Endothelial cell oxidant generation during K(+)-induced membrane depolarization

Primary Graft Dysfunction

Primary graft dysfunction (PGD) is a form of acute lung injury after transplantation characterized by hypoxemia and the development of alveolar infiltrates on chest radiograph that occurs within 72 hours of reperfusion. PGD is among the most common early complications following lung transplantation and significantly contributes to increased short-term morbidity and mortality. In addition, severe PGD has been associated with higher 90-day and 1-year mortality rates compared with absent or less severe PGD and is a significant risk factor for the subsequent development of chronic lung allograft dysfunction. The International Society for Heart and Lung Transplantation released updated consensus guidelines in 2017, defining grade 3 PGD, the most severe form, by the presence of alveolar infiltrates and a ratio of PaO₂:FiO₂ less than 200. Multiple donor-related, recipient-related, and perioperative risk factors for PGD have been identified, many of which are potentially modifiable. Consistently identified risk factors include donor tobacco and alcohol use; increased recipient body mass index; recipient history of pulmonary hypertension, sarcoidosis, or pulmonary fibrosis; single lung transplantation; and use of cardiopulmonary bypass, among others. Several cellular pathways have been implicated in the pathogenesis of PGD, thus presenting several possible therapeutic targets for preventing and treating PGD. Notably, use of ex vivo lung perfusion (EVLP) has become more widespread and offers a potential platform to safely investigate novel PGD treatments while expanding the lung donor pool. Even in the presence of significantly prolonged ischemic times, EVLP has not been associated with an increased risk for PGD.

Primary Graft Dysfunction After Lung Transplantation

Primary graft dysfunction is a form of acute injury after lung transplantation that is associated with significant short- and long-term morbidity and mortality. Multiple mechanisms contribute to the pathogenesis of primary graft dysfunction, including ischemia reperfusion injury, epithelial cell death, endothelial cell dysfunction, innate immune activation, oxidative stress, and release of inflammatory cytokines and chemokines. This article reviews the epidemiology, pathogenesis, risk factors, prevention, and treatment of primary graft dysfunction.

Primary graft dysfunction: lessons learned about the first 72 h after lung transplantation

PURPOSE OF REVIEW

In 2005, the International Society for Heart and Lung Transplantation published a standardized definition of primary graft dysfunction (PGD), facilitating new knowledge on this form of acute lung injury that occurs within 72 h of lung transplantation. PGD continues to be associated with significant morbidity and mortality. This article will summarize the current literature on the epidemiology of PGD, pathogenesis, risk factors, and preventive and treatment strategies.

RECENT FINDINGS

Since 2011, several manuscripts have been published that provide insight into the clinical risk factors and pathogenesis of PGD. In addition, several transplant centers have explored preventive and treatment strategies for PGD, including the use of extracorporeal strategies. More recently, results from several trials assessing the role of extracorporeal lung perfusion may allow for much-needed expansion of the donor pool, without raising PGD rates.

SUMMARY

This article will highlight the current state of the science regarding PGD, focusing on recent advances, and set a framework for future preventive and treatment strategies.

Mechanosignaling in the vasculature: emerging concepts in sensing, transduction and physiological responses

Cells are constantly exposed to mechanical forces that play a role in modulating cellular structure and function. The cardiovascular system experiences physical forces in the form of shear stress and stretch associated with blood flow and contraction, respectively. These forces are sensed by endothelial cells and cardiomyocytes and lead to responses that control vascular and cardiac homeostasis. This was highlighted at the Pan American Physiological Society meeting at Iguassu Falls, Brazil, in a symposium titled "Mechanosignaling in the Vasculature." This symposium presented recent research that showed the existence of a vital link between mechanosensing and downstream redox sensitive signaling cascades. This link helps to transduce and transmit the physical force into an observable physiological response. The speakers showcased how mechanosensors such as ion channels, membrane receptor kinases, adhesion molecules, and other cellular components transduce the force via redox signals (such as reactive oxygen species and nitric oxide) to receptors (transcription factors, growth factors, etc.). Receptor activated pathways then lead to cellular responses including cellular proliferation, contraction, and remodeling. These responses have major relevance to the physiology and pathophysiology of various cardiovascular diseases. Thus an understanding of the complex series of events, from the initial sensing through the final response, is essential for progress in this field. Overall, this symposium addressed some important emerging concepts in the field of mechanosignaling and the eventual pathophysiological responses.

Shear stress-related mechanosignaling with lung ischemia: lessons from basic research can inform lung transplantation

Cessation of blood flow represents a physical event that is sensed by the pulmonary endothelium leading to a signaling cascade that has been termed "mechanotransduction." This paradigm has clinical relevance for conditions such as pulmonary embolism, lung bypass surgery, and organ procurement and storage during lung transplantation. On the basis of our findings with stop of flow, we postulate that normal blood flow is "sensed" by the endothelium by virtue of its location at the interface of the blood and vessel wall and that this signal is necessary to maintain the endothelial cell membrane potential. Stop of flow is sensed by a "mechanosome" consisting of PECAM-VEGF receptor-VE cadherin that is located in the endothelial cell caveolae. Activation of the mechanosome results in endothelial cell membrane depolarization that in turn leads to activation of NADPH oxidase (NOX2) to generate reactive oxygen species (ROS). Endothelial depolarization additionally results in opening of T-type voltage-gated Ca(2+) channels, increased intracellular Ca(2+), and activation of nitric oxide (NO) synthase with resultant generation of NO. Increased NO causes vasodilatation whereas ROS provide a signal for neovascularization; however, with lung transplantation overproduction of ROS and NO can cause oxidative injury and/or activation of proteins that drive inflammation and cell death. Understanding the key events in the mechanosignaling cascade has important lessons for the design of strategies or interventions that may reduce injury during storage of donor lungs with the goal to increase the availability of lungs suitable for donation and thus improving access to lung transplantation.

Mechanotransduction drives post ischemic revascularization through K(ATP) channel closure and production of reactive oxygen species

AIMS

We reported earlier that ischemia results in the generation of reactive oxygen species (ROS) via the closure of a K(ATP) channel which causes membrane depolarization and NADPH oxidase 2 (NOX2) activation. This study was undertaken to understand the role of ischemia-mediated ROS in signaling.

RESULTS

Angiogenic potential of pulmonary microvascular endothelial cells (PMVEC) was studied in vitro and in the hind limb in vivo. Flow adapted PMVEC injected into a Matrigel matrix showed significantly higher tube formation than cells grown under static conditions or cells from mice with knockout of K(ATP) channels or the NOX2. Blocking of hypoxia inducible factor-1 alpha (HIF-1α) accumulation completely abrogated the tube formation in wild-type (WT) PMVEC. With ischemia in vivo (femoral artery ligation), revascularization was high in WT mice and was significantly decreased in mice with knockout of K(ATP) channel and in mice orally fed with a K(ATP) channel agonist. In transgenic mice with endothelial-specific NOX2 expression, the revascularization observed was intermediate between that of WT and knockout of K(ATP) channel or NOX2. Increased HIF-1α activation and vascular endothelial growth factor (VEGF) expression was observed in ischemic tissue of WT mice but not in K(ATP) channel and NOX2 null mice. Revascularization could be partially rescued in K(ATP) channel null mice by delivering VEGF into the hind limb.

INNOVATION

This is the first report of a mechanosensitive ion channel (K(ATP) channel) initiating endothelial signaling that drives revascularization.

CONCLUSION

The K(ATP) channel responds to the stop of flow and activates signals for revascularization to restore the impeded blood flow.

Dietary sodium restriction and association with urinary marinobufagenin, blood pressure, and aortic stiffness

BACKGROUND AND OBJECTIVES

Systolic BP and large elastic artery stiffness both increase with age and are reduced by dietary sodium restriction. Production of the natriuretic hormone marinobufagenin, an endogenous α1 Na+,K+-ATPase inhibitor, is increased in salt-sensitive hypertension and contributes to the rise in systolic BP during sodium loading.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

The hypothesis was that dietary sodium restriction performed in middle-aged/older adults (eight men and three women; 60 ± 2 years) with moderately elevated systolic BP (139 ± 2/83 ± 2 mmHg) would reduce urinary marinobufagenin excretion as well as systolic BP and aortic pulse-wave velocity (randomized, placebo-controlled, and crossover design). This study also explored the associations among marinobufagenin excretion with systolic BP and aortic pulse-wave velocity across conditions of 5 weeks of a low-sodium (77 ± 9 mmol/d) and 5 weeks of a normal-sodium (144 ± 7 mmol/d) diet.

RESULTS

Urinary marinobufagenin excretion (weekly measurements; 25.4 ± 1.8 versus 30.7 ± 2.1 pmol/kg per day), systolic BP (127 ± 3 versus 138 ± 5 mmHg), and aortic pulse-wave velocity (700 ± 40 versus 843 ± 36 cm/s) were lower during the low- versus normal-sodium condition (all P<0.05). Across all weeks, marinobufagenin excretion was related with systolic BP (slope=0.61, P<0.001) and sodium excretion (slope=0.46, P<0.001). These associations persisted during the normal- but not the low-sodium condition (both P<0.005). Marinobufagenin excretion also was associated with aortic pulse-wave velocity (slope=0.70, P=0.02) and endothelial cell expression of NAD(P)H oxidase-p47phox (slope=0.64, P=0.006).

CONCLUSIONS

These results show, for the first time in humans, that dietary sodium restriction reduces urinary marinobufagenin excretion and that urinary marinobufagenin excretion is positively associated with systolic BP, aortic stiffness (aortic pulse-wave velocity), and endothelial cell expression of the oxidant enzyme NAD(P)H oxidase. Importantly, marinobufagenin excretion is positively related to systolic BP over ranges of sodium intake typical of an American diet, extending previous observations in rodents and humans fed experimentally high-sodium diets.

Stop the flow: a paradigm for cell signaling mediated by reactive oxygen species in the pulmonary endothelium

The lung endothelium is exposed to mechanical stimuli through shear stress arising from blood flow and responds to altered shear by activation of NADPH (NOX2) to generate reactive oxygen species (ROS). This review describes the pathway for NOX2 activation and the downstream ROS-mediated signaling events on the basis of studies of isolated lungs and flow-adapted endothelial cells in vitro that are subjected to acute flow cessation (ischemia). Altered mechanical stress is detected by a cell-associated complex involving caveolae and other membrane proteins that results in endothelial cell membrane depolarization and then the activation of specific kinases that lead to the assembly of NOX2 components. ROS generated by this enzyme amplify the mechanosignal within the endothelial cell to regulate activation and/or synthesis of proteins that participate in cell growth, proliferation, differentiation, apoptosis, and vascular remodeling. These responses indicate an important role for NOX2-derived ROS associated with mechanotransduction in promoting vascular homeostasis.

Membrane depolarization is the trigger for PI3K/Akt activation and leads to the generation of ROS

Loss of fluid shear stress (ischemia) to the lung endothelium causes endothelial plasma membrane depolarization via ATP-sensitive K(+) (K(ATP)) channel closure, initiating a signaling cascade that leads to NADPH oxidase (NOX2) activation and ROS production. Since wortmannin treatment significantly reduces ROS production with ischemia, we investigated the role of phosphoinositide 3-kinase (PI3K) in shear-associated signaling. Pulmonary microvascular endothelial cells in perfused lungs subjected to abrupt stop of flow showed membrane depolarization and ROS generation. Stop of flow in flow-adapted mouse pulmonary microvascular endothelial cells in vitro resulted in the activation of PI3K and Akt as well as ROS generation. ROS generation in the lungs in situ was almost abolished by the PI3K inhibitor wortmannin and the PKC inhibitor H7. The combination of the two (wortmannin and H7) did not have a greater effect. Activation of NOX2 was greatly diminished by wortmannin, knockout of Akt1, or dominant negative PI3K, whereas membrane depolarization was unaffected. Ischemia-induced Akt activation (phosphorylation) was not observed with K(ATP) channel-null cells, which showed minimal changes in membrane potential with ischemia. Activation of Akt was similar to wild-type cells in NOX2-null cells, which do not generate ROS with ischemia. Cromakalim, a K(ATP) channel agonist, prevented both membrane depolarization and Akt phosphorylation with ischemia. Thus, Akt1 phosphorylation follows cell membrane depolarization and precedes the activation of NOX2. These results indicate that PI3K/Akt and PKC serve as mediators between endothelial cell membrane depolarization and NOX2 assembly.

The hyperpolarizing impact of glycine on endothelial cells may be anti-atherogenic

Studies to date indicate that endothelial cells express glycine-activated chloride channels, which promote hyperpolarization of the endothelial plasma membrane. If such channels are expressed by endothelial cells lining conduit arteries, glycine is likely to have anti-atherogenic activity. This reflects the fact that endothelial hyperpolarization promotes calcium influx, activating the endothelial isoform of nitric oxide synthase, while also down-regulating the activity of the membrane-bound NADPH oxidase, chief endothelial source of superoxide. Since macrophages express glycine-activated chloride channels that suppress production of oxidants and cytokines, glycine may also oppose atherogenesis by influencing intimal macrophage function. In rats, supplemental glycine exerts anti-inflammatory and anti-angiogenic effects attributed to chloride channel activation. Administration of large daily doses of glycine would appear to be practical and safe, and has already been shown to inhibit protein glycation in human diabetics.

Lung ischemia: a model for endothelial mechanotransduction

Endothelial cells in vivo are constantly exposed to shear associated with blood flow and altered shear stress elicits cellular responses (mechanotransduction). This review describes the role of shear sensors and signal transducers in these events. The major focus is the response to removal of shear as occurs when blood flow is compromised (i.e., ischemia). Pulmonary ischemia studied with the isolated murine lung or flow adapted pulmonary microvascular endothelial cells in vitro results in endothelial generation of reactive oxygen species (ROS) and NO. The response requires caveolae and is initiated by endothelial cell depolarization via K(ATP) channel closure followed by activation of NADPH oxidase (NOX2) and NO synthase (eNOS), signaling through MAP kinases, and endothelial cell proliferation. These physiological mediators can promote vasodilation and angiogenesis as compensation for decreased tissue perfusion.

High-dose but not low-dose mainstream cigarette smoke suppresses allergic airway inflammation by inhibiting T cell function

Epidemiological studies have identified childhood exposure to environmental tobacco smoke as a significant risk factor for the onset and exacerbation of asthma, but studies of smoking in adults are less conclusive, and mainstream cigarette smoke (MCS) has been reported to both enhance and attenuate allergic airway inflammation in animal models. We sensitized mice to ovalbumin (OVA) and exposed them to MCS in a well-characterized exposure system. Exposure to MCS (600 mg/m(3) total suspended particulates, TSP) for 1 h/day suppresses the allergic airway response, with reductions in eosinophilia, tissue inflammation, goblet cell metaplasia, IL-4 and IL-5 in bronchoalveolar lavage (BAL) fluid, and OVA-specific antibodies. Suppression is associated with a loss of antigen-specific proliferation and cytokine production by T cells. However, exposure to a lower dose of MCS (77 mg/m(3) TSP) had no effect on the number of BAL eosinophils or OVA-specific antibodies. This is the first report to demonstrate, using identical smoking methodologies, that MCS inhibits immune responses in a dose-dependent manner and may explain the observation that, although smoking provokes a systemic inflammatory response, it also inhibits T cell-mediated responses involved in a number of diseases.

Multifocal angiostatic therapy: an update

Multifocal angiostatic therapy (MAT) is a strategy that seeks to impede cancer-induced angiogenesis by addressing multiple targets that regulate the angiogenic capacity of a cancer and/or the angiogenic responsiveness of endothelial cells, using measures that are preferentially, but not exclusively, nutraceutical. A prototype of such a regimen has been proposed previously, composed of green tea polyphenols, fish oil, selenium, and high-dose glycine, complementing a low-fat vegan diet, exercise training, and the copper-sequestering drug tetrathiomolybdate (TM). A review of more recent evidence suggests additional agents that could appropriately be included in this regimen and clarifies to some extent the mechanisms of action of its constituents. Diindolylmethane, a widely available crucifera-derived nutraceutical, has inhibited cancer growth in several mouse xenograft models; this effect may be largely attributable to an angiostatic action, as concentrations as low as 5 to 10 muM inhibit proliferation, migration, and tube-forming capacity of human endothelial cells in vitro, and a parenteral dose of 5 mg/kg markedly impairs matrigel angiogenesis in mice. Silymarin/silbinin, which has slowed the growth of human xenografts in a number of studies, suppresses the proliferation, migration, and tube-forming capacity of endothelial cells and inhibits vascular endothelial growth factor (VEGF) secretion by a range of human cancer cell lines, in concentrations that should be clinically feasible. The angiostatic activity of orally administered green tea now appears likely to reflect inhibition of the kinase activity of VEGFR-2. Glycine's angiostatic activity may be attributable to a hyperpolarizing effect on endothelial cells that decreases the activity of NADPH oxidase, now known to promote tyrosine kinase signaling in endothelial cells. The ability of TM to suppress cancer cell production of a range of angiogenic factors results at least in part from a down regulation of NF-kappaB activation. Dual-purpose molecular targets, whose inhibition could be expected to decrease the aggressiveness and chemoresistance of cancer cells while simultaneously impeding angiogenesis, include NF-kappaB, cox-2, c-Src, Stat3, and hsp90; drugs that can address these targets are now in development, and salicylates are notable for the fact that they can simultaneously inhibit NF-kappaB and cox-2. The potential complementary of the components of MAT should be assessed in nude mouse xenograft models.

Hypercholesterolemia suppresses inwardly rectifying K+ channels in aortic endothelium in vitro and in vivo

Inwardly rectifying K+ (Kir) channels are responsible for maintaining endothelial membrane potential and play a key role in endothelium-dependent vasorelaxation. In this study, we show that endothelial Kir channels are suppressed by hypercholesterolemic levels of lipoproteins in vitro and by serum hypercholesterolemia in vivo. Specifically, exposing human aortic endothelial cells to acetylated low-density lipoprotein or very low density lipoprotein resulted in a time- and concentration-dependent decrease in Kir current that correlated with the degree of cholesterol loading. The suppression was fully reversible by cholesterol depletion. Furthermore, a decrease in Kir current resulted in depolarization of endothelial membrane potential. Most important, the flow sensitivity of Kir currents was also impaired by cholesterol loading. Specifically, flow-induced increase in Kir current was suppressed by 70%, and flow-induced hyperpolarization was almost completely abrogated. Furthermore, we show that hypercholesterolemia in vivo also strongly suppresses endothelial Kir currents and causes a shift in endothelial membrane potential, as determined by comparing the currents in aortic endothelial cells freshly isolated from healthy or hypercholesterolemic pigs. Therefore, we suggest that suppression of Kir current is one of the important factors in hypercholesterolemia-induced endothelial dysfunction.

Lung endothelial cell proliferation with decreased shear stress is mediated by reactive oxygen species

Acute cessation of flow (ischemia) leads to depolarization of the endothelial cell (EC) membrane mediated by K(ATP) channels and followed by production of reactive oxygen species (ROS) from NADPH oxidase. We postulated that ROS are a signal for initiating EC proliferation associated with the loss of shear stress. Flow cytometry was used to identify proliferating CD31-positive pulmonary microvascular endothelial cells (mPMVECs) from wild-type, Kir6.2-/-, and gp91phox-/- mice. mPMVECs were labeled with PKH26 and cultured in artificial capillaries for 72 h at 5 dyn/cm2 (flow adaptation), followed by 24 h of stop flow or continued flow. ROS production during the first hour of ischemia was markedly diminished compared with wild-type mice in both types of gene-targeted mPMVECs. Cell proliferation was defined as the proliferation index (PI). After 72 h of flow, >98% of PKH26-labeled wild-type mPMVECs were at a single peak (PI 1.0) and the proportion of cells in the S+G2/M phases were at 5.8% on the basis of cell cycle analysis. With ischemia (24 h), PI increased to 2.5 and the ratio of cells in S+G2/M phases were at 35%. Catalase, diphenyleneiodonium, and cromakalim markedly inhibited ROS production and cell proliferation in flow-adapted wild-type mPMVECs. Significant effects of ischemia were not observed in Kir6.2-/- and gp91phox-/- cells. ANG II activation of NADPH oxidase was unaffected by KATP gene deletion. Thus loss of shear stress in flow-adapted mPMVECs results in cell division associated with ROS generated by NADPH oxidase. This effect requires a functioning cell membrane KATP channel.

Marinobufagenin may mediate the impact of salty diets on left ventricular hypertrophy by disrupting the protective function of coronary microvascular endothelium

Individuals who eat salty diets and who are "salt-sensitive" tend to have increased left ventricular mass, independent of blood pressure; this phenomenon awaits an explanation. It is clear that local up-regulation of angiotensin II (AngII) production and activity play a key role in the induction of left ventricular hypertrophy (LVH). Recent evidence suggests that a healthy coronary microvascular endothelium opposes this effect by serving as a paracrine source of nitric oxide (NO), a natural antagonist of AngII activity, and that up-regulation of this mechanism can account for the protective role of bradykinin with respect to LVH. The coronary microvasculature also possesses NAD(P)H oxidase activity that can generate superoxide, inimical to the bioactivity of endothelial NO. There is now good reason to believe that the triterpenoid marinobufagenin (MBG), a selective inhibitor of the alpha-1 isoform of the sodium pump, mediates the impact of salty diets on blood pressure; production of MBG by the adrenal cortex is boosted when salt-sensitive animals are fed salty diets. It is hypothesized that coronary microvascular endothelium expresses the alpha-1 isoform of the sodium pump, and that MBG thus can target this endothelium. If that is the case, MBG would be expected to decrease membrane potential in these cells; as a consequence, superoxide production would be up-regulated, NO synthase activity would be down-regulated, and myocardial NO bioactivity would thus be suppressed. This would offer a satisfying explanation for the impact of salt and salt-sensitivity on risk for LVH. If expression of the alpha-1 isoform of the sodium pump is a more general property of vascular endothelium, MBG may suppress NO bioactivity in other regions of the vascular tree, thereby contributing to other adverse effects elicited by salty diets: reduced arterial compliance, medial hypertrophy, impaired endothelium-dependent vasodilation, hypertensive/diabetic glomerulopathy, increased risk for stroke, and hypertension.

Membrane depolarization and NADPH oxidase activation in aortic endothelium during ischemia reflect altered mechanotransduction

We previously showed that "ischemia" (abrupt cessation of flow) leads to rapid membrane depolarization and increased generation of reactive oxygen species (ROS) in lung microvascular endothelial cells. This response is not associated with anoxia but, rather, reflects loss of normal shear stress. This study evaluated whether a similar response occurs in aortic endothelium. Plasma membrane potential and production of ROS were determined by fluorescence microscopy and cytochrome c reduction in flow-adapted rat or mouse aorta or monolayer cultures of rat aortic endothelial cells. Within 30 s after flow cessation, endothelial cells that had been flow adapted showed plasma membrane depolarization that was inhibited by pretreatment with cromakalim, an ATP-sensitive K(+) (K(ATP)) channel agonist. Flow cessation also led to ROS generation, which was inhibited by cromakalim and the flavoprotein inhibitor diphenyleneiodonium. Aortic endothelium from mice with "knockout" of the K(ATP) channel (K(IR)6.2) showed a markedly attenuated change in membrane potential and ROS generation with flow cessation. In aortic endothelium from mice with knockout of NADPH oxidase (gp91(phox)), membrane depolarization was similar to that in wild-type mice but ROS generation was absent. Thus rat and mouse aortic endothelial cells respond to abrupt flow cessation by K(ATP) channel-mediated membrane depolarization followed by NADPH oxidase-mediated ROS generation, possibly representing a cell-signaling response to altered mechanotransduction.

Marinobufagenin may mediate the impact of salty diets on left ventricular hypertrophy by disrupting the protective function of coronary microvascular endothelium

Individuals who eat salty diets and who are "salt-sensitive" tend to have increased left ventricular mass, independent of blood pressure; this phenomenon awaits an explanation. It is clear that local up-regulation of angiotensin II (AngII) production and activity play a key role in the induction of left ventricular hypertrophy (LVH). Recent evidence suggests that a healthy coronary microvascular endothelium opposes this effect by serving as a paracrine source of nitric oxide (NO), a natural antagonist of AngII activity, and that up-regulation of this mechanism can account for the protective role of bradykinin with respect to LVH. The coronary microvasculature also possesses NAD(P)H oxidase activity that can generate superoxide, inimical to the bioactivity of endothelial NO. There is now good reason to believe that the triterpenoid marinobufagenin (MBG), a selective inhibitor of the alpha-1 isoform of the sodium pump, mediates the impact of salty diets on blood pressure;production of MBG by the adrenal cortex is boosted when salt-sensitive animals are fed salty diets. It is hypothesized that coronary microvascular endothelium expresses the alpha-1 isoform of the sodium pump, and that MBG thus can target this endothelium. If that is the case, MBG would be expected to decrease membrane potential in these cells;as a consequence, superoxide production would be up-regulated, NO synthase activity would be down-regulated, and myocardial NO bioactivity would thus be suppressed. This would offer a satisfying explanation for the impact of salt and salt-sensitivity on risk for LVH. If expression of the alpha-1 isoform of the sodium pump is a more general property of vascular endothelium, MBG may suppress NO bioactivity in other regions of the vascular tree, thereby contributing to other adverse effects elicited by salty diets: reduced arterial compliance, medial hypertrophy, impaired endothelium-dependent vasodilation, hypertensive/diabetic glomerulopathy, increased risk for stroke, and hypertension.

Decreased 4-aminopyridine sensitive K+ currents in endothelial cells from hypertensive rats

Endothelial cell function is altered in hypertension. The present study was performed to evaluate the alterations in K+ channels in endothelial cells from hypertensive rats. Currents and membrane potentials were recorded in endothelial cells freshly dissociated from the aorta of stroke-prone spontaneously hypertensive rats (SHR-SP) and Wistar-Kyoto rats (WKY). Ca2+-dependent K+ channel blockers, charybdotoxin and apamin, a voltage-dependent K+ channel blocker, 4-aminopyridine, and a non-selective K+ channel blocker, tetrabutylammonium, were used to characterize K+ currents. Depolarizing command steps evoked delayed K+ outward currents in cells from both strains. The current density of 4-aminopyridine sensitive K+ currents was significantly smaller in SHR-SP than in WKY (1.5 +/- 0.4 vs. 4.9 +/- 0.6 pA/pF, at 36 mV, n = 13, p < 0.01), whereas that of other K+ current components did not differ between strains. The resting membrane potential of cells was significantly less negative in SHR-SP than in WKY (-25.0 +/- 1.7, n = 54 vs. -33.5 +/- 1.4 mV, n = 50, p < 0.01). Depolarization by 4-aminopyridine, but not that by charybdotoxin+apamin, abolished the difference in membrane potentials between SHR-SP and WKY (n=7-10 in each strain). Immunostaining of endothelial cells by anti-Kv1.5 antibody was decreased in SHR-SP compared to WKY. In summary, the 4-aminopyridine sensitive K+ currents in aortic endothelial cells were decreased in SHR-SP, which could contribute to the membrane depolarization. Decreased expression of Kv1.5 in SHR-SP might be associated with this alteration.

Shear stress and endothelial cell activation

We have shown previously that ischemia in an isolated rat lung that is normally oxygenated by continued ventilation results in lipid and protein oxidation, indicating the generation of reactive oxygen species. By using a variety of biochemical and imaging techniques, we determined that the initial response, which occurs within the first second of ischemia, is partial depolarization of the endothelial cell plasma membrane. This event is followed within several seconds by activation of endothelial NADPH oxidase and generation of superoxide anion at the extracellular surface of the cell membrane where it is dismutated to freely diffusible H2O2. Approximately 15 secs after the onset of ischemia, we detected an elevation of intracellular Ca2+ caused by its release from intracellular stores, followed by Ca2+ influx, possibly through T-type voltage-dependent Ca2+ channels. Increased nitric oxide generation through activation of endothelial nitric oxide synthase is detected after about 45 secs of ischemia. These changes (endothelial membrane depolarization, reactive oxygen species production, elevation of intracellular Ca2+ levels, and nitric oxide generation) were confirmed in isolated endothelial cells that had been adapted to shear stress in vitro. The in vitro model also demonstrates reactive oxygen species-dependent activation of nuclear factor-kappaB and activator protein-1 and that 24 hrs of ischemia results in increased cell division. These results indicate a novel cell-signaling pathway in response to loss of shear stress. The basis for these changes in endothelial function is related to mechanotransduction, i.e., altered shear stress rather than a metabolic response to ischemia. The biological function for the response may be an attempt to restore blood flow through vasodilatation and new capillary formation.

Update on pulmonary edema: the role and regulation of endothelial barrier function

Discovery of the pathophysiologic mechanisms leading to pulmonary edema and identification of effective strategies for prevention remain significant clinical concerns. Endothelial barrier function is a key component for maintenance of the integrity of the vascular boundary in the lung, particularly since the gas exchange surface area of the alveolar-capillary membrane is large. This review is focused on new insights in the pulmonary endothelial response to injury and recovery, reversible activation by edemagenic agents, and the biochemical/structural basis for regulation of endothelial barrier function. This information is discussed in the context of fundamental concepts of lung fluid balance and pulmonary function.

Nitric oxide synthase-independent release of nitric oxide induced by KCl in the perfused mesenteric bed of the rat

The aim of the present study was to test whether the contractile responses elicited by KCl in the rat mesenteric bed are coupled to the release of nitric oxide (NO). Contractions induced by 70 mM KCl were coincident with the release of NO to the perfusate. The in vitro exposure to the nitric oxide synthase (NOS) inhibitor L-N(omega)-nitro-L-arginine methyl ester, L-NAME (1-100 microM) potentiated the vascular responses to 70 mM KCl and, unexpectedly, increased the KCl-stimulated release of NO. Moreover, even after the chronic treatment with L-NAME (70 mg/kg/day during 4 weeks), the KCl-induced release of NO was not reduced, whereas the potentiation of contractile responses was indeed achieved. The possibility that NOS had not been completely inhibited under our experimental conditions can be precluded because NOS activity was significantly inhibited after both L-NAME treatments. After the in vitro treatment with 1 to 100 microM L-NAME, the inhibition of NOS was concentration-dependent (from 50% to 90%). With regard to the basal release of NO, the inhibition caused by L-NAME was not concentration-dependent and reached a maximum of 40%, suggesting that basal NO outflow is only partially dependent on NOS activity. An eventual enhancement of NOS activity caused by KCl was disregarded because the activity of this enzyme measured in homogenates from mesenteric beds perfused with 70 mM KCl was significantly reduced. On the other hand, endothelium removal, employed as a negative control, almost abolished NOS activity, whereas the incubation with the Ca(2+) ionophore A23187, employed as a positive control, induced an increase in NOS activity. It is concluded that in the mesenteric arterial bed of the rat, the contractile responses elicited by depolarization through KCl are coincident with a NOS-independent release of NO. This observation, which differs from the results obtained with noradrenaline, do not support the use of KCl as an alternative contractile agent whenever the participation of NO is under study.

The effects of various organ preservation solutions on hepatocyte membrane potentials, intracellular calcium concentrations, and outcome following liver transplantation

BACKGROUND

Hepatocyte membrane potential differences (PDs) may be altered by the preservation solutions used in liver transplantation. Such alterations could impact on the survival of the donor liver, extent of biochemical injury, and flux of important ionic compounds. The purpose of the present study was to document these outcomes in the presence of four different preservation solutions.

METHODS

Livers of adult male Sprague-Dawley rats (N = 3 to 4 per group) were impaled with intracellular microelectrodes prior to and at various time periods for 6 hours following complete hepatic resection. Just prior to resection, each liver was perfused with preservation solutions associated with high (normal saline [NS]), moderate (Euro-Collins [EC]), and low (University of Wisconsin solution [UW]) risks of reperfusion injury.

RESULTS

Baseline (in situ) PDs were similar in all groups (-37 +/- 4 mV, mean +/- SD). Ten minutes postresection, hepatic PDs were as follows: NS, -23.8 +/- 3.5 mV; EC, -11.4 +/- 0.4 mV; and UW, -8.7 +/- 0.3 mV (P <0.01 for all groups). Maximum depolarization occurred at 6 hours postresection (NS, -8.1 +/- 1.1 mV; EC, -7.7 +/- 1.3 mV; and UW, -8.6 +/- 1.0 mV). To determine whether these changes are of pathophysiologic importance, the NS solution was modified (addition of 0.1% ethanol) to achieve similar PD changes as those observed with UW. Liver transplants were then performed where the donor livers had been perfused and preserved for 6 hours with either NS or the modified NS (MNS) solution. Posttransplant (10 day) survival was 1 of 6 (17%) in the NS group and 4 of 6 (67%) in the MNS group (P <0.05). Regarding the effects of PD changes on ionic flux, intracellular calcium levels were documented for up to 4 hours by fluorescence video microscopy using Fura-2 in isolated hepatocytes exposed to NS, UW, and MNS solutions. Intracellular calcium levels were similar in all solutions at each time point studied.

CONCLUSIONS

The results of this study indicate that hepatocytes undergo prompt and marked depolarization following hepatic resection, and the extent of the depolarization correlates with survival following transplantation.

Endothelial membrane potential regulates production of both nitric oxide and superoxide--a fundamental determinant of vascular health

There is recent evidence that the membrane potential of vascular endothelium regulates not only nitric oxide (NO) synthesis, but also superoxide generation, such that hyperpolarization stimulates NO production while suppressing that of superoxide. Given that NO works in a variety of ways to inhibit atherothrombotic disease and hypertension, whereas superoxide not only vetoes the benefits of NO but also disrupts endothelial metabolism and promotes LDL oxidation through its oxidant activity, it is thus evident that endothelium membrane potential is a crucial determinant of cardiovascular risk. Membrane polarization can be enhanced by measures which increase the synthesis or availability of the Na+-K+-ATPase, moderately enhance serum K+ and increase the conductance of membrane K+ channels. Such measures may include high-K+/low-Na+ natural diets, insulin sensitizing modalities, 'euthyroid replacement therapy' and ACE inhibitors. Epidemiological correlations of insulin resistance with hypertension and cardiovascular risk may reflect the low membrane potential of insulin-resistant vascular endothelium. Adjunctive measures for suppressing the generation or half-life of endothelial superoxide are suggested.

Beneficial effects of leukocyte-depleted blood and low-potassium dextran solutions on microvascular permeability in preserved porcine lung

Modified Euro-Collins (EC) solution, a crystalloid intracellular-type solution, has been commonly used for pulmonary preservation. Several experimental studies have shown the advantages of using extracellular colloid-based solutions. The aim of this study was to compare the quality of preservation of two extracellular colloid solutions, leukocyte-depleted blood (BL) and low-potassium dextran (LPD) solutions, with that of EC solution. Lungs of 22 domestic pigs were flushed and preserved with EC (n = 8), BL (n = 7), or LPD (n = 7) solution. After harvesting, one of the lungs was reperfused immediately in an ex vivo circuit (control lungs), whereas the contralateral lung was reperfused after 8 h of cold (4 degrees C) storage (preserved lungs). Besides the lung function parameters (gas exchange, pulmonary hemodynamics and mechanics), the permeability of the endothelial-epithelial barrier was assessed by determining the transferrin leak index (TLI) using a double radioisotopic method, by measuring the alveolar/arterial protein concentration ratio, and by analyzing histopathologic changes. The functional quality (oxygenation, airway resistance, dynamic compliance [CL, dyn]) of both BL and LPD lungs was slightly but significantly superior to that of EC lungs. However, pulmonary vascular resistance was lower in BL-preserved than in EC- or LPD-preserved lungs. The TLI was increased in EC control and preserved lungs, whereas it was low in BL and LPD control lungs and did not increase after preservation. The alveolar/arterial protein concentration ratio was not different between control groups, but was increased fourfold in EC-preserved compared with BL- or LPD-preserved lungs. Finally, EC-preserved lungs presented a weight gain about twice that of BL- and LPD-preserved lungs. Morphologic analysis confirmed these results, because in the EC-preserved lungs, rupture of alveolar septa and severe alveolar edema and hemorrhage were observed, whereas BL- and LPD-preserved lungs showed a relatively well-preserved structure. The results demonstrate that both BL and LPD flush solutions preserve the endothelial-epithelial barrier better than does EC solution. Although the quality of preservation is similar, pulmonary vascular resistance is higher in LPD-preserved than in BL-preserved lungs.

In situ imaging of intracellular calcium with ischemia in lung subpleural microvascular endothelial cells

We propose that generation of reactive oxygen species (ROS) during ischemia is associated with an increase in intracellular calcium ([Ca2+]i) in pulmonary capillary endothelial cells. We used an isolated rat lung model and epifluorescence microscopy to evaluate [Ca2+]i in subpleural microvascular endothelial cells in situ by ratio imaging of the fluorophores, Calcium Green and Fura Red (CG/FR). Lungs were ventilated continuously under control (continuously perfused) or global ischemia (no perfusion) and thus remained adequately oxygenated even with ischemia. Ischemia for 5 min led to increase in CG/FR, indicating increase in [Ca2+]i in endothelial cells in situ; CG/FR remained elevated during a subsequent 10 min of ischemia. Ca(2+)-free perfusion and gadolinium (100 microM) inhibited the increase in [Ca2+]i, while thapsigargin (250 nM) had no effect. These results indicate that increase in endothelial cell [Ca2+]i with ischemia was due to influx from the extracellular medium. Perfusion with N-acetyl-L-cysteine (20 mM) or diphenyleneiodonium chloride (10 microM) prevented the ischemia-mediated [Ca2+]i increase, suggesting a role for ROS in the Ca2+ changes with ischemia. Membrane depolarization by perfusion with high potassium (K+) or glyburide also resulted in increased [Ca2+]i whereas the K(+)-channel agonist cromakalim, inhibited ischemia-mediated Ca2+ influx. We conclude that increased ROS generation with 'oxygenated' lung ischemia is associated with influx of Ca2+ and an increase in endothelial cell cytosolic calcium concentration.

Endothelial NADPH oxidase as the source of oxidants in lungs exposed to ischemia or high K+

We have previously demonstrated the generation of reactive oxygen species (ROS) in cultured bovine pulmonary artery endothelial cells (BPAECs) and in isolated perfused rat lungs exposed to high K+ and during global lung ischemia. The present study evaluates the NADPH oxidase pathway as a source of ROS in these models. ROS production, detected by oxidation of the fluorophore, dichlorodihydrofluorescein, increased 2.5-fold in BPAECs and 6-fold in rat or mouse lungs exposed to high (24 mmol/L) K+. ROS generation was markedly inhibited by diphenyliodonium, a flavoprotein inhibitor, and by the synthetic peptide PR-39, an inhibitor of NADPH oxidase assembly, whereas allopurinol had no effect. With ischemia (1 hour), ROS generation by rat and mouse lungs increased 7-fold; PR-39 showed concentration-dependent inhibition of ROS production, with 50% inhibition at 3 micromol/L PR-39. ROS production in lungs exposed to high K+ or ischemia was essentially abolished in mice with a "knockout" of gp91(phox), a membrane-localized cytochrome component of NADPH oxidase; increased ROS production by these lungs after anoxia/reoxygenation was similar to control. PR-39 also inhibited ischemia and the high K+-mediated increase in lung thiobarbituric acid reactive substance. Western blotting of BPAECs and immunocytochemistry of BPAECs and rat and mouse lungs showed the presence of p47phox, a cytoplasmic component of NADPH oxidase and the putative target for PR-39 inhibition. In situ fluorescence imaging in the intact lung demonstrated that the increased dichlorofluorescein fluorescence in these models of ROS generation was localized primarily to the pulmonary endothelium. These studies demonstrate that ROS production in lungs exposed to ischemia or high K+ results from assembly and activation of a membrane-associated NAPDH oxidase of the pulmonary endothelium.

Divalent ion block of inward rectifier current in human capillary endothelial cells and effects on resting membrane potential

1. Cultured human capillary endothelial cells (HCEC) contain a large inward rectifier current, IK(IR), that can be abolished by removing external K+ or by adding 50 microM Ba2+. 2. We show that IK(IR) is responsible for maintaining the hyperpolarized potential (-60.6 +/- 0.5 mV, n = 83) of HCEC. Blocking IK(IR) with 50 microM Ba2+ shifts the zero current level and depolarizes HCEC by 36.5 +/- 1.3 mV (n = 4). 3. Increasing external Ca2+ concentration ([Ca2+]o) from 0.5 to 7 mM reduces the magnitude of IK(IR) by 36.5 +/- 2.3 % (n = 5) and depolarizes the cells by 10.33 +/- 2.4 mV (n = 3), whereas decreasing [Ca2+]o from 1.8 to 0.5 mM increases the amplitude of IK(IR) by 6.9 +/- 1.9 % (n = 4). The relationship between [Ca2+]o and the percentage block of IK(IR) gives a Kd value of 5.4 +/- 0.6 mM at -120 mV. 4. IK(IR) is also blocked by other divalent ions, with Ba2+ >> Sr2+ > Mg2+ > Mn2+ = Ca2+, and the block of peak current at -120 mV being 85.3 +/- 3.2 % (n = 5) for 50 microM Ba2+, 62.9 +/- 2.2 % (n = 5) for 5 mM Sr2+, 40.7 +/- 2.5 % (n = 9) for 5 mM Mg2+, 33.4 +/- 2.1 % (n = 5) for 5 mM Mn2+ and 32.9 +/- 2.1 % (n = 5) for 5 mM Ca2+. 5. The voltage dependence of Sr2+ block of peak IK(IR) occurred with a Kd value of 1.0 +/- 0.09 mM for -140 mV, 1.9 +/- 0.16 mM for -130 mV, 3.1 +/- 0.28 mM for -120 mV, 4.6 +/- 0.34 mM for -110 mV and 6.4 +/- 0.5 mM for -100 mV (n = 5), with a calculated electrical distance (delta) of 0.44 from the outside.

Immunotargeting of catalase to ACE or ICAM-1 protects perfused rat lungs against oxidative stress

The pulmonary endothelium is susceptible to oxidative insults. Catalase conjugated with monoclonal antibodies (MAbs) against endothelial surface antigens, angiotensin-converting enzyme (MAb 9B9) or intercellular adhesion molecule-1 (MAb 1A29), accumulates in the lungs after systemic injection in rats (V. Muzykantov, E. Atochina, H. Ischiropoulos, S. Danilov, and A. Fisher. Proc. Natl. Acad. Sci. USA 93: 5213-5218, 1996). The present study characterizes the augmentation of antioxidant defense by these antibody-catalase conjugates in isolated rat lungs perfused for 1 h with catalase conjugated with either MAb 9B9, MAb 1A29, or control mouse IgG. Approximately 20% of the injected dose of Ab-125I-catalase accumulated in the perfused rat lungs (vs. <5% for IgG-125I-catalase). After elimination of nonbound material, the lungs were perfused further for 1 h with 5 mM hydrogen peroxide (H2O2). H2O2 induced an elevation in tracheal and pulmonary arterial pressures (126 +/- 7 and 132 +/- 5%, respectively, of the control level), lung wet-to-dry weight ratio (7.1 +/- 0.4 vs. 6.0 +/- 0.01 in the control lungs), and ACE release into the perfusate (436 +/- 20 vs. 75 +/- 7 mU in the control perfusates). Both MAb 9B9-catalase and MAb 1A29-catalase significantly attenuated the H2O2-induced elevation in 1) angiotensin-converting enzyme release to the perfusate (215 +/- 14 and 217 +/- 38 mU, respectively), 2) lung wet-to-dry ratio (6.25 +/- 0.1 and 6.3 +/- 0.3, respectively), 3) tracheal pressure (94 +/- 4 and 101 +/- 4%, respectively, of the control level), and 4) pulmonary arterial pressure (103 +/- 3 and 104 +/- 7%, respectively, of the control level). Nonconjugated catalase, nonconjugated antibodies, nonspecific IgG, and IgG-catalase conjugate had no protective effect, thus confirming the specificity of the effect of MAb-catalase. These results support a strategy of catalase immunotargeting for protection against pulmonary oxidative injury.

ATP-independent membrane depolarization with ischemia in the oxygen-ventilated isolated rat lung

We hypothesize that lung ischemic injury is related to cessation of flow leading to endothelial cell membrane depolarization and activation of oxidant-generating systems. Cell membrane potential was assessed in isolated, oxygen ventilated, Krebs-Ringer bicarbonate buffer-dextran-perfused rat lungs by lung surface fluorescence after infusion of bis-oxonol or 5,5',6,6'-tetrachloro-1, 1',3,3'-tetraethylbenzimidazolyl-carbocyanine iodide (JC-1), voltage-sensitive dyes. Surface fluorometry showed increased bis-oxonol fluorescence (34.7 +/- 3.3% above baseline) and decreased JC-1 fluorescence (24.5 +/- 4.5% below baseline) with ischemia, compatible with membrane depolarization. Fluorescence change was initiated within 1-2 min of the onset of ischemia and was rapidly reversible with reperfusion. Fluorescence changes varied with perfusion flow rate; maximal increase occurred with the transition from 1.8 ml/min to zero flow. Elevation of static intravascular pressure resulted in only a minor increase of bis-oxonol fluorescence. In situ subpleural fluorescence microscopy showed that endothelial cells are the major site of the increased bis-oxonol fluorescence signal with ischemia. These results indicate that endothelial cell membrane depolarization represents an early event with lung ischemia. Since the adenosine triphosphate content of lung was unchanged with ischemia in the O2-ventilated lungs, we postulate that membrane depolarization results from elimination of shear stress, possibly via inactivation of flow-sensitive K+-channels.

Normoxic lung ischemia/reperfusion accelerates shedding of angiotensin converting enzyme from the pulmonary endothelium

Normoxic lung ischemia/reperfusion (I/R) leads to oxidative injury of the pulmonary tissue. We analyzed angiotensin-converting enzyme (ACE) in perfused rat lungs upon I/R in order to assess the endothelial injury produced. I/R led to a time-dependent increase in ACE activity in the perfusate, from 145+/-14 mU to 252+/-1 mU, and to reduction of ACE activity in the lung tissue homogenate, from 29.7+/-2.3 U to 22.7+/-1.7 U. About 80% of ACE activity in control and I/R rat lungs was associated with an aqueous phase of extracted perfusates, thus indicating that I/R accelerates shedding of the hydrophilic form of ACE from the plasma membrane. To specifically assess ACE localized on the luminal surface of the pulmonary endothelium, we perfused rat lungs with a radiolabeled monoclonal antibody (mAb) to ACE (anti-ACE mAb 9B9). Pulmonary uptake of mAb 9B9 with I/R was reduced from 32.1+/-1.7% to 24.8+/-0.9%. In contrast, I/R led to a marked increase in the pulmonary uptake of nonspecific [125I]IgG, from 0.17+/-0.02% to 0.67+/-0.04%. Lung wet weight was equal to 0.78+/-0.08% of body weight in the I/R group versus 0.57+/-0.02% at the control level. The observed increase in [125I]IgG uptake and wet lung weight indicate that I/R causes an increase in lung vascular permeability. These results indicate that normoxic lung I/R induces injury to the pulmonary vascular endothelium.

Reduced lipid peroxidation and ischemia-reperfusion injury after lung transplantation using low-potassium dextran solution for lung preservation

Ischemia-reperfusion injury is one of the significant problems in clinical lung transplantation. We investigated the effect of lung preservation with Euro-Collins solution (EC group) or low-potassium dextran solution (LPD group) on lipid peroxidation and ischemia-reperfusion injury in a pig model of lung allotransplantation. The donor lungs were preserved at 4 degrees C for 18 h. Left-sided single lung transplantation was performed, followed by 6 h of reperfusion. Lipid peroxidation was measured as thiobarbituric acid-reactive materials (TBARM) in bronchoalveolar lavage (BAL) fluid and effluent solutions from pulmonary artery (Effluent). After 18 h of ischemia, the LPD group showed lower TBARM in BAL and Effluent than the EC group (p < 0.05). After ischemia plus reperfusion, lung wetto-dry weight ratios and TBARM levels in BAL in the LPD group were lower than those of the EC group (p < 0.05). Lung wet-to-dry weight ratios correlated with TBARM levels in BAL (p < 0.05, r = 0.50). We conclude lipid peroxidation in BAL and Effluent may reflect the degree of ischemia-reperfusion injury, and lung preservation with LPD can reduce lipid peroxidation and lung injury as compared with EC.

Modulation of potassium channel function by methionine oxidation and reduction

Oxidation of amino acid residues in proteins can be caused by a variety of oxidizing agents normally produced by cells. The oxidation of methionine in proteins to methionine sulfoxide is implicated in aging as well as in pathological conditions, and it is a reversible reaction mediated by a ubiquitous enzyme, peptide methionine sulfoxide reductase. The reversibility of methionine oxidation suggests that it could act as a cellular regulatory mechanism although no such in vivo activity has been demonstrated. We show here that oxidation of a methionine residue in a voltage-dependent potassium channel modulates its inactivation. When this methionine residue is oxidized to methionine sulfoxide, the inactivation is disrupted, and it is reversed by coexpression with peptide methionine sulfoxide reductase. The results suggest that oxidation and reduction of methionine could play a dynamic role in the cellular signal transduction process in a variety of systems.

Hyperpolarization of isolated capillaries from guinea-pig heart induced by K+ channel openers and glucose deprivation

1. The present study was designed to test if microvascular coronary endothelial cells express ATP-sensitive K+ channels (KATP channels). We performed microfluorometric measurements of the membrane potential of freshly isolated guinea-pig coronary capillaries equilibrated with the voltage-sensitive dye bis-oxonol (bis-[1,3-dibutylbarbituric acid] trimethineoxonol, [DiBAC4(3)]). 2. The resting membrane potential of capillaries in physiological salt solution was -46 +/- 4.2 mV (n = 8) at room temperature (22 degrees C) as determined after calibration of the fluorescence using the Na(+)-K+ ionophore gramicidin in the presence of different K+ concentrations. Spontaneous membrane potential fluctuations of 10-20 mV amplitude were often observed. 3. A reversible, sustained hyperpolarization to a new membrane potential close to the K+ equilibrium potential (EK) could be induced by application of the K+ channel openers HOE 234 (100 nM to 1 microM), diazoxide (10 PM to 100 nM) or pinacidil (100 nM). Subsequent addition of glibenclamide (200 nM to 2 microM) reversed this hyperpolarization. 4. A glibenclamide-sensitive hyperpolarization of coronary capillaries to values near EK was also observed upon omission of D-glucose (10 mM) from the superfusing solution or by substituting L-glucose for D-glucose. Maximum hyperpolarization was reached in less than 10 min. 5. Our results suggest that microvascular coronary endothelial cells express KATP channels which may be activated during hypoglycaemia.

Oxidant generation with K(+)-induced depolarization in the isolated perfused lung

This study evaluated whether cell membrane depolarization can induce oxidant generation in the isolated perfused rat lung as has been demonstrated with bovine pulmonary artery endothelial cells. Depolarization was produced by perfusing the lungs with high [K+] or with glyburide and was evaluated with bis-oxonol lung surface fluorometry. Lung surface bis-oxonol fluorescence increased above baseline (at 5.9 mM K+) by 18.5% with 24 mM K+, 35% with 48 mM K+, and 67% with 96 mM K+, indicating graded membrane depolarization, and by 75% during perfusion with 10 microM glyburide. Oxidant generation was evaluated with hydroethidine lung surface fluorometry, and with assay of tissue thiobarbituric acid reactive substance (TBARS), conjugated dienes, and perfusate H2O2. Depolarization by high K+ or glyburide led to significant increases in generation of tissue oxidants and lipid peroxidation. Bodipy-FL-glyburide microfluorography showed localization of glyburide binding primarily to vascular endothelial cells vascular and airway smooth muscle cells, alveolar type II cells, and to nonciliated cells of the airway epithelium. These results indicate that cellular depolarization is associated with oxidant generation by the lung and suggests a role for K(+)-channels in these events.