Fast inactivation of Na+ current in rat adrenal chromaffin cells involves two independent inactivation pathways

Voltage-dependent sodium (Nav) current in adrenal chromaffin cells (CCs) is rapidly inactivating and TTX-sensitive. The fractional availability of CC Nav current has been implicated in regulation of action potential (AP) frequency and the occurrence of slow-wave burst firing. To ascertain whether features of CC Nav inactivation might influence AP firing, we recorded Nav current in rat CCs, primarily from adrenal medullary slices. A key feature of CC Nav current is that recovery from inactivation, even following brief (5 ms) inactivation steps, exhibits two exponential components of generally similar amplitude. Variations of standard paired pulse protocols support the view that entry into the fast and slower recovery processes result from largely independent, competing inactivation pathways, both of which occur with similar onset times at depolarizing potentials. Over voltages from -120 to -80 mV, faster recovery varies from ~3 to 30 ms, while slower recovery from about 50-400 ms. At strong activation voltages (+0 mV and more positive), the relative entry into slow or fast recovery pathways is similar and independent of voltage. Trains of brief inactivating steps result in cumulative increases in the slower recovery fraction. This supports idea that brief recovery intervals preferentially allow recovery of channels from fast recovery pathways, thereby increasing the fraction of channels in the slow recovery pathway with each subsequent inactivation step. This provides a mechanism whereby differential rates of recovery produce use-dependent accumulation in slower recovery pathways. Consistent with use-dependent accumulation of channels in slow recovery pathways, repetitive AP clamp waveforms at 1-10 Hz frequencies reduce Nav availability to 10-20% of initial amplitude dependent on holding potential. The results indicate that there are two distinct pathways of fast inactivation, one that leads to normal fast recovery and the other with a slower time course, which together are well-suited to mediate use-dependent changes in Nav availability.