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 ontog...Continue Reading
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