NaI prevents and reverses transmission failure in low Ca2+ Ringer

Synaptic transmission in the bullfrog sympathetic ganglion fails progressively as Ca2+ is reduced from the normal (1.8 mM) to 0.47 mM in an NaCl Ringer. This Ca2+ dependence of synaptic transmission is shifted to the left (lower Ca2+ levels) when NaCl is replaced by NaI. Replacement of NaCl by NaI also reverses pre-existing transmission failure in low Ca2+. As reported previously by us, NaI also causes repetitive postganglionic responses to each single preganglionic stimulus. It is of interest that iodide's ability to cause repetitive postganglionic responses and to shift the Ca2+ dependence of synaptic transmission duplicate the effects of aminopyridines, notably 3,4-diaminopyridine.