The effects of stimulation rate on calcium-dependent action potentials recorded from chick embryo heart cell aggregates.
1. Action potentials were recorded from aggregates of heart cells prepared from 3- to 7-day chick embryos. At 3 days the maximum rate of rise (+ Vmax) was insensitive to TTX; at 7 days it was considerably reduced by TTX. 2. In the presence of TTX the action potential overshoot was dependent on [Ca]0; the results may be fitted using constant field theory and assuming that the membrane is over a hundred times more permeable to Ca than to Na or K. 3. An increase in stimulation rate in the range 0.2-2 Hz led to an increase in both overshoot and + Vmax. This effect was not seen after addition of 20 mM-tetraethylammonium ions, nor when Sr was substituted for Ca in the external medium. We suggest that these rate-dependent changes may result from partial inactivation of an outward K current.
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D002118:Calcium / Q000494:pharmacology / Q000502:physiology*; D002449:Cell Aggregation; D002478:Cells, Cultured; D002642:Chick Embryo; D006321:Heart / Q000187:drug effects / Q000196:embryology* / Q000502:physiology; D007700:Kinetics; D009206:Myocardium / Q000166:cytology; D013268:Stimulation, Chemical; D013324:Strontium / Q000494:pharmacology; D013757:Tetraethylammonium Compounds / Q000494:pharmacology; D013779:Tetrodotoxin / Q000494:pharmacology
D013757:Tetraethylammonium Compounds; D013779:Tetrodotoxin; D002118:Calcium; D013324:Strontium