2+) signals was recorded by confocal microscopy. Kidney slices were also obtained from transgenic mice expressing the GCaMP2 Ca(2+)-sensor in their endothelial cells and real time Ca(2+) transients stimulated by physiological stimuli.

RESULTS

Wide spread loading of Ca(2+) indicators was achieved in the tubular and vascular structures of both the medulla and cortex. Real time Ca(2+) signals were successfully recorded in different intracellular compartments of both rat and mouse cortical and medullary tubules in response to physiological stimuli (ATP and angiotensin II). Glomerular Ca(2+) transients were similarly recorded in kidney slices taken from the transgenic mouse expressing the GCaMP2 Ca(2+)-sensor.

CONCLUSION

We present new approaches that can be adopted to image cytosolic and mitochondrial Ca(2+) signals within various cell types in intact kidney tissue. Moreover, techniques described in this study can be used to facilitate future detailed investigations of intracellular Ca(2+) homeostasis in renal health and disease.

Effects of glycine supplementation on myocardial damage and cardiac function after severe burn

BACKGROUND

Glycine has been shown to participate in protection from hypoxia/reoxygenation injury. However, the cardioprotective effect of glycine after burn remains unclear. This study aimed to explore the protective effect of glycine on myocardial damage in severely burned rats.

METHODS

Seventy-two Wistar rats were randomly divided into three groups: normal controls (C), burned controls (B), and glycine-treated (G). Groups B and G were given a 30% total body surface area full-thickness burn. Group G was administered 1.5 g/(kg d) glycine and group B was given the same dose of alanine via intragastric administration for 3d. Serum creatine kinase (CK), lactate dehydrogenase (LDH), aspartate transaminase (AST), and blood lactate, as well as myocardial ATP and glutathione (GSH) content, were measured. Cardiac contractile function and histopathological changes were analyzed at 12, 24, 48, and 72 hours.

RESULTS

Serum CK, LDH, AST, and blood lactate increased, while myocardial ATP and GSH content decreased in both burned groups. Compared with group B, the levels of CK, LDH, and AST significantly decreased, whereas blood lactate as well as myocardial ATP and GSH content increased in group G. Moreover, cardiac contractile function inhibition and myocardial histopathological damage in group G significantly decreased compared with group B.

CONCLUSION

Myocardial histological structure and function were damaged significantly after burn. Glycine is beneficial to myocardial preservation by improving cardiomyocyte energy metabolism and increasing ATP and GSH abundance.

Effects of glutamine treatment on myocardial damage and cardiac function in rats after severe burn injury

Treatment with glutamine has been shown to reduce myocardial damage associated with ischemia/reperfusion injury. However, the cardioprotective effect of glutamine specifically after burn injury remains unclear. The present study explores the ability of glutamine to protect against myocardial damage in rats that have been severely burned. Seventy-two Wistar rats were randomly divided into three groups: normal controls (C), burned controls (B) and a glutamine-treated group (G). Groups B and G were subjected to full thickness burns comprising 30% of total body surface area. Group G was administered 1.5 g/ (kg•d) glutamine and group B was given the same dose of alanine via intragastric administration for 3 days. Levels of serum creatine kinase (CK), lactate dehydrogenase (LDH), aspartate transaminase (AST) and blood lactic acid were measured, as well as myocardial ATP and glutathione (GSH) contents. Cardiac function indices and histopathological changes were analyzed at 12, 24, 48 and 72 post-burn hours. In both burned groups, levels of serum CK, LDH, AST and blood lactic acid increased significantly, while myocardial ATP and GSH contents decreased. Compared with group B, CK, LDH, and AST levels were lower and blood lactic acid, myocardial ATP and GSH levels were higher in group G. Moreover, cardiac contractile function inhibition and myocardial histopathological damage were significantly reduced in group G compared to B. Taken together, these results show that glutamine supplementation protects myocardial structure and function after burn injury by improving energy metabolism and by promoted the synthesis of ATP and GSH in cardiac myocytes.

Oxidative stress, calcium homeostasis, and altered gene expression in human lung epithelial cells exposed to ZnO nanoparticles

The influence of 20nm ZnO nanoparticles on cytotoxicity, oxidative stress, intracellular calcium homeostasis, and gene expression was studied in human bronchial epithelial cells (BEAS-2B). ZnO caused a concentration- and time-dependent cytotoxicity while elevating oxidative stress and causing membrane damage (cellular LDH release). There was a remarkably steep relationship between concentration and toxicity at concentrations from 5 to 10microg/ml. Cytotoxicity was completely abolished by the antioxidant N-acetylcysteine (NAC). Exposure to ZnO also increased intracellular calcium levels ([Ca(2+)](in)) in a concentration- and time-dependent manner that was partially attenuated by NAC. Nifedipine, a calcium channel blocker, partially attenuated the elevated [Ca(2+)](in), indicating that some of the excess [Ca(2+)](in) is a result of influx from outside the cell. The relationships between oxidative stress, [Ca(2+)](in), and cytotoxicity are discussed. Exposure to a sublethal concentration of ZnO increased the expression of four genes that are involved in apoptosis and oxidative stress responses BNIP, PRDX3, PRNP, and TXRND1, by at least 2.5-fold. Thus, ZnO alters transcriptional regulation in BEAS-2B cells.

Nitrite-induced methemoglobinaemia affects blood ionized and total magnesium level by hydrolysis of plasma adenosine triphosphate in rat

The objective of this study was to evaluate the effects of sodium nitrite (NaNO(2))-induced methemoglobinaemia on plasma ATP (adenosine triphosphate) and corresponding changes of blood-ionized magnesium (iMg(2+)) as well as total magnesium (tMg(2+)) in a time-dependent manner. This study was performed on male Sprague-Dawley rats to which NaNO(2) was injected (10 mg/kg i.p.) to induce methemoglobinaemia. Methemoglobin (MetHb) in blood was measured before (0 min.) and after 10, 30, 60 and 120 min. of NaNO(2) injection. At respective time points, the tMg(2+), blood ions and gases were measured by atomic absorption spectrometry and ion selective electrode, respectively. Haematological parameters were checked by automatic blood cell count, and blood films were observed under light microscope. Plasma ATP was measured by bioluminescence assay using a luminometer, and plasma proteins were measured by an automatic analyser. Blood cell count (RBC, WBC and platelet), haematocrit, and haemoglobin were found to be decreased with the advancement of MetHb concentration. With the gradual increase of MetHb concentration, the plasma ATP decreased and blood iMg(2+) and plasma tMg(2+) increased significantly as time passed by in comparison with the pre-drug values. A significant decrease of the ratio of ionized calcium to iMg(2+), Na(+) and increase of K(+) was observed. In conclusion, NaNO(2)-induced methemoglobinaemia is a cause of hydrolysis of plasma ATP which is responsible for the increase of blood iMg(2+) and plasma tMg(2+) in rats.

Calcium in cell injury and death

Loss of Ca(2+) homeostasis, often in the form of cytoplasmic increases, leads to cell injury. Depending upon cell type and the intensity of Ca(2+) toxicity, the ensuing pathology can be reversible or irreversible. Although multiple destructive processes are activated by Ca(2+), lethal outcomes are determined largely by Ca(2+)-induced mitochondrial permeability transition. This form of damage is primarily dependent upon mitochondrial Ca(2+) accumulation, which is regulated by the mitochondrial membrane potential. Retention of the mitochondrial membrane potential during Ca(2+) increases favors mitochondrial Ca(2+) uptake and overload, resulting in mitochondrial permeability transition and cell death. In contrast, dissipation of mitochondrial membrane potential reduces mitochondrial Ca(2+) uptake, retards mitochondrial permeability transition, and delays death, even in cells with large Ca(2+) increases. The rates of mitochondrial membrane potential dissipation and mitochondrial Ca(2+) uptake may determine cellular sensitivity to Ca(2+) toxicity under pathological conditions, including ischemic injury.

Maleate nephrotoxicity: mechanisms of injury and correlates with ischemic/hypoxic tubular cell death

Maleate injection causes dose-dependent injury in proximal tubular cells. This study sought to better define underlying pathogenic mechanisms and to test whether maleate toxicity recapitulates critical components of the hypoxic/ischemic renal injury cascade. CD-1 mice were injected with maleate or used as a source for proximal tubule segments (PTS) for in vitro studies. Maleate induced dose-dependent PTS injury [lactate deydrogenase (LDH) release, ATP reductions, nonesterified fatty acid (NEFA) accumulation]. These changes were partially dependent on maleate metabolism (protection conferred by metabolic inhibitors: succinate, acetoacetate). Maleate toxicity reproduced critical characteristics of the hypoxia/ATP depletion-induced injury cascade: 1) glutathione (GSH) conferred protection, but due to its glycine, not cysteine (antioxidant), content; 2) ATP reductions reflected decreased production, not Na-K-ATPase-driven increased consumption; 3) cell death was completely blocked by extracellular acidosis (pH 6.6); 4) intracellular Ca(2+) chelation (BAPTA) mitigated cell death; 5) maleate and hypoxia each caused plasma membrane cholesterol shedding and in both instances, this was completely glycine suppressible; 6) maleate + hypoxia caused neither additive NEFA accumulation nor LDH release, implying shared pathogenic pathways; and 7) maleate, like ischemia, induced renal cortical cholesterol loading; increased HMG CoA reductase (HMGCR) activity (statin inhibitable), increased HMGCR mRNA levels, and increased RNA polymerase II recruitment to the HMGCR locus (chromatin immunoprecipitation, ChIP, assay) were involved. These results further define critical determinants of maleate nephrotoxicity and suggest that it can serve as a useful adjunct for studies of ischemia/ATP depletion-induced, proximal tubule-specific, cell death.

Determining calcium concentration in heterogeneous model systems using multiple indicators

Intracellular free calcium concentrations ([Ca2+]i) are assessed by measuring indicator fluorescence in entire cells or subcellular regions using fluorescence microscopy. [Ca2+]i is calculated using equations which link fluorescence intensities (or intensity ratios) to calcium concentrations [G. Grynkiewicz, M. Poenie, R.Y. Tsien, A new generation of Ca2+ indicators with greatly improved fluorescence properties, J. Biol. Chem. 260 (1985) 3440-3450]. However, if calcium ions are heterogeneously distributed within a region of interest, then the observed average fluorescence intensity may not reflect average [Ca2+]i. We assessed potential calcium determination errors in mathematical and experimental models consisting of 'low' and 'high' calcium compartments, using indicators with different affinity for calcium. [Ca2+] calculated using average fluorescence intensity was lower than the actual mean concentrations. Low affinity indicators reported higher (more accurate) values than their high affinity counterparts. To estimate compartment dimensions and respective [Ca2+], we extended the standard approach by using different indicator responses to the same [Ca2+]. While two indicators were sufficient to provide a partial characterization of two-compartment model systems, the use of three or more indicators offered full description of the model provided compartmental [Ca2+] were within the indicator sensitivity ranges. These results show that uneven calcium distribution causes underestimation of actual [Ca2+], and offers novel approaches to estimating calcium heterogeneity.

Molecular mechanisms of acute renal failure following ischemia/reperfusion

Acute renal failure (ARF) necessitating renal replacement therapy is a common problem associated with high morbidity and mortality in the critically ill. Hypotension, followed by resuscitation, is the most common etiologic factor, mimicked by ischemia/reperfusion (I/R) in animal models. Although knowledge of the pathophysiology of ARF in the course of this condition is increasingly detailed, the intracellular and molecular mechanisms leading to ARF are still incompletely understood. This review aims at describing the role of cellular events and signals, including collapse of the cytoskeleton, mitochondrial and nuclear changes, in mediating cell dysfunction, programmed cell death (apoptosis), necrosis and others. Insight into the molecular pathways in the various elements of the kidney, such as vascular endothelium and smooth muscle and tubular epithelium leading to cell damage upon I/R will, hopefully, open new therapeutic modalities, to mitigate the development of ARF after hypotensive episodes and to promote repair and resumption of renal function once ARF has developed.

Voltage-controlled Ca2+ release and entry flux in isolated adult muscle fibres of the mouse

The voltage-activated fluxes of Ca(2+) from the sarcoplasmic reticulum (SR) and from the extracellular space were studied in skeletal muscle fibres of adult mice. Single fibres of the interosseus muscle were enzymatically isolated and voltage clamped using a two-electrode technique. The fibres were perfused from the current-passing micropipette with a solution containing 15 mm EGTA and 0.2 mm of either fura-2 or the faster, lower affinity indicator fura-FF. Electrical recordings in parallel with the fluorescence measurements allowed the estimation of intramembrane gating charge movements and transmembrane Ca(2+) inward current exhibiting half-maximal activation at -7.60 +/- 1.29 and 3.0 +/- 1.44 mV, respectively. The rate of Ca(2+) release from the SR was calculated after fitting the relaxation phases of fluorescence ratio signals with a kinetic model to quantify overall Ca(2+) removal. Results obtained with the two indicators were similar. Ca(2+) release was 2-3 orders of magnitude larger than the flux carried by the L-type Ca(2+) current. At maximal depolarization (+50 mV), release flux peaked at about 3 ms after the onset of the voltage pulse and then decayed in two distinct phases. The slower phase, most likely resulting from SR depletion, indicated a decrease in lumenal Ca(2+) content by about 80% within 100 ms. Unlike in frog fibres, the kinetics of the rapid phase of decay showed no dependence on the filling state of the SR and the results provide little evidence for a substantial increase of SR permeability on depletion. The approach described here promises insight into excitation-contraction coupling in future studies of genetically altered mice.

The metabolic inhibitor antimycin A can disrupt cell-to-cell communication by an ATP- and Ca(2+)-independent mechanism

In cardiac myocytes of new-born rats, the degree of intercellular communication through gap junctional channels closely depends on the metabolic state of the cells. In contrast, in stably transfected HeLa cells expressing rat cardiac connexin43 (Cx43, the main channel-forming protein present in ventricular myocytes), a major part of junctional communication persisted in ATP-depleted conditions, in the presence of a metabolic inhibitor (KCN) or of a broad spectrum inhibitor of protein kinases (H7). However, another metabolic inhibitor, antimycin A, which like cyanide inhibits electron transfer in the respiratory chain, totally interrupted cell-to-cell communication between Cx43-HeLa cells, even in whole-cell conditions, when ATP (5 mM) was present. Antimycin A caused a modest increase in cytosolic calcium concentration; however, junctional uncoupling still occurred when this rise was prevented. Conditions of ischemic insult (e.g. ischemia or chemical hypoxia) frequently cause the activation of protein kinases, particularly of Src and MAP kinases, and such activations are known to markedly disrupt gap junctional communication. Antimycin-induced junctional uncoupling occurred even in the presence of inhibitors of these kinases. Antimycin A appears able to cause junctional uncoupling either through the ATP depletion it induces as a metabolic poison or via a direct action on gap junction constituents.

Evidence that store-operated Ca2+ channels are more effective than intracellular messenger-activated non-selective cation channels in refilling rat hepatocyte intracellular Ca2+ stores

Liver cells possess store-operated Ca2+ channels (SOCs) with a high selectivity for Ca2+ compared with Na+, and several types of intracellular messenger-activated non-selective cation channels with a lower selectivity for Ca2+ (NSCCs). The main role of SOCs is thought to be in refilling depleted endoplasmic reticulum Ca2+ stores [Cell Calcium 7 (1986) 1]. NSCCs may be involved in refilling intracellular stores but are also thought to have other roles in regulating the cytoplasmic-free Ca2+ and Na+ concentrations. The ability of SOCs to refill the endoplasmic reticulum Ca2+ stores in hepatocytes has not previously been compared with that of NSCCs. The aim of the present studies was to compare the ability of SOCs and maitotoxin-activated NSCCs to refill the endoplasmic reticulum in rat hepatocytes. The experiments were performed using fura-2FF and fura-2 to monitor the free Ca2+ concentrations in the endoplasmic reticulum and cytoplasmic space, respectively, a Ca2+ add-back protocol, and 2-aminoethyl diphenylborate (2-APB) to inhibit Ca2+ inflow through SOCs. In cells treated with 2,5-di-t-butylhydroquinone (DBHQ) or vasopressin to deplete the endoplasmic reticulum Ca2+ stores, then washed to remove DBHQ or vasopressin, the addition of Ca2+ caused a substantial increase in the concentration of Ca2+ in the endoplasmic reticulum and cytoplasmic space due to the activation of SOCs. These increases were inhibited 80% by 2-APB, indicating that Ca2+ inflow is predominantly through SOCs. In the presence of 2-APB (to block SOCs), maitotoxin induced a substantial increase in [Ca2+](cyt), but only a modest and slower increase in [Ca2+](er). Under these conditions, Ca2+ inflow is predominantly through maitotoxin-activated NSCCs. It is concluded that SOCs are more effective than maitotoxin-activated NSCCs in refilling the endoplasmic reticulum Ca2+ stores. The previously developed concept of a specific role for SOCs in refilling the endoplasmic reticulum is consistent with the results reported here.

Blockade of maitotoxin-induced endothelial cell lysis by glycine and L-alanine

The maitotoxin (MTX)-induced cell death cascade in bovine aortic endothelial cells (BAECs) is a model for oncotic/necrotic cell death. The cascade is initiated by an increase in cytosolic free Ca(2+) concentration ([Ca(2+)](i)), which is followed by the biphasic uptake of vital dyes. The initial phase of dye entry reflects activation of large pores and correlates with surface membrane bleb formation; the second phase reflects cell lysis. In the present study, the effect of the cytoprotective amino acid glycine was examined. Glycine had no effect on MTX-induced change in [Ca(2+)](i) or on the first phase of vital dye uptake but produced a concentration-dependent (EC(50) approximately 1 mM) inhibition of the second phase of dye uptake. No cytoprotective effect was observed with l-valine, l-proline, or d-alanine, whereas l-alanine was equieffective to glycine. Furthermore, glycine had no effect on MTX-induced bleb formation. To test the hypothesis that glycine specifically blocks formation of a lytic "pore," the loss of fluorescence from BAECs transiently expressing GFP and concatemers of GFP ranging in size from 27 to 162 kDa was examined using time-lapse videomicroscopy. MTX-induced loss of GFP was rapid, correlated with the second phase of dye uptake, and was relatively independent of molecular size. The MTX-induced loss of GFP from BAECs was completely blocked by glycine. The data suggest that the second "lytic" phase of MTX-induced endothelial cell death reflects formation of a novel permeability pathway that allows macromolecules such as GFP or LDH to escape, yet can be prevented by the cytoprotective agents glycine and l-alanine.

Implication of Ref-1 in the repression of renin gene transcription by intracellular calcium

OBJECTIVE

The production of renin, which catalyzes the rate-limiting step of the renin-angiotensin system, is tightly regulated by intracellular second messengers. Among them, an increase of intracellular calcium represses renin gene expression. This inhibition of gene expression by intracellular calcium is exceptional, and the molecular mechanism supporting this phenomenon has not yet been identified. As the renin gene is negatively regulated by calcium in the same way as the parathormone (PTH) gene, we hypothesized that a similar molecular transcriptional mechanism could be involved.

RESULTS

Analysis of the human renin proximal promoter led to the identification of a negative calcium response element (nCaRE), which is identical to the region of the PTH promoter and is involved in its repression by calcium. Transfection experiments in renin-expressing chorio-decidual cells demonstrated the transcriptional functionality of the human renin promoter nCaRE. In addition, mutation of nCaRE suppressed the sensitivity of the renin promoter to the increase in intracellular calcium. Gel shift assays demonstrated that Redox factor 1, a multifunctional protein involved in the repair of damaged DNA and the redox activation of AP-1 transcriptional factors, binds specifically to nCaRE. Immunostaining showed that this factor is translocated from the cytoplasm to the nucleus in response to an increase in the intracellular calcium concentration.

CONCLUSION

Thus, the repression of renin expression by intracellular calcium may be mediated by the calcium-induced translocation of Ref-1 to the nucleus, where it binds to the renin promoter nCaRE, to repress the transcription of the renin gene.

Long-term alteration of calcium homeostatic mechanisms in the pilocarpine model of temporal lobe epilepsy

The pilocarpine model of temporal lobe epilepsy is an animal model that shares many of the clinical and pathophysiological characteristics of temporal lobe or limbic epilepsy in humans. This model of acquired epilepsy produces spontaneous recurrent seizure discharges following an initial brain injury produced by pilocarpine-induced status epilepticus. Understanding the molecular mechanisms mediating these long lasting changes in neuronal excitability would provide an important insight into developing new strategies for the treatment and possible prevention of this condition. Our laboratory has been studying the role of alterations in calcium and calcium-dependent systems in mediating some of the long-term neuroplasticity changes associated with epileptogenesis. In this study, [Ca(2+)](i) imaging fluorescence microscopy was performed on CA1 hippocampal neurons acutely isolated from control and chronically epileptic animals at 1 year after the induction of epileptogenesis with two different fluorescent dyes (Fura-2 and Fura-FF) having high and low affinities for [Ca(2+)](i). The high affinity Ca(2+) indicator Fura-2 was utilized to evaluate [Ca(2+)](i) levels up to 900 nM and the low affinity indicator Fura-FF was employed for evaluating [Ca(2+)](i) levels above this range. Baseline [Ca(2+)](i) levels and the ability to restore resting [Ca(2+)](i) levels after a brief exposure to several glutamate concentrations in control and epileptic neurons were evaluated. Epileptic neurons demonstrated a statistically significantly higher baseline [Ca(2+)](i) level in comparison to age-matched control animals. This alteration in basal [Ca(2+)](i) levels persisted up to 1 year after the induction of epileptogenesis. In addition, the epileptic neurons were unable to rapidly restore [Ca(2+)](i) levels to baseline following the glutamate-induced [Ca(2+)](i) loads. These changes in Ca(2+) regulation were not produced by a single seizure and were not normalized by controlling the seizures in the epileptic animals with anticonvulsant treatment. Peak [Ca(2+)](i) levels in response to different concentrations of glutamate were the same in both epileptic and control neurons. Thus, glutamate produced the same initial [Ca(2+)](i) load in both epileptic and control neurons. Characterization of the viability of acutely isolated neurons from control and epileptic animals utilizing standard techniques to identify apoptotic or necrotic neurons demonstrated that epileptic neurons had no statistically significant difference in viability compared to age-matched controls. These results provide the first direct measurement of [Ca(2+)](i) levels in an intact model of epilepsy and indicate that epileptogenesis in this model produced long-lasting alterations in [Ca(2+)](i) homeostatic mechanisms that persist for up to 1 year after induction of epileptogenesis. These observations suggest that altered [Ca(2+)](i) homeostatic mechanisms may underlie some aspects of the epileptic phenotype and contribute to the persistent neuroplasticity changes associated with epilepsy.

Protection of ATP-depleted cells by impermeant strychnine derivatives: implications for glycine cytoprotection

Glycine and structurally related amino acids with activities at chloride channel receptors in the central nervous system also have robust protective effects against cell injury by ATP depletion. The glycine receptor antagonist strychnine shares this protective activity. An essential step toward identification of the molecular targets for these compounds is to determine whether they protect cells through interactions with intracellular targets or with molecules on the outer surface of plasma membranes. Here we report cytoprotection by a cell-impermeant derivative of strychnine. A strychnine-fluorescein conjugate (SF) was synthesized, and impermeability of plasma membranes to this compound was verified by fluorescence confocal microscopy. In an injury model of Madin-Darby canine kidney cells, ATP depletion led to lactate dehydrogenase release. SF prevented lactate dehydrogenase leakage without ameliorating ATP depletion. This was accompanied by preservation of cellular ultrastructure and exclusion of vital dyes. SF protection was also shown for ATP-depleted rat hepatocytes. On the other hand, when a key structural motif in the active site of strychnine was chemically blocked, the SF lost its protective effect, establishing strychnine-related specificity for SF protection. Cytoprotective effects of the cell-impermeant strychnine derivative provide compelling evidence suggesting that molecular targets on the outer surface of plasma membranes may mediate cytoprotection by strychnine and glycine.

Progressive disruption of the plasma membrane during renal proximal tubule cellular injury

The goal of this study was to examine the progression of plasma membrane disruption during cell injury using rabbit renal proximal tubules (RPT). The results demonstrated that the plasma membrane became permeable to larger and larger molecules as anoxia proceeded. At least three distinctive phases of membrane disruption were differentiated during anoxia. In phases 1, 2, and 3, plasma membranes became permeable to propidium iodide (PI, molecular weight = 668), 3 kDa dextrans, and 70 kDa dextrans or lactate dehydrogenase (LDH, molecular weight = 140 kDa), respectively. Phase 1 was reversible by reoxygenation but not prevented by the glycine. Phase 2 was inhibited by glycine. Phase 3 was inhibited by several membrane-permeable homobifunctional crosslinkers, dimethyl-pimelimidate (DMP), ethylene-glycolbis(succinimidylsuccinate), and dithiobis(succinimidylpropionate), but not by the membrane-impermeable crosslinker dithiobis(sulfosuccinimidylpropionate). In addition, DMP decreased RPT LDH release produced by mitochondrial inhibition (antimycin A), an oxidant (t-butylhydroperoxide) and a nephrotoxicant that is metabolized to an electrophile (tetrafluoroethyl-l-cysteine). These results identify (1) different phases of plasma membrane damage with increasing permeability during cell injury, (2) the reversibility of phase 1, (3) the relative site of action of the cytoprotectant glycine (prevents phase 2), and (4) the protective effects of chemical crosslinkers in RPT cell death produced by different toxicants.

Induction of spontaneous recurrent epileptiform discharges causes long-term changes in intracellular calcium homeostatic mechanisms

Calcium and calcium-dependent systems have been long implicated in the induction of epilepsy. We have previously observed that intracellular calcium ([Ca2+]i) levels remain elevated in cells undergoing epileptogenesis in the hippocampal neuronal culture (HNC) model. In this study, we employed the hippocampal neuronal culture (HNC) model of in vitro 'epilepsy' which produces spontaneous recurrent epileptiform discharges (SREDs) for the life of the neurons in culture to investigate alterations in [Ca2+]i homeostatic mechanisms that may be associated with the 'epileptic' phenotype. [Ca2+]i imaging fluorescence microscopy was performed on control and 'epileptic' neurons with two different fluorescent dyes ranging from high to low affinities for [Ca2+]i. We measured baseline [Ca2+]i levels and the ability to restore resting [Ca2+]i levels after a brief 2-min exposure to the excitatory amino acid glutamate in control neurons and neurons with SREDs. Neurons manifesting SREDs had statistically significantly higher baseline [Ca2+]i levels that persisted for the life of the culture. In addition, the 'epileptic' phenotype was associated with an inability to rapidly restore [Ca2+]i levels to baseline following a glutamate induced [Ca2+]i load. The use of the low affinity dye Fura-FF demonstrated that the difference in restoring baseline [Ca2+]i levels was not due to saturation of the high affinity dye Indo-1, which was utilized for evaluating the [Ca2+]i kinetics at lower [Ca2+]i levels. Peak [Ca2+]i levels in response to glutamate were the same in both 'epileptic' and control neurons. While [Ca2+]i levels recovered in approximately 30 min in control cells, it took more than 90 min to reach baseline levels in cells manifesting SREDs. Alterations of [Ca2+]i homeostatic mechanisms observed with the 'epileptic' phenotype were shown to be independent of the presence of continuous SREDs and persisted for the life of the neurons in culture. Epileptogenesis was shown not to affect the degree or duration of glutamate induced neuronal depolarization in comparing control and 'epileptic' neurons. The results indicate that epileptogenesis in this in vitro model produced long-lasting alterations in [Ca2+]i regulation that may underlie the 'epileptic' phenotype and contribute to the persistent neuroplasticity changes associated with epilepsy.

Ascorbic acid promotes recovery of cellular functions following toxicant-induced injury

We have shown that renal proximal tubular cells (RPTC) recover cellular functions following sublethal injury induced by the oxidant t-butylhydroperoxide but not by the nephrotoxic cysteine conjugate dichlorovinyl-L-cysteine (DCVC). This study investigated whether L-ascorbic acid phosphate (AscP) promotes recovery of RPTC functions following DCVC-induced injury. DCVC exposure (200 microM; 100 min) resulted in 60% RPTC death and loss from the monolayer at 24 h independent of physiological (50 microM) or pharmacological (500 microM) AscP concentrations. Likewise, the DCVC-induced decrease in mitochondrial function (54%), active Na(+) transport (66%), and Na(+)-K(+)-ATPase activity (77%) was independent of the AscP concentration. Analysis of Na(+)-K(+)-ATPase protein expression and distribution in the plasma membrane using immunocytochemistry and confocal laser scanning microscopy revealed the loss of Na(+)-K(+)-ATPase protein from the basolateral membrane of RPTC treated with DCVC. DCVC-injured RPTC cultured in the presence of 50 microM AscP did not proliferate nor recover their physiological functions over time. In contrast, RPTC cultured in the presence of 500 microM AscP proliferated, recovered all examined physiological functions, and the basolateral membrane expression of Na(+)-K(+)-ATPase by day 4 following DCVC injury. These results demonstrate that pharmacological concentrations of AscP do not prevent toxicant-induced cell injury and death but promote complete recovery of mitochondrial function, active Na(+) transport, and proliferation following toxicant-induced injury.

Ionic selectivity of low-affinity ratiometric calcium indicators: mag-Fura-2, Fura-2FF and BTC

Accurate measurement of elevated intracellular calcium levels requires indicators with low calcium affinity and high selectivity. We examined fluorescence spectral properties and ionic specificity of three low-affinity, ratiometric indicators structurally related to Fura-2: mag-Fura-2 (furaptra), Fura-2FF, and BTC. The indicators differed in respect to their excitation wavelengths, affinity for Ca2+ (Kd approximately 20 microM, 6 microM and 12 microM respectively) and selectivity over Mg2+ (Kd approximately 2 mM for mag-Fura-2, > 10 mM for Fura-2FF and BTC). Among the tested indicators, BTC was limited by a modest dynamic range upon Ca2+ binding, susceptibility to photodamage, and sensitivity to alterations in pH. All three indicators bound other metal ions including Zn2+, Cd2+ and Gd3+. Interestingly, only in the case of BTC were spectral differences apparent between Ca2+ and other metal ions. For example, the presence of Zn2+ increased BTC fluorescence 6-fold at the Ca2+ isosbestic point, suggesting that this dye may be used as a fluorescent Zn2+ indicator. Fura-2FF has high specificity, wide dynamic range, and low pH sensitivity, and is an optimal low-affinity Ca2+ indicator for most imaging applications. BTC may be useful if experimental conditions require visible wavelength excitation or sensitivity to other metal ions including Zn2+.

Acute renal failure. II. Experimental models of acute renal failure: imperfect but indispensable

Acute renal failure (ARF) due to ischemic or toxic renal injury, a clinical syndrome traditionally referred to as acute tubular necrosis (ATN), is a common disease with a high overall mortality of approximately 50%. Little progress has been made since the advent of dialysis more than 30 years ago in improving this outcome. During this same period, a considerable amount of basic research has been devoted to elucidating the pathophysiology of ATN. The ultimate goal of this research is to facilitate the development of therapeutic interventions that either prevent ARF, ameliorate the severity of tubular injury following an acute ischemic or toxic renal insult, or accelerate the recovery of established ATN. This research endeavor has been highly successful in elucidating many vascular and tubular abnormalities that are likely to be involved in ischemic and toxic ARF. This information has led to impressive advances in the development of a number of different pharmacological interventions that are highly effective in ameliorating the renal dysfunction in animal models of ARF. Although these developments are exciting and promising, enthusiasm of investigators involved in this endeavor has been tempered somewhat by the results of a few recent clinical studies of patients with ATN. These trials, designed to examine the efficacy in humans of some of the interventions effective in animal models of ARF, have resulted in little or no benefit. This is therefore an important time to reevaluate the approaches we have taken over the past three to four decades to develop new and effective treatments for ATN in humans. The major goals of this review are 1) to evaluate the relevance and utility of the experimental models currently available to study ischemic and toxic renal injury, 2) to suggest novel experimental approaches and models that have the potential to provide advantages over methods currently available, 3) to discuss ways of integrating results obtained from different experimental models of acute renal injury and of evaluating the relevance of these findings to ATN in humans, and 4) to discuss the difficulties inherent in clinical studies of ATN and to suggest how studies should be best designed to overcome these problems.

A role for intracellular calcium in tight junction reassembly after ATP depletion-repletion

The integrity of the tight junction (TJ), which is responsible for the permeability barrier of the polarized epithelium, is disrupted during ischemic injury and must be reestablished for recovery. Recently, with the use of an ATP depletion-repletion model for ischemia and reperfusion injury in Madin-Darby canine kidney cells, TJ proteins such as zonula occludens-1 (ZO-1) were shown to reversibly form large complexes and associate with cytoskeletal proteins (T. Tsukamoto and S. K. Nigam, J. Biol. Chem. 272: 16133-16139, 1997). In this study, we examined the role of intracellular calcium in TJ reassembly after ATP depletion-repletion by employing the cell-permeant calcium chelator 1, 2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-AM (BAPTA-AM). Lowering intracellular calcium during ATP depletion is associated with significant inhibition of the reestablishment of the permeability barrier following ATP repletion as measured by transepithelial electrical resistance and mannitol flux, marked alterations in the subcellular localization of occludin by immunofluorescent analysis, and decreased solubility of ZO-1 and other TJ proteins by Triton X-100 extraction assay, suggesting that lowering intracellular calcium potentiates the interaction of TJ proteins with the cytoskeleton. Coimmunoprecipitation studies indicated that decreased solubility may partly result from the stabilization of large TJ protein-containing complexes with fodrin. Although ionic detergents (SDS and deoxycholate) appeared to cause a dissociation of ZO-1-containing complexes from the cytoskeleton, sucrose gradient analyses of the solubilized proteins suggested that calcium chelation leads to self-association of these complexes. Together, these results raise the possibility that intracellular calcium plays an important facilitatory role in the reassembly of the TJ damaged by ischemic insults. Calcium appears to be necessary for the dissociation of TJ-cytoskeletal complexes, thus permitting functional TJ reassembly and paracellular permeability barrier recovery.

Thresholds for cellular disruption and activation of the stress response in renal epithelia

Renal ischemia causes a rapid fall in cellular ATP, increased intracellular calcium (Ca(i)), and dissociation of Na(+)-K(+)-ATPase from the cytoskeleton along with initiation of a stress response. We examined changes in Ca(i), Na(+)-K(+)-ATPase detergent solubility, and activation of heat-shock transcription factor (HSF) in relation to graded reduction of ATP in LLC-PK(1) cells to determine whether initiation of the stress response was related to any one of these perturbations alone. Ca(i) increased first at 75% of control ATP. Triton X-100 solubility of Na(+)-K(+)-ATPase increased below 70% control ATP. Reducing cellular ATP below 50% control consistently activated HSF. Stepped decrements in cellular ATP below the respective thresholds caused incremental increases in Ca(i), Na(+)-K(+)-ATPase solubility, and HSF activation. ATP depletion activated both HSF1 and HSF2. Proteasome inhibition caused activation of HSF1 and HSF2 in a pattern similar to ATP depletion. Lactate dehydrogenase release remained at control levels irrespective of the degree of ATP depletion. Progressive accumulation of nonnative proteins may be the critical signal for the adaptive induction of the stress response in renal epithelia.

Isoflurane alters proximal tubular cell susceptibility to toxic and hypoxic forms of attack

BACKGROUND

Fluorinated anesthetics can profoundly alter plasma membrane structure and function, potentially impacting cell injury responses. Because major surgery often precipitates acute renal failure, this study assessed whether the most commonly used fluorinated anesthetic, isoflurane, alters tubular cell responses to toxic and hypoxic attack.

METHODS

Mouse proximal tubule segments were incubated under control conditions or with a clinically relevant isoflurane dose. Cell viability (lactate dehydrogenase release), deacylation (fatty acid, such as C20:4 levels), and adenosine triphosphate (ATP) concentrations were assessed under one or more of the following conditions: (a) exogenous phospholipase A2 (PLA2) or C20:4 addition, (b) Ca2+ overload (A23187 ionophore), (c) increased metabolic work (Na ionophore), and (d) hypoxia- or antimycin A-induced attack. Isoflurane's effect on NBD phosphatidylserine uptake (an index of plasma membrane aminophospholipid translocase activity) was also assessed.

RESULTS

Isoflurane alone caused trivial deacylation and no lactate dehydrogenase release. However, it strikingly sensitized to both PLA2- and A23187-induced deacylation and cell death. Isoflurane also exacerbated C20:4's direct membrane lytic effect. Under conditions of mild ATP depletion (Na ionophore-induced increased ATP consumption; PLA2-induced mitochondrial suppression), isoflurane provoked moderate/severe ATP reductions and cell death. Conversely, under conditions of maximal ATP depletion (hypoxia, antimycin), isoflurane conferred a modest cytoprotective effect. Isoflurane blocked aminophospholipid translocase activity, which normally maintains plasma membrane lipid asymmetry (that is, preventing its "flip flop").

CONCLUSIONS

Isoflurane profoundly and differentially affects tubular cell responses to toxic and hypoxic attack. Direct drug-induced alterations in lipid trafficking/plasma membrane orientation and in cell energy production are likely involved. Although the in vivo relevance of these findings remains unknown, they have potential implications for intraoperative renal tubular cell structure/function and how cells may respond to superimposed attack.

Intracellular Ca2+ thresholds that determine survival or death of energy-deprived cells

Increase of intracellular ionized or free Ca2+ is thought to play a central role in cell death due to ATP depletion. However, concurrently operative mechanisms of injury that do not require intracellular Ca2+ increases have made it difficult to test this hypothesis or to determine the concentrations at which intracellular Ca2+ becomes lethal. The predominant Ca2+-independent mechanism of injury during ATP depletion involves the loss of cellular glycine. This type of damage can be fully inhibited by adding the amino acid exogenously. Using glycine to suppress Ca2+-independent plasma membrane damage, we have examined the effect of intracellular Ca2+ elevations on cell viability during ATP depletion. Madin-Darby canine kidney (MDCK) cells were depleted of ATP by incubation with a mitochondrial uncoupler in glucose-free medium. Free Ca2+ concentration in the medium was varied between 26 nmol/L and 1.25 mmol/L in the presence of a Ca2+ ionophore. Measurements with the Ca2+ probes fura-2, furaptra, and fura-2FF showed that intracellular Ca2+ was clamped at extracellular levels under these conditions. Cell survival during ATP depletion was indicated by viable cells recovered 24 hours later. The results show that ATP-depleted cells can sustain high levels of intracellular Ca2+ (100 micromol/L) for prolonged periods and remain viable if plasma membrane damage is prevented by glycine. Cell death was observed only when intracellular free Ca2+ was allowed to increase beyond 100 micromol/L, and this was associated with dramatic nuclear alterations: chromatin condensation, loss of nuclear lamins, and breakdown of DNA into large 50- to 150-kb fragments. Our studies demonstrate unexpectedly high resistance of cells to calcium cytotoxicity if glycine that is lost during ATP depletion is restored. In addition, they provide insights into novel mechanisms of nuclear disintegration and DNA damage that are triggered when the high thresholds of intracellular Ca2+ required for cell death are exceeded.