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

Cardiac hyporesponsiveness in severe sepsis is associated with nitric oxide-dependent activation of G protein receptor kinase

G protein-coupled receptor kinase isoform 2 (GRK2) has a critical role in physiological and pharmacological responses to endogenous and exogenous substances. Sepsis causes an important cardiovascular dysfunction in which nitric oxide (NO) has a relevant role. The present study aimed to assess the putative effect of inducible NO synthase (NOS2)-derived NO on the activity of GRK2 in the context of septic cardiac dysfunction. C57BL/6 mice were submitted to severe septic injury by cecal ligation and puncture (CLP). Heart function was assessed by isolated and perfused heart, echocardiography, and β-adrenergic receptor binding. GRK2 was determined by immunofluorescence and Western blot analysis in the heart and isolated cardiac myocytes. Sepsis increased NOS2 expression in the heart, increased plasma nitrite + nitrate levels, and reduced isoproterenol-induced isolated ventricle contraction, whole heart tension development, and β-adrenergic receptor density. Treatment with 1400W or with GRK2 inhibitor prevented CLP-induced cardiac hyporesponsiveness 12 and 24 h after CLP. Increased labeling of total and phosphorylated GRK2 was detected in hearts after CLP. With treatment of 1400W or in hearts taken from septic NOS2 knockout mice, the activation of GRK2 was reduced. 1400W or GRK2 inhibitor reduced mortality, improved echocardiographic cardiac parameters, and prevented organ damage. Therefore, during sepsis, NOS2-derived NO increases GRK2, which leads to a reduction in β-adrenergic receptor density, contributing to the heart dysfunction. Isolated cardiac myocyte data indicate that NO acts through the soluble guanylyl cyclase/cGMP/PKG pathway. GRK2 inhibition may be a potential therapeutic target in sepsis-induced cardiac dysfunction.

NEW & NOTEWORTHY The main novelty presented here is to show that septic shock induces cardiac hyporesponsiveness to isoproterenol by a mechanism dependent on nitric oxide and mediated by G protein-coupled receptor kinase isoform 2. Therefore, G protein-coupled receptor kinase isoform 2 inhibition may be a potential therapeutic target in sepsis-induced cardiac dysfunction.

Direct Loading of the purified endogenous inhibitor into the cytoplasm of patched cardiomyocytes blocks the ion currents and calcium transport through the NCX1 protein

The Na(+)-Ca(2+) exchanger in mammalian heart muscle (NCX1) is the central transporter protein that regulates extrusion of Ca(2+) from the heart cell. However, the functional biochemistry and physiology of NCX1 have been severely hampered by the absence of any specific high-affinity inhibitor. Here we describe advanced procedures for purifying a candidate inhibitor, previously called endogenous inhibitor factor (NCX(IF)), and demonstrate its direct actions on NCX1 activities in the single-cell system. A combination of advanced HILIC (hydrophilic interaction liquid chromatography) procedures with analytical tests suggests that the properties of NCX(IF) resemble those of a small (disaccharide size) polar molecule lacking any aromatic rings, conjugated bonds, or a primary amino group. The effects of NCX(IF) on the NCX1-mediated ion currents (I(NCX)) and cytosolic Ca(2+) extrusion were detected by a combination of patch-clamp and confocal microscopy under conditions in which the purified NCX(IF) was directly loaded into the cytoplasm of patched cardiomyocytes. It was demonstrated that cytosolic NCX(IF) blocks the Ca(2+)-activated NCX1 inward current and the accompanying extrusion of Ca(2+) from the cell with high efficacy. A constant fraction of NCX1 inhibition was observed under conditions in which the cytosolic [Ca(2+)](i) was varied at fixed doses of NCX(IF), suggesting that the degree of inhibition is controlled by NCX(IF) dose and not by cytosolic Ca(2+) concentration. NCX(IF) blocks equally well both the Ca(2+) extrusion and Ca(2+) entry modes of NCX1, consistent with thermodynamic principles expected for the functioning of a bidirectional "carrier-type" transport system. We concluded that NCX(IF) interacts with a putative regulatory domain from the cytosolic side and, thus, may play an important regulatory role in controlling Ca(2+) signaling in the heart. This may represent a new potential tool for developing novel treatments for cardiac Ca(2+) signaling dysfunction.

Effects of purified endogenous inhibitor of the Na+/Ca2+ exchanger on ouabain-induced arrhythmias in the atria and ventricle strips of guinea pig

Previous studies demonstrated that the purified endogenous inhibitor (NCX(IF)) of the cardiac Na(+)/Ca(2+) exchanger (NCX1) has the capacity to modulate cardiac muscle contractility. Here, we tested the effects of purified NCX(IF) on arrhythmias induced by ouabain in the atria and ventricle strips of guinea pig. For the sake of comparison NCX(IF) was compared to lidocaine and KB-R7943. In the ventricle strip, NCX(IF) ( approximately 10 U/ml) results in rapid, complete and stable inhibition of ouabain-induced arrhythmias (the inhibition of arrhythmia is not followed by revival of irregular contractions). Under similar experimental conditions the atria strips require somewhat higher doses of NCX(IF) (25-50 U/ml) for complete suppression of arrhythmia. In the atria strip, NCX(IF) (10-25 U/ml) increases the threshold dose (1 microM) of ouabain for arrhythmia onset 2.2+/-0.5-fold (n=5, p<0.05) as well as prolongs the lag-phase for arrhythmia appearance 4.0+/-0.5-fold (n=5, p<0.01). The lag period for arrhythmia onset was also lengthened (2.0+/-0.4-fold) by NCX(IF) in the ventricle strips (n=6, p<0.002). At low frequency of pacing (1 Hz), all three tested substances, lidocaine, KB-R7943, and NCX(IF) can effectively suppress the ouabain-induced arrhythmia. However, at higher frequency (2 Hz), lidocaine is ineffective in suppressing arrhythmia, whereas KB-R7943 becomes pro-arrhythmic. In contrast to reference drugs, NCX(IF) retains its anti-arrhythmic capacity at high frequencies, either in the atria (n=6, p<0.01) or ventricle (n=5, p<0.05) strips. In conclusion, NCX(IF) results in rapid, effective and stable suppression of arrhythmia both in the atria and ventricle preparations under conditions at which the reference drugs become ineffective.

Purified endogenous inhibitor of the Na/Ca exchanger can enhance the cardiomyocytes contractility and calcium transients

Previous studies have shown that the newly found endogenous inhibitor (NCX(IF)) of the cardiac Na/Ca exchanger (NCX1) is capable of regulating the muscle strip's contractility and relaxation. Here, the effects of purified NCX(IF) were tested on single cell shortening-lengthening (by using the IR CCD camera coupled with the two-edge video-detector) and [Ca]i-transients (by monitoring the changes in fluo-3 fluorescence). A perfusion of isolated cardiomyocytes (paced at 0.5-1.0 Hz) with NCX(IF) results in 4-6-fold enhancement in the amplitude of cell shortening-lengthening reaching the steady-state levels within 5-8 min (n=20, p<0.009). Simultaneous recordings of cell shortening-lengthening and [Ca]i-transients from the same cell show that the amplitude enhancement is associated with accelerated decay of both signals. Therefore, the NCX(IF)-dependent modulation of the single cell contractility is primarily governed by Ca-related mechanisms. The observed data are consistent with a proposal suggesting that the inhibition of NCX1 by NCX(IF) results in Ca-dependent activation of SERCA (SR Ca ATPase), yielding the accelerated decay of the [Ca]i-transients. The subsequent increase in the SR Ca content may result in enhanced Ca-release reflecting the manifested promotion of [Ca]i-transients. More systematic study is required for confirming this working hypothesis.

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).

The low molecular weight inhibitor of NCX1 interacts with a cytosolic domain that differs from the ion-transport site of the Na/Ca exchanger

The endogenous inhibitory factor (NCX(IF)) of the cardiac Na/Ca exchanger (NCX1) is a low molecular weight substance, which has a strong capacity to modulate the ventricle muscle contractility. Previously, we have shown that NCX(IF) can completely inhibit either the forward (Na(i)-dependent Ca-uptake) or reverse (Na(o)-dependent Ca-release) mode of Na/Ca exchange as well as its partial reaction, the Ca/Ca exchange. Although the preliminary studies have shown that NCX(IF) can rapidly (within few milliseconds) interact with a putative inhibitory site of the Na/Ca exchanger protein (or within its vicinity), it was not clear whether the NCX(IF) can directly interact with the ion transport sites of the exchanger protein or the interaction site of NCX(IF) is distinct from the ion-binding/transport site of NCX1. In order to segregate between these possibilities the NCX(IF) was tested for its capacity to compete with Ca at the cytosolic side by using the preparation of sarcolemma vesicles having predominantly the inside-out orientation. For this goal, the initial rates of Na(i)-dependent (45)Ca-uptake were measured in the presence of extravesicular (cytosolic) NCX(IF) under conditions in which the concentration of extravesicular Ca was varied (2-200 microM) and intravesicular Na was kept fixed at saturating concentration (160 mM). Under these conditions the NCX(IF) results in several fold decrease in V(max) values, while having no significant effect on the K(m). Taking into account the molecular weight of 350-550 Da (derived from the gel-filtration and mass-spectra data), the experimentally measured inhibitory potency of NCX(IF) can be estimated as the IC(50) = 0.3-0.6 microM. Therefore, it is concluded that the NCX(IF) is reasonably potent blocker, which interacts with cytosolic domain thereby preventing the ion-translocation (and not ion-binding) events.