Effect of extracellular acidosis on 45Ca uptake in isolated hypoxic proximal tubules

Hypoxia and Its Acid-Base Consequences: From Mountains to Malignancy

Hypoxia, depending upon its magnitude and circumstances, evokes a spectrum of mild to severe acid-base changes ranging from alkalosis to acidosis, which can alter many responses to hypoxia at both non-genomic and genomic levels, in part via altered hypoxia-inducible factor (HIF) metabolism. Healthy people at high altitude and persons hyperventilating to non-hypoxic stimuli can become alkalotic and alkalemic with arterial pH acutely rising as high as 7.7. Hypoxia-mediated respiratory alkalosis reduces sympathetic tone, blunts hypoxic pulmonary vasoconstriction and hypoxic cerebral vasodilation, and increases hemoglobin oxygen affinity. These effects and others can be salutary or counterproductive to tissue oxygen delivery and utilization, based upon magnitude of each effect and summation. With severe hypoxia either in the setting of profound arterial hemoglobin desaturation and reduced O2 content or poor perfusion (ischemia) at the global or local level, metabolic and hypercapnic acidosis develop along with considerable lactate formation and pH falling to below 6.8. Although conventionally considered to be injurious and deleterious to cell function and survival, both acidoses may be cytoprotective by various anti-inflammatory, antioxidant, and anti-apoptotic mechanisms which limit total hypoxic or ischemic-reperfusion injury. Attempts to correct acidosis by giving bicarbonate or other alkaline agents under these circumstances ahead of or concurrent with reoxygenation efforts may be ill advised. Better understanding of this so-called "pH paradox" or permissive acidosis may offer therapeutic possibilities. Rapidly growing cancers often outstrip their vascular supply compromising both oxygen and nutrient delivery and metabolic waste disposal, thus limiting their growth and metastatic potential. However, their excessive glycolysis and lactate formation may not necessarily represent oxygen insufficiency, but rather the Warburg effect-an attempt to provide a large amount of small carbon intermediates to supply the many synthetic pathways of proliferative cell growth. In either case, there is expression and upregulation of many genes involved in acid-base homeostasis, in part by HIF-1 signaling. These include a unique isoform of carbonic anhydrase (CA-IX) and numerous membrane acid-base transporters engaged to maintain an optimal intracellular and extracellular pH for maximal growth. Inhibition of these proteins or gene suppression may have important therapeutic application in cancer chemotherapy.

Renal Vascular Resistance in Sepsis

AIMS

To assess changes in renal vascular resistance (RVR) in human and experimental sepsis and to identify determinants of RVR.

METHODS

We performed a systematic interrogation of two electronic reference libraries using specific search terms. Subjects were animals and patients involved in experimental and human studies of sepsis and septic acute renal failure, in which the RVR was assessed. We obtained all human and experimental articles reporting RVR during sepsis. We assessed the role of various factors that might influence the RVR during sepsis using statistical methods.

RESULTS

We found no human studies, in which the renal blood flow (and, therefore, the RVR) was measured with suitably accurate direct methods. Of the 137 animal studies identified, 52 reported a decreased RVR, 16 studies reported no change in RVR, and 69 studies reported an increased RVR. Consciousness of animals, duration of measurement, method of induction of sepsis, and fluid administration had no effect on the RVR. On the other hand, on univariate analysis, size of the animals (p < 0.001), technique of measurement (p = 0.017), recovery after surgery (p = 0.004), and cardiac output (p < 0.001) influenced the RVR. Multivariate analysis, however, showed that only cardiac output (p = 0.028) and size of the animals (p = 0.031) remained independent determinants of the RVR.

CONCLUSIONS

Changes in RVR during sepsis in humans are unknown. In experimental sepsis, several factors not directly related to sepsis per se appear to influence the RVR. A high cardiac output and the use of large animals predict a decreased RVR, while a decreased cardiac output and the use of small animals predict an increased RVR.

Acidic pH inhibits ATP depletion-induced tubular cell apoptosis by blocking caspase-9 activation in apoptosome

Tubular cell apoptosis has been implicated in the development of ischemic renal failure. In in vitro models, ATP depletion-induced apoptosis of tubular cells is mediated by the intrinsic pathway involving Bax translocation, cytochrome c release, and caspase activation. While the apoptotic cascade has been delineated, much less is known about its regulation. The current study has examined the regulation of ATP depletion-induced tubular cell apoptosis by acidic pH, a common feature of tissue ischemia. Cultured renal tubular cells were subjected to 3 h of ATP depletion with azide and then recovered in full culture medium. The treatment led to apoptosis in approximately 40% of cells. Apoptosis was significantly reduced, if the pH of ATP depletion buffer was lowered from 7-7.4 to 6-6.5. This was accompanied by the inhibition of caspase activation. However, acidic pH did not prevent Bax translocation and oligomerization in mitochondria. Cytochrome c release from mitochondria was not blocked either, suggesting that acidic pH inhibited apoptosis at the postmitochondrial level. To determine the postmitochondrial events that were blocked by acidic pH, we conducted in vitro reconstitution experiments. Exogenous cytochrome c, when added into isolated cell cytosol, induced caspase activation. Such activation was abrogated, when pH during the reconstitution was lowered to 6 or 6.5. Nevertheless, acidic pH did not prevent the recruitment and association of caspase-9 by Apaf-1, as shown by coimmunoprecipitation. Together, this study demonstrated the inhibition of tubular cell apoptosis following ATP depletion by acidic pH. A critical step blocked by acidic pH seems to be caspase-9 activation in apoptosome.

Vascular hyporesponsiveness of the renal circulation during endotoxemia in anesthetized pigs

OBJECTIVE

To compare the vascular reactivity of the renal circulation in control and septic conditions.

DESIGN

Prospective, randomized, controlled animal study.

SETTING

University research laboratory.

SUBJECTS

Anesthetized pigs (n = 17).

INTERVENTIONS

Ten pigs received a continuous intravenous infusion of endotoxin from Escherichia coli (160 ng x kg(-1) x hr(-1)) during 18 hrs, whereas seven control animals received a saline infusion. To test the vascular reactivity, norepinephrine (NE) (1 microg x kg(-1)), acetylcholine (10 microg x kg(-1)), and sodium nitroprusside (10 microg x kg(-1)) were intravenously injected for 20 secs and changes of mean arterial pressure and renal blood flow were observed during the 200 secs after the drug administration. To compare the evolution of the vascular reactivity over time, three tests were performed 5 hrs, 11 hrs, and 17 hrs after initial endotoxin or saline administration.

MEASUREMENTS AND MAIN RESULTS

Endotoxin infusion induced a hypotensive and hypokinetic syndrome with renal hypoperfusion. The mean arterial pressure increase after NE injection and the mean arterial pressure decrease after acetylcholine and nitroprusside were lower in endotoxin than in control pigs. In the renal circulation, the increase of resistance after NE injection and the decrease of renal resistance after acetylcholine and nitroprusside injections were lower in endotoxin than in control pigs.

CONCLUSIONS

This study shows a hyporesponsiveness of the renal circulation to vasoactive agents during endotoxemia. Vasoconstriction to NE, endothelium-dependent as well as endothelium-independent relaxations are altered during endotoxemia but not abolished, and despite the continuous infusion of endotoxin for 18 hrs, no recovery was observed over time.

Factors maintaining a pH gradient within the kidney: role of the vasculature architecture

BACKGROUND

The architecture of the vasa rectae produces significant oxygen (O2) "shunting" and marked decreases in renal medullary pO2 values. We hypothesized that carbon dioxide (CO2) trapping and increases in medullary pCO2 along with decreases in medullary pH values should also accompany this O2 shunting.

METHODS

We developed computer simulations employing a model of gas exchange through the countercurrent vasculature that predicted trapping of CO2 along with O2 shunting. To test the validity of this model directly, medullary pH was measured by using needle electrodes in the in situ kidney before and after the administration of mannitol or furosemide, or by decreasing blood flow with a transient decrease of renal perfusion pressure with a suprarenal clamp. Data are expressed as mean +/- SD.

RESULTS

Medullary pH was lower than cortical pH (7.20 +/- 0.09 vs. 7.39 +/- 0.08, P < 0.01). Mannitol caused a decrease in medullary pH to 7.02 +/- 0.07 (P < 0.01), whereas furosemide increased medullary pH to 7. 31 +/- 0.09 (P < 0.01). Brief periods of severe hypotension decreased medullary pH to 6.90 +/- 0.09 (P < 0.01).

CONCLUSIONS

These data demonstrate that a significant pH gradient exists within the kidney parenchyma. This gradient is related to the metabolic activity of the thick ascending limb of Henle and the countercurrent vascular architecture, and may be relevant to a variety of physiological phenomena involved in volume, electrolyte, and acid-based homeostasis.

Ischemic preconditioning attenuates functional, metabolic, and morphologic injury from ischemic acute renal failure in the rat

Ischemic preconditioning has been shown to ameliorate injury due to subsequent ischemia in several organs. However, relatively little is known about preconditioning and the kidney. To address this, rats were randomized to control (C, N = 14), 2 min of ischemic preconditioning (P2 N = 10), 3 periods of 2 min of ischemia separated by 5 min periods of reflow (P2,3 N = 7), or three 5 min periods of ischemia separated by 5 min of reflow (P5,3 N = 6) prior to 45 min of bilateral renal ischemia followed by 24 hours of reperfusion. We observed a lower serum creatinine after 24 hours of reflow in P2, P2, 3 but not P5, 3 rats compared with C. Histology was examined in the C and P2, 3 groups and demonstrated less severe injury in the P2, 3 group. To gain insight into the mechanism by which preconditioning ameliorated ischemic injury, we performed near IR spectroscopy and 31P NMR spectroscopy. Based on near IR spectroscopy, the P2, 3 group had closer coupling of cytochrome aa3 redox state with that of hemoglobin during reflow. In the 31P NMR studies, the changes in ATP and pHi were similar during ischemia, but the P2, 3 group recovered ATP and pHi faster than C. These data suggest that ischemic preconditioning may ameliorate ischemic renal injury as assessed by functional, metabolic and morphological methods. The mechanism(s) by which this occurs requires additional study.

Differential expression of inducible nitric oxide synthase messenger RNA along the longitudinal and crypt-villus axes of the intestine in endotoxemic rats

OBJECTIVES

To characterize the mechanisms leading to excessive production of nitric oxide within the gut as a consequence of endotoxemia. We sought to: a) determine the time course of inducible nitric oxide synthase (iNOS) messenger RNA (mRNA) expression in the intestine after challenging rats with lipopolysaccharide (LPS); and b) investigate whether there is differential expression of iNOS in enterocytes along the longitudinal or crypt-villus axes of the intestine in rats after LPS administration.

DESIGN

Prospective, randomized, unblinded study.

SETTING

Research laboratories at a large university-affiliated medical center.

SUBJECTS

Male Sprague-Dawley rats.

INTERVENTIONS

At T = 0 hr, rats were injected with O111:B4 Escherichia coli LPS (5 mg/kg) or a similar volume of the saline vehicle. At various time points thereafter, samples of duodenum, jejunum, ileum, colon, and liver were harvested for subsequent extraction of RNA. In some cases, populations of enterocytes enriched in either crypt or villus cells were harvested from the ileum. In some studies, rats were injected with cycloheximide (25 mg i.p.) 15 mins before being challenged with LPS or dexamethasone (2 mg i.p.) 30 mins before being injected with LPS.

MEASUREMENTS AND MAIN RESULTS

iNOS mRNA was undetectable in ileal tissue from rats under basal conditions, but was evident by T = 1 hr and was maximal at T = 2 hrs after injection of LPS. Thereafter, ileal iNOS mRNA concentrations decreased and were undetectable again at T = 24 hrs. At T = 2 hrs after LPS injection, there was marked expression of iNOS mRNA in the ileum, whereas much lower concentrations of iNOS mRNA were detected in the jejunum and colon, and no iNOS mRNA was detected in the duodenum. At T = 3 hrs after LPS injection, expression of iNOS mRNA was up-regulated in both villus and crypt cells, although LPS-induced iNOS mRNA was more prominent in the former than the latter cell type. Pretreatment of rats with dexamethasone virtually abrogated the expression of iNOS mRNA in ileal samples obtained 3 hrs after the injection of LPS. Prior treatment of rats with the protein synthesis inhibitor, cycloheximide, also blunted LPS-induced iNOS mRNA expression.

CONCLUSIONS

LPS-induced iNOS expression is differentially regulated along both the longitudinal and crypt villus axes of the intestinal mucosa, being most prominent in the villus cells of the ileum. LPS-induced iNOS expression is blunted by pretreating rats with dexamethasone or cycloheximide. The latter finding suggests that LPS-induced expression of iNOS mRNA in the gut requires new protein synthesis. Differential regulation of nitric oxide production along the longitudinal and crypt-villus axes of the gut may be a determinant of the pattern of sepsis-induced intestinal damage.

Intestinal epithelial hyperpermeability. Mechanisms and relevance to disease

Pathologic increases in intestinal permeability to hydrophilic macromolecules has been identified in a number of clinical conditions. The significance of gut barrier dysfunction as a clinical issue remains to be delineated, although it seems likely that alterations in intestinal epithelial permeability play a causative role in a number of conditions ranging from inflammatory bowel disease to the development of complications after cardiopulmonary bypass. It is unlikely that any one mechanism can account for all cases of intestinal hyperpermeability. Rather, it is more probable that myriad factors or combinations of factors, including mesenteric ischemia and cytokine-induced phenomena, lead to alterations in permeability in different clinical entities. Nevertheless, from a purely mechanistic standpoint, some common themes, notably the role of ATP depletion, increases in [Ca2+]i, and cytoskeletal derangements in enterocytes, have emerged as being particularly important.

The nature of renal cell injury

The main functional change in patients with acute renal failure (ARF) is a decrease in glomerular filtration rate (GFR). The virtual complete recovery of renal function in those patients who survive ARF, as well as the minimal renal histological abnormalities during ARF when the GFR is less than 10 ml/min, suggest that a major component of the renal tubular cell injury is sublethal or reversible. Experimental models of acute tubular necrosis frequently have placed the emphasis on irreversible proximal tubular cell death. The nature of the renal tubular cell injury in ischemic acute renal failure, however, includes not only cell death (necrosis or apoptosis) but also sublethal injury causing cell dysfunction. The role of intracellular calcium, the calcium-dependent enzymes calpain, phospholipase A2 and nitric oxide synthase (NOS), in the pathophophysiology of this renal tubular cell injury during hypoxia/ischemia is described. The effects of calpain and nitric oxide (NO) on the cytoskeleton and cell adhesion are discussed. Potential mechanisms whereby tubular injury leads to a profound fall in GFR, including increased tubuloglomerular feedback and increased distal tubular obstruction, in ischemic acute renal failure are proposed.

Modulation of hypoxia-induced calpain activity in rat renal proximal tubules

The effect of the newly developed, nonpeptide, calpain inhibitor, PD 150606, on hypoxia and ionomycin-induced increases in calpain activity in rat proximal tubules (PT) was determined. PD150606 inhibited both hypoxia and ionomycin-induced calpain activity as determined by the fluorescent substrate N-succinyl-Leu-Leu-Val-Tyr-7-amido-4-methyl coumarin (N-succinyl-Leu-Leu-Val-Tyr-AMC). This decrease in calpain activity was accompanied by dose-dependent cytoprotection against hypoxia and ionomycin-induced cell membrane damage. PD150606 had no effect on cathepsin B and L activity in PT as measured by the fluorescent substrate, benzyloxycarbonyl-L-phenylalanyl-L-arginine-7-amido-4-methyl coumarin (Z-Phe-Arg-AMC). The effects of low intracellular pH (pHi) or low free cytosolic calcium [Ca2+]i on this hypoxia-induced calpain activity were also determined. Both low pHi and low [Ca2+]i attenuated the hypoxia-induced increase in calpain activity. This attenuation of calpain activity was observed early before hypoxia-induced membrane damage and was associated with marked reduction in the typical pattern of hypoxia-induced cell membrane damage observed in this model. To identify the isoform of calpain activated in rat proximal tubules, normoxic, hypoxic and ionomycin treated tubules were fractionated by MONO-Q anion exchange chromatography and the fractions were assayed for calpain activity. A single peak of calpain activity characteristic of mu-calpain was found. The calcium dependency of the calpain activity was in the nanomolar range, further confirming that the activity was the low Ca(2+)-sensitive mu-calpain. The present study suggests that in rat proximal tubules: (1) PD 150606 is a specific inhibitor of calpain and not cathepsins B and L; (2) the cytoprotective effects of low pHi and low [Ca2+]i are mediated, at least in part, by inhibition of calpain activity; and (3) the predominant active form of calpain is the isoenzyme mu-calpain.

Cyclooxygenase inhibition and vascular reactivity in a rat model of hyperdynamic sepsis

We postulated that the attenuated pulmonary and systemic vascular contractility observed in sepsis was secondary to the release of vasodilator prostaglandins. We used the cyclooxygenase inhibitor meclofenamate to inhibit prostaglandin synthesis in an unanesthetized, chronically instrumented model of hyperdynamic sepsis. Sixteen male Sprague-Dawley rats (300-350 g) were randomized to either sepsis induced by cecal ligation and perforation (CLP, n = 8) or a sham procedure (Sham, n = 8). Vascular reactivity was assessed by measuring the hypoxic (FiO2 = 0.08) pulmonary pressor response (HPV), and the systemic pressor response to an intravenous infusion of phenylephrine (1.5-7.5 micrograms/kg/min) before and after the administration of meclofenamate (5 mg/kg intravenously, i.v.). Twenty-four hours postoperatively, CLP animals had significantly increased cardiac output (CO) as compared with Sham animals (204 +/- 12 vs. 148 +/- 5 ml/min, p < 0.05), slightly decreased mean arterial pressure (MAP) (109 +/- 4 vs. 118 +/- 3 mm Hg, p < 0.05), and decreased total systemic vascular resistance (TSVR) (0.546 +/- 0.046 vs. 0.805 +/- 0.030 mm Hg.min.ml-1, p < 0.05). Mean pulmonary artery pressure (MPAP) and total pulmonary vascular resistance (TPVR) were similar in both groups (p > 0.05). In response to hypoxia, the change in MPAP (delta MPAP) was 3.6 +/- 1.0 and 6.9 +/- 0.8 (mm Hg) in CLP and Sham animals, respectively (p < 0.05). Similarly, the change in TPVR (delta TPVR) during hypoxia was 0.012 +/- 0.006 and 0.038 +/- 0.009 mm Hg.min.ml-1 in CLP and Sham (p < 0.05). The pulmonary and systemic blood pressure (BP) response to phenylephrine was also attenuated in CLP as compared with Sham animals. After treatment with meclofenamate, differences were no longer apparent in the HPV response between CLP and Sham animals, due to a slight increase in the HPV response of CLP animals and a slight decrease in the HPV response in Sham animals. The attenuated pressor response to phenylephrine was not changed in either the pulmonary or the systemic circulation after the administration of meclofenamate. These data suggest that vasodilator prostaglandins may contribute to the attenuated pulmonary pressor response in sepsis. However, the mechanism of the attenuated HPV may be different than the attenuated response to exogenous catecholamines since meclofenamate had no effect on either the pulmonary or systemic response to a phenylephrine infusion in septic animals.

Nitric oxide kinetics during hypoxia in proximal tubules: effects of acidosis and glycine

In the present study, we directly monitored nitric oxide (NO) with an amperometric NO-sensor in suspensions of rat proximal tubules. Hypoxia-stimulated NO generation was characterized by an initial rise and a subsequent sustained increase which preceded cell membrane damage as assessed by lactic dehydrogenase (LDH) release. In contrast, the NO concentration remained unmeasurable in normoxic controls. Nitro-L-arginine-methyl ester (L-NAME) prevented the hypoxia-induced increase in NO in a dose dependent manner in parallel with incremental cytoprotection. The hypoxia-induced elevation in NO and the associated membrane injury were both markedly prevented by extracellular acidosis (pH 6.95). In vitro proximal tubular nitric oxide synthase (NOS) activity (3H-arginine to 3H-citrulline assay) was pH dependent with optimum activity at pH 8.0 and greatly reduced activity at acidic pH even in the presence of calcium and co-factors. However, glycine, a well recognized cytoprotective agent, did not attenuate the NO concentration during hypoxia. The present study therefore provides direct evidence that NO is generated by rat proximal tubules during hypoxia and demonstrates that the protective effect of low pH against hypoxic rat tubular injury is associated with an inhibition of this NO production.

Arachidonic acid protects against hypoxic injury in rat proximal tubules

Free fatty acids (FFA) and lysophospholipids accumulate during hypoxia (H) in rat proximal tubular epithelial cells partly as a result of increased phospholipase A2 (PLA2) activity. The role of FFA in mediating hypoxic injury and modulating PLA2 activity is not clear. In the present study, the effect of several FFA including arachidonic acid (AA, 20:4) on hypoxia-induced injury and PLA2 activity was assessed in freshly isolated rat proximal tubules. Hypoxia (H) was induced in the presence of either an unsaturated free fatty acid (uFFA) [AA or linoleic acid (LA, 18:2)] or a saturated FFA (sFFA) [palmitic (PA, 16:0) or myristic acid (MA, 14:0)]. Cell membrane injury was assessed by measuring lactate dehydrogenase release (LDH). AA markedly reduced LDH release during hypoxia in a dose dependent manner, while sFFA had no protective effect. LA showed similar protection to that observed with AA. AA did not affect buffer calcium concentration, buffer pH, intracellular pH or intracellular calcium concentration. Neither inhibiting the cyclooxygenase pathway with indomethacin, nor the lipoxygenase pathway with nordihydroguaiaretic acid (NDGA) had any effect on the AA observed cytoprotection. In vitro PLA2 activity in the control tubular extracts was compared to that following addition of AA or PA. PLA2 activity decreased significantly with AA but not with PA in a dose dependent manner. These data suggest that: (1) AA protects against hypoxic injury in rat proximal tubules. (2) This cytoprotection is not specific for AA and other uFFA have a similar effect. (3) AA significantly inhibits PLA2 activity, (4) AA induced cytoprotection may be related to a negative feedback inhibition of PLA2 activity.

Induced alterations in calcium uptake rate in normoxic rate proximal tubules

This study is well-oxygenated, freshly isolated rat proximal tubules (RPT), examined the effects of several drugs that alter the transmembrane K+ and Na+ gradients across cell membranes, including valinomycin (VAL), amphotericin B (AMPHO), and ouabain (OUAB). The effects of high extracellular potassium chloride (KCl) concentrations (45 mM) and low extracellular sodium concentration (100mM) were also studied. After 10 min of drug exposure Ca2+ uptake rate (nmol/mg/min) increased from 2.7 to 3.8 with VAL (p < .02), from 2.9 to 3.7 with AMPHO (p < .05), from 3.6 to 4.1 with OUAB (p < .05), and from 3.2 to 4.8 with 45 mM KCl (p < .001). Ca2+ uptake rate was sustained at these high levels at 20 min in all treated RPT except those exposed to OUAB. LDH release averaged less than 15% in control tubules and did not increase significantly except in RPT treated with VAL, where LDH release at 10 min was 48% and at 20 min was 57% (both p < .001). Of importance, only in VAL-treated RPT did ATP decrease to low levels (6.7 nmol/mg in control to 2.0 +/- 0.3 nmol/mg in VAL, p < .001). Treatment with verapamil reduced Ca2+ uptake rates at 10 min in VAL-treated RPT (from 3.8 to 3.1, p < .02, in AMPHO-treated RPT (from 3.8 to 3.1 p < .001), in OUAB-treated tubules (from 4.0 to 3.4, p < .01), and in KCl-treated RPT (from 3.7 to 3.2, p < .01). These results indicate that acute changes in the transmembrane ion gradient in RPT are accompanied by increased Ca2+ uptake rates. Ca2+ uptake rates are also increased during O2 deprivation in RPT, a situation in which the transmembrane ion gradient is likewise altered. The increased Ca2+ uptake rate observed in the present study and during hypoxia may have a common basis, that is, altered transmembrane ion gradients or some function thereof.

Role of [Ca2+]i in lethal oxidative injury in rat cultured inner medullary collecting duct cells

Reactive oxygen metabolites have been implicated in the pathogenesis of toxic, ischaemic and immunologically mediated renal injury. An increase in the cytosolic free Ca2+ concentration ([Ca2+]i) has been proposed as a mechanism of oxidative stress-induced cell injury. We used a fluorescence spectrometer and a fluorescence probe to measure the [Ca2+]i and viability of rat primary cultured inner medullary collecting duct (IMCD) cells during oxidative stress induced by 5 mM tert-butyl hydroperoxide (TBHP). Initially, this oxidative stress evoked a small increase in [Ca2+]i which was followed by a slower sustained increase from the resting level of 170.8 +/- 38.8 nM to 1490.5 +/- 301.7 nM after 60 min, and this preceded the loss of plasma membrane integrity, measured by the propidium iodide fluorescence method. The elimination of extracellular Ca2+ from the culture medium prevented the TBHP-induced [Ca2+]i increase and improved cell viability. Restoration of extracellular Ca2+ resulted in an immediate and large increase in [Ca2+]i and extensive cell death. Verapamil, a Ca2+ channel blocker, inhibited the [Ca2+]i increase and afforded significant protection against cellular injury following exposure to TBHP-induced oxidative stress. Extracellular acidosis also prevented the increase in [Ca2+]i and cell death caused by this oxidative stress. These results are consistent with the hypothesis that oxidative stress-induced IMCD cellular injury may be the result of increased [Ca2+]i caused, in part, by activation of voltage-dependent Ca2+ channels.

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)

Renal impairment after spontaneous bacterial peritonitis in cirrhosis: incidence, clinical course, predictive factors and prognosis

Although spontaneous bacterial peritonitis is considered a precipitating factor of renal impairment in cirrhosis, no study specifically addressing this problem has been reported. This study was aimed at assessing the incidence, clinical course, predictive factors and prognosis of renal impairment in cirrhotic patients with peritonitis. Therefore, 252 consecutive episodes of spontaneous bacterial peritonitis in 197 patients were analyzed. Clinical and laboratory data obtained before and after diagnosis of peritonitis were considered as possible predictors of renal impairment and hospital mortality. Renal impairment occurred in 83 (33%) episodes, and in every instance it fulfilled the criteria of functional kidney failure. Renal impairment was progressive in 35 episodes, steady in 27 and transient in 21. Blood urea nitrogen and serum sodium concentration before peritonitis and band neutrophils count in blood at diagnosis were independent predictors for the development of renal impairment. Renal impairment was the strongest independent predictor of mortality during hospitalization. Other independent prognostic factors were blood urea nitrogen level before peritonitis, age, positive ascitic fluid culture and serum bilirubin level during infection. These results indicate that renal impairment is a frequent event in cirrhotic patients with spontaneous bacterial peritonitis that occurs mainly in patients with kidney failure before infection. Renal impairment is the most important predictor of hospital mortality in cirrhotic patients with spontaneous bacterial peritonitis.

Neuroprotective effects of glutamate antagonists and extracellular acidity

Glutamate antagonists protect neurons from hypoxic injury both in vivo and in vitro, but in vitro studies have not been done under the acidic conditions typical of hypoxia-ischemia in vivo. Consistent with glutamate receptor antagonism, extracellular acidity reduced neuronal death in murine cortical cultures that were deprived of oxygen and glucose. Under these acid conditions, N-methyl-D-aspartate and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-kainate antagonists further reduced neuronal death, such that some neurons tolerated prolonged oxygen and glucose deprivation almost as well as did astrocytes. Neuroprotection induced by this combination exceeded that induced by glutamate antagonists alone, suggesting that extracellular acidity has beneficial effects beyond the attenuation of ionotropic glutamate receptor activation.

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.

Physiological pH. Effects on posthypoxic proximal tubular injury

After O2 deprivation, tissue acidosis rapidly self-corrects. This study assessed the effect of this pH correction on the induction, and pathways, of posthypoxic proximal tubular injury. In addition, ways to prevent the resultant injury were explored. Isolated rat proximal tubular segments (PTSs) were subjected to hypoxia/reoxygenation (50/30 or 30/50 minutes) under the following incubation conditions: 1) continuous pH 7.4, 2) continuous pH 6.8, or 3) hypoxia at pH 6.8 and reoxygenation at pH 7.4 (NaHCO3 or Tris base addition). Continuously oxygenated PTSs maintained under these same pH conditions served as controls. Lethal cell injury was assessed by lactate dehydrogenase (LDH) release. pH effects on several purported pathways of hypoxia/reoxygenation injury were also assessed (ATP depletion, lipid peroxidation, and membrane deacylation). Acidosis blocked hypoxic LDH release (pH 7.4, 50 +/- 2%; pH 6.8, 6 +/- 1%) without mitigating membrane deacylation or ATP depletion. During reoxygenation, minimal LDH was released (3-5%) if pH was held constant. However, if posthypoxic pH was corrected, immediate (< or = 5 minutes) and marked cell death (e.g., 55 +/- 3% with Tris) occurred. This was dissociated from lipid peroxidation or new deacylation, and it was preceded by a depressed ATP/ADP ratio (suggesting an acidosis-associated defect in hypoxic/posthypoxic cell energetics). Realkalinization injury was not inevitable, since it could be substantially blocked by 1) posthypoxic glycine addition, 2) transient posthypoxic hypothermia, or 3) allowing a 10-minute reoxygenation (cell recovery) period before base addition. Neither mannitol nor graded buffer Ca2+ deletion conferred protection. Acute pH correction caused no injury to continuously oxygenated PTSs. Conclusions are as follows: 1) Posthypoxic "pH shock" causes virtually immediate cell death, not by causing de novo injury but, rather, by removing the cytoprotective effect of acidosis. 2) This injury can be prevented by a variety of methods, indicating a great potential for salvaging severely damaged posthypoxic PTSs.

Hypoxia and metabolic acidosis in the isolated heart: evidence for synergistic injury

Although hypoxia and metabolic acidosis have both been shown to impair cardiac function, some workers have suggested that acidosis during a period of hypoxia will actually accelerate physiologic recovery from this insult. To address the interactions of metabolic acidosis and hypoxia further, isolated isovolumic rat hearts were exposed to normal perfusion conditions for 30 min to establish baseline conditions, then either continued normal conditions, metabolic acidosis, hypoxia, or combined acidosis and hypoxia for 30 min and subsequently reperfused under normal perfusion conditions for an additional 30 min. We observed that acidosis + hypoxia impaired recovery of cardiac contraction more than acidosis or hypoxia alone following experimental perfusion. The combination of acidosis and and hypoxia also impaired cardiac energy metabolism more than acidosis or hypoxia alone as assessed by increases in tissue inorganic phosphate during experimental perfusion as well as during reperfusion. These data suggest that during hypoxia, acidosis appears to primarily impair cardiac energy production as we have previously observed in the normoxic isolated rat heart. Therefore, in the intact beating heart, acidosis may not protect from hypoxic injury as has been suggested in simpler systems but may not protect from hypoxic injury as has been suggested in simpler systems but rather may exacerbate at.

Acute phosphate depletion and in vitro rat proximal tubule injury: protection by glycine and acidosis

The effects of phosphate (PO4) removal from Krebs Henseleit buffer on freshly isolated rat proximal tubules (rPT) were assessed by measuring Ca2+ uptake (nmol/mg protein), cellular adenosine triphosphate (ATP) (nmol/mg), tissue K+ content (nmol/mg) and lactate dehydrogenase (LDH) as an index of cell integrity. Ca2+ uptake increased by 50% in rPT incubated in zero PO4 medium as compared to control (2.6 +/- 0.1 vs. 3.9 +/- 0.19, P less than 0.001) and LDH release increased 2.5-fold from 14.2 +/- 0.6 to 31.6 +/- 1.6%, P less than 0.001. Neither verapamil (200 microM) nor mepacrine (50 microM) reduced Ca2+ uptake or decreased LDH release suggesting that the increased Ca2+ uptake was not occurring through potential operated channels and that phospholipase-induced cell injury was not the cause of increased LDH release. Either glycine (2 mM) or extracellular fluid acidosis (pH 7.06), however, significantly diminished rPT injury and Ca2+ uptake. Specifically, as compared to the increased LDH released in untreated. PO4-depleted rPT, LDH release was diminished significantly by glycine treatment (31.0 +/- 0.9 vs. 15.5 +/- 1.6%, P less than 0.001) or acidosis (30.3 +/- 0.04 vs. 19.2 +/- 0.9%, P less than 0.01). Ca2+ uptake did not increase in glycine treated tubules (2.6 +/- 0.1 vs. 2.8 +/- 0.2 nmol/mg, NS) or in the presence of acidosis (2.6 +/- 0.1 vs. 2.97 +/- 0.17 nmol/mg, NS). ATP concentrations were markedly reduced by PO4 depletion (2.8 +/- 0.2 vs. 4.8 +/- 0.3 nmol/mg, P less than 0.001) and remained at low levels during either acidosis or glycine-induced protection.(ABSTRACT TRUNCATED AT 250 WORDS)

A selective thromboxane synthetase inhibitor, OKY-046, fails to improve blood rheology in endotoxin-shocked rabbits

Effects of a selective thromboxane synthetase inhibitor, (E)-3-[4-(1-imidazolylmethyl)phenyl]-2-propenoic acid hydrochloride monohydrate (OKY-046), were studied hemorheologically in endotoxin shocked-rabbits. The animals were intravenously administrated with 0.1 mg of endotoxin 3 times at intervals of 3 days. At 7 days after the last endotoxin injection, endotoxin (0.2 mg.kg(-1)) was intravenously administrated to induce a shock. OKY-046 (30 mg.kg(-1)) was administrated after hypotension was developed by the endotoxin treatment and, then, it was continuously injected at 0.03 mg.kg(-1).min(-1). Blood pressure remained unchanged and hypotensive was maintained during the treatment with OKY-046. Blood was sampled from the femoral artery 15 (before the administration of OKY-046), 45, and 120 minutes after the final administration of endotoxin. Pa(O)(2) increased, and Pa(CO)(2), arterial pH, and base excess (BE) decreased during the endotoxin shock. The decrease of pH and BE was prevented by the administration of OKY-046. In the endotoxin-shocked animals, hematocrit, whole blood viscosity, erythrocyte deformability, plasma fluidity, and the ratio of hematocrit to whole blood viscosity showed no significant differences between the OKY-046 treated animals and non-treated ones. These data show that a selective thromboxane synthetase inhibitor (OKY-046) does not improve the blood rheology during endotoxin shock, although it seems to prevent the acidosis in some extent.

Calcium handling by renal tubules during oxygen deprivation injury to the kidney prior to reoxygenation

Efforts to more precisely define the mechanism(s) of ischemic injury to renal epithelial tissue, both during O2 deprivation and reflow, have led to the expanding use of freshly isolated renal tubules. This tissue is prepared in a manner that eliminates the impact of changes in vascular resistance and influences of hormones, as well as temporal changes in pH and in regional pO2, and has permitted investigators to focus on the definitive cellular responses to O2 deprivation itself. When such studies are evaluated it becomes clear that in vitro anoxia, for up to 60 minutes, is not associated with any increase in total tissue Ca2+ as measured by atomic absorption techniques, whereas severe hypoxia is attended by a time-dependent increase in total tissue Ca2+. In both severe hypoxia and anoxia, however, the Ca2+ uptake rate is increased, but during hypoxia the less severe acidosis, as well as the continued, albeit modest, mitochondrial energization, appears to facilitate mitochondrial (and thus total tissue) Ca2+ accumulation. In vivo and in vitro, the administration of calcium channel blockers (CCB) attenuates renal oxygen-deprivation-induced injury and one, often overlooked, effect of verapamil, a prototypical CCB, is to reduce K+ loss from treated tissue via inhibition of Ca2(+)-mediated K+ efflux pathways. This may be the cause of the higher levels of K+ reported for several tissues, including kidney tubules, during CCB treatment. In addition, reduced rates of Ca2+ uptake effected by CCB may modify cytosolic free Ca2+ levels such that activation of phospholipases is impaired.(ABSTRACT TRUNCATED AT 250 WORDS)

Enzyme release from chick myocytes during hypoxia and reoxygenation: dependence on pH

On reoxygenation of ischemic or hypoxic hearts a sudden release of cytosolic enzymes coupled with hypercontraction and cell injury occurs, which has been termed the "oxygen paradox". We have attempted to imitate this phenomenon in cultured chick myocytes to try to find the cause of this sudden enzyme release. During 4 hours of normoxic perfusion (pH 7.4) monolayer cultures of chick embryonic myocytes retain their normal morphology, beat rhythmically, and show no release of creatine kinase (CK) into the perfusate. Hypoxic perfusion (O2 less than or equal to 0.25 microliter/ml) stops cell contraction (15-20 min) and causes "blebbing" of the sarcolemma (20-30 min). Membrane blebs increase in size and number with continuing hypoxia and eventually the cells become irreversibly damaged. Perfusion at pH 7.4 leads to a release of CK shortly after membrane damage occurs (30-40 min), with peak enzyme levels at 60-90 min. Reoxygenation after 120 min hypoxia does not exacerbate release. Hypoxic perfusion at pH 7.0 suppresses the release of CK from the cells despite extensive membrane blebbing. Normoxic perfusion at pH 7.4 after 100 min hypoxia (pH 7.0) causes an efflux of enzyme from the irreversibly injured cells. This can be prevented by reoxygenating the cells at pH 7.0 and stimulated by raising the pH of the hypoxic perfusate to 7.4. Shorter hypoxic periods (30 mins) at pH 7.0 followed by normoxic perfusion at pH 7.4 lead to a sudden large efflux of CK, arrhythmic contractions and hypercontraction of myofilaments, i.e. the typical symptoms of the "oxygen paradox". Thus changes in external pH can influence the release of intracellular enzymes during hypoxia and reoxygenation.