Secretion of digestive enzyme derived from two parallel intracellular pools.

ROTHMAN, S. S., AND L. D. ISENMAN. Secretion of dz’gestive enzyme derived from two parallel intracellular pools. Am. J. Physiol. 226(5) : 1082-l 087. 1974.-Both methacholine chloride (MCh) and cholecystokinin-pancreozymin (CCK-PZ) greatly decreased the specific radioactivity of digestive enzymes secreted by the rabbit pancreas in vitro when these stimulants were added to the bathing medium 2 h after labeled amino acid had been introduced. Inhibition was approximately 97% for MCh and 78 yO for CCK-PZ. This observation indicates that secreted proteins are derived from two parallel intracellular pools, slowly (SEP) and rapidly (REP) equilibrating, probably zymogen granules and cytoplasm, respectively. The secretory stimulants preferentially increased protein secretion from the SEP. In the unstimulated state secretion from the REP accounted for no less than from 18 to 7 1 y0 of the total protein secreted for individual experiments. A 5-min delay in the response to MCh was observed. During this delay, secretion from the REP was inhibited relative to secretion from the SEP, even though overall protein output was unchanged. This suggests that competitive inhibition occurred between secretory proteins derived from the two different pools for a common exit process.