Distinct heterotrimeric GTP-binding-proteins act in series to control the exocytotic machinery in chromaffin cells.

Regulated exocytosis requires both calcium and MgATP. Although the biochemical events responsible for ATP-dependent calcium-activated secretion have not been elucidated yet, some progress has been made in determining the relative order of the ATP- and calcium-dependent steps. Studies on permeabilized secretory cells have shown that MgATP acts before calcium and maintains the secretory apparatus in a "primed" state. In this paper, we examine the possible role of heterotrimeric G-proteins in these two steps of exocytosis in permeabilized chromaffin cells. We show that mastoparan and other activators of heterotrimeric G-proteins inhibit the MgATP-dependent reaction, but stimulate the late calcium-dependent step of exocytosis. Non-hydrolyzable GTP analogues (GTP-gamma-S and GMP-PNP) mimic the dual effects of mastoparan on secretion, but with different potencies, suggesting the involvement of two distinct heterotrimeric G-proteins in regulated exocytosis. GPAnt-2, a substance P related peptide known to inhibit the stimulation of Gi and Go by mastoparan, reverses, in a dose-dependent manner, both the inhibitory and stimulatory effects of mastoparan on secretion. These results indicate that two distinct heterotrimeric G-proteins from the Gi/o family may act in series in the exocytotic pathway in chromaffin cells: one controls the ATP-dependent priming step, whereas the second is involved in the late calcium-dependent fusion step which does not require ATP.