AJ: Action potentials in chick atria. Ontogenetic changes in the dependence of tetrodotoxin-resistant action potentials on calcium, strontium, barium

SUMMARY Action potentials were recorded from chick embryo atrial muscle cells bathed in Tyrode's solution. Tetrodotoxin (TTX), 3.1 fiM, was added to Mock tbe early, transient, Na*-dependent conductance system. Rectangular stimuli were used to evoke action potentials the peak amplitude (Ep) of which depend on the external concentration of divalent cations, [Me1*] , Tbe relationship between Ep and [Me1*], shifted to the right with increasing age. For example, the slope of Ep was 33 ± 2, 22 ± 1 and 11 ± 3 mV per 10-fold change in [Ca'+]0 on the 9th, 12th, and 18th incubation days, respectively. In solutions with reduced |C»'+J0, Ep increased when BaI+ or Sr1+ was added to tbe bath. The potency of Me14 in generating action potentials was B»'+> Sr'+> Ca'+ and this sequence did not change during development. Action potential amplitude, which was reduced in 18-day preparations, was increased by isoproterenol (increased Ca1+ conductance, gc.«+) and by tetraetbylammoniu