Photoelectric recording of mechanical responses of cardiac myocytes

Meyer, Rainer; Wiemer, J.; Dembski, J; Haas, Hans

doi:10.1007/bf00581134

Cited by 10 publications

(3 citation statements)

References 20 publications

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“…In some experiments, simultaneously with the electrophysiological measurements, the tonic contraction of single cells was recorded optically as unloaded shortening (Meyer et al 1987). The measured values have been normalized to their respective control shortening during a voltage‐clamp step to 0 mV.…”

Section: Methodsmentioning

confidence: 99%

Control of L‐type calcium current during the action potential of guinea‐pig ventricular myocytes

Linz

Meyer

1998

The Journal of Physiology

101

122

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The L-type calcium current (ICa,L) plays an important role in excitation-contraction coupling of heart cells, as it forms the major trigger for Ca¥-induced Ca¥ release from the sarcoplasmic reticulum (SR) and provides Ca¥ for refilling of the SR (Callewaert, 1992;Barry & Bridge, 1993). The voltage relation of this current is bell shaped with an inward current maximum at about 10 mV and a reversal potential around 60 mV (McDonald et al. 1994). At the top of the action potential between 40 and 50 mV, when ICa,L is activated, ICa,L is already near its reversal potential and thus small. Moreover, the potential of maximum ICa,L is not reached before the repolarization of the cell. The obvious contradiction that ICa,L is important for excitationcontraction coupling on the one hand, but is small during the action potential on the other hand, leads to the idea of applying voltage-clamp pulses consisting of digitized action potentials to record the time course of ICa,L directly during the action potential. This so-called action potential clamp 1. During an action potential the L-type Ca¥ current (ICa,L) activates rapidly, then partially declines leading to a sustained inward current during the plateau phase. The reason for the sustained part of ICa,L has been investigated here. 2. In the present study the mechanisms controlling the ICa,L during an action potential were investigated quantitatively in isolated guinea-pig ventricular myocytes by whole-cell patch clamp. To measure the actual time courses of ICa,L and the corresponding L-type channel inactivation (fAP) during an action potential, action potential-clamp protocols combined with square pulses were applied. 3. Within the first 10 ms of the action potential the ICa,L rapidly inactivated by about 50%; during the plateau phase inactivation proceeded to 95%. Later, during repolarization, the L_type channels recovered up to 25%. 4. The voltage-dependent component of inactivation during an action potential was determined from measurements of L-type current carried by monovalent cations. This component of inactivation proceeded rather slowly and contributed only a little to fAP. ICa,L during an action potential is thus mainly controlled by Ca¥-dependent inactivation. 5. In order to investigate the source of the Ca¥ controlling fAP, internal Ca¥ homeostasis was manipulated by the use of Ca¥ buffers (EGTA, BAPTA), by blocking Na¤-Ca¥ exchange, or by blocking Ca¥ release from the sarcoplasmic reticulum (SR). Internal BAPTA markedly reduced the L-type channel inactivation during the entire action potential, whereas EGTA affected fAP only during the middle and late plateau phases. Inhibition of Na¤-Ca¥ exchange markedly increased inactivation of L-type channels. Although blocking SR Ca¥ release decreased the fura_2-measured cytoplasmic Ca¥ concentration ([Ca¥]é) transient by about 90%, it reduced L-type channel inactivation only during the initial 50 ms of the action potential. Thus, it is Ca¥ entering the cell through the L-type channels that controls the inactivation proc...

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Section: Methodsmentioning

confidence: 99%

Control of L‐type calcium current during the action potential of guinea‐pig ventricular myocytes

Linz

Meyer

1998

The Journal of Physiology

101

122

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“…For about 90% compensation of series resistance a fraction of clamp current was fed back to the control amplifier and subtracted from the command signal, An inverted microscope (Zeiss IM 35) was used for visual control of the experiments. Mechanical activity (shortening) of a cell was continuously monitored by transmicroscopic photometry (24). In this arrangement the contraction signal is in proportion to the edge movement of a cell.…”

Section: Methodsmentioning

confidence: 99%

Excitation-contraction coupling in voltage-clamped cardiac myocytes

1989

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Myocytes isolated from guinea pig ventricles were voltage-clamped using patch pipettes in the whole-cell configuration. For proper voltage control fast Na+ current was blocked by TTX or inactivated by an appropriate prepulse. Zero-load cell shortening was monitored by a photoelectric device. The mechanical response to a short depolarizing clamp was mainly a phasic (transient) contraction. Long-lasting depolarizations caused a tonic (sustained) shortening of a cell. Different clamp patterns were used to study the mode of activation of phasic contraction. 1) With a constant Ca2+ preload established by a train of conditioning pulses, the shortening-voltage relation measured with test pulses of varying height was a bell-shaped curve reflecting the slow inward current (ICa)-voltage relation. The test pulse had a striking influence on the first contraction of the following conditioning series, resulting in an S-shaped relation between post-test contraction and test potential. 2) With series of identical clamps of varying height, steady-state contraction was maximal around 40 mV and not in proportion to ICa. In these measurements Ca2+ preload was likely to increase with increasing potential. It is concluded that ICa initiates phasic contraction by inducing a release of Ca2+ from internal stores while replenishment of the stores is largely determined by an electrogenic transsarcolemmal Na+-Ca2+ exchange. The data suggest that Na+-Ca2+ exchange is not only involved in long-term changes of cardiac contractility but also in beat-to-beat regulation.

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“…The phasic contraction of single cells was recorded optically as unloaded shortening [36]. With this method relative changes in cell length could be measured while electrophysiological recordings were being made.…”

Section: Optical Shortening Recordingsmentioning

confidence: 99%

The late component of L-type calcium current during guinea-pig cardiac action potentials and its contribution to contraction

Linz

Meyer

1998

Pfl�gers Archiv European Journal of Physiology

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L-Type Ca2+ current (ICa,L) elicited during the action potential (AP) of guinea-pig ventricular myocytes exhibits an early and a late component. The whole-cell patch-clamp technique was used to characterize the process regulating the late ICa,L component and to assess its contribution to excitation-contraction coupling. A stepwise decrease in repolarization rate of AP-like voltage-clamp pulses led to an exponential increase in Ca2+ charge carried by ICa, L. This saturation behaviour was significantly reduced or absent when Ba2+ or monovalent cations were used as charge carriers, which suggests that the late component of ICa,L is controlled mainly by Ca2+-dependent processes. Simultaneously recording ICa,L and zero-load shortening or the internal Ca2+ concentration (fura-2) revealed that Ca2+ carried by the late component of ICa,L markedly contributes to the Ca2+ content of the sarcoplasmic reticulum (SR). Reducing the charge transfer by late ICa,L during a series of AP-like conditioning clamp pulses by 48% reduced the shortening amplitude during a subsequent test stimulation by 56%. This relationship was absent during long rectangular depolarizing conditioning clamps, during which Na+/Ca2+ exchange increased its influence on SR Ca2+ loading. The late component of ICa,L developed only a minor direct influence on the simultaneous cell shortening. Thus, the main contribution of the late ICa,L component is to supply Ca2+ for SR loading.

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Photoelectric recording of mechanical responses of cardiac myocytes

Cited by 10 publications

References 20 publications

Control of L‐type calcium current during the action potential of guinea‐pig ventricular myocytes

Control of L‐type calcium current during the action potential of guinea‐pig ventricular myocytes

Excitation-contraction coupling in voltage-clamped cardiac myocytes

The late component of L-type calcium current during guinea-pig cardiac action potentials and its contribution to contraction

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