Developmental changes in subthreshold pace-maker currents in chick embryonic heart cells.

1. Small re-aggregates of cells dissociated from the ventricles of 12-or 17-day-old chick embryonic hearts have beating properties in low potassium tissue culture medium which differ from re-aggregates prepared from 7-day-old hearts; 12-day preparations either beat with a slower rate than comparable-size 7-day preparations or they are quiescent; 17-day preparations usually do not beat. 2. A two-micro-electrode voltage-clamp technique was used to analyse the membrane currents in these preparations in the -120 to -40 mV potential range. 3. A potassium ion membrane current was measured in 12-day and 17-day preparations which was qualitatively similar to the potassium ion current measured in 7-day preparations (Clay & Shrier, 1981). It differed in that its amplitude was diminished in the ratio 1:0.2:0.03 over the 7-, 12- and 17-day sequence. Furthermore, its maximum time constant was 2 sec at both 12 and 17 days, as compared to 1 sec at 7 days, and its membrane potential range of activation was -80 to -70 mV at 12 and 17 days, as compared to -90 to -60 mV at 7 days. 4. The background current also changed with development, becoming less inward over the 7-, 12- and 17-day sequence. The voltage dependence of this change suggested that it was caused by a developmental decrease of the background sodium current. 5. The change in the background component is primarily responsible for the loss of automaticity over the 7-, 12-, and 17-day sequence.