Hearts from chick embryos aged 4,7, or 14 days were dissociated into their component cells, and the cells allowed to reassociate in the form of smooth-surfaced spheroidal aggregates on a gyratory shaker. Records from intracellular electrodes inserted into two widely spaced cells in a spontaneously beating aggregate indicated that the action potentials occurred virtually simultaneously. In aggregates made quiescent with tetrodotoxin, the voltage response to a current pulse injected in one cell could be noted by recording with a second microelectrode at various distance from the current source. The magnitude of the response was found not to vary with distance. It is concluded that the component cells in an aggregate are normally tightly coupled electrically; the cell boundaries do not constitute an appreciable resistive barrier. Such ag-regates behave as virtually isopential systems, with properties similar to those of single spherical cells, as modeled by Eisenberg and Engel (1970. J. Gen. Physiol. 55:736-757). Passive membrane time constant ranged from 11 to 31 ms, with a mean value of 17 ms; this value did not vary with aggregate size. Input resistance (V/I) varied inversely with aggregate size, as predicted, but with much scatter in the measured values. Specific membrane resistance was calculated as either 13,000 or 800 ohm-cm2 depending on whether input resistance was attributed to the total cell surface membrane area or to the outer surface of the sphere alone. No systematic difference in passive electrical properties of aggregates composed of 4-, 7-, and 14-day cells was seen. It is concluded that these aggregates may be suitable for voltage clamp analysis of their excitable membrane properties.

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