Search for exotic decays of the Higgs boson to a pair of pseudoscalars in the μμbb and ττbb final states

A search for exotic decays of the Higgs boson (H ) with a mass of 125Ge V to a pair of light pseudoscalars a 1 is performed in final states where one pseudoscalar decays to two b quarks and the other to a pair of muons or τ leptons. A data sample of proton-proton collisions at s = 13 Te V corresponding to an integrated luminosity of 138fb - 1 recorded with the CMS detector is analyzed. No statistically significant excess is observed over the standard model backgrounds. Upper limits are set at 95% confidence level (CL ) on the Higgs boson branching fraction to μ μ b b and to τ τ b b , via a pair of a 1 s. The limits depend on the pseudoscalar mass m a 1 and are observed to be in the range (0.17-3.3) × 10 - 4 and (1.7-7.7) × 10 - 2 in the μ μ b b and τ τ b b final states, respectively. In the framework of models with two Higgs doublets and a complex scalar singlet (2HDM+S), the results of the two final states are combined to determine upper limits on the branching fraction B ( H → a 1 a 1 → ℓ ℓ b b ) at 95% CL , with ℓ being a muon or a τ lepton. For different types of 2HDM+S, upper bounds on the branching fraction B ( H → a 1 a 1 ) are extracted from the combination of the two channels. In most of the Type II 2HDM+S parameter space, B ( H → a 1 a 1 ) values above 0.23 are excluded at 95% CL for m a 1 values between 15 and 60Ge V .

Observation of WWγ Production and Search for Hγ Production in Proton-Proton Collisions at sqrt[s]=13 TeV

The observation of WWγ production in proton-proton collisions at a center-of-mass energy of 13 TeV with an integrated luminosity of 138  fb^{-1} is presented. The observed (expected) significance is 5.6 (5.1) standard deviations. Events are selected by requiring exactly two leptons (one electron and one muon) of opposite charge, moderate missing transverse momentum, and a photon. The measured fiducial cross section for WWγ is 5.9±0.8(stat)±0.8(syst)±0.7(modeling)  fb, in agreement with the next-to-leading order quantum chromodynamics prediction. The analysis is extended with a search for the associated production of the Higgs boson and a photon, which is generated by a coupling of the Higgs boson to light quarks. The result is used to constrain the Higgs boson couplings to light quarks.

New Structures in the J/ψJ/ψ Mass Spectrum in Proton-Proton Collisions at sqrt[s]=13 TeV

A search is reported for near-threshold structures in the J/ψJ/ψ invariant mass spectrum produced in proton-proton collisions at sqrt[s]=13  TeV from data collected by the CMS experiment, corresponding to an integrated luminosity of 135  fb^{-1}. Three structures are found, and a model with quantum interference among these structures provides a good description of the data. A new structure is observed with a local significance above 5 standard deviations at a mass of 6638_{-38}^{+43}(stat)_{-31}^{+16}(syst)  MeV. Another structure with even higher significance is found at a mass of 6847_{-28}^{+44}(stat)_{-20}^{+48}(syst)  MeV, which is consistent with the X(6900) resonance reported by the LHCb experiment and confirmed by the ATLAS experiment. Evidence for another new structure, with a local significance of 4.7 standard deviations, is found at a mass of 7134_{-25}^{+48}(stat)_{-15}^{+41}(syst)  MeV. Results are also reported for a model without interference, which does not fit the data as well and shows mass shifts up to 150 MeV relative to the model with interference.

Search for Scalar Leptoquarks Produced via τ-Lepton-Quark Scattering in pp Collisions at sqrt[s]=13 TeV

The first search for scalar leptoquarks produced in τ-lepton-quark collisions is presented. It is based on a set of proton-proton collision data recorded with the CMS detector at the LHC at a center-of-mass energy of 13 TeV corresponding to an integrated luminosity of 138  fb^{-1}. The reconstructed final state consists of a jet, significant missing transverse momentum, and a τ lepton reconstructed through its hadronic or leptonic decays. Limits are set on the product of the leptoquark production cross section and branching fraction and interpreted as exclusions in the plane of the leptoquark mass and the leptoquark-τ-quark coupling strength.

Search for Inelastic Dark Matter in Events with Two Displaced Muons and Missing Transverse Momentum in Proton-Proton Collisions at sqrt[s]=13 TeV

A search for dark matter in events with a displaced nonresonant muon pair and missing transverse momentum is presented. The analysis is performed using an integrated luminosity of 138  fb^{-1} of proton-proton (pp) collision data at a center-of-mass energy of 13 TeV produced by the LHC in 2016-2018. No significant excess over the predicted backgrounds is observed. Upper limits are set on the product of the inelastic dark matter production cross section σ(pp→A^{'}→χ_{1}χ_{2}) and the decay branching fraction B(χ_{2}→χ_{1}μ^{+}μ^{-}), where A^{'} is a dark photon and χ_{1} and χ_{2} are states in the dark sector with near mass degeneracy. This is the first dedicated collider search for inelastic dark matter.

Luminosity determination using Z boson production at the CMS experiment

The measurement of Z boson production is presented as a method to determine the integrated luminosity of CMS data sets. The analysis uses proton-proton collision data, recorded by the CMS experiment at the CERN LHC in 2017 at a center-of-mass energy of 13Te V . Events with Z bosons decaying into a pair of muons are selected. The total number of Z bosons produced in a fiducial volume is determined, together with the identification efficiencies and correlations from the same data set, in small intervals of 20pb - 1 of integrated luminosity, thus facilitating the efficiency and rate measurement as a function of time and instantaneous luminosity. Using the ratio of the efficiency-corrected numbers of Z bosons, the precisely measured integrated luminosity of one data set is used to determine the luminosity of another. For the first time, a full quantitative uncertainty analysis of the use of Z  bosons for the integrated luminosity measurement is performed. The uncertainty in the extrapolation between two data sets, recorded in 2017 at low and high instantaneous luminosity, is less than 0.5%. We show that the Z boson rate measurement constitutes a precise method, complementary to traditional methods, with the potential to improve the measurement of the integrated luminosity.

Observation of the Rare Decay of the η Meson to Four Muons

A search for the rare η→μ^{+}μ^{-}μ^{+}μ^{-} double-Dalitz decay is performed using a sample of proton-proton collisions, collected by the CMS experiment at the CERN LHC with high-rate muon triggers during 2017 and 2018 and corresponding to an integrated luminosity of 101  fb^{-1}. A signal having a statistical significance well in excess of 5 standard deviations is observed. Using the η→μ^{+}μ^{-} decay as normalization, the branching fraction B(η→μ^{+}μ^{-}μ^{+}μ^{-})=[5.0±0.8(stat)±0.7(syst)±0.7(B_{2μ})]×10^{-9} is measured, where the last term is the uncertainty in the normalization channel branching fraction. This work achieves an improved precision of over 5 orders of magnitude compared to previous results, leading to the first measurement of this branching fraction, which is found to agree with theoretical predictions.

Search for Higgs Boson Decay to a Charm Quark-Antiquark Pair in Proton-Proton Collisions at sqrt[s]=13 TeV

A search for the standard model Higgs boson decaying to a charm quark-antiquark pair, H→cc[over ¯], produced in association with a leptonically decaying V (W or Z) boson is presented. The search is performed with proton-proton collisions at sqrt[s]=13  TeV collected by the CMS experiment, corresponding to an integrated luminosity of 138  fb^{-1}. Novel charm jet identification and analysis methods using machine learning techniques are employed. The analysis is validated by searching for Z→cc[over ¯] in VZ events, leading to its first observation at a hadron collider with a significance of 5.7 standard deviations. The observed (expected) upper limit on σ(VH)B(H→cc[over ¯]) is 0.94 (0.50_{-0.15}^{+0.22})pb at 95% confidence level (C.L.), corresponding to 14 (7.6_{-2.3}^{+3.4}) times the standard model prediction. For the Higgs-charm Yukawa coupling modifier, κ_{c}, the observed (expected) 95% C.L. interval is 1.1<|κ_{c}|<5.5 (|κ_{c}|<3.4), the most stringent constraint to date.

A portrait of the Higgs boson by the CMS experiment ten years after the discovery

In July 2012, the ATLAS and CMS collaborations at the CERN Large Hadron Collider announced the observation of a Higgs boson at a mass of around 125 gigaelectronvolts. Ten years later, and with the data corresponding to the production of a 30-times larger number of Higgs bosons, we have learnt much more about the properties of the Higgs boson. The CMS experiment has observed the Higgs boson in numerous fermionic and bosonic decay channels, established its spin-parity quantum numbers, determined its mass and measured its production cross-sections in various modes. Here the CMS Collaboration reports the most up-to-date combination of results on the properties of the Higgs boson, including the most stringent limit on the cross-section for the production of a pair of Higgs bosons, on the basis of data from proton-proton collisions at a centre-of-mass energy of 13 teraelectronvolts. Within the uncertainties, all these observations are compatible with the predictions of the standard model of elementary particle physics. Much evidence points to the fact that the standard model is a low-energy approximation of a more comprehensive theory. Several of the standard model issues originate in the sector of Higgs boson physics. An order of magnitude larger number of Higgs bosons, expected to be examined over the next 15 years, will help deepen our understanding of this crucial sector.

Gut microbiota depletion exacerbates cholestatic liver injury via loss of FXR signalling

Primary sclerosing cholangitis (PSC) is a chronic cholestatic liver disease of unknown aetiology for which there are no approved therapeutic options. Patients with PSC display changes in gut microbiota and in bile acid (BA) composition; however, the contribution of these alterations to disease pathogenesis remains controversial. Here we identify a role for microbiota-dependent changes in BA synthesis that modulates PSC pathophysiology. In a genetic mouse model of PSC, we show that loss of microbiota-mediated negative feedback control of BA synthesis results in increased hepatic BA concentrations, disruption of bile duct barrier function and, consequently, fatal liver injury. We further show that these changes are dependent on decreased BA signalling to the farnesoid X receptor, which modulates the activity of the rate-limiting enzyme in BA synthesis, CYP7A1. Moreover, patients with advanced stages of PSC show suppressed BA synthesis as measured by serum C4 levels, which is associated with poor disease prognosis. Our preclinical data highlight the microbiota-dependent dynamics of BA metabolism in cholestatic liver disease, which could be important for future therapies targeting BA and gut microbiome interactions, and identify C4 as a potential biomarker to functionally stratify patients with PSC and predict disease outcomes.

Influence of Liver Fibrosis on Lobular Zonation

Little is known about how liver fibrosis influences lobular zonation. To address this question, we used three mouse models of liver fibrosis, repeated CCl4 administration for 2, 6 and 12 months to induce pericentral damage, as well as bile duct ligation (21 days) and mdr2-/- mice to study periportal fibrosis. Analyses were performed by RNA-sequencing, immunostaining of zonated proteins and image analysis. RNA-sequencing demonstrated a significant enrichment of pericentral genes among genes downregulated by CCl4; vice versa, periportal genes were enriched among the upregulated genes. Immunostaining showed an almost complete loss of pericentral proteins, such as cytochrome P450 enzymes and glutamine synthetase, while periportal proteins, such as arginase 1 and CPS1 became expressed also in pericentral hepatocytes. This pattern of fibrosis-associated 'periportalization' was consistently observed in all three mouse models and led to complete resistance to hepatotoxic doses of acetaminophen (200 mg/kg). Characterization of the expression response identified the inflammatory pathways TGFβ, NFκB, TNFα, and transcription factors NFKb1, Stat1, Hif1a, Trp53, and Atf1 among those activated, while estrogen-associated pathways, Hnf4a and Hnf1a, were decreased. In conclusion, liver fibrosis leads to strong alterations of lobular zonation, where the pericentral region adopts periportal features. Beside adverse consequences, periportalization supports adaptation to repeated doses of hepatotoxic compounds.

Bile Microinfarcts in Cholestasis Are Initiated by Rupture of the Apical Hepatocyte Membrane and Cause Shunting of Bile to Sinusoidal Blood

Bile duct ligation (BDL) is an experimental procedure that mimics obstructive cholestatic disease. One of the early consequences of BDL in rodents is the appearance of so-called bile infarcts that correspond to Charcot-Gombault necrosis in human cholestasis. The mechanisms causing bile infarcts and their pathophysiological relevance are unclear. Therefore, intravital two photon-based imaging of BDL mice was performed with fluorescent bile salts (BS) and non-BS organic anion analogues. Key findings were followed up by matrix-assisted laser desorption ionization imaging, clinical chemistry, immunostaining, and gene expression analyses. In the acute phase, 1-3 days after BDL, BS concentrations in bile increased and single-cell bile microinfarcts occurred in dispersed hepatocytes throughout the liver caused by the rupture of the apical hepatocyte membrane. This rupture occurred after loss of mitochondrial membrane potential, followed by entry of bile, cell death, and a "domino effect" of further death events of neighboring hepatocytes. Bile infarcts provided a trans-epithelial shunt between bile canaliculi and sinusoids by which bile constituents leaked into blood. In the chronic phase, ≥21 days after BDL, uptake of BS tracers at the sinusoidal hepatocyte membrane was reduced. This contributes to elevated concentrations of BS in blood and decreased concentrations in the biliary tract. Conclusion: Bile microinfarcts occur in the acute phase after BDL in a limited number of dispersed hepatocytes followed by larger infarcts involving neighboring hepatocytes, and they allow leakage of bile from the BS-overloaded biliary tract into blood, thereby protecting the liver from BS toxicity; in the chronic phase after BDL, reduced sinusoidal BS uptake is a dominant protective factor, and the kidney contributes to the elimination of BS until cholemic nephropathy sets in.

Interstitial pressure and lung oedema in chronic hypoxia

We evaluated how the increase in lung interstitial pressure correlates with the pulmonary vascular response to chronic hypoxia. In control and hypoxic (30 days; 10% O₂) Wistar male rats, we measured: pulmonary interstitial pressure (P(ip)), cardiac and haemodynamic parameters by echocardiography, and performed lung morphometry on tissue specimens fixed in situ. In control animals, mean ± sd P(ip), air/tissue volume ratio and capillary vascularity index in the air-blood barrier were -12 ± 2.03 cmH₂O, 3.9 and 0.43, respectively. After hypoxia exposure, the corresponding values of these indices in apparently normal lung regions were 2.6 ± 1.7 cmH₂O, 3.6, and 0.5, respectively. In oedematous regions, the corresponding values were 12 ± 4 cmH₂O, 0.4 and 0.3, respectively. Furthermore, in normal regions, the density of pre-capillary vessels (diameter ~50-200 μm) increased and their thickness/internal diameter ratio decreased, while opposite results were found in oedematous regions. Pulmonary artery pressure increased in chronic hypoxia relative to the control (39.8 ± 5.9 versus 26.2 ± 2.2 mmHg). Heterogeneity in local lung vascular response contributes to developing pulmonary hypertension in chronic hypoxia. In oedematous regions, the decrease in capillary vascularity correlated with the remarkable increase in interstitial pressure and morphometry of the pre-capillary vessels suggested an increase in vascular resistance; the opposite was true in apparently normal regions.

Rate dependency of beta-adrenergic modulation of repolarizing currents in the guinea-pig ventricle

Beta-adrenergic stimulation modulates ventricular currents and sinus cycle length (CL). We investigated how changes in CL affect the current induced by isoprenaline (Iso) during the action potential (AP) of guinea-pig ventricular myocytes. Action-potential clamp was applied at CLs of 250 and 1000 ms to measure: (1) the net current induced by 0.1 microm Iso (I(Iso)); (2) the L-type Ca2+ current I(CaL) and slow delayed rectifier current I(Ks) components of I(Iso) (I(IsoCa) and I(IsoK)), identified as the Iso-induced current sensitive to nifedipine and HMR1556, respectively; and (3) I(Iso) persisting after inhibition of both I(Ca) and I(Ks) (I(isoR)). The pause dependency of I(Ks) and its modulation were evaluated in voltage-clamp experiments. The rate dependency of the duration of the action potential at 90% repolarization (APD90) and its modulation by isoprenaline were tested in current-clamp experiments. At a CL of 250 ms I(Iso) was inward during initial repolarization and reversed at 59% of APD90. At a CL of 1000 ms I(Iso) became mostly inward in all cells. Switching to shorter CL did not change I(IsoCa) and I(IsoK) amplitudes, but moved their peak amplitudes to earlier repolarization; I(IsoR) was independent of CL. Acceleration of I(IsoK) at shorter CL was based on faster pause dependency of I(Ks) activation rate. The 'restitution' of activation rates was modulated by isoprenaline. The APD90-CL relation was rotated anticlockwise by isoprenaline and crossed the control curve at a CL of 150 ms (400 beats min(-1)). We conclude that: (1) isoprenaline induced markedly different current profiles according to pacing rate, involving CL-dependent I(Ca) and I(Ks) modulation; (2) the effect of isoprenaline on APD90 was CL dependent, and negligible during tachycardia; and (3) during sympathetic activation, repolarization stability may involve matched modulation of sinus rate and repolarizing currents.

Diverse toxicity associated with cardiac Na+/K+ pump inhibition: evaluation of electrophysiological mechanisms

(E,Z)-3-((2-Aminoethoxy)imino)androstane-6,17-dione hydrochloride (PST2744) is a novel Na(+)/K(+) pump inhibitor with positive inotropic effects. Compared with digoxin in various experimental models, PST2744 was consistently found to be less arrhythmogenic, thus resulting in a significantly higher therapeutic index. The present work compares the electrophysiological effects of PST2744 and digoxin in guinea pig ventricular myocytes, with the aim to identify a mechanism for their different toxicity. The work showed that 1) the action potential was transiently prolonged and then similarly shortened by both agents; 2) the ratio between Na(+)/K(+) pump inhibition and inotropy was somewhat larger for PST2744 than for digoxin; 3) both agents accelerated inactivation of high-threshold Ca(2+) current (I(CaL)), without affecting its peak amplitude; 4) the transient inward current (I(TI)) induced by a Ca(2+) transient in the presence of complete Na(+)/K(+) pump blockade was inhibited (-43%) by PST2744 but not by digoxin; 5) the conductance of Na(+)/Ca(2+) exchanger current (I(NaCa)), recorded under Na(+)/K(+) pump blockade, was only slightly inhibited by PST2744 (-14%) and unaffected by digoxin; and 6) both agents inhibited delayed rectifier current I(Ks) (<or=-21%); delayed rectifier current I(Kr) was inhibited by PST2744 only, but the effect was marginal (-6%). Thus, 1) the higher therapeutic index of PST2744 may be accounted for by inhibition of I(TI), a current directly involved in digitalis-induced arrhythmias. Indeed, the other differences observed concern quantitatively small effects; and 2) I(TI) suppression by PST2744 may be only partly accounted for by inhibition of the Na(+)/Ca(2+) exchanger.

Characterization of the non-linear rate-dependency of QT interval in humans

AIMS

Repolarization has rate-dependent and rate-independent components. A function considering such components separately was validated in canine Purkinje fibres and applied to the QT/RR relation in humans.

METHODS AND RESULTS

Action potential duration (APD) was measured in Purkinje fibres during steady-state pacing at different cycle lengths (CL) and after prolonged quiescence (APD(0)). The APD/CL relationship was expressed by this function: APD=APD(max)(*)CL(S)/(CL(50)(S)+CL(S)), where APD(max) (APD extrapolated at infinite CL) is a rate-independent measure of repolarization, CL(50) (CL at which 50% of APD(max) is achieved) and S evaluates the rate dependency of APD. The same function was used to fit the QT/RR relation in 46 normal subjects (20 males, 26 females) and in 7 amiodarone-treated subjects undergoing a bicycle stress test. RR and QT (V(5)) were measured at the end of each load step; QT(c) (Bazett's formula) was obtained at rest. The APD/CL and QT/RR relations were equally well expressed by the function with high correlation coefficients (R>or=0.90). In Purkinje fibres, APD(max) was 461+/-37 ms, CL(50) was 394+/-54 ms and S was 0.98+/-0.11. APD(max) and APD(0) correlated (R=0.96) and were similar. The corresponding values in humans were: QT(max) 432+/-63 ms, RR(50) 345+/-60 ms and S 2.6+/-0.8. While QT(c) and QT(max) were longer in females, RR(50) and S were similar between genders. Amiodarone increased QT(c), QT(max) and RR(50) and decreased S. In QT(max) and QT(c) distributions generated by pooling data from treated and untreated subjects, 86% of treated subjects were correctly identified by QT(max) and 28% by QT(c).

CONCLUSIONS

Canine and human repolarization showed a saturating dependency on cycle length, described by the proposed function. Gender and amiodarone independently affected QT(max), RR(50) and S: therefore they might reflect specific ionic mechanisms. Finally, QT(max) identified drug-induced repolarization abnormalities in individual subjects better than QT(c).

Pharmacological profile of the novel inotropic agent (E,Z)-3-((2-aminoethoxy)imino)androstane-6,17-dione hydrochloride (PST2744)

The novel Na(+)/K(+)-ATPase inhibitor (E,Z)-3-((2-aminoethoxy)imino)androstane-6,17-dione hydrochloride (PST2744) was characterized for its inotropic and toxic properties. Inhibition potency on dog kidney Na(+)/K(+)-ATPase was comparable (0.43 microM) to that of digoxin (0.45 microM). PST2744 concentration-dependently increased force of contraction in guinea pig atria and twitch amplitude in isolated guinea pig myocytes; in the latter, aftercontractions developed significantly less than with digoxin. Intravenous infusion of 0.2 mg/kg/min PST2744 in anesthetized guinea pigs exerted an immediate and long-lasting inotropic effect (ED(80) of 1.89 +/- 0.37 mg/kg) without causing lethal arrhythmias up to a cumulative dose of 18 mg/kg. Conversely, an equieffective infusion of digoxin (0.016 mg/kg/min; ED(80) of 0.32 mg/kg) caused lethal arrhythmias at a cumulative dose of 0.81 mg/kg. At a higher rate (0.4 mg/kg/min), PST2744 induced lethal arrhythmias, with a lethal dose/ED(80) ratio significantly greater than digoxin (20.2 +/- 6.3 versus 3.23 +/- 0.55, p < 0.05). Decay of the inotropic effect (t(1/2), min) was significantly faster for PST2744 (6.0 +/- 0.39) than for digoxin (18.3 +/- 4.5, p < 0.05). In anesthetized dogs, PST2744 dose-dependently increased maximum velocity of pressure rise (+dP/dt(max)) in the range 32 to 500 microg/kg i.v. and was safer than digoxin. In conscious dogs with a healed myocardial infarction, PST2744 significantly increased resting values of +dP/dt(max), left ventricular pressure, and SPB, and increased +dP/dt(max) throughout treadmill exercise while reverting the increase in left ventricular end diastolic pressure seen in control animals. Digoxin significantly decreased basal heart rate, while not affecting the hemodynamic response to exercise. Thus, PST2744 represents a new class of Na(+)/K(+)-ATPase inhibitors endowed with inotropic activity comparable with that of digitalis but having greater safety.

Rate dependency of delayed rectifier currents during the guinea-pig ventricular action potential

1. The action potential clamp technique was exploited to evaluate the rate dependency of delayed rectifier currents (I(Kr) and I(Ks)) during physiological electrical activity. I(Kr) and I(Ks) were measured in guinea-pig ventricular myocytes at pacing cycle lengths (CL) of 1000 and 250 ms. 2. A shorter CL, with the attendant changes in action potential shape, was associated with earlier activation and increased magnitude of both I(Kr) and I(Ks). Nonetheless, the relative contributions of I(Kr) and I(Ks) to total transmembrane current were independent of CL. 3. Shortening of diastolic interval only (constant action potential shape) enhanced I(Ks), but not I(Kr). 4. I(Kr) was increased by a change in the action potential shape only (constant diastolic interval). 5. In ramp clamp experiments, I(Kr) amplitude was directly proportional to repolarization rate at values within the low physiological range (< 1.0 V s(-1)); at higher repolarization rates proportionality became shallower and finally reversed. 6. When action potential duration (APD) was modulated by constant current injection (I-clamp), repolarization rates > 1.0 V s(-1) were associated with a reduced effect of I(Kr) block on APD. The effect of changes in repolarization rate was independent of CL and occurred in the presence of I(Ks) blockade. 7. In spite of its complexity, the behaviour of I(Kr) was accurately predicted by a numerical model based entirely on known kinetic properties of the current. 8. Both I(Kr) and I(Ks) may be increased at fast heart rates, but this may occur through completely different mechanisms. The mechanisms identified are such as to contribute to abnormal rate dependency of repolarization in prolonged repolarization syndromes.

Autonomic indexes based on the analysis of heart rate variability: a view from the sinus node

OBJECTIVE

Clinical indexes of autonomic activity are based on the analysis of sinus cycle length and of its variability. A common assumption underlying this practice is that neural activity and cycle length may be linearly related. Recent experimental evidence suggests that such an assumption may not be correct; indeed, the relation linking autonomic agonist concentration to cycle length was found to be strongly non-linear in single sinoatrial myocytes. In the present work, we (i) test compatibility of non-linearity of neural modulation of cycle length (CL) with previous experimental and clinical observations; (ii) evaluate its implications for what concerns the interpretation of time- and frequency-domain parameters of heart rate variability (HRV) and baroreflex sensitivity (BRS).

CONCLUSION

Non-linearity of neural modulation of CL may result in an intrinsic rate-dependency of autonomic indexes, with the exception of normalised frequency-domain indexes (e.g. the low frequency/high frequency (LF/HF) ratio), which appear to be devoid of intrinsic rate-dependency. This characteristic may not limit the value of HRV indexes and BRS in risk stratification, but has to be taken into account in their pathophysiological interpretation.

Role of the input/output relation of sinoatrial myocytes in cholinergic modulation of heart rate variability

INTRODUCTION

Modulation of sinus rate may be viewed as the transduction of an input signal (receptor stimulation) into an output signal (cycle length [CL]) by the sinus node. This study analyzes the input/output (I/O) relation of sinoatrial pacemaking elements and tests its impact on cholinergic modulation of heart rate variability.

METHODS AND RESULTS

Spontaneous activity of isolated rabbit sinoatrial myocytes was recorded by patch clamp techniques. CL and all the action potential parameters determining CL were automatically measured from >100 consecutive cycles. Acetylcholine (ACh, 5 to 50 nM) increased CL by decreasing diastolic depolarization rate (DDR) only. This was associated with a substantial increase in the coefficient of variation of CL and minor changes in the coefficient of variation of other parameters. A simple function relating CL to action potential parameters accurately described CL response to ACh (I/O relation). Numerical simulations based on this I/O relation showed that ACh-induced depression of DDR might, by its own, increase CL variability.

CONCLUSION

Time-domain measurements of CL variability may not necessarily reflect variability of the neural input to the sinoatrial node, but also may be affected by its tonic level. Properties of the I/O relation of sinoatrial myocytes may fully account for the dependency of CL variability on mean heart rate, previously described in humans. Any condition depressing DDR may enhance CL variability, independent of changes in the pattern of neural activity.

Lidocaine inhibition of the hyperpolarization-activated current (I(f)) in sinoatrial myocytes

The aim of this study was to provide information on the dose dependence and biophysical details of lidocaine blockade of the hyperpolarization-activated current (I(f)) in the sinoatrial node. Isolated rabbit sinoatrial myocytes were patch-clamped in the whole-cell configuration at 36+/-0.5 degrees C, in the presence of 1 mM Ba2+ and 2 mM Mn2+ to minimize contamination by K+ and Ca2+ currents, respectively. Lidocaine inhibited I(f) dose-dependently with a maximal inhibition of 69.5% at 75 microM and a half-maximal effect at 38.2 microM. Lidocaine reduced the conductance of fully activated I(f), without affecting the current reversal potential; the blocking effect was independent of membrane potential. Voltage dependence of I(f) activation gating was not affected by lidocaine, whose effect was independent of use and rate. Lidocaine did not modify the time course of I(f) activation. At therapeutic concentrations, lidocaine significantly inhibited I(f) by reducing fully activated channel conductance. Lack of voltage and rate dependence of effect differentiates lidocaine from most of other blockers of this current.

A toxin to nervous, cardiac, and endocrine ERG K+ channels isolated from Centruroides noxius scorpion venom

Toxins isolated from a variety of venoms are tools for probing the physiological function and structure of ion channels. The ether-a-go-go-related genes (erg) codify for the K+ channels (ERG), which are crucial in neurons and are impaired in human long-QT syndrome and Drosophila 'seizure' mutants. We have isolated a peptide from the scorpion Centruroides noxius Hoffmann that has no sequence homologies with other toxins, and demonstrate that it specifically inhibits (IC50=16+/-1 nM) only ERG channels of different species and distinct histogenesis. These results open up the possibility of investigating ERG channel structure-function relationships and novel pharmacological tools with potential therapeutic efficacy.

Sulfonylureas blockade of neural and cardiac HERG channels

The human ether-a-go-go-related gene (herg) encodes a K+ current (I(HERG)) which plays a fundamental role in heart excitability and in neurons by contributing to action potential repolarization and to spike-frequency adaptation, respectively. In this paper we show that I(HERG), recorded in neuroblastoma cells and guinea-pig ventricular myocytes, was reversibly inhibited by the K(ATP) channel blocker glibenclamide (IC50 = 74 microM). The voltage and use dependence of glibenclamide blockade were also evaluated. Another sulfonylurea, glimepiride, had less effective results in blocking I(HERG). The findings of this study are relevant to the interpretation of glibenclamide effects on cellular electrophysiology and suggest that oral antidiabetic therapy with sulfonylureas may contribute to iatrogenic QT prolongation and related arrhythmias.

Selective cardiodepressant activity of fluodipine, a fluorenone-1,4-dihydropyridine derivative

The effect of the dihydropyridine derivative, 1,4-dihydro-2,6-dimethyl-4-(fluorenon-4-yl)pyridine-3,5-dicarboxyl ic acid diallyl ester (fluodipine) was studied in vitro in different rabbit, rat and guinea pig preparations and in vivo in the rabbit in order to characterize its pharmacological profile at cardiac and at vascular sites. Compared to nifedipine, fluodipine showed a similar cardiodepressant activity, and a much lower inhibitory activity on vascular contraction. The highest tissue selectivity was observed in guinea pig preparations: fluodipine was about 2-3 times more effective than nifedipine on chronotropism and inotropism in isolated atria, and about 150 times less effective on aortic strip contraction. Accordingly, fluodipine (i) showed high-affinity binding to guinea pig ventricular L-type cardiac Ca2+ channels (Ki=2.57 nM), (ii) was about 80 times less effective than nifedipine to inhibit Ca2+ influx in vascular smooth muscle cells and (iii) induced a significant reduction of heart rate in the anesthetized rabbit (ID25=8.5 mg kg(-1), i.v.) without affecting the blood pressure up to 20 mg kg(-1), whereas nifedipine showed a significant hypotensive effect at very low doses (ID25=0.18 mg kg(-1), i.v.). The pacemaker current If of rabbit sino-atrial node myocytes was not affected by fluodipine. These findings demonstrate that fluodipine exerts selective cardiodepressant activity, likely due to a higher affinity for cardiac than for vascular Ca2+ channels.

Dynamic Ca2+-induced inward rectification of K+ current during the ventricular action potential

Inward rectification, an important determinant of cell excitability, can result from channel blockade by intracellular cations, including Ca2+. However, mostly on the basis of indirect arguments, Ca2+-mediated rectification of inward rectifier K+ current (IK1) is claimed to play no role in the mammalian heart. The present study investigates Ca2+-mediated IK1 rectification during the mammalian ventricular action potential. Guinea pig ventricular myocytes were patch-clamped in the whole-cell configuration. The action potential waveform was recorded and then applied to reproduce normal excitation under voltage-clamp conditions. Subtraction currents obtained during blockade of K+ currents by either 1 mmol/L Ba2+ (IBa) or K+-free solution (I0K) were used to estimate IK1. Similar time courses were observed for IBa and I0K; both currents were strongly reduced during depolarization (inward rectification). Blockade of L-type Ca2+ current by dihydropyridines (DHPs) increased systolic IBa and I0K by 50.7% and 254.5%, respectively. beta-Adrenergic stimulation, when tested on I0K, had an opposite effect; ie, it reduced this current by 66.5%. Ryanodine, an inhibitor of sarcoplasmic Ca2+ release, increased systolic IBa by 47.7%, with effects similar to those of DHPs. Intracellular Ca2+ buffering (BAPTA-AM) increased systolic IBa by 87.7% and blunted the effect of DHPs. Thus, IK1 may be significantly reduced by physiological Ca2+ transients determined by both Ca2+ influx and release. Although Ca2+-induced effects may represent only a small fraction of total IK1 rectification, they are large enough to affect excitability and repolarization. They may also contribute to facilitation of early afterdepolarizations by conditions increasing Ca2+ influx.

Ionic currents during sustained pacemaker activity in rabbit sino-atrial myocytes

1. The contribution of various ionic currents to diastolic depolarization (DD) in rabbit sinoatrial myocytes was evaluated by the action potential clamp technique. Individual currents were identified, during sustained pacemaking activity reproduced under voltage clamp conditions, according to their sensitivity to specific channel blockers. 2. The current sensitive to dihydropyridines (DHPs), blockers of L-type Ca2+ current (ICa,L), was small and outward during most of DD. Diastolic DHP-sensitive current was affected by changes in the driving force for K+, but it was insensitive to E-4031, which blocks the current termed IK,r; it was abolished by cell dialysis with a Ca2+ chelator. 3. The current sensitive to 2 mM Cs+ (ICs), a blocker of hyperpolarization-activated current (I(f)), was inward during the whole DD and it was substantially larger than the net inward current flowing during this phase. However, diastolic IK,r, identified in the same cells as the current sensitive to the blocker E-4031, exceeded ICs 2-fold. 4. These findings suggest that: (a) Ca2+ influx during the pacemaker cycle increases a K+ conductance, thus inverting the direction of the net current generated by L-type Ca2+ channel activity during DD; (b) the magnitude of I(f) would be adequate to account fully for DD; however, the coexistence of a larger IK,r suggests that other channels besides I(f) contribute inward current during this phase.

Muscarinic effects on action potential duration and its rate dependence in canine Purkinje fibers

Studies of the autonomic influence on action potential duration (APD) in the ventricles show direct effects of muscarinic stimulation on epicardial, but not endocardial, APD and conflicting results regarding direct vagal effects on the conduction system. In canine Purkinje fibers, we analyzed the action of the M2 agonist oxotremorine (OXO, 0.1 microM) on APD and on its cycle length (CL) dependence. Fibers were impaled with glass microelectrodes and superfused with Tyrode's solution. APD90 was measured after 3 minutes of drive at CL between 0.3 and 5 seconds. The best fit for the APD/CL relationship at steady state was a hyperbole: APD = APDmax*CL/(CL+CL50), where APDmax (APD at infinite CL) is a rate independent measure of APD, and CL50 (CL at which 50% APDmax is reached) is an index of the rate dependence of APD. In five fibers, OXO reduced APD at all CL (P < 0.05), APDmax was also reduced to 377 +/- 41 ms from 447 +/- 34 ms (P < 0.05), while CL50 was unchanged (405 +/- 46 ms from 437 +/- 28 ms). No effects of OXO on APD and APDmax were seen in two fibers obtained from dogs pretreated with pertussis toxin (PTX). In conclusion, stimulation of M2 receptors in intact, and not PTX treated, Purkinje fibers affects APD but not its CL dependence. This may reflect the activation of a rate independent, background current through a GTP binding protein-linked pathway, such as, IK,ACh. These data differ from those obtained in endocardial and epicardial muscle, stressing the regional differences in vagal modulation of ventricular electrophysiological properties.

Modulation of the hyperpolarization-activated current (I(f)) by adenosine in rabbit sinoatrial myocytes

BACKGROUND

Modulation of sinoatrial pacemaking by adenosine (Ado) in the absence of concomitant adrenergic stimulation (direct modulation) has been attributed to activation of a K+ conductance. In the present study, we evaluated the direct effects of Ado on the pacemaking current I(f) and tested their interaction with those of acetylcholine (ACh).

METHODS AND RESULTS

Rabbit sinoatrial myocytes were patch-clamped at 35 degrees C in the presence of 1 mmol/L BaCl2 and 2 mmol/ L MnCl2, Ado (1 mumol/L) reversibly reduced I(f) by 33.1 +/- 5.7% of control (n = 5; P < .05). Ado (1 mumol/L) reversibly shifted I(f) midactivation potential by -6.63 +/- 1.18 mV (n = 4; P < .05). Fully activated I(f) conductance (0.262 +/- 0.037 versus 0.254 +/- 0.036 nS/ pF; n = 6, NS) and reversal potential (-17.35 +/- 0.99 versus -18.01 +/- 1.42 mV; n = 6, NS) were not changed by 10 mumol/L Ado. The Ado receptor antagonist 8-PST (10 mumol/L) reversed the effect of 0.3 mumol/L Ado by 64.9 +/- 4.2% (n = 6; P < .05). Ado maximally shifted the I(f) activation curve by -5.85 mV, with a half-maximal concentration of 0.0796 mumol/L (n = 93). The shifts in I(f) activation induced by Ado (0.3 mumol/L) and ACh (1 mumol/ L) separately were -4.89 +/- 0.05 and -8.84 +/- 0.51 mV, respectively; concomitant Ado and ACh superfusion shifted activation by -9.7 +/- 0.45 mV (NS versus ACh alone; n = 9). Threshold Ado concentrations dose-dependently reduced the rate of spontaneous pacemaker activity (eg, -18.8 +/- 3.4% at Ado 0.03 mumol/L).

CONCLUSIONS

Submicromolar Ado directly inhibits I(f) and slows pacemaking in sinoatrial myocytes; the mode of I(f) inhibition is similar to that previously described for ACh. Thus, Ado may exert local modulation of sinus rate through signaling pathways similar to those used by ACh.

Basal responses of the L-type Ca2+ and hyperpolarization-activated currents to autonomic agonists in the rabbit sino-atrial node

1. The dose dependence of the cholinergic agonist acetylcholine (ACh) and the beta-adrenergic agonist isoprenaline (Iso) were determined for the hyperpolarization-activated current (If) and the L-type Ca2+ current (ICa,L) in single cells isolated from the rabbit sino-atrial (SA) node. 2. ACh inhibited If by a negative shift of its activation curve with a maximal effect of -9.9 mV; half-maximal effect was produced by 0.019 microM ACh. High ACh concentrations were required to inhibit ICa,L only partially (31% inhibition at 300 microM). 3. In contrast, If and ICa,L responded to Iso over a similar dose range, with concentrations for half-maximal enhancement of 0.0136 and 0.0070 microM, respectively. 4. The effects on spontaneous activity of ACh (range 0.001-0.03 microM) and Iso (range 0.001-1 microM) were investigated. ACh decreased the slope of diastolic depolarization at concentrations similar to those inhibiting If (> 50% at 0.03 microM). Iso enhanced diastolic depolarization at concentrations similar to those affecting both If and ICa,L (half-maximal effect at 0.027 microM). 5. In a ramp-clamp protocol simulating diastolic depolarization, the threshold for activation of inward nifedipine-sensitive current was -41.22 +/- 0.68 mV. Although enhancing ICa,L, Iso did not affect this threshold. 6. Half-maximal ACh concentrations for inhibition of automaticity and If are similar and are lower than the threshold concentrations for modulation of ICa,L; this argues against a role of ICa,L in direct muscarinic modulation of pacemaking. In contrast, modulation of If, ICa,L and automaticity occur at similar Iso concentrations. The difference between maximum diastolic potential (-61.95 +/- 0.93 mV) and the threshold for Iso-stimulated ICa,L (-39.54 +/- 1.03 mV) suggests that this current plays a role only at later stages of diastolic depolarization.

Effects on atrial repolarization of the interaction between K+ channel blockers and muscarinic receptor stimulation

We have tested, in guinea pig atria, how muscarinic stimulation by oxotremorine (Oxo) modifies the effects on action potential duration (APD) of two iK blockers: d-sotalol (5 microM) and ambasilide (1 microM). APD was prolonged by d-sotalol (+34.8 +/- 2.9%) and ambasilide (+54.2 +/- 5.5%). Simultaneous superfusion with Oxo 0.5 microM markedly shortened APD; this effect was larger in the presence of d-sotalol than in the presence of ambasilide (-69 +/- 2% vs. -37.4 +/- 5%; P < .05). Moreover ambasilide, but not d-sotalol, antagonized APD shortening induced by Oxo. The basis for such a difference between the two drugs was studied in patch-clamp experiments on isolated rabbit atrial and sinoatrial myocytes. Besides blocking iK (half-effective concentration: EC50 = 2 microM), ambasilide almost completely inhibited iKACh (-86 +/- 2% at 10 microM; EC50 = 1.6 microM), which was minimally affected by d-sotalol. Ambasilide 2 microM increased 10-fold the acetylcholine (ACh) required for 50% iKACh activation, and reduced maximally activated iKACh by 18.8 +/- 6.3%. When iKACh was activated through a receptor-independent mechanism, 10 microM ambasilide reduced this current by only 18.7 +/- 1.4% of its control value. Moreover, ambasilide, although not affecting the current i(f), in basal conditions, reversed its inhibition by ACh. Thus, 1) the effect of K+ channel blockers on atrial APD may be blunted by ACh; ambasilide effects are less sensitive to ACh than those of d-sotalol; 2) ambasilide, but not d-sotalol, inhibits iKACh; this probably occurs largely, although not exclusively, through muscarinic receptor antagonism. Inhibition of iKACh may account for the persistence of the effects of this drug on atrial APD despite muscarinic stimulation.

Modulation of the electrical restitution of canine Purkinje fibers by local anesthetic drugs: a study with flecainide and propafenone

Some antiarrhythmic drugs that depress conduction also influence action potential duration (APD). This could modify the time course of changes in the APD of premature stimuli (electrical restitution) and affect dispersion of repolarization and homogeneity of refractoriness. The effects of two potent local anesthetic drugs, i.e., flecainide and propafenone, on electrical restitution were studied in canine Purkinje fibers, superfused with Tyrode's solution and impaled with glass microelectrodes. APD was measured at 90% repolarization (APD90): fibers were stimulated for 3 minutes at cycle lengths (CLs) between 350 msec and 1.5 seconds, and kept quiescent for 5 minutes in between. For each run we calculated the percent ratio of the second APD (APD test = APDt) to the first APD after quiescence (APDo) (APDt/APDo x 100). The ratio was correlated with the CL of the run (i.e., the coupling interval between APDo and APDt = CI) by the monoexponential function APDt/APDo x 100 = 100-exp(-tau/CI), whose time constant tau indicates the speed of electrical restitution. At 1 microM, flecainide decreased tau by 26% +/- 4% (to 310 +/- 41 msec from 245 +/- 30 msec, n = 6, P < 0.05), while propafenone did not change it. Thus, unlike propafenone, flecainide slows the process of electrical restitution in Purkinje fibers. This may derive from the drug's action on currents other than Na current (i.e., IK), relevant to the duration of action potential. In a reentrant circuit, despite similar effects on conduction, APD and refractoriness of the first beat would be differently modulated by flecainide and propafenone.

Diverse electrophysiologic effects of propafenone and flecainide in canine Purkinje fibers: implications for antiarrhythmic drug classification

Propafenone and flecainide are assigned to class Ic of the Campbell-Vaughan Williams classification because of their effects on ventricular muscle. The authors compared the use-dependent local anesthetic properties and the effects on repolarization of these drugs (1 and 5 microM) in Purkinje fibers. A reduction in maximum upstroke velocity was used as an index of the local anesthetic action. The rate dependency of the drug's effects on repolarization was evaluated by analyzing the relationship between action potential duration during steady-state stimulation and cycle length (CL). Tonic block was higher for propafenone (n = 10) than for flecainide (n = 7) at both concentrations tested (19 +/- 3% vs. 4 +/- 1% at 1 microM; 59 +/- 10% vs. 24 +/- 4% at 5 microM). Use-dependent block onset and dissipation were significantly slower for flecainide than for propafenone (e.g., at 1 microM and CL = 500 ms, time constant of block onset = 31 +/- 6 vs. 9 +/- 1 beats; time constant of recovery from block = 7.7 +/- 0.2 vs. 2.8 +/- 0.2 sec; P < .05). Steady-state block measured at each CL was compared with that predicted by a theoretical model of use dependency. Predictions approximated the experimentally results only for flecainide. At 1 microM, propafenone shortened action potential duration at all rates; flecainide had biphasic effects. At 5 microM, the effect of flecainide was similar to that of 1 microM propafenone. Thus, in Purkinje fibers, the kinetics of use-dependent local anesthetic effects and the effects on repolarization discriminate flecainide from propafenone.