Electrical activity in embryonic heart cell aggregates developmental aspects

Action potential parameters were measured in beating heart cell aggregates which were formed from trypsin-dissociated cells of embryonic chick hearts aged \(2{\raise0.5ex\hbox{\(\scriptstyle 1\)}\kern-0.1em/\kern-0.15em\lower0.25ex\hbox{\(\scriptstyle 2\)}}\) , 4 or 7 days. In aggregates composed of cells from the whole heart there was an increase in the maximum diastolic potential, overshoot, maximum rate of rise of the action potential ( \(\dot V_{\max }\) ), and action potential duration between days \(2{\raise0.5ex\hbox{\(\scriptstyle 1\)}\kern-0.1em/\kern-0.15em\lower0.25ex\hbox{\(\scriptstyle 2\)}}\) and 7. Action potential parameters from 4-or 7-day aggregates composed exclusively of artial or ventricular cells were similar to those in whole heart aggregates of the same age with the exception of the action potential duration in which atrial <whole heart <ventricular. Between days 4 and 7 the increases in duration were approximately 14% in atrial, 35% in whole heart, and 50% in ventricular aggregates. Differences in action potential duration, within or between ages, were not due solely to differences in the rate of beating. Action potentials in whole heart aggregates aged \(2{\raise0.5ex\hbox{\(\scriptstyle 1\)}\kern-0.1em/\kern-0.15em\lower0.25ex\hbox{\(\scriptstyle 2\)}}\) days were insensitive to TTX (10⁻⁵ g/ml) but abolished by D600 (1 μg/ml). Conversely, at 7 days activity was suppressed by TTX (2×10⁻⁸ g/ml) while D600 (1 μg/ml) shortened the action potential duration and reduced the overshoot without influencing \(\dot V_{\max }\) . Adrenaline (1 μg/ml) restored the action potential overshoot and duration in 7-day aggregates treated with D600. Action potential development in embryonic heart cells appears to be characterized by the functional appearance of fast inward channels. The slow channel mechanism, previously utilized in action potential generation, may gradually assume its adult role of carrying inward current during the plateau phase. In contrast to monolayer cultures, embryonic heart cells cultured in aggregate form seem to have membrane properties similar to those of intact tissue.