Effects of thienopyridines and thienopyrimidinones on L-type calcium current in isolated cardiomyocytes.

Thienopyridines (ticlopidine, clopidogrel) are frequently used drugs in antiplatelet therapy and have been shown to exert a more pronounced negative inotropic effect than thienopyrimidinones. We hypothesized that these differences are due to a differential impact of thienopyridines and thienopyrimidinones on L-type calcium current at the single-cell level. The effects of thienopyridines and thienopyrimidinones were studied on L-type calcium current and action potential parameters with the whole-cell patch-clamp technique in isolated myocytes from guinea pig ventricle and human atrial appendage. Ticlopidine showed the greatest impact on the L-type calcium current in guinea pig myocytes. It significantly reduced L-type calcium current density as well as shifted half maximal inactivation potential to more negative potentials compared to clopidogrel (at 30 µmol/L) and to all thienopyrimidinones (30 and 100 µmol/L). Clopidogrel significantly reduced the L-type calcium current density as well as shifted the half maximal inactivation potential to more negative potentials compared to all thienopyrimidinones at 100 µmol/L only. In contrast, thienopyrimidinones did not affect L-type calcium current properties. The significant different effects of thienopyridines and thienopyrimidinones could also be demonstrated in human atrial myocytes. The more pronounced negative inotropic effect of thienopyridines is well explained by our results demonstrating a differential impairment of L-type calcium current by thienopyridines and thienopyrimidinones. L-type calcium current impairment by thienopyridines may be of special relevance for patients with cardiac diseases characterized by ionic remodelling.

Impaired regulation of cardiac function in sepsis, SIRS, and MODS.

In sepsis, systemic inflammatory response syndrome (SIRS), and multiorgan dysfunction syndrome (MODS), a severe prognostically relevant cardiac autonomic dysfunction exists, as manifested by a strong attenuation of sympathetically and vagally mediated heart rate variability (HRV). The mechanisms underlying this attenuation are not limited to the nervous system. They also include alterations of the cardiac pacemaker cells on a cellular level. As shown in human atrial cardiomyocytes, endotoxin interacts with cardiac hyperpolarization-activated cyclic nucleotide-gated (HCN) ion channels, which mediate the pacemaker current If and play an important role in transmitting sympathetic and vagal signals on heart rate and HRV. Moreover, endotoxin sensitizes cardiac HCN channels to sympathetic signals. These findings identify endotoxin as a pertinent modulator of the autonomic nervous regulation of heart function. In MODS, the vagal pathway of the autonomic nervous system is particularly compromised, leading to an attenuation of the cholinergic antiinflammatory reflex. An amelioration of the blunted vagal activity appears to be a promising novel therapeutic target to achieve a suppression of the inflammatory state and thereby an improvement of prognosis in MODS patients. Preliminary data revealed therapeutic benefits (increased survival rates and improvements of the depressed vagal activity) of the administration of statins, beta-blockers, and angiotensin-converting enzyme inhibitors in patients with MODS.

Endotoxin impairs the human pacemaker current If.

LPSs trigger the development of sepsis by gram-negative bacteria and cause a variety of biological effects on host cells, including alterations on ionic channels. Because heart rate variability is reduced in human sepsis and endotoxemia, we hypothesized that LPS affects the pacemaker current I(f) in human heart, which might--at least in part--explain this phenomenon. Isolated human myocytes from right atrial appendages were incubated for 6 to 10 h with LPS (1 and 10 microg/mL) and afterwards used to investigate the pacemaker current I(f). I(f) was measured with the whole-cell patch-clamp technique (at 37 degrees C). Incubation of atrial myocytes with 10 microg/mL LPS was found to significantly impair I(f) by suppressing the current at membrane potentials positive to -80 mV and slowing down current activation, but without effecting maximal current conductance. Furthermore, in incubated cells (10 microg/mL), the response of I(f) to [beta]-adrenergic stimulation (1 microM isoproterenol) was significantly larger compared with control cells (shift of half-maximal activation voltage to more positive potentials amounted to -10 and -14 mV in untreated and treated cells, respectively). Simulations using a spontaneously active sinoatrial cell model demonstrated that LPS-induced I(f) impairment reduced the responsiveness of the model cell to fluctuations of autonomic input. This study showed a direct impact of LPS on the cardiac pacemaker current I(f). The LPS-induced I(f) impairment may contribute to the clinically observed reduction in heart rate variability under septic conditions and in cardiac diseases such as heart failure, where endotoxin can be of pathophysiological relevance.

Oxidized LDL induces ventricular myocyte damage and abnormal electrical activity--role of lipid hydroperoxides.

OBJECTIVE: It was our aim to investigate effects of human LDL, copper-, or AAPH-oxidized over different periods of time to different degrees (ox-LDL), on viability and electrophysiological parameters of isolated ventricular myocytes of guinea pigs. METHODS: Guinea pig ventricular myocytes were incubated with ox-LDL or native LDL (at 0.5 mg/ml) for 12 h, and afterwards myocyte damage, action potentials, and transmembrane ion currents were studied (at 37 degrees C). RESULTS: Ox-LDL was found to induce severe myocyte damage, whereas native LDL had no effect. Myocyte damage was dependent on the content of total lipid hydroperoxides in both copper-oxidized and AAPH-oxidized LDL. Incubation with ox-LDL led to intense contractile and electrophysiological effects including prolongation of action potential duration, depolarization of resting membrane potential, spontaneous activity, generation of afterdepolarizations, and modification of transmembrane ion currents (e.g. inward rectifier, calcium, and background currents). CONCLUSIONS: Ox-LDL induced cell damage and irregular electrical activity in ventricular myocytes. These effects were dependent on the lipid hydroperoxide content of ox-LDL and were similar to oxidative stress (OS) induced by various OS-generating systems. The observed effects may play a role for functional cardiac abnormalities in patients with increased ox-LDL levels.

An ionic model for rhythmic activity in small clusters of embryonic chick ventricular cells.

We recorded transmembrane potential in whole cell recording mode from small clusters (2-4 cells) of spontaneously beating 7-day embryonic chick ventricular cells after 1-3 days in culture and investigated effects of the blockers D-600, diltiazem, almokalant, and Ba2+. Electrical activity in small clusters is very different from that in reaggregates of several hundred embryonic chick ventricular cells, e.g., TTX-sensitive fast upstrokes in reaggregates vs. TTX-insensitive slow upstrokes in small clusters (maximum upstroke velocity approximately 100 V/s vs. approximately 10 V/s). On the basis of our voltage- and current-clamp results and data from the literature, we formulated a Hodgkin-Huxley-type ionic model for the electrical activity in these small clusters. The model contains a Ca2+ current (ICa), three K+ currents (IKs, IKr, and IK1), a background current, and a seal-leak current. ICa generates the slow upstroke, whereas IKs, IKr, and IK1 contribute to repolarization. All the currents contribute to spontaneous diastolic depolarization, e.g., removal of the seal-leak current increases the interbeat interval from 392 to 535 ms. The model replicates the spontaneous activity in the clusters as well as the experimental results of application of blockers. Bifurcation analysis and simulations with the model predict that annihilation and single-pulse triggering should occur with partial block of ICa. Embryonic chick ventricular cells have been used as an experimental model to investigate various aspects of spontaneous beating of cardiac cells, e.g., mutual synchronization, regularity of beating, and spontaneous initiation and termination of reentrant rhythms; our model allows investigation of these topics through numerical simulation.

If in left human atrium: a potential contributor to atrial ectopy.

OBJECTIVE: The left human atrium plays an important role in initiation of atrial fibrillation (AF) and the hyperpolarization activated cation current (I(f)) is a candidate for contributing to abnormal automaticity. However, electrophysiological data concerning I(f) are not available in this cardiac region and we therefore investigated I(f) in human left atrial tissue. METHODS: Human atrial myocytes were isolated from the left atrial appendage (LAA) and the left atrial wall (LAW) obtained from patients undergoing open heart surgery. I(f) was measured with the whole-cell patch-clamp technique. RESULTS: I(f) densities between -70 and -110 mV were found to be significantly higher in LAA than in LAW cells. Furthermore, in the group of LAA cells the half maximal activation potential (V(1/2)) was found to be less negative (V(1/2) of -84.3+/-1.9 mV, n=14/9) compared to LAW cells (V(1/2) of -97.8+/-2.1 mV, n=28/9). Beta-adrenergic receptor stimulation with isoproterenol (1 microM) caused an acceleration of current activation and a V(1/2) shift to more positive potentials in cells of both regions (LAA: 8.8+/-2.3 mV, n=6/4 and LAW: 8.9+/-2.6 mV, n=6/4). Simulations using a mathematical model of the human atrial myocyte demonstrated that I(f) was able to induce spontaneous activity in the model at a regular rhythm due to the interplay of I(f), Na(+)/Ca(2+) exchange current and Ca(2+) release of the sarcoplasmic reticulum (SR). CONCLUSIONS: Our study revealed the presence of I(f) in left atrial myocytes and showed that I(f) parameters depend on atrial region. I(f) current densities were sufficient to convert the mathematical model of a quiescent human atrial cell into a "pacemaker cell". These data support the hypothesis of I(f) as a contributor to abnormal automaticity in human atrial tissue.

Influence of preservatives and topical steroids on ciliary beat frequency in vitro.

OBJECTIVE: To measure the influence of topical steroids and the preservative potassium sorbate on the ciliary beat frequency (CBF) of human nasal mucosa in vitro. DESIGN: In vitro study of cultured ciliated cells of human nasal mucosa. METHODS: Human nasal mucosa was removed endoscopically and cultured for 10 days. Cell cultures with ciliated cells grown on an object slide were exposed to benzalkonium chloride and topical steroids in an exposure chamber. The CBF was measured with a photometer. RESULTS: The preservative potassium sorbate did not influence CBF in different concentrations. The glucocorticoid budesonide spray containing potassium sorbate did not affect CBF at 10% dilution and showed moderate reversible decrease of CBF at 50% dilution. The glucocorticoid sprays fluticasone propionate and mometasone fuorate containing the preservative benzalkonium chloride caused a reversible decrease of CBF at 10% dilution and a complete irreversible standstill at 50% dilution. CONCLUSIONS: In vitro, the steroid sprays containing fluticasone or mometasone, both with benzalkonium chloride, caused slowing or standstill of CBF depending on the concentration. The isolated preservative potassium sorbate and the budesonide nasal spray containing this preservative did not have negative influence on CBF in vitro. Potassium sorbate can therefore be considered harmless to the motility of ciliated cells.

A hyperpolarization activated inward current (If) is present in infant ventricular myocytes.

I(f) was shown to be present in adult human atrial and ventricular myocytes but data obtained from infant myocytes are lacking. We have studied I(f) in isolated ventricular myocytes from children undergoing surgical correction of tetralogy of Fallot (TOF; n = 5; mean age: 15.3 months). All recordings were made with the patch clamp technique in the whole cell mode at a temperature of 36-37 degrees C. A modified Tyrode solution containing 25 mM KCl was used to amplify I(f). Considering I(f) to be present when its current density at -120 mV was greater than 0.5 pA/pF, I(f) could be found in 28 out of 32 myocytes (88%). The mean current density was -2.01 +/- 0.3 pA/pF (mean +/- S.E.M.). First current activation occurred at -70 mV and I(f) could be reversibly inhibited by superfusing the myocytes with CsCl (2 mM). Half maximal activation (V(1/2)) of I(f) was at -80.3 +/- 1.0 mV (n = 28). Beta-adrenergic receptor stimulation with isoproterenol (1 microM) caused an acceleration of current activation and a shift of V(1/2) by 7.88 +/- 1.8 mV (n = 10) to less negative potentials. This study provides first evidence that the hyperpolarization-activated pacemaker current I(f) is present in infant human ventricular myocytes. Our results suggest that I(f) in ventricle of infants suffering from TOF has similar properties as I(f) in adult ventricle.

NADH supplementation decreases pinacidil-primed I K ATP in ventricular cardiomyocytes by increasing intracellular ATP.

1 The aim of this study was to investigate the effect of nicotinamide-adenine dinucleotide (NADH) supplementation on the metabolic condition of isolated guinea-pig ventricular cardiomyocytes. The pinacidil-primed ATP-dependent potassium current I(K(ATP)) was used as an indicator of subsarcolemmal ATP concentration and intracellular adenine nucleotide contents were measured. 2 Membrane currents were studied using the patch-clamp technique in the whole-cell recording mode at 36-37 degrees C. Adenine nucleotides were determined by HPLC. 3 Under physiological conditions (4.3 mM ATP in the pipette solution, ATP(i)) I(K(ATP)) did not contribute to basal electrical activity. 4 The ATP-dependent potassium (K((ATP))) channel opener pinacidil activated I(K(ATP)) dependent on [ATP](i) showing a significantly more pronounced activation at lower (1 mM) [ATP](i). 5 Supplementation of cardiomyocytes with 300 micro g ml(-1) NADH (4-6 h) resulted in a significantly reduced I(K(ATP)) activation by pinacidil compared to control cells. The current density was 13.8+/-3.78 (n=6) versus 28.9+/-3.38 pA pF(-1) (n=19; P<0.05). 6 Equimolar amounts of the related compounds nicotinamide and NAD(+) did not achieve a similar effect like NADH. 7 Measurement of adenine nucleotides by HPLC revealed a significant increase in intracellular ATP (NADH supplementation: 45.6+/-1.88 nmol mg(-1) protein versus control: 35.4+/-2.57 nmol mg(-1) protein, P<0.000005). 8 These data show that supplementation of guinea-pig ventricular cardiomyocytes with NADH results in a decreased activation of I(K(ATP)) by pinacidil compared to control myocytes, indicating a higher subsarcolemmal ATP concentration. 9 Analysis of intracellular adenine nucleotides by HPLC confirmed the significant increase in ATP.

Selectivity of different calcium antagonists on T- and L-type calcium currents in guinea-pig ventricular myocytes.

Both L- and T-type calcium channels are present in the heart. In cardiac myocytes L-type calcium channels are blocked by the classical calcium channel blockers, while T-type calcium channels are thought to be insensitive to these drugs and to be selectively blocked by mibefradil. We aimed to compare the T/L calcium channel blocking selectivity of several calcium channel blockers by evaluating their effects on both components evoked in the same cell from a holding potential corresponding to the normal physiological value (-90mV). Currents were recorded in single patch-clamped guinea-pig ventricular myocytes, superfused with a Na(+)- and K(+)-free solution to abolish overlapping currents. Two dihydropyridines (amlodipine and lacidipine), verapamil diltiazem and mibefradil were tested; for each compound concentrations equieffective on L-type Ca(2+) current were used. All calcium channel blockers, at concentrations blocking less than 30% of L-type Ca(2+) current, inhibited a significant amount of T-type Ca(2+) current, varying from 0.8% (diltiazem) to 28% (mibefradil). We calculated for each compound the T/L ratio. As expected, mibefradil showed the highest T selectivity; lacidipine and diltiazem resulted to be L selective. Verapamil and amlodipine were not selective. Thus, the calcium channel blockers can be differentiated on the basis of their T/L selectivity.