SLOW FREQUENCY-DEPENDENCE OF ACTION-POTENTIAL AFTERHYPERPOLARIZATION IN BULLFROG SYMPATHETIC-GANGLION NEURONS

The afterhyperpolarization (AHP) which follows the action potential (AP) in bullfrog sympathetic ganglion B-cells involves activation of Ca2+-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-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 Ca2+ from intracellular stores. Ca2+ currents (I(Ca)), 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 Ca2+-induced I(Ca) inactivation and I(Ca) run-down. There was excellent correlation (r = 0.964) between the estimated changes in Ca2+ influx and the expected activation of the Ca2+-sensitive K+ current, I(AHP). This result is consistent with the hypothesis that the frequency dependence of the AHP is a consequence of the slow inactivation Of I(Ca).