Slow frequency-dependence of action potential after hyperpolarization in bullfrog sympathetic ganglion neurones

The afterhyperpolarization (AHP) which follows the action potential (AP) in bullfrog sympathetic ganglion B-cells involves activation of Ca super(2+)-sensitive K super(+) conductances following Ca super(2+) influx via Ca super(2+) channels. The duration of AHPs evoked at 2-s stimulus intervals were 70.05 plus or minus 3.76% of those evoked at 90-s stimulus intervals (n = 35). Since there was no consistent effect of ryanodine (5 mu M), ruthenium red, (300 mu M) or dantrolene Na (35 mu M) on this frequency dependence, it is unlikely to result from release of Ca super(2+) from intracellular stores. The result is consistent with the hypothesis that the frequency dependence of the AHP is a consequence of the slow inactivation of I sub(Ca).