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

Endoplasmic reticulum Ca(2+) signaling and calpains mediate renal cell death

The goal of the current study was to determine the roles of ATP content, endoplasmic reticulum (ER) Ca(2+) stores, cytosolic free Ca(2+) (Ca(2+)(f)) and calpain activity in the signaling of rabbit renal proximal tubular (RPT) cell death (oncosis). Increasing concentrations (0.3-10 microM) of the mitochondrial inhibitor antimycin A produced rapid ATP depletion that correlated to a rapid and sustained increase in Ca(2+)(f), but not phospholipase C activation. The ER Ca(2+)-ATPase inhibitors thapsigargin (5 microM) or cyclopiazonic acid (100 microM) alone produced similar but transient increases in Ca(2+)(f). Pretreatment with thapsigargin prevented antimycin A-induced increases in Ca(2+)(f) and antimycin A pretreatment prevented thapsigargin-induced increases in Ca(2+)(f). Calpain activity increased in conjunction with ER Ca(2+) release. Pretreatment, but not post-treatment, with thapsigargin or cyclopiazonic acid prevented antimycin A-induced cell death. These data demonstrate that extensive ATP depletion signals oncosis through ER Ca(2+) release, a sustained increase in Ca(2+)(f) and calpain activation. Depletion of ER Ca(2+) stores prior to toxicant exposure prevents increases in Ca(2+)(f) and oncosis.

Cytoprotective effects of calbindin-D(28k) against antimycin-A induced hypoxic injury in proximal tubular cells

Intracellular calcium plays an important role on the pathogenesis of hypoxia-induced cellular injury. Calbindin-D(28k), a cytosolic vitamin D-dependent calcium binding protein, can serve as a buffer to limit a surge in intracellular Ca2+ concentration ([Ca2+]i) induced by various stimulations. To evaluate the possible cytoprotective effect of calbindin-D(28k) against hypoxic injury in proximal tubular cells, a plasmid containing calbindin-D(28k) cDNA under the control of CMV immediate-early gene promoter was transfected into the murine proximal tubular epithelial (MCT) cells. The expression of calbindin-D(28k) in the transfected cells was verified with Northern blot analysis, Western blot analysis, and immunofluorescent staining. The non-transfected and transfected MCT cells were subjected to chemical hypoxia induced by antimycin A (10 microM) and glucose deprivation for 30-120 min. The transfection of calbindin-D(28k) reduced lactate dehydrogenase (LDH) release by 41%, 41%, 24%, and 24%, respectively, at 30, 60, 90 and 120 min after hypoxia when compared to the non-transfected cells (all p < 0.05). Cell viability after hypoxic injury was also significantly higher in transfected cells than non-transfected cells. Transfection with the plasmid without calbindin-D(28k) cDNA did not affect LDH release or cell viability after chemical hypoxic injury. [Ca+2]i was measured ratiometrically with fura-2 after exposure to chemical hypoxia. The rate of initial rise in [Ca2+]i and final [Ca+2]i at 30-120 min were significantly lowered in transfected cells. In conclusion, this study demonstrated that transfection of calbindin-D(28k) gene into MCT cells provide protective effects against chemical hypoxic injury probably through its buffering effects on [Ca+2]i.

Nephrotoxicants Induce Endothelin Release and Signaling in Renal Proximal Tubules: Effect on Drug Efflux

We previously used killifish proximal tubules, fluorescent substrates, and confocal microscopy to demonstrate that transport mediated by the multidrug resistance protein (Mrp2) and by P-glycoprotein was reduced by nanomolar concentrations of endothelin-1 (ET), acting through a basolateral B-type ET receptor and protein kinase C (PKC). Here we show that representatives of two classes of nephrotoxicants decrease transport by activating the endothelin-PKC signaling pathway. Exposing tubules to radiocontrast agents (iohexol, diatrizoate) or aminoglycoside antibiotics (gentamicin, amikacin) reduced Mrp2-mediated fluorescein methotrexate (FL-MTX) transport from cell to tubular lumen. Pretreating the tubules with an ET(B)-receptor antagonist or with PKC-selective inhibitors abolished these effects. The nephrotoxicants activated signaling by inducing release of ET from the tubules, because adding of an antibody against ET to the medium abolished the effects. Elevating medium Ca(2+) also reduced FL-MTX transport; this reduction was abolished when tubules were pretreated with an ET antibody, an ET(B)-receptor antagonist, PKC-selective inhibitors, or the Ca(2+) channel blocker, nifedipine. None of these drugs by themselves affected FL-MTX transport. Importantly, nifedipine also blocked the ET(B)-receptor/PKC-dependent reduction in FL-MTX transport caused by gentamicin and diatrizoate. These results for two classes of structurally unrelated nephrotoxicants suggest that Ca(2+)-dependent ET release and subsequent action through an autocrine mechanism may be an early response to tubular injury.

Ca(2+)-dependent inhibition of inwardly rectifying K(+) channel in opossum kidney cells

The effect of intracellular Ca(2+) on the activity of the inwardly rectifying ATP-regulated K(+) channel with an inward conductance of about 90 pS was examined by using the patch-clamp technique in opossum kidney proximal tubule (OKP) cells. The activity of the inwardly rectifying K(+) channel rapidly declined with an application of ionomycin (1 microM) in the presence of 10(-6) M Ca(2+) in cell-attached patches. The application of 10 microM phorbor-12-myristate-acetate (PMA) with 10(-6) M Ca(2+) reduced the K(+) channel activity. Although the channel activity was not influenced by an increase of bath Ca(2+) from 10(-7.5) to 10(-6) M, the activity was inhibited by protein kinase C (PKC, 1 U/ml) with 10(-6) M Ca(2+) in inside-out patches. The inhibitory effect of Ca(2+) with ionomycin on the channel activity was diminished by the pretreatment with a specific PKC inhibitor, GF 109203X (5 microM), in cell-attached patches. By contrast, the application of Ca(2+)/calmodulin kinase II (CaMK II, 300 pM) dramatically increased this channel activity in inside-out patches. In cell-attached patches, the addition of both GF 109203X and cyclospolin A (5 microM), a potent inhibitor of protein phosphatase 2B (calcineurin), instead stimulated the K(+) channel activity with ionomycin and 10(-6) M Ca(2+). The addition of protein phosphatase 2B (calcineurin) (2 U/ml) to the bath with calmodulin (1 microM) and Ni(2+) (10 microM) to stimulate calcineurin inhibited the channel activity in inside-out patches. Furthermore, the inhibitory effect of PKC or calcineurin on this channel activity was abolished by a removal of Ca(2+) from bath solution. These results suggest that Ca(2+)-dependent inhibitory effect on the inwardly rectifying K(+) channel in OKP cells was mainly mediated by Ca(2+)-PKC-mediated phosphorylation, and that the Ca(2+)-calmodulin-dependent phosphorylation process may be counterbalanced by the Ca(2+)-calmodulin-dependent dephosphorylation process.

A role for sodium in hypoxic but not in hypothermic injury to hepatocytes and LLC-PK1 cells

BACKGROUND

Hypothermia is considered to be responsible for sodium influx during cold hypoxic incubation. However, we have previously shown that hypothermia alone leads to a pronounced decrease in cellular sodium content when liver endothelial cells or hepatocytes are incubated under such conditions. In the research described here, we therefore studied the effects of hypothermia and hypoxia, alone or combined, on cellular sodium homeostasis and assessed the role sodium plays in the pathogenesis of hypoxic and hypothermic injury to cultured liver and kidney cells.

METHODS

Isolated hepatocytes and LLC-PK1 cells were incubated in Krebs-Henseleit buffer or a sodium-free modification thereof under normoxic and hypoxic conditions at 4 degrees C as well as at 37 degrees C. Cytosolic sodium concentration was determined in isolated hepatocytes under both warm and cold conditions using digital fluorescence microscopy and the Na+-sensitive dye sodium-binding benzofuran isophthalate.

RESULTS

When hepatocytes were incubated under cold normoxic conditions the cellular sodium concentration decreased. However, it increased strongly under hypoxic conditions at 4 degrees C and at 37 degrees C. When either hepatocytes or LLC-PK1 cells were incubated under hypoxic conditions at 4 degrees C or 37 degrees C, sodium-free medium provided protection. In contrast, sodium-free medium did not alleviate the hypothermic injury observed when cells were incubated under cold normoxia.

CONCLUSIONS

The sodium influx observed during cold hypoxia is triggered by hypoxia and not by hypothermia. Sodium plays a prominent role in hypoxic injury to cultured liver and kidney cells, although hypothermic injury of these cells is independent of sodium homeostasis.

Effects of renal cytoprotective agents on erythrocyte membrane stability

To elucidate potential mechanisms of ischemic renal injury, investigators often use drugs that interfere with specific pathological pathways and study their protective efficacy in in vitro models of ischemia, such as isolated renal proximal tubules subjected to hypoxia. However, the protective effects of certain drugs may depend on non-specific membrane-stabilizing properties. We have studied the effects of several drugs on membrane integrity using osmotic lysis of erythrocytes as a model system. Freshly isolated rabbit erythrocytes were subjected to a hypotonic shock, and the protective effects of various calcium channel blockers, phospholipase inhibitors, free fatty acids, the NO-synthase inhibitor L-NAME, the amino acid glycine and its receptor-analogue strychnine, and two chloride channel blockers were examined. Most agents protected erythrocytes against hypotonic hemolysis when added to the medium in the same concentration range as used in suspensions of hypoxic proximal tubules. Only the protective agents that proposedly act via a blockade of chloride influx (glycine, strychnine and the chloride channel blockers), did not attenuate hypotonic hemolysis. The erythrocyte hemolysis assay may provide an easy and rapid method to screen for non-specific membrane-stabilizing effects of potentially cytoprotective agents.

Cold preservation of isolated rabbit proximal tubules induces radical-mediated cell injury

BACKGROUND

Reactive oxygen species (ROS) are involved in reperfusion injury after preservation. Recent studies in isolated endothelial cells and hepatocytes suggested the occurrence of ROS-mediated injury during the period of cold incubation. In the present study, formation of ROS and subsequent cell injury were studied in freshly isolated rabbit proximal tubules (PTs).

METHODS

PTs were incubated in University of Wisconsin (UW) solution, Euro-Collins solution, or a modified Krebs-Henseleit buffer under aerobic conditions for up to 94 hr at 4 degrees C. ROS formation and cell death were assessed as lipid peroxidation (formation of thiobarbituric acid-reactive substances [TBARS]) and release of lactate dehydrogenase, respectively. The involvement of ROS was further investigated in UW solution using compounds that might interfere with ROS formation. In addition, tubules were studied under anaerobic conditions (gassing with 95% N2/5% CO2).

RESULTS

Cold preservation of rabbit PTs in any of the solutions under aerobic conditions caused progressive lipid peroxidation and concomitant cell injury. Addition to UW solution of inhibitors of ROS formation, in particular 2,2'-dipyridyl, or removal of oxygen by gassing with 95% N2/5% CO2, prevented lipid peroxidation and protected rabbit PTs against cold injury. Both the nitric oxide (NO) synthase inhibitor L-NAME and dexamethasone, which blocks the inducible NO synthase, were ineffective. The cytoprotectant glycine affected neither TBARS formation nor lactate dehydrogenase release.

CONCLUSIONS

Cold preservation of renal PTs under aerobic conditions caused cell injury even in the specially designed preservation solution UW. Cell injury is caused by iron-dependent, NO synthase-independent ROS formation.

Hypoxia decreases calcium influx into rat proximal tubules

Renal ischemia results in adenosine triphosphate (ATP) depletion, particularly in cells of the proximal tubule (PT), which rely heavily on oxidative phosphorylation for energy supply. Lack of ATP leads to a disturbance in intracellular homeostasis of Na+, K+ and Cl-. Also, cytosolic Ca2+ levels in renal PTs may increase during hypoxia [1], presumably by a combination of impaired extrusion and enhanced influx [2]. However, Ca2+ influx was previously measured using radiolabeled Ca2+ and at varying partial oxygen tension [2]. We have now used to Mn2(+)-induced quenching of fura-2 fluorescence to study Ca2+ influx in individual rat PTs during normoxic and hypoxic superfusion. Normoxic Ca2+ influx was indeed reflected by the Mn2+ quenching of fura-2 fluorescence and this influx could be inhibited by the calcium entry blocker methoxyverapamil (D600; inhibition 50 +/- 2% and 35 +/- 3% for 10 and 100 mumol, respectively). La3+ completely blocked normoxic Ca2+ influx. Hypoxic superfusion or rat PTs did not induce an increase in Ca2+ influx, but reduced this influx to 79 +/- 3% of the normoxic control. We hypothesize that reducing Ca2+ influx during hypoxia provides the cell with a means to prevent cellular Ca2+ overload during ATP-depletion, where Ca2+ extrusion is limited.

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.

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.

Modulation of c-fos and egr-1 expression in the isolated perfused kidney by agents that alter tubular work

The isolated perfused rat kidney provides a model of selective hypoxia to the medullary thick ascending limb. To investigate the relationship between immediate early gene expression and the extent of hypoxic damage, we determined expression of the immediate early genes (IEG) c-fos and egr-1 in isolated perfused kidneys during standard perfusion and after various measures shown previously to be protective. mRNA levels of c-fos and egr-1 were markedly increased in kidneys after 90 minutes of standard perfusion with Krebs-Henseleit buffer containing albumin. Gene expression was most prominent in the outer medulla followed by papilla and cortex, a pattern reflected by the immunohistochemical demonstration of a prominent accumulation of both egr-1 and c-fos polypetides mainly in the medullary thick ascending limb (mTAL). Protective measures known to minimize morphological damage to the mTAL, including hyperoncotic perfusion, perfusion with glycine, or perfusion with a mixture of amino acids, decreased mRNA levels of c-fos and egr-1 in the outer medulla (by 50% and 35%, respectively) and the papilla (by 60 and 30%, respectively). Renal cortex showed only minor changes. In contrast, prevention of tubular transport by perfusion with 1 mM ouabain increased mRNA levels of c-fos and egr-1 in the outer medulla by 100% and 60%, respectively. Ouabain also dramatically increased mRNA levels of both IEGs in two lines of cultured renal epithelial cells. Changes in the level and distribution of the protein products of these IEGs were not detectable in perfused kidneys by immunohistochemistry. Hypoxic injury of the kidney stimulates IEG expression even in the absence of reperfusion. Protection against hypoxic injury in the mTAL correlates with suppression of IEG mRNA levels when protection is provided by amino acids or hyperoncotic perfusion, but not when provided by inhibition of Na,K-ATPase, which stimulates IEG expression. We conclude that diminished IEG expression is not a necessary concomitant of protection against hypoxic injury.

Effect of Ca2+ channel blockers, external Ca2+ and phospholipase A2 inhibitors on t-butylhydroperoxide-induced lipid peroxidation and toxicity in rat liver slices

OBJECTIVES

This study was undertaken to examine the effect of oxidant on lipid peroxidation and lethal cell injury in rat liver slices.

METHODS

t-Butylhydroperoxide (t-BHP) was employed as a model of an oxidant. The lipid peroxidation and lethal cell injury were estimated by measuring the formation of malondialdehyde (MDA) and lactate dehydrogenase (LDH) release, respectively.

RESULTS

t-BHP increased lipid peroxidation and LDH release in a dose-dependent manner over concentrations of 0.5-10 mM. t-BHP-induced lipid peroxidation was completely prevented by an antioxidant, N,N-diphenyl-p-phenylenediamine (DPPD), but LDH release was partially decreased. Both t-BHP-induced lipid peroxidation and LDH release were significantly protected by iron chelator, deferoxamine, sulfhydryl reducing agent, dithiothreitol and glutathione. Ca2+ channel blockers, verapamil, diltiazem and nifedipine exerted a significant protective effect against t-BHP-induced lipid peroxidation and LDH release. By contrast, addition of external Ca2+ chelator, ethylene glycol bis(b-aminoethyl ether)-N,N-tetraacetic acid (EGTA) did not alter t-BHP-induced lipid peroxidation, whereas t-BHP-induced lethal cell injury was significantly prevented. Phospholipase A2 (PLA2) inhibitors, mepacrine and butacaine produced a partial protective effect.

CONCLUSIONS

These results suggest that t-BHP induces cell injury by lipid peroxidation-dependent and -independent mechanisms which can be partially prevented by Ca2+ channel blockers and PLA2 inhibitors.

Hypoxia induces intercellular adhesion molecule-1 on cultured human tubular cells

The adverse effects of acute renal ischemia are partly mediated through an infiltration of inflammatory cells into the tubulointerstitium. The expression of adhesion molecules such as intercellular adhesion molecule-1 (ICAM-1) by resident renal cells (endothelial cells and tubular cells) may facilitate this process. We investigated whether hypoxia stimulates the expression of ICAM-1 by cultured human proximal tubular cells (HPTC). Hypoxic culture conditions (PO2 < 4 kPa) stimulated the expression of ICAM-1 by HPTC in a time-dependent manner (P < 0.0001) as demonstrated by quantitative flow cytometry analysis. Quantitative PCR demonstrated an increase in ICAM-1 transcription. Re-oxygenation of tubular cells did not increase ICAM-1 expression further. TNF alpha concentration in culture supernatants increased with hypoxia, but blocking experiments demonstrated that TNF alpha was not implicated in hypoxia-induced expression of ICAM-1. Furthermore, the cytokines IL-6 and IL-1 beta were not involved, but the effect of hypoxia was blocked by PDTC, an antioxidant that may inhibit the activation of the transcription factor NF-kappa B. These data demonstrate that hypoxia is a stimulus that induces the synthesis and expression of the adhesion molecule ICAM-1, presumably via the activation of NF-kappa B.

Molecular dissection of Ca2+ efflux in immortalized proximal tubule cells

Plasma membrane Ca(2+)-ATPase (PMCA) and the Na+/Ca2+ exchanger participate in regulating cell function by maintaining proper intracellular Ca2+ concentrations ([Ca2+]i). In renal epithelial cells these proteins have been additionally implicated in cellular calcium absorption. The purpose of the present studies was to determine the Ca2+ extrusion mechanisms in cells derived from the proximal tubule. Homology-based RT-PCR was used to amplify PMCA transcripts from RNA isolated from mouse cell lines originating from the S1, S2, and S3 proximal tubule segments. S1, S2, and S3 cells exhibited only PMCA1 and PMCA4 products. PCR product identity was confirmed by sequence analysis. Northern analysis of proximal tubule cell RNAs revealed appropriate transcripts of 7.5 and 5.5 kb for PMCA1 and 8.5 and 7.5 kb for PMCA4, but were negative for PMCA2 and PMCA3. Western analysis with a monoclonal antibody to PMCA showed that all proximal cell lines expressed a reacting plasma membrane protein of 140 kD, the reported PMCA molecular mas. Na+/Ca2+ exchanger (NCX1) mRNA expression, analyzed by RT-PCR, protein expression by Western analysis, and functional exchange activity were uniformly absent from all proximal tubule cell lines. These observations support the idea that immortalized cells derived from the proximal tubule express PMCA1 and PMCA4, which may serve as the primary mechanism of cellular Ca2+ efflux.

Effects of nifedipine and platelet activating factor antagonist (BN 52021) in glycerol-induced acute renal failure in rats

We studied the actions of nifedipine and the platelet activating factor (PAF) antagonist BN 52021 on renal and tubular function in glycerol-induced acute renal failure (Gly-ARF). The tubular handling of sodium was evaluated through the lithium clearance method in awake rats in metabolic cages. The sequential analysis of tubular function 3, 6, 12, and 24 h after Gly-ARF showed a sharp decrease in fractional proximal Na reabsorption (FPRNa)--control 74.1 +/- 12.5%, 3 h: 79.5 +/- 6.0%; 6 h: 41.8 +/- 15.9%; 12 h: 22.9 +/- 17.9%; and 24 h: 31.1 +/- 16.2% (p < 0.001) while fractional distal Na reabsorption (FDRNa) did not change during the study. The effect of nifedipine (20 mg/kg p.o.) and BN 52021 (1 mg/kg i.p.) were evaluated 24 h after the induction of Gly-ARF. Both drugs attenuated the reduction in creatinine clearance (control 431.8 +/- 108.2, glycerol 96.7 +/- 43.8, glycerol plus nifedipine 264.9 +/- 103.5, and glycerol plus BN 52021 188.9 +/- 69.8 microL/min/100 g, p < 0.001). However, only nifedipine could keep FPRNa higher than untreated rats (58.3 +/- 13.2 vs. 31.1 +/- 16.2%, p < 0.05) and reduced the tubular necrosis on histologic semiquantitative analysis. Our data showed that nifedipine and BN 52021 could protect against filtration failure in Gly-ARF but that only nifedipine reduced the proximal tubular lesion.

Protection against hypoxic injury of rat proximal tubules by felodipine via a calcium-independent mechanism

Most evidence for a key role of calcium entry in hypoxia-induced renal damage stems from studies with calcium channel blockers. In proximal tubules, a primary site of renal ischaemic injury, only phenyl-alkylamines, especially verapamil, have been studied. In the present study the effect of the dihydropyridine felodipine on hypoxic injury in isolated rat proximal tubules was investigated. To discriminate between the block of calcium entry and other effects, the enantiomers and a non-calcium blocking derivative of felodipine (H186/86) were included. Cell membrane injury was assessed by measuring the release of lactate dehydrogenase (LDH). At high concentrations (100 microM) felodipine, H186/86 and the two enantiomers all protected rat proximal tubules against hypoxia-induced injury to the same extent. Absence of extracellular calcium did not offer protection, but rather enhanced hypoxic injury. All dihydropyridines used increased the intracellular potassium concentration during normoxia. Felodipine attenuated the hypoxia-induced loss of cellular potassium. We have tried to mimic the effects of felodipine by using potassium channel blockers. The potassium channel blockers quinidine and glibenclamide afforded some protection against hypoxic injury, although their effects on cellular potassium were equivocal. We conclude that the dihydropyridine calcium channel blocker felodipine protects rat proximal tubules against hypoxic injury via a calcium-independent mechanism. We propose that high levels of intracellular potassium and attenuation of potassium loss during hypoxia are important in this protection.

Cellular acidification occurs during anoxia in cultured, but not in freshly isolated, rabbit proximal tubular cells

In a variety of cells it has been shown that acidosis is protective against anoxic injury. We have demonstrated previously that proximal tubule (PT) cells in primary culture were more resistant to anoxia-induced cell injury than were freshly isolated cells. Therefore, we asked the question of whether a difference in cellular acidification during anoxia could explain this difference in susceptibility to anoxia. To answer this question, intracellular pH (pHi) was measured during anoxic incubation of PT cells in culture and those that were freshly isolated. PT cells were incubated in an anoxic chamber at 37 degrees C after loading with 2',7'-bis-(2-carboxyethyl)-5,6-carboxyfluorescein acetoxymethyl ester (BCECF-AM) or fura-2 acetoxymethyl ester (fura-2-AM). pHi and cytosolic free Ca2+ ([Ca2+]i) were measured by digital imaging fluorescence microscopy. During anoxia, pHi in cultured PT cells decreased from 7.3 +/- 0.1 to 6.8 +/- 0.1, whereas pHi in freshly isolated cells did not decrease significantly. In addition, the intrinsic buffering capacities (beta i) in cultured and freshly isolated PT cells were determined and turned out to be the same at a pHi greater than or equal to 7.3. Below pHi 7.3, beta i increased several fold in freshly isolated PT cells, and rose to significantly higher levels than in cultured PT cells. During 1 h of anoxia, cell viability of freshly isolated PT cells decreased significantly to 54% +/- 2% (P < 0.05), while no loss in viability was observed in cultured PT cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of substrate-free anoxia and veratridine on intracellular calcium concentration in isolated rat ventricular cardiomyocytes

Cytosolic free Ca2+ concentration ([Ca2+]i) was measured in freshly isolated rat ventricular cardiomyocytes during substrate-free anoxia. Cardiomyocytes were loaded with fura-2 and incubated in an anoxic chamber in which a pO2 equal to 0 mmHg was realized by inclusion of Oxyrase. [Ca2+]i was measured in individual cells using digital imaging fluorescence microscopy. During anoxia, the shape of cardiomyocytes changed from a relaxed-elongated form into a rigor configuration within 15 min after the onset of anoxia. After the cells had developed the rigor state, a delayed rise in [Ca2+]i reached a stable maximal level within 45 min. The mean values for the pre-anoxic and maximal anoxic [Ca2]i were 52 +/- 3 nM (N = 42) and 2115 +/- 59 nM (N = 45), respectively. The purported Na+ overload blocker R 56865, significantly reduced maximal anoxic [Ca2+]i to 533 +/- 56 nM (P < 0.05), implicating a role of elevated intracellular Na+ in the anoxia-induced increase in [Ca2+]i. Veratridine (30 microM), with induces Na+ overload, increased [Ca2+]i to 787 +/- 39 nM. The compound R 56865 reduced veratridine-induced increases in [Ca2+]i to 152 +/- 38 nM. Upon reperfusion, after 45 min of anoxia, two distinct responses were observed. Most often, [Ca2+]i decreased upon reperfusion without a change in morphology or viability, while in the minority of cases, [Ca2+]i increased further followed by hypercontraction and loss of cell viability. The mean value for [Ca2+]i 10 min after reperfusion of the former group, was 752 +/- 46 nM (N = 38).(ABSTRACT TRUNCATED AT 250 WORDS)