Acidification and intracellular sodium ion activity during stimulated myocardial ischemia

Rap1b-loss increases neutrophil lactate dehydrogenase activity to enhance neutrophil migration and acute inflammation in vivo

Introduction

Neutrophils are critical for host immune defense; yet, aberrant neutrophil tissue infiltration triggers tissue damage. Neutrophils are heterogeneous functionally, and adopt 'normal' or 'pathogenic' effector function responses. Understanding neutrophil heterogeneity could provide specificity in targeting inflammation. We previously identified a signaling pathway that suppresses neutrophilmediated inflammation via integrin-mediated Rap1b signaling pathway.

Methods

Here, we used Rap1-deficient neutrophils and proteomics to identify pathways that specifically control pathogenic neutrophil effector function.

Results

We show neutrophil acidity is normally prevented by Rap1b during normal immune response with loss of Rap1b resulting in increased neutrophil acidity via enhanced Ldha activity and abnormal neutrophil behavior. Acidity drives the formation of abnormal invasive-like protrusions in neutrophils, causing a shift to transcellular migration through endothelial cells. Acidity increases neutrophil extracellular matrix degradation activity and increases vascular leakage in vivo. Pathogenic inflammatory condition of ischemia/reperfusion injury is associated with increased neutrophil transcellular migration and vascular leakage. Reducing acidity with lactate dehydrogenase inhibition in vivo limits tissue infiltration of pathogenic neutrophils but less so of normal neutrophils, and reduces vascular leakage.

Discussion

Acidic milieu renders neutrophils more dependent on Ldha activity such that their effector functions are more readily inhibited by small molecule inhibitor of Ldha activity, which offers a therapeutic window for antilactate dehydrogenase treatment in specific targeting of pathogenic neutrophils in vivo.

pH modification of human T-type calcium channel gating

External pH (pH(o)) modifies T-type calcium channel gating and permeation properties. The mechanisms of T-type channel modulation by pH remain unclear because native currents are small and are contaminated with L-type calcium currents. Heterologous expression of the human cloned T-type channel, alpha1H, enables us to determine the effect of changing pH on isolated T-type calcium currents. External acidification from pH(o) 8.2 to pH(o) 5.5 shifts the midpoint potential (V(1/2)) for steady-state inactivation by 11 mV, shifts the V(1/2) for maximal activation by 40 mV, and reduces the voltage dependence of channel activation. The alpha1H reversal potential (E(rev)) shifts from +49 mV at pH(o) 8.2 to +36 mV at pH(o) 5.5. The maximal macroscopic conductance (G(max)) of alpha1H increases at pH(o) 5.5 compared to pH(o) 8.2. The E(rev) and G(max) data taken together suggest that external protons decrease calcium/monovalent ion relative permeability. In response to a sustained depolarization alpha1H currents inactivate with a single exponential function. The macroscopic inactivation time constant is a steep function of voltage for potentials < -30 mV at pH(o) 8.2. At pH(o) 5.5 the voltage dependence of tau(inact) shifts more depolarized, and is also a more gradual function of voltage. The macroscopic deactivation time constant (tau(deact)) is a function of voltage at the potentials tested. At pH(o) 5.5 the voltage dependence of tau(deact) is simply transposed by approximately 40 mV, without a concomitant change in the voltage dependence. Similarly, the delay in recovery from inactivation at V(rec) of -80 mV in pH(o) 5.5 is similar to that with a V(rec) of -120 mV at pH(o) 8.2. We conclude that alpha1H is uniquely modified by pH(o) compared to other calcium channels. Protons do not block alpha1H current. Rather, a proton-induced change in activation gating accounts for most of the change in current magnitude with acidification.

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

Cardiac ionic currents and acute ischemia: from channels to arrhythmias

The aim of this review is to provide basic information on the electrophysiological changes during acute ischemia and reperfusion from the level of ion channels up to the level of multicellular preparations. After an introduction, section II provides a general description of the ion channels and electrogenic transporters present in the heart, more specifically in the plasma membrane, in intracellular organelles of the sarcoplasmic reticulum and mitochondria, and in the gap junctions. The description is restricted to activation and permeation characterisitics, while modulation is incorporated in section III. This section (ischemic syndromes) describes the biochemical (lipids, radicals, hormones, neurotransmitters, metabolites) and ion concentration changes, the mechanisms involved, and the effect on channels and cells. Section IV (electrical changes and arrhythmias) is subdivided in two parts, with first a description of the electrical changes at the cellular and multicellular level, followed by an analysis of arrhythmias during ischemia and reperfusion. The last short section suggests possible developments in the study of ischemia-related phenomena.

Effect of acidotic blood reperfusion on reperfusion injury after coronary artery occlusion in the dog heart

A prolongation of the intracellular acidosis after myocardial ischemia can protect the myocardium against reperfusion injury. In isolated hearts, this was achieved by prolongation of the extracellular acidosis. The aim of this study was to investigate whether regional reperfusion with acidotic blood after coronary artery occlusion can reduce infarct size and improve myocardial function in vivo. Anesthetized open-chest dogs were instrumented for measurement of regional myocardial function, assessed by sonomicrometry as systolic wall thickening (sWT). Infarct size was determined by triphenyltetrazolium staining after 3 h of reperfusion. The left anterior descending coronary artery (LAD) was perfused through a bypass from the left carotid artery. The animals underwent 1 h of LAD occlusion and subsequent bypass-reperfusion with normal blood (control, n = 6) or blood equilibrated to pH = 6.8 by using 0.1 mM HCl during the first 30 min of reperfusion (HCl, n = 5). Regional collateral blood flow (RCBF) at 30-min occlusion was measured by using colored microspheres. There was no difference in recovery of sWT in the LAD-perfused area between the two groups at the end of the experiments [-2.8+/-1.2% (HCl) vs. -4.4+/-2.5% (control); mean +/- SEM; p = NS]. RCBF was comparable in both groups. Infarct size (percentage of area at risk) was reduced in the treatment group (12.8+/-2.8%) compared with the control group (26.2+/-4.8%; p < 0.05). These results indicate that reperfusion injury after coronary artery occlusion can be reduced by a prolonged local extracellular acidosis in vivo.

Effects of stilbene derivatives SITS and DIDS on development of intracellular acidosis during ischemia in isolated guinea pig ventricular papillary muscle in vitro

Using ion-selective microelectrode techniques, we investigated the effects of 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS) and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), which are known as Cl(-)-HCO3- exchange blockers, on action potentials and intracellular pH (pHi) in guinea pig ventricular papillary muscles subjected to simulated ischemia. Simulated ischemia was produced by stopping the flow of superfusing solution and then covering the preparations with mineral oil. Simulated ischemia induced a progressive decrease in the maximum upstroke rate and resting membrane potentials, shortened action potential duration, and resulted in cessation of action potentials within 10-12 min after the onset of simulated ischemia. The pHi-measurements revealed progressive intracellular acidosis during the period of simulated ischemia. SITS (0.5 mM) or DIDS (0.1 mM) delayed the onset of ischemia-induced deterioration of action potentials and prolonged the time to cessation of action potentials. SITS or DIDS (0.1-0.5 mM) induced an increase in pHi in HCO3(-)-buffered solution and suppressed the development of intracellular acidosis during ischemia. Under the external Cl(-)-free condition, the time to cessation of action potentials caused by ischemia was significantly delayed, and the development of intracellular acidosis during ischemia was attenuated. The present results indicate that activation of the Cl(-)-HCO3- exchange system would be involved, in part, in the development of intracellular acidosis during cardiac ischemia.

Effects of trimetazidine on pHi regulation in the rat isolated ventricular myocyte

1. We have examined the effects of trimetazidine (TMZ) on intracellular pH (pHi) regulation in rat isolated ventricular myocytes. pHi was recorded ratiometrically by use of the pH-sensitive fluoroprobe, carboxy-SNARF-1 (carboxy-seminaphtorhodafluor). 2. Following an intracellular acid load (induced by 10 mM NH4Cl removal), pHi recovery in HEPES-buffered Tyrode solution was significantly slowed down upon application of 0.3 mM TMZ only when myocytes were pretreated for 5 h 30 min (slowing by approximately 50%; P < 0.01). This effect of TMZ on pHi recovery was shown to be not only time- but also dose-dependent with a large, quickly reversible, effect obtained with 1 mM TMZ applied for 2-3 h (slowing by approximately 64%; P < 0.001). This slowing of pHi recovery was also associated with a decrease of the NH4+ removal-induced acidification. 3. Relationship between intracellular intrinsic buffering power (beta i) and pHi was assessed in absence or presence of TMZ (0.3 mM or 1 mM). As expected, beta i increased roughly linearly with a decrease in pHi in all cases. However, both concentrations of TMZ significantly increased beta i (by approximately 55 and 65% at pHi 7.1, respectively). 4. When Na+/H+ exchange was inhibited by dimethyl amiloride (DMA; 40 microM), trimetazidine (1 mM) did not change the H+ flux estimated at pHi 7.1 (0.31 +/- 0.03 mequiv l-1 min-1, n = 5, control, versus 0.30 +/- 0.025 mequiv l-1 min-1, n = 5, TMZ), ruling out any effect of TMZ on background acid loading. 5. Acid efflux carried by Na+/H+ exchange was significantly decreased only when myocytes were pretreated with 1 mM TMZ, for 2-3 h (JeH = 2.86 +/- 0.38 mequiv l-1 min-1, n = 26, control, versus 1.66 +/- 0.26 mequiv l-1 min-1, n = 10, TMZ, estimated at pHi 7.1; P < 0.05). 6. In conclusion, the present work demonstrates that, following an intracellular acid load in HEPES-buffered medium, trimetazidine slows down pHi recovery in rat isolated ventricular myocytes, primarily through an increase of beta i. An effect on Na+/H+ exchange is also detected but only after long-term incubation of the myocytes with TMZ.

Electrophysiologic changes in ischemic ventricular myocardium: I. Influence of ionic, metabolic, and energetic changes

Myocardial ischemia leads to significant changes in the intracellular and extracellular ionic milieu, high-energy phosphate compounds, and accumulation of metabolic by-products. Changes are measured in extracellular pH and K+, and intracellular pH, Ca2+, Na+, Mg2+, ATP, ADP, and inorganic phosphate. Alterations of membrane currents occur as a consequence of these ionic changes, adrenergic receptor stimulation, and accumulation of lactate, amphipathic compounds, and adenosine. Changes in the volume of the extracellular and intracellular spaces contribute further to the ultimate perturbations of active and passive membrane properties that underlie alterations in excitability, abnormal automaticity, refractoriness, and conduction. These characteristic changes of electrophysiologic properties culminate in loss of excitability and failure of impulse propagation and form the substrate for ventricular arrhythmias mediated through abnormal impulse formation and reentry. The ability to detail the changes in ions, metabolites, and high-energy phosphate compounds in both the extracellular and intracellular spaces and to correlate them directly with the simultaneously occurring electrophysiologic changes have greatly enhanced our understanding of the electrical events that characterize the ischemic process and hold promise for permitting studies aimed at developing interventions that may lessen the lethal consequences of ischemia.

Effect of Na+ reduction and monensin on ion content and contractile response in normoxic and ischaemic reperfused rat hearts

The possibility was explored whether the functional properties of Na+/Ca2+ exchange are altered after ischaemia, thereby contributing to the elevated intracellular (i) Ca2+ levels in ischaemic reperfused hearts. The intracellular Na+, K+ and Ca2+ contents in rat Langendorff heart preparations were determined by atomic absorption spectrometry under normoxic conditions, after ischaemia (30 min) and after ischaemia (30 min) plus reperfusion (30 min). In addition, the influence of modulating the Na+ gradient (Na+o/Na+i) across the sarcolemma was studied with respect to cardiac contractility and intracellular ion content. This was done by either decreasing extracellular (o) Na+ or by increasing Na+i with monensin, both in normoxic and reperfused hearts. Both Na+o reduction and monensin led to an increase in contractility and coronary flow, an effect which was nearly abolished in reperfused hearts. Under normoxic conditions the intracellular ion contents amounted to Na+ = 12.4 +/- 0.4, K+ = 99.0 +/- 3.1 and Ca2+ = 0.64 +/- 0.02 mmol/kg cell (means +/- SEM, n = 7). In normoxic hearts, lowering Na+o reduced and monensin increased Na+i, thereby both leading to a decrease in Na+ gradient; no effect on total Ca2+i content was observed. Na+i increased twofold after ischaemia as compared to the normoxic situation, an effect which was aggravated (4 fold increase) in reperfused hearts. The opposite effects were observed for K+i with a 25% decrease after ischaemia and a 40% decrease in reperfused hearts. Only after ischaemia plus reperfusion was Ca2+i increased (6 fold).(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in myoplasmic pH and calcium concentration during exposure to lactate in isolated rat ventricular myocytes

1. We investigated the mechanisms involved in the rise of myoplasmic calcium concentration ([Ca2+]i) when isolated rat ventricular myocytes were exposed to lactate. The intracellular pH (pHi) and [Ca2+]i were measured using the fluorescent indicators 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF) and fura-2, respectively. Cell shortening was used as a measure of contractile performance. 2. Exposure to 20 mM lactate at the normal extracellular pH (pHo 7.4) for 10 min caused the pHi to fall rapidly by 0.24 pH units and cell shortening was reduced. Thereafter, pHi partially recovered by 0.16 pH units, which was paralleled by a recovery of shortening. 3. Exposure to lactate at a reduced extracellular pH (pHo 6.4) induced a very large acidosis of 0.70 pH units and cell shortening was abolished. During maintained exposure to lactate the pHi remained constant and cell shortening did not recover. 4. Application of Na(+)-H+ exchanger inhibitors, amiloride or ethylisopropyl-amiloride (EIPA), abolished the recovery of pHi and shortening during maintained exposure to lactate at pHo 7.4 and caused an additional acidosis during maintained application of lactate at pHo 6.4. 5. Application of lactate at both the normal and reduced pHo resulted in a rapid, followed by a slower, rise in [Ca2+]i. The diastolic and systolic [Ca2+]i and the amplitude of the systolic rise in the [Ca2+]i (the Ca2+ transient) all increased in both the rapid and the slow phase. 6. When lactate was applied at pHo 7.4, in the presence of EIPA, the initial rise of [Ca2+]i still occurred but the slower increase was abolished. This suggests an involvement of the Na(+)-H+ exchanger in the slower rise of [Ca2+]i. 7. In conclusion, the Na(+)-H+ exchanger is an important regulator of pHi during a lactate-induced intracellular acidosis. The rise of [Ca2+]i involves at least two mechanisms: (i) a rapid component which may represent reduced myoplasmic Ca2+ buffering, impaired Ca2+ removal by the sarcoplasmic reticulum or a direct inhibitory effect of protons on the Na(+)-Ca2+ exchanger; (ii) a slower component linked to stimulation of Na(+)-H+ exchanger which causes an increased [Na+]i and stimulates the Na(+)-Ca2+ exchanger, resulting in an enhanced Ca2+ influx.

Vasodilative response to hypoxia and simulated ischemia is mediated by ATP-sensitive K+ channels in guinea pig thoracic aorta

Local vasodilation in response to hypoxia or ischemia improves perfusion and O2 supply of the affected tissue. This local vasodilation thus constitutes the most important mechanism in the prevention of ischemic cell injury. The regulation of vascular tone has mainly been attributed to changes of cytoplasmatic Ca2+ ((Ca2+)i) concentrations in vascular smooth muscle cells. The mechanism underlying these changes has not, however, been elucidated so far. Using aortic strips of guinea pigs (transversally cut in spirals; normal Tyrode, in mM: NaCl 150, KCl 4.5, MgCl2 2, CaCl2 2.5, glucose 10; buffered with 10 mM HEPES at pH 7.4; equilibrated with 100% O2 at 31 degrees C) the authors could show that metabolic blockade (glucose replaced by 10 mM 2-deoxyglucose (DOG) led to a relaxation of the preparation. Thus, in four experiments, resting tension decreased from 0.75 g by 27% +/- 12% within two hours (% of maximal contractile force developed by each preparation when depolarized with 43 mM KCl and 101.5 mM NaCl). When the same experiment was carried out in the presence of 1 mM tolbutamide (a known blocker of ATP-dependent K+ channels) in vascular smooth muscle no such relaxation could be seen (n = 4). Furthermore, in the same type of preparation, similar results have been obtained upon hypoxic relaxation (100% O2 replaced by 100% N2), where 1 mM tolbutamide also prevented vasodilation. Thus, hypoxic/ischemic vasodilation in response to glycolytic inhibition (DOG) and hypoxia (N2) is based upon the opening of K+ ATP channels and hence can be prevented by sulfonylureas (the opening of K+ ATP channels would lead to hyperpolarization (increased K+ conductance, Goldmann equation), thus diminishing the open probability of voltage-gated Ca2+ channels with subsequent vasodilation). This inhibition by sulfonylureas of vasodilative response to ischemia may also constitute the so far unknown cause of the increased cardiovascular mortality seen under sulfonylurea treatment.

Intracellular calcium and myocardial function during ischemia

Cardiac ischemia causes a rapid decline in mechanical performance and, if prolonged, myocardial cell death occurs on reperfusion. The early decline in mechanical performance could, in principle, be caused either by reduced intracellular calcium release or by reduced responsiveness of the myofibrillar proteins to calcium. It is now known that intracellular calcium rises during ischemia and that the early decline in mechanical performance is caused largely by the inhibitory effects of phosphate and protons on the myofibrillar proteins. The rise of intracellular calcium during ischemia is related to the acidosis and is probably caused by calcium influx on the Na/Ca exchanger. This is triggered by a rise in intracellular sodium which enter the cell in exchange for protons on the Na/H exchanger. Intracellular calcium rises still further on reperfusion the elevation of calcium and the degree of muscle damage are closely correlated.