Inhibition of Na+/Ca2+ exchange in pituitary plasma membrane vesicles by analogues of amiloride

Amiloride inhibits the vasopressin-induced increase in epithelial water permeability

The vasopressin (VP)-induced increase in water permeability in high-resistance, amphibian epithelia is not altered by the abolition of net Na+ flux caused by amiloride added to the apical bathing medium. In this work we looked at the effects on water transport of amiloride added to the serosal medium at a concentration (10(-3) M) known to inhibit Na+/H+ exchange. In urinary bladders of Bufo marinus, amiloride partially blocked the hydrosmotic response to VP. A similar inhibition was found with cyclic adenosine 5'-monophosphate (cAMP) or serosal hypertonicity. We hypothesized that this effect of amiloride could be due to an inhibition of Na+/H+ and/or Na+/Ca2+ antiporters present in the epithelial basolateral membrane and looked at the effects of the diuretic in Na(+)-free media. A similar degree of inhibition of water flow was still found, thus showing that amiloride acts on a cell target other than the antiporters. In toad skin, amiloride did not inhibit the hydrosmotic response to VP and to isoproterenol; however the response to high K+ was significantly reduced. Among the amiloride cell targets described so far, adenylate cyclase and protein kinase A appear to be the best candidates to explain the inhibition of the hydrosmotic response reported here. Direct measurements of intracellular cAMP are needed however to substantiate this hypothesis.

Stimulation of Ca2+ uptake into epididymal bull spermatozoa by analogues of amiloride

Certain amiloride analogues 3',4'-dichlorobenzamil 2',4'-dimethylbenzamil and alpha',2'-benzobenzamil hydrochloride (ATBB) stimulate calcium accumulation and motility by epididymal bovine spermatozoa. This stimulation can be seen at a range of 0.1-0.4 mM, while at higher concentration there is inhibition of calcium uptake by these amiloride analogues. The amiloride derivative 5-(4-chlorobenzyl)-2',4'-dimethylbenzamil (CBDMB), which bears a 4-chlorobenzyl substituent on the 5-amino nitrogen atom, did not stimulate calcium uptake. The amiloride analogue 3',4'-dichlorobenzamil inhibits the Na+/Ca2(+)-exchange activity in isolated plasma membrane vesicles, and the stimulatory effect of 3',4'-dichlorobenzamil on calcium uptake into epididymal sperm could be seen in Na(+)-free medium. Thus, the stimulation of Ca2+ accumulation in the cells caused by 3',4'-dichlorobenzamil is not a result of inhibiting the Na(+)-dependent Ca2+ clearance. There is no stimulation of Ca2+ uptake into ejaculated cells by adding 3',4'-dichlorobenzamil, which is not due to the presence of the calcium-transport inhibitor (caltrin) in these cells [Rufo, G.A., Schoff, P.K. & Lardy, H.A. (1984) J. Biol. Chem. 259, 2547-2552]. The stimulatory effect of 3',4'-dichlorobenzamil on Ca2+ uptake is inhibited by the voltage-dependent Ca2(+)-channel blockers nifedipin and diltiazem. This indicates that the stimulation of Ca2+ uptake by the amiloride analogues is due to the activation of a voltage-dependent Ca2+ channel of the plasma membrane.

Alpha 1-adrenoceptor stimulation increases intracellular pH and Ca2+ in cardiomyocytes through Na+/H+ and Na+/Ca2+ exchange

The effects of alpha 1-adrenergic stimulation on intracellular pH (pHi) and Ca2+ concentration ([Ca2+]i) were investigated in isolated rat cardiomyocytes with fluorescence dyes, BCECF and fura-2, respectively. In the presence of 5 or 25 mM HCO3- norepinephrine (NE) increased pHi in a dose-dependent manner. Intracellular alkalinization was inhibited by prazosin and phentolamine but not by yohimbine. NE-induced alkalinization was inhibited in the presence of a Na+/H+ exchange inhibitor (5-(N,N-hexamethylene) amiloride (HMA)), a C kinase inhibitor (H-7) or a calmodulin inhibitor (W-7), or in the absence of extracellular Na+. NE also increased [Ca2+]i following the pHi increase, which was abolished in the absence of extracellular Na+ or Ca2+. This Ca2+ influx was inhibited by HMA but not by diltiazem (10(-5) M). Thus, we conclude that alpha 1-adrenergic stimulation enhances Na+/H+ exchange by activation of C kinase, thereby allowing intracellular alkalinization, and that subsequent activation of Na+/Ca2+ exchange increases Ca2+ influx.

Inositol metabolism during neuroblastoma B50 cell differentiation: effects of differentiating agents on inositol uptake

Inositol uptake was studied in the rat CNS neuroblastoma B50 cell line. Eadie-Hofstee analysis of the uptake pattern reveals two defined modes of inositol entry into the cell. The high-affinity uptake component requires the presence of extracellular sodium and is inhibited by phloridzin. Analysis of the uptake velocities of the high-affinity uptake component provided the following apparent kinetic parameters: Km = 13.7 microM and Vmax = 14.7 pmol/mg of protein/min (without correcting for residual diffusion) and Km = 12.9 microM and Vmax = 12.3 pmol/mg of protein/min (with correction). At physiological concentrations, the high-affinity transport process contributes approximately 70% to total uptake; the remainder is due to a low-affinity diffusion-like process. Uptake inhibition studies reveal that the uptake process is sensitive to ouabain, amiloride, and dichlorobenzamil inhibition but relatively insensitive to cytochalasin B or phloretin. When neuroblastoma B50 cells are induced to differentiate morphologically with high extracellular calcium or with dibutyryl cyclic AMP, a significant decrease in inositol uptake is observed. The dibutyryl cyclic AMP-mediated inhibition of uptake affects only the high-affinity uptake component and is noncompetitive in nature. The high extracellular calcium-mediated inhibition is less specific; it involves "disappearance" of the high-affinity process, some inhibition of the low-affinity process, and an increase of inositol efflux. The significance of these observations is discussed in the context of neuroblastoma B50 cell differentiation.

Effects of amiloride on metabolism and contractility during reoxygenation in perfused rat hearts

Myocardial recovery after hypoxia may be determined not only by the extent of metabolic depression during the hypoxic period but also by changes in cation contents as well. Calcium overload during reoxygenation, mediated in part by Na-Ca exchange and supported by the rise in cell sodium during hypoxia, may be one factor. The effects of amiloride (0.1 mM), a diuretic that inhibits Na(+)-H+ and Na-Ca exchanges in cardiac sarcolemma and mitochondria preparations, were studied during hypoxia-reoxygenation in the isovolumic, isolated rat heart. During hypoxia, cell sodium, measured using potassium ethylenediamine tetraacetate cobaltate as an extracellular marker, increased in amiloride and amiloride-free hearts, but there was no increase in cell calcium (3.3 +/- 0.3 vs. 3.6 +/- 0.9 mumol/g dry wt; p = NS). Amiloride did not alter developed pressure (DP), end-diastolic pressure (EDP), pH, or integrated areas of adenosine triphosphate (ATP) and phosphocreatine (PCr) (determined by phosphorus-31-nuclear magnetic resonance spectroscopy) during hypoxia or normal perfusion conditions. Forty minutes after reoxygenation, however, cell calcium was significantly lower in the amiloride (5.1 +/- 1.3 mumol/g dry wt) than in the amiloride-free group (10.4 +/- 1.8 mumol/g dry wt; p less than 0.001), and there was improved recovery of DP (percent of initial) (72 +/- 12% vs. 41 +/- 12%; p less than 0.001), PCr (99 +/- 9% vs. 70 +/- 14%; p less than 0.001), and pH (7.17 +/- 0.17 vs. 6.88 +/- 0.16; p less than 0.001) in the amiloride group. To determine whether this dose of amiloride inhibits the manifestations of sodium-mediated calcium gain in the same model during normoxia, the metabolic and functional sequelae of lithium-substituted low sodium (50 mM) perfusion were studied. Amiloride significantly limited the manifestations of sodium-mediated calcium gain as indexed (all expressed as percent of control) by a lower peak DP (221 +/- 25% vs. 284 +/- 20%) at 3 minutes, improved preservation of PCr (85 +/- 10% vs. 68 +/- 9%) and ATP (104 +/- 12% vs. 84 +/- 9%), lower rise in inorganic phosphate (201 +/- 74% vs. 332 +/- 106%), and a smaller fall in intracellular pH (7.01 +/- 0.04 vs. 6.70 +/- 0.15, p less than 0.05) for all metabolic parameters during a 20-minute period.(ABSTRACT TRUNCATED AT 400 WORDS)

The relationship between mitogen-induced changes in the cytoplasmic pH and free Ca2+ concentration in rat thymocytes

The relationship between pHi and [Ca]i signals generated in rat thymocytes by the mitogen Con A has been investigated. It is shown that the mitogen-induced [Ca]i rise is dependent on Na+/H+ exchange or some other Na(+)-sensitive process. This conclusion is based on the following findings: (i) [Ca]i response to Con A weakens upon decreasing the concentration of extracellular Na+, or inhibiting Na+/H+ exchange; (ii) agents that alkalinize the cytoplasm (the phorbol ester TPA, the Na+/H+ ionophore monensin and NH4Cl) cause an increase in [Ca]i (Klip, A., Rothstein, A. and Mack, E. (1984) Biochem. Biophys. Res. Commun. 124, 14-22; Grinstein, S. and Goetz, J.D. (1985) Biochim. Biophys. Acta 819, 267-270); (iii) The effects of Con A, TPA and monensin on [Ca]i are not additive. The last observation suggests that all these agents activate the same Na+/H+ (Na+ and/or H+)-dependent system of Ca2+ transport. It is found that the pH i and [Ca]i responses in rat thymocytes are sensitive to changes in the intracellular levels of cyclic nucleotides, ATP and in temperature. These regulatory effects on the ionic signals are different for Con A, TPA and monensin. In particular, both the stimulation of Na+/H+ antiport and the [Ca]i rise brought about by Con A or TPA are inhibited upon elevating the cellular cAMP. In contrast, the monensin-induced [Ca]i signal is almost independent of cAMP but is highly sensitive to changes in cGMP and temperature. Reducing the ATP level eliminates both the pHi and [Ca]i responses to Con A but not to monensin. These different characteristics of [Ca]i signals elicited by the mitogen and the Na+/H+ ionophore indicate that these agents use different mechanisms to activate the Na+/H(+)-dependent Ca2+ transporting system. A [Ca]i response to monensin has been obtained in some other cell types, namely, in lymphoblastoid Raji cells, Ehrlich ascites tumor cells and also in platelets.

The effect of amiloride analogs on taste responses in gerbil

Amiloride analogs that were designed to inhibit three types of Na+ transport systems (the epithelial Na+ channel, the Na+/H+ antiporter, and the Na+/Ca++ exchanger) were applied to the tongue of the gerbil to determine their effects of electrophysiological taste responses to NaCl, CaCl2, sucrose, and glutamic acid. The pattern of responses from the chorda tympani nerve indicates that the taste of NaCl is almost totally accounted for by the epithelial Na+ channel. Phenamil, an amiloride analog which specifically blocks the epithelial Na+ channel at low concentrations, suppressed the taste responses to 0.03 M NaCl by 97%. The pattern of responses also indicates that the Na+/H+ antiporter and the Na+/Ca2+ exchanger do not mediate salt taste in the gerbil. None of the amiloride analogs blocked taste responses to CaCl2, sucrose, or glutamic acid. It is concluded that the salty taste of NaCl in the gerbil is almost totally mediated by the epithelial Na+ channel, and the kinetics of this channel are identical to amiloride-sensitive sodium channels in other systems.

Influence of isolation media on synaptosomal properties: intracellular pH, pCa, and Ca2+ uptake

Preparations of synaptosomes isolated in sucrose or in Na(+)-rich media were compared with respect to internal pH (pHi), internal Ca2+ concentration ([Ca2+]i), membrane potential and 45Ca2+ uptake due to K+ depolarization and Na+/Ca2+ exchange. We found that synaptosomes isolated in sucrose media have a pHi of 6.77 +/- 0.04 and a [Ca2+]i of about 260 nM, whereas synaptosomes isolated in Na(+)-rich ionic media have a pHi of 6.96 +/- 0.07 and a [Ca2+]i of 463 nM, but both types of preparations have similar membrane potentials of about -50 mV when placed in choline media. The sucrose preparation takes up Ca2+ only by voltage sensitive calcium channels (VSCC'S) when K(+)-depolarized, while the Na(+)-rich synaptosomes take up 45Ca2+ both by VSCC'S and by Na+/Ca2+ exchange. The amiloride derivative 2',4'-dimethylbenzamil (DMB), at 30 microM, inhibits both mechanisms of Ca2+ influx, but 5-(N-4-chlorobenzyl)-2',4' dimethylbenzamil (CBZ-DMB), at 30 microM, inhibits the Ca2+ uptake by VSCC'S, but not by Na+/Ca2+ exchange. Thus, DMB and CBZ-DMB permit distinguishing between Ca2+ flux through channels and through Na+/Ca2+ exchange. We point out that the different properties of the two types of synaptosomes studied account for some of the discrepancies in results reported in the literature for studies of Ca2+ fluxes and neurotransmitter release by different types of preparations of synaptosomes.

Cation transport probes: the amiloride series

The use of amiloride and its analogs in the study of ion transport requires a knowledge of the pharmacology of inhibition of transport proteins, and of effects on enzymes, receptors, and other cellular processes, such as DNA, RNA, and protein synthesis, and cellular metabolism. We have reviewed the pharmacology of inhibition of these processes by amiloride an its analogs, as well as the use of amiloride analogs as potential probes for the characterization of ion transport systems.

Distinct epitopes on amiloride

Most Na(+)-selective transport proteins are inhibited by the drug amiloride. Studies using amiloride analogues suggest that specific regions of amiloride might participate in binding to receptors on these transport proteins. To determine whether certain domains of this drug are recognized as distinct epitopes, amiloride was coupled to albumin through either its C-5 NH2-group on the pyrazine ring or through a terminal NH2-group of the guanidino moiety, and antibodies were raised against these amiloride-albumin conjugates. Studies of antibody binding to amiloride analogues identified the 3,5-diaminopyrazinyl, the guanidinocarbonyl, and the C-6 halo moieties as distinct epitopes, although the antibodies required the presence of both the 3,5-diaminopyrazinyl as well as the guanidinocarbonyl moiety for binding.

Effects of amiloride analogues on the production of prostacyclin by aortic endothelial cells

1. The release of prostacyclin (PGI2) from bovine aortic endothelial cells stimulated by adenosine 5'-triphosphate (ATP) was decreased by amiloride analogues bearing alkyl groups on the 5-amino nitrogen atom, like 5-(N-ethyl-N-isopropyl)amiloride (EIPA), which are inhibitors of the Na+/H+ exchanger. Analogues substituted on a terminal guanidino nitrogen atom were not inhibitory. 2. The release of PGI2 induced by ATP was not significantly depressed in a Na+-poor medium or in a medium acidified to pH 6.9, two conditions known to inhibit the Na+/H+ exchanger. 3. Cytoplasmic alkalinization by ammonium chloride did not suppress the inhibitory action of EIPA. By itself, ammonium chloride decreased the response of endothelial cells to ionophore A23187 and ATP, whereas sodium acetate had no effect. 4. EIPA did not decrease the mobilization of free arachidonic acid induced by ATP. It inhibited the conversion of exogenous arachidonate into PGI2 and prostaglandin E2 (PGE2). 5. Although the intracellular pH was not measured in this study, it seems unlikely that cytoplasmic alkalinization via the activation of the Na+/H+ exchanger plays a significant role in the stimulatory action of ATP on the release of PGI2 from endothelial cells. The inhibition of that release by EIPA and other amiloride analogues might involve a direct effect on cyclo-oxygenase, although an action on the reacylation of free arachidonic acid cannot be excluded.

Inhibition of Na-Ca exchange by general anesthetics

General anesthetics, typically octanol, were found to inhibit the influx of calcium in isolated sodium-loaded adult rat heart cells, using 45Ca, quin 2, or indo 1. Inhibition by octanol, like inhibition by sodium, was competitive with calcium. Octanol and sodium together inhibited calcium influx synergistically. At physiological levels of extracellular calcium and sodium, the EC50 was 177 +/- 37 microM for octanol and 48 +/- 5 microM for decanol. These values are threefold to fourfold larger than those reported to cause 50% loss of righting reflex in tadpoles, a measure of their anesthetic effectiveness. We conclude that general anesthetics inhibit Na-Ca exchange at the sarcolemma. We suggest that octanol inhibits like sodium, and the synergism stems from the cooperativity of sodium inhibition at the binding and regulatory sites of the exchanger. Insofar as Na-Ca exchange may regulate inotropy, the inhibition of Na-Ca exchange by general anesthetics could contribute to their negative inotropic effect.

Chemical modification of cell proliferation and fluid secretion in renal cysts

We used an in vitro model, MDCK cyst, to determine the extent to which pharmacologic compounds known to inhibit plasma membrane solute transport mechanisms could alter the enlargement of renal epithelial cysts. Solitary MDCK cells cultured within collagen gel undergo clonal growth to form true epithelial cysts in which a single layer of polarized cells (apex toward lumen) encloses a fluid-filled cavity. Repeated observations by light microscopy were used to quantitate the rate of cyst growth in diameter, and demonstrated that cyst enlargement involved an increase in cell number (proliferation) and a net increase in intracystic volume (fluid secretion). Intracyst pressure was greater than the interstitium (6.7 mm H2O +/- 3.1 SD), indicating that fluid entry was secondary to net solute accumulation. Amiloride and seven amiloride analogs that inhibited to different degrees conductive Na+ transport, Na+-dependent H+ transport and Na+-dependent Ca++ transport reversibly decreased the rate of cyst enlargement. The effectiveness of these agents to retard cyst enlargement correlated with their relative potencies to inhibit Na+-dependent Ca++ transport. Morphologic examination indicated that amiloride and amiloride analogs decreased cell proliferation and fluid secretion to the same degree. Ouabain and vanadate (Na+K,ATPase inhibitors), and L-645,695 (Na+-dependent Cl-/HCO3- inhibitor) potently slowed cyst expansion. In contrast to amiloride and amiloride analogs, these agents caused an unusual degree of cellular stratification within the cyst walls, a finding consistent with the notion that fluid secretion was inhibited to a greater extent then cellular proliferation. We conclude that chemical inhibitors of primary and secondary active solute transport can diminish or halt the enlargement of epithelial cysts in vitro by decreasing the rate of cellular proliferation and/or net fluid secretion.

Agonist-specific role for Na+-H+ antiport in prostaglandin release from microvessel endothelium

Rabbit coronary microvascular endothelial (RCME) cells synthesize prostaglandin (PG) I2 and PGE2 in response to stimulation with human thrombin, ATP, and the Ca2+ ionophore, A23187. Replacement of extracellular Na+ with choline or N-methylglucamine reduced thrombin-stimulated PG secretion but did not significantly affect either ATP- or A23187-stimulated PG secretion. Pretreatment of RCME cells with Na+ channel or Na+ -Ca2+ exchange blockers did not alter PG release in response to any of these three agonists. Pretreatment of RCME cells with the specific Na+ -H+ antiport blockers 5-(N-ethyl-N-isopropyl)-amiloride (EIPA, 10 microM) and 5-(N,N-hexamethylene)-amiloride (HMA, 0.1 microM) significantly reduced thrombin but not A23187- or ATP-stimulated PG secretion. Studies with the intracellular pH indicator dye 2,7-bis(carboxyethyl)-5(6)-carboxyfluorescein demonstrated thrombin activation of Na+ -H+ antiport, an effect blocked by either HMA or EIPA. Since manipulations known to inhibit Na+ -H+ exchange (EIPA, HMA, replacement of Na+ with choline or N-methylglucamine) reduced thrombin-stimulated RCME cell PG release, we conclude that activation of Na+ -H+ exchange is involved in the coupling of thrombin interaction with RCME cells to subsequent phospholipase activation and PG release.

Inhibition of Na+/Ca2+ exchange by amiloride acting from opposite sides of cardiac sarcolemma

Amiloride inhibited the Na+Ca2+ exchange activity of cardiac sarcolemmal vesicles with similar affinities at the cis and trans sides of the membrane, estimated apparent Ki on both sides of the sarcolemma being similar. The extent of amiloride inhibition on Na+/Ca2+ exchange activity was decreased by alkaline pH only when the drug was acting from the external side of the vesicle sarcolemma, whereas when vesicles were preincubated with the drug at different pH values, amiloride appeared to act as a weak permeant base, being a more effective inhibitor at alkaline pH values. In fact, a rise in the pH of the preincubation medium may favour the entry and consequently the effect of the drug on the exchanger. The pH dependence of the inhibition of Na+/Ca2+ exchange activity by either extravesicular or intravesicular amiloride was consistent with the hypothesis that in both cases the protonated drug was the active form. Evidence is presented that the pattern of interaction of amiloride on the Na+/Ca2+ exchange system strictly depended on the sidedness of drug action. In fact, while Na+ protected against inhibition by amiloride when it was acting on the same side of the vesicle membrane as the drug, it synergically interacted with amiloride to inhibit exchange activity when it was acting on the opposite side of the sarcolemma as the drug. Furthermore, only extravesicular amiloride removed the stimulation of Na+/Ca2+ exchange activity in Ca2+-treated vesicles.

Ionomycin releases calcium from the sarcoplasmic reticulum and activates Na+/Ca2+ exchange in vascular smooth muscle cells

Ionomycin (1 microM) produced a large spike in cytosolic free Ca2+ [( Ca2+]i). The ionophore had no effect on [Ca2+]i if the sarcoplasmic reticulum had previously been Ca2+ depleted by stimulating neurohormone receptors. Ionomycin markedly increased 45Ca2+ efflux and decreased total cell Ca2+ by 60 to 70% in 1 min. Replacing extracellular Na+ [( Na+]o) with choline or N-methyl-D-glucamine strongly inhibited the effects of ionomycin on 45Ca2+ efflux and total Ca2+. Ionomycin caused similar peak increases in [Ca2+]i in the presence and absence of [Na+]o, but the exponential fall from the peak was faster in the presence of [Na+]o. Dimethylbenzamil, a potent blocker of Na+/Ca2+ exchange in these cells, strongly inhibited the effects of ionomycin on 45Ca2+ efflux and total cell Ca2+. We conclude that the increase in cytosolic free Ca2+ produced by ionomycin may be sufficient to activate the plasma membrane Na+/Ca2+ exchanger which removes Ca2+ from the cytosol and helps restore basal [Ca2+]i.

Na-Ca exchanger as a calcium influx pathway in adrenal glomerulosa cells

When aequorin-loaded glomerulosa cells were incubated in isotonic Na2+-free medium containing N-methyl-D-glucamine instead of NaCl, there was an increase in cytoplasmic free calcium concentration, [Ca2+] c, which was not observed when extracellular calcium concentration was reduced to 1 microM. Upon removal of extracellular sodium, there was nearly five-fold increase in fractional efflux ratio of calcium. The reduction of extracellular sodium resulted in a stimulation of calcium influx rate, the magnitude of which was dependent on extracellular sodium concentration. Similar stimulation of calcium influx was observed when extracellular sodium was replaced with lithium. Nitrendipine did not affect the calcium influx induced by the reduction of extracellular sodium while a derivative of amiloride 3',4'-dichlorobenzamil, which inhibits Na-Ca exchange, attenuated calcium influx observed in sodium-free medium. These results indicate that removal of extracellular sodium leads to an increase in [Ca2+] c by stimulating calcium influx and that calcium enters the cell via Na-Ca exchanger.

Evidence for the involvement of Na+/Ca2+ exchange in the stimulation of inositol phospholipid hydrolysis by sodium channel activation and depolarization

Amiloride, an inhibitor of the Na+/Ca2+ exchanger, blocked the hydrolysis of inositol phospholipids in mouse cerebrocortical slices induced by the sodium channel activator veratridine, by KCl, or by the sodium ionophore monensin; there was no inhibition by A 23187, a Ca2+ ionophore, or by serotonin. It is concluded that agents that increase intracellular Na2+ stimulate inositide hydrolysis by an indirect effect via Na+/Ca2+ exchange.

Relationship between cytosolic free Ca2+ and Na+-Ca2+ exchange in aortic muscle cells

We examined the relationship between extracellular Na+ ([Na+]o) and cytosolic free Ca2+ ([Ca2+]i) in primary and passaged cultures of aortic muscle cells. Removing [Na+]o increased [Ca2+]i by approximately 10-fold in cells that were Na+ loaded. Decreasing [Na+]o from 140 to 32 mM caused the half-maximal increase in [Ca2+]i. [Ca2+]i exhibited a sigmoidal dependence on [Na+]o. Mg2+, a competitive inhibitor of Na2+-Ca2+ antiport in these cells, antagonized the increase in [Ca2+]i produced by lowering [Na+]o. High K+ decreased the potency of Mg2+ 6.5-fold as previously reported for Na+ gradient-dependent 45Ca2+ influx. In contrast to the Na+-loaded cells, removing [Na+]o caused no detectable change in [Ca2+]i in cells with normal Na+ even though the calculated electrochemical driving force for Na+-Ca2+ exchange was large enough to almost maximally increase [Ca2+]i in the Na+-loaded cells. We conclude that Na+-Ca2+ antiport activity is latent in the unstimulated cell at basal intracellular Na+ and Ca2+.

Amiloride-blockable sodium currents in isolated taste receptor cells

Isolated taste receptor cells from the frog tongue were investigated under whole-cell patch-clamp conditions. With the cytosolic potential held at -80 mV, more than 50% of the cells had a stationary inward Na current of 10 to 700 pA in Ringer's solution. This current was in some cells partially, in others completely, blockable by low concentrations of amiloride. With 110 mM Na in the external and 10 mM Na in the internal solution, the inhibition constant of amiloride was (at -80 mV) near 0.3 microM. In some cells the amiloride-sensitive conductance was Na specific; in others it passed both Na and K. The Na/K selectivity (estimated from reversal potentials) varied between 1 and 100. The blockability by small concentrations of amiloride resembled that of channels found in some Na-absorbing epithelia, but the channels of taste cells showed a surprisingly large range of ionic specificities. Receptor cells, which in situ express these channels in their apical membrane, may be competent to detect the taste quality "salty." The same cells also express TTX-blockable voltage-gated Na channels.

Odorant response of isolated olfactory receptor cells is blocked by amiloride

Olfactory receptor cells were isolated from the nasal mucosa of Rana esculenta and patch clamped. Best results were obtained with free-floating cells showing ciliary movement. 1) On-cell mode: Current records were obtained for up to 50 min. Under control conditions they showed only occasional action potentials. The odorants cineole, amyl acetate and isobutyl methoxypyrazine were applied in saline by prolonged superfusion. At 500 nanomolar they elicited periodic bursts of current transients arising from cellular action potentials. The response was rapidly, fully and reversibly blocked by 50 microM amiloride added to the odorant solution. With 10 microM amiloride, the response to odorants was only partially abolished. 2) Whole-cell mode: Following breakage of the patch, the odorant response was lost within 5 to 15 min. Prior to this, odorants evoked a series of slow transient depolarizations (0.1/sec, 45 mV peak to peak) which reached threshold and thus elicited the periodic discharge of action potentials. These slow depolarizing waves were reversibly blocked by amiloride, which stabilized the membrane voltage between -80 and -90 mV. We conclude that amiloride inhibits chemosensory transduction of olfactory receptor cells, probably by blocking inward current pathways which open in response to odorants.

Characterization of the mitochondrial Na+-H+ exchange. The effect of amiloride analogues

The kinetic properties and inhibitor sensitivity of the Na+-H+ exchange activity present in the inner membrane of rat heart and liver mitochondria were studied. (1) Na+-induced H+ efflux from mitochondria followed Michaelis-Menten kinetics. In heart mitochondria, the Km for Na+ was 24 +/- 4 mM and the Vmax was 4.5 +/- 1.4 nmol H+/mg protein per s (n = 6). Basically similar values were obtained in liver mitochondria (Km = 31 +/- 2 mM, Vmax = 5.3 +/- 0.2 nmol H+/mg protein per s, n = 4). (2) Li+ proved to be a substrate (Km = 5.9 mM, Vmax = 2.3 nmol H+/mg protein per s) and a potent competitive inhibitor with respect to Na+ (Ki approximately 0.7 mM). (3) External H+ inhibited the mitochondrial Na+-H+ exchange competitively. (4) Two benzamil derivatives of amiloride, 5-(N-4-chlorobenzyl)-N-(2',4'-dimethyl)benzamil and 3',5'-bis(trifluoromethyl)benzamil were effective inhibitors of the mitochondrial Na+-H+ exchange (50% inhibition was attained by approx. 60 microM in the presence of 15 mM Na+). (5) Three 5-amino analogues of amiloride, which are very strong Na+-H+ exchange blockers on the plasma membrane, exerted only weak inhibitory activity on the mitochondrial Na+-H+ exchange. (6) The results indicate that the mitochondrial and the plasma membrane antiporters represent distinct molecular entities.

Amiloride and its analogs as tools in the study of ion transport

Amiloride inhibits most plasma membrane Na+ transport systems. We have reviewed the pharmacology of inhibition of these transporters by amiloride and its analogs. Thorough studies of the Na+ channel, the Na+/H+ exchanger, and the Na+/Ca2+ exchanger, clearly show that appropriate modification of the structure of amiloride will generate analogs with increased affinity and specificity for a particular transport system. Introduction of hydrophobic substituents on the terminal nitrogen of the guanidino moiety enhances activity against the Na+ channel; whereas addition of hydrophobic (or hydrophilic) groups on the 5-amino moiety enhances activity against the Na+/H+ exchanger. Activity against the Na+/Ca2+ exchanger and Ca2+ channel is increased with hydrophobic substituents at either of these sites. Appropriate modification of amiloride has produced analogs that are several hundred-fold more active than amiloride against specific transporters. The availability of radioactive and photoactive amiloride analogs, anti-amiloride antibodies, and analogs coupled to support matrices should prove useful in future studies of amiloride-sensitive transport systems. The use of amiloride and its analogs in the study of ion transport requires a knowledge of the pharmacology of inhibition of transport proteins, as well as effects on enzymes, receptors, and other cellular processes, such as DNA, RNA, and protein synthesis, and cellular metabolism. One must consider whether the effects seen on various cellular processes are direct or due to a cascade of events triggered by an effect on an ion transport system.

Na/Ca exchange and excitation--contraction coupling in frog fast fibres

A 3 Na/Ca exchanger in the transverse tubular wall is modelled as the coupling mechanism between transverse tubular depolarization and Ca release from the sarcoplasmic reticulum. At rest, the Ca-occupied site faces the transverse tubular lumen. Upon depolarization, the difference in chemical potentials of Na and Ca gives a net inward force on Ca resulting in a reorientation of the exchanger so the Ca site now faces the myoplasm and releases Ca to stimulate Ca-induced Ca release from the sarcoplasmic reticulum. The rotation of the exchanger's asymmetrical charge could generate the 'charge movement' signal. As depolarization continues, the site is depleted of Ca and contraction ends spontaneously. Repolarization reorients the exchanger; the depleted Ca site now faces the transverse tubular lumen and slowly refills with Ca (repriming). A kinetic model is capable of controlling both twitch and contracture tension. The Na/Ca exchange blocker dichlorobenzamil (Merck) (10 microM), elevated external Na and low pH all slowed the rate of rise of potassium contracture tension. The ratios of rates of tension rise were dCB/control = 0.4 +/- 0.1, elevated external Na/Tris = 0.6 +/- 0.1, pH 6.3/control = 0.7 +/- 0.01. These results can be mimicked with the kinetic model by slowing the rate of 'rotation' (and hence charge movement) by 50%. Elevated internal Na increases the rate of rise of contracture tension; elevated internal Na/control 1.6 +/- 0.3. Dichlorobenzamil also slows the recovery following spontaneous relaxation; the time constant (68 s) of repriming is unchanged but shifted to longer recovery times. Reduced external Na and pH 6.3 also slow recovery in a similar manner, consistent with delayed rotation of the Ca-depleted site. These results suggest that Na/Ca exchange is a step in both the excitation contraction coupling chain and the repolarization-repriming sequence.

Amiloride analogues cause endothelium-dependent relaxation in the canine coronary artery in vitro: possible role of Na+/Ca2+ exchange

1. A number of amiloride analogues were used to test the proposal that Na+/Ca2+ exchange may play a role in the secretion of endothelium-derived relaxing factor (EDRF). The analogues used were those substituted on either the 5-amino group or the terminal guanidino nitrogen atom. The former block both Na+/Ca2+ and Na+/H+ exchange whilst the latter block the Na+ channel and the Na+/Ca2+ exchange. 2. Both series of compounds caused relaxation in isolated rings of dog coronary artery (EC50 values, 1-10 microM) presumably due to release of EDRF since removal of endothelium greatly attenuated the response. 3. Amiloride (1-100 microM) had little effect on either endothelium-intact or denuded arteries. 4. The guanidino substituted analogues also appeared to block selectively the relaxation response to acetylcholine in the coronary artery, independently of their EDRF-releasing activity. 5. It is proposed that endothelial cells have an active Na+/Ca2+ exchange operating in the forward mode to extrude Ca2+. This mechanism may be important in the control of EDRF release.

Amiloride and its analogs as tools to inhibit Na+ transport via the Na+ channel, the Na+/H+ antiport and the Na+/Ca2+ exchanger

Amiloride analogs inhibit a number of transmembrane Na+ transport systems: 1) the epithelium Na+ channel, 2) the Na+/H+ exchange system and 3) the Na+/Ca2+ exchange system. Structure--activity relationships using amiloride derivatives with selected modification of each of the functional groups of the molecule indicate that the 3 Na+ transporting systems have distinct pharmacological profiles. 5-N Disubstituted derivatives of amiloride, such as ethylisopropylamiloride are the most potent inhibitors of the Na+/H+ exchange system. Conversely, amiloride derivatives that are substituted on the guanidino moiety, such as phenamil, are potent inhibitors of the epithelium Na+ channel. It is thus possible, by using selected amiloride derivatives to inhibit selectively one or another of the Na+ transport systems.

An endogenous sodium current may mediate wound healing in Xenopus neurulae

We have studied healing of Xenopus neurulae in order to investigate the role of endogenous sodium-carried electric currents in wound closure. Transected embryos healed completely within 7 hr in an artificial pond water medium. Wound closure was biphasic, with a rapid purse string-like contraction that was independent of sodium concentration followed by a slower sodium-dependent phase. The initial contraction was reversibly inhibited by cytochalasin B. Healing was prevented when neurulae were wounded and left to heal in sodium-free medium. Healing was also prevented in the presence of amiloride, benzamil, or ouabain, drugs that inhibit sodium flux through the epithelium. The transepithelial potential measured in intact neurulae fell rapidly and reversibly by 70% in response to 10 microM amiloride. Currents measured leaving the wound also decreased by 70% following amiloride addition. Our results indicate that at least one phase of wound healing in Xenopus neurulae is dependent upon an endogenous sodium-carried electric current. We hypothesize that the current may act by guiding epithelial cells to the wound site.

Na+-Ca2+ exchange in human neutrophils

The pathway for inward Ca2+ movements in isolated human neutrophils was investigated. One-way 45Ca2+ influx into resting, steady-state cells amounted to approximately 6 mumol.l cell water-1.min-1. This uptake can be entirely accounted for on the basis of a carrier-mediated exchange of external Ca2+ for internal Na+. The counter transport exhibited trans stimulation of 45Ca2+ influx by internal Na+ (Km approximately equal to 26 mM) with a Hill coefficient of approximately equal to 2.6. There was substrate saturation by external Ca2+ (Km approximately equal to 0.15 mM) and by external Na+ (Km approximately equal to 30 mM); the two ions competed for the same binding sites. The exchange was noncompetitively inhibited by benzamil (Ki approximately 75 microM) and by some other amiloride analogues bearing a substituent on the terminal nitrogen atom of the guanidino group. Membrane depolarization enhanced the rate of 45Ca2+ entry, suggesting an electrogenic process; this voltage dependence was consistent with a coupling ratio of approximately 3 Na+:1 Ca2+. Hence, the Na+-Ca2+ exchange mechanism of neutrophils displays many features in common with those of other cell types. Studies in cells exposed to the tripeptide N-formylmethionyl-leucyl-phenylalanine (FMLP) indicate activation of the counter-transport system by the chemotactic factor. Thus, Na+-Ca2+ exchange may be at least partly responsible for the increase in 45Ca2+ influx and transient rise in intracellular free Ca2+ that are seen following stimulation.

Phenamil, an amiloride analogue, inhibits differentiation of Friend murine erythroleukemic cells

Amiloride has been reported to inhibit Friend murine erythroleukemic (MEL) cell commitment to differentiate by inhibiting the MEL cell plasma membrane Na+-Ca2+ antiporter (R. L. Smith, I. G. Macara, R. Levenson, D. Housman, and L. Cantley. J. Biol. Chem. 257: 773-780, 1982). We therefore screened a series of amiloride analogues to determine whether a more potent and specific inhibitor of MEL cell differentiation could be found. In our experiments, as in those of Lubin (J. Cell. Physiol. 124: 539-544, 1985), amiloride itself did not inhibit MEL cell differentiation. However, we did find that the amiloride analogue phenamil reversibly inhibits dimethyl sulfoxide (DMSO)-induced MEL cell commitment to differentiate with a K1/2 of 2.5-5.0 microM (in plasma clot assay). At an extracellular concentration of 15 microM, phenamil inhibits commitment to differentiate by approximately 90% in the plasma clot assay while having a minimal effect on growth. Phenamil is not metabolized but is rapidly taken up by MEL cells. Phenamil was most effective as an inhibitor when present during the first 12 h of DMSO treatment, indicating that phenamil affects the early commitment process rather than later steps involved in hemoglobin synthesis. Phenamil does not, however, inhibit the early differentiation-induced decrease in [Na+]i and the concomitant drop in the Na+-K+ pump rate. A specific binding site for phenamil is suggested because some analogues in which the phenamil structure is slightly modified are unable to inhibit differentiation.

Activation of sodium-proton exchange is not a prerequisite for Ca2+ mobilization and aggregation in human platelets

Recently it has been suggested [(1987) Nature 325, 456-458; (1987) FEBS Lett. 212, 123-126] that the activation of Na+/H+ exchange is a prerequisite for platelet aggregation and the development of the Ca2+ signal. As direct evidence for the role of the Na+/H+-exchange pathway the inhibition of the Ca2+ signal by EIPA, a specific inhibitor of Na+/H+ exchange, was offered. Here we demonstrate that low concentrations of EIPA (below 1 microM) completely block Na+/H+ exchange while EIPA inhibits aggregation or Ca2+ mobilization only in concentrations 100-times greater than 1 microM. Moreover, another amiloride analogue, CBDMB, developed to act predominantly on Na+/Ca2+ exchange, does not affect Na+/H+ exchange in platelets but blocks aggregation and Ca2+ mobilization. We conclude that while Na+/H+ exchange has a fundamental role in platelet functions it is not prerequisite for the development of Ca2+ signal and aggregation.

Actions of amiloride analogues on prostacyclin synthesis by rat aortic rings

1 Fresh rat aortic rings were incubated in HEPES-buffered salt solutions (pH 8.0) in the presence or absence of amiloride analogues. The effect of these drugs on prostacyclin (PGI2) synthesis was determined by radioimmunoassay of the stable hydrolysis product 6-oxo-prostaglandin (PG)F1 alpha. 2 Amiloride and phenamil (potent inhibitors of epithelial Na+ transport) had no significant effect on basal or Ca2+-stimulated PGI2 synthesis. 3 Several analogues previously reported to inhibit Na+/Ca2+ exchange caused a dose-related increase in 6-oxo-PGF1 alpha production in media containing NaCl 120 mM and CaCl2 2.5 mM. 2',3'-Benzobenzamil was the most potent analogue with a maximum stimulation of 4.51 +/- 0.89 fold, and an EC50 of 3 x 10(-5) M. 4 Amiloride analogues bearing substituents on the 5-amino group of the pyrazine ring have been reported to inhibit Na+/H+ exchange more potently than Na+/Ca2+ exchange. Three of these compounds inhibited Ca2+-stimulated 6-oxo-PGF1 alpha production at concentrations that did not significantly influence basal 6-oxo-PGF1 alpha production.

A derivative of amiloride blocks both the light-regulated and cyclic GMP-regulated conductances in rod photoreceptors

Vertebrate rod photoreceptors in the dark maintain an inward current across the outer segment membrane. The photoresponse results from a light-induced suppression of this dark current. The light-regulated current is not sensitive to either tetrodotoxin or amiloride, potent blockers of Na+ channels. Here, we report that a derivative of amiloride, 3',4'-dichlorobenzamil (DCPA), completely suppresses the dark current and light response recorded from rod photoreceptors. DCPA also blocks a cyclic GMP-activated current in excised patches of rod plasma membrane and a cGMP-induced Ca++ flux from rod disk membranes. These results are consistent with the notion that the Ca++ flux mechanism in the disk membrane and the light-regulated conductance in the plasma membrane are identical. DCPA also inhibits the Na/Ca exchange mechanism in intact rods, but at a 5-10-fold-higher concentration than is required to block the cGMP-activated flux and current. The blocking action of DCPA in 10 nM Ca++ is different from that in 1 mM Ca++, which suggests either that the conductance state of the light-regulated channel may be modified in high and low concentrations of Ca++, or that there may be two ionic channels in the rod outer segment membrane.

Inhibition by amiloride analogues of Na+-dependent hexose uptake in LLC-PK1/Cl4 cells

Amiloride and four analogues of amiloride were shown to inhibit Na+-dependent, phlorizin-sensitive hexose uptake by a clone of pig kidney cells, LLC-PK1/Cl4. The analogues tested were: 5-(N-ethyl-N-isopropyl)amiloride (EIPA), 5-(N-methyl-N-isobutyl)amiloride (MIBA), 3',4'-dichlorobenzamil, and phenamil. The transport substrate was the nonmetabolizable glucose analogue alpha-methyl-D-glucoside. Blockade of Na+-K+ transport at the basolateral membranes or removal of divalent cations from the assay medium had little effect on the initial rate of hexose uptake, whereas MIBA remained an effective inhibitor under both conditions. The inhibitions by EIPA of Na+-H+ exchange and hexose-dependent Na+ uptake could be distinguished by appropriate choice of concentrations of the inhibitor. Hexose transport inhibition does not appear to be secondary to other known effects of the amilorides. Inhibition by all analogues is enhanced when they are tested in low (2 mM) Na+ medium, where they show half-maximal inhibition in the range of 100-300 microM. More detailed kinetic analysis of inhibition by EIPA shows it to be competitive with Na+ with a Ki of 73-107 microM. It is concluded that the amilorides are acting directly on the hexose transporter.

Amiloride and 5-N,N-dimethylamiloride inhibit the carrier mediated uptake of choline in Ehrlich ascites tumor cells

Amiloride and 5-N,N-dimethylamiloride (DMA) inhibit the choline uptake of Ehrlich ascites tumor cells. The inhibition by DMA is competitive with a KI value of 20 microM. The apparent KM value for choline was determined as 15 microM. Amiloride is approximately three times less potent. Amiloride uptake is not antagonized by choline or impaired in cells characterized by a deficient choline carrier. This indicates that amiloride is not transported into the cell by the choline carrier. The inhibition of the choline uptake by DMA cannot be attributed to a depression of choline kinase (EC 2.7.1.32) and is therefore considered to be due to a direct interaction between DMA and the choline carrier. DMA does not compete with sodium ions for its effect on the choline carrier. It is suggested that the choline carrier of Ehrlich ascites tumor cells exhibits a binding site for DMA similar to the one on the Na+/H+ antiporter.

Benzodiazepine interactions at neuronal and smooth muscle Ca2+ channels

Ro 5-4864 and PK 11195, ligands for the peripheral benzodiazepine binding site, blocked 45Ca2+ uptake into depolarized guinea-pig cortical synaptosomes. The fast phase of uptake was significantly more sensitive than the slow phase. The central benzodiazepine ligands, Ro 15-1788 and beta-CCE at 10(-4) and 5 X 10(-5) M respectively, were less effective and did not discriminate between fast and slow uptake phases. Ro 5-4864 and PK 11195 inhibited and potentiated respectively the binding of [3H]nitrendipine to synaptosomes by effects on the KD value. PK 11195 (10(-5) M), blocked K+ depolarization and Bay K 8644-induced tension responses in guinea-pig ileal longitudinal smooth muscle. Benzodiazepines do interact with voltage-dependent Ca2+ channels but the present data are insufficient to prove direct coupling of peripheral benzodiazepine sites with Ca2+ channels.