Regulation of Na(+)-H+ exchange by ATP depletion and calmodulin antagonism in renal epithelial cells

Functional role of glucose metabolism, osmotic stress, and sodium-glucose cotransporter isoform-mediated transport on Na+/H+ exchanger isoform 3 activity in the renal proximal tubule

Na(+)-glucose cotransporter 1 (SGLT1)-mediated glucose uptake leads to activation of Na(+)-H(+) exchanger 3 (NHE3) in the intestine by a process that is not dependent on glucose metabolism. This coactivation may be important for postprandial nutrient uptake. However, it remains to be determined whether SGLT-mediated glucose uptake regulates NHE3-mediated NaHCO3 reabsorption in the renal proximal tubule. Considering that this nephron segment also expresses SGLT2 and that the kidneys and intestine show significant variations in daily glucose availability, the goal of this study was to determine the effect of SGLT-mediated glucose uptake on NHE3 activity in the renal proximal tubule. Stationary in vivo microperfusion experiments showed that luminal perfusion with 5 mM glucose stimulates NHE3-mediated bicarbonate reabsorption. This stimulatory effect was mediated by glycolytic metabolism but not through ATP production. Conversely, luminal perfusion with 40 mM glucose inhibited NHE3 because of cell swelling. Notably, pharmacologic inhibition of SGLT activity by Phlorizin produced a marked inhibition of NHE3, even in the absence of glucose. Furthermore, immunofluorescence experiments showed that NHE3 colocalizes with SGLT2 but not SGLT1 in the rat renal proximal tubule. Collectively, these findings show that glucose exerts a bimodal effect on NHE3. The physiologic metabolism of glucose stimulates NHE3 transport activity, whereas, supraphysiologic glucose concentrations inhibit this exchanger. Additionally, Phlorizin-sensitive SGLT transporters and NHE3 interact functionally in the proximal tubule.

Lipid- and mechanosensitivities of sodium/hydrogen exchangers analyzed by electrical methods

Sodium/hydrogen exchangers (NHEs) are ubiquitous ion transporters that serve multiple cell functions. We have studied two mammalian isoforms, NHE1 (ubiquitous) and NHE3 (epithelial-specific), by measuring extracellular proton (H+) gradients during whole-cell patch clamp with perfusion of the cell interior. Maximal Na(+)-dependent H+ fluxes (JH+) are equivalent to currents >20 pA for NHE1 in Chinese hamster ovary fibroblasts, >200 pA for NHE1 in guinea pig ventricular myocytes, and 5-10 pA for NHE3 in opossum kidney cells. The fluxes are blocked by an NHE inhibitor, ethylisopropylamiloride, and are absent in NHE-deficient AP-1 cells. NHE1 activity is stable with perfusion of nonhydrolyzable ATP [adenosine 5'-(beta,gamma-imido)triphosphate], is abolished by ATP depletion (2 deoxy-D-glucose with oligomycin or perfusion of apyrase), can be restored with phosphatidylinositol 4,5-bisphosphate, and is unaffected by actin cytoskeleton disruption (latrunculin or pipette perfusion of gelsolin). NHE3 (but not NHE1) is reversibly activated by phosphatidylinositol 3,4,5-trisphosphate. Both NHE1 and NHE3 activities are disrupted in giant patches during gigaohm seal formation. NHE1 (but not NHE3) is reversibly activated by cell shrinkage, even at neutral cytoplasmic pH without ATP, and inhibited by cell swelling. NHE1 in Chinese hamster ovary fibroblasts (but not NHE3 in opossum kidney cells) is inhibited by agents that thin the membrane (L-alpha-lysophosphatidylcholine and octyl-beta-D-glucopyranoside) and activated by cholesterol enrichment, which thickens membranes. Expressed in AP-1 cells, however, NHE1 is insensitive to these agents but remains sensitive to volume changes. Thus, changes of hydrophobic mismatch can modulate NHE1 but do not underlie its volume sensitivity.

Cellular mechanisms for apical ATP effects on intracellular pH in human bronchial epithelium

The effect of external ATP on intracellular pH (pH(i)) was investigated using a pH imaging system in a human bronchial epithelial cell line (16HBE14o-) loaded with BCECF-AM. The steady-state pH(i) of 16HBE14o- epithelial monolayers was 7.137 +/- 0.027 (n = 46). Apical addition of ATP (10(-4) M) to epithelial monolayers induced a rapid and sustained pH(i) decrease of 0.164 +/- 0.024 pH units (n = 17; P < 0.001). The intracellular acidification was rapidly reversed upon removal of external ATP. In contrast, the non-hydrolysable ATP analogue AMP-PNP did not produce any significant change in pH(i). Inhibition of purinoreceptors by suramin did not affect the acidification induced by apical ATP. Inhibition of Na+-H+ exchange by apical Na+ removal or addition of amiloride (0.5 mM) reduced the apical ATP-induced pH(i) decrease, suggesting the involvement of a Na+-H+ exchanger or surface pH effects on the ATP-induced pH(i) response. Inhibitors of proton channels such as ZnCl2 (10(-4) M) also partially inhibited the ATP response. The pH(i) response to ATP was dependent on the external pH (pH(o)), with increasing acidification produced at lower pH(o) values. Neither the basal pH(i) nor the ATP-induced intracellular acidification was affected by thapsigargin (a Ca2+-ATPase inhibitor), chelerythrine chloride (a protein kinase C (PKC) inhibitor), RpcAMP (a protein kinase A (PKA) inhibitor) or PMA (a PKC activator). Therefore, the intracellular acidification of human bronchial epithelial cells induced by apical ATP does not involve signalling via Ca2+, PKC or PKA nor binding to a purinoreceptor. We interpret the effect of ATP to produce an intracellular acidification as a three step process: activation of H+ channels, inhibition of Na+-H+ exchange and influx of protonated ATP.

Role of the Na/H antiport in pH-dependent cell death in pulmonary artery endothelial cells

We investigated the role of intracellular pH (pH(i)) and Na/H exchange in cell death in human pulmonary artery endothelial cells (HPAEC) following a metabolic insult (inhibition-oxidative phosphorylation, glycolysis). Metabolic inhibition in medium at pH 7. 4 decreased viability (0-15% live cells) over 6 h. Cell death was attenuated by maneuvers that decreased pH(i) and inhibited Na/H exchange (acidosis, Na/H antiport inhibitors). In contrast, cell death was potentiated by maneuvers that elevated pH(i) or increased Na/H exchange (monensin, phorbol ester treatment) before the insult. HPAEC demonstrated a biphasic pH(i) response following a metabolic insult. An initial decrease in pH(i) was followed by a return to baseline over 60 min. Maneuvers that protected HPAEC and inhibited Na/H exchange (acidosis, Na(+)-free medium, antiport inhibitors) altered this pattern. pH(i) decreased, but no recovery was observed, suggesting that the return of pH(i) to normal was mediated by antiport activation. Although Na/H antiport activity was reduced (55-60% of control) following a metabolic insult, the cells still demonstrated active Na/H exchange despite significant ATP depletion. Phorbol ester pretreatment, which potentiated cell death, increased Na/H antiport activity above the level observed in monolayers subjected to a metabolic insult alone. These results demonstrate that HPAEC undergo a pH-dependent loss of viability linked to active Na/H exchange following a metabolic insult. Potentiation of cell death with phorbol ester treatment suggests that this cell death pathway involves protein kinase C-mediated phosphorylation events.

Adenosine inhibits the transfected Na+-H+ exchanger NHE3 in Xenopus laevis renal epithelial cells (A6/C1)

1. Adenosine influences the vectorial transport of Na+ and HCO3- across kidney epithelial cells. However, its action on effector proteins, such as the Na+-H+ exchanger NHE3, an epithelial brush border isoform of the Na+-H+ exchanger (NHE) gene family, is not yet defined. 2. The present study was conducted in Xenopus laevis distal nephron A6 epithelia which express both an apical adenosine receptor of the A1 type (coupled to protein kinase C (PKC)) and a basolateral receptor of the A2 type (coupled to protein kinase A (PKA)). The untransfected A6 cell line expresses a single NHE type (XNHE) which is restricted to the basolateral membrane and which is activated by PKA. 3. A6 cell lines were generated which express exogenous rat NHE3. Measurements of side-specific pHi recovery from acid loads in the presence of HOE694 (an inhibitor with differential potency towards individual NHE isoforms) detected an apical resistant Na+-H+ exchange only in transfected cell lines. The sensitivity of the basolateral NHE to HOE694 was unchanged, suggesting that exogenous NHE3 was restricted to the apical membrane. 4. Stimulation of the apical A1 receptor with N 6-cyclopentyladenosine (CPA) inhibited both apical NHE3 and basolateral XNHE. These effects were mimicked by the addition of the protein kinase C (PKC) activator phorbol 12-myristate 13-acetate (PMA) and partially prevented by the PKC inhibitor calphostin C which also blocked the effect of PMA. 5. Stimulation of the basolateral A2 receptor with CPA inhibited apical NHE3 and stimulated basolateral XNHE. These effects were mimicked by 8-bromo-cAMP and partially prevented by the PKA inhibitor H89 which entirely blocked the effect of 8-bromo-cAMP. 6. In conclusion, CPA inhibits rat NHE3 expressed apically in A6 epithelia via both the apical PKC-coupled A1 and the basolateral PKA-coupled A2 adenosine receptors.

Hypoxia and smooth muscle function: key regulatory events during metabolic stress

Hypoxia rapidly reduces force in many smooth muscles and we review recent data that shed light on the mechanisms involved. As many regulated cellular processes are integrated to co-ordinate smooth muscle contractility, the processes responsible for decreased force output with altered metabolism are also likely to be many, acting in concert, rather than the actions of one altered parameter. Nevertheless the aim of this study is to elucidate the hierarchical series of events that contribute to reduced smooth muscle force production during altered metabolism. We conclude that in many phasic smooth muscles the decrease in force can be attributed to impaired electro-mechanical coupling whereby the Ca2+ transient is reduced. A direct effect of hypoxia on the Ca2+ channel may be of key importance. In tonic vascular smooth muscles KATP channels may also play a role in the integrated functional responses to hypoxia. There are also many examples of force being reduced, in tonically activated preparations, without a fall in steady-state Ca2+; indeed it usually increases. We examine the roles of altered [ATP], pH, myosin phosphorylation, inorganic phosphate and proteolytic activity on the [Ca2+]-force relationship during hypoxia. We find no defining force-inhibitory role for any one factor acting alone, and suggest that force most probably falls as a result of the combination of myriad factors.

ATP dependence of Na+/H+ exchange. Nucleotide specificity and assessment of the role of phospholipids

We studied the ATP dependence of NHE-1, the ubiquitous isoform of the Na+/H+ antiporter, using the whole-cell configuration of the patch-clamp technique to apply nucleotides intracellularly while measuring cytosolic pH (pHi) by microfluorimetry. Na+/H+ exchange activity was measured as the Na(+)-driven pHi recovery from an acid load, which was imposed via the patch pipette. In Chinese hamster ovary (CHO) fibroblasts stably transfected with NHE-1, omission of ATP from the pipette solution inhibited Na+/H+ exchange. Conversely, ATP perfusion restored exchange activity in cells that had been metabolically depleted by 2-deoxy-D-glucose and oligomycin. In cells dialyzed in the presence of ATP, no "run-down" was observed even after extended periods, suggesting that the nucleotide is the only diffusible factor required for optimal NHE-1 activity. Half-maximal activation of the antiporter was obtained at approximately 5 mM Mg-ATP. Submillimolar concentrations failed to sustain Na+/H+ exchange even when an ATP regenerating system was included in the pipette solution. High ATP concentrations are also known to be required for the optimal function of other cation exchangers. In the case of the Na/Ca2+ exchanger, this requirement has been attributed to an aminophospholipid translocase, or "flippase.". The involvement of this enzyme in Na+/H+ exchange was examined using fluorescent phosphatidylserine, which is actively translocated by the flippase. ATP depletion decreased the transmembrane uptake of NBD-labeled phosphatidylserine (NBD-PS), indicating that the flippase was inhibited. Diamide, an agent reported to block the flippase, was as potent as ATP depletion in reducing NBD-PS uptake. However, diamide had no effect on Na+/H+ exchange, implying that the effect of ATP is not mediated by changes in lipid distribution across the plasma membrane. K-ATP and ATP gamma S were as efficient as Mg-ATP in sustaining NHE-1 activity, while AMP-PNP and AMP-PCP only partially substituted for ATP. In contrast, GTP gamma S was ineffective. We conclude that ATP is the only soluble factor necessary for optimal activity of the NHE-1 isoform of the antiporter. Mg2+ does not appear to be essential for the stimulatory effect of ATP. We propose that two mechanisms mediate the activation of the antiporter by ATP: one requires hydrolysis and is likely an energy-dependent event. The second process does not involve hydrolysis of the gamma-phosphate, excluding mediation by protein or lipid kinases. We suggest that this effect is due to binding of ATP to an as yet unidentified, nondiffusible effector that activates the antiporter.

Effect of hyperoxia and exogenous oxidant stress on pulmonary artery endothelial cell Na+/H+ antiport activity

Little is known about the mechanisms of altered cell membrane function after hyperoxic exposure. We determined the effects of hyperoxic exposure and exogenous oxidant stress with xanthine/xanthine oxidase (X/XO) on Na+/H+ antiport activity. Pulmonary artery endothelial cell monolayers were incubated in 95% O2/5% CO2 (24 to 72 hours) simultaneously with controls placed in 21 % O2/5% CO2. Monolayers were then incubated for 2 hours in MEM with or without X/XO (100 micromol/L X; 0.01 U/ml XO). Antiport activity was determined as the rate of recovery from intracellular acidosis by measurement of intracellular pH (pH,) with 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein (BCECF). Hyperoxic exposure (72 hours) decreased Na+/H+ antiport activity as compared with that in control monolayers. Exogenous oxidant stress also decreased antiport activity in both control and hyperoxic cells, but this effect was more pronounced in hyperoxic cells at all time points. These changes occurred in the absence of overt cytotoxicity. Incubation with antioxidants (polyethylene glycol-superoxide dismutase (PEG-SOD), PEG-catalase, vitamin E), N-acetylcysteine, or phospholipase A2 (PLA2) inhibitors did not prevent the decrease in antiport activity after hyperoxic exposure. Conditioned medium experiments demonstrated that the diminished antiport activity was not related to release of a soluble mediator after hyperoxic exposure. These findings suggest that the diminished Na+/H+ antiport activity represents a sublethal form of membrane dysfunction that may be a component of the increased endothelial cell susceptibility to injury after hyperoxic exposure.