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

The use of inotropes for correcting hemodynamic dysfunction in patients with congestive heart failure has been described over many decades. Drugs such as cardiac glycosides, cathecolamines, phosphodiestherase inhibitors, and calcium sensitizers have been in turn proposed. However, the number of new chemical entities in this therapeutic field has been surprisingly low, and the current selection of drugs is limited. One of the paradigm shifts in the discovery for new inotropes was to focus on 'calcium sensitizers' instead of 'calcium mobilizers'. This was designed to lead to the development of safer inotropes, devoid of the complications that arise due to increased intracellular calcium levels. However, only three such calcium sensitizers have been fully developed over the latest 30 years. Moreover, two of these, levosimendan and pimobendan, have multiple molecular targets and other pharmacologic effects in addition to inotropy, such as peripheral vasodilation. More recently, omecamtiv mecarbil was described, which is believed to have a pure inotropy action that is devoid of pleiotropic effects. When the clinical data of these three calcium sensitizers are compared, it appears that the less pure inotropes have the cutting edge over the purer inotrope, due to additional effects during the treatment of a complex syndrome such as acute congested heart failure. This review aims to answer the question whether calcium sensitization per se is a sufficient strategy for bringing required clinical benefits to patients with heart failure. This review is dedicated to the memory of Heimo Haikala, a true and passionate innovator in this challenging field.

Comparison of the efficiency of Na+/Ca2+ exchanger or Na+/H+ exchanger inhibition and their combination in reducing coronary reperfusion-induced arrhythmias.

During ischaemia/reperfusion, the rise in [Na(+)](i), induced by simultaneous depression of the Na(+)/K(+)-ATPase and activation of the Na(+)/H(+) exchanger (NHE), shifts the Na(+)/Ca(2+) exchanger (NCX) into reverse transport mode, resulting in Ca(2+)(i)overload, which is a critical factor in enhancing the liability to cardiac arrhythmias. The inhibition of NHE, and recently NCX has been suggested to effectively protect the heart from reperfusion-induced arrhythmias. In this study, we investigated and compared the efficacy of individual or the simultaneous inhibition of the NHE and NCX against reperfusion-induced arrhythmias in Langendorff-perfused rat hearts by applying a commonly used regional ischaemia-reperfusion protocol. The NHE and NCX were inhibited by cariporide and SEA0400 or the novel, more selective ORM-10103, respectively. Arrhythmia diagrams calculated for the reperfusion period were analysed for the incidence and duration of extrasystoles (ESs), ventricular tachycardia (VT) and ventricular fibrillation (VF). NHE inhibition by cariporide was highly efficient in reducing the recorded reperfusion-induced arrhythmias. Following the application of SEA0400 or ORM-10103, the number and duration of arrhythmic periods were efficiently or moderately decreased. While both NCX inhibitors effectively reduced ESs, the most frequently triggered arrhythmias, they exerted limited or no effect on VTs and VFs. Of the NCX inhibitors, ORM-10103 was more effective. Surprisingly, the simultaneous inhibition of the NCX and NHE failed to significantly improve the antiarrhythmic efficacy reached by NCX blockade alone. In conclusion, although principal simultaneous NHE+NCX inhibition should be highly effective against all types of the recorded reperfusion-induced arrhythmias, NCX inhibitors, alone or in combination with cariporide, seem to be moderately suitable to provide satisfactory cardioprotection - at least in the present arrhythmia model. Since ORM-10103 and SEA0400 are known to effectively inhibit after-depolarisations, it is suggested that their efficacy and that of other NCX inhibitors may be higher and more pronounced in the predominantly Ca(2+)(i)-dependent triggered arrhythmias.

ORM-10103, a novel specific inhibitor of the Na+/Ca2+ exchanger, decreases early and delayed afterdepolarizations in the canine heart.

BACKGROUND AND PURPOSE: At present there are no small molecule inhibitors that show strong selectivity for the Na(+) /Ca(2+) exchanger (NCX). Hence, we studied the electrophysiological effects of acute administration of ORM-10103, a new NCX inhibitor, on the NCX and L-type Ca(2+) currents and on the formation of early and delayed afterdepolarizations. EXPERIMENTAL APPROACH: Ion currents were recorded by using a voltage clamp technique in canine single ventricular cells, and action potentials were obtained from canine and guinea pig ventricular preparations with the use of microelectrodes. KEY RESULTS: ORM-10103 significantly reduced both the inward and outward NCX currents. Even at a high concentration (10 µM), ORM-10103 did not significantly change the L-type Ca(2+) current or the maximum rate of depolarization (dV/dtmax ), indicative of the fast inward Na(+) current. At 10 µM ORM-10103 did not affect the amplitude or the dV/dtmax of the slow response action potentials recorded from guinea pig papillary muscles, which suggests it had no effect on the L-type Ca(2+) current. ORM-10103 did not influence the Na(+) /K(+) pump or the main K(+) currents of canine ventricular myocytes, except the rapid delayed rectifier K(+) current, which was slightly diminished by the drug at 3 µM. The amplitudes of pharmacologically- induced early and delayed afterdepolarizations were significantly decreased by ORM-10103 (3 and 10 µM) in a concentration-dependent manner. CONCLUSIONS AND IMPLICATIONS: ORM-10103 is a selective inhibitor of the NCX current and can abolish triggered arrhythmias. Hence, it has the potential to be used to prevent arrhythmogenic events.

Potassium channels sensitive to combination of charybdotoxin and apamin regulate the tone of diabetic isolated canine coronary arteries.

AIMS: Functional roles of calcium-activated potassium channels on the mechanical activity of epicardial coronary arteries obtained from a canine model of diabetes were investigated. METHODS: Coronary arteries were isolated from healthy, alloxan-diabetic and insulin-treated diabetic dogs. Basal tensions, contractions induced by the prostaglandin (PG) analogue, U46619, and endothelium-dependent relaxations to acetylcholine (ACh) were modified with charybdotoxin (CHTX) + apamin (APA), inhibitors of calcium-activated potassium channels, as well as with N(omega)-nitro-l-arginine (LNA) + indomethacin (INDO) to suppress the synthesis of nitric oxide (NO) and PGs. The relaxing effect of nitroprusside-sodium (SNP), an NO donor, was also determined. RESULTS: In diabetic coronary arteries, CHTX + APA did not change while LNA + INDO elevated the basal tension. PG-induced contractions were enhanced by CHTX + APA and by LNA + INDO in all the three groups of animals. CHTX + APA decreased the maximal relaxations to ACh in a partly insulin-dependent manner. LNA + INDO abolished the endothelium-dependent relaxations to ACh. In diabetic coronary arteries, the sensitivity to SNP-induced relaxation was enhanced, insulin independently, suggesting that NO could be partly responsible for maintaining intact ACh-induced vasorelaxation. CONCLUSION: In diabetic canine coronary artery, the vasomotor responses reflect up-regulation of calcium-activated potassium channels. This endothelial mechanism of the canine epicardial coronary artery may oppose vasoconstrictions in diabetic vascular tissue.

Role of slow delayed rectifier K+-current in QT prolongation in the alloxan-induced diabetic rabbit heart.

AIM: In diabetes mellitus, several cardiac electrophysiological parameters are known to be affected. In rodent experimental diabetes models, changes in these parameters were reported, but only limited relevant information is available in other species, having cardiac electrophysiological properties more resembling the human, including the rabbit. The present study was designed to analyse the effects of experimental type 1 diabetes on ventricular repolarization and the underlying transmembrane potassium currents in rabbit hearts. METHODS: Diabetes was induced by a single injection of alloxan (145 mg kg(-1) i.v.). After the development of diabetes (3 weeks), electrophysiological studies were performed using whole cell voltage clamp and ECG measurements. RESULTS: The QT(c) interval in diabetic rabbits was moderately but statistically significantly longer than measured in the control animals (155 +/- 1.8 ms vs. 145 +/- 2.8 ms, respectively, n = 9-10, P < 0.05). This QT(c)-lengthening effect of diabetes was accompanied by a significant reduction in the density of the slow delayed rectifier K(+) current, I(Ks) (from 1.48 +/- 0.35 to 0.86 +/- 0.17 pA pF(-1) at +50 mV, n = 19-21, P < 0.05) without changes in current kinetics. No differences were observed either in the density or in the kinetics of the inward rectifier K(+) current (I(K1)), the rapid delayed rectifier K(+) current (I(Kr)), the transient outward current (I(to)) and the L-type calcium current (I(CaL)) between the control and alloxan-treated rabbits. CONCLUSION: It is concluded that type 1 diabetes mellitus, although only moderately, lengthens ventricular repolarization. Diabetes attenuates the repolarization reserve by decreasing the density of I(Ks) current, and thereby may enhance the risk of sudden cardiac death.

Detection of nanobacteria-like particles in human atherosclerotic plaques.

Recent and historical evidence is consistent with the view that atherosclerosis is an infectious disease or microbial toxicosis impacted by genetics and behavior. Because small bacterial-like particles, also known as nanobacteria have been detected in kidney stones, kidney and liver cyst fluids, and can form a calcium apatite coat we posited that this agent is present in calcified human atherosclerotic plaques. Carotid and aortic atherosclerotic plaques and blood samples collected at autopsy were examined for nanobacteria-like structures by light microscopy (hematoxylin-eosin and a calcium-specific von Kossa staining), immuno-gold labeling for transmission electron microscopy (TEM) for specific nanobacterial antigens, and propagation from homogenized, filtered specimens in culture medium. Nanobacterial antigens were identified in situ by immuno-TEM in 9 of 14 plaque specimens, but none of the normal carotid or aortic tissue (5 specimens). Nanobacteria-like particles were propagated from 26 of 42 sclerotic aorta and carotid samples and were confirmed by dot immunoblot, light microscopy and TEM. [3H]L-aspartic acid was incorporated into high molecular weight compounds of demineralized particles. PCR amplification of 16S rDNA sequences from the particles was unsuccessful by traditional protocols. Identification of nanobacteria-like particles at the lesion supports, but does not by itself prove the hypothesis that these agents contribute to the pathogenesis of atherosclerosis, especially vascular calcifications.

Effect of the antifibrillatory compound tedisamil (KC-8857) on transmembrane currents in mammalian ventricular myocytes.

The cellular mechanism of action of tedisamil (KC-8857) (TED), a novel antiarrhythmic/antifibrillatory compound, was studied on transmembrane currents in guinea pig, rabbit and dog ventricular myocytes by applying the patch-clamp and the conventional microelectrode technique. In guinea pig myocytes the rapid component of the delayed rectifier potassium current (IKr) was largely diminished by 1 microM TED (from 0.88+/-0.17 to 0.23+/-0.07 pA/pF, n=5, p<0.05), while its slow component (IKs) was reduced only by 5 microM TED (from 8.1+/-0.3 to 4.23+/-0.07 pA/pF, n=5, p<0.05). TED did not significantly change the IKr and IKs kinetics. In rabbit myocytes 1 microM TED decreased the amplitude of the transient outward current (I(to)) from 20.3+/-4.9 to 13.9+/-2.8 pA/pF (n=5, p<0.05), accelerated its fast inactivation time constant from 8.3+/-0.6 to 3.5+/-0.5 ms (n=5, p<0.05) and reduced the ATP-activated potassium current (IKATP) from 38.2+/-11.8 to 18.4+/-4.7 pA/pF (activator: 50 microM cromakalim; n=5, p<0.05). In dog myocytes 2 microM TED blocked the fast sodium current (INa) with rapid onset and moderately slow offset kinetics, while the inward rectifier potassium (IK1), the inward calcium (ICa) and even the I(to) currents were not affected by TED in concentration as high as 10 microM. The differences in I(to) responsiveness between dog and rabbit are probably due to the different alpha-subunits of I(to) in these species. It is concluded that inhibition of several transmembrane currents, including IKr, IKs, I(to), IKATP and even INa, can contribute to the high antiarrhythmic/antifibrillatory potency of TED, underlying predominant Class III combined with I A/B type antiarrhythmic characteristics.

Comparison of regional vasomotor responses to acetazolamide and CO2 in rabbit cerebrum and cerebellum, measured by a hydrogen clearance method.

AIM: Many investigators have proved the usefulness of acetazolamide provocation and the carbon dioxide test for assessment of the local cerebrovascular reactivity by measurement of the regional cerebral blood flow in patients with occlusive cerebrovascular disease. Data originating from a comparison of these two different vasomotor stimuli as concerns the differences in sensitivity to them in various parts of the central nervous system are scarce. Our aim was to compare the cerebral blood flow responses to hypercapnic and acetazolamide stimuli in different brain regions. METHODS: The cerebral blood flow was measured in the cerebrum (cortex and caudate nucleus) and cerebellum (cortex), as measured by a hydrogen clearance method in anaesthetized, artificially ventilated rabbits. RESULTS: In normocapnia, the cerebral blood flow values in the cerebrum and the cerebellum differed significantly. The cerebral blood flow responses to both vasodilatory stimuli were to be significantly higher in the cerebrum than in the cerebellum, but the relative increases, i.e. the mean relative reactivities, were similar in the different regions measured. CONCLUSION: The regional dissimilarity might explain to some extent the different sensitivities of the various brain areas to sudden blood pressure changes (infarction or haemorrhage). The results further suggest that heterogeneity in cerebrovascular reactivity should be considered in the assessment of vasoreactivity in patients with occlusive cerebrovascular disease. Since the comparison of the carbon dioxide and acetazolamide-induced cerebrovascular reactivities revealed a strong linear relationship, it was concluded that acetazolamide provocation is equivalent to the carbon dioxide test in the evaluation of cerebrovascular reactivity.

Role of calcium-activated potassium channels in the regulation of basal and agonist-elevated tones in isolated conduit arteries. Short communication.

Functional role of calcium-activated potassium (KCa) channels on the basal and agonist-elevated arterial tones was investigated in isolated rabbit aorta, porcine and canine coronary arteries as well as in human internal mammary artery. The vascular tones enhanced by contractile agents were increased further by preincubation of these conduit blood vessels with selective (charybdotoxin or iberiotoxin) or nonselective (tetraethylammonium) inhibitors of KCa channels. The basal tone (without an agonist) was increased only in the canine coronary artery. The results indicate a feed-back regulatory role of KCa channels counteracting the vasospasm of conduit arteries.

Decreased cutaneous vasodilatation to isometric handgrip exercise in Alzheimer's disease.

BACKGROUND: Cutaneous active vasodilatation is a cholinergic nerve mediated function of the sympathetic nervous system and the disturbed function of cholinergic neurotransmission is known as a prominent feature of Alzheimer's disease (AD). METHODS: To assess this relationship, skin blood flow (SkBF) and other haemodynamic parameters were determined by a simple vasodilatory test, isometric handgrip exercise (IHG), in 22 late-onset sporadic type AD and 20 aged control persons (AC). RESULTS: Significantly higher cutaneous vascular resistance and decreased SkBF were found after the stimulus in the AD group. A smaller reduction (p < 0.03) of R wave intervals on the electrocardiogram was observed in the AD group compared to the AC one. After IHG, change in systolic blood pressure was less in the AD (p < 0.01) than in the AC group. CONCLUSION: Our results suggest that autonomic dysfunction affecting active vasodilator sympathetic, as well as parasympathetic functions is present in AD.

The cellular electrophysiologic effect of a new amiodarone like antiarrhythmic drug GYKI 16638 in undiseased human ventricular muscle: comparison with sotalol and mexiletine.

The cellular electrophysiologic effect of GYKI 16638, a new antiarrhythmic compound was studied and compared with that of sotalol and mexiletine in undiseased human right ventricular muscle preparation by applying the conventional microelectrode technique. GYKI 16638 (5 microM), at stimulation cycle length of 1000 ms, lengthened action potential duration (APD(90)) from 338.9 +/- 28.6 ms to 385.4 +/- 24 ms (n = 9, p > 0.05). This APD lengthening effect, unlike that of sotalol (30 microM), was rate-independent. GYKI 16638, contrary to sotalol and like mexiletine (10 microM), exerted a use-dependent depression of the maximal rate of depolarization (V(max)) which amounted to 36.4 +/- 11.7% at cycle length of 400 ms (n = 5, p < 0.05) and was characterised with an offset kinetical time constant of 298.6 +/- 70.2 ms. It was concluded that GYKI 16638 in human ventricular muscle shows combined Class IB and Class III antiarrhythmic properties, resembling the electrophysiological manifestation seen after chronic amiodarone treatment.