MgATP acts before Ca2+ to prime amylase secretion from permeabilized rat pancreatic acini.

The time course of Ca(2+)-dependent amylase secretion from alpha-toxin-permeabilized rat pancreatic acini was biphasic, consisting of an initial burst of secretion, which lasted approximately 2.5 min, followed by a slower, sustained release of amylase. The initial, rapid phase of secretion did not appear to require MgATP, whereas the second, sustained phase of secretion was entirely MgATP dependent. The initial, rapid, apparently MgATP-independent response was labile in the prolonged absence of MgATP and was abolished when the acini were metabolically poisoned before permeabilization. These findings suggest that the initial phase of secretion does not require the presence of MgATP but is actually dependent on an MgATP-requiring event that occurred within the acini before permeabilization. Our studies also demonstrated that MgATP acts before Ca2+ to prime amylase secretion. Thus the initial, rapid phase of secretion most probably reflects release via exocytotic sites primed by MgATP before permeabilization. The slower kinetics of the second, sustained phase of secretion may, at least in part, reflect the repriming of the exocytotic machinery. The results of these studies also indicate that Ca(2+)-dependent secretion (regulated exocytosis) in the pancreatic acinar cell is composed of at least two biochemically distinct steps. The first step is MgATP dependent and primes exocytosis and is followed by a Ca(2+)-dependent, but MgATP-independent, step that triggers exocytosis.