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.