Na(+)-K(+)-Cl- cotransporter of brain capillary endothelial cells. Properties and regulation by endothelins, hyperosmolar solutions, calyculin A, and interleukin-1

Na+ -K+ -2Cl- Cotransporter (NKCC) Physiological Function in Nonpolarized Cells and Transporting Epithelia

Two genes encode the Na+ -K+ -2Cl- cotransporters, NKCC1 and NKCC2, that mediate the tightly coupled movement of 1Na+ , 1K+ , and 2Cl- across the plasma membrane of cells. Na+ -K+ -2Cl- cotransport is driven by the chemical gradient of the three ionic species across the membrane, two of them maintained by the action of the Na+ /K+ pump. In many cells, NKCC1 accumulates Cl- above its electrochemical potential equilibrium, thereby facilitating Cl- channel-mediated membrane depolarization. In smooth muscle cells, this depolarization facilitates the opening of voltage-sensitive Ca2+ channels, leading to Ca2+ influx, and cell contraction. In immature neurons, the depolarization due to a GABA-mediated Cl- conductance produces an excitatory rather than inhibitory response. In many cell types that have lost water, NKCC is activated to help the cells recover their volume. This is specially the case if the cells have also lost Cl- . In combination with the Na+ /K+ pump, the NKCC's move ions across various specialized epithelia. NKCC1 is involved in Cl- -driven fluid secretion in many exocrine glands, such as sweat, lacrimal, salivary, stomach, pancreas, and intestine. NKCC1 is also involved in K+ -driven fluid secretion in inner ear, and possibly in Na+ -driven fluid secretion in choroid plexus. In the thick ascending limb of Henle, NKCC2 activity in combination with the Na+ /K+ pump participates in reabsorbing 30% of the glomerular-filtered Na+ . Overall, many critical physiological functions are maintained by the activity of the two Na+ -K+ -2Cl- cotransporters. In this overview article, we focus on the functional roles of the cotransporters in nonpolarized cells and in epithelia. © 2018 American Physiological Society. Compr Physiol 8:871-901, 2018.

Fluid and ion transfer across the blood-brain and blood-cerebrospinal fluid barriers; a comparative account of mechanisms and roles

The two major interfaces separating brain and blood have different primary roles. The choroid plexuses secrete cerebrospinal fluid into the ventricles, accounting for most net fluid entry to the brain. Aquaporin, AQP1, allows water transfer across the apical surface of the choroid epithelium; another protein, perhaps GLUT1, is important on the basolateral surface. Fluid secretion is driven by apical Na+-pumps. K+ secretion occurs via net paracellular influx through relatively leaky tight junctions partially offset by transcellular efflux. The blood-brain barrier lining brain microvasculature, allows passage of O2, CO2, and glucose as required for brain cell metabolism. Because of high resistance tight junctions between microvascular endothelial cells transport of most polar solutes is greatly restricted. Because solute permeability is low, hydrostatic pressure differences cannot account for net fluid movement; however, water permeability is sufficient for fluid secretion with water following net solute transport. The endothelial cells have ion transporters that, if appropriately arranged, could support fluid secretion. Evidence favours a rate smaller than, but not much smaller than, that of the choroid plexuses. At the blood-brain barrier Na+ tracer influx into the brain substantially exceeds any possible net flux. The tracer flux may occur primarily by a paracellular route. The blood-brain barrier is the most important interface for maintaining interstitial fluid (ISF) K+ concentration within tight limits. This is most likely because Na+-pumps vary the rate at which K+ is transported out of ISF in response to small changes in K+ concentration. There is also evidence for functional regulation of K+ transporters with chronic changes in plasma concentration. The blood-brain barrier is also important in regulating HCO3- and pH in ISF: the principles of this regulation are reviewed. Whether the rate of blood-brain barrier HCO3- transport is slow or fast is discussed critically: a slow transport rate comparable to those of other ions is favoured. In metabolic acidosis and alkalosis variations in HCO3- concentration and pH are much smaller in ISF than in plasma whereas in respiratory acidosis variations in pHISF and pHplasma are similar. The key similarities and differences of the two interfaces are summarized.

Endothelin inhibits renin release from juxtaglomerular cells via endothelin receptors A and B via a transient receptor potential canonical-mediated pathway

Renin is the rate-limiting step in the production of angiotensin II: a critical element in the regulation of blood pressure and in the pathogenesis of hypertension. Renin release from the juxtaglomerular (JG) cell is stimulated by the second messenger cAMP and inhibited by increases in calcium (Ca). Endothelins (ETs) inhibit renin release in a Ca-dependent manner. JG cells contain multiple isoforms of canonical transient receptor potential (TRPC) Ca-permeable channels. The proposed hypothesis is that endothelin inhibits renin release by activating TRPC store-operated Ca channels. RT-PCR and immunofluorescence revealed expression of both ETA and ETB receptors in mouse JG cells. Incubation of primary cultures of JG cells with ET-1 (10 nmol/L) decreased renin release by 28%. Addition of either an ETA or an ETB receptor blocker completely prevented the ET inhibition of renin release. Incubation with the TRPC blocker (SKF 96365, 50 μmol/L) completely reversed the Ca-mediated inhibition of renin release by ETs. These results suggest that endothelin inhibits renin release from JG cells via both ETA and ETB receptors, which leads to the activation of TRPC store-operated Ca channels.

Blood-brain barrier Na transporters in ischemic stroke

Blood-brain barrier (BBB) endothelial cells form a barrier that is highly restrictive to passage of solutes between blood and brain. Many BBB transport mechanisms have been described that mediate transcellular movement of solutes across the barrier either into or out of the brain. One class of BBB transporters that is all too often overlooked is that of the ion transporters. The BBB has a rich array of ion transporters and channels that carry Na, K, Cl, HCO3, Ca, and other ions. Many of these are asymmetrically distributed between the luminal and abluminal membranes, giving BBB endothelial cells the ability to perform vectorial transport of ions across the barrier between blood and brain. In this manner, the BBB performs the important function of regulating the volume and composition of brain interstitial fluid. Through functional coupling of luminal and abluminal transporters and channels, the BBB carries Na, Cl, and other ions from blood into brain, producing up to 30% of brain interstitial fluid in healthy brain. During ischemic stroke cerebral edema forms by processes involving increased activity of BBB luminal Na transporters, resulting in "hypersecretion" of Na, Cl, and water into the brain interstitium. This review discusses the roles of luminal BBB Na transporters in edema formation in stroke, with an emphasis on Na-K-Cl cotransport and Na/H exchange. Evidence that these transporters provide effective therapeutic targets for reduction of edema in stroke is also discussed, as are recent findings regarding signaling pathways responsible for ischemia stimulation of the BBB Na transporters.

High brain ammonia tolerance and down-regulation of Na+:K+:2Cl(-) Cotransporter 1b mRNA and protein expression in the brain of the Swamp Eel, Monopterus albus, exposed to environmental ammonia or terrestrial conditions

Na(+):K(+):2Cl(-) cotransporter 1 (NKCC1) has been implicated in mediating ischemia-, trauma- or ammonia-induced astrocyte swelling/brain edema in mammals. This study aimed to determine the effects of ammonia or terrestrial exposure on ammonia concentrations in the plasma and brain, and the mRNA expression and protein abundance of nkcc/Nkcc in the brain, of the swamp eel Monopterusalbus. Ammonia exposure led to a greater increase in the ammonia concentration in the brain of M. albus than terrestrial exposure. The brain ammonia concentration of M. albus reached 4.5 µmol g(-1) and 2.7 µmol g(-1) after 6 days of exposure to 50 mmol l(-1) NH4Cl and terrestrial conditions, respectively. The full cDNA coding sequence of nkcc1b from M. albus brain comprised 3276 bp and coded for 1092 amino acids with an estimated molecular mass of 119.6 kDa. A molecular characterization indicated that it could be activated through phosphorylation and/or glycosylation by osmotic and/or oxidative stresses. Ammonia exposure for 1 day or 6 days led to significant decreases in the nkcc1b mRNA expression and Nkcc1b protein abundance in the brain of M. albus. In comparison, a significant decrease in nkcc1b mRNA expression was observed in the brain of M. albus only after 6 days of terrestrial exposure, but both 1 day and 6 days of terrestrial exposure resulted in significant decreases in the protein abundance of Nkcc1b. These results are novel because it has been established in mammals that ammonia up-regulates NKCC1 expression in astrocytes and NKCC1 plays an important role in ammonia-induced astrocyte swelling and brain edema. By contrast, our results indicate for the first time that M. albus is able to down-regulate the mRNA and protein expression of nkcc1b/Nkcc1b in the brain when confronted with ammonia toxicity, which could be one of the contributing factors to its extraordinarily high brain ammonia tolerance.

Characterization of neuroprotective effects of biphalin, an opioid receptor agonist, in a model of focal brain ischemia

Approximately 795,000 people experience a new or recurrent stroke in the United States annually. The purpose of this study was to assess the protective effect of a nonselective opioid receptor agonist, biphalin, in brain edema and infarct damage by using both in vitro and in vivo models of stroke. In an in vivo model of ischemia, biphalin significantly decreased edema (66.6 and 58.3%) and infarct (52.2 and 56.4%) ratios in mouse transient (60-min occlusion/24-h reperfusion) and permanent (6 h) middle cerebral artery occlusion models, respectively. Biphalin administration also showed decreased neurodegeneration in hippocampal, cortical, and striatal brain tissue after ischemia, evidenced by reduced Fluoro-Jade C staining. In addition, biphalin improved neurological function after stroke injury evidenced by neurological score and locomotor activity evaluation. Biphalin significantly decreased penumbral expression of Na(+), K(+), 2Cl(-) cotransporter (NKCC) and the translocation of the conventional isoforms of protein kinase C (PKC). It also reversed the activation of PKC-induced cell volume increase during ischemia in primary neuronal cell cultures exposed to 1 h of oxygen glucose deprivation. These data suggest that opioid receptor activation provides neuroprotection during stroke, and a possible explanation of this mechanism could be the inhibition of NKCC function via the regulation of PKC-dependent cell signaling.

Molecular biology of the blood-brain and the blood-cerebrospinal fluid barriers: similarities and differences

Efficient processing of information by the central nervous system (CNS) represents an important evolutionary advantage. Thus, homeostatic mechanisms have developed that provide appropriate circumstances for neuronal signaling, including a highly controlled and stable microenvironment. To provide such a milieu for neurons, extracellular fluids of the CNS are separated from the changeable environment of blood at three major interfaces: at the brain capillaries by the blood-brain barrier (BBB), which is localized at the level of the endothelial cells and separates brain interstitial fluid (ISF) from blood; at the epithelial layer of four choroid plexuses, the blood-cerebrospinal fluid (CSF) barrier (BCSFB), which separates CSF from the CP ISF, and at the arachnoid barrier. The two barriers that represent the largest interface between blood and brain extracellular fluids, the BBB and the BCSFB, prevent the free paracellular diffusion of polar molecules by complex morphological features, including tight junctions (TJs) that interconnect the endothelial and epithelial cells, respectively. The first part of this review focuses on the molecular biology of TJs and adherens junctions in the brain capillary endothelial cells and in the CP epithelial cells. However, normal function of the CNS depends on a constant supply of essential molecules, like glucose and amino acids from the blood, exchange of electrolytes between brain extracellular fluids and blood, as well as on efficient removal of metabolic waste products and excess neurotransmitters from the brain ISF. Therefore, a number of specific transport proteins are expressed in brain capillary endothelial cells and CP epithelial cells that provide transport of nutrients and ions into the CNS and removal of waste products and ions from the CSF. The second part of this review concentrates on the molecular biology of various solute carrier (SLC) transport proteins at those two barriers and underlines differences in their expression between the two barriers. Also, many blood-borne molecules and xenobiotics can diffuse into brain ISF and then into neuronal membranes due to their physicochemical properties. Entry of these compounds could be detrimental for neural transmission and signalling. Thus, BBB and BCSFB express transport proteins that actively restrict entry of lipophilic and amphipathic substances from blood and/or remove those molecules from the brain extracellular fluids. The third part of this review concentrates on the molecular biology of ATP-binding cassette (ABC)-transporters and those SLC transporters that are involved in efflux transport of xenobiotics, their expression at the BBB and BCSFB and differences in expression in the two major blood-brain interfaces. In addition, transport and diffusion of ions by the BBB and CP epithelium are involved in the formation of fluid, the ISF and CSF, respectively, so the last part of this review discusses molecular biology of ion transporters/exchangers and ion channels in the brain endothelial and CP epithelial cells.

Intracellular pH regulation in human Sertoli cells: role of membrane transporters

Sertoli cells are responsible for regulating a wide range of processes that lead to the differentiation of male germ cells into spermatozoa. Intracellular pH (pHi) is an important parameter in cell physiology regulating namely cell metabolism and differentiation. However, pHi regulation mechanisms in Sertoli cells have not yet been systematically elucidated. In this work, pHi was determined in primary cultures of human Sertoli cells. Sertoli cells were exposed to weak acids, which caused a rapid acidification of the intracellular milieu. pHi then recovered by a mechanism that was shown to be particularly sensitive to the presence of the inhibitor DIDS (4,4′-diisothiocyanostilbene disulfonic acid). In the presence of amiloride and PSA (picrylsulfonic acid), pHi recovery was also significantly affected. These results indicate that, in the experimental conditions used, pHi is regulated by the action of an Na+-driven HCO3−/Cl−exchanger and an Na+/HCO3−co-transporter and also by the action of the Na+/H+exchanger. On the other hand, pHi recovery was only slightly affected by concanamycin A, suggesting that V-Type ATPases do not have a relevant action on pHi regulation in human Sertoli cells, and was independent of the presence of bumetanide, suggesting that the inhibition of the Na+/K+/Cl−co-transporter does not affect pHi recovery, not even indirectly via the shift of ionic gradients. Finally, pHi was shown to be sensitive to the removal of external Cl−, but not of Na+or K+, evidencing the presence of a membrane Cl−-dependent base extruder, namely the Na+-independent HCO3−/Cl−exchanger, and its role on pHi maintenance on these cells.

Shear stress and 17β-estradiol modulate cerebral microvascular endothelial Na-K-Cl cotransporter and Na/H exchanger protein levels

Ion transporters of blood-brain barrier (BBB) endothelial cells play an important role in regulating the movement of ions between the blood and brain. During ischemic stroke, reduction in cerebral blood flow is accompanied by transport of Na and Cl from the blood into the brain, with consequent brain edema formation. We have shown previously that a BBB Na-K-Cl cotransporter (NKCC) participates in ischemia-induced brain Na and water uptake and that a BBB Na/H exchanger (NHE) may also participate. While the abrupt reduction of blood flow is a prominent component of ischemia, the effects of flow on BBB NKCC and NHE are not known. In the present study, we examined the effects of changes in shear stress on NKCC and NHE protein levels in cerebral microvascular endothelial cells (CMECs). We have shown previously that estradiol attenuates both ischemia-induced cerebral edema and CMEC NKCC activity. Thus, in the present study, we also examined the effects of estradiol on NKCC and NHE protein levels in CMECs. Exposing CMECs to steady shear stress (19 dyn/cm2) increased the abundance of both NKCC and NHE. Estradiol abolished the shear stress-induced increase in NHE but not NKCC. Abrupt reduction of shear stress did not alter NKCC or NHE abundance in the absence of estradiol, but it decreased NKCC abundance in estradiol-treated cells. Our results indicate that changes in shear stress modulate BBB NKCC and NHE protein levels. They also support the hypothesis that estradiol attenuates edema formation in ischemic stroke in part by reducing the abundance of BBB NKCC protein.

Characterization of SPAK and OSR1, regulatory kinases of the Na-K-2Cl cotransporter

Our recent studies demonstrate that SPAK (Ste20p-related Proline Alanine-rich Kinase), in combination with WNK4 [With No lysine (K) kinase], phosphorylates and stimulates the Na-K-2Cl cotransporter (NKCC1), whereas catalytically inactive SPAK (K104R) fails to activate the cotransporter. The catalytic domain of SPAK contains an activation loop between the well-conserved DFG and APE motifs. We speculated that four threonine residues (T231, T236, T243, and T247) in the activation loop might be sites of phosphorylation and kinase activation; therefore, we mutated each residue into an alanine. In this report, we demonstrate that coexpression of SPAK (T243A) or SPAK (T247A) with WNK4 not only prevented, but robustly inhibited, cotransporter activity in NKCC1-injected Xenopus laevis oocytes. These activation loop mutations produced an effect similar to that of the SPAK (K104R) mutant. In vitro phosphorylation experiments demonstrate that both intramolecular autophosphorylation of SPAK and phosphorylation of NKCC1 are significantly stronger in the presence of Mn2+ rather than Mg2+. We also show that SPAK activity is markedly inhibited by staurosporine and K252a, partially inhibited by N-ethylmaleimide and diamide, and unaffected by arsenite. OSR1, a kinase closely related to SPAK, exhibited similar kinase properties and similar functional activation of NKCC1 when coexpressed with WNK4.

Protein kinase C family members as a target for regulation of blood-brain barrier Na,K,2Cl-cotransporter during in vitro stroke conditions and nicotine exposure

PURPOSE

The aim of the study is to identify specific protein kinase C (PKC) isoforms involvement in K(+) transport mediated at altered blood-brain barrier (BBB) response to stroke conditions with prior nicotine exposure, which provides ways to intervene pharmacologically in PKC-mediated molecular pathways that could lead to effective treatment for smoking stroke patients.

METHODS

Changes in PKC isoform levels were studied in the cytosolic and membrane fractions of bovine brain microvessel endothelial cells subjected to stroke conditions as well as nicotine/cotinine exposure. Furthermore, abluminal Na,K,2Cl-cotransporter (NKCC) activity regulated by specific conventional PKC isoform activators and inhibitors was investigated using rubidium ((86)Rb) uptake studies.

RESULTS

Membrane-bound PKCalpha, PKCbetaI, and PKCepsilon levels were increased after 6 h hypoxia/aglycemia, and this was attenuated by 24-h nicotine/cotinine exposure. Interestingly, membrane-bound PKCgamma protein level was decreased after 6 h hypoxia/aglycemia and increased by 24-h nicotine/cotinine exposure. (86)Rb uptake studies showed that basolateral NKCC activity was down-regulated by both a conventional PKC inhibitor and specific inhibitors for PKCalpha, PKCbeta, and PKCvarepsilon and was up-regulated by an activator of conventional PKCs during 6-h hypoxia/aglycemia treatment.

CONCLUSION

Specific PKC inhibitors or activators might be designed to individualize stroke therapies and improve health outcome for smokers by rebalancing ion transport into and out of the brain.

Tobacco smoke chemicals attenuate brain-to-blood potassium transport mediated by the Na,K,2Cl-cotransporter during hypoxia-reoxygenation

Smoking tobacco, including cigarettes, has been associated with an increased incidence and relative risk for cerebral infarction in both men and women. Recently, we have shown that nicotine and cotinine attenuate abluminal (brain facing) K(+) uptake mediated by the Na,K,2Cl-cotransporter (NKCC) in bovine brain microvessel endothelial cells (BBMECs) after hypoxic/aglycemic exposure (stroke conditions). The purpose of the current study was to explore the effects of nicotine and tobacco smoke chemicals on K(+) movement through the blood-brain barrier during both hypoxia/aglycemia and reoxygenation. BBMECs were exposed to nicotine/cotinine, nicotine-containing cigarette smoke extract (N-CSE), or nicotine-free cigarette smoke extract (NF-CSE) in quantities designed to mimic plasma concentrations of smokers. Stroke conditions were mimicked in vitro in BBMECs through 6 h of hypoxia/aglycemia with or without 12 h of reoxygenation, after which NKCC-mediated K(+) uptake and paracellular integrity were measured with (86)Rb and [(14)C]sucrose, respectively. In addition, K(+) concentrations in brain extracellular fluid were estimated in (86)Rb-injected rats that were administered nicotine, N-CSE, or NF-CSE and on whom global ischemia/reperfusion by in vivo four-vessel occlusion was performed. Both in vitro and in vivo paradigms showed nicotine, the major alkaloid present in tobacco smoke, to be the determining factor of an inhibited response of abluminal NKCC in BBMECs during and after stroke conditions. This was measured as a decrease in abluminal brain endothelial cell NKCC activity and as an increase in brain extracellular K(+) concentration measured as the brain extracellular fluid (86)Rb/plasma ratio after in vivo four-vessel occlusion with reperfusion.

Endothelin-1 in plasma, cisternal CSF and microdialysate following aneurysmal SAH

OBJECTIVE

Endothelin-1 (ET-1) is postulated to play an important role in the development of cerebral vasospasm (CVS) following SAH. This study was conducted to investigate the time course of ET-release in three different sources: CSF, plasma and microdialysate.

METHODS

In a prospective study ET-1-concentrations were measured in plasma, cisternal CSF and microdialysate in 20 patients with aneurysmal SAH for at least 8 days after hemorrhage.

RESULTS

ET-1 concentration in microdialysate was almost four times higher compared to CSF and plasma. (p<0.001) Only in CSF ET-1-release showed a significant increase over time with highest values on day 5 post ictus (p = 0.03). This was parallel to the increase of transcranial Doppler velocities. ET-1 in plasma and microdialysate did not change over time.

CONCLUSION

ET-1 may have a different biological function in different biological tissues. Only ET-1 in CSF seemed to be associated with CVS.

Arginine vasopressin stimulation of cerebral microvascular endothelial cell Na-K-Cl cotransporter activity is V1receptor and [Ca] dependent

Ischemia-induced brain edema formation is mediated by increased transport of Na and Cl across an intact blood-brain barrier (BBB). Our previous studies have provided evidence that a luminally located BBB Na-K-Cl cotransporter is stimulated during cerebral ischemia to increase transport of Na and Cl into the brain. The main focus of the present study was to evaluate the effects of arginine vasopressin (AVP), previously shown to be increased in the brain during ischemia and to promote edema formation, on activity of the BBB cotransporter. Cerebral microvascular endothelial cell (CMEC) monolayers were cultured in astroglial cell conditioned medium, and Na-K-Cl cotransporter activity was assessed as bumetanide-sensitive86Rb influx. In both human and bovine CMECs, as well as in freshly isolated microvessels, AVP stimulated cotransport activity. This stimulatory effect was mimicked by V1but not V2vasopressin agonists and was blocked by V1but not V2vasopressin antagonists. Consistent with a V1vasopressin receptor mechanism of action, AVP caused an increase in CMEC intracellular [Ca] that was blocked by a V1antagonist. Exposing the cells to [Ca]-free media and/or reducing intracellular [Ca] by BAPTA also blocked AVP stimulation of CMEC cotransporter activity, as did the phospholipase C inhibitor U-73122. Finally, we found that while stimulation of CMEC cotransporter activity by AVP occurred within minutes, it was also sustained for hours in the continued presence of AVP. These findings support the hypothesis that AVP, through a V1receptor- and [Ca]-dependent mechanism, stimulates the BBB Na-K-Cl cotransporter to participate in ischemia-induced edema formation.

Bumetanide reduces cerebral edema formation in rats with diabetic ketoacidosis

The mechanisms responsible for cerebral edema formation in diabetic ketoacidosis (DKA) are not well understood, although evidence suggests ischemia as a contributing factor. Previous studies have shown that the Na-K-Cl cotransporter of cerebral microvascular endothelial cells and astrocytes is a major participant in ischemia-induced cerebral edema in stroke. The present study was conducted to test the hypothesis that the Na-K-Cl cotransporter also contributes to cerebral edema in DKA. Sprague-Dawley rats were administered streptozotocin to induce DKA, and then cerebral edema was assessed by determination of apparent diffusion coefficients (ADC) with magnetic resonance diffusion-weighted imaging. Cerebral ADC values in DKA rats were significantly reduced in both cortex and striatum compared with non-DKA control rats, indicating the presence of cerebral edema. Intravenous administration of bumetanide to DKA rats abolished the drop in cortical ADC values, while having no significant effect in the striatum. Insulin and saline treatment had no effect when given after bumetanide but increased both cortical and striatal ADC values when given before bumetanide. Evidence is also presented here that acetoacetate and beta-hydroxybutyrate stimulate brain microvascular Na-K-Cl cotransporter activity. These findings suggest that the Na-K-Cl cotransporter contributes to brain edema in DKA.

Bumetanide inhibition of the blood-brain barrier Na-K-Cl cotransporter reduces edema formation in the rat middle cerebral artery occlusion model of stroke

Increased transport of Na+ across an intact blood-brain barrier (BBB) participates in edema formation during the early hours of cerebral ischemia. In previous studies, the authors showed that the BBB Na-K-Cl cotransporter is stimulated by factors present during ischemia, suggesting that the cotransporter may contribute to the increased brain Na+ uptake in edema. The present study was conducted to determine (1) whether the Na-K-Cl cotransporter is located in the luminal membrane of the BBB, and (2) whether inhibition of the BBB cotransporter reduces brain edema formation. Perfusion-fixed rat brains were examined for cotransporter distribution by immunoelectron microscopy. Cerebral edema was evaluated in rats subjected to permanent middle cerebral artery occlusion (MCAO) by magnetic resonance diffusion-weighted imaging and calculation of apparent diffusion coefficients (ADC). The immunoelectron microscopy studies revealed a predominant (80%) luminal membrane distribution of the cotransporter. Magnetic resonance imaging studies showed ADC ratios (ipsilateral MCAO/contralateral control) ranging from 0.577 to 0.637 in cortex and striatum, indicating substantial edema formation. Intravenous bumetanide (7.6-30.4 mg/kg) given immediately before occlusion attenuated the decrease in ADC ratios for both cortex and striatum (by 40-67%), indicating reduced edema formation. Bumetanide also reduced infarct size, determined by TTC staining. These findings suggest that a luminal BBB Na-K-Cl cotransporter contributes to edema formation during cerebral ischemia.

Regulation of Blood-Brain Barrier Na,K,2Cl-Cotransporter through Phosphorylation during in Vitro Stroke Conditions and Nicotine Exposure

Nicotine, a major constituent of tobacco smoke, has important effects on brain recovery after focal ischemia (Wang et al., 1997). The purpose of this work is to systematically test the effects of nicotine during stroke conditions on blood-brain barrier (BBB) potassium transport, protein expression of the Na,K,2Cl-cotransporter (NKCC), and cell signaling pathways that control NKCC activity at the BBB. Confluent bovine brain microvessel endothelial cells (BBMECs) were exposed to both a hypoxic/aglycemic (H/A) environment to model BBB function during stroke conditions and nicotine and cotinine (N/C) to model plasma levels seen in smokers. BBMECs exhibit both Na,K-ATPase and NKCC activity (60 and 34 nmol/min/g, respectively) that contribute to 98% of the K(+) uptake in cultured endothelial cells. An adaptive up-regulation of NKCC activity was identified to occur on the basolateral surface of the BBB after in vitro stroke conditions. Twenty-four hours of N/C exposure, at doses equivalent to plasma levels of smokers, combined with 6 h of H/A, reduced NKCC protein expression and total NKCC activity (shown by bumetanide-sensitive (86)RB uptake) compared with 6 h of H/A alone (P < 0.01). Basolateral K(+) transport was found to be modulated by nicotinic acetylcholine receptors expressed at the BBB. NKCC activity on the basolateral surface of the BBB is controlled by an ongoing phosphorylation/dephosphorylation processes. We have identified a potential mechanism in altered BBB response to stroke conditions with prior N/C exposure directly implicating damage to brain-to-blood K(+) transport mediated at the BBB and perhaps neuronal recovery after stroke.

Blood-brain barrier permeability to ammonia in liver failure: a critical reappraisal

In patients with acute liver failure (ALF), hyperammonemia is related to development of cerebral edema and herniation. The present review discusses the mechanisms for the cerebral uptake of ammonia. A mathematical framework is provided to allow a quantitative examination of whether published studies can be explained by the conventional view that cerebral uptake of ammonia is restricted to diffusion of the unprotonated form (NH(3)) (the diffusion hypothesis). An increase in cerebral blood flow (CBF) enhanced ammonia uptake more than expected, possibly due to recruitment or heterogeneity of brain capillaries. Reported effects of pH on ammonia uptake were in the direction predicted by the diffusion hypothesis, but often less pronounced than expected. The published effects of mannitol, cooling, and indomethacin in experimental animals and patients were difficult to explain by the diffusion hypothesis alone, unless dramatic changes of capillary surface area or permeability for ammonia were induced. Therefore we considered the possible role of membrane protein mediated transport of NH(4)(+) across the blood-brain barrier (BBB). Early tracer studies in Rhesus monkeys suggested that NH(4)(+) is responsible for 20% or even more of the transport of ammonia from plasma to brain. In other locations, such as in the thick ascending limb of Hendle's loop and in isolated astrocytes, transport protein mediated translocation of NH(4)(+) is predominant. Many of the ion-transporters involved in renal NH(4)(+) reabsorbtion are also present in brain capillary membranes and could mediate uptake of NH(4)(+). Astrocytic uptake of NH(4)(+) is associated with increased extracellular K(+), which is a potent cerebral vasodilator. Such interference between transport of NH(4)(+) and other cations could be clinically important because increased cerebral blood flow often precedes cerebral herniation in acute liver failure. We suggest that protein mediated transport of NH(4)(+) through the brain capillary wall is a realistic possibility that should be more intensely studied.

New therapeutics that antagonize endothelin: promises and frustrations

The discovery of endothelin--a highly potent endogenous vasoconstrictor - in 1988 has led to considerable efforts to develop antagonists of endothelin receptors that could have therapeutic potential in disorders including hypertension, heart failure and renal diseases. However, in general, the results of trials in humans have not mirrored the highly promising effects in animal disease models. Here, we discuss preclinical and clinical results with endothelin antagonists, and consider possible approaches to fully realizing the potential of endothelin antagonism.

Role of EDHF in the vasodilatory effect of loop diuretics in guinea-pig mesenteric resistance arteries

1. Relaxing effect of loop diuretics, piretanide and furosemide in comparison with acetylcholine (ACh) was investigated in guinea-pig isolated mesenteric resistance arteries. 2. Concentration-response curves to ACh (0.001 - 10 microM) and diuretics (0.0001 - 1 microM) were constructed in noradrenaline (10 - 30 microM)-precontracted arteries incubated either in normal physiological salt solution (PSS) or in 30 mM KCl PSS (K-PSS). 3. In PSS, maximal relaxations (R(max)) and pD(2) to ACh were 87+/-2% and 7.1+/-0.1 (n=10). L-N(G)-nitro-arginine methyl ester (L-NAME, 100 microM) reduced R(max) by 20% (P<0.01, n=7) and pD(2) by 10% (P<0.01). In contrast, indomethacin (10 microM) increased R(max) by 19% (P<0.01, n=8) and pD(2) by 10% (P<0.05). Combination of L-NAME+indomethacin reversed the effect observed with either of these inhibitors used alone. In K-PSS, R(max) was attenuated by 40% (P<0.001, n=6) compared to PSS. L-NAME reduced R(max) by 65% (P<0.01, n=5) and increased pD(2) by 15 fold. L-NAME+indomethacin suppressed the resistant relaxation. 4. In PSS+L-NAME+indomethacin, inhibitors of small (SK(Ca); apamin, 0.1 microM) and large (BK(Ca); iberiotoxin and charybdotoxin, 0.1 microM) conductance Ca(2+)-sensitive K(-)-channels used alone had little effect on the ACh-response. Combination of apamin+iberiotoxin reduced R(max) by 40% (P<0.05, n=7) while apamin+charybdotoxin fully abolished the resistant relaxation. 5. In PSS, piretanide and furosemide induced relaxation with R(max): 89+/-3% vs 84+/-5% and pD(2): 8.5+/-0.1 vs 7.7+/-0.2 (P<0.01) for piretanide (n=11) and furosemide (n=10), respectively. Endothelial abrasion suppressed relaxation to diuretics. L-NAME and indomethacin used alone or in combination did not significantly modify the response to diuretics. 6. In K-PSS, piretanide-induced relaxation was abolished whereas that to furosemide was reduced by 70% (P<0.001, n=9) compared to PSS and was suppressed by L-NAME+indomethacin. In PSS+L-NAME+indomethacin, apamin slightly reduced relaxation to diuretics whereas charybdotoxin or iberiotoxin abolished the response. 7. These results indicate that ACh-evoked relaxation is mediated by both NO/PGl(2)-dependent and -independent mechanisms. The EDHF-dependent component relies on activation of Ca(2+)-activated K(+) channels, is sensitive to a combination of apamin+charybdotoxin and to a smaller degree to a combination of apamin+iberiotoxin. Loop diuretic-induced relaxation is endothelium-dependent, appears to be mediated by NO, PGl(2) and EDHF for furosemide and EDHF only for piretanide. For the two diuretics, opening of BK(Ca) channels may be involved in the relaxation.

Genistein activates CFTR-mediated Cl(-) secretion in the murine trachea and colon

The action of the isoflavone genistein on the cystic fibrosis transmembrane conductance regulator (CFTR) has been studied in many cell systems but not in intact murine tissues. We have investigated the action of genistein on murine tissues from normal and cystic fibrosis (CF) mice. Genistein increased the short-circuit current (I(sc)) in tracheal (16.4 +/- 2.8 microA/cm(2)) and colonic (40.0 +/- 4.4 microA/cm(2)) epithelia of wild-type mice. This increase was inhibited by furosemide, diphenylamine-2-carboxylate, and glibenclamide, but not by DIDS. In contrast, genistein produced no significant change in the I(sc) of the tracheal epithelium (0.9 +/- 1.1 microA/cm(2)) and decreased the I(sc) of colons from CF null (-13.1 +/- 2.3 microA/cm(2)) and DeltaF508 mice (-10.3 +/- 1.3 microA/cm(2)). Delivery of a human CFTR cDNA-liposome complex to the airways of CF null mice restored the genistein response in the tracheas to wild-type levels. Tracheas from DeltaF508 mice were also studied: 46% of trachea showed no response to genistein, whereas 54% gave an increase in I(sc) similar to that in wild type. We conclude that genistein activates CFTR-mediated Cl(-) secretion in the murine trachea and distal colon.

Characterization of an endothelin ET(B) receptor in the gill of the dogfish shark Squalus acanthias

Endothelins (ETs) are potent vasoconstrictive peptides that are secreted by the vascular endothelium and other tissues in vertebrates. Previous studies have demonstrated that ETs are expressed in a variety of fish tissues and contract various blood vessels. In order to determine if receptors for ET are expressed in fish gill tissue, we examined the binding kinetics of (125)I-labeled, human ET-1 to membrane fragments isolated from the gill of the dogfish shark, Squalus acanthias. (125)I-ET-1 bound at a single site, with a dissociation constant (K(d)) and binding site number (B(max)) very similar to those described in a variety of mammalian blood vessels. ET-1 and ET-3 competed equally with (125)I-ET-1, suggesting that the receptor was ET(B), which has been shown in mammalian systems to bind to both ligands equally. The ET(B)-specific agonists sarafotoxin S6c, IRL-1620, and BQ-3020 also competed against (125)I-ET-1 at a single site, supporting this hypothesis. We conclude that the shark gill expresses an ET(B) receptor with substantial homology to the mammalian receptor and that ET may play an important role in modulating such vital gill functions as gas exchange, ion regulation, acid-base balance, and excretion of nitrogen.

Blood-brain barrier, ion homeostatis and epilepsy: possible implications towards the understanding of ketogenic diet mechanisms

The finding that epileptic seizures alter blood-brain barrier (BBB) properties has stimulated interest into the possibility that phenotypic changes in brain endothelium may constitute a pathological initiator leading to seizures. Recent evidence obtained from epileptic patients undergoing cortical resection, demonstrated abnormal expression of glucose transporter molecules (GLUT1), while [18F]deoxyglucose PET studies demonstrated regions of decreased glucose uptake and hypometabolism in seizure foci. The properties of other 'nonexcitable CNS cells' are also altered in epileptic tissue, and glial cells from epileptic brain displayed diminished capacity for ionic homeostasis; voltage-dependent mechanisms were primarily affected, increasing reliance on energy-dependent mechanisms. Diminished ion homeostasis together with increased metabolic demand of hyperactive neurons may further aggravate the neuropathological consequences of BBB loss of glucose uptake mechanisms. Since ketone bodies can provide an alternative to glucose to support brain energy requirements, it is hypothesized that one of the mechanisms of the ketogenic diet in epilepsy may relate to increased availability of beta-hydroxybutyrate, a ketone body readily transported at the BBB. This hypothesis is supported by the fact that the ketogenic diet is the treatment of choice for the glucose transporter protein syndrome and pyruvate dehydrogenase deficiency, both associated with cerebral energy failure and seizures.

An in situ cytochemical evaluation of blood-brain barrier sodium, potassium-activated adenosine triphosphatase polarity

It is presently believed that sodium, potassium-activated adenosine triphosphatase (Na+, K+-ATPase) is localized on the abluminal plasma membrane of brain endothelial cells. But there have been contrary reports from some cytochemical studies. We examined the localization of the enzyme in rat cerebral microvessel endothelium using the in situ model originally employed to establish the abluminal polarity concept. Alterations in fixation and incubation media from the original reports were conducted to determine the effect on localization pattern. With the Ernst indirect incubation method as originally used, three types of localization patterns were obtained: abluminal only, luminal only, and on both surfaces of endothelial cells. With the direct incubation method of Mayahara, reaction product was seen on both surfaces. Reduction in fixation time followed by the use of the indirect incubation method resulted in a complete loss of the reaction product. The same reduction in fixation time followed by the use of the direct method did not alter the localization pattern of the enzyme. Our results demonstrated that Na+, K+-ATPase is localized on both surfaces of brain endothelial cells. The localization pattern of Na+, K+-ATPase is significantly dependent upon fixation and the incubation medium used in the in situ model. Data discrepancies for the enzyme as reported in the literature appear to be caused by differences in cytochemical protocols, rather than the biological reasons advocated by other investigators. We conclude that past cytochemical reports of blood-brain barrier (BBB) Na+, K+-ATPase abluminal localization were incomplete. The currently held abluminal polarity theory of the enzyme needs to be reexamined. Past basic and clinical cytochemical studies of BBB Na+, K+-ATPase should be viewed and interpreted with caution.

Na+-K+-2Cl- cotransport in Ehrlich cells: regulation by protein phosphatases and kinases

To identify protein kinases (PK) and phosphatases (PP) involved in regulation of the Na+-K+-2Cl- cotransporter in Ehrlich cells, the effect of various PK and PP inhibitors was examined. The PP-1, PP-2A, and PP-3 inhibitor calyculin A (Cal-A) was a potent activator of Na+-K+-2Cl- cotransport (EC50 = 35 nM). Activation by Cal-A was rapid (<1 min) but transient. Inactivation is probably due to a 10% cell swelling and/or the concurrent increase in intracellular Cl- concentration. Cell shrinkage also activates the Na+-K+-2Cl- cotransport system. Combining cell shrinkage with Cal-A treatment prolonged the cotransport activation compared with stimulation with Cal-A alone, suggesting PK stimulation by cell shrinkage. Shrinkage-induced cotransport activation was pH and Ca2+/calmodulin dependent. Inhibition of myosin light chain kinase by ML-7 and ML-9 or of PKA by H-89 and KT-5720 inhibited cotransport activity induced by Cal-A and by cell shrinkage, with IC50 values similar to reported inhibition constants of the respective kinases in vitro. Cell shrinkage increased the ML-7-sensitive cotransport activity, whereas the H-89-sensitive activity was unchanged, suggesting that myosin light chain kinase is a modulator of the Na+-K+-2Cl- cotransport activity during regulatory volume increase.

Na-K-Cl cotransporter expression in the developing and senescent gerbil cochlea

Changes in the cellular expression pattern of the Na-K-Cl cotransporter (NKCC) were investigated during postnatal development and with advancing age in the gerbil cochlea. At birth, faint immunostaining for NKCC was discernable in the developing stria vascularis (StV), Reissner's membrane, interdental cells and some relatively undifferentiated cells lining the cochlear partition. Between 2 and 4 days after birth (DAB) immunostaining persisted and increased in the future interdental, inner and outer sulcus and claudius cells but then disappeared from these sites by 8 DAB. In contrast, NKCC immunoreactivity in the StV increased progressively during development and approached adult levels by 12 DAB. Immunostaining for NKCC in subpopulations of fibrocytes in the inferior portion of the spiral ligament, the suprastrial region and the spiral limbus was first detectable between 10 and 12 DAB and staining intensity reached adult levels around 16 DAB. Changes in NKCC expression with advancing age generally mimicked those previously observed for Na,K-ATPase in focal regions of atrophic lateral wall. Diminished immunostaining was first seen in the StV, presumably associated with the involution of the marginal cell's basolateral processes. Further atrophy culminated in complete loss of immunostaining in the StV and an associated down-regulation of NKCC expression in spiral ligament transport fibrocytes. The marked similarities in the developmental and age-related expression patterns of NKCC and Na,K-ATPase point to a high level of functional cooperativity between these two ion transport mediators, which together provide an efficient mechanism for generating and maintaining high K+ levels in endolymph and the endocochlear potential.

Salidiuretic action by genistein in the isolated, perfused rat kidney

The urinary isoflavonoid genistein inhibits membrane Na-K-Cl cotransporters at similar concentrations as furosemide, but the significance of this action is unknown. Genistein was therefore investigated in rats for its potential salidiuretic actions. In the isolated, perfused rat kidney, genistein induced a maximal salidiuretic action similar to that of furosemide but was 3 to 5 times less potent than furosemide in terms of active doses (natriuresis EC50, 237+/-92 versus 56+/-20 micromol/L for genistein and furosemide, respectively). Genistein and furosemide had no additive salidiuretic actions. Genistein had no significant effect on glomerular filtration rate but was able to significantly reduce renal vascular resistance with respect to vehicle isolated perfused kidney. Indomethacin (10 micromol/L), a blocker of prostaglandin biosynthesis, reduced salidiuresis and renal vasorelaxation by genistein. Subcutaneous genistein (15 mg/kg) induced a statistically significant increase in diuresis and natriuresis with respect to vehicle during the first 6 hours of administration in rats. In conclusion, genistein compares well with furosemide in vitro for its salidiuretic profile and potency in the isolated perfused rat kidney and is also natriuretic by the subcutaneous route in the rat. Further studies are required to investigate potential natriuretic and perhaps hypotensive actions of dietary genistein.

The effect of endothelins on ion transport systems in cultured rat brain capillary endothelial cells

Brain capillary endothelial cells regulate the movement of ions and water across the blood-brain barrier via specific ion transport systems. Disturbances in these ion transport systems are involved in the formation of ischemic brain edema. This study describes the effects of endothelins (i.e., ET-1 and ET-3) on ion transport systems in cultured rat brain capillary endothelial cells using 86Rb+ and 22Na+ as markers for K+ and Na+, respectively. ET-1 stimulated K+ uptake and efflux with EC50 values of 0.6 nM and 0.5 nM, respectively. The potencies of ET-3 on these responses were considerably lower. Both ET-1 and ET-3 stimulated Na+ uptake through a Na+/H+ exchange system with similar potencies (i.e., EC50 = 0.80 nM and 1.89 nM, respectively). ET-stimulated K+ uptake, K+ efflux, and Na+ uptake activities were all inhibited by BQ123 (selective ETA receptor antagonist). ET-1 stimulated K+ uptake and efflux, in contrast to Na+ uptake, were also reduced by protein kinase C inhibitors and by an intracellular Ca2+ chelator. The results suggest that ETs can affect the activities of ion and water transport at the blood-brain barrier through different signal transduction mechanisms.

Functional characterization of endothelin receptors on cultured brain capillary endothelial cells of the rat

This report describes the effects of endothelins (ET-1 and ET-3) on ion transport systems expressed on cultured rat brain capillary endothelial cells (RBEC) and includes investigation of pharmacological properties of ET receptors, their reactivity and induction of signal transduction pathways. ET-1 stimulated IP3 formation and Ca2+ uptake with half-maximal effective concentrations (EC50) of 0.68 and 0.93 nM, respectively; the effects of ET-3 on these responses were much weaker. ET-1-stimulated IP3 formation and Ca2+ uptake were inhibited by an ETA antagonist (BQ123) and a phospholipase C (PLC) inhibitor (U73122), indicating the presence of ETA receptors coupled to PLC. ET-1 stimulated K+ efflux (through a quinine-sensitive mechanism) and K+ uptake (through both ouabain-sensitive and bumetanide-sensitive mechanisms) with EC50 of 0.59 and 0.68 nM, respectively. The potencies of ET-3 on these responses were considerably lower than those of ET-1. By contrast, ET-1 or ET-3 stimulated Na+ uptake with similarly high potencies (EC50 = 0.80 and 1.89 nM, respectively) through EIPA (a Na+/H+ exchange inhibitor)-sensitive mechanisms. ET-stimulated K+ efflux, K+ uptake and Na+ uptake activities were all inhibited by BQ123 (but not by BQ788), suggesting the involvement of ETA (and not ETB) receptors in all these responses. ET-1 stimulated K+ uptake and efflux were inhibited by either U73122 or an intracellular Ca2+ chelator, suggesting that these two responses were mediated via PLC. In contrast, ET stimulation of Na+ uptake was unaffected by PLC inhibition or intracellular Ca2+ chelation. These data suggest the presence of two distinct subtypes of ETA receptors on RBEC; one appears to be a typical ETA receptor which is coupled to PLC and has higher binding affinity for ET-1 than ET-3. The other (ETA-like) receptor is similarly activated by ET-1 and ET-3 with high potencies but is independent of PLC. This possibility was further confirmed by the [125I]ET-1 binding studies demonstrating the presence of high- and low-affinity ET-3 binding sites.

Potassium homeostasis and glial energy metabolism

Since capillaries appear not to contribute significantly to rapid removal of K+ from brain tissue, the K+ released into extracellular clefts by neurons at the onset of electrical activity is presumably removed either by redistribution in the clefts or by uptake into cells. What appear to be the three major processes require no energy from the glial cells. These are diffusion through the extracellular clefts, spatial buffering by glial cells, and net uptake of K+ into glial cells through glial K+ channels associated with uptake of Cl- through an independent Cl- conductance. There is a relatively slow uptake by the Na+/K+-ATPase, which directly consumes ATP. In addition, some glial cells take up K+ on the Na+/K+/2Cl- cotransporter, which leads indirectly to energy consumption when the Na+ is subsequently pumped out. Currently available data suggest that the glial energy metabolism devoted to K+ homeostasis is less than a tenth of the total tissue energy metabolism, even under conditions of pathologically high extracellular [K+]. Hence, in situ, it is possible that glial cells could function with much less ATP than neurons do. All the various routes of muffling of changes in extracellular [K+] can be modulated, directly or indirectly, by transmitters liberated by neurons. A consequence of this could be regulation of the entry of Na+ into glial cells such that the Na+/K+-ATPase is activated. The degree of activation might be adjusted so that the resulting activation of the glial glycolytic pathway is appropriate to the provision of the quantity of metabolic substrates required by the neurons.

Genistein activates CFTR Cl- channels via a tyrosine kinase- and protein phosphatase-independent mechanism

Previous studies have revealed an adenosine 3',5'-cyclic monophosphate (cAMP)-independent activation of cystic fibrosis transmembrane conductance regulator (CFTR) Cl- channels by the tyrosine kinase inhibitor genistein. To further explore its mechanism of action, we have reconstituted genistein activation of CFTR in excised inside-out membrane patches. In the presence or absence of ATP, genistein appeared unable to open silent CFTR Cl- channels. However, on CFTR prephosphorylation by cAMP-dependent protein kinase (cAK), genistein enhanced CFTR activity by twofold, resulting from a prolonged burst duration. Genistein could also hyperactivate partially phosphorylated CFTR in the absence of cAK and therefore is different from 5'-adenylylimidodiphosphate, which required fully phosphorylated CFTR. Phosphatase-resistant thiophosphorylation likewise primed the CFTR Cl- channel for hyperactivation by genistein in the absence of cAK. Replacement of ATP by GTP as a hydrolyzable nucleotide triphosphate for CFTR did not impair the ability of genistein to activate thiophosphorylated CFTR, despite the fact that GTP is a poor substrate for tyrosine kinases. These findings argue against a role of protein phosphatases or tyrosine kinases but suggest a more direct interaction of genistein with CFTR, possibly at the level of the second nucleotide-binding domain.

Characteristics of membrane transport processes of macula densa cells

1. Macula densa (MD) cells are located within the thick ascending limb (TAL) and have their apical surface in contact with tubular fluid and their basilar region in contact with the glomerulus. These cells sense changes in luminal fluid sodium chloride concentration ([NaCl]) and transmit signals resulting in changes in vascular resistance (tubuloglomerular feedback) and renin release. 2. Current efforts have focused on understanding the cellular transport mechanisms of MD cells. Progress in this area has benefited from the use of the isolated perfused TAL-glomerular preparation, which permits direct access to MD cells. 3. Using microelectrodes to measure basolateral membrane potential (VBL) of MD cells, it was found that VBL was very sensitive to changes in luminal fluid [NaCl]. As [NaCl] was elevated from 20 to 150 mmol/L, VBL was found to depolarize by over 30 mV. 4. Basolateral membrane potential measurements were also used to identify an apical Na+:2Cl-:K+ cotransport pathway in MD cells that is the major pathway for NaCl entry into these cells. 5. Other work identified a basolateral chloride channel that is presumed to be responsible for changes in VBL during alterations in luminal [NaCl]. This channel, which is the predominant conductance across the basolateral membrane, may be regulated by intracellular Ca2+ and cAMP. 6. An apical Na+:H+ exchanger in MD cells was detected by measuring changes in intracellular pH using the fluorescent probe 2',7'-bis-(2-carboxyethyl)-5(and-6) carboxyfluorescein. 7. Using patch-clamp techniques, a high density of pH- and Ca(2+)-sensitive K+ channels was observed at the apical membrane of MD cells. 8. Other studies found that, at the normal physiological conditions prevailing at the end of the TAL (luminal [NaCl] of 20-60 mmol/L), reabsorption mediated by MD cells is very sensitive to changes in luminal [NaCl].

Molecular characterization of the Na-K-Cl cotransporter of bovine aortic endothelial cells

The Na-K-Cl cotransporter is an important regulator of endothelial cell volume and may also contribute to flux of Na and Cl across the endothelium of the blood-brain barrier. To date, two Na-K-Cl cotransport isoforms have been identified, the cotransporter in secretory epithelia, NKCC1, and that in absorptive renal epithelia, NKCC2. Our previous studies showed that a monoclonal antibody to the cotransporter of human colonic T84 epithelial cells, an NKCC1 isoform, recognizes a 170-kDa glycoprotein from endothelial cells. The molecular identity of the Na-K-Cl cotransporter present in endothelial cells, however, has been unknown. In addition, although evidence has been provided that phosphorylation of the endothelial cotransporter plays a role in regulating its activity, little is known about potential sites for protein kinase interaction with the cotransporter. The present study was conducted to determine the molecular structure of the endothelial Na-K-Cl cotransporter. Using a 1.0-kilobase (kb) cDNA fragment from a conserved region of the T84 cell cotransporter, we screened a bovine aortic endothelial cell cDNA library and subsequently identified and sequenced two overlapping clones that together spanned the entire coding region. The endothelial cotransporter is a 1,201-amino acid protein with 12 putative transmembrane segments and large amino and carboxy termini, each containing several consensus sites for phosphorylation by protein kinases. Comparison of the endothelial cotransporter amino acid sequence with known NKCC1 and NKCC2 sequences revealed a 96% identity with NKCC1. Northern blot analysis using a cDNA probe from the endothelial cotransporter revealed high expression of approximately 7.5-kb transcripts in a number of bovine tissues. Finally, a prominent expression of Na-K-Cl cotransporter was found by Western blot analysis in both cultured and freshly isolated endothelial cells of bovine aorta and cerebral microvessels.

Endothelin-1 as a mediator of endothelial cell-pericyte interactions in bovine brain capillaries

Endothelial cells and pericytes are closely associated in brain capillaries. Together with astrocytic foot processes, they form the blood-brain barrier. Capillaries were isolated from bovine brain cortex. Pure populations of endothelial cells and pericytes were isolated and cultured in vitro. Polarized monolayers of endothelial cells preferentially secreted immunoreactive endothelin-1 (Et-1) at their abluminal (brain-facing) membrane. They did not express receptors for Et-1. Pericytes expressed BQ-123-sensitive ETA receptors for endothelins as evidenced by 125I-Et-1 binding experiments. These receptors were coupled to phospholipase C as demonstrated by intracellular calcium measurements using indo-1-loaded cells. Addition of Et-1 to pericytes induced marked changes in the cell morphology that were associated with a reorganization of F-actin and intermediate filaments. It is concluded that Et-1 is a paracrine mediator at the bovine blood-brain barrier and that capillary pericytes are target cells for endothelium-derived Et-1.

Electrogenic hyperpolarization-elicited chloride transporter current in blue cones of zebrafish retinal slices

Voltage-activated currents in blue cones of the retinal slice of zebrafish were characterized using whole cell recording techniques. Depolarizing-elicited currents were recorded: an outward tetraethylammonium (TEA)-sensitive K+ current (IKx), an outward Ca(2+)-activated Cl- current (ICl(Ca)), from which we inferred an inward Ca2+ current (ICa) as well as a hyperpolarizing-elicited nonselective inward cation current (Ih). In addition, hyperpolarizing steps elicited an outward current (Iout-h) in about one-third of the blue cones. Iout-h seems to be carried by inward transported Cl- because it was abolished by equimolar substitution of bath Cl- with acetate; equimolar substitution of Na+ with choline or TEA had no effect; it was not affected by Cl- channel blockers, anthracene-9-carboxylic acid, 4,4'-diisothiocyanostilbene-2.2'-disulfonic acid, N-phenylanthranilic acid (DPC), niflumic acid, and 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid but was suppressed by Cl- transporter blockers acetalzolamide, bumetanide, N-ethylmaleimide, furosemide, and vanadate, and no reversal potential was found. In addition, this current was suppressed by ouabains but unrelated to their Na(+)-K(+)-ATPase inhibitory effect, was not suppressed by Co2+ or nifedipine, was not affected by the gap junction decoupler, 2-octanol, was increased by bath application of Cs+, presumably due to suppression of Ih, which was masked by Iout-h, and was suppressed by intensive light. Similar current also was found in the short cones and double cones. As Iout-h operates over the same voltage range, and with similar magnitude and time course as Ih, we suggest that Iout-h contributes to the modulation of the photoresponse of cones.

Expression of the Na(+)-K(+)-2Cl- cotransporter BSC2 in the nervous system

We used in situ hybridization and immunocytochemistry with polyclonal antibodies against the mouse bumetanide-sensitive Na(+)-K(+)-2Cl- cotransporter (mBSC2) to determine the location of this cotransporter in rat brain. Northern blots and in situ hybridization showed the presence of cotransporter mRNA in the brain, with an especially high level of expression in the choroid plexus (CP). Affinity-purified anti-BSC2 antibody identified proteins of 145-155 kDa on Western blot analysis and immunoprecipitation of brain and CP membrane protein. Indirect immunofluorescence demonstrated that BSC2 protein is located on the apical surface of the CP and is heterogeneously distributed in cell bodies and dendrites of neurons in the central and peripheral nervous system. The apical localization of BSC2 in the CP was confirmed by 86Rb+ uptakes in primary cultures of CP cells grown on permeable filters and confocal immunofluorescence microscopy. The apical localization of the cotransporter in CP epithelium suggests a role for the cotransporter in cerebrospinal fluid K+ homeostasis. In neurons, the cotransporter may help regulate intracellular Cl- concentration and thereby affect neuronal response to gamma-aminobutyric acid.

Functional properties of cultured endothelial cells derived from large microvessels of human brain

This report describes the fractional separation of microvessels from human brain for establishment of segmentally derived endothelial cell (EC) cultures. The investigation comprised evaluation of media constituents and purity of the cell culture and focused on functional biochemical characterization of endothelium derived from large microvessels (EC) Cells contained endothelial marker factor VIII (von Willebrand antigen), secreted endothelin-1 (ET-1) and prostaglandins, and took up 86Rb+ as a measure of K+. Exogenous ET-1 stimulated phosphatidylinositol hydrolysis and K+ uptake; BQ-123 (selective ETA receptor antagonist) but not IRL-1038 or BQ-788 (selective ETB receptor antagonists) inhibited both. Ouabain (inhibitor of Na(+)-K(+)-ATPase) and bumetanide (inhibitor of Na(+)-K(+)-Cl- cotransport) reduced (74-80 and 20-40%, respectively) the ET-1-stimulated K+ uptake. Staurosporine [protein kinase C (PKC) inhibitor] selectively reduced Na(+)-K(+)-Cl- cotransport, whereas verapamil but not nifedipine (L-type voltage-dependent Ca2+ channel blockers) decreased Na(+)-K(+)-ATPase activity induced by ET-1. Phorbol 12-myristate 13-acetate (PMA; activator of PKC) stimulated K+ uptake, which was only decreased with bumetanide. N-ethylisopropylamiloride (inhibitor of Na+/H+ exchange) reduced the ET-1-stimulated but not the PMA-induced K+ uptake. Results indicate that phosphatidylinositol hydrolysis and ion transport systems in large microvascular EC are stimulated by ET-1 through activation of ETA receptors. The findings also suggest that the ET-1-stimulated Na(+)-K(+)-ATPase activity, in contrast to Na(+)-K(+)-Cl- cotransport, is not mediated by PKC. In addition, the data suggest a linkage between Na(+)-K(+)-ATPase activity and Na+/H+ exchange.

Na(+)-K(+)-Cl- cotransport system in brain capillary endothelial cells: response to endothelin and hypoxia

Effect of endothelin-1 and chemically induced hypoxia on Na(+)-K(+)-Cl- cotransport activity in cultured rat brain capillary endothelial cells was examined by using 86Rb+ as a tracer for K+; bumetanide-sensitive K+ uptake was defined as Na(+)-K(+)-Cl- cotransport activity. Endothelin-1, phorbol 12-myristate 13-acetate (PMA), or thapsigargin increased Na(+)-K(+)-Cl- cotransport activity. A protein kinase C inhibitor, bisindolylmaleimide, inhibited PMA- and endothelin-1- (but not thapsigargin-) induced Na(+)-K(+)-Cl- cotransport activity, indicating the presence of both protein kinase C-dependent regulatory mechanisms and protein kinase C-independent mechanisms which involve intracellular Ca2+. Oligomycin, sodium azide, or antimycin A increased Na(+)-K(+)-Cl- cotransport activity by 80-200%. Oligomycin-induced Na(+)-K(+)-Cl- cotransport activity was reduced by an intracellular Ca2+ chelator (BAPTA/AM) but not affected by bisindolylmaleimide, suggesting the involvement of intracellular Ca2+, and not protein kinase C, in hypoxia-induced Na(+)-K(+)-Cl- cotransport activity.

Astroglial-mediated phosphorylation of the Na-K-Cl cotransporter in brain microvessel endothelial cells

Our previous studies have shown that cerebral microvessel endothelial cells (CMEC) express a Na-K-Cl cotransporter and that exposure of CMEC to astroglial cells causes a nearly 2-fold increase in activity of the cotransporter but only 1.5-fold increase in expression of cotransport protein [D. Sun, C. Lytle, and M. E. O'Donnell. Am. J. Physiol. 269 (Cell Physiol. 38): C1506-C1512, 1995]. This finding suggests that the astroglial cell effects may be mediated by mechanisms involving cotransporter activation in addition to increased protein expression. In the present study, we evaluated the role of protein phosphorylation in elevation of CMEC cotransport activity by astroglial cells and extracellular hypertonicity. We also examined the effects of protein phosphatase and protein kinase inhibitors on both cotransporter activity and phosphorylation in CMEC. The phosphorylation level of Na-K-Cl cotransport protein was quantitatively evaluated by immunoprecipitation analysis with the use of a monoclonal antibody to the cotransporter after 32P labeling of cultured CMEC. Activity of the cotransporter was assessed as bumetanide-sensitive K influx. We found that the phosphatase inhibitors calyculin A and okadaic acid significantly increased both cotransport activity and phosphorylation of cotransport protein. Activity and phosphorylation level of the cotransporter were also markedly increased by exposing the cells to astroglial cell-conditioned or hypertonic medium. Moreover, the astroglial-induced stimulation of the CMEC cotransporter was inhibited by the protein kinase inhibitor K-252a. These findings suggest that phosphorylation of cotransport protein plays an important role in regulation of Na-K-Cl cotransport activity and that astroglial-induced elevation of cotransport activity involves both phosphorylation-associated stimulation of cotransport activity and increased expression of the cotransporter protein.

Astroglial cell-induced expression of Na-K-Cl cotransporter in brain microvascular endothelial cells

Endothelial cells of the blood-brain barrier (BBB) are characterized by extensive tight junctions and asymmetric distribution of specific enzymes and transport systems. Maintenance of the BBB endothelial phenotype depends on astrocyte-endothelial interactions. We showed previously that cultured cerebral microvascular endothelial cells (CMEC) exhibit robust Na-K-Cl cotransport activity. In the present study, we evaluated the expression of Na-K-Cl cotransport protein in CMEC by quantitative Western blot analysis and found that a protein of approximately 170 kDa was recognized by a monoclonal antibody against the cotransporter. Exposure of CMEC to astroglial cells or their conditioned media increased the expression of the CMEC cotransport protein by approximately 55%. Using a monoclonal antibody against the alpha-subunit of chicken Na-K-ATPase, we found that these treatments also increased expression of Na-K-ATPase protein by a similar amount. By comparing bumetanide-sensitive K influx and [3H]bumetanide binding apical vs. basolateral surfaces of CMEC, we found both cotransporter activity and [3H]bumetanide binding to be approximately 90% apical and 10% basolateral. Coculture of the CMEC with astroglial cells increased cotransport activity and [3H]bumetanide binding at both surfaces, with the asymmetric distribution maintained. These results indicate that the cotransporter is regulated by astroglial cells and that an apically distributed CMEC cotransporter may function in tandem with the basolateral Na-K-ATPase to mediate vectorial transport of Na and Cl across the BBB.

[Cl-]i-dependent phosphorylation of the Na-K-Cl cotransport protein of dog tracheal epithelial cells

Basolateral Na-K-Cl cotransport activity in primary cultures of dog tracheal epithelial cells is stimulated by beta-adrenergic agents, such as isoproterenol, and by apical UTP, which acts through an apical P2-purinergic receptor. While at least part of the stimulatory effect of isoproterenol appears to involve direct activation of the cotransporter via cAMP-dependent protein kinase, cotransport stimulation by apical UTP is entirely secondary to apical Cl- efflux and a resultant decrease in intracellular [Cl-] ([Cl-]i) and/or cell shrinkage (Haas, M., and McBrayer, D. G. (1994) Am. J. Physiol. 266, C1440-C1452). In the secretory epithelia of the shark rectal gland and avian salt gland, Na-K-Cl cotransport activation by both cAMP-dependent and cAMP-independent secretagogues has been shown to be accompanied by phosphorylation of the cotransport protein itself (Lytle, C., and Forbush, B., III (1992) J. Biol. Chem. 267, 25438-25443; Torchia, J., Lytle, C., Pon, D. J., Forbush, B., III, and Sen, A. K. (1992) J. Biol. Chem. 267, 25444-25450). In the present study, we immunoprecipitate the approximately 170-kDa Na-K-Cl cotransport protein of dog tracheal epithelial cells with a monoclonal antibody against the cotransporter of the intestinal cell line T84. Incubation of confluent primary cultures of tracheal cells with isoproterenol and apical UTP increases basolateral-to-apical 36Cl- flux 3.4- and 2.6-fold, respectively, and produces similar increases (3.2- and 2.8-fold, respectively) in 32P incorporation into the approximately 170-kDa cotransport protein. Decreasing [Cl-]i (without concomitant cell shrinkage) by incubating cultures with apical nystatin and reduced apical [Cl-] ([Cl-]alpha) likewise increases both cotransport activity and cotransport protein phosphorylation. These effects become more pronounced with greater reductions in [Cl-]alpha; after 20 min of incubation with nystatin and 32 mM [Cl-]alpha, cotransport activity and 32P incorporation into the cotransport protein are increased 2.8- and 2.7-fold, respectively, similar to increases seen with apical UTP. 2-3-fold increases in cotransporter activity and phosphorylation are also seen in nystatin-treated cells under hypertonic conditions (50 mM sucrose added apically and basolaterally). These findings suggest a close correlation between Na-K-Cl cotransport activity and phosphorylation of the approximately 170-kDa cotransport protein. The latter is phosphorylated in response to both reduced [Cl-]i and cell shrinkage, either or both of which are likely to be involved in secondary cotransport activation in response to apical UTP.

Properties and functions of a neuromedin-B-preferring bombesin receptor in brain microvascular endothelial cells

Endothelial cells were isolated from rat brain microvessels and grown in vitro. They expressed a high density of [125I-Tyr4]bombesin receptor (Bmax = 0.9 pmol/mg protein) with an apparent Kd value of 10 nM. The pharmacological profile of inhibition of the specific [125I-Tyr4]bombesin binding [bombesin = neuromedin B > gastrin releasing peptide (GRP)] was consistent with the presence of a neuromedin-B-preferring receptor. Addition of bombesin, neuromedin B and GRP increased the activity of phospholipase C as measured by the production of total inositol phosphates and from intracellular Ca2+ measurements. They increase 86Rb+ uptake by the Na+, K+, 2Cl- cotransporter and by a charybdotoxin-sensitive, Ca(2+)-activated K+ channel and 22Na+ uptake by the Na+/H+ exchanger. The pharmacological profiles of activation of phospholipase C, Na+, K+, 2Cl- cotransport and Na+/H+ exchange by bombesin-like peptide were consistent with an involvement of the neuromedin-B-preferring receptor characterized in binding experiments. It is suggested that one of the actions of neuromedin B in brain vessels could be to control K+ secretion by the blood/brain barrier.

Endothelins stimulate sodium uptake into rat brain capillary endothelial cells through endothelin A-like receptors

The effect of endothelins (ETs) on sodium/hydrogen (Na+/H+) antiport system was examined in cultured rat brain capillary endothelium (RBEC). ET-1, ET-2, and ET-3 stimulated Na+ uptake into RBEC with similar half-maximal stimulation (EC50) values (0.7, 0.6, and 1.1 nM, respectively). This reaction was inhibited by the Na+/H+ antiport inhibitor, N-(ethyl-N-isopropyl)-amiloride (EIPA). The selective endothelin A (ETA) receptor-antagonist (cyclo-D-Trp-D-Asp-Pro-D-Val-Leu (BQ123)), but not endothelin B (ETB) receptor-antagonists ((Cys11, Cys15)-ET-1 (IRL1038) or N-cis-2,6-dimethylpiperidinocarbonyl-L-gamma MeLeu-D-Trp(COOMe)-D-Nle-ONa (BQ788)), inhibited both ET-1- and ET-3-stimulated Na+ uptake, indicating ETA-receptor mediation. The protein kinase C (PKC) activator (phorbol 12-myristate 13-acetate (PMA)) failed to stimulate Na+ uptake. The calcium-calmodulin (CaM) inhibitor (W7) reduced ET-1-stimulated Na+ uptake by 50%, whereas the PKC inhibitor (staurosporine) had no effect, indicating that ET-1 stimulation of the Na+/H+ antiport system is linked to a CaM-dependent and PKC-independent pathway.

Sensitization by calyculin A of brain capillary endothelial cells to endothelin-1

1. Cultured brain capillary endothelial cells of the rat respond to endothelin-1 (ET-1) by an increased activity of the Na+,K+,2Cl-, cotransporter and a mobilization of intracellular Ca2+ stores. 2. Calyculin A (1-30 nM), but not okadaic acid, sensitizes up to 100 fold the Na+,K+,2Cl- cotransporter to the action of ET-1. 3. Calyculin A (30 nM) does not modify the binding properties of ET-1 to ETA receptors. 4. Calyculin A (30 nM) inhibits ET-1 induced intracellular Ca2+ mobilization. 5. It is concluded that inhibition of protein phosphatase 1 selectively modifies the repertoire of intracellular actions of ET-1 and favours actions that are unrelated to the phospholipase C signalling cascade.

Endothelin receptor subtypes and their role in transmembrane signaling mechanisms

The endothelins (ETs) are potent vasoactive peptides that appear to be involved in diverse biological actions, for example, contraction, neuromodulation, and neurotransmission, as well as in various pathophysiological conditions, such as renal and heart failure. The diversity of actions of ETs may be explained in terms of (1) the existence of several receptor subtypes and (2) the activation of different signal transduction pathways. This review summarizes the state of the art in this intensively studied field, with particular focus on structural aspects, receptor heterogeneity, coupling of receptors to G-proteins, and signal transduction mechanisms mediated by the activation of ET-receptors.