title: Action potentials in chick atria. Ontogenetic changes in the dependence of tetrodotoxin-resistant action potentials on calcium, strontium, barium.
abstract: Action potentials were recorded from chick embryo atrial muscle cells bathed in Tyrode's solution. Tetrodotoxin (TTX), 3.1 muM, was added to block the 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, [Me2+]0. The relationship between Ep and [Me2+]0 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 [Ca2+]0 on the 9th, 12th, and 18th incubation days, respectively. In solutions with reduced [Ca2+]0, Ep increased when Ba2+ or Sr2+ was added to the bath. The potency of Me2+ in generating action potentials was Ba2+ greater than Sr2+ greater than Ca2+ and this sequence did not change during development. Action potential amplitude, which was reduced in 18-day preparations, was increased by isoproterenol (increased Ca2+ conductance, gCa2+) and by tetraethylammonium (TEA) ion (decreased K+ conductance, gK). The results show that (1) Me2+-dependent action potentials support membrane excitation in chick atrial cells treated with TTX, and (2) the ability of Me2+ to support action potentials decreases during ontogenesis. We conclude from these experiments that the ontogenetically related decrease in Me2+-induced action potentials is the result of a reduction in gMe2+/gK+ during stimulation.
mesh_terms: D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D016275:Atrial Function; D001464:Barium / Q000494:pharmacology*; D002118:Calcium / Q000494:pharmacology*; D002642:Chick Embryo; D004558:Electric Stimulation; D006321:Heart / Q000196:embryology*; D006325:Heart Atria / Q000196:embryology; D007545:Isoproterenol / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects; D013324:Strontium / Q000494:pharmacology*; D013757:Tetraethylammonium Compounds / Q000494:pharmacology; D013779:Tetrodotoxin / Q000494:pharmacology*
chemical_list: D013757:Tetraethylammonium Compounds; D001464:Barium; D013779:Tetrodotoxin; D007545:Isoproterenol; D002118:Calcium; D013324:Strontium