A photoelectric recording set for pulsation curves of heart muscle cultures in vitro

Singular behavior of slow dynamics of single excitable cells

In various kinds of cultured cells, it has been reported that the membrane potential exhibits fluctuations with long-term correlations, although the underlying mechanism remains to be elucidated. A cardiac muscle cell culture serves as an excellent experimental system to investigate this phenomenon because timings of excitations can be determined over an extended time period in a noninvasive manner through visualization of contractions, although the properties of beat-timing fluctuations of cardiac muscle cells at the single-cell level remains to be fully clarified. In this article, we report on our investigation of spontaneous contractions of cultured rat cardiac muscle cells at the single-cell level. It was found that single cells exhibit several typical temporal patterns of contractions and spontaneous transitions among them. Detrended fluctuation analysis on the time series of interbeat intervals revealed the presence of 1/f(beta) noise at sufficiently large timescales. Furthermore, multifractality was also found in the time series of interbeat intervals. These experimental trends were successfully explained using a simple mathematical model, incorporating correlated noise into ionic currents. From these findings, it was established that singular fluctuations accompanying 1/f(beta) noise and multifractality are intrinsic properties of single cardiac muscle cells.

A simple technique for on-line measurement of contractions of single smooth muscle fibers under current or voltage clamp

This paper describes a simple technique for routine on-line measurement of length and unloaded, isotonic contractions of single smooth muscle fibers during electrophysiological experiments. The fiber is held by the recording electrode itself, stretched straight in a fast-flowing stream of solution. The video image of the fiber is measured on-line by a simple computer program. Unlike other optical methods of tracking fiber length, the technique does not require a rigid one-dimensional contraction of the fiber. The technique is reliable and easy to use, and readily compatible with current clamp, voltage clamp, and rapid reversible application of neurotransmitters and drugs.

A computerized device for long-term measurements of the contraction frequency of cultured rat heart cells under stable incubating conditions

A new computer-controlled measurement system for assessing beat rates of spontaneously contracting cultured heart cells is presented. The system overcomes several disadvantages of established techniques such as: (i) lack of precise control of the environment (pH, temperature, humidity); (ii) restriction to the measurement of one culture at a time; (iii) inability to obtain long-term measurements. The beat rate is recorded by subjecting monolayer cultures to dark field illumination and recording contraction-related changes in light scattering. A maximum of environmental stability is achieved because measurements are performed in the incubator. Beat rates of up to 16 individual culture dishes can be assessed repeatedly during freely selectable time intervals. Control of the experiments, data acquisition and data analysis are carried out by a computer. The specific advantage of the method lies in the ability to measure the beat rate of several culture dishes continuously over time intervals limited only by the viability of the cultures, i. e. up to several weeks.

Closed loop television tracking of beating heart cells in vitro

The motion of beating heart cells in vitro has been used as a sensitive indicator of the presence of drugs. The combination of closed loop television video tracking and latex microspheres placed on the cells as markers results in an improved measurement of cell motion. Latex microspheres, 5.2 micrometers, are allowed to settle onto beating myocardial heart cells in vitro, where they move in concert with the cell motion. These microspheres, when viewed by televised transmission microscopy with normal optics, present high contrast circular targets that are excellent for closed loop television video tracking techniques. With video tracking, an analog voltage is generated proportional to the horizontal center of intensity of the target being tracked. This signal represents the location of a video demodulator gate that is maintained centered over the video target by feedback control.

Voltage-sensitive dyes. Discerning contraction and electrical signals in myocardium

Muscle contraction can introduce artifact in attempted optical measurements of the action potential in heart tissue stained with voltage-sensitive dyes. Using rabbit sinus node and atrial tissue in vitro, we found that the voltage-sensitive part of the optical signal remains relatively unchanged by variations in the rate of external stimulation or by the application of transmural stimulation (TS), while the contraction-related component can be significantly increased by these same interventions. The relative contributions of membrane voltage and contraction to the optical signal can thus be determined. In particular, the rapid upstroke component of the action potential can be easily identified using this technique.

An optical monitor of tension for small cardiac preparations

When a light beam is focused on a muscle preparation which is allowed to contract, large changes in the intensity of the emerging light accompany the contraction. These movement-related optical signals were studied and compared to simultaneous measurements of force in isolated cardiac Purkinje fibers. The two signals were compared in response to action potentials and to graded changes in membrane potential controlled under voltage clamp. These experiments indicate that the optical signal is a sensitive monitor of tension development under these conditions. This technique is particularly well-suited to force measurements in smaller preparations in which direct mechanical techniques are not feasible.

The mechanical activity of chick embryonic myocardial cell aggregates

1. Simultaneous recordings of membrane potential and edge movement were obtained in spontaneously beating chick embryonic myocardial cell aggregates, which are known to behave as an isopotential syncytium.2. The time course of edge movement was similar in different aggregates, and in different regions of the same aggregate.3. Peak amplitude was increased by 10(-6)m-ouabain, and by rapid reduction of the external sodium concentration.4. Peak amplitude was decreased during single premature action potentials, but sustained rapid pacing produced an ascending staircase.5. Depolarizing current pulses increased both the amplitude and duration of the contraction, and caused potentiation of the next spontaneous beat. Edge displacement during a series of pulses was a monotonic function of membrane potential.6. Edge movement between action potentials (diastolic movement) was well fitted by an exponential with a mean time constant of 69 msec. Diastolic edge movement was due to a weak, slowly decaying contractile force, which was demonstrated in cells grown on a linear-elastic nylon bristle.7. The time course of diastolic edge movement remained constant, or nearly constant, during variations in peak amplitude that resulted from prematurity of the action potential, exposure to 10(-6)m-ouabain, spontaneous mechanical alternans, or prolongation of the action potential by current pulses.8. In contrast, reduction of the external sodium concentration produced marked, selective slowing of the diastolic edge movement. Similar slowing occurred during cooling and during staircase. Diastolic edge movement was selectively accelerated when the preceding interbeat interval was prolonged by a hyperpolarizing current pulse.9. The above observations are consistent with the hypothesis that edge displacement is a monotonic function of contractile force.10. The slow relaxation between action potentials probably reflects removal of intracellular calcium across the surface membrane in exchange for sodium. Changes in the rate of calcium removal may play a role in the regulation of contractility in this tissue.

Correlation between relaxation and automaticity in embryonic heart cell aggregates

Diastolic depolarization in cardiac muscle is due to a decline in potassium permeability that has been ascribed to removal of intracellular free calcium. A continued decline in tension during the pacemaker potential might therefore occur. In this study, contractile responses of chicken embryonic heart cell aggregates are recorded with a photodiode. Photodiode output is well correlated with the position of the aggregate's edge. Movements of different edges are synchronous, and their amplitude and duration vary appropriately during experimental maneuvers that alter the magnitude and duration of contractile force. Edge movement during relaxation has two phases, a rapid phase lasting about 100 msec and a slow phase that may last over 10 sec. The slow phase is not due to viscoelasticity because its time course does not depend on the magnitude or duration of the initial deformation. The rate of relaxation is correlated with the rate of depolarization during the pacemaker potential. Reduction in automaticity during cooling, spontaneous variation, and overdrive pacing are associated with impairment of the slow component of relaxation. Electrophysiological evidence suggests that the diastolic potassium permeability of the aggregates is controlled by intracellular calcium. A possible explanation for the correlation between the slope of the pacemaker potential and the slow component of relaxation is that both phenomena reflect a common physiological process-i.e., the removal of free calcium from the cytoplasm.

Excitation-contraction coupling of isolated cardiac fibers with disrupted or closed sarcolemmas. Calcium-dependent cyclic and tonic contractions

Single cells of adult rat ventricle were separated with 0.1% trypsin and 0.05-0.1% collagenase. Ten percent of these cells were tubular and striated, contracted spontaneously in the presence of 0.05 mM CaCl 2 or after addition of 0.1-0.5 mM CaCl 2 , and developed contracture in 1 mM CaCl 2 . A resting potential of -30 to -50 mv and an action potential more than 100 msec in duration were recorded in some of the cells. These relatively intact cells were contrasted with myocardial fibers (10-100 µ ) obtained by homogenization. In the presence of 0.025 mM ethyleneglycol bis(β-aminoethylether)-N, N'-tetraacetic acid (EGTA), localized cyclic contractions with intracellular asynchrony were observed under the phase microscope in obviously disrupted areas of these mechanically separated fibers. These contractions were insensitive to the ratio of Na + to K + , and neither resting potentials nor action potentials were observed. Thus, these disrupted fibers were considered skinned fibers of cardiac muscle; this assumption was further supported by the absence of modification of fiber sensitivity to Ca 2+ by simultaneous skinning with EDTA. Effects of Ca 2+ were studied with control of pCa in the medium by EGTA-Ca buffers, either directly on the homogenate or after depletion of Ca 2+ content of the tissue. At 22°C, pH 7.0, and in the presence of 4 mM MgCl 2 , 5 mM adenosine triphosphate, and 2 mM EGTA, cyclic contractions were inhibited and a tonic contraction was induced at a pCa of 6.25-6.5, which represented the threshold for activation of myofilaments. With the same medium but with low EGTA (less than 0.2 mM), cyclic contractions were obtained if pCa was equal to or lower than 7.4-7.6. Therefore, these contractions corresponded to a cyclic release of Ca 2+ from cellular stores, yielding a transient activation of myofilaments. This large release of Ca 2+ within the fiber was induced by a small addition of Ca 2+ to the medium. It was concluded that a regenerative release of Ca 2+ could be demonstrated in heart muscle under these conditions. Localized cyclic contractions were not observed in disrupted fibers of frog heart, in which spontaneously beating cells could be separated with enzymes.