Cell volume and ph regulation by the Amphiuma red blood cell: A model for hypoxia-induced cell injury

The SLC9A-C Mammalian Na+/H+ Exchanger Family: Molecules, Mechanisms, and Physiology

Na⁺/H⁺ exchangers play pivotal roles in the control of cell and tissue pH by mediating the electroneutral exchange of Na⁺ and H⁺ across cellular membranes. They belong to an ancient family of highly evolutionarily conserved proteins, and they play essential physiological roles in all phyla. In this review, we focus on the mammalian Na⁺/H⁺ exchangers (NHEs), the solute carrier (SLC) 9 family. This family of electroneutral transporters constitutes three branches: SLC9A, -B, and -C. Within these, each isoform exhibits distinct tissue expression profiles, regulation, and physiological roles. Some of these transporters are highly studied, with hundreds of original articles, and some are still only rudimentarily understood. In this review, we present and discuss the pioneering original work as well as the current state-of-the-art research on mammalian NHEs. We aim to provide the reader with a comprehensive view of core knowledge and recent insights into each family member, from gene organization over protein structure and regulation to physiological and pathophysiological roles. Particular attention is given to the integrated physiology of NHEs in the main organ systems. We provide several novel analyses and useful overviews, and we pinpoint main remaining enigmas, which we hope will inspire novel research on these highly versatile proteins.

The ClC-3 chloride channels in cardiovascular disease

ClC-3 is a member of the ClC voltage-gated chloride (Cl(-)) channel superfamily. Recent studies have demonstrated the abundant expression and pleiotropy of ClC-3 in cardiac atrial and ventricular myocytes, vascular smooth muscle cells, and endothelial cells. ClC-3 Cl(-) channels can be activated by increase in cell volume, direct stretch of β1-integrin through focal adhesion kinase and many active molecules or growth factors including angiotensin II and endothelin-1-mediated signaling pathways, Ca(2+)/calmodulin-dependent protein kinase II and reactive oxygen species. ClC-3 may function as a key component of the volume-regulated Cl(-) channels, a superoxide anion transport and/or NADPH oxidase interaction partner, and a regulator of many other transporters. ClC-3 has been implicated in the regulation of electrical activity, cell volume, proliferation, differentiation, migration, apoptosis and intracellular pH. This review will highlight the major findings and recent advances in the study of ClC-3 Cl(-) channels in the cardiovascular system and discuss their important roles in cardiac and vascular remodeling during hypertension, myocardial hypertrophy, ischemia/reperfusion, and heart failure.

Effect of complete protein 4.1R deficiency on ion transport properties of murine erythrocytes

Moderate hemolytic anemia, abnormal erythrocyte morphology (spherocytosis), and decreased membrane stability are observed in mice with complete deficiency of all erythroid protein 4.1 protein isoforms (4.1(-/-); Shi TS et al. J Clin Invest 103: 331, 1999). We have examined the effects of erythroid protein 4.1 (4.1R) deficiency on erythrocyte cation transport and volume regulation. 4.1(-/-) mice exhibited erythrocyte dehydration that was associated with reduced cellular K and increased Na content. Increased Na permeability was observed in these mice, mostly mediated by Na/H exchange with normal Na-K pump and Na-K-2Cl cotransport activities. The Na/H exchange of 4.1(-/-) erythrocytes was markedly activated by exposure to hypertonic conditions (18.2 +/- 3.2 in 4.1(-/-) vs. 9.8 +/- 1.3 mmol/10(13) cell x h in control mice), with an abnormal dependence on osmolality (EC(50) = 417 +/- 42 in 4.1(-/-) vs. 460 +/- 35 mosmol/kgH(2)O in control mice), suggestive of an upregulated functional state. While the affinity for internal protons was not altered (K(0.5) = 489.7 +/- 0.7 vs. 537.0 +/- 0.56 nM in control mice), the V(max) of the H-induced Na/H exchange activity was markedly elevated in 4.1(-/-) erythrocytes (V(max) 91.47 +/- 7.2 compared with 46.52 +/- 5.4 mmol/10(13) cell x h in control mice). Na/H exchange activation by okadaic acid was absent in 4.1(-/-) erythrocytes. Altogether, these results suggest that erythroid protein 4.1 plays a major role in volume regulation and physiologically downregulates Na/H exchange in mouse erythrocytes. Upregulation of the Na/H exchange is an important contributor to the elevated cell Na content of 4.1(-/-) erythrocytes.

Physiology and pathophysiology of Na+/H+ exchange and Na+ -K+ -2Cl- cotransport in the heart, brain, and blood

Maintenance of a stable cell volume and intracellular pH is critical for normal cell function. Arguably, two of the most important ion transporters involved in these processes are the Na+/H+ exchanger isoform 1 (NHE1) and Na+ -K+ -2Cl- cotransporter isoform 1 (NKCC1). Both NHE1 and NKCC1 are stimulated by cell shrinkage and by numerous other stimuli, including a wide range of hormones and growth factors, and for NHE1, intracellular acidification. Both transporters can be important regulators of cell volume, yet their activity also, directly or indirectly, affects the intracellular concentrations of Na+, Ca2+, Cl-, K+, and H+. Conversely, when either transporter responds to a stimulus other than cell shrinkage and when the driving force is directed to promote Na+ entry, one consequence may be cell swelling. Thus stimulation of NHE1 and/or NKCC1 by a deviation from homeostasis of a given parameter may regulate that parameter at the expense of compromising others, a coupling that may contribute to irreversible cell damage in a number of pathophysiological conditions. This review addresses the roles of NHE1 and NKCC1 in the cellular responses to physiological and pathophysiological stress. The aim is to provide a comprehensive overview of the mechanisms and consequences of stress-induced stimulation of these transporters with focus on the heart, brain, and blood. The physiological stressors reviewed are metabolic/exercise stress, osmotic stress, and mechanical stress, conditions in which NHE1 and NKCC1 play important physiological roles. With respect to pathophysiology, the focus is on ischemia and severe hypoxia where the roles of NHE1 and NKCC1 have been widely studied yet remain controversial and incompletely elucidated.

Age-related differences in Na+-dependent Ca2+ accumulation in rabbit hearts exposed to hypoxia and acidification

In this study, we test the hypothesis that in newborn hearts (as in adults) hypoxia and acidification stimulate increased Na(+) uptake, in part via pH-regulatory Na(+)/H(+) exchange. Resulting increases in intracellular Na(+) (Na(i)) alter the force driving the Na(+)/Ca(2+) exchanger and lead to increased intracellular Ca(2+). NMR spectroscopy measured Na(i) and cytosolic Ca(2+) concentration ([Ca(2+)](i)) and pH (pH(i)) in isolated, Langendorff-perfused 4- to 7-day-old rabbit hearts. After Na(+)/K(+) ATPase inhibition, hypoxic hearts gained Na(+), whereas normoxic controls did not [19 +/- 3.4 to 139 +/- 14.6 vs. 22 +/- 1.9 to 22 +/- 2.5 (SE) meq/kg dry wt, respectively]. In normoxic hearts acidified using the NH(4)Cl prepulse, pH(i) fell rapidly and recovered, whereas Na(i) rose from 31 +/- 18.2 to 117.7 +/- 20.5 meq/kg dry wt. Both protocols caused increases in [Ca](i); however, [Ca](i) increased less in newborn hearts than in adults (P < 0.05). Increases in Na(i) and [Ca](i) were inhibited by the Na(+)/H(+) exchange inhibitor methylisobutylamiloride (MIA, 40 microM; P < 0.05), as well as by increasing perfusate osmolarity (+30 mosM) immediately before and during hypoxia (P < 0.05). The data support the hypothesis that in newborn hearts, like adults, increases in Na(i) and [Ca](i) during hypoxia and after normoxic acidification are in large part the result of increased uptake via Na(+)/H(+) and Na(+)/Ca(2+) exchange, respectively. However, for similar hypoxia and acidification protocols, this increase in [Ca](i) is less in newborn than adult hearts.

Hypertonic perfusion inhibits intracellular Na and Ca accumulation in hypoxic myocardium

Much evidence supports the view that hypoxic/ischemic injury is largely due to increased intracellular Ca concentration ([Ca](i)) resulting from 1) decreased intracellular pH (pH(i)), 2) stimulated Na/H exchange that increases Na uptake and thus intracellular Na (Na(i)), and 3) decreased Na gradient that decreases or reverses net Ca transport via Na/Ca exchange. The Na/H exchanger (NHE) is also stimulated by hypertonic solutions; however, hypertonic media may inhibit NHE's response to changes in pH(i) (Cala PM and Maldonado HM. J Gen Physiol 103: 1035-1054, 1994). Thus we tested the hypothesis that hypertonic perfusion attenuates acid-induced increases in Na(i) in myocardium and, thereby, decreases Ca(i) accumulation during hypoxia. Rabbit hearts were Langendorff perfused with HEPES-buffered Krebs-Henseleit solution equilibrated with 100% O(2) or 100% N(2). Hypertonic perfusion began 5 min before hypoxia or normoxic acidification (NH(4)Cl washout). Na(i), [Ca](i), pH(i), and high-energy phosphates were measured by NMR. Control solutions were 295 mosM, and hypertonic solutions were adjusted to 305, 325, or 345 mosM by addition of NaCl or sucrose. During 60 min of hypoxia (295 mosM), Na(i) rose from 22+/-1 to 100+/-10 meq/kg dry wt while [Ca](i) rose from 347+/-11 to 1,306+/-89 nM. During hypertonic hypoxic perfusion (325 mosM), increases in Na(i) and [Ca](i) were reduced by 65 and 60%, respectively (P<0.05). Hypertonic perfusion also diminished Na uptake after normoxic acidification by 87% (P<0.05). The data are consistent with the hypothesis that mild hypertonic perfusion diminishes acid-induced Na accumulation and, thereby, decreases Na/Ca exchange-mediated Ca(i) accumulation during hypoxia.

Cloning and expression of the Na+/H+ exchanger from Amphiuma RBCs: resemblance to mammalian NHE1

The cDNA encoding the Na+/H+ exchanger (NHE) from Amphiuma erythrocytes was cloned, sequenced, and found to be highly homologous to the human NHE1 isoform (hNHE1), with 79% identity and 89% similarity at the amino acid level. Sequence comparisons with other NHEs indicate that the Amphiuma tridactylum NHE isoform 1 (atNHE1) is likely to be a phylogenetic progenitor of mammalian NHE1. The atNHE1 protein, when stably transfected into the NHE-deficient AP-1 cell line (37), demonstrates robust Na+-dependent proton transport that is sensitive to amiloride but not to the potent NHE1 inhibitor HOE-694. Interestingly, chimeric NHE proteins constructed by exchanging the amino and carboxy termini between atNHE1 and hNHE1 exhibited drug sensitivities similar to atNHE1. Based on kinetic, sequence, and functional similarities between atNHE1 and mammalian NHE1, we propose that the Amphiuma exchanger should prove to be a valuable model for studying the control of pH and volume regulation of mammalian NHE1. However, low sensitivity of atNHE1 to the NHE inhibitor HOE-694 in both native Amphiuma red blood cells (RBCs) and in transfected mammalian cells distinguishes this transporter from its mammalian homologue.

The amiloride-sensitive Na+/H+ exchange antiporter and control of intracellular pH in hippocampal brain slices

The intracellular pH, 7.54 +/- 0.03 (mean +/- S.D., n = 15), determined with the Neutral red method, of the hippocampal brain slice preparation under baseline incubation conditions is considerably more alkaline than the bath buffer pH. Neutralization by amiloride suggests that the alkalinity was due to Na+/H+ exchange antiporter activation. To characterize the brain Na+/H+ exchange antiporter we compared the inhibitory effects of MIA, amiloride and other 5-N substituted analogues on proton extrusion after acid loading by transient exposure to ammonium chloride in the isolated hippocampal brain slice preparation. The potencies of amiloride compounds on the initial recovery rate of intracellular pH after acid-loading were DMA > MIA > HMA = MHA > or = IPA-HCI > IPA > MNPA = Amil > Benzamil. The greater potency of the 5-N substituted analogs of amiloride over amiloride and benzamil strongly suggest that Na+/H+ exchange antiporter is the mechanism responsible for alkalinization in the isolated hippocampal brain slice in vitro.

Effects of Na-K-2Cl cotransport inhibition on myocardial Na and Ca during ischemia and reperfusion

In the context of the "pump-leak" hypothesis (37), changes in myocardial intracellular Na (Nai) during ischemia and reperfusion have historically been interpreted to be the result of changes in Na efflux via the Na-K pump. We investigated the alternative hypothesis that changes in Nai during ischemia are the result of changes in the Na "leak" rather than changes in the pump. More specifically, we hypothesize that the increase in Nai during ischemia is in part the result of increased Na uptake mediated by Na/H exchange. Furthermore, we present data consistent with the interpretation that the Na-K-2Cl cotransporter is active (or, alternatively, displaced from equilibrium) during ischemia and may contribute an additional Na efflux pathway during reperfusion. Thus inhibition of Na efflux via Na-K-2Cl cotransport during ischemia and reperfusion could result in increased Nai and therefore decreased force driving Ca efflux via Na/Ca exchange and ultimately increased intracellular Ca concentration ([Ca]i). Nai (in meq/kg dry wt) and [Ca]i (in nM) were measured in isolated Langendorff-perfused rabbit hearts using nuclear magnetic resonance spectroscopy. Except, during the 65 min of ischemia, hearts were perfused with N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid-buffered Krebs-Henseleit solution equilibrated with 100% O2 at 23 degrees C and pH 7.4 +/- 0.05. During ischemia, Nai rose from 16.6 +/- 0.3 to 62.9 +/- 5.1 (delta Nai approximately 46) meq/kg dry wt and decreased during subsequent reperfusion (mean +/- SE, n = 3 hearts). To measure Na uptake ("leak") in the absence of efflux via the Na-K pump, in all of the protocols described below, the perfusate was nominally K-free solution containing 1 mM ouabain for 10 min before ischemia and during the 30-min reperfusion. After K-free perfusion, Nai rose from 20.2 +/- 0.5 to 79.1 +/- 5.3 (delta Nai approximately 59) meq/kg dry wt (n = 3) during ischemia and decreased during K-free reperfusion. When amiloride (1 mM) was added to the K-free perfusate to inhibit Na/H exchange, Nai rose from 16.3 +/- 0.9 to 44.7 +/- 5.1 (delta Nai approximately 28) meq/kg dry wt (n = 3) during ischemia; i.e., amiloride decreased Na uptake. When bumetanide (20 microM) was added to the nominally K-free perfusate to inhibit Na-K-2Cl contransport, Nai rose from 22.5 +/- 3.9 to 83.8 +/- 13.9 (delta Nai approximately 61 meq/kg dry wt (n = 3) during ischemia and did not decrease during reperfusion; i.e., bumetanide inhibited Na recovery during reperfusion (P < 0.05 compared with bumetanide free). For the same protocol, the presence of bumetanide resulted in increased [Ca]i during ischemia and reperfusion (P < 0.05); these increases in [Ca]i are interpreted to be the result of increased Nai. Thus the results are consistent with the hypotheses.