Abstract
We have studied the functional role of Ca V3 channels in triggering fast exocytosis in rat chromaffin cells (RCCs). Ca V3 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 μM Ni 2+, which selectively blocks T-type currents in RCCs. This suggests that the “low-threshold exocytosis” induced by cAMP is due to increased Ca 2+ influx through cAMP-recruited T-type channels, rather than to an enhanced secretion downstream of Ca 2+ 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 Ca 2+ dependence of exocytosis, the initial speed of vesicle depletion, and the mean quantal size of single secretory events. All this indicates that cAMP-recruited Ca V3 channels enhance the secretory activity of RCCs at low voltages by coupling to the secretory apparatus with a Ca 2+ efficacy similar to that of already existing high-threshold Ca 2+ channels. Finally, using RT-PCRs we found that the fast inactivating low-threshold Ca 2+ current component recruited by cAMP is selectively associated to the α 1H (Ca V3.2) channel isoform.
Published Version (
Free)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call