Calcium sparks in human ventricular cardiomyocytes from patients with terminal heart failure

Cardiomyocytes from terminally failing hearts display significant abnormalities in e-c-coupling, contractility and intracellular Ca(2+) handling. This study is the first to demonstrate the influence of end-stage heart failure on specific properties of Ca(2+) sparks in human ventricular cardiomyocytes. We investigated the frequency and characteristics of spontaneously arising Ca(2+) sparks in single isolated human myocytes from terminally failing (HF) and non-failing (NF) control myocardium by using the Ca(2+) indicator Fluo-3. The Ca(2+) sparks were recorded by line-scan images along the longitudinal axis of the myocytes at a frequency of 250Hz. After loading the sarcoplasmic reticulum (SR) with Ca(2+) by repetitive field stimulation (10 pulses at 1Hz) the frequency of the Ca(2+) sparks immediately after stimulation (t = 0s) was reduced significantly in HF compared to NF (4.15 +/- 0.42 for NF vs. 2.81 +/- 0.20 for HF sparks s(-1), P = 0.05). This difference was present constantly in line-scan recordings up to 15s duration (t = 15s: 2.75 +/- 0.65 for NF vs. 1.36 +/- 0.34 for HF sparks s(-1), P = 0.05). The relative amplitude (F/F(0)) of Ca(2+) sparks was also significantly lower in HF cardiomyocytes (1.33 +/- 0.015 NF vs. 1.19 +/- 0.003 HF, t = 0s) and during subsequent recordings of 15s. Significant differences between HF and NF were also present in calculations of specific spark properties. The time to peak was estimated at 25.75 +/-0.88ms in HF and 18.68 +/- 0.45ms in NF cardiomyocytes (P = 0.05). Half-time of decay was 66.48 +/- 1.89ms (HF) vs. 44.15 +/- 1.65ms (NF, P < 0.05), and the full width at half-maximum (FWHM) was 3.99 +/- 0.06 microm (HF) vs. 3.5 +/- 0.07 microm (NF, P < 0.05). These data support the hypothesis that even in the absence of cardiac disease, Ca(2+) sparks from human cardiomyocytes differ from previous results of animal studies with respect to the time-to-peak, half-time of decay and FWHM. The role of elevated external Ca(2+) in HF was studied by recording Ca(2+) sparks in HF cardiomyocytes with 10mmol external Ca(2+) concentration. Under these conditions, the average spark amplitude was increased from 1.19 +/- 0.003 (F/F(0), 2mmol Ca(2+)) to 1.26 +/- 0.01 (F/F(0), 10mmol Ca(2+)). We conclude that human heart failure causes distinct changes in Ca(2+) spark frequency and characteristics comparable to results established in animal models of heart failure. A reduced Ca(2+) load of the SR alone is unlikely to account for the observed differences between HF and NF and additional alterations in intracellular Ca(2+) release mechanisms must be postulated.

Functional expression of GFP-linked human heart sodium channel (hH1) and subcellular localization of the a subunit in HEK293 cells and dog cardiac myocytes

Recent evidence suggests that biosynthesis of the human heart Na+ channel (hH1) protein is rapidly modulated by sympathetic interventions. However, data regarding the intracellular processing of hH1 in vivo are lacking. In this study we sought to establish a model that would allow us to study the subcellular localization of hH1 protein. Such a model could eventually help us to better understand the trafficking of hH1 in vivo and its potential role in cardiac conduction. We labeled the C-terminus of hH1 with the green fluorescent protein (GFP) and compared the expression of this construct (hH1-GFP) and hH1 in transfected HEK293 cells. Fusion of GFP to hH1 did not alter its electrophysiological properties. Confocal microscopy revealed that hH1-GFP was highly expressed in intracellular membrane structures. Immuno-electronmicrographs showed that transfection of hH1-GFP and hH1 induced proliferation of three types of endoplasmic reticulum (ER) membranes to accommodate the heterologously expressed proteins. Labeling with specific markers for the ER and the Golgi apparatus indicated that the intracellular channels are almost exclusively retained within the ER. Immunocytochemical labeling of the Na+ channel in dog cardiomyocytes showed strong fluorescence in the perinuclear region of the cells, a result consistent with our findings in HEK293 cells. We propose that the ER may serve as a reservoir for the cardiac Na+ channels and that the transport from the ER to the Golgi apparatus is among the rate-limiting steps for sarcolemmal expression of Na+ channels.

Identification of the cardiac ryanodine receptor channel in membrane blebs of sarcoplasmic reticulum

Blebs of the sarcoplasmic reticulum (SR) membrane of heart muscle cells were generated after saponin perforation of the plasma membrane followed by complete hypercontraction of the cell. Although characteristic proteins of the plasma membrane, namely the beta1-adrenoreceptor and Galphai, were stained by monoclonal antibodies in the hypercontracted cells, these proteins could not be detected in the adjacent blebs. Monoclonal antibodies to the cardiac ryanodine receptor (RyR2), calsequestrin and SERCA2 bound at different amounts to surface components of the blebs and to components of the hypercontracted cells. From the immunofluorescence signals we conclude that the blebs are mainly constituted of corbular and junctional SR membrane, and only to a lesser extent of network SR membrane. Deconvolution microscopy revealed that the membrane location of RyR2, calsequestrin and SERCA2 in the bleb is comparable to native SR membrane. At the bleb membrane giga-ohm seals could be obtained and patches could be excised in a way that single-channel currents could be measured, although these are not completely identified.

The ultrarapid and the transient outward K(+) current in human atrial fibrillation. Their possible role in postoperative atrial fibrillation

Atrial fibrillation (AF) causes distinct changes in atrial conduction, characterized as electrical remodeling. Experimental data on the possible significance of alterations of specific K(+)outward currents in this process are still limited in human AF. The ultra-rapid delayed rectifier current (I(Kur)) has not been studied in AF with respect to its sensitivity to 4-Aminopyridine (4-AP). To clarify the role of (1) the 4-AP sensitive I(Kur)current, compared to recordings without using 4-AP (I(Kur*)), and (2) the transient outward current (I(to)) in changes of atrial repolarization associated with AF, whole cell voltage-clamp recordings were obtained from atrial myocytes of patients undergoing elective cardiac surgery, with and without a history of atrial fibrillation (AF/non-AF). Further, a possible relation between experimental data and postoperative AF was studied. In AF patients, I(Kur*)was reduced by 40% [5.00+/-0.32 pA/pF (non-AF) and 2.91+/-0. 45 pA/pF (AF) at +50 mV, P<0.0001, n=22/11], I(Kur)by 55% [3.81+/-0. 30 pA/pF (non-AF) and 1.71+/-0.20 pA/pF (AF) at +50 mV, P<0.0001, n=22/11]. The mean amplitude of I(Kur)was significantly smaller than I(Kur*). Consistently, I(to)was reduced by 44% [11.57+/-0.77 pA/pF (non-AF) and 6.51+/-1.31 pA/pF (AF), P<0.01, n=25/11]. In 48% of non-AF patients, postoperative AF was detected. The corresponding voltage-clamp recordings showed a trend to reduced I(Kur*)and I(Kur)currents, although it did not reach statistical significance. The consistent reduction of all three K(+)currents investigated due to the presence of AF indicates an important association of abnormalities in cellular repolarization with the onset and the self-sustaining nature of human AF.

Anoxia generates rapid and massive opening of KATP channels in ventricular cardiac myocytes

OBJECTIVE

The aim was to improve the measurement of both the time course and amplitude of anoxia-induced KATP-channel current (IKATP) in isolated heart cells to specify the role of these channels in the time course of K+ accumulation in the ischemic myocardium.

METHODS

Ionic currents in isolated ventricular heart cells of the mouse were measured with a patch clamp technique under normoxic conditions (atmospheric pO2), during wash-out of oxygen, and under anoxic conditions (pO2 < 0.2 mmHg). During the measurement, the actual pO2 in the close proximity of the cell was determined with an optical technique by exciting Pd-meso-tetra(4-carboxyphenyl)porphin with light flashes of 508-570 nm and evaluating the quenching kinetics of the emitted phosphorescence signal at 630-700 nm. These quenching kinetics steeply depend on pO2 and can be evaluated best at pO2 values near 0 mmHg.

RESULTS

Out of 28 cells, 23 cells started to develop IKATP at pO2 values between 0 and 0.4 mmHg, i.e. in the range of the level of half maximum activity of the cytochrome oxidase. The remaining five cells developed IKATP between 0.4 and 1.8 mmHg. With respect to the time course, 18 out of 27 cells started to develop IKATP within the first minute after pO2 had decreased to values below 0.2 mmHg. The amplitude of IKATP induced by anoxia and various metabolic inhibitors was large, 29 +/- 12 and 48 +/- 21 nA (+40 mV), respectively. The anoxia-induced IKATP was significantly smaller than IKATP induced by metabolic inhibitors. During the pulses of 50 ms duration to +40 mV, the amplitude of IKATP decayed and, after clamping back to -80 mV, IKATP generated large tail currents. This suggests a notable change in the concentration gradient of K+ ions in the time range of tens of milliseconds.

CONCLUSIONS

The results in isolated myocytes indicate that KATP channels open sufficiently rapidly after starting anoxia and generate sufficiently large conductance at maintained anoxia to explain both the time course and magnitude of the ischemic K+ accumulation if an appropriate counter-ion flux is available.

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.

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.

The effect of the benzothiazepine diltiazem on force and Ca2+ current in isolated frog skeletal muscle fibres

1. The action of the D-cis and L-cis isomers of the benzothiazepine derivative diltiazem on isometric force and L-type Ca2+ inward current in short muscle fibres from toe muscles of the frog (Rana esculenta and R. temporaria) was investigated under voltage clamp control with two internal microelectrodes. The experiments were performed at 10 degrees C in TEA-sulphate solution, in which the concentration of ionized Ca2+ was about 4 mM. 2. In the presence of diltiazem (D-cis, 0.25-30 microM; L-cis, 5-100 microM) normal Ca2+ currents and phasic contractures could be elicited by a depolarizing voltage step. However, after long-lasting depolarizations both the Ca2+ channel and the force controlling system (voltage sensor) remained refractory for minutes instead of seconds as under control conditions, i.e. they were 'paralysed'. L-cis diltiazem was about 20 times less effective in comparison with the D-cis isomer. 3. Speed of restoration of force and Ca2+ current was retarded by up to two orders of magnitude, depending on the applied concentration of diltiazem. 4. The steady state potential dependence of force restoration (V0.5 = -49 mV; the potential at which half of the voltage sensors are restored) and that of Ca2+ current restoration (V0.5 = -57 mV) were not altered by diltiazem. 5. The time course of the transition to paralysis, which was quite slow under control conditions, could be accelerated by diltiazem in a concentration-dependent manner. 6. Paralysis developed at the same speed when the fibre was depolarized from -90 mV to -30 mV or to +30 mV. 7. The voltage dependence of Ca2+ current inactivation (inactivation curve; conditioning pulses of 2 min duration) was shifted by about 14 mV to more negative potentials (V0.5 = -31 mV; V'0.5 = -45 mV) under the influence of 1 microM-D-cis diltiazem. It is assumed that no steady state was reached during the conditioning pulse. 8. The voltage dependence of force activation (V0.5 = -45 mV) was not altered by diltiazem. 9. Since the effects of both isomers of diltiazem could only be disclosed during the adjustment of a new steady state and since the drugs acted in a similar way on the inactivation kinetics of force and Ca2+ current, it is concluded that, in depolarized fibres, the drug facilitates the transition to a stabilized inactivated form of the voltage sensor/Ca2+ channel molecule.