Role of cytosolic Ca in renal tubule damage induced by anoxia

Hypoxic Rise in Cytosolic Calcium and Renal Proximal Tubule Injury Mediated by a Nickel-Sensitive Pathway

In the kidney, cell injury resulting from ischemia and hypoxia is thought to be due, in part, to increased cytosolic Ca2+ levels, [Ca2+]i, leading to activation of lytic enzymes, cell dysfunction, and necrosis. We report evidence of a progressive and exponential increase in [Ca2+]i (from 245 ± 10 to 975 ± 100 nM at 45 mins), cell permeabilization and propidium iodide (PI) staining of the nucleus, and partial loss of cell transport functions such as Na+-gradient–dependent uptakes of 14C-alpha-methyiglucopyranoside and inorganic phosphate (32Pi) in proximal convoluted tubules of adult rabbits subjected to hypoxia. The rise in [Ca2+]i depended on the presence of extracellular [Ca2+] and could be blocked by 50 μM Ni2+ but not by verapamil (100 μM). Presence of 50 μM Ni2+ also reduced the hypoxia-induced morphological and functional injuries. We also used HEK 293 cells, a kidney cell line, incubated in media without glucose and exposed for 3.5 hrs to 1% O2–5% CO2 and then returned to glucose-containing media for another 3.5 hrs in an air–5% CO2 atmosphere and finally exposed for 1 min to media containing 1 μM Pl. NiCl2 (50 μM) or pentobarbital (300 μM) more than phenobarbital (1.5 mM), when present in the incubation medium during both the hypoxic and the reoxygenation periods, induced significant (P < 0.001) reductions in the number of cell nuclei stained with Pl, similar to their relative potency as inhibitors of T channels. Our findings indicate that hypoxia-induced alterations in calcium level and subsequent cell injury in the proximal convoluted tubule and in HEK cells involve a nickel-sensitive and dihydropyridine insensitive pathway or channel.

Apoptosis versus necrosis in acute pancreatitis

Acute pancreatitis is a disease of variable severity in which some patients experience mild, self-limited attacks, whereas others manifest a severe, highly morbid, and frequently lethal attack. The events that regulate the severity of acute pancreatitis are, for the most part, unknown. It is generally believed that the earliest events in acute pancreatitis occur within acinar cells and result in acinar cell injury. Other processes, such as recruitment of inflammatory cells and generation of inflammatory mediators, are believed to occur subsequent to acinar cell injury, and these "downstream" events are believed to influence the severity of the disease. Several recently reported studies, however, have suggested that the acinar cell response to injury may, itself, be an important determinant of disease severity. In these studies, mild acute pancreatitis was found to be associated with extensive apoptotic acinar cell death, whereas severe acute pancreatitis was found to involve extensive acinar cell necrosis but very little acinar cell apoptosis. These observations led to the hypothesis that apoptosis could be a favorable response to acinar cells and that interventions that favor induction of apoptotic, as opposed to necrotic, acinar cell death might reduce the severity of an attack of acute pancreatitis. Indeed, in an experimental setting, the induction of pancreatic acinar cell apoptosis protects mice against acute pancreatitis. Little is known about the mechanism of apoptosis in the pancreatic acinar cell, although some early attempts have been made in that direction. Also, clinical relevance of these experimental studies remains to be investigated.

Cell death induced by acute renal injury: a perspective on the contributions of apoptosis and necrosis

In humans and experimental models of renal ischemia, tubular cells in various nephron segments undergo necrotic and/or apoptotic cell death. Various factors, including nucleotide depletion, electrolyte imbalance, reactive oxygen species, endonucleases, disruption of mitochondrial integrity, and activation of various components of the apoptotic machinery, have been implicated in renal cell vulnerability. Several approaches to limit the injury and augment the regeneration process, including nucleotide repletion, administration of growth factors, reactive oxygen species scavengers, and inhibition of inducers and executioners of cell death, proved to be effective in animal models. Nevertheless, an effective approach to limit or prevent ischemic renal injury in humans remains elusive, primarily because of an incomplete understanding of the mechanisms of cellular injury. Elucidation of cell death pathways in animal models in the setting of renal injury and extrapolation of the findings to humans will aid in the design of potential therapeutic strategies. This review evaluates our understanding of the molecular signaling events in apoptotic and necrotic cell death and the contribution of various molecular components of these pathways to renal injury.

Necrotic volume increase and the early physiology of necrosis

Whether a lethally injured mammalian cell undergoes necrosis or apoptosis may be determined by the early activation of specific ion channels at the cell surface. Apoptosis requires K+ and Cl- efflux, which leads to cell shrinking, an active phenomenon termed apoptotic volume decrease (AVD). In contrast, necrosis has been shown to require Na+ influx through membrane carriers and more recently through stress-activated non-selective cation channels (NSCCs). These ubiquitous channels are kept dormant in viable cells but become activated upon exposure to free-radicals. The ensuing Na+ influx leads to cell swelling, an active response that may be termed necrotic volume increase (NVI). This review focuses on how AVD and NVI become conflicting forces at the beginning of cell injury, on the events that determine irreversibility and in particular, on the ion fluxes that decide whether a cell is to die by necrosis or by apoptosis.

Neurosteroid inhibition of cell death

Diverse gamma-aminobutyric acid (GABAA) receptor modulators exhibited novel cytoprotective effects and mechanisms of action in rabbit renal proximal tubules subjected to mitochondrial inhibition (antimycin A) or hypoxia. Cytoprotective potencies (50% effective concentration, EC50) were 0.3 nM allopregnanolone (AP) > 0.4 nM 17 alpha-OH-allopregnanolone (17 alpha-OH-AP) > 30 nM dehydroepiandrosterone sulfate (DHEAS) = 30 nM pregnenolone sulfate (PS) > 500 nM pregnenolone (PREG) > 30 microM muscimol > 10 mM GABA following antimycin A exposure. Maximal protection with AP and 17 alpha-OH-AP was 70%, whereas DHEAS, PS, PREG, and muscimol produced 100% cytoprotection. Experiments with AP, PS, and muscimol revealed the return of mitochondrial function and active Na+ transport following hypoxia/reoxygenation. Muscimol inhibited the antimycin A-induced influx of both extracellular Ca2+ and Cl- that occurs during the late phase of cell injury, whereas the neurosteroids only inhibited influx of Cl-. Radioligand binding studies with AP and PS did not reveal a specific binding site; however, structural requirements were observed for cytoprotective potency and efficacy. In conclusion, we suggest that the GABAA receptor modulators muscimol and neurosteroids are cytoprotective at different cellular sites in the late phase of cell injury; muscimol inhibits Ca2+ and subsequent Cl- influx, whereas the neurosteroids inhibit Cl- influx.

Effect of glycine on prelethal and postlethal increases in calpain activity in rat renal proximal tubules

The effect of glycine on hypoxia- and ionomycin-induced increases in calpain activity in rat proximal tubules was determined. Calpain activity was determined both in vitro and in the intact cell using the fluorescent substrate N-succinyl-Leu-Leu-Val-Tyr-7-amido-4-methyl coumarin (N-succinyl-Leu-Leu-Val-Tyr-AMC) and Western blotting for calpain-specific spectrin breakdown products (BDP), respectively. At 7.5 minutes of hypoxia (prelethal injury model) there was a significant (10-fold) increase in in vitro calpain activity that was not inhibited by glycine. At 15 minutes of hypoxia (postlethal injury model) there was a further increase in calpain activity that was inhibited by glycine. Normoxic tubules incubated with the calcium ionophore ionomycin (5 microM) for two minutes and 10 minutes had a significant increase in calpain activity that was not inhibited by glycine. After 15 minutes of hypoxia in the presence of glycine, there was an increase in calpain-specific spectrin breakdown products (BDP) in both Triton X-100 soluble and cytosolic extracts from proximal tubules. Glycine in concentrations up to 10 mM had no direct effect on the in vitro calpain activity of purified calpains. The present study demonstrates that: (1) prelethal increases in calpain activity stimulated by hypoxia and ionomycin treatment are not affected by glycine; (2) calpain-mediated spectrin breakdown during hypoxia occurs in the presence of glycine; (3) glycine does prevent the additional postlethal increase in calpain activity probably by maintaining membrane integrity to calcium influx.

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

Previous studies have shown that cytosolic-free Ca2+ (Caf) increases to at least low micromolar concentrations during ATP depletion of isolated kidney proximal tubules. However, peak levels could not be determined precisely with the Ca2+-sensitive fluorophore, fura-2, because of its high affinity for Ca2+. Now, we have used two low affinity Ca2+ fluorophores, mag-fura-2 (furaptra) and fura-2FF, to quantitate the full magnitude of Caf increase. Between 30 and 60 min after treatment with antimycin to deplete ATP in the presence of glycine to prevent lytic plasma membrane damage, Caf measured with mag-fura-2 exceeded 10 microM in 91% of tubules studied and 68% had increases to greater than 100 microM. Caf increases of similar magnitude that were dependent on influx of medium Ca2+ were also seen using the new low Ca2+ affinity, Mg2+-insensitive, fluorophore fura-2FF in tubules depleted of ATP by hypoxia, and these increases were reversed by reoxygenation. Total cell Ca2+ levels in antimycin-treated or hypoxic tubules did not change, suggesting that mitochondria were not buffering the increased Caf during ATP depletion. Considered in the context of the high degree of structural preservation of glycine-treated tubule cells during ATP depletion and the commonly assumed Ca2+ requirements for phospholipid hydrolysis, actin disassembly, and Ca2+-mediated structural damage, the remarkable elevations of Caf demonstrated here suggest an unexpected resistance to the deleterious effects of increased Caf during energy deprivation in the presence of glycine.

Susceptibility of human proximal tubular cells to hypoxia: effect of hypoxic preconditioning and comparison to glomerular cells

In animals models, exposure of the brain, heart, or kidneys to sublethal ischemia induces tolerance for subsequent ischemia. However, the ability of human renal cells to undergo hypoxic preconditioning has not been evaluated. In addition, it is unclear if renal ischemic preconditioning induces resistance at the cellular level, or if preconditioning is a result of altered postischemic hemodynamics or the azotemic environment. In this study, we tested the ability of cultured human proximal tubular epithelial cells (PTEC) to undergo hypoxic preconditioning at the cellular level. Hypoxia was induced by incubating cells in an anaerobic incubator in glucose-free buffer (combined oxygen-glucose deprivation; COGD). Cell injury was assessed by lactate dehydrogenase (LDH) efflux, release of arachidonic acid metabolites, and light microscopy. PTEC preconditioned with 12 h of COGD and a 24-h recovery period had less LDH efflux than control PTEC after subsequent exposure to 20 h of COGD (15.0 +/- 2.5% vs. 44.0 +/- 3.4%, p < 0.05). Preconditioned PTEC also retained relatively normal morphology and had less release of arachidonic acid metabolites than control PTEC. Because renal ischemia is characterized predominately by tubular injury with relative sparing of the glomerulus, we determined if PTEC are more susceptible to hypoxic injury than glomerular cells. For further comparison, we also assessed the susceptibility to hypoxia of the porcine tubular epithelial cell line LLC-PK1. After exposure to 18 h of COGD, LDH efflux from PTEC (25.5 +/- 3.3%, mean +/- SEM) was lower than from LLC-PK1 cells (47.6 +/- 4.0%; p < 0.01), but not mesangial cells (22.7 +/- 5.0%) or glomerular endothelial cells (38.2 +/- 6.2%). In conclusion, we have demonstrated that cultured PTEC are as resistant to hypoxic injury as glomerular cells, and that PTEC attain cytoresistance after hypoxic preconditioning. Characterization of the molecular changes that occur in human PTEC after hypoxic preconditioning may reveal innate survival mechanisms that can be manipulated to promote protection from renal ischemia in patients.

Age dependence of tolerance to anoxia and changes in cytosolic calcium in rabbit renal proximal tubules

Calcium(Ca2+)-dependent processes mediate, in part, anoxic cell injury. These may account for the difference in sensitivity to anoxia between certain immature and mature renal cells. To address this question, we studied the effects of anoxia on cytosolic free Ca2+ concentration ([Ca2+]i), cell integrity, and transport functions in microdissected proximal convoluted tubules (PCT) of < 3-week-old (newborn) and > 12-week-old (adult) rabbits. Tubules were loaded with 10 microM fura-2 AM by incubation for 60 min at 37 degrees C, and then superfused with isosmotic saline solution gassed with either 95%O2-5%CO2 (control group) or 95%N2-5%CO2 (anoxia group) for 30 min. [Ca2+]i was measured ratiometrically; cell damage was assessed by nuclear binding of propidium iodide (PI). Anoxia resulted in a fourfold increase in [Ca2+]i in adult tubules (from resting values of 245 +/- 10 to 975 +/- 100 nM, P < 0.001), whereas in newborn tubules the rise was significantly less (from resting values of 137 +/- 5 to 165 +/- 5 nM, P < 0.001 between anoxic groups). Transient exposure to 100 mM potassium chloride, which depolarizes the PCT cells, induced increases in [Ca2+]i from baseline, to 920 +/- 90 nM in tubules from adult and to 396 +/- 16 nM in those from newborn rabbits (P < 0.001 between age groups). After exposure to ligands such as parathyroid hormone (PTH) and ATP, [Ca2+]i increased in both newborn and adult tubules, but to lower levels in newborn tubules. The response to PTH and ATP was transient in both age groups, [Ca2+]i returning to baseline levels after 2 min. Following anoxia, tubules from adult animals exhibited staining of all cell nuclei by 1 min exposure to PI, indicative of gross permeabilization of the cells. Nuclei of anoxic immatures tubules did not stain with PI. The sodium-dependent uptakes of a glucose analogue (14C-alpha-methyl-glucopyranoside) and phosphate (32Pi) were preserved in agarose-filled tubules of newborns after anoxia, whereas in those of adults recovery from anoxia was associated with drastic reduction in the uptake of these solutes. Overall, our results suggest that: (1) during anoxia, cell Ca2+ rises to critical levels in PCTs of adults compared with those of < 3-week-old animals, (2) Ca2+ influx occurs via a pathway activated by exposure to high [K+]o, presumably voltage-sensitive Ca2+ channels or reversal of Na(+)-Ca2+ exchange, (3) these pathways are either less active or less abundant in proximal tubules of newborn compared with adult rabbits, and (4) secondary active transport activity and cellular integrity are well preserved after anoxia in PCT cells of newborn but not of adult rabbits.

Studies of renal injury. II. Activation of the glucose transporter 1 (GLUT1) gene and glycolysis in LLC-PK1 cells under Ca2+ stress

Injury to the renal proximal tubule is common and may be followed by either recovery or cell death. The survival of injured cells is supported by a transient change in cellular metabolism that maintains life even when oxygen tension is reduced. This adaptive process involves the activation of the gene encoding the glucose transporter GLUT1, which is essential to maintain the high rates of glucose influx demanded by glycolysis. We hypothesized that after cell injury increases of cell Ca2+ (Ca2+i) initiate the flow of information that culminates with the upregulation of the stress response gene GLUT1. We found that elevations of Ca2+i caused by the calcium ionophore A23187 activated the expression of the GLUT1 gene in LLC-PK1 cells. The stimulatory effect of Ca2+i on GLUT1 gene expression was, at least in part, transcriptional and resulted in higher levels of GLUT1 mRNA, cognate protein, cellular hexose transport activity, glucose consumption, and lactate production. This response was vital to the renal cells, as its interruption severely increased Ca2+-induced cytotoxicity and cell mortality. We propose that increases of Ca2+i initiate stress responses, represented in part by activation of the GLUT1 gene, and that disruption to the flow of information originating from Ca2+-induced stress, or to the coordinated expression of the stress response, prevents cell recovery after injury and may be an important cause of permanent renal cell injury and cell death.

Mitochondrial dysfunction in ischaemia-reperfusion

The mitochondrial dysfunction in ischaemia-reperfusion is shortly reviewed. During ischaemia the ATP level and pH drops, phospholipids are degraded, membrane permeabilities increased and the cytosolic levels of Na+ and Ca2+ raised. During the following reperfusion the Ca2+ levels may further increase while pH is raised. The oxidative phosphorylation is resumed and the ATP used for membrane repair and ion pumping. The mitochondrial Ca2+ handling is important in removing Ca2+ from the cytosol since the mitochondria are able to take up substantial amounts of Ca2+. However, if a certain threshold is exceeded, mitochondria undergo a so-called permeability transition (MPT), release their Ca2+, undergo swelling and become uncoupled. MPT has been shown to be due to the opening of large pore allowing passage of substances with a M(R) < 1500. Data are presented showing by electron microscopy swelling of mitochondria in cells in perfused liver before other gross morphological changes have taken place. There are a number of factors lowering the threshold for Ca2+ in inducing the MPT: inorganic phosphate, pro-oxidants that oxidize membrane SH-groups, oxidation of NAD(P)H and GSH, while a protective effect is exerted by Mg2+, ADP (and ATP), some antioxidants, carnitine, decrease in pH, and cyclosporin A that binds to cyclophilin. The potential benefit of these in minimizing reperfusion-induced tissue damage is discussed.

Changes in [Ca2+]i in cultured rat proximal tubular epithelium: an in vitro model for renal ischemia

Two components of ischemia, oxygen deprivation and glycolytic inhibition, were studied in primary cultures of rat proximal tubular epithelial cells (PTE). Changes in cytosolic Ca2+ ([Ca2+]i) and its relationship to loss of mitochondrial membrane potential (delta psi m) and cell killing were characterized in single cells whereas ATP and LDH release were determined in populations of monolayer PTE. (1) Inhibition of mitochondrial respiration with KCN or anoxia resulted in little decrease in ATP or cell killing and slight change in [Ca2+]i over many hours. (2) Inhibition of respiration and glycolysis with anoxic HBSS minus glucose resulted in decreased ATP (54.4%) and cell killing (20%) during 5 h anoxic exposure. In all cases, but at highly variable times (113 +/- 62 min), [Ca2+]i initially rose to > 1 microM. In some cases it immediately dropped, stabilizing at about 500 nM for up to 1 h and rising again just prior to cell death. (3) Inhibition with anoxia + 1 mM IAA resulted in rapid depletion of ATP and cell killing, with increases in [Ca2+]i to > 1 microM by 20 +/- 2 min. (4) Depletion of glycolytic metabolites by depriving cells of substrate for 12 h (in HBSS minus glucose) before subjecting to anoxia minus glucose resulted in increases in [Ca2+]i at 40 +/- 17 min followed by cell killing. (5) Injury with anoxic HBSS minus glucose was reversed by reaeration before or during the initial rise in [Ca2+]i. Later reaeration resulted in rapid cell killing. In all cases, delta psi m was dissipated only after [Ca2+]i was significantly elevated.

Chemical hypoxia increases cytosolic Ca2+ and oxygen free radical formation

'Chemical hypoxia' was produced in isolated rat hepatocytes. The cells were immobilized in agarose gel threads and perfused with Krebs-Henseleit bicarbonate buffer equilibrated with 95% O2 + 5% CO2 or 95% air + 5% CO2. During 'chemical hypoxia', 2 mM KCN + 0.5 mM iodoacetate (CN-IAA) were added to the perfusate for 30 min. Cytosolic ionized Ca2+ (Cai2+) was measured with aequorin, the formation of oxygen free radicals by lucigenin-enhanced chemiluminescence and cell injury by the rate of LDH released by the cells in the effluent perfusate. As soon as the cells were exposed to CN-IAA in the presence of 95% O2 + 5% CO2, Cai2+ increased rapidly to reach 1.5 microM within 10 min, and oxygen free radical formation increased 5-fold. The increase in LDH release was temporally delayed and occurred only during the recovery phase. The results were not significantly different when the cells were perfused with KHB equilibrated with 95% air + 5% CO2, except that oxygen free radical formation increased 13-fold. These results suggest that both a rise in Cai2+ and a formation of reactive oxygen species could be responsible for the cell injury and the cell death induced by CN-IAA poisoning.

Arachidonic acid release in renal proximal tubule cell injuries and death

Arachidonic acid release and the effect of phospholipase inhibitors on various types of cell injuries and death to rabbit renal proximal tubule suspensions were determined. Proximal tubules were exposed to the mitochondrial inhibitor antimycin A (0.1 microM), the protonophore carbonyl cyanide p-trifluoromethoxyphenylhydrazone (1 microM FCCP), the oxidant tert-butyl hydroperoxide (0.5 mM TBHP), or the calcium ionophore ionomycin (5 microM) in the absence or presence of the putative phospholipase inhibitors dibucaine, mepacrine, chlorpromazine, or U-26384. The phospholipase inhibitors had no effect on the proximal tubule lactate dehydrogenase (LDH) release (a marker of cell death) produced by FCCP, antimycin A, or ionomycin after 1,2, or 2 hours of exposure, respectively. Only dibucaine and mepacrine decreased LDH release in TBHP-treated proximal tubules without decreasing TBHP-induced lipid peroxidation. Antimycin A and ionomycin did not release arachidonic acid from proximal tubules prelabeled with [1-14C] arachidonic acid. In contrast, TBHP released arachidonic acid from proximal tubules prior to the onset of cell death, and dibucaine and mepacrine decreased the TBHP-induced release. Thus, phospholipase inhibitors were cytoprotective in those injuries that produced arachidonic acid release. These results suggest that arachidonic acid release and phospholipase A2 activation play a contributing role in oxidant-induced renal proximal tubule cell injury and death but not in mitochondrial inhibitor- or calcium ionophore-induced proximal tubule cell injury and death.

Effects of Ca2+ channel blockers, low Ca2+ medium and glycine on cell Ca2+ and injury in anoxic rabbit proximal tubules

L-type Ca2+ channel blockers (CCBs) have been shown to be protective against ischemia-induced injury of the kidney, suggesting that increased intracellular Ca2+ levels ([Ca2+]i) play an important role in the pathogenesis of ischemic cell injury. To assess the role of [Ca2+]i in anoxic injury of the proximal tubule (PT) and the protective effect of CCBs, digital imaging fluorescence microscopy was used to monitor [Ca2+]i in individual PT cells during 60 minutes of anoxia. [Ca2+]i started to rise within 10 minutes and reached maximal levels between 30 to 45 minutes of anoxia. The onset of this increase and the maximal levels reached varied markedly among individual cells. The mean values for initial and maximal anoxic [Ca2+]i were 109 +/- 2 (N = 209) and 422 +/- 14 (N = 240) nM, respectively. Methoxyverapamil (D600; 1 microM) significantly reduced anoxic [Ca2+]i to 122 +/- 5 nM (P < 0.05; N = 79). Removal of extracellular Ca2+ completely abolished anoxia-induced increases in [Ca2+]i, confirming that these increases in [Ca2+]i result from Ca2+ influx. During 60 minutes of anoxia, PT cells showed a gradual decrease in cell viability to 54 +/- 2%. D600 (1 microM) significantly increased cell viability to 64 +/- 3% (P < 0.05). Glycine (5 mM), however, increased cell viability to 77 +/- 4% without a significant reduction in anoxic [Ca2+]i levels. Low Ca2+ medium only protected when 0.1 mM La3+ was included, a condition which increased cell viability to 82 +/- 5%.(ABSTRACT TRUNCATED AT 250 WORDS)

Age and development-related changes in osteopontin and nitric oxide synthase mRNA levels in human kidney proximal tubule epithelial cells: contrasting responses to hypoxia and reoxygenation

Osteopontin (OPN) encodes a secreted glycosylated phosphoprotein containing a GRGDS motif that can mediate cell attachment through the alpha v beta 3 integrin, and has recently been shown to down-regulate nitric oxide synthase (NOS) expression. We report here that primary cultures of renal proximal tubule epithelial (PTE) cells prepared from human kidneys of different developmental stages and ages show a positive correlation between developmental age and the expression, at the mRNA level, of both OPN and constitutive NOS. However, OPN and NOS responded in different manners, as assessed by mRNA measurements, to hypoxia-reoxygenation injury. The OPN mRNA level, assessed by Northern blotting, increased slightly during 60 min of hypoxia and more substantially during subsequent reoxygenation of primary PTE cells derived from the kidneys of young but not of aged donors. The abundance of NOS mRNA, measured using a cDNA probe to the constitutive form of the enzyme, was enhanced during hypoxia in kidneys derived from humans of all ages, and then decreased during reoxygenation--possibly as the result of increased OPN expression. PTE cells from aged kidneys are more susceptible to cell death under hypoxic conditions that PTE cells from young kidneys. An investigation of the effect of an oxidant on OPN and NOS mRNA levels revealed that within 30 min of exposure to H2O2, NOS mRNA levels decreased simultaneously with an increase in OPN mRNA levels. Nitric oxide (NO), the product of NOS, is at low levels an important signal transduction molecule participating in the regulation of vascular tone and renal reabsorption; at high levels it is cytotoxic. We suggest that the diminished ability of cells from old kidneys to down-regulate NO production and to increase OPN expression after hypoxia-reoxygenation may contribute to their increased susceptibility to oxidant injury.

Evidence for role of cytosolic free calcium in hypoxia-induced proximal tubule injury

The role of cytosolic free Ca2+ ([Ca2+]i) in hypoxic injury was investigated in rat proximal tubules. [Ca2+]i was measured using fura-2 and cell injury was estimated with propidium iodide (PI) in individual tubules using video imaging fluorescence microscopy. [Ca2+]i increased from approximately 170 to approximately 390 nM during 5 min of hypoxia. This increase preceded detectable cell injury as assessed by PI and was reversible with reoxygenation. 1,2-Bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA; 100 microM) reduced [Ca2+]i under basal conditions (approximately 80 nM) and during hypoxia (approximately 120 nM) and significantly attenuated hypoxic injury. When [Ca2+]i and hypoxic cell injury were studied concurrently in the same individual tubules, the 10 min [Ca2+]i rise correlated significantly with subsequent cell damage observed at 20 min. 2 mM glycine did not block the rise in [Ca2+]i, yet protected the tubules from hypoxic injury. These results indicate that in rat proximal tubules, hypoxia induces an increase of [Ca2+]i which occurs before cell damage. The protective effect of BAPTA supports a role for [Ca2+]i in the initiation of hypoxic proximal tubule injury. The glycine results, however, implicate calcium-independent mechanisms of injury and/or blockade of calcium-mediated processes of injury such as activation of phospholipases or proteases.

The effect of L-type Ca2+ channel blockers on anoxia-induced increases in intracellular Ca2+ concentration in rabbit proximal tubule cells in primary culture

Ca2+ channel blockers (CCB) have been shown to be protective against ischaemic damage of the kidney, suggesting an important role for intracellular Ca2+ ([Ca2+]i) in generating cell damage. To delineate the mechanism behind this protective effect, we studied [Ca2+]i in cultured proximal tubule (PT) cells during anoxia in the absence of glycolysis and the effect of methoxyverapamil (D 600) and felodipine on [Ca2+]i during anoxia. A method was developed whereby [Ca2+]i in cultured PT cells could be measured continuously with a fura-2 imaging technique during anoxic periods up to 60 min. Complete absence of O2 was realised by inclusion of a mixture of oxygenases in an anoxic chamber. [Ca2+]i in PT cells started to rise after 10 min of anoxia and reached maximal levels at 30 min, which remained stable up to 60 min. The onset of this increase and the maximal levels reached varied markedly among individual cells. The mean values for normoxic and anoxic [Ca2+]i were 118 +/- 2 (n = 98) and 662 +/- 22 (n = 160) nM, respectively. D 600 (1 microM), but not felodipine (10 microM), significantly reduced basal [Ca2+]i in normoxic incubations. During anoxia 1 microM and 100 microM D 600 significantly decreased anoxic [Ca2+]i levels by 22 and 63% respectively. Felodipine at 10 microM was as effective as 1 microM D 600. Removal of extracellular Ca2+ and addition of 0.1 mM La3+ completely abolished anoxia-induced increases in [Ca2+]i. We conclude that anoxia induces increases in [Ca2+]i in rabbit PT cells in primary culture, which results from Ca2+ influx. Since this Ca2+ influx is partially inhibited by low doses of CCBs, L-type Ca2+ channels may be involved.

Cytoprotection by inhibition of chloride channels: the mechanism of action of glycine and strychnine

Previous studies have demonstrated that strychnine mimics the cytoprotective effects of glycine (1) and that strychnine binds specifically to renal proximal tubules (RPT) at cytoprotective concentrations (2). The goal of this study was to determine a mechanism by which strychnine and glycine are cytoprotective. Antimycin A (0.1 microM) caused chloride influx subsequent to mitochondrial inhibition and prior to the release of lactate dehydrogenase (LDH) activity (a marker of cell death/lysis). The addition of strychnine or glycine prevented the chloride influx and LDH release. The chloride channel inhibitors ethacrynic acid, furosemide, anthracene-9-carboxylic acid, DIDS, and SITS decreased LDH release in RPT exposed to antimycin A with a rank order of potency of DIDS > ethacrynic acid = furosemide = anthracene-9-carboxylic acid > SITS. These data, in conjunction with the preceeding paper, indicate a critical role for chloride influx in cell death/lysis; support the existence of a novel strychnine binding site on the plasma membrane of RPT that is coupled to a chloride channel; and suggest that glycine and strychnine are cytoprotective through their inhibition of chloride influx.

Inhibition of organic anion transport in endothelial cells by hydrogen peroxide

ATP loss is a prominent feature of cellular injury induced by oxidants or ischemia. How reduction of cellular ATP levels contributes to lethal injury is still poorly understood. In this study we examined the ability of H2O2 to inhibit in a dose-dependent manner the extrusion of fluorescent organic anions from bovine pulmonary artery endothelial cells. Extrusion of fluorescent organic anions was inhibited by probenecid, suggesting an organic anion transporter was involved. In experiments in which ATP levels in endothelial cells were varied by treatment with different degrees of metabolic inhibition, it was determined that organic anion transport was ATP-dependent. H2O2-induced inhibition of organic anion transport correlated well with the oxidant's effect on cellular ATP levels. Thus H2O2-mediated inhibition of organic anion transport appears to be via depletion of ATP, a required substrate for the transport reaction. Inhibition of organic anion transport directly by probenecid or indirectly by metabolic inhibition with reduction of cellular ATP levels was correlated with similar reductions of short term viability. This supports the hypothesis that inhibition of organic anion transport after oxidant exposure or during ischemia results from depletion of ATP and may significantly contribute to cytotoxicity.

Cell swelling, blebbing, and death are dependent on ATP depletion and independent of calcium during chemical hypoxia in a glial cell line (ROC-1)

The morphological and biochemical changes that occur during chemical hypoxic injury in a neural cell line were studied in the presence and absence of calcium. Oligodendroglial-glioma hybrid cells (ROC-1) were subjected to inhibitors of glycolytic and oxidative ATP synthesis (chemical hypoxia). Complete respiratory inhibition depleted [ATP] to less than 5% of control by 4 min. Blebs appeared on the cell surfaces and cells began to swell within a few minutes of ATP depletion. A 200% increase in cell volume and bleb coalescence preceded irreversible cell injury (lactate dehydrogenase release) which began at approximately 20 min with 50% cell death by 40 min. In energized cells an equivalent degree of osmotic swelling induced by ouabain inhibition of the Na+, K(+)-ATPase pump did not produce blebbing or cell death. Partial inhibition of respiration decreased [ATP] to approximately 10% of control by 40 min. Blebbing and swelling began at 40 min and bleb coalescence preceded plasma membrane disruption which began at approximately 55 min. ATP depletion, blebbing, swelling, and death followed similar time courses in the presence or absence of extracellular calcium ([Ca2+]e). Intracellular calcium ([Ca2+]i) was measured using fura-2. In calcium-containing medium metabolic inhibition caused a transient increase in resting [Ca2+]i (100 +/- 17 nM) followed by a low steady-state level preceding plasma membrane disruption. Following deenergization in calcium-free medium, [Ca2+]i remained below 60 nM throughout injury and death. These data suggest that decreased ATP initiates a sequence of events including bleb formation and cell swelling that lead to irreversible cell injury in the absence of large increases in [Ca2+]i.