Enzyme changes in zymogen granules and in pancreatic secretion throughout long-term CCK treatment

Pancreatic enzyme storage and secretion were studied in rats treated twice daily with s.c. injections (5 micrograms/kg) of CCK-8 for 3, 7, and 15 days. Isolated zymogen granules were analyzed by flow cytometry to determine their FSC (forward scatter), SSC (side scatter), and amylase and trypsinogen contents. DNA content, pancreatic weight, and both basal and stimulated pancreatic secretion under i.v. CCK infusion (1.25 micrograms/kg/h) were also studies. Two subsets of zymogen granules were identified by flow cytometry in both control and CCK-treated rats on the basis of FSC and SSC parameters: Z1 (smaller and less complex) and Z2. Both subsets displayed a high degree of heterogeneity with respect to their enzyme content per zymogen granule. During the first 7 days of CCK treatment, hyperplasia and hypertrophy developed in the rats together with changes in the zymogen granules, reflected by a significantly decreased FSC, and increased SSC, and an increase in the mean trypsinogen/amylase ratio per granule. A rise in pancreatic enzyme secretion, especially of trypsin, was observed. After 15 days of CCK administration, a simultaneous decrease in amylase content and increase in trypsinogen content per zymogen granule was observed. A desensitization of the pancreas to CCK happened after 15 days of CCK administration, reflected by a reduction of all the pancreatic functions that had been increased at shorter CCK administration periods. Nevertheless, trypsinogen appeared resistant to desensitization because its secretion significantly increased in response to an i.v. infusion of CCK. CCK treatment displayed a differential packaging of the enzymes in individual zymogen granules; the trypsinogen/amylase ration was significantly higher in Z2 zymogen granules than in Z1 subset throughout the treatment.