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.