Glow-in-the-Dark Infectious Disease Diagnostics Using CRISPR-Cas9-Based Split Luciferase Complementation.

Nucleic acid detection methods based on CRISPR and isothermal amplification techniques show great potential for point-of-care diagnostic applications. However, most current methods rely on fluorescent or lateral flow assay readout, requiring external excitation or postamplification reaction transfer. Here, we developed a bioluminescent nucleic acid sensor (LUNAS) platform in which target dsDNA is sequence-specifically detected by a pair of dCas9-based probes mediating split NanoLuc luciferase complementation. LUNAS is easily integrated with recombinase polymerase amplification (RPA), providing attomolar sensitivity in a rapid one-pot assay. A calibrator luciferase is included for a robust ratiometric readout, enabling real-time monitoring of the RPA reaction using a simple digital camera. We designed an RT-RPA-LUNAS assay that allows SARS-CoV-2 RNA detection without the need for cumbersome RNA isolation and demonstrated its diagnostic performance for COVID-19 patient nasopharyngeal swab samples. Detection of SARS-CoV-2 from samples with viral RNA loads of ∼200 cp/µL was achieved within ∼20 min, showing that RPA-LUNAS is attractive for point-of-care infectious disease testing.

A plug-and-play platform of ratiometric bioluminescent sensors for homogeneous immunoassays.

Heterogeneous immunoassays such as ELISA have become indispensable in modern bioanalysis, yet translation into point-of-care assays is hindered by their dependence on external calibration and multiple washing and incubation steps. Here, we introduce RAPPID (Ratiometric Plug-and-Play Immunodiagnostics), a mix-and-measure homogeneous immunoassay platform that combines highly specific antibody-based detection with a ratiometric bioluminescent readout. The concept entails analyte-induced complementation of split NanoLuc luciferase fragments, photoconjugated to an antibody sandwich pair via protein G adapters. Introduction of a calibrator luciferase provides a robust ratiometric signal that allows direct in-sample calibration and quantitative measurements in complex media such as blood plasma. We developed RAPPID sensors that allow low-picomolar detection of several protein biomarkers, anti-drug antibodies, therapeutic antibodies, and both SARS-CoV-2 spike protein and anti-SARS-CoV-2 antibodies. With its easy-to-implement standardized workflow, RAPPID provides an attractive, fast, and low-cost alternative to traditional immunoassays, in an academic setting, in clinical laboratories, and for point-of-care applications.

Risk of Metformin-Associated Lactic Acidosis (MALA) in Patients After Gastric Bypass Surgery.

BACKGROUND: Pharmacokinetic data suggest that the risk of metformin-associated lactic acidosis (MALA) may be increased after Roux-en-Y gastric bypass (RYGB) surgery. The aim of this study was to examine the impact of metformin on plasma lactate levels before and after RYGB surgery. METHODS: Retrospective study of plasma lactate levels before and 3 months after RYGB surgery in patients with type 2 diabetes mellitus (T2DM) not using metformin (MET-0, N = 58), on a stable dose (MET-S, N = 138), or on a decreasing dose (MET-D, N = 85) of metformin. RESULTS: Preoperatively, lactate levels were similar in patients on metformin (1.8 ± 0.05 mmol/L) and those not on metformin (1.7 ± 0.08 mmol/L), P = 0.21. Three months postoperatively, lactate levels had decreased in all groups (P < 0.001) to 1.3 ± 0.07 (SE), 1.4 ± 0.05, and 1.2 ± 0.05 mmol/l in MET-0, MET-S, and MET-D, respectively. Lactate levels differed between the groups (P = 0.03), with the lowest level in MET-D. The number of patients with hyperlactatemia (lactate > 2 mmol/l) decreased from 31 to 14%, from 22 to 8.6%, and from 26 to 4.7% in MET-S, MET-0, and MET-D, respectively. CONCLUSION: Mild hyperlactatemia (lactate > 2 mmol/l) is common in morbidly obese patients with T2DM. It is probably related to increase lactate production by adipocytes. Lactate levels decreased after RYGB-induced weight loss, irrespective of the use of metformin. We therefore conclude that there is no need for routinely lowering of the metformin dose after uncomplicated RYGB surgery, as long as normal renal function is preserved.

Sphingosine-1-Phosphate Receptor 1 Regulates Cardiac Function by Modulating Ca2+ Sensitivity and Na+/H+ Exchange and Mediates Protection by Ischemic Preconditioning.

BACKGROUND: Sphingosine-1-phosphate plays vital roles in cardiomyocyte physiology, myocardial ischemia-reperfusion injury, and ischemic preconditioning. The function of the cardiomyocyte sphingosine-1-phosphate receptor 1 (S1P1) in vivo is unknown. METHODS AND RESULTS: Cardiomyocyte-restricted deletion of S1P1 in mice (S1P1 (α) (MHCC) (re)) resulted in progressive cardiomyopathy, compromised response to dobutamine, and premature death. Isolated cardiomyocytes from S1P1 (α) (MHCC) (re) mice revealed reduced diastolic and systolic Ca(2+) concentrations that were secondary to reduced intracellular Na(+) and caused by suppressed activity of the sarcolemmal Na(+)/H(+) exchanger NHE-1 in the absence of S1P1. This scenario was successfully reproduced in wild-type cardiomyocytes by pharmacological inhibition of S1P1 or sphingosine kinases. Furthermore, Sarcomere shortening of S1P1 (α) (MHCC) (re) cardiomyocytes was intact, but sarcomere relaxation was attenuated and Ca(2+) sensitivity increased, respectively. This went along with reduced phosphorylation of regulatory myofilament proteins such as myosin light chain 2, myosin-binding protein C, and troponin I. In addition, S1P1 mediated the inhibitory effect of exogenous sphingosine-1-phosphate on ß-adrenergic-induced cardiomyocyte contractility by inhibiting the adenylate cyclase. Furthermore, ischemic precondtioning was abolished in S1P1 (α) (MHCC) (re) mice and was accompanied by defective Akt activation during preconditioning. CONCLUSIONS: Tonic S1P1 signaling by endogenous sphingosine-1-phosphate contributes to intracellular Ca(2+) homeostasis by maintaining basal NHE-1 activity and controls simultaneously myofibril Ca(2+) sensitivity through its inhibitory effect on adenylate cyclase. Cardioprotection by ischemic precondtioning depends on intact S1P1 signaling. These key findings on S1P1 functions in cardiac physiology may offer novel therapeutic approaches to cardiac diseases.

Ca(2+) cycling properties are conserved despite bradycardic effects of heart failure in sinoatrial node cells.

BACKGROUND: In animal models of heart failure (HF), heart rate decreases due to an increase in intrinsic cycle length (CL) of the sinoatrial node (SAN). Pacemaker activity of SAN cells is complex and modulated by the membrane clock, i.e., the ensemble of voltage gated ion channels and electrogenic pumps and exchangers, and the Ca(2+) clock, i.e., the ensemble of intracellular Ca(2+) ([Ca(2+)]i) dependent processes. HF in SAN cells results in remodeling of the membrane clock, but few studies have examined its effects on [Ca(2+)]i homeostasis. METHODS: SAN cells were isolated from control rabbits and rabbits with volume and pressure overload-induced HF. [Ca(2+)]i concentrations, and action potentials (APs) and Na(+)-Ca(2+) exchange current (INCX) were measured using indo-1 and patch-clamp methodology, respectively. RESULTS: The frequency of spontaneous [Ca(2+)]i transients was significantly lower in HF SAN cells (3.0 ± 0.1 (n = 40) vs. 3.4 ± 0.1 Hz (n = 45); mean ± SEM), indicating that intrinsic CL was prolonged. HF slowed the [Ca(2+)]i transient decay, which could be explained by the slower frequency and reduced sarcoplasmic reticulum (SR) dependent rate of Ca(2+) uptake. Other [Ca(2+)]i transient parameters, SR Ca(2+) content, INCX density, and INCX-[Ca(2+)]i relationship were all unaffected by HF. Combined AP and [Ca(2+)]i recordings demonstrated that the slower [Ca(2+)]i transient decay in HF SAN cells may result in increased INCX during the diastolic depolarization, but that this effect is likely counteracted by the HF-induced increase in intracellular Na(+). ß-adrenergic and muscarinic stimulation were not changed in HF SAN cells, except that late diastolic [Ca(2+)]i rise, a prominent feature of the Ca(2+) clock, is lower during ß-adrenergic stimulation. CONCLUSIONS: HF SAN cells have a slower [Ca(2+)]i transient decay with limited effects on pacemaker activity. Reduced late diastolic [Ca(2+)]i rise during ß-adrenergic stimulation may contribute to an impaired increase in intrinsic frequency in HF SAN cells.

Increased sarcolemmal Na(+)/H(+) exchange activity in hypertrophied myocytes from dogs with chronic atrioventricular block.

Dogs with compensated biventricular hypertrophy due to chronic atrioventricular block (cAVB), are more susceptible to develop drug-induced Torsade-de-Pointes arrhythmias and sudden cardiac death. It has been suggested that the increased Na(+) influx in hypertrophied cAVB ventricular myocytes contribute to these lethal arrhythmias. The increased Na(+) influx was not mediated by Na(+) channels, in fact the Na(+) current proved reduced in cAVB myocytes. Here we tested the hypothesis that increased activity of the Na(+)/H(+) exchanger type 1 (NHE-1), commonly observed in hypertrophic hearts, causes the elevated Na(+) influx. Cardiac acid-base transport was studied with a pH-sensitive fluorescent dye in ventricular myocytes isolated from control and hypertrophied cAVB hearts; the H(+) equivalent flux through NHE-1, Na(+)-HCO(-) 3 cotransport (NBC), Cl(-)/OH(-) exchange (CHE), and Cl(-)/HCO(-) 3 exchange (AE) were determined and normalized per liter cell water and corrected for surface-to-volume ratio. In cAVB, sarcolemmal NHE-1 flux was increased by 65 ± 6.3% in the pH i interval 6.3-7.2 and NBC, AE, and CHE fluxes remained unchanged. Accordingly, at steady-state intracellular pH the total sarcolemmal Na(+) influx by NHE-1 + NBC increased from 8.5 ± 1.5 amol/µm(2)/min in normal myocytes to 15 ± 2.4 amol/µm(2)/min in hypertrophied cAVB myocytes. We conclude that compensated cardiac hypertrophy in cAVB dogs is accompanied with an increased sarcolemmal NHE-1 activity. This in conjunction with unchanged activity of the other acid-base transporters will raise the intracellular Na(+) in hypertrophied cAVB myocytes.

Calcium transient and sodium-calcium exchange current in human versus rabbit sinoatrial node pacemaker cells.

There is an ongoing debate on the mechanism underlying the pacemaker activity of sinoatrial node (SAN) cells, focusing on the relative importance of the "membrane clock" and the "Ca(2+) clock" in the generation of the small net membrane current that depolarizes the cell towards the action potential threshold. Specifically, the debate centers around the question whether the membrane clock-driven hyperpolarization-activated current, I f , which is also known as the "funny current" or "pacemaker current," or the Ca(2+) clock-driven sodium-calcium exchange current, I NaCa, is the main contributor to diastolic depolarization. In our contribution to this journal's "Special Issue on Cardiac Electrophysiology," we present a numerical reconstruction of I f and I NaCa in isolated rabbit and human SAN pacemaker cells based on experimental data on action potentials, I f , and intracellular calcium concentration ([Ca(2+)] i ) that we have acquired from these cells. The human SAN pacemaker cells have a smaller I f , a weaker [Ca(2+)] i transient, and a smaller I NaCa than the rabbit cells. However, when compared to the diastolic net membrane current, I NaCa is of similar size in human and rabbit SAN pacemaker cells, whereas I f is smaller in human than in rabbit cells.

Dietary Omega-3 Polyunsaturated Fatty Acids Suppress NHE-1 Upregulation in a Rabbit Model of Volume- and Pressure-Overload.

BACKGROUND: Increased consumption of omega-3 polyunsaturated fatty acids (ω3-PUFAs) from fish oil (FO) may have cardioprotective effects during ischemia/reperfusion, hypertrophy, and heart failure (HF). The cardiac Na(+)/H(+)-exchanger (NHE-1) is a key mediator for these detrimental cardiac conditions. Consequently, chronic NHE-1 inhibition appears to be a promising pharmacological tool for prevention and treatment. Acute application of the FO ω3-PUFAs eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) inhibit the NHE-1 in isolated cardiomyocytes. We studied the effects of a diet enriched with ω3-PUFAs on the NHE-1 activity in healthy rabbits and in a rabbit model of HF induced by volume- and pressure-overload. METHODS: Rabbits were allocated to four groups. The first two groups consisted of healthy rabbits, which were fed either a diet containing 1.25% (w/w) FO (ω3-PUFAs), or 1.25% high-oleic sunflower oil (ω9-MUFAs) as control. The second two groups were also allocated to either a diet containing ω3-PUFAs or ω9-MUFAs, but underwent volume- and pressure-overload to induce HF. Ventricular myocytes were isolated by enzymatic dissociation and used for intracellular pH (pH(i)) and patch-clamp measurements. NHE-1 activity was measured in HEPES-buffered conditions as recovery rate from acidosis due to ammonium prepulses. RESULTS: In healthy rabbits, NHE-1 activity in ω9-MUFAs and ω3-PUFAs myocytes was not significantly different. Volume- and pressure-overload in rabbits increased the NHE-1 activity in ω9-MUFAs myocytes, but not in ω3-PUFAs myocytes, resulting in a significantly lower NHE-1 activity in myocytes of ω3-PUFA fed HF rabbits. The susceptibility to induced delayed afterdepolarizations (DADs), a cellular mechanism of arrhythmias, was lower in myocytes of HF animals fed ω3-PUFAs compared to myocytes of HF animals fed ω9-MUFAs. In our rabbit HF model, the degree of hypertrophy was similar in the ω3-PUFAs group compared to the ω9-MUFAs group. CONCLUSION: Dietary ω3-PUFAs from FO suppress upregulation of the NHE-1 activity and lower the incidence of DADs in our rabbit model of volume- and pressure-overload.

Activated human platelet products induce proarrhythmic effects in ventricular myocytes.

Sudden cardiac death remains one of the most prevalent modes of death and is mainly caused by ventricular fibrillation (VF) in the setting of acute ischemia resulting from coronary thrombi. Animal experiments have shown that platelet activation may increase susceptibility of ischemic myocardium to VF, but the mechanism is unknown. In the present study, we evaluated the effects of activated blood platelet products (ABPPs) on electrophysiological properties and intracellular Ca(2+) (Ca(2+)(i)) homeostasis. Platelets were collected from healthy volunteers. After activation, their secreted ABPPs were added to superfusion solutions. Rabbit ventricular myocytes were freshly isolated, and membrane potentials and Ca(2+)(i) were recorded using patch-clamp methodology and indo-1 fluorescence measurements, respectively. ABPPs prolonged action potential duration and induced early and delayed afterdepolarizations. ABPPs increased L-type Ca(2+) current (I(Ca,L)) density, but left densities of sodium current, inward rectifier K(+) current, transient outward K(+) current, and rapid component of the delayed rectifier K(+) current unchanged. ABPPs did not affect kinetics or (in)activation properties of membrane currents. ABPPs increased systolic Ca(2+)(i), Ca(2+)(i) transient amplitude, and sarcoplasmic reticulum Ca(2+) content. ABPPs did not affect the Na(+)-Ca(2+) exchange current (I(NCX)) in Ca(2+)-buffered conditions. Products secreted from activated human platelets induce changes in I(Ca,L) and Ca(2+)(i), which result in action potential prolongation and the occurrence of early and delayed afterdepolarizations in rabbit myocytes. These changes may trigger and support reentrant arrhythmias in ischemia models of coronary thrombosis.

Effects of muscarinic receptor stimulation on Ca2+ transient, cAMP production and pacemaker frequency of rabbit sinoatrial node cells.

We investigated the contribution of the intracellular calcium (Ca (i) (2+) ) transient to acetylcholine (ACh)-mediated reduction of pacemaker frequency and cAMP content in rabbit sinoatrial nodal (SAN) cells. Action potentials (whole cell perforated patch clamp) and Ca (i) (2+) transients (Indo-1 fluorescence) were recorded from single isolated rabbit SAN cells, whereas intracellular cAMP content was measured in SAN cell suspensions using a cAMP assay (LANCE((R))). Our data show that the Ca (i) (2+) transient, like the hyperpolarization-activated "funny current" (I (f)) and the ACh-sensitive potassium current (I (K,ACh)), is an important determinant of ACh-mediated pacemaker slowing. When I (f) and I (K,ACh) were both inhibited, by cesium (2 mM) and tertiapin (100 nM), respectively, 1 micro M ACh was still able to reduce pacemaker frequency by 72%. In these I (f) and I (K,ACh)-inhibited SAN cells, good correlations were found between the ACh-mediated change in interbeat interval and the ACh-mediated change in Ca (i) (2+) transient decay (r (2) = 0.98) and slow diastolic Ca (i) (2+) rise (r (2) = 0.73). Inhibition of the Ca (i) (2+) transient by ryanodine (3 microM) or BAPTA-AM (5 microM) facilitated ACh-mediated pacemaker slowing. Furthermore, ACh depressed the Ca (i) (2+) transient and reduced the sarcoplasmic reticulum (SR) Ca(2+) content, all in a concentration-dependent fashion. At 1 microM ACh, the spontaneous activity and Ca (i) (2+) transient were abolished, but completely recovered when cAMP production was stimulated by forskolin (10 microM) and I (K,ACh) was inhibited by tertiapin (100 nM). Also, inhibition of the Ca (i) (2+) transient by ryanodine (3 microM) or BAPTA-AM (25 microM) exaggerated the ACh-mediated inhibition of cAMP content, indicating that Ca (i) (2+) affects cAMP production in SAN cells. In conclusion, muscarinic receptor stimulation inhibits the Ca (i) (2+) transient via a cAMP-dependent signaling pathway. Inhibition of the Ca (i) (2+) transient contributes to pacemaker slowing and inhibits Ca (i) (2+) -stimulated cAMP production. Thus, we provide functional evidence for the contribution of the Ca (i) (2+) transient to ACh-induced inhibition of pacemaker activity and cAMP content in rabbit SAN cells.

Ae2(a,b)-deficient mice exhibit osteopetrosis of long bones but not of calvaria.

Extracellular acidification by osteoclasts is essential to bone resorption. During proton pumping, intracellular pH (pH(i)) is thought to be kept at a near-neutral level by chloride/bicarbonate exchange. Here we show that the Na(+)-independent chloride/bicarbonate anion exchanger 2 (Ae2) is relevant for this process in the osteoclasts from the long bones of Ae2(a,b)(-/-) mice (deficient in the main isoforms Ae2a, Ae2b(1), and Ae2b(2)). Although the long bones of these mice had normal numbers of multinucleated osteoclasts, these cells lacked a ruffled border and displayed impaired bone resorption activity, resulting in an osteopetrotic phenotype of long bones. Moreover, in vitro osteoclastogenesis assays using long-bone marrow cells from Ae2(a,b)(-/-) mice suggested a role for Ae2 in osteoclast formation, as fusion of preosteoclasts for the generation of active multinucleated osteoclasts was found to be slightly delayed. In contrast to the abnormalities observed in the long bones, the skull of Ae2(a,b)(-/-) mice showed no alterations, indicating that calvaria osteoclasts may display normal resorptive activity. Microfluorimetric analysis of osteoclasts from normal mice showed that, in addition to Ae2 activity, calvaria osteoclasts--but not long-bone osteoclasts--possess a sodium-dependent bicarbonate transporting activity. Possibly, this might compensate for the absence of Ae2 in calvaria osteoclasts of Ae2(a,b)(-/-) mice.

Is sodium current present in human sinoatrial node cells?

Pacemaker activity of the sinoatrial node has been studied extensively in various animal species, but is virtually unexplored in man. As such, it is unknown whether the fast sodium current (I(Na)) plays a role in the pacemaker activity of the human sinoatrial node. Recently, we had the unique opportunity to perform patch-clamp experiments on single pacemaker cells isolated from a human sinoatrial node. In 2 out of the 3 cells measured, we observed large inward currents with characteristics of I(Na). Although we were unable to analyze the current in detail, our findings provide strong evidence that I(Na) is present in human sinoatrial node pacemaker cells, and that this I(Na) is functionally available at potentials negative to -60 mV.

Intracellular calcium modulation of voltage-gated sodium channels in ventricular myocytes.

AIMS: Cardiac voltage-gated sodium channels control action potential (AP) upstroke and cell excitability. Intracellular calcium (Ca(i)(2+)) regulates AP properties by modulating various ion channels. Whether Ca(i)(2+) modulates sodium channels in ventricular myocytes is unresolved. We studied whether Ca(i)(2+) modulates sodium channels in ventricular myocytes at Ca(i)(2+) concentrations ([Ca(i)(2+)]) present during the cardiac AP (0-500 nM), and how this modulation affects sodium channel properties in heart failure (HF), a condition in which Ca(i)(2+) homeostasis is disturbed. METHODS AND RESULTS: Sodium current (I(Na)) and maximal AP upstroke velocity (dV/dt(max)), a measure of I(Na), were studied at 20 and 37 degrees C, respectively, in freshly isolated left ventricular myocytes of control and HF rabbits, using whole-cell patch-clamp methodology. [Ca(i)(2+)] was varied using different pipette solutions, the Ca(i)(2+) buffer BAPTA, and caffeine administration. Elevated [Ca(i)(2+)] reduced I(Na) density and dV/dt(max), but caused no I(Na) gating changes. Reductions in I(Na) density occurred simultaneously with increase in [Ca(i)(2+)], suggesting that these effects were due to permeation block. Accordingly, unitary sodium current amplitudes were reduced at higher [Ca(i)(2+)]. While I(Na) density and gating at fixed [Ca(i)(2+)] were not different between HF and control, reductions in dV/dt(max) upon increases in stimulation rate were larger in HF than in control; these differences were abolished by BAPTA. CONCLUSION: Ca(i)(2+) exerts acute modulation of I(Na) density in ventricular myocytes, but does not modify I(Na) gating. These effects, occurring rapidly and in the [Ca(i)(2+)] range observed physiologically, may contribute to beat-to-beat regulation of cardiac excitability in health and disease.

Sodium ion transporters as new therapeutic targets in heart failure.

Sodium ion transporters in sarcolemma are involved in numerous vital cell functions, such as excitability, excitation-contraction coupling, energy metabolism, pH and volume regulation, development and growth. In a number of cardiac pathologies, the intracellular sodium concentration ([Na+]i) is elevated. Since [Na+]i and intracellular Ca2+ concentration ([Ca2+]i are coupled through the Na+/Ca(2+)-exchanger, these cardiac pathologies display disturbed calcium handling. For instance, [Na+]i is increased in heart failure (HF) leading to Na+/Ca(2+)-exchanger mediated increase in [Ca2+]i, reduced contractility and increased propensity to arrhythmias. Several studies support the contention that an increase in [Na+]i and [Ca2+]i transduces a signal the nucleus, that triggers development of cardiac remodelling and hypertrophy. Pharmacological intervention, which favourably interferes with [Na+]i and [Ca2+]i homeostasis, might prevent hypertrophy, cardiac remodelling, arrhythmias and HF. The most important sodium transport mechanisms that may underlie increased [Na+]i are: Na+/H(+)-exchanger (NHE-1), Na+-HCO(3)(-) co-transporter (NBC), Na(+)-K(+)-Cl(-) co-transporter (NKCC), Na(+)-channel, Na+/K(+)-ATPase and Na+/Ca(2+)-exchanger (NCX). Preclinical studies showed that pharmacological interventions, targeted against sarcolemmal sodium ion transporters, proved effective in ameliorating heart failure. In this respect: 1) NHE-1 inhibition reduces cardiac remodelling, hypertrophy and HF, although, in the patients following coronary artery bypass graft surgery, it was associated with an increase of stroke. 2) The activity of NBC is up-regulated, during the development of hypertrophy and may be a therapeutic strategy to prevent the development of hypertrophy and HF. 3) NKCC is increased in post-infarction HF, and the inhibition of NKCC attenuated post-infarction remodelling. 4) Inactivation of sodium channels is impaired in HF, which may result, in increased Na+ influx and prolongation of the action potential. 5) Blockade of NCX may be useful as a part of a combined therapeutic approach. Inhibition of reversed mode, or activation of forward mode NCX reduce Ca2+ overload. 6) Inhibition of Na+/K(+)-ATPase (digoxin), is used to increase contractility, however, it enhances progression of HF. Oppositely, new drugs which increase activity of Na+/K(+)-ATPase may prevent the development of cardiac remodelling hypertrophy and HF.

Single cells isolated from human sinoatrial node: action potentials and numerical reconstruction of pacemaker current.

Pacemaker activity of the sinoatrial node has extensively been studied in laboratory animals of various species, but is virtually unexplored in man. Most experimental data have been obtained from rabbit, where the hyperpolarization-activated 'funny' current (If), also known as the 'pacemaker current', plays an important role in diastolic depolarization and thus in setting pacing rate. Recently, we isolated pacemaker cells from excised human sinoatrial node tissue, and recorded action potentials and If using the whole-cell patch-clamp technique in current clamp and voltage clamp mode, respectively. Single sinoatrial node pacemaker cells showed a spontaneous beating rate of 73 +/- 3 beats/min (mean +/- SEM, n = 3) with a remarkably slow diastolic depolarization. If was identified in voltage clamp experiments as the 2 mmol/L Cs+-sensitive inward current activating upon 2-s hyperpolarizing voltage clamp steps. The If reversal potential and (de)activation kinetics were similar to those in rabbit. However, the fully-activated If conductance was 3-4 times smaller than typically found in rabbit. Furthermore, the half-maximal activation voltage was approximately 20 mV more negative than in rabbit. These differences would both act to reduce the functional role of If in human pacemaker cells. To assess this functional role, we carried out a numerical reconstruction of the If time course during an experimentally recorded human sinoatrial node action potential, based on the obtained data on If amplitude and kinetics. This reconstruction revealed that If provides a small but significant inward current in the voltage range of diastolic depolarization. We conclude that human sinoatrial node pacemaker cells functionally express If and that this If contributes to pacemaking in human sinoatrial node.

Pacemaker current (I(f)) in the human sinoatrial node.

AIMS: Animal studies revealed that the hyperpolarization-activated pacemaker current, I(f), contributes to action potential (AP) generation in sinoatrial node (SAN) and significantly determines heart rate. I(f) is becoming a novel therapy target to modulate heart rate. Yet, no studies have demonstrated that I(f) is functionally present and contributes to pacemaking in human SAN. We aimed to study I(f) properties in human SAN. METHODS AND RESULTS: In a patient undergoing SAN excision, we identified SAN using epicardial activation mapping. From here, we isolated myocytes and recorded APs and I(f) using patch-clamp techniques. Pacemaker cells generated spontaneous APs (cycle length 828 +/- 15 ms) following slow diastolic depolarization, maximal diastolic potential - 61.7 +/- 4.3 mV, and maximal AP upstroke velocity 4.6 +/- 1.2 V/s. They exhibited an hyperpolarization-activated inward current, blocked by external Cs(+) (2 mmol/L), characterizing it as I(f). Fully-activated conductance was 75.2 +/- 3.8 pS/pF, reversal potential - 22.1 +/- 2.4 mV, and half-maximal activation voltage and slope factor of steady-state activation - 96.9 +/- 2.7 and - 8.8 +/- 0.5 mV. Activation time constant ranged from approximately 350 ms (-130 mV) to approximately 1 s (-100 mV), deactivation time constant 156 +/- 45 ms (-40 mV). The role of I(f) in pacemaker activity was demonstrated by slowing of pacemaker cell diastolic depolarization and beating rate by Cs(+). CONCLUSION: I(f) is functionally expressed in human SAN and probably contributes to pacemaking in human SAN.

Contribution of NHE-1 to cell length shortening of normal and failing rabbit cardiac myocytes.

At the same intracellular pH (pHi) Na+/H+ exchange (NHE-1) fluxes of ventricular myocytes of hypertrophied failing hearts (HFH) are increased. We assessed how NHE-1 affected cell length shortening. pHi was measured fluorimetrically in resting and twitching (1-3 Hz) normal and HFH rabbit myocytes. In HEPES-buffered solutions, increased NHE-1 fluxes (P=0.001, n=14) made HFH resting pHi 0.2+/-0.03 units more alkaline than control (n=27). In CO2/HCO3--buffered solutions, HFH resting pHi was not different (7.05+/-0.02, n=30). Twitching myocytes of both groups shortened 15-16% less per 0.1 pH unit acidification. In HEPES-buffered solutions, cariporide depressed cell length shortening of normal myocytes (1-3 Hz) by 16+/-5.4% (n=9, P=0.005). In HFH myocytes cariporide restored the positive force-frequency relationship (n=7, P=0.009), by depressing twitch amplitudes at 1 Hz (16+/-11%, P=0.047) but not at 2 and 3 Hz. The depressions were all caused by pHi acidification. In CO2/HCO3- buffered solutions the cariporide-induced acidification was too small to explain the cell length shortening depression of normal (19+/-5.0%, n=11, P=0.006) and HFH myocytes (14+/-4.7%, n=11, P=0.001). When compared to HEPES-buffered solutions, HFH myocytes in CO2/HCO3--buffered solutions shortened 12+/-6.8% better than expected given the 0.16+/-0.02 units more acidic pHi's at which they twitched. We conclude that in CO2/HCO3--buffered solutions NHE-1 improved cell length shortening of unstretched normal and HFH myocytes via a pHi-independent mechanism. Although NHE-1 was increased in HFH myocytes, the magnitude of the pHi-independent effect of NHE-1 inhibition on cell length shortening was similar in both groups.

Chronic inhibition of Na+/H+-exchanger attenuates cardiac hypertrophy and prevents cellular remodeling in heart failure.

OBJECTIVE: In patients with heart disease, the transition from compensatory hypertrophy to heart failure (HF) is associated with altered calcium handling. Up-regulated Na(+)/H(+)-exchanger (NHE-1) activity underlies increased [Na(+)](i) and disturbance of cellular calcium handling in HF. We hypothesize that chronic inhibition of NHE-1 activity prevents the hypertrophic response, cellular remodeling, and development of HF. METHODS: Rabbits received a control or cariporide (inhibitor of NHE-1) diet for 3 months, starting after induction of combined volume and pressure overload. Age-matched animals served as control. Development of HF was examined echocardiographically and electrocardiographically after 3 months. [Na(+)](i), [Ca(2+)](i), pH(i), and action potentials were measured in left ventricular midmural myocytes with SBFI, indo-1, SNARF, and di-4-anepps. Sarcoplasmic reticulum calcium content was calculated from the response of [Ca(2+)](i) to rapid cooling. Calcium after-transients were elicited by cessation of rapid stimulation (3 Hz) in the presence of 100 nmol/l noradrenalin. RESULTS: Chronic treatment of rabbits with the specific Na(+)/H(+)-exchanger activity inhibitor cariporide greatly attenuated development of hypertrophy and entirely abolished development of HF; the heart/body weight ratio increased only little, no change in lung weight occurred, left ventricular dimensions and fractional shortening changed mildly, ascites was not present, QT duration did not increase, and sudden death did not occur. Chronic cariporide treatment also prevented cellular electrical and ionic remodeling. Myocyte dimensions were unaltered, action potentials were not prolonged, cytoplasmic sodium and NHE-1 activity did not increase, cytoplasmic and SR calcium handling remained undisturbed, and no increase of the incidence of calcium after-transient dependent delayed after depolarizations (DADs) occurred. CONCLUSION: We conclude that enhanced activity of NHE-1 underlies cardiac cellular electrical and ionic remodeling in experimental heart failure, and that chronic dietary treatment with cariporide attenuates hypertrophy, development of HF, and cellular remodeling.

NHE-1 and NBC during pseudo-ischemia/reperfusion in rabbit ventricular myocytes.

Despite many studies into the pathophysiology of cardiac ischemia-reperfusion injury, a number of key details are as yet undisclosed. These include the timing and magnitude of the changes in both Na(+)/H(+) exchange (NHE-1) and Na(+) -- HCO(3)(-) -cotransport (NBC) transport rates. We fluorimetrically measured H(i)(+) fluxes (J(NHE-1) and J(NBC)) and Na(i)(+) fluxes in single contracting rabbit ventricular myocytes subjected to metabolic inhibition, pseudo-ischemia (i.e. metabolic inhibition and extracellular acidosis of 6.4), and pseudo-reperfusion. Metabolic inhibition and pseudo-ischemia inhibited NHE-1 by 43 +/- 3.1% and 91 +/- 3.6%, and NBC by 66 +/- 5.4% and 100%, respectively. Inhibition was due to both an acidic shift of the pH(i) at which NHE-1 and NBC become quiescent (set-point pH(i)) and a reduction of the steepness of the pH(i) -- H(i)(+) flux profiles. NHE-1 and NBC did not contribute to Na(i)(+) loading during metabolic inhibition (Na(i)(+) 18 +/- 1.7 mM) or pseudo-ischemia (Na(i)(+) 21 +/- 1.7 mM), because pH(i) acidified less than set-point pH(i)'s. Upon pseudo-reperfusion NBC recovered to 54 +/- 7.3% but NHE-1 to 193 +/- 11% of aerobic control flux, and set-point pH(i)'s returned to near neutral values. Metabolic inhibition and reperfusion caused an acid load of 18 +/- 3.2 mM H(+) 94% of which were extruded by the hyperactive NHE-1. At pseudo-reperfusion Na(i)(+) rose sharply to 31 +/- 5.8 mM within 1.5 min and that coincided with hypercontracture. Cariporide not only prevented the Na(i)(+) transient, but also inhibited pH(i) recovery and prevented hypercontracture. Our results are consistent with the view that NHE-1 is active during metabolic inhibition if, like in whole hearts, pH(i) is driven more acidic than NHE-1 set-point pH(i). Furthermore, either an acidic pH(i) or absence of additional Na(i)(+) loading during reperfusion, or both, limit ischemia-reperfusion injury.

Ca(2+)-activated Cl(-) current in rabbit sinoatrial node cells.

The Ca(2+)-activated Cl(-) current (I(Cl(Ca))) has been identified in atrial, Purkinje and ventricular cells, where it plays a substantial role in phase-1 repolarization and delayed after-depolarizations. In sinoatrial (SA) node cells, however, the presence and functional role of I(Cl(Ca)) is unknown. In the present study we address this issue using perforated patch-clamp methodology and computer simulations. Single SA node cells were enzymatically isolated from rabbit hearts. I(Cl(Ca)) was measured, using the perforated patch-clamp technique, as the current sensitive to the anion blocker 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS). Voltage clamp experiments demonstrate the presence of I(Cl(Ca)) in one third of the spontaneously active SA node cells. The current was transient outward with a bell-shaped current-voltage relationship. Adrenoceptor stimulation with 1 microM noradrenaline doubled the I(Cl(Ca)) density. Action potential clamp measurements demonstrate that I(Cl(Ca)) is activate late during the action potential upstroke. Current clamp experiments show, both in the absence and presence of 1 microM noradrenaline, that blockade of I(Cl(Ca)) increases the action potential overshoot and duration, measured at 20 % repolarization. However, intrinsic interbeat interval, upstroke velocity, diastolic depolarization rate and the action potential duration measured at 50 and 90 % repolarization were not affected. Our experimental data are supported by computer simulations, which additionally demonstrate that I(Cl(Ca)) has a limited role in pacemaker synchronization or action potential conduction. In conclusion, I(Cl(Ca)) is present in one third of SA node cells and is activated during the pacemaker cycle. However, I(Cl(Ca)) does not modulate intrinsic interbeat interval, pacemaker synchronization or action potential conduction.