Non-selective effects of amiloride and its analogues on ion transport systems and their cytotoxicities in cardiac myocytes

Effect of Temperature and Ionic Substitutions on the Tegumental Potentials of Protoscoleces of Echinococcus granulosus

The protoscolex (PSC) is generated by asexual reproduction at the larval stage of taeniid Echinococcus granulosus that causes cystic echinococcosis or hydatidosis, a worldwide zoonosis. The PSC is enveloped by a complex cellular syncytial tegument responsible for ionic movements and the hydroelectrolytic balance of the parasite. We recently reported on two electrical potentials in bovine lung protoscoleces (PSCs) that reflect differences in ionic movements between the parasite's invaginated and evaginated developmental stages. Here, we explored the effect of temperature and ionic substitutions on the tegumental potentials of bovine lung PSCs of Echinococcus granulosus by microelectrode impalements. We observed that the transient peak potential was temperature-dependent, consistent with an active transport component in the invaginated state only. Further changes in the electrical potentials by high K⁺ depolarization, low external Ca²⁺, and addition of the diuretic amiloride are in agreement with the presence of a Ca²⁺-sensitive cation-selective electrodiffusional pathway in the outer surface of the parasite. Variations in electrical potential differences through the tegument provide an accessible and valuable parameter for studying ionic transport mechanisms and, therefore, potential targets for developing novel antiparasitic drugs.

Polycystin-2 (TRPP2) regulates primary cilium length in LLC-PK1 renal epithelial cells

Polycystin-2 (PC2, TRPP2) is a Ca²⁺ permeable nonselective cation channel whose dysfunction generates autosomal dominant polycystic kidney disease (ADPKD). PC2 is present in different cell locations, including the primary cilium of renal epithelial cells. However, little is known as to whether PC2 contributes to the primary cilium structure. Here, we explored the effect(s) of external Ca²⁺, PC2 channel blockers, and PKD2 gene silencing on the length of primary cilia in wild-type LLC-PK1 renal epithelial cells. Confluent cell monolayers were fixed and immuno-labeled with an anti-acetylated α-tubulin antibody to identify primary cilia and measure their length. Although primary cilia length measurements did not follow a Normal distribution, the data were normalized by Box-Cox transformation rendering statistical differences under all experimental conditions. Cells exposed to high external Ca²⁺ (6.2 mM) decreased a 13.5% (p < 0.001) primary cilia length as compared to controls (1.2 mM Ca²⁺). In contrast, the PC2 inhibitors amiloride (200 μM) and LiCl (10 mM), both increased primary ciliary length by 33.2% (p < 0.001), and 17.4% (p < 0.001), respectively. PKD2 gene silencing by siRNA elicited a statistically significant, 10.3% (p < 0.001) increase in primary cilia length compared to their respective scrambled RNA transfected cells. The data indicate that conditions that regulate PC2 function or gene expression modify the length of primary cilia in renal epithelial cells. Blocking of PC2 mitigates the effects of elevated external Ca²⁺ concentration on primary cilia length. Proper regulation of PC2 function in the primary cilium may be essential in the onset of mechanisms that trigger cyst formation in ADPKD.

Amiloride derivatives are effective blockers of insect odorant receptors

Heteromeric insect odorant receptors (ORs) form ligand-activated nonselective cation channels in recombinant expression systems. We performed a pharmacological characterization of Drosophila melanogaster and Bombyx mori ORs expressed in the Xenopus laevis oocyte expression system and characterized them using the 2-electrode voltage clamp. We identified amiloride derivatives as high-affinity blockers, which inhibit the ion current through the channel in a low micromolar range. For the heteromeric Drosophila Or47a + DmelOrco receptor, the potency sequence (IC(50)) is HMA [5-(N,N-hexamethylene)amiloride] (3.9 µM), MIA [5-(N-methyl-N-isobutyl)amiloride] (11.0 µM), and DMA [5-(N,N-dimethyl)amiloride] (113.3 µM). Amiloride itself is nearly ineffective. Other tested insect ORs (Drosophila Or49b + DmelOrco, B. mori BmorOr1 + BmorOrco) were blocked in a similar fashion suggesting that the amiloride derivatives were potential general blockers of all receptor combinations. Our results suggest that pyrazine derivatives of amiloride are useful probes to study the mechanism of chemosensory transduction in insects in more detail.

Acid-sensing ionic-channel functional expression in the vestibular endorgans

In the vestibular system, the electrical discharge of the afferent neurons has been found to be highly sensitive to external pH changes, and acid-sensing ionic-channels (ASIC) have been found to be functionally expressed in afferent neurons. No previous attempt to assay the ASIC function in vestibular afferent neurons has been done. In our work we studied the electrical discharge of the afferent neuron of the isolated inner ear of the axolotl (Ambystoma tigrinum) to determine the participation of proton-gated currents in the postransductional information processing in the vestibular system. Microperfusion of FMRF-amide significantly increased the resting activity of the afferent neurons of the semicircular canal indicating that ASIC currents are tonically active in the resting condition. The use of ASIC antagonists, amiloride and acetylsalicylic acid (ASA), significantly reduced the vestibular-nerve discharge, corroborating the idea that the afferent neurons of the vestibular system express ASICs that are sensitive to amiloride, ASA, and to FMRF-amide. The sensitivity of the vestibular afferent-resting discharge to the microperfusion of ASIC acting agents indicates the participation of these currents in the establishment of the afferent-resting discharge.

Inhibition of TRPP3 Channel by Amiloride and Analogs

TRPP3, a member of the transient receptor potential (TRP) superfamily of cation channels, is a Ca2+-activated channel permeable to Ca2+, Na+, and K+. TRPP3 has been implicated in sour tasting in bipolar cells of tongue and in regulation of pH-sensitive action potential in spinal cord neurons. TRPP3 is also present in excitable and nonexcitable cells of other tissues, including retina, brain, heart, testis, and kidney, with unknown functions. In this study, we examined the functional modulation of TRPP3 channel by amiloride and its analogs, known to inhibit several ion channels and transporters and respond to all taste stimuli, using Xenopus laevis oocyte expression, electrophysiology, and radiotracer measurements. We found that amiloride and its analogs inhibit TRPP3 channel activities with different affinities. Radiolabeled (45)Ca2+ uptake showed that TRPP3-mediated Ca2+ transport was inhibited by amiloride, phenamil, benzamil, and 5-(N-ethyl-N-isopropyl)amiloride (EIPA). Two-microelectrode voltage clamp experiments revealed that TRPP3-mediated Ca2+-activated currents are substantially inhibited by amiloride analogs, in an order of potency of phenamil > benzamil > EIPA > amiloride, with IC50 values of 0.14, 1.1, 10.5, and 143 microM, respectively. The inhibition potency positively correlated with the size of inhibitors. Using cell-attached patch clamping, we showed that the amiloride analogs decrease the open probability and mean open time but have no effect on single-channel conductance. Study of inhibition by phenamil in the presence of previously reported inhibitor tetrapentylammonium indicates that amiloride and organic cation inhibitors compete for binding the same site on TRPP3. TRPP3 may contribute to previously reported in vivo amiloride-sensitive cation transport.

Mechanisms of cardiodepression by an Na+-H+ exchange inhibitor methyl-N-isobutyl amiloride (MIA) on the heart: lack of beneficial effects in ischemia-reperfusion injury

Although Na+-H+ exchange (NHE) inhibitors such as methyl-N-isobutyl amiloride (MIA) are known to depress the cardiac function, the mechanisms of their negative inotropic effect are not completely understood. In this study, isolated rat hearts were perfused with MIA to study its action on cardiac performance, whereas isolated subcellular organelles such as sarcolemma, myofibrils, sarcoplasmic reticulum, and mitochondria were treated with MIA to determine its effect on their function. The effect of MIA on intracellular Ca2+ mobilization was examined in fura-2-AM-loaded cardiomyocytes. MIA was observed to depress cardiac function in a concentration-dependent manner in HCO3- -free buffer. On the other hand, MIA had an initial positive inotropic effect followed by a negative inotropic effect in HCO3-containing buffer. MIA increased the basal concentration of intracellular Ca2+ ([Ca2+]i) and augmented the KCl-mediated increase in [Ca2+]i. MIA did not show any direct effect on myofibrils, sarcolemma, and sarcoplasmic reticulum ATPase activities; however, this agent was found to decrease the intracellular pH, which reduced the myofibrils Ca2+-stimulated ATPase activity. MIA also increased Ca2+ uptake by mitochondria without having any direct effect on sarcoplasmic reticulum Ca2+ uptake. In addition, MIA did not protect the hearts subjected to mild Ca2+ paradox as well as ischemia-reperfusion-mediated injury. These results suggest that the increase in [Ca2+]i in cardiomyocytes may be responsible for the initial positive inotropic effect of MIA, but its negative inotropic action may be due to mitochondrial Ca2+ overloading as well as indirect depression of myofibrillar Ca2+ ATPase activity. Thus the accumulation of [H+]i as well as occurrence of intracellular and mitochondrial Ca2+ overload may explain the lack of beneficial effects of MIA in preventing the ischemia-reperfusion-induced myocardial injury.

Modification of intracellular calcium concentration in cardiomyocytes by inhibition of sarcolemmal Na+/H+exchanger

Although the Na+/H+exchanger (NHE) is considered to be involved in regulation of intracellular Ca2+concentration ([Ca2+]i) through the Na+/Ca2+exchanger, the exact mechanisms of its participation in Ca2+handling by cardiomyocytes are not fully understood. Isolated rat cardiomyocytes were treated with or without agents that are known to modify Ca2+movements in cardiomyocytes and exposed to an NHE inhibitor, 5-( N-methyl- N-isobutyl)amiloride (MIA). [Ca2+]iin cardiomyocytes was measured spectrofluorometrically with fura 2-AM in the absence or presence of KCl, a depolarizing agent. MIA increased basal [Ca2+]iand augmented the KCl-induced increase in [Ca2+]iin a concentration-dependent manner. The MIA-induced increase in basal [Ca2+]iwas unaffected by extracellular Ca2+, antagonists of the sarcolemmal (SL) L-type Ca2+channel, and inhibitors of the SL Na+/Ca2+exchanger, SL Ca2+pump ATPase and mitochondrial Ca2+uptake. However, the MIA-induced increase in basal [Ca2+]iwas attenuated by inhibitors of SL Na+-K+-ATPase and sarcoplasmic reticulum (SR) Ca2+transport. On the other hand, the MIA-mediated augmentation of the KCl response was dependent on extracellular Ca2+concentration and attenuated by agents that inhibit SL L-type Ca2+channels, the SL Na+/Ca2+exchanger, SL Na+-K+-ATPase, and SR Ca2+release channels and the SR Ca2+pump. However, the effect of MIA on the KCl-induced increase in [Ca2+]iremained unaffected by treatment with inhibitors of SL Ca2+pump ATPase and mitochondrial Ca2+uptake. MIA and a decrease in extracellular pH lowered intracellular pH and increased basal [Ca2+]i, whereas a decrease in extracellular pH, in contrast to MIA, depressed the KCl-induced increase in [Ca2+]iin cardiomyocytes. These results suggest that NHE may be involved in regulation of [Ca2+]iand that MIA-induced increases in basal [Ca2+]i, as well as augmentation of the KCl-induced increase in [Ca2+]i, in cardiomyocytes are regulated differentially.

Pharmacology of Brain Na+/Ca2+Exchanger: From Molecular Biology to Therapeutic Perspectives

In the last two decades, there has been a growing interest in unraveling the role that the Na+/Ca2+ exchanger (NCX) plays in the function and regulation of several cellular activities. Molecular biology, electrophysiology, genetically modified mice, and molecular pharmacology have helped to delve deeper and more successfully into the physiological and pathophysiological role of this exchanger. In fact, this nine-transmembrane protein, widely distributed in the brain and in the heart, works in a bidirectional way. Specifically, when it operates in the forward mode of operation, it couples the extrusion of one Ca2+ ion with the influx of three Na+ ions. In contrast, when it operates in the reverse mode of operation, while three Na+ ions are extruded, one Ca2+ enters into the cells. Different isoforms of NCX, named NCX1, NCX2, and NCX3, have been described in the brain, whereas only one, NCX1, has been found in the heart. The hypothesis that NCX can play a relevant role in several pathophysiological conditions, including hypoxia-anoxia, white matter degeneration after spinal cord injury, brain trauma and optical nerve injury, neuronal apoptosis, brain aging, and Alzheimer's disease, stems from the observation that NCX, in parallel with selective ion channels and ATP-dependent pumps, is efficient at maintaining intracellular Ca2+ and Na+ homeostasis. In conclusion, although studies concerning the involvement of NCX in the pathological mechanisms underlying brain injury during neurodegenerative diseases started later than those related to heart disease, the availability of pharmacological agents able to selectively modulate each NCX subtype activity and antiporter mode of operation will provide a better understanding of its pathophysiological role and, consequently, more promising approaches to treat these neurological disorders.

Na+ overload during ischemia and reperfusion in rat hearts: comparison of the Na+/H+ exchange blockers EIPA, cariporide and eniporide

Intracellular myocardial Na+ overload during ischemia is an important cause of reperfusion injury via reversed Na+/Ca2+ exchange. Prevention of this Na+ overload can be accomplished by blocking the different Na+ influx routes. In this study the effect of ischemic inhibition of the Na+/H+ exchanger (NHE) on [Na+]i, pH, and post-ischemic contractile recovery was tested, using three different NHE-blockers: EIPA, cariporide and eniporide. pHi and [Na+]i were measured using simultaneous 31P and 23Na NMR spectroscopy, respectively, in paced (5 Hz) isolated, Langendorff perfused rat hearts while contractility was assessed by an intraventricular balloon. NHE-blockers (3 microM) were administered during 5 min prior to 30 min of global ischemia followed by 30 min drug-free reperfusion. NHE blockade markedly reduced ischemic Na+ overload; after 30 min of ischemia [Na+]i had increased to 293 +/- 26, 212 +/- 6, 157 +/- 5 and 146 +/- 6% of baseline values in untreated and EIPA (p < 0.01 vs. untreated), cariporide (p < 0.01 vs. untreated) and eniporide (p < 0.01 vs. untreated) treated hearts, respectively. Ischemic acidosis did not differ significantly between groups. During reperfusion, however, recovery of pH, was significantly delayed in treated hearts. The rate pressure product recovered to 12.0 +/- 1.9, 12.1 +/- 2.1, 19.5 +/- 2.8 and 20.4 +/- 2.5 x 10(3) mmHg/min in untreated and EIPA, cariporide (p < 0.01 vs. untreated) and eniporide (p < 0.01 vs. untreated) treated hearts, respectively. In conclusion, blocking the NHE reduced ischemic Na+ overload and improved post-ischemic contractile recovery. EIPA, however, was less effective and exhibited more side effects than cariporide and eniporide in the concentrations used.

Clinical potential of sodium-calcium exchanger inhibitors as antiarrhythmic agents

The Na(+)/Ca(2+) exchanger (NaCaX) plays an important role in calcium handling in myocytes, but in the setting of calcium overload NaCaX can also contribute to the activation of an arrhythmogenic transient inward current (I(ti)). Therefore, approaches to inhibit NaCaX could have potential antiarrhythmic effects in pathophysiological states such as heart failure (HF) or myocardial ischaemia and reperfusion. NaCaX typically functions in a forward (Ca(2+) extrusion) mode but can also function in a reverse (Ca(2+) influx) mode. The determining factors for the directionality of NaCaX ion movement are the electrochemical gradients of calcium and sodium, and membrane potential (E(m)). In HF, upregulated NaCaX plays a dual role: it decreases sarcoplasmic reticulum (SR) calcium load, which leads to contractile dysfunction, and it underlies the I(ti) responsible for delayed after-depolarisations (DADs) and ventricular arrhythmias. In myocardial ischaemia and reperfusion, increases in [Na(+)](i) (as a result of acidosis and activation of the Na(+)/H(+) exchanger [NHE]) lead to calcium overload via the NaCaX and arrhythmogenesis is probably mediated by I(ti) activation due to NaCaX. As such, inhibition of NaCaX could provide a novel therapeutic approach to the prevention and treatment of arrhythmias. Unfortunately, it is difficult to assess the efficacy of such an approach since there are no specific NaCaX inhibitors. Currently available agents are hampered by their nonspecific effects on other ion channels and carriers. The potential utility of specific inhibition of forward or reverse mode NaCaX as an antiarrhythmic approach in the settings of HF and ischaemia/ reperfusion is discussed within the context of current knowledge of myocyte calcium and sodium handling. NaCaX is a challenging and complex therapeutic target because of the delicate balance of SR calcium load (too little contributes to contractile dysfunction and too much leads to calcium overload and arrhythmogenesis). Further understanding of NaCaX function, [Na(+)](i) and [Ca(2+)](i) in HF and ischaemia/reperfusion, combined with the development and assessment of specific NaCaX inhibitors, will ultimately define the potential role of NaCaX inhibition in the prevention and treatment of ventricular arrhythmias.

Cytostatic potential of novel agents that inhibit the regulation of intracellular pH

Cells within the acidic extracellular environment of solid tumours maintain their intracellular pH (pHi) through the activity of membrane-based ion exchange mechanisms including the Na(+)/H(+) antiport and the Na(+)-dependent Cl(-)/HCO(3)(-) exchanger. Inhibition of these regulatory mechanisms has been proposed as an approach to tumour therapy. Previously available inhibitors of these exchangers were toxic (e.g. 4,4-diisothiocyanstilbene-2,2-disulphonic acid), and/or non-specific (e.g. 5-N-ethyl-N-isopropyl amiloride). Using two human (MCF7, MDA-MB231) and one murine (EMT6) breast cancer cell lines, we evaluated the influence of two new agents, cariporide (an inhibitor of the Na(+)/H(+) antiport) and S3705 (an inhibitor of the Na(+)-dependent Cl(-)/HCO(3)(-) exchanger) on the regulation of intracellular pH (pHi). The cytotoxicity of the two agents was assessed by using clonogenic assays. Our results suggest that cariporide has similar efficacy and potency to 5-N-ethyl-N-isopropyl amiloride for inhibition of Na(+)/H(+) exchange while S3705 is more potent and efficient than 4,4-diisothiocyanstilbene-2,2-disulphonic acid in inhibiting Na+-dependent Cl(-)/HCO3(-) exchange. The agents inhibited the growth of tumour cells when they were incubated at low pHe (7.0-6.8), but were non-toxic to cells grown at doses that inhibited the regulation of pHi. Our results indicate that cariporide and S3705 are selective cytostatic agents under in vitro conditions that reflect the slightly acidic microenvironment found in solid tumours.

Localization and functional relevance of system a neutral amino acid transporters in cultured hippocampal neurons

Glutamine and alanine are important precursors for the synthesis of glutamate. Provided to neurons by neighboring astrocytes, these amino acids are internalized by classical system A amino acid carriers. In particular, System A transporter (SAT1) is a highly efficient glutamine transporter, whereas SAT2 exhibits broad specificity for neutral amino acids with a preference for alanine. We investigated the localization and the functional relevance of SAT1 and SAT2 in primary cultures of hippocampal neurons. Both carriers have been expressed since early developmental stages and are uniformly distributed throughout all neuronal processes. However, whereas SAT1 is present in axonal growth cones and can be detected at later developmental stages at the sites of synaptic contacts, SAT2 does not appear to be significantly expressed in these compartments. The non-metabolizable amino acid analogue alpha-(methylamino)-isobutyric acid, a competitive inhibitor of system A carriers, significantly reduced miniature excitatory postsynaptic current amplitude in neurons growing on top of astrocytes, being ineffective in pure neuronal cultures. alpha-(Methylamino)-isobutyric acid did not alter neuronal responsitivity to glutamate, thus excluding a postsynaptic effect. These data indicate that system A carriers are expressed with a different subcellular distribution in hippocampal neurons and play a crucial role in controlling the astrocyte-mediated supply of glutamatergic neurons with neurotransmitter precursors.

The effect of leptin on Na(+)-H(+) antiport (NHE 1) activity of obese and normal subjects erythrocytes

Obesity is currently considered as a chronic metabolic disease, associated with a high risk of cardiovascular complications. Leptin, an adipocyte-derived hormone has a variety target cells influencing a wide range of processes. Possible counteractions of hyperleptinaemia are currently investigated. The Na(+)-H(+) exchanger (NHE 1) is involved in multiple cellular functions and its activation has been related to hypertension and obesity. NHE 1 is present on erythrocytes and can be stimulated by various hormones. Erythrocytes have on their surface a variety of receptors with mostly unknown function. In the present paper, the effect of leptin on erythrocytes NHE 1 activity has been investigated. For this reason, the intracellular pH and sodium influxes were measured before and after addition of leptin in erythrocyte suspensions from normal and obese individuals. Amiloride, a specific NHE 1 inhibitor, and staurosporine a protein kinase C inhibitor were used to inhibit erythrocyte NHE 1. For the binding study leptin was labeled with fluorescein isothiocyanate (FITC) and the binding on erythrocytes was estimated by Scatchard analysis. NHE 1 activity increased in the presence of leptin but significantly less in the obese than in the control group. Furthermore the concentrations of leptin binding sites on the surface of erythrocytes were lower in erythrocytes drawn from obese individuals than in erythrocytes drawn from normal subjects. Since NHE 1 activity has been associated with insulin resistance and hypertension, the activation of this antiport by leptin may represent a link between adipose tissue hypertrophy and cardiovascular complications of obesity.

Involvement of anion exchange in the hypoxia/reoxygenation-induced changes in pH(i) and

The involvement of Cl(-)/HCO(3)(-) exchange in hypoxia/reoxygenation-induced changes in pH(i) and Ca(2+) concentration ([Ca(2+)](i)) was examined in rat ventricular myocytes. During 10-min hypoxia, the initial pH(i) (7.21+/-0.04) fell to below 6.8. Subsequent reperfusion with reoxygenated buffer returned this acidic pH(i) to the neutral range with increases in [Ca(2+)](i). These responses were reduced by the removal of Cl(-) or HCO(3)(-) and by the addition of anion exchange inhibitors, SITS (4-acetamido-4'isothiocyanato-stilbene-2,2'disulfonic acid) and DIDS (4,4'-diisothiocyano-stilbene-2,2'-disulfonic acid), while inhibitors for the Cl(-) channel and Na(+)/K(+)/2Cl(-) cotransport were without effects. The hypoxia-induced acidification was attenuated by protein kinase C inhibitors, calphostin C and chelerythrine, but not by a protein kinase A inhibitor, KT5720. Under normoxic condition, protein kinase C activation induced a SITS-sensitive acidification. Furthermore, in electrically driven rat papillary muscle, SITS and DIDS improved the recovery of developed tension during the reoxygenation. These results suggest that the hypoxia-induced decrease in pH(i) is mediated at least in part by anion exchange stimulation through protein kinase C activation, and this exchange takes part in the reoxygenation-induced Ca(2+) overload as well as contractile dysfunction.

Effect of ethylisopropyl amiloride, a Na+ - H+ exchange inhibitor, on cardioprotective effect of ischaemic and angiotensin preconditioning

The present study is designed to investigate the role of Na+ -H+ exchanger in the cardioprotective effect of ischaemic and angiotensin (Ang II) preconditioning. Isolated perfused rat heart was subjected to global ischaemia for 30 min followed by reperfusion for 120 min. Coronary effluent was analysed for LDH and CK release to assess the degree of cardiac injury. Myocardial infarct size was estimated macroscopically using TTC staining. Left ventricular developed pressure (LVDP) and dp/dt were recorded to evaluate myocardial contractility. Four episodes of ischaemic or Ang II preconditioning markedly reduced LDH and CK release in coronary effluent and decreased myocardial infarct size. 5-(N-ethyl-N-isopropyl)amiloride (EIPA), a Na+ - H+ exchange inhibitor, produced no marked effect on ischaemic preconditioning and Ang II preconditioning induced cardioprotection. On the other hand, EIPA administration prior to global ischaemia produced a similar reduction in myocardial injury as was noted with ischaemic preconditioning or Ang II preconditioning. On the basis of these results, it may be concluded that inhibition of Na+ - H+ exchanger protects against ischaemia-reperfusion induced myocardial injury whereas activation of Na+ - H+ exchanger may not mediate the cardioprotective effect of ischaemic and Ang II preconditioning.

Influence of an extracellular acidosis on excitatory synaptic transmission and long-term potentiation in the CA1 region of rat hippocampal slices

The effects of extracellular acidification on the synaptic function and neuronal excitability were investigated on the hippocampal CA1 neurons. A decrease of extracellular pH from 7.4 to 6.7 did not alter either the resting membrane potential or the neuronal membrane input resistance. Extracellularly recorded field excitatory postsynaptic potentials (fEPSPs) and population spikes (PSs) were significantly reduced by acidosis. Additionally, the amplitude of presynaptic fiber volley was also reduced. The sensitivity of postsynaptic neurons to N-methyl-D-aspartate, but not to alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid, was depressed by acidosis. Lowering of extracellular pH did not significantly affect the magnitude of paired-pulse facilitation (PPF) of synaptic transmission. Acidosis also reversibly limited the sustained repetitive firing (RF) of Na(+)-dependent action potentials elicited by injection of depolarizing current pulses into the pyramidal cells. The limitation of RF by extracellular acidification was accompanied by the reduction of the maximal rate of rise (;V(max)) of the action potentials and the amplitude of afterhyperpolarization. Neither the Na (+)/H (+) antiporter blocker 5-(N -ethyl -N -isopropyl)-amiloride nor the selective adenosine A (1) receptor antagonist 1,3-dipropyl -8-cyclopentylxanthine, however, affected the acidosis -induced synaptic depression. It was also found that acidosis did not affect either the induction r maintenance of long -term potentiation (LTP) at Schaffer collateral -CA 1 synapses. These results suggest that the extracellular acidosis -induced synaptic depression is likely to result from an inhibition of presynaptic Na (+) conductance, thereby decreasing the amplitude of action potentials in individual afferent fibers or the number of afferent fiber activation to stimuli and then indirectly affecting the signaling processes contributing to trigger neurotransmitter release.

Intracellular pH modulates spontaneous and epileptiform bioelectric activity of hippocampal CA3-neurones

A growing body of evidence hints at intracellular free protons to be involved in the modulation of electric activity of cortical neurones. In this study we demonstrate that application of the weak acid propionate (2.5-20 mM) transiently lowers intracellular pH (pH(i)) of BCECF-AM loaded CA3-neurones in hippocampal slices. The predictability of this acidification prompted us to use propionate as a tool to investigate effects of pH(i) on spontaneous bioelectric activity (SBA) and epileptiform activity (EA, induced by bicuculline, caffeine or low magnesium) of CA3 neurones: SBA and EA were transiently suppressed by 2-20 mM propionate - coinciding with the transient neuronal acidification. As activation of Na(+)/H(+)-exchangers (NHE) is involved in the recovery from neuronal acidosis and NHE-inhibition alone is known to increase the activity of intracellular free protons of hippocampal neurones, we tested the effect of the NHE-blockers amiloride (0.5-1 mM) or HOE642 (200 microM) on SBA and EA of CA3-neurones. Long-term application of NHE-inhibitors alone continuously suppressed SBA and EA, which recovered during additional exposure to the weak base trimethylamine (5-10 mM). Simultaneous administration of propionate and NHE-blockers intensified the inhibition of neuronal activity. Together, these results indicate that intracellular acidification inhibits bioelectric activity of hippocampal CA3-neurones. This supports the hypothesis that pH(i) contributes to the control of cortical excitability.

Blocking Na(+)-H+ exchange by cariporide reduces Na(+)-overload in ischemia and is cardioprotective

In myocardial ischemia, rapid inactivation of Na(+)-K(+)-ATPase and continuing influx of sodium induce Na(+)-overload which is the basis of Ca(2+)-overload and irreversible tissue injury following reperfusion. The Na(+)-H(+)-exchanger of subtype 1 (NHE-1) is assumed to play a major role in this process, but previously available inhibitors were non-specific and did not allow to verify this hypothesis. Cariporide (HOE 642) is a recently synthesized NHE-1 inhibitor. We have investigated its effects on Na+ homeostasis (23Na NMR spectroscopy), cardiac function and energy metabolism (31P NMR) in ischemia and reperfusion. In the well-oxygenated, isolated guinea-pig heart, cariporide (10 microM) had no effect on intracellular Na+, pH or cardiac function. NHE-1 inhibition by cariporide was demonstrated using the NH4Cl prepulse technique. When hearts were subjected to 15 min of ischemia, cariporide markedly inhibited intracellular Na(+)-accumulation (1.3 +/- 0.1 vs 2.1 +/- 0.1-fold rise) but had no effect on the decline in pH. In reperfusion, NHE-1-blockade significantly delayed pH recovery. With longer periods of ischemia (36 min), cariporide delayed the onset of contracture, reduced ATP depletion, Na(+)-overload and again had no effect on pH. In reperfusion, hearts treated with cariporide showed an improved recovery of left ventricular pressure (60 +/- 1 vs 16 +/- 8 mmHg): end-diastolic pressure was normalized and phosphocreatine fully recovered, while there was only a partial recovery in controls. The data demonstrate that Na(+)-H(+)-exchange is an important port of Na(+)-entry in ischemia and contributes to H(+)-extrusion in reperfusion. By reducing Na(+)-overload in ischemia and prolonging acidosis in reperfusion, NHE-blockade represents a promising cardioprotective principle.

A novel role for cyclic nucleotide-gated cation channels in lung liquid homeostasis in sheep

1. Sheep lungs were artificially perfused in situ with warmed whole oxygenated sheep blood. The airspaces of the lungs were filled with liquid containing an impermeant tracer, to allow measurement of the rate of net transepithelial liquid movement under various conditions. 2. Dichlorobenzamil (1.5 x 10-5 M), a blocker of cyclic nucleotide-gated cation channels, inhibited the resting absorption of lung liquid in sheep aged 6 months (n = 5) (from -36.47 +/- 4.62 to -4.36 +/- 5.27 ml h-1, means +/- s.e.m.; P < 0.005, paired t test). Amiloride (10-4 M), a blocker of epithelial sodium channels, had no additive effect to that of dichlorobenzamil. 3. In the lungs of sheep aged 6 months (n = 4), amiloride (10-4 M) partially inhibited the resting absorption of liquid (from -35.21 +/- 8.57 to -11.05 +/- 4.91 ml h-1; P < 0.05, one-tailed paired t test), and dichlorobenzamil (1.5 x 10-5 M) exerted an additive effect to that of amiloride resulting in secretion at +6.29 +/- 3.05 ml h-1 (P < 0. 01, paired t test). 4. In the lungs of sheep aged 6 weeks (n = 3), amiloride (10-4 M) also inhibited the resting absorption of liquid (from -26.36 +/- 14.05 to -5.17 +/- 8.27 ml h-1; P < 0.05, one-tailed paired t test); however, dichlorobenzamil (1.5 x 10-5 M) did not exert an additive effect to that of amiloride. 5. In the lungs of sheep aged 6 months (n = 4), amiloride (10-4 M) partially inhibited the resting absorption of liquid (from -35.70 +/- 8.58 to -6.79 +/- 4.28 ml h-1; P < 0.05, paired t test), and pimozide (1.5 x 10-4 M), another blocker of cyclic nucleotide-gated cation channels, also exerted an additive effect to that of amiloride, resulting in secretion of lung liquid at +15.36 +/- 9.14 ml h-1 (P < 0.05, paired t test). 6. We conclude that cyclic nucleotide-gated cation channels mediate a component of lung liquid absorption in sheep aged 6 months (but not in sheep aged 6 weeks), and that a mechanism for lung liquid secretion (present in fetuses) is retained at 6 months of age.

Effect of Na+/Ca2+ exchange inhibitor, KB-R7943 on ouabain-induced arrhythmias in guinea-pigs

1. We investigated protective effects of KB-R7943, a Na+/Ca2+ exchange (NCX) inhibitor, on ouabain-induced tonotropy and arrhythmias in isolated whole atria and ouabain-induced changes in electrocardiogram (ECG) in the guinea-pig. 2. KB-R7943 (10 and 30 microM) suppressed the tonotropic effect of ouabain, and prolonged the onset time of extra-systole induced by ouabain in isolated atria. 3. The intravenous injection of KB-R7943 (1 and 3 mg kg-1) significantly increased the doses of ouabain required to induce ventricular premature beats (VPB), ventricular tachycardia (VT), ventricular fibrillation (VF) and cardiac arrest (CA) in anaesthetized guinea-pigs. 4. Lidocaine (Na+channel inhibitor) and R56865 (Na+ and Ca2+ overload inhibitor) also suppressed the ouabain-induced tonotropic effect and extra-systole in isolated atria, but Hoe-694 (Na+/H+ exchange inhibitor) or diltiazem (Ca2+ channel inhibitor) did not affect them. 5. Lidocaine also increased the doses of ouabain required to induce VPB, VT, VF and CA in anaesthetized guinea-pigs. 6. From these results, we conclude that KB-R7943 suppresses ouabain-induced arrhythmias through inhibition of the reverse-mode NCX.

Activation of neurotransmitter release in hippocampal nerve terminals during recovery from intracellular acidification

Intracellular pH may be an important variable regulating neurotransmitter release. A number of pathological conditions, such as anoxia and ischemia, are known to influence intracellular pH, causing acidification of brain cells and excitotoxicity. We examined the effect of acidification on quantal glutamate release. Although acidification caused only modest changes in release, recovery from acidification was associated with a very large (60-fold) increase in the frequency of miniature excitatory postsynaptic currents (mEPSCs) in cultured hippocampal neurons. This was accompanied by a block of evoked EPSCs and a rise in intracellular free Ca2+ ([Ca2+]i). The rise in mEPSC frequency required extracellular Ca2+, but influx did not occur through voltage-operated channels. Because acidic pH is known to activate the Na+/H+ antiporter, we hypothesized that a resulting Na+ load could drive Ca2+ influx through the Na+/Ca2+ exchanger during recovery from acidification. This hypothesis is supported by three observations. First, intracellular Na+ rises during acidification. Second, the elevation in [Ca2+]i and mEPSC frequency during recovery from acidification is prevented by the Na+/H+ antiporter blocker EIPA applied during the acidification step. Third, the rise in free Ca2+ and mEPSC frequency is blocked by the Na+/Ca2+ exchanger blocker dimethylbenzamil. We thus propose that during recovery from intracellular acidification a massive activation of neurotransmitter release occurs because the successive activation of the Na+/H+ and Na+/Ca2+ exchangers in nerve terminals leads to an elevation of intracellular calcium. Our results suggest that changes in intracellular pH and especially recovery from acidification have extensive consequences for the release process in nerve terminals. Excessive release of glutamate through the proposed mechanism could be implicated in excitotoxic insults after anoxic or ischemic episodes.

Na-H exchange inhibition with cariporide limits functional impairment caused by repetitive ischemia

Intracellular calcium ([Ca]i) overload on reperfusion may be one of the mechanisms responsible for ischemia-induced regional myocardial dysfunction. Because inhibiting the Na-H exchanger (NHE) limits intracellular sodium ([Na]i) and subsequent [Ca]i accumulation, we hypothesized that NHE inhibition would attenuate regional dysfunction in response to 25 cycles of ischemia (I, 2-min) and reperfusion (R, 8-min) of the left circumflex coronary artery (LCx) in conscious swine. Six animals were instrumented to measure arterial pressure, regional myocardial blood flow (colored microspheres), systolic wall thickening (WTh) in the normally perfused (left anterior descending, LAD) and LCx regions (sonomicrometry), LCx blood flow velocity (Doppler), and to reversibly occlude the LCx (hydraulic occluder). Each animal completed three protocols separated by 7 days: ISC, 25 I/R cycles; CAR, 25 I/R cycles + NHE inhibition (cariporide); and VEH, vehicle administration for 4.2 h. Regional myocardial blood flow was measured during LCx occlusion in the first protocol and 10 min after I/R 25 in all protocols. Systemic hemodynamics were similar among and within each protocol. Blood flow measured during LCx occlusion confirmed that perfusion was reduced (p < 0.05) to this compared with the LAD region. During ISC, LCx WTh was reduced (p < 0.05) after five IR cycles, and a stable reduction (approximately 55% of baseline; p < 0.05) was present after 20 I/R cycles. During CAR, LCx systolic WTh was reduced (p < 0.05) only after 15 and 25 I/R cycles (approximately 80 and 72%, respectively). The decrease in LCx WTh was greater in ISC than in CAR (p < 0.05). LCx WTh was not altered during VEH, while LAD WTh was similar within and among all protocols. Regional blood flow measured after 25 I/R cycles was not different among protocols. Our results indicate that NHE inhibition delays the onset and limits the degree of regional dysfunction in response to repeated bouts of ischemia and reperfusion.

Na+/Ca2+ exchange in catfish retina horizontal cells: regulation of intracellular Ca2+ store function

The role of the Na+/Ca2+ exchanger in intracellular Ca2+ regulation was investigated in freshly dissociated catfish retinal horizontal cells (HC). Ca2+-permeable glutamate receptors and L-type Ca2+ channels as well as inositol 1,4,5-trisphosphate-sensitive and caffeine-sensitive intracellular Ca2+ stores regulate intracellular Ca2+ in these cells. We used the Ca2+-sensitive dye fluo 3 to measure changes in intracellular Ca2+ concentration ([Ca2+]i) under conditions in which Na+/Ca2+ exchange was altered. In addition, the role of the Na+/Ca2+ exchanger in the refilling of the caffeine-sensitive Ca2+ store following caffeine-stimulated Ca2+ release was assessed. Brief applications of caffeine (1-10 s) produced rapid and transient changes in [Ca2+]i. Repeated applications of caffeine produced smaller Ca2+ transients until no further Ca2+ was released. Store refilling occurred within 1-2 min and required extracellular Ca2+. Ouabain-induced increases in intracellular Na+ concentration ([Na+]i) increased both basal free [Ca2+]i and caffeine-stimulated Ca2+ release. Reduction of external Na+ concentration ([Na+]o) further and reversibly increased [Ca2+]i in ouabain-treated HC. This effect was not abolished by the Ca2+ channel blocker nifedipine, suggesting that increases in [Na+]i promote net extracellular Ca2+ influx through a Na+/Ca2+ exchanger. Moreover, when [Na+]o was replaced by Li+, caffeine did not stimulate release of Ca2+ from the caffeine-sensitive store after Ca2+ depletion. The Na+/Ca2+ exchanger inhibitor 2',4'-dimethylbenzamil significantly reduced the caffeine-evoked Ca2+ response 1 and 2 min after store depletion.

A novel antagonist, No. 7943, of the Na+/Ca2+ exchange current in guinea-pig cardiac ventricular cells

1. The effects of No. 7943 on the Na+/Ca2+ exchange current and on other membrane currents were investigated in single cardiac ventricular cells of guinea-pig with the whole-cell voltage-clamp technique. 2. No. 7943 at 0.1-10 microM suppressed the outward Na+/Ca2+ exchange current in a concentration-dependent manner. The suppression was reversible and the IC50 value was approximately 0.32 microM. 3. No. 7943 at 5-50 microM suppressed also the inward Na+/Ca2+ exchange current in a concentration-dependent manner but with a higher IC50 value of approximately 17 microM. 4. In a concentration-response curve, No. 7943 raised the K(m)Ca2+ value, but did not affect the Imax value, indicating that No. 7943 is a competitive antagonist with external Ca2+ for the outward Na+/ Ca2+ exchange current. 5. The voltage-gated Na+ current, Ca2+ current and the inward rectifier K+ current were also inhibited by No. 7943 with IC50S of approximately 14, 8 and 7 microM, respectively. 6. In contrast to No. 7943, 3', 4'-dichlorobenzamil (DCB) at 3-30 microM suppressed the inward Na+/Ca2+ exchange current with IC50 of 17 microM, but did not affect the outward exchange current at these concentrations. 7. We conclude that No. 7943 inhibits the outward Na+/Ca2+ exchange current more potently than any other currents as a competitive inhibitor with external Ca2+. This effect is in contrast to DCB which preferentially inhibits the inward rather than the outward Na+/Ca2+ exchange current.