An interaction between ATP and high K+: mutual impairment of ATP- and high K(+)-evoked [Ca2+]i increase in NG 108-15 cells

The interaction between ATP- and high K(+)-evoked increase in intracellular free calcium concentration ([Ca2+]i) was investigated to gain an insight into the mechanism of interaction of ATP with voltage-sensitive calcium channels. [Ca2+]i was measured in the neuronal model, neuroblastoma x glioma hybrid cells (NG 108-15), using the fluorescence indicator fura-2. In the presence of 1.8 mM extracellular Ca2+, ATP induced a rapid, concentration-dependent increase in [Ca2+]i. High K+ (50 mM) evoked a [Ca2+]i rise from 109 +/- 11 nM to 387 +/- 81 nM (n = 16). The application of either of these two [Ca2+]i-increase provoking agents in sequence with the other caused impairment of the latter effect. The mutual desensitization of the responses to ATP and high K+ strongly suggests that both agents rely at least in part on the same source of Ca2+ for elevation of [Ca2+]i in NG 108-15 cells.

Specific beta(2)AR blocker ICI 118,551 actively decreases contraction through a G(i)-coupled form of the beta(2)AR in myocytes from failing human heart

BACKGROUND

We have observed direct (noncatecholamine-blocking) negative inotropic effects of the selective beta(2)-adrenoceptor (AR) antagonist ICI 118,551 in myocytes from failing human ventricle. In this study we characterize the effect in parallel in human myocytes and in myocytes from animal models where beta(2)ARs or G(i) proteins are overexpressed.

METHODS AND RESULTS

Enzymatically isolated, superfused ventricular myocytes were exposed to betaAR agonists and antagonists/inverse agonists, and contraction amplitude was measured. ICI 118,551 decreased contraction in ventricular myocytes from failing human hearts by 45.3+/-4.1% (n=20 hearts/31 myocytes, P<0.001) but had little effect in nonfailing hearts (4.9+/-4%, n=5 myocytes/3 hearts). Effects were significantly larger in patients classified as end-stage. Transgenic mice with high beta(2)AR number and increased G(i) levels had normal basal contractility but showed a similar negative inotropic response to ICI 118,551. Overexpression of human beta(2)AR in rabbit myocytes using adenovirus potentiated the negative inotropic effect of ICI 118,551. In human, rabbit, and mouse myocytes, the negative inotropic effects were blocked after treatment of cells with pertussis toxin to inactivate G(i), and overexpression of G(i)alpha(2) induced the effect de novo in normal rat myocytes.

CONCLUSIONS

We hypothesize that ICI 118,551 binding directs the beta(2)AR to a G(i)-coupled form and away from the G(s)-coupled form (ligand-directed trafficking). ICI 118,551 effectively acts as an agonist at the G(i)-coupled beta(2)AR, producing a direct negative inotropic effect. Conditions where beta(2)ARs are present and G(i) is raised (failing human heart, TGbeta(2) mouse heart) predispose to the appearance of the negative inotropic effect.

Physiological antagonism between ventricular beta 1-adrenoceptors and alpha 1-adrenoceptors but no evidence for beta 2- and beta 3-adrenoceptor function in murine heart

1. Murine left atrium lacks inotropic beta(2)-adrenoceptor function. We investigated whether beta(2)-adrenoceptors are involved in the cardiostimulant effects of (-)-adrenaline on spontaneously beating right atria and paced right ventricular myocardium of C57BL6 mice. We also studied a negative inotropic effect of (-)-adrenaline. 2. Sinoatrial tachycardia, evoked by (-)-adrenaline was resistant to blockade by beta(2)-selective ICI 118,551 (50 nM) but antagonized by beta(1)-selective CGP 20712A (300 nM). This pattern was unaffected by pretreatment with pertussis toxin (PTX, 600 microg kg(-1) i.p. 24 h) which reversed carbachol-evoked bradycardia to tachycardia. 3. Increases of ventricular force by (-)-adrenaline and (-)-noradrenaline were not blocked by ICI 118,551 but antagonized by CGP 20712A. 4. Under blockade of beta-adrenoceptors, (-)-adrenaline and (-)-noradrenaline depressed ventricular force (-logIC(50)M=7.7 and 6.9). The cardiodepressant effects of (-)-adrenaline were antagonized by phentolamine (1 microM) and prazosin (1 microM) but not by (-)-bupranolol (1 microM). Prazosin potentiated the positive inotropic effects of (-)-adrenaline (in the absence of beta-blockers) from -logEC(50)M=6.2 - 6.8. 5. PTX-treatment reduced carbachol-evoked depression of ventricular force in the presence of high catecholamine concentrations. Inhibition of ventricular function of G(i) protein was verified by 82% reduction of in vitro ADP-ribosylation. PTX-treatment tended to increase the positive inotropic potency of (-)-adrenaline under all conditions investigated, including the presence of ICI 118,551. 6. (-)-Adrenaline causes murine cardiostimulation through beta(1)-adrenoceptors but not through beta(2)-adrenoceptors. The negative inotropic effects of (-)-adrenaline are mediated through ventricular alpha(1)-adrenoceptors but not through beta(3)-adrenoceptors. Both G(i) protein and alpha(1)-adrenoceptors restrain (-)-adrenaline-evoked increases in right ventricular force mediated through beta(1)-adrenoceptors.

Cardiac tissue engineering

Recent progress in implantations of differentiated cardiac and non-cardiac cells as well as adult stem cells into the heart suggests that the irreversible loss of viable cardiac myocytes that occurs during myocardial infarction can be at least partly substituted. We evaluated an alternative approach by reconstituting cardiac tissue grafts in vitro and implanting them as spontaneously and coherently contracting tissues. For this purpose we have optimized a method to generate ring-shaped three-dimensional engineered heart tissue (EHT) in vitro from neonatal rat cardiac myocytes. When subjected to isometric force measurements in organ baths, electrically stimulated EHTs exhibit a Frank-Starling behavior, a positive inotropic response to increases in extracellular calcium, a positive inotropic and lusitropic response to isoprenaline, and a negative inotropic response to the muscarinic agonist carbachol ('accentuated antagonism'). Twitch tension under maximal calcium amounts to 1-2 mN/ mm2. Importantly, passive (resting) tension is low, yielding a ratio of active/passive tension of approximately 1.5 under basal and 14 under maximal calcium. Morphologically, EHTs represent a highly interconnected three-dimensional network of cardiac myocytes resembling loose cardiac tissue with a high fraction of binucleated cardiac myocytes, strong eosin staining and elongated centrally located nuclei. Electron microscopy demonstrated well developed sarcomeric structures, T-tubules, SR vesicles, T-tubule-SR-junctions, all types of intercellular connective structures, and a basement membrane. Thus, EHTs comprise functional and morphological properties of intact, ventricular myocardium. First implantation experiments of EHTs in the peritoneum of Fischer 344 rats showed that EHTs survived for at least 14 days, maintained a network of differentiated cardiac myocytes, and were strongly vascularized. Thus, EHTs may serve as material for a novel tissue replacement approach.

Human inward rectifier potassium channels in chronic and postoperative atrial fibrillation

OBJECTIVE

We showed recently that the 825T allele of the G-protein beta 3-subunit C825T polymorphism is associated with large inward rectifier K(+) currents I(K1) but low acetylcholine-activated K(+) current I(K,ACh) amplitudes. During chronic atrial fibrillation (AF), I(K1) and I(K,ACh) current densities were increased when compared to sinus rhythm (SR). It is unknown whether chronic AF and G beta 3 gene status are independent contributors to atrial K(+) current activity. We measured I(K1) and I(K,ACh) in tissue from AF patients with different G beta 3 genotypes and assessed the relation between the I(K1) and I(K,ACh) amplitudes and the incidence of postoperative AF.

METHODS

We measured the amplitudes of I(K1) and I(K,ACh) in atrial myocytes from 26 patients with sinus rhythm (SR) and from 16 patients with chronic AF (>6 months). The K(+) currents were measured with standard patch-clamp techniques. The G beta 3 gene status of the patients was determined by PCR and restriction analysis.

RESULTS

At -100 mV, the amplitude of I(K1) was larger in AF (10.9+/-1.0 pA/pF, n=49/16, cells/patients) than in SR (6.3+/-0.6 pA/pF, n=68/26, P<0.05), whereas the amplitude of I(K,ACh) was smaller in chronic AF (2.9+/-0.7 pA/pF, n=49/16) than in SR (6.3+/-0.7 pA/pF, n=68/26, P<0.05). These changes were independent of the patient G beta 3 gene status. Eight patients out of 26 in the SR group (31%) developed postoperative AF. When analysed based on incidence of postoperative AF, current amplitudes did not differ significantly.

CONCLUSION

We provide evidence for up-regulation of I(K1) but down-regulation of I(K,ACh) in chronic AF which are independent of G beta 3 gene status. Atrial myocytes from patients who are in SR but later develop postoperative AF have no manifestation of altered I(K1) and I(K,ACh) at the time of cardiac surgery. Our results suggest that the AF-related changes of I(K1) and I(K,ACh) may be a consequence of or a contributory factor to chronic AF.

Task Force on Sudden Cardiac Death, European Society of Cardiology

The European Society of Cardiology has convened a Task Force on Sudden Cardiac Death in order to provide a comprehensive, educational document on this important topic. The main document has been published in the European Heart Journal in August 2001. The Task Force has now summarized the most important clinical issues on sudden cardiac death and provided tables with recommendations for risk stratification and for prophylaxis of sudden cardiac death. The present recommendations are specifically intended to encourage the development and revision of national guidelines on prevention of sudden cardiac death. The common challenge for cardiologists, physicians of other medical specialties and health professionals throughout Europe is to realize the potential for sudden cardiac death prevention and to contribute to public health efforts to reduce its burden.

Molecular basis of downregulation of G-protein-coupled inward rectifying K(+) current (I(K,ACh) in chronic human atrial fibrillation: decrease in GIRK4 mRNA correlates with reduced I(K,ACh) and muscarinic receptor-mediated shortening of action potentials

BACKGROUND

Clinical and experimental evidence suggest that the parasympathetic nervous system is involved in the pathogenesis of atrial fibrillation (AF). However, it is unclear whether changes in G-protein-coupled inward rectifying K(+) current (I(K,ACh)) contribute to chronic AF.

METHODS AND RESULTS

In the present study, we used electrophysiological recordings and competitive reverse-transcription polymerase chain reaction to study changes in I(K,ACh) and the level of the I(K,ACh) GIRK4 subunit in isolated human atrial myocytes and the atrial tissue of 39 patients with sinus rhythm and 24 patients with chronic AF. The density of I(K,ACh) was approximately 50% smaller in myocytes from patients with AF compared with those in sinus rhythm, and this was accompanied by decreased levels of GIRK4 mRNA. The current density of the inward rectifying K(+) current (I(K1)) was 2-fold larger during AF than in sinus rhythm, in correspondence with an increase in Kir2.1 mRNA. The larger I(K1) in AF is consistent with more negative membrane potentials in right atrial trabeculae from AF patients. Moreover, action potential duration was reduced in AF, and the action potential shortening produced by muscarinic receptor stimulation was attenuated, indicating that the changes of I(K1) and I(K,ACh) were functionally relevant.

CONCLUSIONS

Chronic human AF induces transcriptionally mediated upregulation of I(K1) but downregulation of I(K,ACh) and attenuates the muscarinic receptor-mediated shortening of atrial action potentials. This suggests that atrial myocytes adapt to a chronically high rate by downregulating I(K,ACh) to counteract the shortening of the atrial effective refractory period due to electrical remodeling.

Three thiadiazinone derivatives, EMD 60417, EMD 66430, and EMD 66398, with class III antiarrhythmic activity but different electrophysiologic profiles

The thiadiazinone derivatives EMD 60417, EMD 66430, and EMD 66398 were developed as class III antiarrhythmic agents. Their chemical structure is closely related to that of their calcium-sensitizing congener [+]-EMD 60263, and EMD 66398 possesses the methylsulfonylaminobenzoyl moiety present in the prototypical IKr blocker E-4031. We compared the electrophysiologic effects of these compounds with standard drugs (almokalant, E-4031, quinidine) in cardiac myocytes from guinea-pig ventricle and human atrium by whole-cell patch-clamp technique. The test compounds' class III action, which is related to impairment of K+ channel function, was confirmed by action potential measurements. EMD 60417, EMD 66430, EMD 66398, and almokalant (1 microM each) reversibly prolonged the action potential duration in guinea-pig myocytes. In the same cells, the rapidly activating component IKr of the delayed rectifier K+ current, which has been defined by its sensitivity to E-4031, was reduced by EMD 60417, EMD 66430, EMD 66398, and almokalant. Inhibition of IKr was concentration-dependent as determined by attenuation of tail currents. The slowly activating component IKs of the delayed rectifier K+ current was not affected. The inward rectifier K+ current IK1 was not influenced at potentials close to the reversal potential. Transient and sustained outward K+ currents (Ito, Iso) measured in human atrial myocytes were not altered by any EMD compound. L-type Ca2+ current was hardly affected at concentrations of 1-10 microM, but sodium current was decreased. Action potential prolongation by EMD 60417, EMD 66430, and EMD 66398 is due to block of IKr. INa is inhibited at higher concentrations by EMD 66430 and EMD 60417. EMD 66398 is more potent and selective for IKr than EMD 60417 and EMD 66430, and thus resembles E-4031 in structure and function.

The hyperpolarization-activated current If in ventricular myocytes of non-transgenic and beta2-adrenoceptor overexpressing mice

In transgenic mice (TG4) overexpressing the human beta2-adrenoceptor (beta2-AR), unoccupied receptors are supposed to activate spontaneously the signalling cascade, leading to enhanced levels of cAMP. This second messenger shifts activation curves of the hyperpolarization-activated current If towards less negative potentials. Here, we characterize If of ventricular myocytes from non-transgenic littermate (LM) and TG4 mice and investigate whether If is modulated by spontaneous beta2-AR signalling. If was activated in whole-cell voltage-clamp experiments during test steps ranging from -65 mV to -135 mV (holding potential: -55 mV; 36 degrees C). In TG4 the maximum amplitude was fivefold larger than in LM myocytes (-1.10 +/- 0.11 pA/pF vs. -0.22 +/- 0.04 pA/pF at -135 mV), and the potential for half-maximum If current (VI0.5) was less negative (-100.5 +/- 1.0 mV in TG4 vs. -108.4 +/- 2.6 mV in LM). (-)-Isoproterenol (1 microM) shifted VI0.5 of LM myocytes by 10.4 mV towards less negative potentials but had no significant effect in TG4. However, the inverse beta2-AR agonist ICI 118,551 (300 nM) shifted VI0.5 of TG4 myocytes to values observed in LM under control conditions, suggesting a relation to spontaneously active beta2-ARs. Enhanced expression of hyperpolarization-activated and cyclic nucleotide gated channels (HCN) could contribute to increased maximum If amplitude in TG4 myocytes. Semi-quantitative RT-PCR analysis demonstrated a 1.8-fold elevation of HCN4 mRNA and no significant change for HCN2 mRNA in TG4 ventricle. Cardiac hypertrophy was not detected in TG4 mice investigated here. We conclude that spontaneous beta2-AR signalling in hearts of TG4 mice shifts If current-voltage relation towards less negative potentials. Increased maximum If amplitude in TG4 myocytes is in line with enhanced expression of HCN channels. Both mechanisms could contribute to larger inward current at physiological diastolic potentials.

Selectivity of blocking of low- versus high-voltage activated calcium currents by the dihydropyridine derivatives Bay E5759 and Bay A4339 in neuroblastoma--glioma NG 108-15 cells

Beneficial therapeutic effects of dihydropyridine derivatives in cardiovascular and neurological disorders are often associated with selective L-type Ca(2+)channel blockade. Here the new dihydropyridine derivatives Bay E5759 (1,4-dihydro-2,6-dimethyl-4-(3-nitrophenyl)-3,5-pyridinedicarboxylic acid ethyl-1-methylethyl ester) and Bay A4339 (1,4-dihydro-2,6-dimethyl-4-(3-nitrophenyl)-3,5-pyridinedicarboxylic acid dimethyl-ester) were tested for their potency and selectivity of blocking of Ba(2+)currents mediated by low-(LVACC)vs high-voltage activated Ca(2+)channels (HVACC) in neuroblastoma-glioma hybrid cells. Nisoldipine and mibefradil served as reference compounds. Bay E5759 and Bay A4339 blocked HVACC at low nanomolar concentrations, whereas LVACC was hardly reduced at up to 10 microM. The order of potency for blockade of HVACC was Bay E5759 (IC(50): 0.4 nM) > Bay A4339 (2.5 nM) approximately = nisoldipine (4 nM) >> mibefradil (3.8 microM). Thus Bay E5759 and Bay A4339 are highly potent and selective blockers of HVACC, presumably L-type Ca(2+)channels.

Autoantibodies against the beta1 adrenoceptor from patients with dilated cardiomyopathy prolong action potential duration and enhance contractility in isolated cardiomyocytes

Autoantibodies against the beta1-adrenoceptor (beta1-AAB) from patients with dilated cardiomyopathy (DCM) increase the beating frequency of cultured neonatal rat cardiomyocytes. This effect is accompanied by only a small increase in cAMP production. Here we have investigated whether beta1-AAB affect electrophysiological properties and cell shortening of isolated cardiomyocytes by interacting with the beta1-adrenoceptor. Beta1-AAB were obtained during immunoadsorption of patients with DCM and were used for experiments in isolated myocytes cultured from neonatal rat hearts, or freshly isolated from adult rat ventricles or from human right atria. The unselective beta -adrenoceptor agonist (-)-isoprenaline was studied for comparison. Immunoglobulin G (IgG) antibodies increased the spontaneous beating frequency of neonatal rat cardiomyocytes to a lesser degree than (-)-isoprenaline, but both effects were maximum and stable after 2 min. In rat ventricular and human atrial myocytes, IgG increased action potential duration (APD) in a concentration-dependent manner with larger effects on late than on early repolarization phases. Similar effects were obtained with purified beta1-AAB, whereas flow through of the chromatography column was ineffective. (-)-isoprenaline prolonged APD to the same extent during plateau and late phase of repolarization. beta1-AAB increased L-Type Ca2+ current in correspondence with the prolongation of APD. The effects of beta1-AAB and (-)-isoprenaline on APD were strongly attenuated after preincubation of the myocytes with the selective beta1-adrenoceptor antagonist (-)-bisoprolol. In addition, beta1-AAB increased cell shortening in ventricular myocytes from adult rat hearts. Beta1-AAB enhancing the beating frequency of cultured cardiomyocytes, increase L-Type Ca2+ current, APD and contractility in freshly isolated cardiomyocytes mediated via beta1-adrenoceptors. These effects may contribute to beta1-adrenoceptor-mediated cardiotoxicity in heart failure.

Murine ventricular L-type Ca(2+) current is enhanced by zinterol via beta(1)-adrenoceptors, and is reduced in TG4 mice overexpressing the human beta(2)-adrenoceptor

1. The functional coupling of beta(2)-adrenoceptors (beta(2)-ARs) to murine L-type Ca(2+) current (I(Ca(L))) was investigated with two different approaches. The beta(2)-AR signalling cascade was activated either with the beta(2)-AR selective agonist zinterol (myocytes from wild-type mice), or by spontaneously active, unoccupied beta(2)-ARs (myocytes from TG4 mice with 435 fold overexpression of human beta(2)-ARs). Ca(2+) and Ba(2+) currents were recorded in the whole-cell and cell-attached configuration of the patch-clamp technique, respectively. 2. Zinterol (10 microM) significantly increased I(Ca(L)) amplitude of wild-type myocytes by 19+/-5%, and this effect was markedly enhanced after inactivation of Gi-proteins with pertussis-toxin (PTX; 76+/-13% increase). However, the effect of zinterol was entirely mediated by the beta(1)-AR subtype, since it was blocked by the beta(1)-AR selective antagonist CGP 20712A (300 nM). The beta(2)-AR selective antagonist ICI 118,551 (50 nM) did not affect the response of I(Ca(L)) to zinterol. 3. In myocytes with beta(2)-AR overexpression I(Ca(L)) was not stimulated by the activated signalling cascade. On the contrary, I(Ca(L)) was lower in TG4 myocytes and a significant reduction of single-channel activity was identified as a reason for the lower whole-cell I(Ca(L)). The beta(2)-AR inverse agonist ICI 118,551 did not further decrease I(Ca(L)). PTX-treatment increased current amplitude to values found in control myocytes. 4. In conclusion, there is no evidence for beta(2)-AR mediated increases of I(Ca(L)) in wild-type mouse ventricular myocytes. Inactivation of Gi-proteins does not unmask beta(2)-AR responses to zinterol, but augments beta(1)-AR mediated increases of I(Ca(L)). In the mouse model of beta(2)-AR overexpression I(Ca(L)) is reduced due to tonic activation of Gi-proteins.

Isolation and characterisation of five neurotoxic and cardiotoxic polypeptides from the sea anemone Anthopleura elegantissima

Five toxins (APE 1 to APE 5) of the sea anemone species Anthopleura elegantissima (Brandt) have been isolated from a toxic by-product fraction of its concentrated crude watery-methanolic extract, prepared previously for the isolation of a neuropeptide (the head-activator) by Schaller and Bodenmüller (Proc. Natl. Acad. Sci. USA 78 (1981) 7000) from 200kg sea anemones. Toxin purification was performed by desalting of the starting material by dialysis (MWCO 3500) against distilled water, anion exchange chromatography on QAE-Sephadex A25 at pH 8, twice gel filtration on Sephadex G50 m, repeated chromatography on QAE-Sephadex at pH 10 and chromatography on the cation exchanger Fractogel EMD SO(3)(-)-650 M.Final purification of the toxins was achieved by HPLC on MN SP 250/10 Nucleosil 500-5 C(18) PPN and MN SP 250/21 Nucleosil 300-7 C(18). Each toxin was composed of at least two isotoxins of which APE 1-1, APE 1-2, APE 2-1, APE 2-2 and APE 5-3 were isolated in preparative scale. With exception of APE 5-3 the sequences of the isotoxins have been elucidated. They resemble the 47 residue type-I long polypeptide toxins native to Anemonia sulcata (Pennant). All isotoxins paralyse the shore crab (Carcinus maenas) by tetanic contractions after i.m. application. The toxins modify current passing through the fast Na(+) channel in neuroblastoma cells, leading to delayed and incomplete inactivation. APE 1-1, APE 2-1 and APE 5-3 produce a positive inotropic effect in mammalian heart muscle, although they differ in potency. The order of potency is APE 2-1>APE 1-1>APE 5-3 (i.e. threshold concentrations are 1, 10 and 300nM, respectively). In addition, they enhance the spontaneous beating frequency in isolated right atria (guinea pig). The most potent cardiotoxic isotoxin is APE 2-1, its sequence is identical with that of AP-C, a toxin isolated and characterised previously by Norton et al. (Drugs and Foods from the Sea, 1978, University of Oklahoma Press, p. 37-50).LD50 APE 2-1:1 micro g/kg b.w. C. maenas (i.m.). LD50 APE 1-1:10 microg/kg b.w. C. maenas (i. m.). LD50 APE 5-3:50 microg/kg b.w. C. maenas (i.m.).

Effects of azelastine on contractility, action potentials and L-type Ca(2+) current in guinea pig cardiac preparations

Azelastine is used for symptomatic relief of allergic rhinitis and asthma bronchiale. In vitro studies in smooth muscle cells from guinea pig trachea and ileum demonstrate that the drug blocks L-type Ca(2+) current (I(Ca, L)). However, for safety reasons, it is important to know whether azelastine also affects cardiac I(Ca, L) in therapeutically relevant concentrations. We have therefore studied the effects of azelastine on I(Ca, L) in guinea pig ventricular myocytes using standard whole-cell patch-clamp technique. Force of contraction and action potentials from isolated papillary muscles of the same species were also investigated at physiological temperature (36 degrees C). Azelastine (30 microM) significantly reduced force of contraction, shortened action potential duration, and depressed maximum upstroke velocity. I(Ca, L) was elicited by 200-ms-long clamp steps from -100 to 0 mV (one pulse every 3 s). Azelastine blocked I(Ca, L) reversibly and concentration-dependently with an IC(50) of 20.2+/-1.3 microM and a Hill coefficient of 1.1. At 10 microM, azelastine shifted steady-state inactivation by 5 mV (n=7) to more negative potentials. The time course of I(Ca, L) inactivation could be described by a double exponential function. Azelastine (10 microM) significantly shortened the slow inactivation time constant (tau(s)) from 54.2+/-2.8 ms under control conditions to 38.7+/-2.9 ms (n=16) in the presence of drug. Azelastine also reduced low-voltage-activated Ca(2+) currents with a similar IC(50) value (24 microM, at -35 mV). Since the therapeutic plasma concentrations are in the order of 10-100 nM, we conclude that azelastine does indeed affect also cardiac I(Ca, L), but the concentrations required are at least two orders of magnitude larger than those obtained during drug therapy.

Acute hemodynamic effects during immunoadsorption in patients with dilated cardiomyopathy positive for beta 1-adrenoceptor autoantibodies

In two recent studies immunoadsorption improved left ventricular function in patients with dilated cardiomyopathy (DCM) who were positive for cardiostimulatory autoantibodies against the human beta 1-adrenoceptor (beta 1-AAB). In this study we invasively measured acute hemodynamics during immunoadsorption. Three patients with DCM, who were positive for beta 1-AAB (NYHA-class III, left ventricular ejection fraction < 0.3), were treated with 1 immunoadsorption session daily on 5 consecutive days. Immunoadsorption induced a strong decrease of cardiac index, whereas systemic vascular resistance increased. Baseline levels were reached again several hours after therapy ended. We conclude that during immunoadsorption substantial hemodynamic changes occur in patients positive for beta 1-AAB.

Ascorbic acid-induced modulation of venous tone in humans

Ascorbic acid appears to have vasodilatory properties, but the underlying mechanisms are not well understood. The aims of this study were to define the acute effects of locally infused ascorbic acid in human veins and to explore underlying mechanisms by using pharmacological tools in vivo. Ascorbic acid was infused in dorsal hand veins submaximally preconstricted with the alpha(1)-adrenoceptor agonist phenylephrine or with prostaglandin F(2alpha) in 23 healthy male nonsmokers, and the venodilator response was measured. Ascorbic acid produced dose-dependent dilation with maximum reversal of constriction of 38+/-4% in phenylephrine-preconstricted veins and of 51+/-13% in prostaglandin F(2alpha)-preconstricted veins. Oral pretreatment with the cyclooxygenase inhibitor acetylsalicylic acid or local coinfusion of ascorbic acid and the nitric oxide synthase inhibitor N:(G)-monomethyl-L-arginine had no effect, but coinfusion of ascorbic acid and methylene blue (to inhibit cGMP generation) abolished venodilation. Coinfusion of ascorbic acid and the nonselective potassium channel blocker quinidine abolished venodilation, whereas the inhibitor of ATP-dependent potassium channels glibenclamide had no effect. In cultured bovine endothelial cells, ascorbic acid did not affect intracellular calcium concentration but blunted the response to ATP or digitonin exposure. Ascorbic acid, in millimolar concentrations, dilates human hand veins, presumably by activation of vascular smooth muscle potassium channels through cGMP. This activation is independent of eNOS-mediated nitric oxide synthesis and cyclooxygenase products and does not involve ATP-dependent potassium channels.

N-desethylamiodarone modulates intracellular calcium concentration in endothelial cells

The main in-vivo metabolite of amiodarone, N-desethylamiodarone (DEAM), possesses clinically relevant class-II antiarrhythmic and vasodilator activities. Vasodilation by DEAM is endothelium dependent and involves a sustained and biphasic increase in cytosolic free Ca2+ concentration ([Ca2+]i). The aims of this study were to explore the mechanisms mediating the DEAM-induced increase in [Ca2+]i in endothelial cells and to determine whether this increase in [Ca2+]i was associated with altered cell proliferation. Cultured bovine aortic endothelial cells were loaded with the Ca2+-sensitive fluorescent dye Fura-2/AM, and [Ca2+]i measured spectrofluorimetrically. DEAM increased [Ca2+]i concentration dependently (EC50 approximately 6 microM) both in the presence and absence of extracellular Ca2+. In the presence of extracellular Ca2+, the response of [Ca2+]i to DEAM (10 microM) consisted of an initial rise to a plateau followed by a second increase to micromolar levels. The initial plateau was reduced by the endoplasmic reticulum Ca2+-ATPase inhibitor thapsigargin (200 nM) and by the antioxidant ascorbic acid (100 microM). The initial rate of rise in [Ca2+]i was decreased by blocking mitochondrial Ca2+ release with cyclosporine A (1 microM). Under Ca2+-free conditions, the response of [Ca2+]i to DEAM (10 microM) was also biphasic, consisting of an initial transient peak and a second slow increase. When extracellular Ca2+ was restored, [Ca2+]i rose to micromolar concentrations. The initial peak was abolished by thapsigargin, but not altered by ascorbic acid or cyclosporine A. Both the second [Ca2+]i increase and that due to restoring extracellular Ca2+ were reduced by ascorbic acid but not affected by thapsigargin or cyclosporine A. The DEAM-induced generation of free radicals and sustained increase in [Ca2+]i might alter cell proliferation and endothelial cell proliferation was indeed concentration-dependently inhibited by DEAM (IC50 approximately 2.5 microM). In conclusion, the DEAM-induced [Ca2+]i increase in endothelial cells is due to Ca2+ influx from the extracellular space and to Ca2+ release from endoplasmic reticulum and mitochondria and involves enhanced generation of free radicals.

The effect of Gi-protein inactivation on basal, and beta(1)- and beta(2)AR-stimulated contraction of myocytes from transgenic mice overexpressing the beta(2)-adrenoceptor

The atria and ventricles of transgenic mice (TGbeta(2)) with cardiac overexpression of the human beta(2)-adrenoceptor (beta(2)AR) were initially reported to show maximum contractility in the absence of beta-AR stimulation. However, we have previously observed a different phenotype in these mice, with myocytes showing normal contractility but reduced betaAR responses. We have investigated the roles of cyclic AMP and Gi in basal and betaAR function in these myocytes. ICI 118,551 at inverse agonist concentrations decreased contraction by 32%. However, the cyclic AMP antagonist Rp-cAMPS had no effect on contraction in TGbeta(2) myocytes, indicating that there was no tonic influence of raised cyclic AMP. These findings cannot be explained by the proposed model for inverse agonism, where the activated receptor (R*) raises cyclic AMP levels and so increases contraction in the absence of agonist. After pertussis toxin (PTX) pretreatment to produce inactivation of Gi, the basal contraction in 1 mM Ca(2+) was increased in TGbeta(2) mice (7.82+/-0.47%, n=23) compared to LM mice (3.60+/-0.59%, n=11) (P<0.001). The contraction amplitude of myocytes to the maximal concentration of isoprenaline was also increased significantly by PTX in TGbeta(2) mice (9.40+/-1.22%, n=8) and was no longer reduced compared to LM mice (8.93+/-1.50%, n=11). Both beta(1)- and beta(2)AR subtypes were affected both by the original desensitization and by the resensitization with PTX. PTX treatment has therefore restored the original phenotype, with high basal contractility and little further effect of isoprenaline. We suggest that both beta-AR desensitization and lack of increased basal contraction in ventricular myocytes from our colony of TGbeta(2) mice were due to increased activity of PTX-sensitive G-proteins.

G-Protein beta(3)-subunit 825T allele is associated with enhanced human atrial inward rectifier potassium currents

BACKGROUND

A C825T polymorphism was recently identified in the human gene encoding for the beta(3)-subunit of heterotrimeric G proteins. The 825T allele is associated with a splice variant of Gbeta(3) and enhanced signal transduction. We hypothesized that patients carrying the 825T allele exhibit the modified Gbeta(3) phenotype. The resulting enhancement of signal transduction should be detectable in the Gbetagamma-dimer-mediated acetylcholine-stimulated K(+) current (I(K,ACh)).

METHODS AND RESULTS

Seventy patients undergoing cardiac surgery were genotyped for the C825T polymorphism. In right atrial myocytes from these patients, the inward rectifier K(+) currents (I(K1), I(K,ACh)) were studied with the whole-cell patch-clamp technique. Background current I(K1) was measured with depolarizing ramp pulses and quantified as inward current at -100 mV; mean amplitudes were (pA/pF) 4.98+/-0.49 (n=30/93 patients/cells) in patients with CC genotype, 4.25+/-0.36 (n=31/121 patients/cells) with TC, and 7. 46+/-1.14 (n=9/32 patients/cells; P<0.05) with TT. Conversely, mean I(K,ACh), which is maximally activated by carbachol (2 micromol/L), was reduced in patients with TT genotype (pA/pF, 4.30+/-1.33, n=9/27 patients/cells; P<0.05) compared with the other 2 groups (6.56+/-0. 54, n=30/80 and 6.16+/-0.45, n=31/117 patients/cells, for CC and TC genotype, respectively). Essentially similar results were obtained with adenosine (1 mmol/L).

CONCLUSIONS

We found an association between the Gbeta(3) 825T allele and amplitude of human atrial I(K1) and I(K,ACh). Increased background current density in TT carriers could shorten action potential duration and may be due to I(K,ACh) being constitutively active in this genotype.

Evidence for Edg-3 receptor-mediated activation of I(K.ACh) by sphingosine-1-phosphate in human atrial cardiomyocytes

Sphingosine-1-phosphate (SPP) and sphingosylphosphorylcholine (SPPC) have been reported to activate muscarinic receptor-activated inward rectifier K(+) current (I(K.ACh)) in cultured guinea pig atrial myocytes with similar nanomolar potency. Members of the endothelial differentiation gene (Edg) receptor family were recently identified as receptors for SPP; however, these receptors respond only to micromolar concentrations of SPPC. Here we investigated the sphingolipid-induced activation of I(K.ACh) in freshly isolated guinea pig, mouse, and human atrial myocytes. SPP activated I(K.ACh) in atrial myocytes from all three species with a similar nanomolar potency (EC(50) values: 4-8 nM). At these low concentrations, SPPC also activated I(K.ACh) in guinea pig myocytes. In contrast, SPPC was almost ineffective in mouse and human myocytes, thus resembling the pharmacology of the Edg receptors. Transcripts of Edg-1, Edg-3, and Edg-5 were detected in human atrial cells. Moreover, activation of I(K.ACh) by SPP was blocked by the Edg-3-selective antagonist suramin, which did not affect basal or carbachol-stimulated K(+) currents. In conclusion, these data indicate that I(K.ACh) activation by SPP and SPPC exhibits large species differences. Furthermore, they suggest that SPP-induced I(K.ACh) activation in human atrial myocytes is mediated by the Edg-3 subtype of SPP receptors.

T-Type and tetrodotoxin-sensitive Ca(2+) currents coexist in guinea pig ventricular myocytes and are both blocked by mibefradil

Under Na(+)-free conditions, low-voltage-activated Ca(2+) currents in cardiomyocytes from various species have been described either as Ni(2+)-sensitive T-type Ca(2+) current (I(Ca(T))) or as tetrodotoxin (TTX)-sensitive Ca(2+) current (I(Ca(TTX))). So far, coexistence of the 2 currents within the same type of myocyte has never been reported. We describe experimental conditions under which I(Ca(T)) and I(Ca(TTX)) can be separated and studied in the same cell. Rat and guinea pig ventricular myocytes were investigated with the whole-cell voltage-clamp technique in Na(+)-free solutions. Whereas rat myocytes lack I(Ca(T)) and exhibit I(Ca(TTX)) only, guinea pig myocytes possess both of these low-voltage-activated Ca(2+) currents, which are separated pharmacologically by superfusion with TTX or Ni(2+). I(Ca(T)) and I(Ca(TTX)) were of similar amplitude but significantly differed in their electrophysiological properties: I(Ca(TTX)) activated at more negative potentials than did I(Ca(T)), the potential for half-maximum steady-state inactivation was more negative, and current deactivation and recovery from inactivation were faster. I(Ca(TTX)) but not I(Ca(T)) increased after membrane rupture ("run-up"). Isolation of I(Ca(TTX)) by application of the bivalent cation Ni(2+) is critical because of possible shifts in voltage dependence. Therefore, we investigated whether the T-type Ca(2+) channel blocker mibefradil (10 micromol/L) is a suitable tool for the study of I(Ca(TTX)). However, mibefradil not only blocked I(Ca(T)) by 85+/-2% but also decreased I(Ca(TTX)) by 48+/-8%. We conclude that under Na(+)-free conditions I(Ca(T)) and I(Ca(TTX)) coexist in guinea pig ventricular myocytes and that both currents are sensitive to mibefradil. Future investigations of I(Ca(T)) will have to consider the TTX-sensitive current component to avoid possible interference.

Beta4-adrenoceptors are more effective than beta1-adrenoceptors in mediating arrhythmic Ca2+ transients in mouse ventricular myocytes

Putative beta4-adrenoceptors mediate cardiostimulation and arrhythmias in mammalian heart. Both beta1- and putative beta4-adrenoceptors mediate arrhythmias but through different mechanisms. To elucidate further the mechanisms of cardiostimulation and arrhythmias we measured Ca2+ transients and L-type Ca2+ currents in mouse ventricular myocytes. We used (-)-CGP 12177, an antagonist of beta1- and beta2-adrenoceptors with agonist properties at the putative beta4-adrenoceptor, and (-)-isoprenaline as an agonist for beta1- and beta2-adrenoceptors. (-)-CGP 12177 increased Ca2+ transients in electrically stimulated cells loaded with Indo-1. The maximum increase of Ca2+ transients caused by (-)-CGP 12177 amounted to approximately one-third of that caused by maximally effective (-)-isoprenaline concentrations. Both (-)-CGP 12177 and (-)-isoprenaline caused concentration-dependent arrhythmic Ca2+ transients. The arrhythmias appeared at paced Ca2+ transients and between paced Ca2+ transients. The arrhythmic potency of (-)-CGP 12177 (-logEC50=9.4) was approximately 40 times greater than that of (-)-isoprenaline (-logEC50=7.8). L-type Ca2+ current was measured in the whole cell configuration of the patch clamp technique. In the presence of both 3-isobutyl 1-methylxanthine (6 micromol/l) and (-)-propranolol (500 nmol/l), (-)-CGP 12177 (100 nmol/l) increased significantly L-type Ca2+ current by 19% of the effect of (-)-isoprenaline. The (-)-CGP 12177-evoked increase of Ca2+ transients contrasts with the smaller effects on L-type Ca2+ current, suggesting that activation of the putative beta4-adrenoceptor causes a more efficient Ca2+-induced Ca2+ release than activation of the beta1-adrenoceptor. Beta4-Adrenoceptors mediate arrhythmias with smaller Ca2+ transients and smaller increases of L-type Ca2+ current than beta1-adrenoceptors, in line with different but still unknown mechanisms as previously suggested for the intact heart.

Four different components contribute to outward current in rat ventricular myocytes

In rat ventricle, two Ca(2+)-insensitive components of K(+) current have been distinguished kinetically and pharmacologically, the transient, 4-aminopyridine (4-AP)-sensitive I(to) and the sustained, tetraethylammonium (TEA)-sensitive I(K). However, a much greater diversity of depolarization-activated K(+) channels has been reported on the level of mRNA and protein. In the search for electrophysiological evidence of further current components, the whole cell voltage-clamp technique was used to analyze steady-state inactivation of outward currents by conditioning potentials in a wide voltage range. Peak (I(peak)) and late (I(late)) currents during the test pulse were analyzed by Boltzmann curve fitting, producing three fractions each. Fractions a and b had different potentials of half-maximum inactivation (V(0.5)); the third residual fraction, r, did not inactivate. Fractions a for I(peak) and I(late) had similar relative amplitudes and V(0.5) values, whereas size and V(0.5) of fractions b differed significantly between I(peak) and I(late). Only b of I(peak) was transient, suggesting a relation with I(to), whereas a, b, and r of I(late) appeared to be three different sustained currents. Therefore, four individual outward current components were distinguished: I(to) (b of I(peak)), I(K) (a), the steady-state current I(ss) (r), and the novel current I(Kx) (b of I(late)). This was further supported by differential sensitivity to TEA, 4-AP, clofilium, quinidine, dendrotoxin, heteropodatoxin, and hanatoxin. With the exception of I(to), none of the currents exhibited a marked transmural gradient. Availability of I(K) was low at resting potential; nevertheless, I(K) contributed to action potential shortening in hyperpolarized subendocardial myocytes. In conclusion, on the basis of electrophysiological and pharmacological evidence, at least four components contribute to outward current in rat ventricular myocytes.

The effects of verapamil and diltiazem on N-, P- and Q-type calcium channels mediating dopamine release in rat striatum

1. The putative inhibitory effects of verapamil and diltiazem on neuronal non-L-type Ca2+ channels were studied by investigating their effects on either K+- or veratridine-evoked [3H]-dopamine ([3H]-DA) release in rat striatal slices. Involvement of N-, P- and Q-type channels was identified by sensitivity of [3H]-DA release to omega-conotoxin GVIA (omega-CTx-GVIA), omega-agatoxin IVA (omega-Aga-IVA) and omega-conotoxin MVIIC (omega-CTx-MVIIC), respectively. 2. KCl (50 mM)-evoked [3H]-DA release was abolished in the absence of Ca2+, and was insensitive to dihydropyridines (up to 30 microM). It was significantly blocked by omega-CTx-GVIA (1 microM), omega-Aga-IVA (30 nM) and was confirmed to be abolished by omega-CTx-MVIIC (3 microM), indicating involvement of N-, P- and Q-type channel subtypes. 3. Verapamil and diltiazem inhibited K+-evoked [3H]-DA release in a concentration-dependent manner. The inhibitory effects of verapamil or diltiazem (each 30 microM) were fully additive to the effect of omega-CTx-GVIA (1 microM), whereas co-application with omega-Aga-IVA (30 nM) produced similar effects to those of omega-Aga-IVA alone. 4. As shown previously, veratridine-evoked [3H]-DA release in Ca2+ containing medium exclusively involves Q-type Ca2+ channels. Here, diltiazem (30 microM) did not inhibit veratridine-evoked [3H]-DA release, whereas verapamil (30 microM) partially inhibited it, indicating possible involvement of Q-type channels in verapamil-induced inhibition. However, verapamil (30 microM) inhibited this release even in the absence of extracellular Ca2+, suggesting that Na+ rather than Q-type Ca2+ channels are involved. 5. Taken together, our results suggest that verapamil can block P- and at higher concentrations possibly N- and Q-type Ca2+ channels linked to [3H]-DA release, whereas diltiazem appears to block P-type Ca2+ channels only.

Voltage-sensitive Ca2+ channels, intracellular Ca2+ stores and Ca2+-release-activated Ca2+ channels contribute to the ATP-induced [Ca2+]i increase in differentiated neuroblastoma x glioma NG 108-15 cells

Activation of ionotropic P2X7 purinoreceptors in NG108-15 cells directly opens non-selective cation channels, leading to an increase in intracellular Ca2+ concentration ([Ca2+]i) and membrane depolarization and, hence, by indirect opening of voltage-stimulated Ca2+ channels (VSCC) to further increases of [Ca2+]i, whereas activation of the metabotropic P2Y receptor causes intracellular Ca2+ release. The quantitative contribution of Ca2+ entry and release to ATP-induced [Ca2+]i increase in differentiated NG108-15 cells is not known. Here we have investigated the Ca2+ influx and Ca2+ release components by studying [Ca2+]i in Fura-2-loaded cells and by using the following tools: nifedipine to block L-type VSCC, omega-conotoxin GVIa (omegaCT) to block N-type VSCC and thapsigargin to deplete intracellular Ca2+ stores. With 1.8 mM Ca2+ in the medium, ATP (600 microM) increased [Ca2+]i by 656 +/- 50 nM (n = 11). This response was reduced to 72% by nifedipine (50 microM), to 63% by omegaCT (1 microM), and to 31% by nifedipine and omegaCT in combination. Since nifedipine and omegaCT completely block VSCC in our model, the remaining 31% of [Ca2+]i increase could be caused by influx via P2X7-activated non-selective channels or by intracellular release mediated by P2Y receptors. When Ca2+-free medium was used to exclude Ca2+ influx, ATP (600 microM) increased [Ca2+]i by only 34 +/- 4 nM (n = 4), indicating that the majority of [Ca2+]i increase depends on Ca2+ influx. A similar rise by 37 +/- 4 nM (n = 4) was observed with the selective P2Y agonist UTP (150 microM). This small response was sensitive to thapsigargin and hence represents Ca2+ release. The remainder (i.e. total [Ca2+]i increase minus nifedipine-, omegaCT- and thapsigargin-sensitive [Ca2+]i increases) should, therefore, represent Ca2+ influx via P2X7 non-selective cation channels.

L-type calcium current and contractility in ventricular myocytes from mice overexpressing the cardiac beta 2-adrenoceptor

OBJECTIVES

The reported increase in basal activity of hearts from transgenic mice (TG4) overexpressing the human beta 2-adrenoceptor (beta 2-AR) was explained by spontaneously active beta 2-ARs that stimulate the beta-adrenergic cascade in the absence of an agonist. In order to examine altered myocardial function on a cellular level, we have investigated L-type calcium current (ICa,L) and cell shortening in ventricular myocytes from TG4 hearts. Myocytes from littermates (LM) and wild type animals (WT) served as controls.

METHODS

Cardiac beta-AR density was measured by [125I]-iodocyanopindolol binding to ventricular membranes. ICa,L was assessed by standard whole-cell voltage clamp technique. Contractility was measured as cell shortening in ventricular myocytes and as force of contraction in electrically stimulated left atria.

RESULTS

Overexpression of beta 2-ARs was confirmed by an almost 400-fold increase in beta-AR density. The beta 1:beta 2-AR ratio in WT mice was 71:29. Myocytes from TG4 and LM mice were similar in size as judged by membrane capacitance and two dimensional cell area. ICa,L amplitude was significantly lower in TG4 than in LM myocytes (with 2 mM [Ca2+]o -4.82 +/- 0.48 vs. -6.56 +/- 0.38 pA/pF, respectively). In TG4 myocytes, the ICa,L response to isoproterenol (1 microM) was almost abolished. Cell shortening was not different in physiological [Ca2+]o, but smaller in maximum [Ca2+]o when comparing TG4 to control myocytes. Basal force of contraction in left atria did not differ between TG4 and LM at any age investigated. In TG4 left atria the inotropic response to isoproterenol was also absent, whereas responses to high [Ca2+]o or dibutyryl-cAMP (1 mM) were present but reduced. The rate of spontaneous beating of right atria was elevated in TG4 mice.

CONCLUSIONS

Since only spontaneous beating rate but neither basal ICa,L amplitude nor basal contractile activity were elevated, our data fail to reveal evidence for spontaneously active, stimulating beta 2-ARs in left atrium and ventricle. A contractile deficit unrelated to the beta-adrenoceptor pathway is evident in TG4 myocytes and left atria.

Drugs preventing Na+ and Ca2+ overload

Cardiac intracellular Na+and Ca2+homeostasis is regulated by the concerted action of ion channels, pumps and exchangers. The Na+, K+-ATPase produces the electrochemical concentration gradient for Na+, which is the driving force for Ca2+removal from the cytosol via the Na+/Ca2+exchange. Reduction of this gradient by increased intracellular Na+concentration leads to cellular Ca2+overload resulting in arrhythmias and contractile dysfunction. Na+and Ca2+overload-associated arrhythmias can be produced experimentally by inhibition of Na+efflux (digitalis-induced intoxication) and by abnormal Na+influx via modulated Na+channels (veratridine, DPI 201-106; hypoxia) or via the Na+, H+exchanger. Theoretically, blockers of Na+and Ca2+channels, inhibitors of abnormal oscillatory release of Ca2+from internal stores or modulators of the Na+, Ca2+and Na+, H+exchanger activities could protect against cellular Na+and Ca2+overload. Three exemplary drugs that prevent Na+and Ca2+overload, i.e. the benzothiazolamine R56865, the methylenephenoxydioxy-derivative CP-060S, and the benzoyl-guanidine Hoe 642, a Na+, H+exchange blocker, are briefly reviewed with respect to their efficacy on digitalis-, veratridine- and ischaemia/reperfusion-induced arrhythmias.

Effects of the calcium sensitizer [+]-EMD 60263 and its enantiomer [-]-EMD 60264 on cardiac ionic currents of guinea pig and rat ventricular myocytes

The thiadiazinone enantiomers [+]-EMD 60263 and [-]-EMD 60264 ((+)-5-(1-(alpha-ethylimino-3,4-dimethoxybenzyl)-1,2,3,4-tetrah ydroquinoline-6-yl)-6-methyl-3,6-dihydro-2H-1,3,4-thiadiazine-2 -on) exhibit distinct stereoselectivity for Ca2+-sensitizing action ([+]-enantiomer) and phosphodiesterase inhibition ([-]-enantiomer). However, in isolated guinea pig papillary muscle, both compounds cause an action-potential prolongation that has been related to a nonselective depression of the delayed rectifier potassium current. Because [-]-EMD 60264 did not increase force of contraction despite phosphodiesterase inhibition, we postulated that one or several additional actions may oppose the anticipated positive inotropic effect. Therefore we investigated whether other membrane currents were also affected in voltage-clamped ventricular cardiomyocytes. Both [+]-EMD 60263 and [-]-EMD 60264 reduced sodium current as well as L-type calcium current in guinea pig ventricular myocytes, but steady-state inactivation or conductance curves of I(Na) and I(Ca) were not shifted along the voltage axis. Inward rectifier and transient outward current were studied in rat myocytes, but neither current was affected. We conclude that the positive inotropic action of [+]-EMD 60263 can be explained by prevalence of the Ca2+-sensitizing effect over its inhibitory actions on Na+ and Ca2+ current, whereas the negative inotropic effect of [-]-EMD 60264 may be caused by inhibition of I(Ca) predominating over PDE inhibition.

Electrophysiological aspects of changes in heart rate

Cardiac action potentials undergo characteristic changes in response to increasing pacing frequency, i.e., the resting potential becomes less negative (depolarization), the amplitude decreases, and the action potential duration (APD) shortens. The electrophysiological properties of the major cardiac inward and outward currents are discussed with respect to their possible contribution to rate-dependent changes in AP shape. Short diastolic intervals may not allow sufficient time for channel recovery. Incomplete recovery from inactivation will reduce both inward (INa, ICa) and outward (Ito) currents and hence lead to APD shortening or prolongation, respectively. Incomplete deactivation during diastole will increase current and, in the case of the delayed rectifier IKs, produces rate-dependent APD shortening. Heterogeneity in current density of myocytes from various regions within the ventricular wall complicates the direct translation of rate-dependent changes in APD into changes of the QT interval of the ECG. With increasing rates of stimulation, K+ accumulates within the tubular system. Membrane depolarization may be attributed to this frequency-dependent increase in [K+]o, whereas APD shortening is not mimicked by high [K+]o. Nevertheless, the various cardiac K+ channels differ in their sensitivity to extracellular [K+]o. Some of the frequency-dependent effects of drugs with class III action may be related to an influence of [K+]o on drug potency to block K+ channels.

Mechanism of block by tedisamil of transient outward current in human ventricular subepicardial myocytes

1. Tedisamil is a new antiarrhythmic drug with predominant class III action. The aim of the present study was to investigate the blocking pattern of the compound on the transient outward current (I(to)) in human subepicardial myocytes isolated from explanted left ventricles. Using the single electrode whole cell voltage clamp technique, I(to) was analysed after appropriate voltage inactivation of sodium current and block of calcium current. 2. Tedisamil reduced the amplitude of peak I(to), but did not affect the amplitude of non-inactivating outward current. The drug accelerated the apparent rate of I(to) inactivation. The reduction in time constant of I(to) inactivation depended on drug concentration, the apparent IC50 value was 4.4 microM. 3. Tedisamil affected I(to) amplitude in a use-dependent manner. After 2 min at -80 mV, maximum block of I(to) was reached after 4-5 clamp steps either at the frequency of 0.2 or 2 Hz, indicating that the block was not frequency-dependent in an experimentally relevant range. Recovery from block was very slow and proceeded with a time constant of 12.1+/-1.8 s. Also in the presence of drug, a fraction of channels recovered from inactivation with a similar time constant as in control myocytes (i.e. 81+/-40 ms and 51+/-8 ms, respectively, n.s.). 4. From the onset of fractional block of I(to) by tedisamil during the initial 60 ms of a clamp step, we calculated k1 = 9 x 10(6) mol(-1) s(-1) for the association rate constant, and k2 = 23 s(-1) for the dissociation rate constant. The resulting apparent KD was 2.6 microM and is similar to the IC50 value. 5. The effects of tedisamil on I(to) could be simulated by assuming a four state channel model where the drug binds to the channel in an open (activated) conformation. It is concluded that in human subepicardial myocytes tedisamil is an open channel blocker of I(to) and that this effect probably contributes to the antiarrhythmic potential of this drug.

Voltage-activated calcium channels involved in veratridine-evoked [3H]dopamine release in rat striatal slices

The present study explored the role of different sub-types of voltage-activated Ca2+ channels (VACCs) in mediating veratridine-evoked [3H]dopamine (DA) release from rat striatal slices. The release of [3H]DA evoked by veratridine (25 microM) decreased by 50.6+/-2.9% (n=8) in the absence of calcium and was completely abolished by 1 microM tetrodotoxin. The L-type Ca2+ channel blockers nifedipine (10 microM), nitrendipine (10 microM), diltiazem (10 microM) and verapamil (10 microM) did not modulate this release. Similarly, [3H]DA release was affected neither by the N-type VACC blocker omega-conotoxin-GVIA (1 microM) nor by the selective P-type channel blockers omega-agatoxin-IVA and omega-agatoxin-TK at low nM concentrations (30 nM), indicating no involvement of N- and P-type Ca2+ channels. In contrast, higher concentrations of omega-agatoxin-IVA that would also inhibit Q-type VACCs, blocked the release of [3H]DA by 27.9+/-8.1% (n=5) and 37.5+/-13.6% (n=3) at 0.3 and 1 microM, respectively. In addition, application of the Q-type Ca2+ channel blocker omega-conotoxin-MVIIC (0.01-3 degrees M) reduced [3H]DA release in a concentration-dependent manner, with maximum inhibition of 35.3+/-4.1% at 3 microM (n=5). On the basis of these results, it is concluded that the Ca2+ channels that participate in veratridine-evoked [3H]DA release are Q-type Ca2+ channels.

Guanine nucleotide-sensitive inhibition of L-type Ca2+ current by lysosphingolipids in RINm5F insulinoma cells

The lysosphingolipids sphingosine-1-phosphate (SPP) and sphingosylphosphorylcholine (SPPC) reportedly increase free cytosolic Ca2+ concentration ([Ca2+]i) in a variety of cell types, apparently by activating G protein-coupled plasma membrane receptors. We investigated whether and how sphingolipids modulate Ca2+ homeostasis in the insulinoma cell line RINm5F. The addition of SPPC and glucopsychosine (GPS) did not affect basal [Ca2+]i but inhibited the KCl (30 mM)-induced increase in [Ca2+]i in a pertussis toxin-insensitive and concentration-dependent manner (EC50 approximately 5 micro M). Similar inhibitory effects were observed with dihydro-SPPC and psychosine, whereas SPP and various N-acylated sphingolipids (at 10 micro M each) had little or no effect on the KCl-induced [Ca2+]i increase. Because in RINm5F cells the primary pathway for depolarization-induced [Ca2+]i increase are L-type Ca2+ channels, we studied whether sphingolipids reduce L-type Ca2+ current (ICa.L). When added to the bath, GPS and SPPC, but not SPP (10 micro M each), rapidly reduced maximal ICa.L by approximately 35%, similar to the alpha2-adrenoceptor agonist clonidine (30 micro M). However, when applied internally, GPS had no effect on ICa. L. When the electrode solution contained the stable GDP analog guanosine-5'-O-(2-thio)diphosphate (1 and 10 mM), the inhibitory effect of GPS was abolished. In conclusion, a novel cellular action of lysosphingolipids is observed in RINm5F cells (i.e., a guanine nucleotide-sensitive inhibition of L-type Ca2+ currents). The pharmacological profile of this inhibition is unique and unlike any known lysosphingolipid receptor-mediated action.

Characterization of excitation-contraction coupling in conscious dogs with pacing-induced heart failure

OBJECTIVE

In isolated cardiac preparations of non-failing hearts from different species, including man, there is a positive force-frequency relation which is reversed into a negative relation in preparation from failing hearts. Whether or not such relations between ventricular function and heart rate hold true in the in situ heart is not clear at present. Mechanical restitution and postextrasystolic potentiation might serve as alternative measures of excitation-contraction coupling.

METHODS

Eleven dogs were instrumented with a left ventricular micromanometer, ultrasonic crystals for the measurement of regional wall thickness, two hydraulic occluders around the descending aorta and the inferior caval vein, and left atrial and ventricular pacing leads with a subcutaneous pacemaker. Left ventricular dP/dtmax, as an isovolumic phase index, and systolic wall thickening, as an ejection phase index, were plotted versus heart rate, and heart rate was increased by left atrial pacing from rest to 200 min-1 in increments of 25 min-1. In a subset of dogs, left ventricular filling was controlled and the frequency range expanded by the bradycardic agent UL-FS 49. Measurements were performed in the presence and absence of autonomic blockade (hexamethonium, atropine). Mechanical restitution and postextrasystolic potentiation were determined as normalized dP/dtmax and systolic wall thickening, respectively, of the extra- and postextrasystolic beat versus defined variations of the extrasystolic time interval (250-550 ms). Following control studies, heart failure was induced by rapid left ventricular pacing at 250 min-1 for 20 days +/- 6 (SD) and measurements repeated. Isolated left ventricular trabeculae from non-failing and failing hearts were studied during stimulation at 0.2-4 Hz.

RESULTS

Only with filling control and in the absence of autonomic blockade, was there a slightly positive relation between dP/dtmax and heart rate in the control state. Otherwise, the relation of dP/dtmax to heart rate was flat both in the control state and in heart failure. The relation between systolic wall thickening and heart rate in the control state was negative, unless filling was controlled, and it was flat in heart failure. In contrast, the time constants of mechanical restitution and postextrasystolic potentiation were increased significantly with heart failure from 91 +/- 25 (SD) to 164 +/- 13 ms and from 107 +/- 18 to 156 +/- 4 ms, respectively, for dP/dtmax and from 76 +/- 22 to 162 +/- 10 ms and from 101 +/- 17 to 160 +/- 17 ms, respectively, for systolic wall thickening. These time constants were, however, insensitive to UL-FS 49 and autonomic blockade. There was a negative force-frequency relation in left ventricular trabeculae from non-failing hearts at higher calcium concentrations, where it was flat in trabeculae from failing hearts.

CONCLUSION

Time constants of mechanical restitution and postextrasystolic potentiation are more sensitive than the steady state relation of ventricular function and heart rate to characterize the impairment of excitation-contraction coupling in heart failure.

Cardiac troponin I plasma levels for diagnosis and quantitation of perioperative myocardial damage in patients undergoing coronary artery bypass surgery

OBJECTIVE

The definition of a reliable and generally accepted diagnostic standard for perioperative myocardial damage is desirable. Cardiac troponin I (cTnI) is highly specific for myocardial tissue and can be measured rapidly. The aim of our study was to evaluate the diagnostic potential of cTnI for myocardial lesions in patients undergoing coronary artery bypass surgery (CABG).

METHODS

A total of 119 patients with diffuse coronary artery disease were operated on using blood cardioplegia. Serial blood samples drawn before and after surgery were analyzed for the activity of creatine kinase MB isoenzyme (CKMB) and the concentrations of CKMB mass, cardiac troponins T and I. On the basis of the biochemical results (except cTnI) and the findings of electrocardiography/echocardiography, patients were classified and cTnI was studied for each group separately: group I, minor myocardial damage; group II, non-transmural infarction; group III, transmural infarction; and group IV, preoperative non-transmural infarction.

RESULTS

In 87 patients of group I (73.1%) cTnI levels remained low; 19 patients (16.0%) were assigned to group II, 8 patients (6.7%) to group III, and 5 patients (4.2%) to group IV. For discrimination of patients without and with perioperative myocardial infarction (PMI) by one cTnI determination, the use of cutoff values of 6.5 ng/ml at 8 h, 9.8 ng/ml at 12 h, and 11.6 ng/ml at 24 h after aortic unclamping resulted in a diagnostic efficiency of 88, 94 and 98%). Especially, a cTnI value at 24 h had a sensitivity of 100% and a specificity of 97%. Cardiac troponin levels at 24 h were found to correlate closely with the well-recognized 2-48 h area-under-the-curve (P < 0.0001; R = 0.993), making serial determinations unnecessary.

CONCLUSIONS

cTnI qualifies as a marker for diagnosis of PMI and quantitation of the amount of myocardial damage, because of the availability of a quick diagnostic test with high specificity, the high diagnostic efficiency, and especially the sufficient information gained by a single determination 24 h after aortic unclamping.

Mechanical restitution in atrial muscle from human and rat hearts: effects of agents that modify sarcoplasmic reticulum function

Force of contraction (Fc) of isolated human and rat atrial myocardium shows characteristic patterns of mechanical restitution when single test intervals are interposed in regular stimulation. With several pharmacological agents that modify the function of the sarcoplasmic reticulum we have investigated the role of the sarcoplasmic reticulum in mechanical restitution in these two species. Caffeine, thapsigargin and 2,5-di-(tert-butyl)-1,4-benzohydroquinone (BHQ) were used to reduce Ca2+ uptake, ryanodine to open Ca2+ release channels, and forskolin to stimulate Ca2+ uptake. Under control conditions, Fc recovered rapidly with test intervals shorter than steady-state, and was potentiated with longer than steady-state intervals. In human atrial tissue the maximum potentiation factor was 1.26 +/- 0.03 after a mean test interval of 9.70 +/- 1.55 s (n = 43) as compared to 3.07 +/- 0.45 after 30 sec. in rat atria (n = 48). Caffeine (3 mM) did not significantly affect steady-state Fc but abolished post-rest potentiation in human and rat preparations. Forskolin (1 microM) enhanced and accentuated the mechanical restitution curve in particular for short test intervals. In the presence of thapsigargin (10 microM), steady-state Fc and mechanical restitution could not be distinguished from time-matched controls exposed to solvent only, indicating that this agent is ineffective in human and rat atrial tissue. In contrast, the putative Ca2+ uptake inhibitor BHQ (100 microM) strongly reduced steady-state Fc and decreased potentiation at all intervals in human muscle, but shifted the mechanical restitution curve in parallel to lower values in rat atria. Ryanodine (10 nM) induced post-rest decay in human and depressed both steady-state Fc and post-rest potentiation in rat atrial muscle. From these results it is concluded that human and rat atrial muscle differ in the Ca2+ handling by the sarcoplasmic reticulum during mechanical restitution.

Stereoselectivity of actions of the calcium sensitizer [+]-EMD 60263 and its enantiomer [-]-EMD 60264

The thiadiazinone derivative [+]-EMD 60263 ((+)-5-(l-(alpha-ethylimino-3,4-dimethoxybenzyl)-1,2,3,4-tetrah ydroquinoline -6-yl)-6-methyl-3,6-dihydro-2H-1,3,4 -thiadiazine-2-on) is a Ca(2+)-sensitizing agent with only minor phosphodiesterase inhibitory activity. Our aim was to characterize the inotropic and electrophysiological effects of [+]-EMD 60263 and its enantiomer [-]-EMD 60264 in several cardiac muscle preparations. The Ca(2+)-sensitizing activity resided in the [+]-enantiomer only. [+]-EMD 60263 (3 microM) shifted the EC50 of Ca2+ for contractile activation of skinned fibers of pig heart from 2.41 microM to 0.73 microM, whereas [-]-EMD 60264 (30 microM) was ineffective. In Langendorff-perfused guinea pig hearts, [+]-EMD 60263 and [-]-EMD 60264 induced concentration-dependent positive and negative inotropic effects, respectively; both enantiomers reduced spontaneous heart rate but did not influence perfusion pressure. The maximum increase in force of human atrial trabeculae was 35% of pre-drug control with [+]-EMD 60263 in comparison to 113% with forskolin. In guinea-pig papillary muscles, [+]-EMD 60263 and [-]-EMD 60264 had opposite inotropic responses, however, both agents similarly prolonged action potential duration. Both enantiomers concentration-dependently blocked the rapidly activating component IKr of the delayed rectifier in guinea-pig myocytes. The block saturated at potentials positive to +30 mV, closely resembling the effects of the antiarrhythmic agent E-4031 which had been originally used to define IKr.

Tachycardia-induced failure alters contractile properties of canine ventricular myocytes

OBJECTIVE

Rapid cardiac pacing has been used as a model for experimentally-induced cardiomyopathy. However, its relevance to human heart failure is not clear at present because little is known about changes in size and function of ventricular myocytes. We have therefore studied the responses to graded increases in frequency and calcium in canine ventricular myocytes from failing hearts. The aim of our study was to evaluate the resemblance between canine pacing-induced and human end-stage heart failure.

METHODS

Myocytes were isolated from the left ventricular wall of dogs that were in heart failure after 6 weeks of pacing at 250 beats/min. Cell shortening was measured by edge detection.

RESULTS

Clinical signs of failure included dyspnea, ascites, and heart dilatation; the hemodynamic parameters were: LVdP/dtmax 1613 +/- 149 vs. 4713 +/- 304 mmHg/s in 6 control dogs; LVEDP 17.2 +/- 4.4 vs 5.6 +/- 1.1 mmHg; LV volume 60.5 +/- 6.2 vs. 30-35 ml. Myocytes from failing hearts were longer and thinner than those from controls (from factor: 0.40 +/- 0.01 vs. 0.47 +/- 0.01, P < 0.001, > 30 cells/heart). With 6 mM Ca2+ and at 0.5 Hz, contraction amplitude was significantly attenuated in myocytes from failing hearts: 6.6 +/- 0.9% cell shortening vs. 10.0 +/- 0.8% in controls (P < 0.05). This deficit was exacerbated at higher stimulation rates. Time-to-peak contraction and time-to-50% relaxation were not altered. There was no difference in sensitivity to thapsigargin.

CONCLUSION

As with cells from human failing hearts, contraction amplitude showed rate-dependent depression in this animal model, whereas features like slowing of contraction and relaxation and reduced sensitivity to thapsigargin, were not reproduced.

Electrophysiology of ion channels of the heart

The contribution of Na+, Ca2+, and various K+ currents to the shape of the cardiac action potential is outlined based on the relation between electrophysiological properties and structure of channel molecules. These currents have also been found in human ventricular myocytes, where the most prominent K+ current is a transient outward current that is not influenced by methylsulfonanilide antiarrhythmic drugs. Combined knowledge of electrophysiological and molecular properties of ion channels is likely to form the basis for rational design of future drugs.

Phosphodiesterase inhibition and Ca2+ sensitization

Inhibitors of phosphodiesterase type III (PDE III) enhance cardiac contractile force by elevating the intracellular calcium concentration [Ca2+]i by impairing cAMP degradation thus increasing cAMP levels. The drugs are more effective in healthy than in failing hearts since basal cAMP production is diminished in the latter. However, long term treatment with PDE-III inhibitors does not appear to be beneficial due to increased risk of potentially lethal arrhythmias caused by augmentation of [Ca2+]i[1). This risk should be absent in Ca2+ sensitizers. Recently, thiadiazinone derivatives have been synthetized in which the potency for Ca2+ sensitization is many-fold larger than the potency for PDE-III inhibition. The Ca(2+)-sensitizing action resides in the [+]-enantiomers, while the [-]-enantiomers show weak PDE-III inhibition. In the enantiomer pair [+]-EMD 60263 and [-]-EMD 60264, only the former concentration-dependently increased force of contraction in isolated cardiac preparations and myocytes. In the Langendorff-perfused guinea-pig heart, force was reversibly increased, whereas [-]-EMD 60264 even produced a negative inotropic response despite of its PDE inhibitory activity. Heart rate, however, was reduced by both enantiomers. Perfusion pressure remained unaffected. The effects were fully reversible upon wash-out of the enantiomers. [+]-EMD 60263 also enhanced cell shortening of human myocytes from both normal and failing hearts. In contrast to the opposite effects on contractility, both enantiomers prolong the action potential duration by blocking the rapidly activating component of the delayed rectifier K+ current. Thus they also possess class III antiarrhythmic activity. The therapeutic potential of these agents has yet to be assessed in clinical studies.

Selectivity of class-III antiarrhythmic action of clofilium in guinea pig ventricular myocytes

Clofilium is an antiarrhythmic agent with a supposedly predominant class-III action which is related to impairment of K+ channel function. We investigated membrane currents in cardiac myocytes isolated from guinea pig ventricle to evaluate the selectivity of action of clofilium on K+ currents. For measurement of action potentials or membrane currents, the single electrode patch clamp technique was applied in current- or voltage-clamp mode, respectively. Clofilium (30 mu M) irreversibly prolonged the action potential duration in guinea pig myocytes. In contrast, the concomitant reduction in plateau phase was completely reversible. The delayed rectifier K+ current Ikappa, was reduced. The rapidly activating component of Ikappa, which has been defined by its sensitivity to the compound E-4031, was also reduced by clofilium. The inward rectifier was slightly inhibited by the drug. Clofilium reversibly reduced L-type Ca2+ current. Sodium current was inhibited in a use-dependent manner. This effect was not reversible but proceeded after washout of the compound. Therefore, clofilium affects both inward and outward currents in mammalian cardiac myocytes in a similar concentration range. The effects on multiple membrane currents may contribute to the antiarrhythmic action of the drug.

Mechanical restitution and recirculation fraction in cardiac myocytes and left ventricular muscle of adult rats

Unloaded cell shortening was measured in electrically stimulated myocytes from adult rat hearts to compare the contractile response to stimulation with that in isometrically contracting left ventricular papillary muscles under similar experimental conditions, but preloaded to produce maximum twitch tension. Mechanical restitution in cells followed a biexponential function with time constants of 0.19 +/- 0.03 s and 36.4 +/- 10.2 s (7 cells from 5 hearts, n = 7/5). The time constants for papillary muscles were 0.58 +/- 0.05 s and 14.6 +/- 1.0 s (n = 6/6). In myocytes, maximum post-rest potentiation occurred after 30 to 60 s of rest. The potentiation after 60 s of rest was 2.48 +/- 0.31 times the steadystate in cells and 2.63 +/- 0.16 in papillary muscles. Recirculation fraction of C2+ as calculated from the decay of post-rest potentiation was 0.84 +/- 0.04 in single cells and 0.59 +/- 0.02 in papillary muscles (p < 0.005). Caffeine (3mM) abolished post-rest potentiation in both types of preparations. The numerical values for the time constants of mechanical restitution, potentiation factor and recirculation fraction in papillary muscles did not depend on preload. It is concluded that interval-dependent changes of contractility are preserved in single cardiac cells but the kinetics of decay of potentiation appear to have changed quantitatively.

Differences between outward currents of human atrial and subepicardial ventricular myocytes

1. Outward currents were studied in myocytes isolated from human atrial and subepicardial ventricular myocardium using the whole-cell voltage clamp technique at 22 degrees C. The Na+ current was inactivated with prepulses to -40 mV and the Ca2+ current was eliminated by both reducing extracellular [Ca2+] to 0.5 mM and addition of 100 microM CdCl2 to the bath solution. 2. In human myocytes, three different outward currents were observed. A slowly inactivating sustained outward current, I(so), was found in atrial but not ventricular myocytes. A rapidly inactivating outward current, I(to), of similar current density was observed in cells from the two tissues. An additional uncharacterized non-inactivating background current of similar size was observed in atrial and in ventricular myocytes. 3. I(to) and I(so) could be differentiated in atrial myocytes by their different kinetics and potential dependence of inactivation, and their different sensitivities to block by 4-amino-pyridine, suggesting that two individual channel types were involved. 4. In atrial cells, inactivation of I(to) was more rapid and steady-state inactivation occurred at more negative membrane potentials than in ventricular cells. Furthermore, the recovery of I(to) from inactivation was slower and without overshoot in atrial myocytes. In addition, 4-aminopyridine-induced block of I(to) was more efficient in atrial than in ventricular cells. These observations suggest that the channels responsible for atrial and ventricular I(to) were not identical. 5. We conclude that the differences in outward currents substantially contribute to the particular shapes of human atrial and ventricular action potentials. The existence of I(so) in atrial cells only provides a clinically interesting target for anti-arrhythmic drug action, since blockers of I(so) would selectively prolong the atrial refractory period, leaving ventricular refractoriness unaltered.

Post-rest potentiation and its decay after inotropic interventions in isolated rat heart muscle

The effects of various inotropic interventions on post-rest potentiation and its decay were investigated in isolated cardiac muscle. The inotropic interventions studied were: reduced extracellular Na+ and elevated extracellular Ca2+ concentration; exposure to ouabain, monensin, isoprenaline, phenylephrine and cirazoline. Force of contraction (stimulation frequency 2 Hz) was measured isometrically in left atria and right ventricular strips of rat hearts. Maximum post-rest potentiation was reached after 10 sec. of rest and amounted to 245 +/- 26% of pre-rest control in ventricle and 192 +/- 15% in atria. Ca(2+)-recirculation fraction was calculated from the decay of post-rest potentiation after resumption of regular stimulation, it was 0.77 +/- 0.01 in 11 control ventricular strips. High concentrations of caffeine (3 mmol/l) completely abolished post-rest potentiation in both tissues. The development of post-rest potentiation was accelerated in the presence of most of the inotropic agents. However, with the exception of ouabain and only in atrial muscle, none of the inotropic interventions produced higher post-rest contraction amplitudes than during controls. In rat heart muscle, the inotropic interventions studied are not any more effective in augmenting force of contraction than prolonged stimulation intervals. This suggests that (1) the distribution of Ca2+ into the sarcoplasmic reticulum is at a maximum during post-rest potentiation; (2) modifications of signal transduction pathways cannot further increase post-rest potentiation; and therefore that (3) shifts in Ca2+ distribution act as a limiting factor.