Na/H exchange-dependent cell volume and pH regulation and disturbances

Ion imaging during axolotl tail regeneration in vivo

Several studies have reported that endogenous ion currents are involved in a wide range of biological processes from single cell and tissue behavior to regeneration. Various methods are used to assess intracellular and local ion dynamics in biological systems, e.g., patch clamping and vibrating probes. Here, we introduce an approach to detect ion kinetics in vivo using a noninvasive method that can electrophysiologically characterize an entire experimental tissue region or organism. Ion-specific vital dyes have been successfully used for live imaging of intracellular ion dynamics in vitro. Here, we demonstrate that cellular pH, cell membrane potential, calcium, sodium and potassium can be monitored in vivo during tail regeneration in the axolotl (Ambystoma mexicanum) using ion-specific vital dyes. Thus, we suggest that ion-specific vital dyes can be a powerful tool to obtain electrophysiological data during crucial biological events in vivo.

Effects of cold cardioplegia on pH, Na, and Ca in newborn rabbit hearts

Many studies suggest myocardial ischemia-reperfusion (I/R) injury results largely from cytosolic proton (Hi)-stimulated increases in cytosolic Na (Nai), which cause Na/Ca exchange-mediated increases in cytosolic Ca concentration ([Ca]i). Because cold, crystalloid cardioplegia (CCC) limits [H]i, we tested the hypothesis that in newborn hearts, CCC diminishes Hi, Nai, and Cai accumulation during I/R to limit injury. NMR measured intracellular pH (pHi), Nai, [Ca]i, and ATP in isolated Langendorff-perfused newborn rabbit hearts. The control ischemia protocol was 30 min for baseline perfusion, 40 min for global ischemia, and 40 min for reperfusion, all at 37°C. CCC protocols were the same, except that ice-cold CCC was infused for 5 min before ischemia and heart temperature was lowered to 12°C during ischemia. Normal potassium CCC solution (NKCCC) was identical to the control perfusate, except for temperature; the high potassium (HKCCC) was identical to NKCCC, except that an additional 11 mmol/l KCl was substituted isosmotically for NaCl. NKCCC and HKCCC were not significantly different for any measurement. The following were different ( P < 0.05). End-ischemia pHi was higher in the CCC than in the control group. Similarly, CCC limited increases in Nai during I/R. End-ischemia Nai values (in meq/kg dry wt) were 115 ± 16 in the control group, 49 ± 13 in the NKCCC group, and 37 ± 12 in the HKCCC group. CCC also improved [Ca]i recovery during reperfusion. After 40 min of reperfusion, [Ca]i values (in nmol/l) were 302 ± 50 in the control group, 145 ± 13 in the NKCCC group, and 182 ± 19 in the HKCCC group. CCC limited ATP depletion during ischemia and improved recovery of ATP and left ventricular developed pressure and decreased creatine kinase release during reperfusion. Surprisingly, CCC did not significantly limit [Ca]i during ischemia. The latter is explained as the result of Ca release from intracellular buffers on cooling.

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.

Heterogeneous patterns of pH regulation in glial cells in the dorsal and ventral medulla

We examined pH regulation in two chemosensitive areas of the brain, the retrotrapezoid nucleus (RTN) and the nucleus tractus solitarius (NTS), to identify the proton transporters involved in regulation of intracellular pH (pHi) in medullary glia. Transverse brain slices from young rats [postnatal day 8 (P8) to P20] were loaded with the pH-sensitive probe 2',7'-bis (2-carboxyethyl)-5,6-carboxyfluorescein after kainic acid treatment removed neurons. Cells were alkalinized when they were depolarized (extracellular K+ increased from 6.24 to 21.24 mM) in the RTN but not in the NTS. This alkaline shift was inhibited by 0.5 mM DIDS. Removal of CO2/HCO3- or Na+ from the perfusate acidified the glial cells, but the acidification after Na+ removal was greater in the RTN than in the NTS. Treatment of the slice with 5-(N-ethyl-N-isopropyl)amiloride (100 microM) in saline containing CO2/HCO3- acidified the cells in both nuclei, but the acidification was greater in the NTS. Restoration of extracellular Cl- after Cl- depletion during the control condition acidified the cells. Immunohistochemical studies of glial fibrillary acid protein demonstrated much denser staining in the RTN compared with the NTS. We conclude that there is evidence of Na+-HCO3- cotransport and Na+/H+ exchange in glia in the RTN and NTS, but the distribution of glia and the distribution of these pH-regulatory functions are not identical in the NTS and RTN. The differential strength of glial pH regulatory function in the RTN and NTS may also alter CO2 chemosensory neuronal function at these two chemosensitive sites in the brain stem.

Mechanisms of endothelial cell swelling from lactacidosis studied in vitro

One of the early sequelae of ischemia is an increase of circulating lactic acid that occurs in response to anaerobic metabolism. The purpose of the present study was to investigate whether lactic acidosis can induce endothelial swelling in vitro under closely controlled extracellular conditions. Cell volume of suspended cultured bovine aortic endothelial cells was measured by use of an advanced Coulter technique employing the "pulse area analysis" signal-processing technique (CASY1). The isosmotic reduction of pH from 7.4 to 6.8 had no effect on cell volume. Lowering of pH to 6.6, 6.4, or 6.0, however, led to significant, pH-dependent increases of cell volume. Swelling was more pronounced in bicarbonate-buffered media than in HEPES buffer. Specific inhibition of Na(+)/H(+) exchange by ethylisopropylamiloride completely prevented swelling in HEPES-buffered media. Pretreatment with ouabain to partially depolarize the cells did not affect the degree of acidosis-induced swelling. In bicarbonate-buffered media, the inhibition of transmembrane HCO(3)(-) transport by DIDS reduced swelling to a level comparable with that seen in the absence of bicarbonate ions. Lactacidosis-induced endothelial swelling, therefore, is a result of intracellular pH regulatory mechanisms, namely, Na(+)/H(+) exchange and bicarbonate-transporting carriers.

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.

A proton-translocating H+-ATPase is involved in C6 glial pH regulation

Glial cells extrude acid equivalents to maintain pHi. Although four mechanisms have been described so far, pHi-control under physiological conditions is still not sufficiently explained. We therefore investigated whether a H+-translocating ATPase is involved in glial pHi homeostasis using an established glial cell line (C6 glioma). In the absence of bicarbonate, the inhibition of H+-ATPases by NEM led to a pHi decrease. The application of a more specific inhibitor (NBD-Cl) showed that the H+-ATPase involved is of the vacuolar type. Inhibition went along with delayed cell swelling. Together with the fact that glial acidification was far more pronounced in Na+-free media, this may serve as evidence for a secondary activation of Na+/H+-exchange once an activation setpoint is reached, which in turn causes secondary swelling from Na+-uptake. Stimulation of Na+/H+-exchange by PMA can increase the setpoint. pHi-recovery after an acid load was blocked by the inhibition of v-type H+-ATPase, if pHi did not reach 6.6 during the acid load. The inhibition of Na+/H+-exchange by amiloride inhibited recovery only if acidification was below the threshold. Finally, in bicarbonate-free media a v-type H+-ATPase contributes to pH-regulation in glial cells, especially during pH-homeostasis at physiological conditions, while Na+/H+-exchange gains significance during severe acid loads.

The effects of hypoxia on pHi in porcine coronary artery endothelium and smooth muscle. A novel method for measurements in endothelial cells in situ

Endothelium-dependent relaxation of porcine coronary arteries is attenuated under hypoxic conditions. Recent evidence also indicates that pHi may modulate the release of the endothelium-derived relaxing factor. We tested the hypothesis that hypoxia-induced attenuation of endothelium-dependent relaxation is mediated by alterations in pHi. We developed a novel method for loading surface cells, whereby endothelial cell pHi could be measured in situ on the intact porcine coronary artery. Endothelial cells of arterial ring segments were selectively loaded with the fluorescent indicator BCECF-AM. Differential loading of the endothelial cell layer was verified by confocal microscopy. pHi of the endothelial cells in situ and of endothelium-denuded arteries was measured with a Photon Technology International spectrofluorimeter. The functional integrity of the endothelium was assessed by the endothelium-dependent relaxation to substance P in a paired adjacent ring. In the experimental protocol for pHi measurements, preparations were perfused with a physiological bicarbonate buffer (pH 7.4), stimulated with KCI (29 mmol/L), and then subjected to hypoxia and reoxygenation. The mean basal pHi in endothelial cells on the intact six arteries was 6.92 +/- 0.07. Addition of KCI to the perfusion medium decreased (P = .025) pHi to 6.79 +/- 0.07. Subsequent bubbling with N2 increased (P = .009) pHi to 7.00 +/- 0.06, which was reversed by reoxygenation. In contrast to the in situ endothelium, pHi of the smooth muscle was not significantly altered from its basal value of 7.24 +/- 0.06 (n = 5) by either KCI or hypoxia. This differential behavior corroborated the confocal data indicating differential dye loading. These data thus suggest that oxygen-sensitive alterations in pHi may be an important mechanism of signal transduction in endothelial cells.

Protective effects of dimethyl amiloride against postischemic myocardial dysfunction in rabbit hearts: phosphorus 31-nuclear magnetic resonance measurements of intracellular pH and cellular energy

The effects of 5-(N,N-dimethyl)amiloride, a potent and specific Na(+)-H+ exchange inhibitor, were investigated in isolated perfused rabbit hearts subjected to ischemia and reperfusion. Phosphorus 31-nuclear magnetic resonance spectroscopy was used to monitor intracellular pH, creatine phosphate, beta-adenosine triphosphate, and inorganic phosphate. After cardioplegic arrest with St. Thomas' Hospital solution, normothermic (37 degrees C) global ischemia was induced for 45 minutes, and the hearts were reperfused for 50 minutes. Dimethyl amiloride at 10 mumol/L, which has minimal inotropic and chronotropic effects on the nonischemic heart, was added to the cardioplegic solution. Treatment with dimethyl amiloride reduced the elevation of left ventricular end-diastolic pressure during and after the ischemia and improved the postischemic recovery of developed pressure from 76% +/- 3.2% at 30 minutes of reperfusion in control hearts (n = 6) up to 99% +/- 1.9% in hearts treated with dimethyl amiloride (n = 8). Dimethyl amiloride did not affect the decline in intracellular pH during ischemia for up to 30 minutes but enhanced the intracellular acidosis thereafter. The intracellular pH at the end of ischemia was 6.21 +/- 0.05 in control hearts compared with 5.24 +/- 0.17 in hearts treated with dimethyl amiloride (p < 0.05). During reperfusion, intracellular pH of hearts treated with dimethyl amiloride was less than control for 5 minutes, but subsequent recovery of intracellular pH was similar to control. Treatment with dimethyl amiloride did not affect creatine phosphate breakdown, inorganic phosphate accumulation, and beta-adenosine triphosphate depletion during 45 minutes of ischemia. The creatine phosphate resynthesis and inorganic phosphate reduction during reperfusion were also unaffected. These findings suggest that Na(+)-H+ exchange plays an important role not only during reperfusion but also during ischemia for the development of postischemic cardiac dysfunction most likely by inducing primary Na+ and secondary Ca2+ overload. Specific Na(+)-H+ exchange inhibitors like dimethyl amiloride would have a potential therapeutic profile in cardiac surgery, especially if added before ischemia.

The role of Na+/H+ exchange in ischemia-reperfusion

In ischemia the cytosol of cardiomyocytes acidifies; this is reversed upon reperfusion. One of the major pH(i)-regulating transport systems involved is the Na+/H+ exchanger. Inhibitors of the Na+/H+ exchanger have been found to more effectively protect ischemic-reperfused myocardium when administered before and during ischemia than during reperfusion alone. It has been hypothesized that the protection provided by pre-ischemic administration is due to a reduction in Na+ and secondary Ca2+ influx. Under reperfusion conditions Na+/H/ exchange inhibition also seems protective since it prolongs intracellular acidosis which can prevent hypercontracture. In detail, however, the mechanisms by which Na+/H+ exchange inhibition provides protection in ischemic-reperfused myocardium are still not fully identified.

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.

Effects of radiologic contrast media on human endothelial and kidney cell lines: intracellular pH and cytotoxicity

RATIONALE AND OBJECTIVES

Radiologic contrast media (CM) are hyperosmotic compounds injected undiluted into a patient's blood, in which they contact endothelial cells. For some types of cultured cells, the application of a hyperosmotic stimulus may cause intracellular pH (pHi) acidification that is related to the extent of hyperosmolality and that ultimately influences cellular function. Accordingly, endothelial and kidney cells, two types of cells known to be exposed to CM effects, were treated at relevant iodine concentrations with various CM (320-1500 mOsm) to determine whether cell exposure to CM can disturb the pH(i) and to examine the contribution of CM to cellular cytotoxicity.

METHODS

Ionic (n = 3) and nonionic (n = 3) CM were compared. Changes in the pH(i) of human vascular endothelial and kidney cell lines were monitored by use of a pH-sensitive fluorescent dye (2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein acetoxymethyl ester). The viability of cells treated with CM was determined by measuring the reduction of a tetrazolium salt (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenlytetrazolium bromide) to violet formazan, a reaction that requires the activity of mitochondrial dehydrogenase; this measurement was made with a microplate reader.

RESULTS

The pH(i) of endothelial and kidney cells exposed to 40-60 mg of iodine per milliliter of CM showed acidification (approximately 0.2 pH unit). Within minutes, gradual pH(i) alkalinization to baseline values occurred. The return to baseline values was slower with ionic compounds than with nonionic CM (P < 0.001). Nonionic agents caused less cellular damage than did ionic CM.

CONCLUSIONS

The pH(i) is involved in the immediate intracellular transduction of CM effects in vitro. The exposure of cells to ionic CM is more detrimental than is exposure to nonionic CM, as demonstrated by disturbances in the cytosolic pH and by long-term effects on cell viability.

pH regulatory Na/H exchange by Amphiuma red blood cells

In Amphiuma red blood cells, the Na/H exchanger has been shown to play a central role in the regulation of cell volume following cell shrinkage (Cala, P. M. 1980. Journal of General Physiology. 76:683-708.) The present study was designed to evaluate the existence of pH regulatory Na/H exchange in the Amphiuma red blood cell. The data illustrate that when the intracellular pHi was decreased below the normal value of 7.00, Na/H exchange was activated in proportion to the degree of acidification. Once activated, net Na/H exchange flux persisted until normal intracellular pH (6.9-7.0) was restored, with a half time of approximately 5 min. These observations established a pHi set point of 7.00 for the pH-activated Na/H exchange of Amphiuma red blood cell. This is in contrast to the behavior of osmotically shrunken Amphiuma red blood cells in which no pHi set point could be demonstrated. That is, when activated by cell shrinkage the Na/H exchange mediated net Na flux persisted until normal volume was restored regardless of pHi. In contrast, when activated by cell acidification, the Na/H exchanger functioned until pHi was restored to normal and cell volume appeared to have no effect on pH-activated Na/H exchange. Studies evaluating the kinetic and inferentially, the molecular equivalence of the volume and pHi-induced Amphiuma erythrocyte Na/H exchanger(s), indicated that the apparent Na affinity of the pH activated cells is four times greater than that of shrunken cells. The apparent Vmax is also higher (two times) in the pH activated cells, suggesting the involvement of two distinct populations of the transporter in pH and volume regulation. However, when analyzed in terms of a bisubstrate model, the same data are consistent with the conclusion that both pH and volume regulatory functions are mediated by the same transport protein. Taken together, these data support the conclusion that volume and pH are regulated by the same effector (Na/H exchanger) under the control of as yet unidentified, distinct and cross inhibitory volume and pH sensing mechanisms.

Importance of tonicity of carbicarb on the functional and metabolic responses of the acidotic isolated heart

In this study, the physiological and metabolic effects of Carbicarb administered as an isotonic (150 mmol/L Na[n[]I+) or hypertonic (1 mol/L Na[n[]I+) solution over 2 minutes in the acidotic isolated heart were compared. Physiological monitoring as well as 31P and 23Na nuclear magnetic resonance spectroscopy were performed. Both isotonic and hypertonic Carbicarb induced comparable dose-dependent increases in intracellular pH as well as decreases in inorganic phosphate and increases in creatine phosphate concentrations, which were sustained for 20 minutes. However, immediate functional improvement was greater in hearts receiving isotonic Carbicarb. Metabolic acidosis conditions resulted in a 27% increase in cytosolic sodium by 30 minutes (P < .05). In this setting, hypertonic Carbicarb induced a large transient increase in cytosolic sodium, whereas isotonic Carbicarb caused immediate and sustained decreases in cytosolic sodium. These data suggest that isotonic Carbicarb may have more beneficial effects on cardiac function than hypertonic Carbicarb. These effects may be related to associated changes in cytosolic sodium.

Na+/H+ exchanger and reperfusion-induced ventricular arrhythmias in isolated perfused heart: possible role of amiloride

The roles of the Na+/H+ exchange system in the development and cessation of reperfusion induced ventricular arrhythmias were studied in the isolated perfused rat heart. The hearts were perfused in the working heart mode with modified Krebs Henseleit bicarbonate (KHB) buffer and whole heart ischemia was induced by a one-way ball valve with 330 beat/min pacing. Ischemia was continued for 15 min followed by 20 min of aerobic reperfusion (control). Amiloride (1.0 mM), an inhibitor of the Na+/H+ exchange system, was added to the KHB buffer only during reperfusion (group B) or only during ischemic periods (group C). Electrocardiographic and hemodynamic parameters were monitored throughout the perfusion. Coronary effluent was collected through pulmonary artery cannulation and PO2, PCO2, HCO3- and pH were measured by blood-gas analyzer. The incidence of reperfusion induced ventricular arrhythmias was 100%, 100% and 0% in control, group B and group C, respectively. The mean onset time of termination of reperfusion arrhythmias was significantly shorter in group B than in control. PCO2 increased from 39.0 +/- 0.9 to 89.3 +/- 6.0 mmHg at the end of ischemia in control and from 40.6 +/- 0.4 to 60.5 +/- 5.8 in group C, the difference between groups was statistically significant. HCO3- level decreased from 21.8 +/- 0.1 to 18.3 +/- 0.5 mmol/l in control, however, this decrease was significantly inhibited in group C (from 22.0 +/- 0.5 to 20.3 +/- 0.2). The increase in PCO2 and the decrease in HCO3- in group B were similar over time to those observed in control.(ABSTRACT TRUNCATED AT 250 WORDS)

Hyperosmotic activation of the Na(+)-H+ exchanger in a rat bone cell line: temperature dependence and activation pathways

1. The hyperosmotic activation of the Na(+)-H+ exchanger was studied in an osteoblast-like rat cell line (RCJ 1.20). The activation was monitored by recording the intracellular pH (pHi) changes employing double excitation of the pH-sensitive fluorescent dye 2'7'-bis(carboxyethyl)-5(6)-carboxyfluorescein acetoxymethyl ester (BCECF-AM). 2. Exposure of the cells to a hyperosmotic HCO(3-)-free medium at 37 degrees C produced an initial cytosolic acidification of 0.05 pH units followed by a lag period and an alkalinization overshoot of about 0.2 pH units, without a concomitant change of the free cytosolic calcium [Ca2+]i by the use of Fura-2 calcium-sensitive probes. This response was completely inhibited by amiloride (0.33 mM) or by Na+ depletion from the external medium and insensitive to the extracellular Cl- replacement, indicating the involvement of a Na(+)-H+ exchanger in the hyperosmotic response. 3. Hyperosmotic stimuli (200 moSM sucrose) applied in the temperature range of 17-37 degrees C demonstrated a shortening of the lag period preceding alkalinization and an increased rate of proton extrusion upon temperature elevation. The biochemical reaction underlying the lag period and the proton extrusion resulted in apparent activation energies of 19 and 29 kcal mol-1, respectively, as calculated from the appropriate Arrhenius plots. 4. Stimulation of the exchanger under isosmotic conditions by 25 nM 4 beta-phorbol 12-myristate 13-acetate (PMA) and 0.1 mM vanadate resulted in an amiloride-sensitive pHi increase of about 0.08 pH units. The hyperosmotic stress was additive to the stimulatory effects of these agents, suggesting an independent hyperosmotic activation pathway. 5. The hyperosmotic activation of the Na(+)-H+ exchanger was independent of cAMP, cGMP, cytosolic Ca2+ and protein kinase C. Thus, none of the classical transduction mechanisms seem to be involved directly in the hyperosmotic activation of the antiporter. 6. The pHi response induced by the hyperosmotic stress was abolished by two calmodulin inhibitors, W-7 and chlorpromazine (50% inhibition, Ki at 28 and 20 microM, respectively), 20 microM cytochalasin B, but not by 10 microM colchicine. The results suggest the involvement of actin and calmodulin-like structural elements of the cytoskeleton in the transduction process leading to the activation of the Na(+)-H+ exchanger.

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

The Amphiuma red blood cell is one of the model systems employed early in the study of vertebrate cell volume regulation. Following both cell swelling and shrinkage the Amphiuma red blood cell demonstrates volume regulation to virtual completion in 90-120 min. When swollen the Amphiuma red blood cell loses K, Cl and osmotically obliged water, while following shrinkage volume regulation is the result of Na, Cl and therefore water uptake. The main contribution of the Amphiuma red cell as a model is that it was the first cell in which volume regulation was demonstrated to be electroneutral and more specifically that K/H and Na/H exchangers were responsible for regulation following cell swelling and shrinkage, respectively. Additionally, the Amphiuma red blood cell K/H and Na/H exchangers have been demonstrated to function in a pH regulatory capacity. The latter observation in turn led to the demonstration of the mutually exclusive and contradictory nature of volume and pH regulation predicted upon Na/H exchanger activity. These observations prompted our recent investigations of the Na/H exchanger as a contributor to hypoxia-induced cell damage, using the rabbit heart as a model. These studies illustrated that Na, and Ca imbalances characteristic of hypoxia-induced cell damage are ultimately referable to the Na/H exchanger's function in a pH regulatory capacity, which contributes fundamentally to cell volume and Ca derangement and ultimately cell injury.

Foreign anions modulate volume set point of sheep erythrocyte K-Cl cotransport

Preequilibration at 37 degrees C in isosmotic media with Cl replaced by lyotropic (foreign) anions reversibly increased Cl-dependent K efflux and Rb influx, the inhibition by furosemide, and thus K-Cl cotransport in low-K but not in high-K sheep erythrocytes with the following order of effectiveness: SCN greater than I greater than NO3 greater than Cl = Br. This effect depended on time, temperature, and anion concentration and was reversible. Preincubation in isosmotic SCN at 37 degrees C stimulated K-Cl flux in anisosmotic Cl media (370-240 mosM) by increasing the volume sensitivity through shifting the point of zero K-Cl flux by approximately 100 mosmol. Thus even shrunken cells exhibited K-Cl cotransport. Preincubation in hyperosmotic SCN or Cl (440 mosM) followed by K flux in hyposmotic Cl (240 mosM) caused a 30-min lag phase that was absent when cells were swollen only. Hence, foreign anions increased the K flux rate in Cl, suggesting upregulation of K-Cl cotransport through new sites or higher turnover per transporter. The anions must act directly on proteins and/or lipids as the accompanying intracellular pH (pHi) changes were too small to attribute the K-Cl flux activation to cellular acidification. After thiol alkylation, which also activates K-Cl cotransport, SCN preexposure at 37 degrees C became ineffective. Carbethoxylation significantly reduced the foreign anion enhancement of K-Cl cotransport and abolished K efflux in Br. It is concluded that interaction of anions through carbethoxylation-sensitive sites with thiols may determine the level of K-Cl cotransport activity.

Na-H exchange in myocardium: effects of hypoxia and acidification on Na and Ca

Historically, increase in cell Na content during ischemic and hypoxic episodes were thought to result from impaired ATP production causing decreased Na(+)-K(+)-ATPase activity. Here we report the results of testing the alternate hypothesis that hypoxia-induced Na uptake is 1) the result of increased entry, as opposed to decreased extrusion 2) via Na-H exchange operating in a pH regulatory capacity and that cell Ca accumulation occurs via Na-Ca exchange secondary to collapse of the Na gradient. We used 23Na-, 19F-, and 31P-nuclear magnetic resonance (NMR) to measure intracellular Na content (Nai), Ca concentration [( Ca]i), pH (pHi), and high-energy phosphates in Langendorff-perfused rabbit hearts. When the Na(+)-K(+)-ATPase was inhibited by ouabain and/or K-free perfusion, hearts subjected to hypoxia gained Na at a rate greater than 10 times that of normoxic controls [during the first 12.5 min Nai increased from 7.9 +/- 5.8 to 34.9 +/- 11.0 (SD) meq/kg dry wt compared with 11.1 +/- 16.3 to 13.6 +/- 9.0 meq/kg dry wt, respectively]. When normoxic hearts were acidified using a 20 mM NH4Cl prepulse technique, pHi rapidly fell from 7.27 +/- 0.24 to 6.63 +/- 0.12 but returned to 7.07 +/- 0.10 within 20 min, while Na uptake was similar in rate and magnitude to that observed during hypoxia (24.5 +/- 13.4 to 132.1 +/- 17.7 meq/kg dry wt). During hypoxia and after NH4Cl washout, increases in [Ca]i were similar in time course to those observed for Na.(ABSTRACT TRUNCATED AT 250 WORDS)

Correlation between cytosolic free calcium, contracture, ATP, and irreversible ischemic injury in perfused rat heart

The relations between ATP depletion, increased cytosolic free calcium concentration [( Cai]), contracture development, and lethal myocardial ischemic injury, as evaluated by enzyme release, were examined using 19F nuclear magnetic resonance to measure [Cai] in 1,2-bis(2-amino-5-fluorophenoxy)ethane-N,N,N',N'-tetraacetic acid (5F-BAPTA)-loaded perfused rat hearts. Total ischemia at 37 degrees C was induced in beating hearts, potassium-arrested hearts, magnesium-arrested hearts, and hearts pretreated with 0.9 microM diltiazem to reduce but not abolish contractility. In the beating hearts, time-averaged [Cai], which is intermediate between the systolic and the basal [Cai], was 544 +/- 74 nM. In contrast, in the potassium- and magnesium-arrested hearts, the time-averaged values are lower than in beating hearts (352 +/- 88 nM for potassium arrest, 143 +/- 22 nM for magnesium arrest). During ischemia, ATP depletion, contracture, and a rise in [Cai] are delayed by cardiac arrest, but all occur more rapidly in the potassium-arrested hearts than in the magnesium-arrested hearts. The diltiazem-treated hearts were generally similar to the magnesium-arrested hearts in their response to ischemia. Under all conditions, contracture development was initiated after tissue ATP had fallen to less than 50% of control; invariably, there was a progressive rise in [Cai] during and following contracture development. Reperfusion with oxygenated perfusate shortly after peak contracture development resulted in a return of [Cai] to its preischemic level, resynthesis of creatine phosphate, no significant enzyme release, and no substantial loss of 5F-BAPTA from the heart. The data demonstrate that an increase in [Cai] precedes lethal myocardial ischemic injury. This rise in [Cai] may accelerate the depletion of cellular ATP and may directly contribute to the development of lethal ischemic cell injury.