Hypoxia and acidosis impair cGMP synthesis in microvascular coronary endothelial cells

Carbonic Anhydrase Inhibition Ameliorates Inflammation and Experimental Pulmonary Hypertension

Inflammation and vascular smooth muscle cell (VSMC) phenotypic switching are causally linked to pulmonary arterial hypertension (PAH) pathogenesis. Carbonic anhydrase inhibition induces mild metabolic acidosis and exerts protective effects in hypoxic pulmonary hypertension. Carbonic anhydrases and metabolic acidosis are further known to modulate immune cell activation. To evaluate if carbonic anhydrase inhibition modulates macrophage activation, inflammation, and VSMC phenotypic switching in severe experimental pulmonary hypertension, pulmonary hypertension was assessed in Sugen 5416/hypoxia (SU/Hx) rats after treatment with acetazolamide or ammonium chloride (NH4Cl). We evaluated pulmonary and systemic inflammation and characterized the effect of carbonic anhydrase inhibition and metabolic acidosis in alveolar macrophages and bone marrow-derived macrophages (BMDMs). We further evaluated the treatment effects on VSMC phenotypic switching in pulmonary arteries and pulmonary artery smooth muscle cells (PASMCs) and corroborated some of our findings in lungs and pulmonary arteries of patients with PAH. Both patients with idiopathic PAH and SU/Hx rats had increased expression of lung inflammatory markers and signs of PASMC dedifferentiation in pulmonary arteries. Acetazolamide and NH4Cl ameliorated SU/Hx-induced pulmonary hypertension and blunted pulmonary and systemic inflammation. Expression of carbonic anhydrase isoform 2 was increased in alveolar macrophages from SU/Hx animals, classically (M1) and alternatively (M2) activated BMDMs, and lungs of patients with PAH. Carbonic anhydrase inhibition and acidosis had distinct effects on M1 and M2 markers in BMDMs. Inflammatory cytokines drove PASMC dedifferentiation, and this was inhibited by acetazolamide and acidosis. The protective antiinflammatory effect of acetazolamide in pulmonary hypertension is mediated by a dual mechanism of macrophage carbonic anhydrase inhibition and systemic metabolic acidosis.

Evaluation of Energy Balance on Human Telomerase Reverse Transcriptase (hTERT) Alternative Splicing by Semi-quantitative RT-PCR in Human Umbilical Vein Endothelial Cells

Background:

Decreased high-energy phosphate level is involved in endothelial cell injury and dysfunction. Reduced telomerase activity in endothelial cells in parallel with reduced energy levels might be due to altered direction of alternative splicing machine as a complication of depleted energy during the process of atherosclerosis.

Materials and Methods:

Isolated human umbilical vein endothelial cells (HUVECs) were treated for 24 hours by oligomycine (OM) and 2-deoxy glucose (2-DG). After 24 hours, the effect of energy depletion on telomerase splicing pattern was evaluated using RT-PCR. Indeed, in both treated and untargeted cells, nitric oxide (NO) and von Willebrand factor (vWF) were measured.

Results:

ATP was depleted in treated cells by 43.9% compared with control group. We observed a slight decrease in NO levels (P = 0.09) and vWF (P = 0.395) in the setting of 49.36% ATP depletion. In both groups, no telomerase gene expression was seen. Telomerase and housekeeping gene expression were found in positive control group (colon cancer tissue) and sample tissue.

Conclusions:

The absence of telomerase gene expression in HUVECs might be due to the mortality of these cells or the low level of telomerase gene expression in these cells under normal circumstances.

Endothelial actions of atrial natriuretic peptide prevent pulmonary hypertension in mice

The cardiac hormone atrial natriuretic peptide (ANP) regulates systemic and pulmonary arterial blood pressure by activation of its cyclic GMP-producing guanylyl cyclase-A (GC-A) receptor. In the lung, these hypotensive effects were mainly attributed to smooth muscle-mediated vasodilatation. It is unknown whether pulmonary endothelial cells participate in the homeostatic actions of ANP. Therefore, we analyzed GC-A/cGMP signalling in lung endothelial cells and the cause and functional impact of lung endothelial GC-A dysfunction. Western blot and cGMP determinations showed that cultured human and murine pulmonary endothelial cells exhibit prominent GC-A expression and activity which were markedly blunted by hypoxia, a condition known to trigger pulmonary hypertension (PH). To elucidate the consequences of impaired endothelial ANP signalling, we studied mice with genetic endothelial cell-restricted ablation of the GC-A receptor (EC GC-A KO). Notably, EC GC-A KO mice exhibit PH already under resting, normoxic conditions, with enhanced muscularization of small arteries and perivascular infiltration of inflammatory cells. These alterations were aggravated on exposure of mice to chronic hypoxia. Lung endothelial GC-A dysfunction was associated with enhanced expression of angiotensin converting enzyme (ACE) and increased pulmonary levels of Angiotensin II. Angiotensin II/AT1-blockade with losartan reversed pulmonary vascular remodelling and perivascular inflammation of EC GC-A KO mice, and prevented their increment by chronic hypoxia. This experimental study indicates that endothelial effects of ANP are critical to prevent pulmonary vascular remodelling and PH. Chronic endothelial ANP/GC-A dysfunction, e.g. provoked by hypoxia, is associated with activation of the ACE-angiotensin pathway in the lung and PH.

Calcium-mediated cell death during myocardial reperfusion

Reperfusion may induce additional cell death in patients with acute myocardial infarction receiving primary angioplasty or thrombolysis. Altered intracellular Ca(2+) handling was initially considered an essential mechanism of reperfusion-induced cardiomyocyte death. However, more recent studies have demonstrated the importance of Ca(2+)-independent mechanisms that converge on mitochondrial permeability transition (MPT) and are shared by cardiomyocytes and other cell types. This article analyses the importance of Ca(2+)-dependent cell death in light of these new observations. Altered Ca(2+) handling includes increased cytosolic Ca(2+) levels, leading to activation of calpain-mediated proteolysis and sarcoplasmic reticulum-driven oscillations; this can induce hypercontracture, but also MPT due to the privileged Ca(2+) transfer between sarcoplasmic reticulum and mitochondria through cytosolic Ca(2+) microdomains. In the opposite direction, permeability transition can worsen altered Ca(2+) handling and favour hypercontracture. Ca(2+) appears to play an important role in cell death during the initial minutes of reperfusion, particularly after brief periods of ischaemia. Developing effective and safe treatments to prevent Ca(2+)-mediated cardiomyocyte death in patients with transient ischaemia, by targeting Ca(2+) influx, intracellular Ca(2+) handling, or Ca(2+)-induced cell death effectors, is an unmet challenge with important therapeutic implications and large potential clinical impact.

Contribution of delayed intracellular pH recovery to ischemic postconditioning protection

Ischemic postconditioning (PoCo) has been proven to be a feasible approach to attenuate reperfusion injury and enhance myocardial salvage in patients with acute myocardial infarction, but its mechanisms have not been completely elucidated yet. Recent studies demonstrate that PoCo may delay the recovery of intracellular pH during initial reperfusion, and that its ability to limit infarct size critically depends on this effect. Prolongation of postischemic intracellular acidosis inhibits hypercontracture, mitochondrial permeability transition, calpain-mediated proteolysis, and gap junction-mediated spread of injury during the first minutes of reflow. This role of prolonged acidosis does not exclude the participation of other pathways in PoCo-induced cardioprotection. On the contrary, it may allow these pathways to act by preventing immediate reperfusion-induced cell death. Moreover, the existence of interactions between intracellular acidosis and endogenous protection signaling cannot be excluded and needs to be investigated. The role of prolonged acidosis in PoCo cardioprotection has important implications in the design of optimal PoCo protocols and in the translation of cardioprotective strategies to patients with on-going myocardial infarction receiving coronary reperfusion.

Heme oxygenase-1 does not mediate the effects of extracellular acidosis on vascular smooth muscle cell proliferation, migration, and susceptibility to apoptosis

BACKGROUND

Unbalanced vascular smooth muscle cell (VSMC) proliferation, migration, and apoptosis contribute to vascular disorders such as atherosclerosis, restenosis, and pulmonary hypertension. The effect of extracellular acidosis (EA) on VSMC homeostasis is incompletely understood but we previously reported that EA increases heme oxygenase-1 (HO-1) expression in VSMCs. Since HO-1 regulates VSMC proliferation and apoptosis we sought to define the role of HO-1 in VSMC responses to EA.

METHODS

Mouse aortic smooth muscle cells (MASMCs) were isolated from wild-type and HO-1-null mice. Cell proliferation and migration assays were done in a physiologic pH (7.4) or EA (pH 6.8). VSMC apoptosis in response to hydrogen peroxide was assessed by JC-1 staining, caspase-3 cleavage, annexin V, and Hoechst staining.

RESULTS

Wild-type MASMCs showed decreased proliferation and migration at pH 6.8 compared to pH 7.4. This observation was also true in HO-1-null MASMCs. Although wild-type and HO-1-null cells showed differences in the mode and kinetics of cell death, both genotypes exhibited increased susceptibility to hydrogen peroxide-induced apoptosis at pH 6.8 compared to 7.4.

CONCLUSIONS

EA inhibits VSMC proliferation and migration and increases susceptibility to oxidant-induced apoptosis. These effects of acidosis on VSMC homeostasis are independent of HO-1.

Calcium and cyclic nucleotides affect TNF-alpha-induced stem cell migration

The purpose of this study was to study the effect of calcium, cyclic AMP (cAMP) and cyclic GMP (cGMP) on embryonic stem cell (ESC) motility during TNF-alpha-induced chemotaxis. ESCs were monitored using a chemotaxis chamber, with different concentrations of calcium or cAMP or cGMP added to the medium. Changes in intracellular calcium ([Ca(2+)](i)) were measured with the fluorescent dye fura-2/AM. We combined migratory parameters in a mathematical model and described it as "mobility". After adding calcium, a dose-dependant increase in cell speed was found. Cyclic AMP increased mobility as well as the [Ca(2+)](i). In contrast, adding dbcGMP resulted in a significant decrease in the mobility of the ESCs. During migration ESCs showed an increase in [Ca(2+)](i). Furthermore, TNF-alpha dramatically increased the movement as well as the directionality of ESCs. These results demonstrate that ESCs are highly motile and respond to different concentrations of calcium in a dose-related manner.

Acidic reoxygenation protects against endothelial dysfunction in rat aortic rings submitted to simulated ischemia

Ischemia-reperfusion causes endothelial dysfunction. Prolongation of acidosis during initial cardiac reperfusion limits infarct size in animal models, but the effects of acidic reperfusion on vascular function are unknown. The present work analyzes the effects of acidic reoxygenation on vascular responses to different agonists in rat aortic rings. Arterial rings obtained from Sprague-Dawley rat aorta were placed in organ baths containing a Krebs solution oxygenated at 37 degrees C (pH 7.4). After equilibration (30 mN, 1 h), the effects of acidosis (pH 6.4) on aortic responses to acetylcholine and norepinephrine were initially assessed under normoxic conditions. Thereafter, the effects of acidosis during hypoxia (1 h) or reoxygenation on aortic responses to acetylcholine, norepinephrine, or sodium nitroprusside were analyzed and compared with those observed in control rings. Acidosis did not modify aortic responses to acetylcholine or adrenaline during normoxia. In contrast, rings submitted to hypoxia and reoxygenated at pH 7.4 showed a reduction in vasodilator responses to acetylcholine and in contractions to norepinephrine with no change in responses to sodium nitroprusside. Reoxygenation at pH 6.4 did not modify the depressed response to norepinephrine but enhanced the recovery of acetylcholine-induced vasorelaxation. Cumulative concentration-response curves to acetylcholine showed an increased responsiveness to this drug in rings reoxygenated at a low pH. This functional improvement was associated with the preservation of aortic cGMP content after stimulation of reoxygenated rings with acetylcholine. In conclusion, acidic reoxygenation preserves endothelial function in arterial rings submitted to simulated ischemia, likely through the preservation of cGMP signaling.

Intermittent antegrade cardioplegia: isolated heart preservation with the Asporto heart preservation device

BACKGROUND

A major problem in procurement of donor hearts is the limited time a donor heart remains viable. After cardiectomy, ischemic hypoxia is the main cause of donor heart degradation. The global myocardial ischemia causes a cascade of oxygen radical formation that cumulates in an elevation in hydrogen ions (decrease in pH), irreversible cellular injury, and potential microvascular changes in perfusion.

OBJECTIVE

To determine the changes of prolonged storage times on donor heart microvasculature and the effects of intermittent antegrade perfusion.

MATERIALS AND METHODS

Using porcine hearts flushed with a Ribosol-based cardioplegic solution, we examined how storage time affects microvascular myocardial perfusion by using contrast-enhanced magnetic resonance imaging at a mean (SD) of 6.1 (0.6) hours (n = 13) or 15.6 (0.6) hours (n = 11) after cardiectomy. Finally, to determine if administration of cardioplegic solution affects pH and microvascular perfusion, isolated hearts (group 1, n = 9) given a single antegrade dose, were compared with hearts (group 2, n = 8) given intermittent antegrade cardioplegia (150 mL, every 30 min, 150 mL/min) by a heart preservation device. Khuri pH probes in left and right ventricular tissue continuously measured hydrogen ion levels, and perfusion intensity on magnetic resonance images was plotted against time.

RESULTS

Myocardial perfusion measured via magnetic resonance imaging at 6.1 hours was significantly greater than at 15.6 hours (67% vs 30%, P = .00008). In group 1 hearts, the mean (SD) for pH at the end of 6 hours decreased to 6.2 (0.2). In group 2, hearts that received intermittent antegrade cardioplegia, pH at the end of 6 hours was higher at 6.7 (0.3) (P = .0005). Magnetic resonance imaging showed no significant differences between the 2 groups in contrast enhancement (group 1, 62%; group 2, 40%) or in the wet/dry weight ratio.

CONCLUSION

Intermittent perfusion maintains a significantly higher myocardial pH than does a conventional single antegrade dose. This difference may translate into an improved quality of donor hearts procured for transplantation, allowing longer distance procurement, tissue matching, improved outcomes for transplant recipients, and ideally a decrease in transplant-related costs.

Intermittent Antegrade Cardioplegia: Isolated Heart Preservation with the Asporto Heart Preservation Device

Background

A major problem in procurement of donor hearts is the limited time a donor heart remains viable. After cardiectomy, ischemic hypoxia is the main cause of donor heart degradation. The global myocardial ischemia causes a cascade of oxygen radical formation that cumulates in an elevation in hydrogen ions (decrease in pH), irreversible cellular injury, and potential microvascular changes in perfusion.

Objective

To determine the changes of prolonged storage times on donor heart microvasculature and the effects of intermittent antegrade perfusion.

Materials and Methods

Using porcine hearts flushed with a Ribosol-based cardioplegic solution, we examined how storage time affects microvascular myocardial perfusion by using contrast-enhanced magnetic resonance imaging at a mean (SD) of 6.1 (0.6) hours (n=13) or 15.6 (0.6) hours (n=11) after cardiectomy. Finally, to determine if administration of cardioplegic solution affects pH and microvascular perfusion, isolated hearts (group 1, n=9) given a single antegrade dose, were compared with hearts (group 2, n=8) given intermittent antegrade cardioplegia (150 mL, every 30 min, 150 mL/min) by a heart preservation device. Khuri pH probes in left and right ventricular tissue continuously measured hydrogen ion levels, and perfusion intensity on magnetic resonance images was plotted against time.

Results

Myocardial perfusion measured via magnetic resonance imaging at 6.1 hours was significantly greater than at 15.6 hours (67% vs 30%, P=.00008). In group 1 hearts, the mean (SD) for pH at the end of 6 hours decreased to 6.2 (0.2). In group 2, hearts that received intermittent antegrade cardioplegia, pH at the end of 6 hours was higher at 6.7 (0.3) ( P=.0005). Magnetic resonance imaging showed no significant differences between the 2 groups in contrast enhancement (group 1, 62%; group 2, 40%) or in the wet/dry weight ratio.

Conclusion

Intermittent perfusion maintains a significantly higher myocardial pH than does a conventional single antegrade dose. This difference may translate into an improved quality of donor hearts procured for transplantation, allowing longer distance procurement, tissue matching, improved outcomes for transplant recipients, and ideally a decrease in transplant-related costs.

A proton-activated, outwardly rectifying chloride channel in human umbilical vein endothelial cells

Extracellular acidic pH-activated chloride channel I(Cl, acid), has been characterized in HEK 293 cells and mammalian cardiac myocytes. This study was designed to characterize I(Cl,acid) in human umbilical vein endothelial cells(HUVECs). The activation and deactivation of the current rapidly and repeatedly follows the change of the extracellular solution at pH 4.3, with the threshold pH 5.3. In addition, at very positive potentials, the current displays a time-dependent facilitation. pH-response relationship for I(Cl,acid) revealed that EC(50) is pH 4.764 with a threshold pH value of pH 5.3 and nH of 14.545. The current can be blocked by the Cl(-) channel inhibitor DIDS (100 microM). In summary, for the first time we report the presence of proton-activated, outwardly rectifying chloride channel in HUVECs. Because an acidic environment can develop in local myocardium under pathological conditions such as myocardial ischemia, I(Cl,acid) would play a role in regulation of EC function under these pathological conditions.

Endothelium and cardiopulmonary resuscitation

The endothelium is a viable target for injury, repair and cellular modulation. Because of its vast extension and active metabolic status of producing mediators for vasomotor tone, coagulation, and inflammation, it is a key target for therapy during ischemia/reperfusion injury. Cardiopulmonary resuscitation is a model of whole-body ischemia/reperfusion injury. It has become apparent that the endothelium participates in a host of responses elicited by ischemia/reperfusion. This review examines the role of the endothelium during and after ischemia/reperfusion and the participation by its mediators and evidence for endothelial involvement during and after cardiopulmonary resuscitation. The strategic location of the endothelium makes it an excellent signal transduction mechanism for a host of disease processes. In addition to biochemical stimuli, mechanical stimulation of the endothelium elicits production of several mediators, including endothelium-derived nitric oxide, prostaglandins, and antithrombotics and anticoagulants. Whole-body, periodic acceleration is a novel method of stimulating the endothelium via pulsatile shear stress. Periodic acceleration has been shown to be an effective experimental method of cardiopulmonary resuscitation, with evidence of postresuscitation cardioprotective effects. This review indicates that understanding endothelial modulation during and after ischemia/reperfusion will significantly improve therapeutic choices.

Role of nitric oxide in capillary perfusion and oxygen delivery regulation during systemic hypoxia

The role of nitric oxide (NO) and reactive oxygen species (ROS) in regulating capillary perfusion was studied in the hamster cheek pouch model during normoxia and after 20 min of exposure to 10% O2-90% N2. We measured PO2 by using phosphorescence quenching microscopy and ROS production in systemic blood. Identical experiments were performed after treatment with the NO synthase inhibitor NG-monomethyl-L-arginine (L-NMMA) and after the reinfusion of the NO donor 2,2'-(hydroxynitrosohydrazono)bis-etanamine (DETA/NO) after treatment with L-NMMA. Hypoxia caused a significant decrease in the systemic PO2. During normoxia, arteriolar intravascular PO2 decreased progressively from 47.0 +/- 3.5 mmHg in the larger arterioles to 28.0 +/- 2.5 mmHg in the terminal arterioles; conversely, intravascular PO2 was 7-14 mmHg and approximately uniform in all arterioles. Tissue PO2 was 85% of baseline. Hypoxia significantly dilated arterioles, reduced blood flow, and increased capillary perfusion (15%) and ROS (72%) relative to baseline. Administration of L-NMMA during hypoxia further reduced capillary perfusion to 47% of baseline and increased ROS to 34% of baseline, both changes being significant. Tissue PO2 was reduced by 33% versus the hypoxic group. Administration of DETA/NO after L-NMMA caused vasodilation, normalized ROS, and increased capillary perfusion and tissue PO2. These results indicate that during normoxia, oxygen is supplied to the tissue mostly by the arterioles, whereas in hypoxia, oxygen is supplied to tissue by capillaries by a NO concentration-dependent mechanism that controls capillary perfusion and tissue PO2, involving capillary endothelial cell responses to the decrease in lipid peroxide formation controlled by NO availability during low PO2 conditions.

Extracellular acidosis induces heme oxygenase-1 expression in vascular smooth muscle cells

Extracellular acidosis (EA) has profound effects on vascular homeostasis, including vascular bed-specific alterations in vascular tone. Regulation of gene expression by EA has been observed in a variety of cells including vascular endothelial cells. Whether EA regulates gene expression in vascular smooth muscle cells (VSMCs) is not known. Heme oxygenase (HO)-1 is expressed in vascular cells, and its expression is regulated by cellular stressors such as heat, radiation, and hypoxia. Increased HO-1 expression in VSMCs leads to increased production of CO and its second messenger cGMP, which are important regulators of vascular tone and paracrine interactions in the vasculature. We examined whether EA regulates the expression of HO-1 in VSMCs. Exposure of VSMCs to acidic medium (pH 6.8) significantly increased HO-1 mRNA and protein compared with exposure to medium of physiological pH (pH 7.4). The acidic induction of HO-1 expression was time dependent and involved both transcriptional activation of the HO-1 gene and enhanced stability of HO-1 mRNA. Nitric oxide did not appear to mediate this response. We conclude that HO-1 is transcriptionally and posttranscriptionally upregulated by EA in VSMCs. This induction is time dependent and reversible. We speculate that EA, as an important tissue and cellular stressor for VSMCs, may elicit changes in gene expression patterns that contribute to the maintenance or disruption of vascular homeostasis.

Microvascular oxygenation, oxidative stress, NO suppression and superoxide dismutase during postischemic reperfusion

Increased formation of reactive oxygen species (ROS) on reperfusion after ischemia underlies ischemia-reperfusion (I/R) damage. We measured, in real time, oxygen tension in both microvessels and tissue and oxidant stress during postischemic reperfusion in the hamster cheek pouch microcirculation. We measured Po2 by using phosphorescence quenching microscopy and ROS production in the systemic blood. We evaluated the effects of a nitric oxide synthase inhibitor (NG-monomethyl-L-arginine, L-NMMA) and SOD on the oxidative stress during reperfusion. Microvascular injury was assessed by measuring diameter change, the perfused capillary length (PCL), and leukocyte adhesion. During early reperfusion, arteriolar Po2 was significantly lower than baseline, whereas capillary Po2 varied between 7 and 0 mmHg. Arterial blood flow did not regain baseline values, whereas Po2 returned to baseline in arterioles and tissue after 30 min of reperfusion. During 5 and 15 min of reperfusion, ROS increased by 72 and 89% versus baseline, respectively, and declined to baseline after 30 min of reperfusion. Pretreatment with SOD maintained ROS at normal levels, increased arteriolar diameter, blood flow, and PCL, and decreased leukocyte adhesion (P < 0.05). L-NMMA decreased ROS only within 5 min of reperfusion, which increased significantly by 72% later during reperfusion. L-NMMA worsened leukocyte adhesion (P < 0.05). In conclusion, our results show that the early reperfusion is characterized by low Po2 linked to increased production of ROS. At early reperfusion both SOD and L-NMMA decreased ROS production, whereas only SOD reduced it during later reperfusion. We suggest that low-flow hypoxia profoundly affects vascular endothelial damage during reperfusion through changes in ROS and nitric oxide production.

Effect of ischemia on soluble and particulate guanylyl cyclase-mediated cGMP synthesis in cardiomyocytes

The effect of simulated ischemia [hypoxia, no glucose, extracellular pH (pH(o)) 6.4] on cGMP synthesis induced by stimulation of soluble (sGC) or particulate guanylyl cyclase (pGC) was investigated in adult rat cardiomyocytes. Intracellular cGMP content was measured after stimulation of sGC by S-nitroso-N-penicillamine (SNAP) or stimulation of pGC by natriuretic peptides [urodilatin (Uro), atrial natriuretic peptide (ANP), or C-type natriuretic peptide (CNP)] for 1 min in the presence of phosphodiesterase inhibitors. After 2 h of simulated ischemia, a decrease of >50% was observed in pGC-dependent cGMP synthesis, but no significant change was observed in sGC-dependent cGMP synthesis. The reduction in cGMP synthesis caused by simulated ischemia was mimicked by extracellular acidosis (pH(o) 6.4), which decreased pGC-mediated cGMP synthesis without altering sGC-mediated cGMP synthesis. An extreme sensitivity of pGC activity to low pH was also observed in membrane cell fractions. Hypoxia without acidosis (pH(o) 7.4) profoundly depressed cellular ATP content but did not change the response to SNAP, Uro, or ANP (selective agonists of pGC type A receptor). Only cGMP synthesis in response to CNP (a selective agonist of pGC type B receptor) was significantly reduced by ATP depletion. These data support the relevance of intracellular pH as a modulator of cGMP and suggest that, in ischemic cardiomyocytes, synthesis of cGMP would be mainly nitric oxide dependent.