Calcium sensitizers: What have we learned over the last 25years?

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An exploratory conformational analysis of 3-mercapto-propanamide and 2-methyl-3-mercapto-propanamide as well as their S-deprotonated conjugate basis: An ab initio study

Ab initio conformational analysis has been carried out on 3-mercapto-propanamide, (R)- and (S)-2-methyl-3-mercaptopropanamide as well as their S-deprotonated conjugate basis. They were carried out at the HF/3-21G level of theory. The topology of the conformational potential energy surfaces and hypersurfaces have been analysed.Fil: Torday, László L.. Albert Szent-Györgyi Medical University; HungríaFil: Penke, Botond. Albert Szent-Györgyi Medical University; HungríaFil: Zamarbide, Graciela Nidia. Universidad Nacional de San Luis; ArgentinaFil: Enriz, Ricardo Daniel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - San Luis. Instituto Multidisciplinario de Investigaciones Biológicas de San Luis. Universidad Nacional de San Luis. Facultad de Ciencias Físico Matemáticas y Naturales. Instituto Multidisciplinario de Investigaciones Biológicas de San Luis; ArgentinaFil: Papp, Julius Gy. Albert Szent-Györgyi Medical University; Hungrí

The reverse mode of the Na+^+/Ca2+^{2+} exchanger contributes to the pacemaker mechanism in rabbit sinus node cells

Sinus node (SN) pacemaking is based on a coupling between surface membrane ion-channels and intracellular Ca$^{2+}$-handling. The fundamental role of the inward Na$^+$/Ca$^{2+}$ exchanger (NCX) is firmly established. However, little is known about the reverse mode exchange. A simulation study attributed important role to reverse NCX activity, however experimental evidence is still missing. Whole-cell and perforated patch-clamp experiments were performed on rabbit SN cells supplemented with fluorescent Ca$^{2+}$-tracking. We established 2 and 8 mM pipette NaCl groups to suppress and enable reverse NCX. NCX was assessed by specific block with 1 μM ORM-10962. Mechanistic simulations were performed by Maltsev–Lakatta minimal computational SN model. Active reverse NCX resulted in larger Ca$^{2+}$-transient amplitude with larger SR Ca$^{2+}$-content. Spontaneous action potential (AP) frequency increased with 8 mM NaCl. When reverse NCX was facilitated by 1 μM strophantin the Ca$^{2+}$$_i$ and spontaneous rate increased. ORM-10962 applied prior to strophantin prevented Ca$^{2+}$$_i$ and AP cycle change. Computational simulations indicated gradually increasing reverse NCX current, Ca$^{2+}$$_i$ and heart rate with increasing Na$^+$$_i$. Our results provide further evidence for the role of reverse NCX in SN pacemaking. The reverse NCX activity may provide additional Ca$^{2+}$-influx that could increase SR Ca$^{2+}$-content, which consequently leads to enhanced pacemaking activity

Increased Short-Term Beat-to-Beat QT Interval Variability in Patients with Impaired Glucose Tolerance

Prediabetic states and diabetes are important risk factors for cardiovascular morbidity and mortality. Determination of short-term QT interval variability (STVQT) is a non-invasive method for assessment of proarrhythmic risk. The aim of the study was to evaluate the STVQT in patients with impaired glucose tolerance (IGT). 18 IGT patients [age: 63 +/- 11 years, body mass index (BMI): 31 +/- 6 kg/m2, fasting glucose: 6.0 +/- 0.4 mmol/l, 120 min postload glucose: 9.0 +/- 1.0 mmol/l, hemoglobin A1c (HbA1c): 5.9 +/- 0.4%; mean +/- SD] and 18 healthy controls (age: 56 +/- 9 years, BMI: 27 +/- 5 kg/m2, fasting glucose: 5.2 +/- 0.4 mmol/l, 120 min postload glucose: 5.5 +/- 1.3 mmol/l, HbA1c: 5.4 +/- 0.3%) were enrolled into the study. ECGs were recorded, processed, and analyzed off-line. The RR and QT intervals were expressed as the average of 30 consecutive beats, the temporal instability of beat-to-beat repolarization was characterized by calculating STVQT as follows: STVQT = Sigma|QTn + 1 - QTn| (30x radical2)-1. Autonomic function was assessed by means of standard cardiovascular reflex tests. There were no differences between IGT and control groups in QT (411 +/- 43 vs 402 +/- 39 ms) and QTc (431 +/- 25 vs 424 +/- 19 ms) intervals or QT dispersion (44 +/- 13 vs 42 +/- 17 ms). However, STVQT was significantly higher in IGT patients (5.0 +/- 0.7 vs 3.7 +/- 0.7, P < 0.0001). The elevated temporal STVQT in patients with IGT may be an early indicator of increased instability of cardiac repolarization during prediabetic conditions

Novel Na+/Ca2+ Exchanger Inhibitor ORM-10962 Supports Coupled Function of Funny-Current and Na+/Ca2+ Exchanger in Pacemaking of Rabbit Sinus Node Tissue

Background and Purpose: The exact mechanism of spontaneous pacemaking is not fully understood. Recent results suggest tight cooperation between intracellular Ca$^{2+}$ handling and sarcolemmal ion channels. An important player of this crosstalk is the Na$^{+}$/ Ca$^{2+}$ exchanger (NCX), however, direct pharmacological evidence was unavailable so far because of the lack of a selective inhibitor. We investigated the role of the NCX current in pacemaking and analyzed the functional consequences of the I$_{f-}$NCX coupling by applying the novel selective NCX inhibitor ORM-10962 on the sinus node (SAN). Experimental Approach: Currents were measured by patch-clamp, Ca$^{2+}$-transients were monitored by fluorescent optical method in rabbit SAN cells. Action potentials (AP) were recorded from rabbit SAN tissue preparations. Mechanistic computational data were obtained using the Yaniv et al. SAN model. Key Results: ORM-10962 (ORM) marginally reduced the SAN pacemaking cycle length with a marked increase in the diastolic Ca$^{2+}$ level as well as the transient amplitude. The bradycardic effect of NCX inhibition was augmented when the funny-current (I$_{f}$) was previously inhibited and vice versa, the effect of I$_{f}$ was augmented when the Ca$^{2+}$ handling was suppressed. Conclusion and Implications: We confirmed the contribution of the NCX current to cardiac pacemaking using a novel NCX inhibitor. Our experimental and modeling data support a close cooperation between I$_{f}$ and NCX providing an important functional consequence: these currents together establish a strong depolarization capacity providing important safety factor for stable pacemaking. Thus, after individual inhibition of I$_{f}$ or NCX, excessive bradycardia or instability cannot be expected because each of these currents may compensate for the reduction of the other providing safe and rhythmic SAN pacemaking

The development of L-type Ca2+ current mediated alternans does not depend on the restitution slope in canine ventricular myocardium

Cardiac alternans have crucial importance in the onset of ventricular fibrillation. The early explanation for alternans development was the voltage‑driven mechanism, where the action potential (AP) restitution steepness was considered as crucial determining factor. Recent results suggest that restitution slope is an inadequate predictor for alternans development, but several studies still claim the role of membrane potential as underlying mechanism of alternans. These controversial data indicate that the relationship of restitution and alternans development is not completely understood. APs were measured by conventional microelectrode technique from canine right ventricular papillary muscles. Ionic currents combined with fluorescent measurements were recorded by patch‑clamp technique. APs combined with fluorescent measurements were monitored by sharp microelectrodes. Rapid pacing evoked restitution‑independent AP duration (APD) alternans. When non‑alternating AP voltage command was used, Ca2+i‑transient (CaT) alternans were not observed. When alternating rectangular voltage pulses were applied, CaT alternans were proportional to ICaL amplitude alternans. Selective ICaL inhibition did not influence the fast phase of APD restitution. In this study we found that ICaL has minor contribution in shaping the fast phase of restitution curve suggesting that ICaL—if it plays important role in the alternans mechanism—could be an additional factor that attenuates the reliability of APD restitution slope to predict alternans

Blockade of sodium‑calcium exchanger via ORM-10962 attenuates cardiac alternans

Repolarization alternans, a periodic oscillation of long-short action potential duration, is an important source of arrhythmogenic substrate, although the mechanisms driving it are insufficiently understood. Despite its relevance as an arrhythmia precursor, there are no successful therapies able to target it specifically. We hypothesized that blockade of the sodium‑calcium exchanger (NCX) could inhibit alternans. The effects of the selective NCX blocker ORM-10962 were evaluated on action potentials measured with microelectrodes from canine papillary muscle preparations, and calcium transients measured using Fluo4-AM from isolated ventricular myocytes paced to evoke alternans. Computer simulations were used to obtain insight into the drug's mechanisms of action. ORM-10962 attenuated cardiac alternans, both in action potential duration and calcium transient amplitude. Three morphological types of alternans were observed, with differential response to ORM-10962 with regards to APD alternans attenuation. Analysis of APD restitution indicates that calcium oscillations underlie alternans formation. Furthermore, ORM-10962 did not markedly alter APD restitution, but increased post-repolarization refractoriness, which may be mediated by indirectly reduced L-type calcium current. Computer simulations reproduced alternans attenuation via ORM-10962, suggesting that it is acts by reducing sarcoplasmic reticulum release refractoriness. This results from the ORM-10962-induced sodium‑calcium exchanger block accompanied by an indirect reduction in L-type calcium current. Using a computer model of a heart failure cell, we furthermore demonstrate that the anti-alternans effect holds also for this disease, in which the risk of alternans is elevated. Targeting NCX may therefore be a useful anti-arrhythmic strategy to specifically prevent calcium driven alternans

Discovery and Characterization of ORM‐11372, a Novel Inhibitor of the Sodium‐Calcium Exchanger with Positive Inotropic Activity

Background and purpose The lack of selective sodium‐calcium exchanger (NCX) inhibitors has hampered the exploration of physiological and pathophysiological roles of cardiac NCX 1.1. We aimed to discover more potent and selective drug like NCX 1.1. inhibitor. Experimental approach A flavan series‐based pharmacophore model was constructed. Virtual screening helped us identify a novel scaffold for NCX inhibition. A distinctively different NCX 1.1 inhibitor, ORM‐11372, was discovered after lead optimization. Its potency against human and rat NCX 1.1 and selectivity against other ion channels was assessed. The cardiovascular effects of ORM‐11372 were studied in normal and infarcted rats, and rabbits. Human cardiac safety was studied ex‐vivo using human ventricular trabeculae. Key results ORM‐11372 inhibited human NCX 1.1 reverse and forward currents; IC50 values were 5 and 6 nM, respectively. ORM‐11372 inhibited human cardiac sodium 1.5 (INa) and hERG KV11.1 currents (IhERG) in a concentration‐dependent manner; IC50 values were 23.2 and 10.0 μM. ORM‐11372 caused no changes in action potential duration; short term variability and triangulation were observed for concentrations of upto 10 μM. ORM‐11372 induced positive inotropic effects in 18 ± 6% and 35 ± 8% anesthetized rats with myocardial infarctions and rabbits, respectively; no other haemodynamic effects were observed, except improved relaxation at the lowest dose. Conclusion and implications ORM‐11372, a unique, novel, and potent inhibitor of human and rat NCX 1.1, is a positive inotropic compound. NCX inhibition can induce clinically relevant improvements in left ventricular contractions without affecting relaxation, heart rate, or blood pressure, without pro‐arrhythmic risk