Low-threshold exocytosis induced by cAMP-recruited Cav3.2 (alpha1H) channels in rat chromaffin cells

We have studied the functional role of CaV3 channels in triggering fast exocytosis in rat chromaffin cells (RCCs). CaV3 T-type channels were selectively recruited by chronic exposures to cAMP (3 days) via an exchange protein directly activated by cAMP (Epac)-mediated pathway. Here we show that cAMP-treated cells had increased secretory responses, which could be evoked even at very low depolarizations (–50, –40 mV). Potentiation of exocytosis in cAMP-treated cells did not occur in the presence of 50 mM Ni21, which selectively blocks T-type currents in RCCs. This suggests that the ‘‘low-threshold exocytosis’’ induced by cAMP is due to increased Ca21 influx through cAMP-recruited T-type channels, rather than to an enhanced secretion downstream of Ca21 entry, as previously reported for short-term cAMP treatments (20 min). Newly recruited T-type channels increase the fast secretory response at low voltages without altering the size of the immediately releasable pool. They also preserve the Ca21 dependence of exocytosis, the initial speed of vesicle depletion, and the mean quantal size of single secretory events. All this indicates that cAMP-recruited CaV3 channels enhance the secretory activity of RCCs at low voltages by coupling to the secretory apparatus with a Ca21 efficacy similar to that of already existing highthreshold Ca21 channels. Finally, using RT-PCRs we found that the fast inactivating low-threshold Ca21 current component recruited by cAMP is selectively associated to the a1H (CaV3.2) channel isoform.