Multiple levels of native cardiac Na+ channels at elevated temperature measured with high-bandwidth/low-noise patch clamp

Towards single cell heat shock response by accurate control on thermal confinement with an on-chip microwire electrode

Metal electrodes with micron scale width enable the heating of less than a dozen cells in a confluent layer at predictable temperatures up to 85 °C with an accuracy of ±2 °C. Those performances were obtained by a preliminary robust temperature calibration based on biotin-rhodamine fluorescence and by controlling the temperature map on the substrate through thermal modeling. The temperature accuracy was proved by inducing the expression of heat shock proteins (HSP) in a few NIH-3T3 cells through a confined and precise temperature rise. Our device is therefore effective to locally induce a heat shock response with almost single-cell resolution. Furthermore, we show that cells heated at a higher temperature than the one of heat shock remain alive without producing HSP. Electrode deposition being one of the most common engineering processes, the fabrication of electrode arrays with a simple control circuit is clearly within reach for parallel testing. This should enable the study of several key mechanisms such as cell heat shock, death or signaling. In nanomedicine, controlled drug release by external stimuli such as for example temperature has attracted much attention. Our device could allow fast and efficient testing of thermoactivable drug delivery systems.

Noise and bandwidth of current recordings from submicrometer pores and nanopores

Nanopores and submicrometer pores have recently been explored for applications ranging from detection of single molecules, assemblies of nanoparticles, nucleic acids, occurrence of chemical reactions, and unfolding of proteins. Most of these applications rely on monitoring electrical current through these pores, hence the noise and signal bandwidth of these current recordings are critical for achieving accurate and sensitive measurements. In this report, we present a detailed theoretical and experimental study on the noise and signal bandwidth of current recordings from glass and polyethylene terephthalate (PET) membranes that contain a single submicrometer pore or nanopore. We examined the theoretical signal bandwidth of two different pore geometries, and we measured the signal bandwidth of the electronics used to record the ionic current. We also investigated the theoretical noise generated by the substrate material, the pore, and the electronics used to record the current. Employing a combination of theory and experimental results, we were able to predict the noise in current traces recorded from glass and PET pores with no applied voltage with an error of less than 12% in a range of signal bandwidths from 1 to 40 kHz. In approximately half of all experiments, application of a voltage did not significantly increase the noise. In the other half of experiments, however, application of a voltage resulted in an additional source of noise. For these pores, predictions of the noise were usually still accurate within 35% error at signal bandwidths of at least 10 kHz. The power spectra of this extra noise suggested a 1/f(alpha) origin with best fits to the power spectrum for alpha = 0.4-0.8. This work provides the theoretical background and experimental data for understanding the bandwidth requirements and the main sources of noise in current recordings; it will be useful for minimizing noise and achieving accurate recordings.

Expression pattern of neuronal and skeletal muscle voltage-gated Na+ channels in the developing mouse heart

In the mammalian heart, a variety of voltage-gated Na(+) channel transcripts and proteins have been detected. However, little quantitative information is available on the abundance of each transcript during development, or the contribution of TTX-sensitive Na(+) channels to the cardiac sodium current (I(Na)). Using competitive and real-time RT-PCR we investigated the transcription of six Na(+) channels (Na(v)1.1-Na(v)1.6) and the beta1 subunit during mouse heart development. Na(v)1.5 was predominantly expressed in the adult heart, whereas the splice variant Na(v)1.5a was the major Na(+) channel isoform in embryonic hearts. The TTX-resistant Na(+) channel transcripts (Na(v)1.5 and Na(v)1.5a) increased 1.7-fold during postnatal development. Transcripts encoding TTX-sensitive Na(+) channels (Na(v)1.1-Na(v)1.4) and the beta1 subunit gradually increased up to fourfold from postnatal day (P)1 to P126, while the Na(v)1.6 transcript level remained low and constant over the same period. In adults, TTX-sensitive channel mRNA accounted for 30-40% of the channel pool in whole-heart preparations (Na(v)1.3 > Na(v)1.4 > Na(v)1.2 >> Na(v)1.1 approximately Na(v)1.6), and 16% in mRNA from isolated cardiomyocytes (Na(v)1.4 > Na(v)1.3 > Na(v)1.2 > Na(v)1.1 > Na(v)1.6). Confocal immunofluorescence on ventricular myocytes suggested that Na(v)1.1 and Na(v)1.2 were localized at the intercalated disks and in the t tubules. Na(v)1.3 labelling predominantly produced a diffuse but strong intracellular signal. Na(v)1.6 fluorescence was detected only along the Z lines. Electrophysiological recordings showed that TTX-sensitive and TTX-resistant Na(+) channels, respectively, accounted for 8% and 92% of the I(Na) in adult ventricular cardiomyocytes. Our data suggest that neuronal and skeletal muscle Na(+) channels contribute to the action potential of cardiomyocytes in the adult mammalian heart.

A novel method of thermal activation and temperature measurement in the microscopic region around single living cells

We present a simple approach to bring fast and reversible temperature steps of a wide range of amplitudes from the temperature of the experimental chamber up to the boiling point of water in a desired position, with rise and fall times of around 10 ms in a microvolume of microm in size, such as in a single cell. For this purpose, we applied a technique for illuminating a metal aggregate (1-2 microm in diameter) placed at the tip of a glass micropipette with a focused infrared (1064 nm) laser beam under an optical microscope. Stable temperature gradients were created around the metal aggregate using an appropriate neutral density filter set for the laser output. To monitor the local temperature, we devised a new microthermometer composed of the tip of a micropipette filled with thermosensitive fluorescent dye Europium-TTA possessing steep temperature-dependent phosphorescence upon 365 nm excitation. The microm size of the tip of this pipette was able to measure the local temperature with 0.1 degrees C precision and microm spatial resolution. This new approach is compatible with standard electrophysiological and imaging techniques.

Mouse heart Na+ channels: primary structure and function of two isoforms and alternatively spliced variants

We isolated two full-length cDNA clones from the adult murine heart that encode two different voltage-gated Na+ channels: mH1 and mH2. Sequence comparisons indicated that mH1 is highly homologous to rat SCN5A, whereas mH2 is highly homologous to SCN4A, expressed in rat skeletal muscle. Electrophysiological properties of mH1 channels strongly resembled the tetrodotoxin (TTX)-resistant Na+ current of mouse ventricular cells, whereas mH2 channels activated at more positive potentials and were highly sensitive to TTX [50% inhibitory constant (IC50) = 11 nM]. We found that mH2 is not expressed in cardiac cells of neonatal mice, but appears to be upregulated during the development. Besides these Na+ channel isoforms, we also detected two alternatively spliced mH1 variants that were characterized by deletions within the sequence coding for the intracellular loop between domains II and III. One of the shortened channels, mH1-2, developed Na+ currents indistinguishable from those of mH1. The other splice variant (mH1-3) did not form functional channels. Quantitative reverse transcriptase-polymerase chain reaction indicated that RNA preparations of the adult mouse heart contain 54% mH1, 25% mH1-2, 16% mH2, and 5% mH1-3. Conclusively, mH1 generates the main portion of the mouse cardiac TTX-resistant Na+ current and mH2 is a candidate for TTX-sensitive currents previously described in adult cardiomyocytes. Furthermore, the presence of mH1-2 and mH1-3 transcripts indicates that alternative splicing plays a role in the regulation of functional Na+ channels in cardiomyocytes.

Temperature dependence of early and late currents in human cardiac wild-type and long Q-T DeltaKPQ Na+ channels

Na+ current (INa) through wild-type human heart Na+ channels (hH1) is important for normal cardiac excitability and conduction, and it participates in the control of repolarization and refractoriness. INa kinetics depend strongly on temperature, but INa for hH1 has been studied previously only at room temperature. We characterized early INa (the peak and initial decay) and late INa of the wild-type hH1 channel and a mutant channel (DeltaKPQ) associated with congenital long Q-T syndrome. Channels were stably transfected in HEK-293 cells and studied at 23 and 33 degreesC using whole cell patch clamp. Activation and inactivation kinetics for early INa were twofold faster at higher temperature for both channels and shifted activation and steady-state inactivation in the positive direction, especially for DeltaKPQ. For early INa (<24 ms), DeltaKPQ decayed faster than the wild type for voltages negative to -20 mV but slower for more positive voltages, suggesting a reduced voltage dependence of fast inactivation. Late INa at 240 ms was significantly greater for DeltaKPQ than for the wild type at both temperatures. The majority of late INa for DeltaKPQ was not persistent; rather, it decayed slowly, and this late component exhibited slower recovery from inactivation compared with peak INa. Additional kinetic changes for early and peak INa for DeltaKPQ compared with the wild type at both temperatures were 1) reduced voltage dependence of steady-state inactivation with no difference in midpoint, 2) positive shift for activation kinetics, and 3) more rapid recovery from inactivation. This study represents the first description of human Na+ channel kinetics near physiological temperature and also demonstrates complex gating changes in the DeltaKPQ that are present at 33 degreesC and that may underlie the electrophysiological and clinical phenotype of congenital long Q-T Na+ channel syndromes.

Inactivation of single cardiac Na+ channels in three different gating modes

In small cell-attached patches containing one and only one Na+ channel, inactivation was studied in three different gating modes, namely, the fast-inactivating F mode and the more slowly inactivating S mode and P mode with similar inactivation kinetics. In each of these modes, ensemble-averaged currents could be fitted with a Hodgkin-Huxley-type model with a single exponential for inactivation (tauh). tauh declined from 1.0 ms at -60 mV to 0.1 ms at 0 mV in the F mode, from 4.6 ms at -40 mV to 1.1 ms at 0 mV in the S mode, and from 4.5 ms at -40 mV to 0.8 ms at +20 mV in the P mode, respectively. The probability of non-empty traces (net), the mean number of openings per non-empty trace (op/tr), and the mean open probability per trace (popen) were evaluated at 4-ms test pulses. net inclined from 30% at -60 mV to 63% at 0 mV in the F mode, from 4% at -90 mV to 90% at 0 mV in the S mode, and from 2% at -60 mV to 79% at +20 mV in the P mode. op/tr declined from 1.4 at -60 mV to 1.1 at 0 mV in the F mode, from 4.0 at -60 mV to 1.2 at 0 mV in the S mode, and from 2.9 at -40 mV to 1.6 at +20 mV in the P mode. popen was bell-shaped with a maximum of 5% at -30 mV in the F mode, 48% at -50 mV in the S mode, and 16% at 0 mV in the P mode. It is concluded that 1) a switch between F and S modes may reflect a functional change of inactivation, 2) a switch between S and P modes may reflect a functional change of activation, 3) tauh is mainly determined by the latency until the first channel opening in the F mode and by the number of reopenings in the S and P modes, 4) at least in the S and P modes, inactivation is independent of pore opening, and 5) in the S mode, mainly open channels inactivate, and in the P mode, mainly closed channels inactivate.

Increased expression of the cardiac L-type calcium channel in estrogen receptor-deficient mice

Steroid hormones control the expression of many cellular regulators, and a role for estrogen in cardiovascular function and disease has been well documented. To address whether the activity of the L-type Ca2+ channel, a critical element in cardiac excitability and contractility, is altered by estrogen and its nuclear receptor, we examined cardiac myocytes from male mice in which the estrogen receptor gene had been disrupted (ERKO mice). Binding of dihydropyridine Ca2+ channel antagonist isradipine (PN200-110) was increased 45.6% in cardiac membranes from the ERKO mice compared to controls, suggesting that a lack of estrogen receptors in the heart increased the number of Ca2+ channels. Whole-cell patch clamp of acutely dissociated adult cardiac ventricular myocytes indicated that Ca2+ channel current was increased by 49% and action potential duration was increased by 75%. Examination of electrocardiogram parameters in ERKO mice showed a 70% increase in the QT interval without significant changes in PQ or QRS intervals. These results show that the membrane density of the cardiac L-type Ca2+ channel is regulated by the estrogen receptor and suggest that decreased estrogen may lead to an increase in the number of cardiac L-type Ca2+ channels, abnormalities in cardiac excitability, and increased risk of arrhythmia and cardiovascular disease.

ATP-dependent potassium channel in rat cardiomyocytes is blocked by lidocaine. Possible impact on the antiarrhythmic action of lidocaine

BACKGROUND

During myocardial ischemia, lidocaine has favorable antiarrhythmic properties. Malignant arrhythmias result from heterogeneity between ischemic and nonischemic regions in extracellular potassium concentration and action potential duration. These effects have been attributed to the activation of ATP-dependent potassium (KATP) channels. In this study, we investigated the action of lidocaine on the KATP channels to test the possible link between the antiarrhythmic properties of lidocaine and its action on the KATP channel.

METHODS AND RESULTS

The patch-clamp technique was employed on enzymatic dissociated cardiomyocytes of adult rats. Lidocaine was applied to the outer side of excised membrane patches by means of a multibarrel perfusion system. Lidocaine reversibly blocked the mean current of the KATP channels in a concentration-dependent manner (IC50 = 43 +/- 4.7 mumol/L, E = 0 mV, n = 6), while the amplitude of the single-channel current remained unchanged. The half-maximum blocking concentration corresponds to the therapeutic range for the antiarrhythmic application of a lidocaine bolus in humans.

CONCLUSIONS

The open probability but not the conductance of the KATP channel in the membrane of rat cardiomyocytes is blocked by lidocaine. This action may explain, in part, the favorable antiarrhythmic properties of lidocaine during acute myocardial ischemia.

Voltage-dependent properties of three different gating modes in single cardiac Na+ channels

Three different modes of Na+ channel action, the F mode (fast inactivating), the S mode (slowly inactivating), and the P mode (persistent), were studied at different potentials in exceptionally small cell-attached patches containing one and only one channel. Switching between the modes was independent of voltage. In the F mode, the mean open time (tau o) at -30 and -40 mV was 0.14 and 0.16 ms, respectively, which was significantly larger than at -60 and 0 mV, where the values were 0.07 and 0.08 ms, respectively. The time before which half of the first channel openings occurred (t 0.5), decreased from 0.58 ms at -60 mV to 0.14 ms at 0 mV. The fit of steady-state activation with a Boltzmann function yielded a half-maximum value (V 0.5) at -48.1 mV and a slope (k) of 5.6 mV. The mean open time in the S mode increased steadily from 0.12 ms at -80 mV to 1.09 ms at -30 mV, but was not prolonged further at -20 mV (1.07 ms). Concomitantly, t 0.5 decreased from 1.61 ms at -80 mV to 0.22 ms at -20mV. Here the midpoint of steady-state activation was found at -61.2 mV, and the slope was 8.7 mV. The mean open time in the P mode increased from 0.07 ms at -60 mV to 0.45 ms at 0 mV and t 0.5 declined from 2.14 ms at -60 mV to 0.19 ms at +20 mV. Steady-state activation had its midpoint at -14.7 mV, and the slope was 10.9 mV. It is concluded that a single Na+ channel may switch among the F, S, and P mode and that the three modes differ by a characteristic pattern of voltage dependence of tau 0, t 0.5, and steady-state activation.

Multimodal action of single Na+ channels in myocardial mouse cells

Unitary Na+ currents of myocardial mouse cells were studied at room temperature in 10 cell-attached patches, each containing one and only one channel. Small-pore patch pipettes (resistance 10-97 M omega when filled with 200% Tyrode's solution) with exceptionally thick walls were used. Observed were both rapidly inactivating (6 patches) and slowly inactivating (3 patches) Na+ currents. In one patch, a slow transition from rather fast to slow inactivation was detected over a time of 0.5 h. A short and a long component of the open-channel life time were recorded at the beginning, but only a short one at the end of the experiment. Concomitantly, the first latency was slowed. Amplitude histograms showed that the electrochemical driving force across the pore of the channel did not change during this time. In three patches, a fast and repetitive switching between different modes of Na+ channel action could be clearly identified by plotting the long-time course of the averaged current per trace. The ensemble-averaged current formed in each mode was different in kinetics and amplitude. Each mode had a characteristic mean open-channel life time and distribution of first latency, but the predominant single-channel current amplitude was unaffected by mode switches. It is concluded that two types of changes in kinetics may happen in a single Na+ channel: fast and reversible switches between different modes, and a slow loss of inactivation.

Facilitated giga-seal formation with a just originated glass surface

A simple technique of tip preparation in patch pipettes is described, which facilitates giga-seal formation. The pipettes were fabricated from thick-walled borosilicate glass tubing (external diameter 2.0 mm, internal diameter 0.5 mm) and the tips could be repeatedly broken in the bath. The pipette resistance correspondingly fell in steps of 3-20 M omega from about 80 M omega to about 2 M omega (double concentrated Tyrode). Scanning electron microscopy showed that the tip obtained after breaking was fairly plain. These broken tips were especially appropriate for patch-clamping. In cardiac myocytes in 11 out of 26 patches with Na+ channel activity, giga-seals developed spontaneously, i.e. without suction. In these patches the amplitude of the mean current with depolarizing pulses to -40 mV was significantly higher in comparison with patches formed under negative pressure. It is concluded that spontaneously sealed patches are most likely of planar configuration and the Na+ channel activity exceeds that in suction-induced patches.

Anoxia decreases the transient K+ outward current in isolated ventricular heart cells of the mouse

Transient K+ outward currents (Ito) were measured in enzymatically isolated ventricular mouse heart cells with a patch clamp technique in the whole cell configuration. Exposure of the cells to substrate-free anoxia gradually decreased both the peak and the late Ito. The inactivation time course of Ito was fitted with two exponentials. After 4-10 min of anoxia, the contribution of the fast and slow exponential decreased to 60 +/- 7% and 62 +/- 4% of the control value and recovered after reoxygenation within 1-3 min to 84 +/- 5% and 75 +/- 6% (n = 10; all mean +/- SEM), respectively. The time constants of the exponentials were invariant to anoxia. Voltage dependence of activation and inactivation of Ito were not influenced by anoxia. Application of stimulators of protein kinase A and C, cGMP- dependent protein kinase, or of the oxidant diamide during anoxia did not recover Ito. It is concluded that under conditions of metabolic stress, Ito is reversibly down-regulated leaving inactivation kinetics unchanged. The underlying mechanism is as yet unknown but does neither involve a decreased activity of protein kinase A, protein kinase C, nor c-GMP dependent protein kinase.

Thermodynamic entropy of two conformational transitions of single Na+ channel molecules

Single cardiac Na+ channel currents were recorded with improved resolution (bandwidth up to 20 kHz) at two temperatures, 10 and 25 degrees C. The mean open time was determined at voltages between -50 and 0 mV by evaluation of the distribution of the event-related gaps in the center of the baseline noise. Fit of the voltage-dependent reciprocal mean open times at both temperatures allowed even for a single channel molecule to separate an entropic from an enthalpic part of activation energy for both deactivation and inactivation. Both entropies are positive and the entropy of deactivation exceeds that of inactivation by more than twice.