Time for a Fully Integrated Nonclinical-Clinical Risk Assessment to Streamline QT Prolongation Liability Determinations: A Pharma Industry Perspective

Defining an appropriate and efficient assessment of drug‐induced corrected QT interval (QTc) prolongation (a surrogate marker of torsades de pointes arrhythmia) remains a concern of drug developers and regulators worldwide. In use for over 15 years, the nonclinical International Council for Harmonization of Technical Requirements for Pharmaceuticals for Human Use (ICH) S7B and clinical ICH E14 guidances describe three core assays (S7B: in vitro hERG current & in vivo QTc studies; E14: thorough QT study) that are used to assess the potential of drugs to cause delayed ventricular repolarization. Incorporating these assays during nonclinical or human testing of novel compounds has led to a low prevalence of QTc‐prolonging drugs in clinical trials and no new drugs having been removed from the marketplace due to unexpected QTc prolongation. Despite this success, nonclinical evaluations of delayed repolarization still minimally influence ICH E14‐based strategies for assessing clinical QTc prolongation and defining proarrhythmic risk. In particular, the value of ICH S7B‐based “double‐negative” nonclinical findings (low risk for hERG block and in vivo QTc prolongation at relevant clinical exposures) is underappreciated. These nonclinical data have additional value in assessing the risk of clinical QTc prolongation when clinical evaluations are limited by heart rate changes, low drug exposures, or high‐dose safety considerations. The time has come to meaningfully merge nonclinical and clinical data to enable a more comprehensive, but flexible, clinical risk assessment strategy for QTc monitoring discussed in updated ICH E14 Questions and Answers. Implementing a fully integrated nonclinical/clinical risk assessment for compounds with double‐negative nonclinical findings in the context of a low prevalence of clinical QTc prolongation would relieve the burden of unnecessary clinical QTc studies and streamline drug development.

Cross-site and cross-platform variability of automated patch clamp assessments of drug effects on human cardiac currents in recombinant cells

Automated patch clamp (APC) instruments enable efficient evaluation of electrophysiologic effects of drugs on human cardiac currents in heterologous expression systems. Differences in experimental protocols, instruments, and dissimilar site procedures affect the variability of IC50 values characterizing drug block potency. This impacts the utility of APC platforms for assessing a drug's cardiac safety margin. We determined variability of APC data from multiple sites that measured blocking potency of 12 blinded drugs (with different levels of proarrhythmic risk) against four human cardiac currents (hERG [IKr], hCav1.2 [L-Type ICa], peak hNav1.5, [Peak INa], late hNav1.5 [Late INa]) with recommended protocols (to minimize variance) using five APC platforms across 17 sites. IC50 variability (25/75 percentiles) differed for drugs and currents (e.g., 10.4-fold for dofetilide block of hERG current and 4-fold for mexiletine block of hNav1.5 current). Within-platform variance predominated for 4 of 12 hERG blocking drugs and 4 of 6 hNav1.5 blocking drugs. hERG and hNav1.5 block. Bland-Altman plots depicted varying agreement across APC platforms. A follow-up survey suggested multiple sources of experimental variability that could be further minimized by stricter adherence to standard protocols. Adoption of best practices would ensure less variable APC datasets and improved safety margins and proarrhythmic risk assessments.

Drug-induced functional cardiotoxicity screening in stem cell-derived human and mouse cardiomyocytes: effects of reference compounds

INTRODUCTION

Early prediction of drug-induced functional cardiotoxicity requires robust in-vitro systems suitable for medium/high throughput and easily accessible cardiomyocytes with defined reproducible properties. The xCELLigence Cardio system uses 96-well plates with interdigitated electrodes that detect the impedance changes of rhythmic contractions of stem cell-derived cardiomyocyte (SC-CM) layers. Here, we report on our initial screening experience in comparison to established (multi)cellular and in-vivo models.

METHODS

Impedance signals from human iPSC-CM (iCells™) and mouse eSC-CM (Cor.At™) were analyzed for contraction amplitude (CA) and duration, rise/fall time, beating rate (BR) and irregularity.

RESULTS

Following solution exchange, impedance signals re-approximated steady-state conditions after about 2 (Cor.At™) and 3h (iCells™); these time points were used to analyze drug effects. The solvent DMSO (≤1%) hardly influenced contraction parameters in Cor.At™, whereas in iCells™ DMSO (>0.1%) reduced CA and enhanced BR. The selective hERG K⁺ channel blockers E-4031 and dofetilide reduced CA and accelerated BR (≥30 nM) according to the analysis software. The latter, however, was due to burst-like contractions (300 nM) that could be detected only by visual inspection of recordings, and were more pronounced in Cor.At™ as in iCells™. In cardiac myocytes and tissue preparations, however, E4031 and dofetilide have been reported to increase cell shortening and contractile force and to reduce BR. Compounds (pentamidine, HMR1556, ATX2, TTX, and verapamil) with other mechanisms of action were also investigated; their effects differed partially between cell lines (e.g. TTX) and compared to established (multi)cellular models (e.g. HMR1556, ouabain).

CONCLUSION

Mouse and human stem cell-derived cardiomyocytes respond differently to drugs and these responses occasionally also differ from those originating from established in-vitro and in-vivo models. Hence, drug-induced cardiotoxic effects may be detected with this system, however, the predictive or even translational value of results is considered limited and not yet firmly established.

Field and action potential recordings in heart slices: correlation with established in vitro and in vivo models

BACKGROUND AND PURPOSE

Action potential (AP) recordings in ex vivo heart preparations constitute an important component of the preclinical cardiac safety assessment according to the ICH S7B guideline. Most AP measurement models are sensitive, predictive and informative but suffer from a low throughput. Here, effects of selected anti-arrhythmics (flecainide, quinidine, atenolol, sotalol, dofetilide, nifedipine, verapamil) on field/action potentials (FP/AP) of guinea pig and rabbit ventricular slices are presented and compared with data from established in vitro and in vivo models.

EXPERIMENTAL APPROACH

Data from measurements of membrane currents (hERG, I(Na) ), AP/FP (guinea pig and rabbit ventricular slices), AP (rabbit Purkinje fibre), haemodynamic/ECG parameters (conscious, telemetered dog) were collected, compared and correlated to complementary published data (focused literature search).

KEY RESULTS

The selected anti-arrhythmics, flecainide, quinidine, atenolol, sotalol, dofetilide, nifedipine and verapamil, influenced the shape of AP/FP of guinea pig and rabbit ventricular slices in a manner similar to that observed for rabbit PF. The findings obtained from slice preparations are in line with measurements of membrane currents in vitro, papillary muscle AP in vitro and haemodynamic/ECG parameters from conscious dogs in vivo, and were also corroborated by published data.

CONCLUSION AND IMPLICATIONS

FP and AP recordings from heart slices correlated well with established in vitro and in vivo models in terms of pharmacology and predictability. Heart slice preparations yield similar results as papillary muscle but offer enhanced throughput for mechanistic investigations and may substantially reduce the use of laboratory animals.

Safety pharmacology assessment of central nervous system function in juvenile and adult rats: effects of pharmacological reference compounds

INTRODUCTION

Recent EU/US pediatric legislation and FDA/EMEA guidelines recognize the potential differences in safety profiles of drugs in adults versus young patients. Hence safety studies are recommended to investigate key functional domains of e.g. the developing CNS.

METHODS

Selected psychoactive stimulants (caffeine, d-amphetamine, scopolamine) and depressants (baclofen, diazepam, haloperidol, chlorpromazine, imipramine, morphine) were characterized upon single administration with regard to behavioural parameters, locomotor activity, body temperature, pro-/anti-convulsive activity (pentylenetetrazole, PTZ), and nocifensive responses (hotplate) in neonatal (2 weeks), juvenile (4 weeks) and adult rats (8-9 weeks).

RESULTS

In vehicle-treated rats, behavioural patterns matured with age, locomotor activity and handling-induced rise in body temperature were enhanced, whereas PTZ convulsion threshold dose and nocifensive response latency decreased. Single test compound treatment elicited behavioural effects characteristic for psychoactive drugs with stimulating and depressing properties regardless of age. However, incidence of certain behaviours, and magnitude of effects on locomotor activity and body temperature varied with age and became generally more pronounced in adult rats. Pro-/anti-convulsive effects and delayed nocifensive responses did not differ between juvenile and adult rats.

CONCLUSION

CNS effects of selected psychoactive reference compounds were qualitatively similar, but quantitatively different in neonatal, juvenile and adult rats.

Suitability of commonly used excipients for electrophysiological in-vitro safety pharmacology assessment of effects on hERG potassium current and on rabbit Purkinje fiber action potential

INTRODUCTION

Regulatory guidelines require investigation of the liability for delayed ventricular repolarization by new chemical entities within a broad concentration range in-vitro. However, investigation can be limited by poor drug aqueous solubility, and by solvent physicochemical attributes that disrupt cell membrane integrity. Although excipients or solubilizing agents may aid to achieve the necessary high concentrations, no comprehensive overview on the suitability of solvents for in-vitro electrophysiological safety studies exists.

METHODS

Excipients were tested for potential interference with the hERG (human ether-a-go-go-related gene) K(+) current (whole-cell voltage-clamp, 23+/-2 degrees C), and the shape of rabbit Purkinje fiber action potentials (conventional glass microelectrode technique, 37+/-1 degrees C).

RESULTS AND DISCUSSION

Water-soluble complexation builders/carriers had little effect on hERG K(+) current at up to 50 mg/ml (BSA, bovine serum albumin) and 11 mg/ml (HP-beta-CD, hydroxypropyl-beta-cyclodextrin; IC(20), concentration of 20% inhibition). Water-soluble organic (co)solvents inhibited hERG K(+) currents (IC(20), %/mM): 0.7/152, ethanol; 0.9/67, Transcutol; 1.2/154, DMSO (dimethylsulfoxide); 1.6/389, acetonitrile; 1.9/48, polyethylene glycol 400; 2.1/660, methanol. Part of their inhibitory effect is attributed to the osmolality of extracellular solutions, because hERG IC(20) and extrapolated osmolality at the hERG IC(20) strongly correlate. Water-soluble non-ionic solubilizers/surfactants are potent inhibitors of hERG K(+) current with IC(20) concentrations of 0.07% (Cremophor EL) or lower (Tween 20, Tween 80: approximately 0.001%). Part of this inhibitory effect is attributed to their interaction with lipid membranes, because hERG inhibition occurs close to critical micelle concentrations (Cremophor, approximately 0.009%; Tween 20, approximately 0.007%). Purkinje fiber action potentials are little affected by HP-beta-CD at up to 2 mg/ml, while DMSO tends to shorten the action potential duration at 1%.

CONCLUSION

When conducting electrophysiological in-vitro assessments of drug effects, solubilizers/surfactants (Cremophor EL, Tween 20, Tween 80) should be avoided. Instead, water-soluble organic (co)solvents (methanol, acetonitrile, DMSO) or complexation builders/carriers (HP-beta-CD, BSA) appear to be more favorable.

Subsensitivity of P2X but not vanilloid 1 receptors in dorsal root ganglia of rats caused by cyclophosphamide cystitis

The application of cyclophosphamide to rats was used to induce interstitial cystitis. Behavioural studies indicated a strong pain reaction that developed within 2 h and levelled off thereafter causing a constant pain during the following 18 h. Neurons prepared from L6/S1 dorsal root ganglia innervating the urinary bladder responded to the application of capsaicin or alpha,beta-methylene ATP (alpha,beta-meATP) with an increase of intracellular Ca2+ ([Ca2+]i). The [Ca2+]i responses to capsaicin were identical in the dorsal root ganglion cells of cyclophosphamide- and saline-treated rats, whereas alpha,beta-meATP induced less increase in [Ca2+]i in the cyclophosphamide-treated animals than in their saline-treated counterparts. Hence, alpha,beta-meATP-sensitive P2X3 and/or P2X2/3 receptors of L6/S1 dorsal root ganglion neurons were functionally downregulated during subacute pain caused by experimental cystitis. In contrast, capsaicin-sensitive vanilloid 1 receptors did not react to the same procedure. Thoracal dorsal root ganglia, not innervating the urinary bladder, were also unaltered in their responsiveness to alpha,beta-meATP by cyclophosphamide treatment.

Changes in morphology and inward rectifier currents in human atrial myocytes depend on culture conditions

Human atrial myocytes were cultured under systematically varied conditions in order to obtain stable cells for future gene manipulation. Transient (I(to)) and sustained outward current (I(so)), and voltage- and muscarinic receptor-activated inward rectifier K(+) currents (I(K1), I(K,ACh)) were measured in freshly isolated cells and after 5 days in culture. Myocytes were grown on polylysin or laminin in medium with or without 10 % serum (medium+S, medium-S). Cultured myocytes dedifferentiated to a greater extent in medium+S than medium-S, but independent of the chemical nature of the adherence surface. Apparent surface area increased in medium+S, whereas membrane capacitance declined under all culture conditions. I(to) of myocytes cultured in medium-S was increased. Myocytes grown on polylysin and laminin exhibited reduced I(K1) current density. Under all culture conditions, I(K,ACh) was attenuated with carbachol but hardly affected with sphingosine-1-phosphate as agonists. In conclusion, morphological and electrophysiological changes depended on serum in the culture medium rather than on adherence surface being coated with laminin or polylysin.

Microvascular endothelial cells from human omentum lack an inward rectifier K+ current

In most macrovascular endothelial cell (EC) preparations, resting membrane potential is determined by the inwardly rectifying K+ current (I(K1)), whereas in microvascular EC the presence of I(K1) varies markedly. Cultured microvascular EC from small vessels of human omentum were examined by means of the voltage-clamp technique to elucidate the putative role of I(K1) in maintaining resting membrane potential. Macrovascular EC from human iliac artery and bovine aorta served as reference. Human omentum EC showed an outwardly rectifying current-voltage relation. Inward current was hardly sensitive to variations of extracellular [K+] and Ba2+ block suggesting lack of I(K1). However, substitution of extracellular [Na+] and/or [Cl-] affected the current-voltage relation indicating that Na+ and Cl- contribute to basal current. Furthermore, outward current was reduced by tetraethylammonium (10 mM), and cell-attached recordings suggested the presence of a Ca2+-activated K+ current. In contrast to human omentum EC, EC from human iliac artery and bovine aorta possessed inwardly rectifying currents which were sensitive to variations of extracellular [K+] and blocked by Ba2+. Thus, the lack of I(K1) in human omentum EC suggests that resting membrane potential is determined by Na+ and Cl- currents in addition to K+ outward currents.

The arginine-rich hexapeptide R4W2 is a stereoselective antagonist at the vanilloid receptor 1: a Ca2+ imaging study in adult rat dorsal root ganglion neurons

Vanilloid receptors (VR) integrate various painful stimuli, e.g., noxious heat, acidic pH, capsaicin, and resiniferatoxin (RTX). Although VR antagonists may be useful analgesics, the available agents capsazepine and ruthenium red lack the necessary potency and selectivity. Recently, submicromolar concentrations of the arginine-rich hexapeptide RRRRWW-NH(2) (R(4)W(2)) blocked VR-mediated ionic currents in a Xenopus expression system in a noncompetitive and nonstereoselective manner. Here, VR-antagonistic effects of L-R(4)W(2) and D-R(4)W(2), hexapeptides consisting entirely of L- and D-amino acids, were characterized in native adult rat dorsal root ganglion neurons using [Ca(2+)](i) imaging (Fura-2/acetoxymethyl ester). Fura-2 fluorescence ratio (R) was increased by RTX and capsaicin by 0.473 +/- 0.098 unit above basal levels of 0.903 +/- 0.011 (R(max), 2.289 +/- 0.031; R(min), 0.657 +/- 0.007) in a concentration-dependent manner (log EC(50): RTX, -10.04 +/- 0.05, n = 10; capsaicin, -6.60 +/- 0.10, n = 11). Agonist concentration-response curves were shifted to the right by L- and D-R(4)W(2) (0.1, 1, and 10 microM each) and by capsazepine (3, 10, 30, and 100 microM), whereas their maximal effects and slopes remained unaffected, indicating competitive antagonism. Schild analysis for L-R(4)W(2) yielded apparent dissociation constants of 4.0 nM (RTX) and 3.7 nM (capsaicin), and slopes smaller than unity (RTX, 0.38; capsaicin, 0.42). Apparent dissociation constants and slopes for D-R(4)W(2) and capsaicin were 153 nM and 0.67 versus 4.1 microM and 1.19 for capsazepine and capsaicin. Thus, VR-mediated effects in native dorsal root ganglion neurons were antagonized by L-R(4)W(2) > D-R(4)W(2) > capsazepine (order of potency). In conclusion, the R(4)W(2) hexapeptide is a potent, stereospecific, and (probably) competitive VR antagonist, although an allosteric interaction cannot be completely ruled out.

Sphingosylphosphorylcholine-biological functions and mechanisms of action

Compared to the lysophospholipid mediators, sphingosine-1-phosphate (S1P) and lysophosphatidic acid (LPA), little information is available regarding the molecular mechanisms of action, metabolism and physiological significance of the related sphingosylphosphorylcholine (SPC). S1P and LPA have recently been established as agonists at several G-protein-coupled receptors of the EDG family, S1P additionally serves an intracellular second messenger function. Several cellular effects of SPC can be explained by low-affinity binding to and activation of S1P-EDG receptors. However, certain cellular and subcellular actions of SPC are not shared by S1P, suggesting that SPC, which has been identified in normal blood plasma, ascites and various tissues, is a lipid mediator in its own right. This concept was corroborated by the recent discovery of specific high-affinity G-protein-coupled SPC receptors. In this article, our present knowledge on cellular actions and biological functions of SPC will be reviewed.

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.

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.

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.

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.

Local venous response to N-desethylamiodarone in humans

OBJECTIVE

Amiodarone, a class III antiarrhythmic agent, is a potent vasodilator in vivo. Its main metabolite, N-desethylamiodarone, contributes to the antiarrhythmic action of amiodarone after long-term treatment. It is unknown whether N-desethylamiodarone has acute vascular effects. The aim of this study was to explore the mechanism of action of N-desethylamiodarone in human hand veins.

METHODS

The dorsal hand vein compliance technique was applied in 36 healthy male volunteers. In hand veins preconstricted with the alpha1-adrenergic receptor agonist phenylephrine or prostaglandin F2alpha, N-desethylamiodarone and an inhibitor of nitric oxide formation (N(G)-monomethyl-L-arginine, L-NMMA) were infused in the presence or absence of a cyclooxygenase inhibitor (acetylsalicylic acid), and the venodilator effect was measured. Furthermore, N-desethylamiodarone was infused after oral treatment with hydrocortisone or coinfused with alpha-tocopherol. Additional experiments were carried out in bovine aortic endothelial cells to explore the effects of N-desethylamiodarone on the intracellular Ca2+ concentration ([Ca2+]i).

RESULTS

N-Desethylamiodarone produced dose-dependent venodilation (47% +/- 4% maximum). In vitro, 10 micromol/L N-desethylamiodarone caused a sustained increase of the endothelial [Ca2+]i. Pretreatment of the volunteers with acetylsalicylic acid reduced the maximum N-desethylamiodarone-induced venodilation to 22% +/- 8%; L-NMMA reduced the maximum N-desethylamiodarone-induced venodilation to 18% +/- 11%. Pretreatment with acetylsalicylic acid and coinfusion of N-desethylamiodarone and L-NMMA abolished the venodilation, whereas hydrocortisone had no effect. Coinfusion of alpha-tocopherol and N-desethylamiodarone reduced the maximum N-desethylamiodarone-induced venodilation to 11% +/- 4%.

CONCLUSIONS

In concentrations estimated to be in the therapeutic range, N-desethylamiodarone dilates preconstricted human hand veins in vivo and increases endothelial [Ca2+]i in vitro. Subsequently the cyclooxygenase (COX-1) and the endothelial nitric oxide synthase pathways are activated. The resulting venodilation does not involve inflammatory cytokines, inducible nitric oxide synthase, or inducible cyclooxygenase (COX-2).

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.

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.

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.

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.

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.

Calcium signalling by G protein-coupled sphingolipid receptors in bovine aortic endothelial cells

Besides its role as a putative second messenger releasing Ca2+ from intracellular stores, sphingosine-1-phosphate (SPP) has recently been identified as an extracellularly acting ligand activating a high affinity G protein-coupled membrane receptor in various cell types. Since SPP can be released from activated platelets, we examined in the present study whether endothelial cells express receptors for SPP and related sphingolipids. In bovine aortic endothelial cells loaded with fura-2, addition of SPP caused a rapid and transient increase in intracellular Ca2+ concentration ([Ca2+]i), amounting to maximally about 230 nM. Removal of extracellular Ca2+ revealed that SPP-induced [Ca2+]i elevations were due to both release of Ca2+ from intracellular stores and influx of extracellular Ca2+. Pretreatment of the cells with pertussis toxin inhibited the SPP-induced increase in [Ca2+]i by 83%, in line with the previously reported involvement of G proteins of the Gi/o family in SPP signalling in other cell types. In contrast to other [Ca2+]i-elevating agonists, e.g., ATP and bradykinin, SPP did not activate phospholipase C in bovine aortic endothelial cells, suggesting the involvement of a novel, unidentified signalling pathway in SPP-induced release of intracellular Ca2+. Furthermore, SPP also did not cause activation of either phospholipase D or A2. Out of various related sphingolipids studied, only sphingosylphosphorylcholine (SPPC) induced a similar maximal increase in [Ca2+]i as SPP, and its effect was also fully pertussis toxin-sensitive. However, the potencies of the two sphingolipids to increase [Ca2+]i differed by more than two orders of magnitude, with the EC50 values being 0.8 nM and 260 nM for SPP and SPPC, respectively. These results identify SPP and SPPC as novel and potent endothelial agonists, inducing calcium signalling by activation of a Gi/o protein-coupled receptor(s). Given the recently reported release of SPP from thrombin-activated platelets, SPP may represent a novel mediator of platelet-endothelial cell interactions.

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.

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.

Agonist-induced changes of [Ca2+]i and membrane currents in single bovine aortic endothelial cells

Cultured bovine aortic endothelial cells (BAECs) possess an inward rectifier K+ current (IK1), a Ca(2+)-activated K+ current, a nonselective cation current (INS), and a Ca(2+)-activated Cl- current; however, their relative roles remain to be established. In single BAECs, cytosolic free Ca2+ concentration ([Ca2+]i) [K5-fura 2 (50 microM), ratio 340/380 nm] was measured simultaneously with whole cell currents at 22 degrees C. Bradykinin (BK, 2 microM), ATP (10 microM), ionomycin (100 nM), or 2,5-di-(tert-butyl)-1,4-benzohydroquinone (10 microM) were used as agonists. In physiological salt solution (PSS), agonist exposure caused a rapid [Ca2+]i increase, followed by an increase in outward current (greater than -50 mV) and a smaller increase in inward current (greater than -80 mV). Chelation of [Ca2+]i with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid attenuated agonist-induced [Ca2+]i and current responses. Inactivation of the cyclooxygenase pathway by acetylsalicyclic acid and ibuprofen (50 microM each) did not affect the BK-induced [Ca2+]i transient but abolished the current response. In K(+)-free solution, agonist-stimulated outward currents (at +50 mV) were 10 times smaller than in PSS and were consistent with the activation of both INS and a Cl- current. In Cl(-)-free solution, the outward current response following agonist exposure was virtually abolished; at the same time, a linear inward current component with a reversal potential near the equilibrium potential for Na+ was activated. The maximal amplitude of the agonist-induced outward current decreased with decreasing symmetrical Cl- concentrations. Our results suggest that 1) IKI is the dominant current in resting BAECs; 2) K+, Cl-, and nonselective cation conductances contribute to the agonist-induced current response; and 3) most of the agonist-induced activation of currents depends on increased [Ca2+]i and is sensitive to cyclooxygenase inhibitors.

Coordinate depression of bradykinin receptor recycling and microtubule-dependent transport by taxol

Significant cardiovascular side effects have limited the use of taxol as an anticancer drug. A link between decreased plasma membrane dynamics and taxol has been implied because taxol can inhibit intracellular vesicle movements. Reduced membrane recycling caused by taxol could inhibit agonist-evoked Ca2+ signaling within endothelial cells, resulting in endothelium-dependent vasodilation. Bradykinin and ATP are two agonists that evoke Ca2+ transients in endothelial cells. Since the bradykinin receptor-agonist complex is internalized and recycled whereas the ATP agonist-receptor complex is not, we expected that a taxol inhibition of recycling would decrease bradykinin but not ATP receptor activity. We found that taxol depresses (i) the frequency (to 41% of control) and velocity (to 55% of control) of microtubule-dependent vesicle transport and (ii) bradykinin-evoked cytosolic Ca2+ transients (to 76% of control) in bovine aortic endothelial cells. In studying bradykinin receptor desensitization, which reflects receptor recycling, we demonstrate that taxol inhibits bradykinin-evoked Ca2+ transients by 50%. Taxol did not significantly alter ATP-evoked Ca2+ transients in either single-exposure or desensitization experiments. We suggest that taxol's reduction of bradykinin-evoked Ca2+ transients is due to altered microtubule-dependent membrane recycling. This report describes taxol's ability to alter plasma membrane composition through effects on vesicle transport and membrane trafficking pathways. This finding provides a possible mechanism by which taxol can substantially alter cardiovascular function.

Effects of the new phosphodiesterase-III inhibitor R80122 on contractility and calcium current in human cardiac tissue

The selective phosphodiesterase III (PDE-III) inhibitor R80122 ((E)-N-cyclohexal-N-methyl-2-[[[phenyl-(1,2,3,5- tetrahydro-2-oxoimidazo-[2,1b]-quinazolin-7-yl)-methylene]-a mino]-oxy]-acetamide) has been reported to possess greater cardiotonic potency and less side effects than the standard compounds milrinone or enoximone. To characterize this new compound further, we investigated the effects of R80122 on force of contraction (Fc) and calcium current (ICa) in human right atrium (HRA) and human left ventricle (HLV) with reference to the nonselective PDE-inhibitor IBMX (3-isobutyl-1-methylxanthine). With "late" exposure (300- to 330-min equilibration) of human atrial trabeculae, R80122 (3 microM) increased Fc by 60 +/- 11%; log EC50 was -6.2 +/- 0.1. R80122 (3 microM) induced a relative leftward shift of forskolin concentration-response curves by 0.34 log units; the respective value for IBMX (20 microM) was 0.46. A positive inotropic effect of R80122 was also shown in guinea pig papillary muscles. ICa was measured in voltage-clamped isolated myocytes of human right atrial and left ventricular (LV) tissue, and, for comparison, guinea pig ventricle. With clamp steps from -40 to +5 mV, R80122 (3 microM) increased peak ICa from 3.1 +/- 0.2 to 5.4 +/- 0.3 pA/pF in HRA cells, from 2.9 +/- 0.4 to 5.1 +/- 0.6 pA/pF in HLV cells, and from 4.4 +/- 0.3 to 6.6 +/- 0.5 pA/pF in guinea pig myocytes. IBMX 20 microM increased ICa to a greater extent. Washout or addition of carbachol 10 microM partially reversed the effect of R80122. Voltage dependence, inactivation time course, and steady-state inactivation of ICa were little changed by either compound. Stimulation of Ca2+ influx by L-type Ca2+ channels contributes to the positive inotropic effect of the selective PDE-III inhibitor R80122.

Effects of extracellular ATP on ICa, [Ca2+]i, and contraction in isolated ferret ventricular myocytes

The effects of extracellular ATP on the voltage-activated "L-type" Ca current (ICa), action potential, resting and transient intracellular Ca2+ levels, and cell contraction were examined in enzymatically isolated myocytes from the right ventricles of ferrets. With the use of the whole cell patch-clamp technique, extracellular ATP (10(-7) to 10(-3) M) inhibited ICa in a time- and concentration-dependent manner. ATP decreased the peak amplitude of ICa without altering the residual current at the end of 500-ms clamp steps. The concentration-response relationship for ATP inhibition of ICa was well described by a conventional Michaelis-Menten relationship with a half-maximal inhibitory concentration of 1 microM and a maximal effect of 50%. Consistent with its inhibitory effect on ICa, ATP hyperpolarized the plateau phase and shortened the action potential duration. In fura-2-loaded myocytes, extracellular ATP did not change the resting myoplasmic Ca2+ levels; however, when current was elicited under voltage-clamp conditions, ATP both decreased the myoplasmic intracellular Ca2+ transient and inhibited the degree of cell shortening. Our results suggest that ATP could be a genuine and potent extracellular modulator of cardiac function in ferret ventricular myocardium.

Intracellular calcium, currents, and stimulus-response coupling in endothelial cells

Vascular endothelium appears to be a unique organ. It not only responds to numerous hormonal and chemical signals but also senses changes in physical parameters such as shear stress, producing mediators that modulate the responses of numerous cells, including vascular smooth muscle, platelets, and leukocytes. In many cases, the initial response of endothelial cells to these diverse signals involves elevation of cytosolic Ca2+ and activation of Ca(2+)-dependent enzymes, including nitric oxide synthase and phospholipase A2. Both the release of Ca2+ from intracellular stores, most likely the endoplasmic reticulum, and the influx of Ca2+ from the extracellular space contribute to the [Ca2+]i increase. The most important trigger for Ca2+ release is inositol 1,4,5-trisphosphate, which is generated by the action of phospholipase C, a plasmalemmal enzyme activated in many cases by the receptor-G protein cascade. Ca2+ influx appears to be related to the activity of receptor-G protein-enzyme complex and to the degree of fullness of the endoplasmic reticulum but does not involve voltage-gated Ca2+ channels. The magnitude of the Ca2+ influx depends on the electrochemical gradient, which is modulated by the membrane potential, Vm. Under basal conditions, Vm is dominated by a large inward rectifier K+ current. Some stimuli, e.g., acetylcholine, have been shown to hyperpolarize Vm, thus increasing the electrochemical gradient for Ca2+, which appears to be modulated by activation of Ca(2+)-dependent K+ and Cl- currents. However, the lack of potent and specific blockers for many of the described or postulated channels (e.g., nonselective cation channel, Ca(2+)-activated Cl- channel) makes an estimation of their effect on endothelial cell function rather difficult. Possible future directions of research and clinical implications are discussed.

Differences in cardiodepressant effects of halothane and isoflurane after inotropic stimulation in guinea-pig papillary muscles

The effects of halothane and isoflurane on action potentials and force of contraction were studied in guinea-pig isolated papillary muscles in order to investigate the cardiodepressant action of the anesthetics in the presence of clinically relevant inotropic drugs. In control conditions, equipotent concentrations of the two volatile anesthetics depressed force of contraction to a similar degree. Halothane (2%) slightly shortened the action potential duration, whereas isoflurane (2.8%) did not influence the shape of the action potential. The muscles were then treated with three different inotropes, digitoxin, amrinone and 3-isobutyl-1-methylxanthine (IBMX) prior to exposure to the anesthetics. After pretreatment with digitoxin and amrinone, halothane was significantly more cardiodepressant than isoflurane. With IBMX, no difference in negative inotropic effect was observed. Neither volatile anesthetic significantly changed the action potential duration after pretreatment with the inotropic drugs. We suggest that halothane is more cardiodepressant than isoflurane in the presence of drugs which enhance force of contraction both by an increase in calcium influx and calcium release because it impairs force of contraction not only via inhibition of the calcium current but also via interference with calcium release.

Characterization of Ca(2+)-antagonistic effects of three metabolites of the new antihypertensive agent naftopidil, (naphthyl)hydroxy-naftopidil, (phenyl)hydroxy-naftopidil, and O-desmethyl-naftopidil

The newly developed antihypertensive agent naftopidil blocks alpha 1-adrenoceptors and inhibits Ca2+ entry via potential-dependent channels in vascular and cardiac muscle. It is extensively metabolized in vivo. Since it is of interest whether its metabolites are still pharmacologically active, we have characterized the effects of (naphthyl)hydroxy-naftopidil (NHN), (phenyl)hydroxy-naftopidil (PHN), and O-desmethyl-naftopidil (DMN) in various isolated preparations of the guinea pig heart. In constant-flow Langendorff hearts, the compounds decreased force of contraction by 66-81% and slowed spontaneous heart rate by 28-48%. DMN reduced perfusion pressure by 33%. The fibrillation threshold, which was measured as the strength of alternating current required to induce ventricular fibrillation, increased more than 10-fold. In papillary muscles, 3 x 10(-5) M of all compounds reduced force of contraction (pD2 values approximately 5.5) and shortened the action potential duration in the plateau phase. The maximum depolarization velocity (dV/dtmax) was slightly reduced (10-21%) by NHN, PHN, and DMN. In voltage-clamped ventricular cardiomyocytes, the calcium current ICa was depressed by the three compounds (10(-6)-10(-4) M) in a concentration-dependent manner. In conclusion, the three naftopidil metabolites investigated have pharmacological activities similar to those of their parent compound and hence could contribute to the in vivo effects of naftopidil.

Quantitation of hypoxanthine in plasma from patients with ischemic heart disease: adaption of a high-performance liquid chromatographic method

A high-performance liquid chromatographic method is described for the separation and quantitation of several purine compounds, including hypoxanthine. The isocratic separation of a standard mixture of nine compounds is achieved within 20 min on a reversed-phase Nucleosil 100-5C18 column, with a mobile phase of KH2PO4 (300 mM, pH 4.0)-methanol-acetonitrile-tetrahydrofuran (97.9:1:1:0.1, v/v). Uric acid, guanine, hypoxanthine, uridine, xanthine, allopurinol, inosine, guanosine and 7-methylxanthine were almost completely baseline-separated, with detection limits in the range 0.5-1.2 pmol per injection. The influence of the concentrations of buffer and tetrahydrofuran on the quality of separation are described. The within-day and the day-to-day precision were satisfactory (e.g. coefficients of variation of less than 1.5 and ca. 6.0%, respectively, for peak heights). The recovery of [3H]hypoxanthine added to samples was 86 +/- 1%. Hypoxanthine was quantified in human plasma samples obtained at various times during coronary artery bypass grafting. The hypoxanthine levels measured immediately after release of the aortic cross-clamp were significantly higher than those determined under control conditions (18.8 +/- 7.0 and 3.4 +/- 1.0 microM, respectively).

Combined effects of volatile anesthetics and phosphodiesterase inhibitors on contractile performance in guinea pig hearts

The cardiotonic effects of the phosphodiesterase inhibitors amrinone, milrinone, and 3-isobutyl-1-methylxanthine were studied in Langendorff-perfused guinea pig hearts exposed to the cardiodepressant concentrations of halothane or isoflurane. Left ventricular pressure, rate of change of pressure (dP/dt), heart rate, and perfusion pressure were measured in the presence of increasing drug concentrations. Under control conditions, both (dP/dt)max and heart rate were increased most with 3-isobutyl-1-methylxanthine and least with amrinone. In isoflurane-pretreated hearts (1.3%, vol/vol), the phosphodiesterase inhibitors increased (dP/dt)max to a larger degree, whereas the increase in heart rate remained similar to that in the control hearts. After exposure to halothane (0.8%, vol/vol), however, amrinone and milrinone were less effective with respect to enhancement of contractile performance. There was no difference in the 3-isobutyl-1-methylxanthine-induced increase of (dP/dt)max between hearts exposed to isoflurane and those exposed to halothane. Our results suggest that contractile performance of isolated hearts is more easily stimulated by milrinone and amrinone in the presence of isoflurane than in the presence of halothane.

Naftopidil, a new alpha-adrenoceptor blocking agent with calcium antagonistic properties: characterization of Ca2+ antagonistic effects

The newly developed antihypertensive agent naftopidil blocks alpha 1-adrenoceptors and inhibits Ca2+ entry via potential-dependent channels in vascular muscle. The aim of our study was to detect possible Ca2+ channel blocking activity in various isolated preparations of the guinea pig heart. Prazosin and verapamil were used for reference. In papillary muscles, 10 microM of all drugs reduced the force of contraction Fc. The action potential duration and the refractory period were hardly affected by naftopidil, decreased by verapamil, and slightly increased by prazosin. In constant-flow Langendorff hearts, the drugs reduced the perfusion pressure, decreased the Fc, and slowed the spontaneous heart rate (order of potency: verapamil much greater than naftopidil greater than prazosin). In voltage-clamped ventricular cardiomyocytes, the calcium current ICa was completely inhibited by verapamil (pD2 value of 6.9) and to 53.5% by naftopidil (pD2 value of 6.4). Prazosin (10 microM) decreased ICa by little more than 10%. There were no differences in the steady-state inhibition of ICa by the two enantiomers of naftopidil. The block of ICa was clearly use dependent. Radioligand binding studies with (+)-[3H]PN 200-110. (-)-[3H]desmethoxy-verapamil, and (+)-cis-[3H]diltiazem in guinea pig skeletal muscle T-tubulus membranes demonstrated that racemic naftopidil exhibited some affinity for the three distinct drug receptor domains of the L-type Ca2+ channel. In conclusion, the present data are consistent with the hypothesis that naftopidil is a weak ligand for L-type calcium channels. It partially blocks ICa and shows no stereoselectivity.