Slow frequency-dependence of action potential afterhyperpolarization in bullfrog sympathetic ganglion neurones.

The after hyperpolarizatin (AHP) which follows the action potential (AP) in bullfrog sympathetic ganglion B-cells involves activation of Ca(2+)-sensitive K+ conductances following Ca2+ influx via Ca2+ channels. The duration of AHPs evoked at 2-s stimulus intervals were 70.05 +/- 3.76% of those evoked at 90-s stimulus intervals (n = 35). Since there was no consistent effect of ryanodine (5 microM), ruthenium red, (300 microM) or dantrolene Na (35 microM) on this frequency dependence, it is unlikely to result from release of Ca2+ from intracellular stores. Ca2+ currents (ICa), studied by means of the whole-cell patch-clamp technique, exhibited a slow frequency dependence as a result of a slow inactivation process which was independent of Ca(2+)-induced ICa inactivation and ICa run-down. There was excellent correlation (r = 0.964) between the estimated changes in Ca2+ influx and the expected activation of the Ca(2+)-sensitive K+ current, IAHP. This result is consistent with the hypothesis that the frequency dependence of the AHP is a consequence of the slow inactivation of ICa.