Effects of amiloride on the cardiovascular system: role of Na+/Ca2+ exchange

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

alpha1-Adrenoceptor-mediated Negative Inotropic Effect Caused by Intracellular Ionic Activities in Guinea-pig Papillary Muscle

In guinea-pig papillary muscle, phenylephrine (PE), an agonist of alpha1-adrenoceptor (alpha1-AR), led to a transient negative inotropic effect (NIE) and a subsequent sustained positive inotropic effect (PIE). To clarify the ionic mechanisms underlying the NIE, we measured the [Na+]i or [pH]i by ion-selective microelectrodes. PE produced a decrease in the intracellular Na+ concentration ([Na+]i) and an increase in intracellular pH ([pH]i). During the phase of NIE, PE produced only a (-) change of [Na+]i (Delta[Na+]i). With a decrease in extracellular Na+ or an increase in extracellular Ca2+, the PE-induced NIE was attenuated and PE produced (+)Delta[Na+]i. The PE-induced NIE and (-)Delta[Na+]i were definitely strengthened by lowering the bath temperature or increasing the stimulation frequency. 2-(2,6-di-methoxyphenoxyethyl)amino-methyl-1,4-benzidioxane HCl, an antagonist of alpha1A-AR, completely abolished the PE-induced NIE and (-)Delta[Na+]i. Phorbol 12,13-dibutyrate, an activator of protein kinase C (PKC), decreased the baseline [Na+]i and twitch force and increased the baseline [pH]i in mimicry of PE. Pretreatment with 1-5(isoquinolinesulphonyl)-2-methylpiperazine, an inhibitor of PKC, abolished the PE-induced NIE and (-)Delta[Na+]i. During pretreatment with benzamil, an inhibitor of Na+/Ca2+ exchange, we found that the PE-induced NIE and (-)Delta[Na+]i were reversibly abolished. Our results indicate the PE-induced NIE may be elicited upon the activation of Na+/Ca2+ exchange which can be attributed to the (-)Delta[Na+]i. (-)Delta[Na+]i is mediated through a PKC-dependent pathway via an activation of alpha1A-AR subtype and its effect could be strengthened remarkably at high [Na+]i and [Ca2+]i values.

Advanced procedures for separation and analysis of low molecular weight inhibitor (NCXIF) of the cardiac sodium-calcium exchanger

A low molecular weight inhibitor (NCX(IF)) of the cardiac Na/Ca exchanger, isolated from the calf ventricle tissue, is capable of regulating the muscle strip's contractility and relaxation without involving the beta-activation pathway. The structural analysis of NCX(IF) requires highly purified preparations that fulfill the demanding requirements for mass spectra and NMR analyses. No such preparation is yet available. To this end, new HPLC procedures were developed by a combination of the reverse phase, normal phase, and HILIC (hydrophilic liquid chromatography) techniques. The specific activity of NCX(IF) is 10(5) times higher in the purified preparations (as compared to the crude extract) showing a 2-5% yield of total inhibitory activity and 20-100 microg content of final material. The purification yield reveals that 1 kg ventricle muscle contains 0.1-0.2 mg NCX(IF), meaning that the tissue concentrations of NCX(IF) may reach 10(-7)-10(-6) M. The diode-array scanning of purified preparations of NCX(IF) shows a homogeneous 3D peak with a maximal absorption at 202 nm. These spectral properties may represent a five-membered ring (e.g., proline, histidine) and/or simple chemical groups (like amine, carbonyl, ester, etc.), but not an aromatic ring or complex conjugates (alkyne, alkene, aldehyde, etc.). NCX(IF) does not respond to phenol/sulfur reagent, suggesting that it lacks reducing (aldo) sugar. NCX(IF) shows a faint response to fluorescamine, meaning that it may contain an amino group (or its derivative). It is believed that a combination of presently developed procedures with LC/MS and LC/MS/MS may provide a useful tool for structural analysis of NCX(IF).

Inotropic and lusitropic effects induced by the inhibitory factor of the Na/Ca exchanger are not mediated by the beta-adrenergic activation

Recently, an endogenous inhibitory factor (NCXIF) of the cardiac Na/Ca exchanger (NCX1) has been isolated, purified, and preliminary characterized. Here, we demonstrate that low doses of NCXIF (10(-7)10(-8) M) induce strong inotropic effects in the guinea and rat ventricle strips, while having no detectable effects in the atria even at 10(-5) M. The inotropic effects of NCXIF are species-specific; the rat ventricle muscle is 20 to 50 times more sensitive to varying doses of NCXIF than the guinea pig. On the other hand the extent of maximal inotropic response is more prominent in the guinea pig model (up to 6-fold enhancement) than in the rat (up to 2-fold enhancement). The NCXIF accelerates the single-twitch relaxation (lusitropic effect) in dose-dependent manner, reaching approximately 2-fold shortening of twitch width at saturating doses. The dose-dependence curves of lusitropic and inotropic effects exhibit a reciprocal relationship, meaning that these two effects might share common mechanisms. To test a possible involvement of catecholamines, the effects of NCXIF were examined in the presence or absence of beta-adrenergic blocker, deralin. The saturating doses of deralin (1- 3 microM) do not alter either the NCXIF-induced acceleration of relaxation or twitch enhancement, meaning that the NCXIF effects cannot be mediated by occasional release of endogenous catecholamines. The capacity of NCXIF to modulate the ventricle contractility unconnectedly to the beta-adrenergic activation may provide new rational clues for future pharmacological interventions.

Contribution of Drosophila DEG/ENaC genes to salt taste

The ability to detect salt is critical for the survival of terrestrial animals. Based on amiloride-dependent inhibition, the receptors that detect salt have been postulated to be DEG/ENaC channels. We found the Drosophila DEG/ENaC genes Pickpocket11 (ppk11) and Pickpocket19 (ppk19) expressed in the larval taste-sensing terminal organ and in adults on the taste bristles of the labelum, the legs, and the wing margins. When we disrupted PPK11 or PPK19 function, larvae lost their ability to discriminate low concentrations of Na(+) or K(+) from water, and the electrophysiologic responses to low salt concentrations were attenuated. In both larvae and adults, disrupting PPK11 or PPK19 affected the behavioral response to high salt concentrations. In contrast, the response of larvae to sucrose, pH 3, and several odors remained intact. These results indicate that the DEG/ENaC channels PPK11 and PPK19 play a key role in detecting Na(+) and K(+) salts.

Drosophila DEG/ENaC pickpocket genes are expressed in the tracheal system, where they may be involved in liquid clearance

The Drosophila tracheal system and mammalian airways are branching networks of tubular epithelia that deliver oxygen to the organism. In mammals, the epithelial Na(+) channel (ENaC) helps clear liquid from airways at the time of birth and removes liquid from the airspaces in adults. We tested the hypothesis that related Drosophila degenerin (DEG)/ENaC family members might play a similar role in the fly. Among 16 Drosophila DEG/ENaC genes, called pickpocket (PPK) genes, we found 9 expressed in the tracheal system. By in situ hybridization, expression appeared in late-stage embryos after tracheal tube formation, with individual PPK genes showing distinct temporal and spatial expression patterns as development progressed. Promoters for several PPK genes drove reporter gene expression in the larval and adult tracheal systems. Adding the DEG/ENaC channel blocker amiloride to the medium inhibited liquid clearance from the trachea of first instar larvae. Moreover, when RNA interference was used to silence PPK4 and PPK11, larvae failed to clear tracheal liquid. These data suggest substantial molecular diversity of DEG/ENaC channel expression in the Drosophila tracheal system where the PPK proteins likely play a role in Na(+) absorption. Extensive similarities between Drosophila and mammalian airways offer opportunities for genetic studies that may decipher further the structure and function of DEG/ENaC proteins and development of the airways.

Low-voltage-activated calcium channels are not involved in capacitation and biological response to progesterone in human sperm

The involvement of voltage-dependent calcium channels in the biological effects exerted by progesterone (P) on human spermatozoa is still a controversial issue. We have investigated the involvement of T-type calcium channels [voltage-operated calcium channels (VOCCT)] in two biological functions of human sperm, responsiveness to P and capacitation, by employing three different pharmacological antagonists of VOCCT, namely mibefradil (Ro 5967), pimozide and amiloride. Intracellular calcium [Ca(2+)]i increase in response to P was essentially unaffected by pre-treatment with mibefradil and pimozide at concentrations previously shown to prevent [Ca(2+)]i increase in response to zona proteins. Amiloride could not be tested in these experiments because it was found to interfere with fura-2 fluorescence. The increase in tyrosine phosphorylation stimulated by P in a protein of about 97 kDa was unaffected by the three antagonists. Acrosome reaction (AR) induced by P was also unaffected by mibefradil or pimozide but was significantly inhibited by amiloride at high concentrations (100 and 500 but not 10 microM). At 100 and 500 microM amiloride also inhibited Na/H exchanger as assessed by a fluorimetric method. We conclude that VOCCT are not involved in calcium increase and AR stimulated by P in human sperm. We next investigated the effect of the three VOCCT inhibitors on sperm capacitation by evaluating tyrosine phosphorylation and AR in basal conditions and in response to P. We found that the presence of pimozide and amiloride during capacitation stimulated a higher increase of tyrosine phosphorylation, whereas mibefradil was less effective. The ability of P to induce the AR, considered an index of occurrence of capacitation, was not affected by pimozide and mibefradil, whereas was inhibited by amiloride at concentrations that inhibit Na/H exchanger. In conclusion, our results do not support a major role of low-voltage-activated calcium channels in capacitation and response to P of human spermatozoa.

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.

Amiloride and vertebrate gustatory responses to NaCl

Amiloride at < or = 1 microM may block epithelial Na+ channels without affecting other cellular mechanisms, and attenuates gustatory responses to lingual NaCl from the chorda tympani nerves (CT) of gerbil, hamster, rhesus monkey, and several strains of laboratory rat and mouse, and from glossopharyngeally innervated frog taste-receptor cells; at 5 microM to 50 microM, also from Wistar rat and mongrel dog CT. Affected units responded more to NaCl than to KCl. Suppression of CT responses to KCl, HCl, NH4Cl, or saccharides also occurred in some mammals, but amiloride did not elicit responses. Taste-dependent behaviors towards NaCl or KCl were altered. DBA and 129/J laboratory mice, and mudpuppy, were unaffected by amiloride. In humans, 10 microM amiloride both produced taste reports and reduced total intensity of NaCl and LiCl by 15-20%. NaCl and LiCl sourness, and KCl and QHCl bitterness declined, but saltiness generally did not change. Effects on sweetness were inconsistent. Amiloride-sensitive gustatory mechanisms were prominent in some mammals, were not necessary for responses to NaCl, and were of minor importance for human taste.

Interactions between benzylamiloride and fura-2: studies in vitro and in cardiac myocytes

Amiloride derivatives are commonly used inhibitors of Na+/H+- and Na+/Ca2+-exchange. Because they are fluorescent molecules the use of benzylamiloride (BZA), an inhibitor of Na+/Ca2+ exchange, in conjunction with Fura-2, a commonly used fluorescent Ca2+ indicator, might complicate interpretation of fluorescence data obtained. In vitro data show that BZA decreases the Fura-2 fluorescence at all useful wavelengths in a concentration-dependent manner. The Fura-2 ratio 340/380 (used to estimate intracellular Ca2+ ([Ca2+]in)) also decreased with increasing BZA concentrations. The Stern-Volmer relation suggests that this phenomenon is due to either static or dynamic quenching. Varying temperatures from 4 to 37 degreesC did not alter Stern-Volmer constants, consistent instead with fluorescence resonance energy transfer (FRET). The in situ relevance of these interactions was evaluated in adult rat cardiac myocytes which exhibit Na+/Ca2+ exchange reflected by rapid [Ca2+]in increase following Na+ removal. Pretreatment with BZA >/= 25 microM decreased the magnitude of Fura-2 changes induced by Na+ removal. Analysis of the individual Fura-2 useful wavelengths indicated that >/= 25 microM BZA altered the Fura-2 signal in a manner consistent with the quenching effects noted in vitro. Together, these data show that BZA interacts with Fura-2 in vitro and in situ and suggest caution when interpreting Fura-2 fluorescence data derived in conjunction with BZA.

Effect of amiloride on inotropic and toxic actions of ouabain in guinea-pig left atria

The effect of amiloride on the positive inotropic and toxic effects of ouabain in guinea-pig left atria has been studied. In atria driven at 1 Hz, amiloride (0.3 and 0.5 mM) decreased the EC50 but did not affect the maximal tension developed by ouabain. At 0.1 Hz, amiloride did not change either the EC50 or the maximal tension developed by ouabain. Ouabain toxicity (onset of arrhythmias) was not changed by amiloride at either frequency of stimulation. Therefore, amiloride did not antagonize either the positive inotropic or the toxic effect of ouabain. The positive inotropic effect of amiloride has been ascribed to the inhibition of the Na+/Ca2+ exchanger. Since amiloride inhibits also the Na+/H+ exchanger, 5-(N-ethyl-N-isopropyl)amiloride (EIPA), an amiloride derivative which selectively inhibits the Na+/H+ exchange, has been tested to evaluate the role of the Na+/H+ exchange in the amiloride-ouabain interaction. EIPA increased the EC50 values of ouabain and decreased the maximal developed tension by the glycoside in atria driven at 0.1 and 1 Hz, but did not antagonize the toxic response (arrhythmias) of atria to ouabain. It is suggested that the inhibition of Ca2+ exit through the Na+/Ca2+ exchange by amiloride and ouabain may explain the observation that the positive inotropic effects of amiloride and ouabain are additive.

Suppression of GABAB receptor function in rat neocortical slices by amiloride

Interactions of amiloride with GABAB receptors have been examined using spontaneously discharging rat neocortical slices. These discharges were suppressed by the GABAB receptor agonist baclofen (10 microM), and were prevented by amiloride and its analogs 5-(N,N-dimethyl)-amiloride, 5-(N-methyl-N-isobutyl)-amiloride and benzamil, but not by triamterene (100-500 microM). Each of these also increased the spontaneous discharge rate and reduced the discharge amplitude. The action of amiloride and its analogs in preventing the action of baclofen, may involve allosteric modification of the receptor binding sites via guanine nucleotide-binding proteins, or an indirect effect through antagonism of co-activated adenosine A1 receptors.