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

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Abstract
Small re-aggregates of cells dissociated from the ventricles of 12 or 17 day old chick embryonic hearts having beating properties in low K tissue culture medium which differ from re-aggregates prepared from 7 day old hearts; 12 day preparations beat with a slower rate than comparable size 7 day preparations or they are quiescent; 17 day preparations usually do not beat. A 2 microelectrode voltage clamp technique was used to analyze the membrane currents in these preparations in the -120 to -40 mV potential range. K+ membrane current was measured in 12 day and 17 day preparations which was qualitatively similar to the K+ current measured in 7 days preparations. It differed in that its amplitude was diminished in the ratio 1:0.2:0.03 over the 7, 12 and 17 days sequence. Its maximum time constant was 2 s at 12 and 17 days, as compared to 1 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. The background current also charged 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 Na current. The change in the background component is primarily responsible for the loss of automaticity over the 7, 12 and 17 day sequence.