The mechanical activity of chick embryonic myocardial cell aggregates

Abstract

SUMMARY1. 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 ofthe 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.

Predicting the onset of period-doubling bifurcations in noisy cardiac systems

“…The ovoid-shaped aggregates, with diameters between 100 μm and 300 μm, beat spontaneously with intrinsic periods between 1 s and 2 s. We monitored the activity of the spontaneously beating aggregates by measuring the variation in light intensity of a pixel on the edge of each aggregate. Estimates of the magnitude of the displacement of the aggregate as a consequence of the beat (or contraction) are on the order of ∼5 μm, which is consistent with previous measurements (37). We analyzed the beat dynamics of the aggregates by computing interbeat intervals, the time between successive beats.…”

External Na-independent Ca extrusion in cultured ventricular cells. Magnitude and functional significance.

“…A second, slower phase, with a time constant ofseveral hundred milliseconds, may be due to Ca extrusion across the sarcolemma (Miura et al, 1981 ;Clusin, 1981). The present results (see also Clusin et al, 1983) indicate that the slow second phase of relaxation in spontaneously contracting cultured cells might well be due to Na-Ca exchange, as previously suggested (Miura et al, 1981 ;Clusin, 1981). It is too rapid, however, to be accounted for by the Ca pump.…”

Interaction of palmitoyl carnitine with calcium antagonists in myocytes

“…This is essentially a measure of edge displacement and would typically be of the order of 5pm. Edge movement has been previously shown to be a function of the force of contraction (Clusin, 1981) and in this study edge movement has been used synonymously with contraction. Aggregates were continually superfused with culture media substitute salt solution as described earlier prewarmed to 370C.…”

“…Ventricles were chopped into fragments and were washed in Hanks Ca2+-and Mg2"-free balanced salt solution (Gibco) and then agitated at 37°C in the presence of 0.05% trypsin as described by Clusin (1981) In experiments where the interaction between palmitoyl carnitine and antagonists was investigated, concentrations of antagonists which caused approximately 85% inhibition of beating were added simulataneously with varying concentrations of palmitoyl carnitine. The scoring protocol was the same as that described for antagonist experiments and the data were again expressed as % of the sham-treated control aggregate beating.…”