Rab3A delayed catecholamine secretion from bovine adrenal chromaffin cells.

To study the role of Rab3A in regulated secretion, recombinant Rab3A protein plus various guanine nucleotides were perfused into bovine adrenal chromaffin cells via a patch pipette, and depolarization-evoked catecholamine secretion from individual cells was measured by amperometry. Rab3A plus GTP, GDP, and GTP gamma S all delayed the occurrence of exocytosis while Rab3A, GTP, or GDP alone had no effect. The delay of evoked-secretion is specific for Rab3A plus guanine nucleotides, since treatments with GTP plus BSA or GDP beta S plus Rab3A had no effect on the appearance of secretion events. Rab3A plus GTP increased the frequency of the exocytosis of smaller packets of catecholamine. These results are consistent with the notion that an excess of Rab3A in the cytoplasm depletes the regulatory proteins responsible for regulating the interconversion between GTP- and GDP-bound forms of Rab3A and that Rab3A is involved in the final steps of regulated exocytosis.