The method of administration of cholecystokinin determines the effects evoked in the pancreas

Earlier studies have shown different effects on cell proliferation and weig ht characteristics by sulfated chole-cystokinin-8 (CCK-8S) in the rat pancr eas when the peptide has been administered continuously rather than intermi ttently. The aim of this study was as follows: (i) to compare the effect of continuous infusion and of intermittent injections of CCK-8S on cell proli feration, weight gain, and induction of apoptosis and (ii) to examine the e ffect of injections of CCK-8S on CCK-A receptor gene expression in the rat pancreas. Male Sprague-Dawley rats had subcutaneous continuous infusion of CCK-8S in a dose of 5 mug/kg/h or 1% bovine serum albumin (BSA) (vehicle) b y implanted osmotic minipumps. The rats were frilled after 4 days. Other ra ts were either injected subcutaneously only once or injected twice daily fo r 3 days with either 6 mug Of CCK dissolved in 0.5 mt BSA or 0.5 mt BSA alo ne. The rats were killed 1, 3, 6, and 12 hours after the last injection. On e hour before death they received 5-bromo-2deoxyuridine (BrdU) intraperiton eally to localize and quanti tate the cell proliferation. Plasma was collec ted for analysis of CCK. The pancreas was dissected and immunohistochemistr y alas performed for analysis of the expression of the apoptosis promoting protein bar and the apoptosis inhibiting protein bcl- 2, and for BrdU and C CK-A receptor localization. In situ hybridization (IS) was used fur examina tion and semiquantification of CCK-A receptor mRNA expression. Continuous i nfusion of CCK-8S led to a sixfold increase in plasma. CCK and a 40% increa se in pancreatic weight without any effect on BrdU labeling, Immunohistoche mistry revealed decreased tissue expression of bax but unaffected expressio n of bcl-2. Intermittent injections of CCK-8S led to hyper-CCK-emia with in creased incorporation of BrdU, indicating increased cell proliferation but no increase in pancreatic weight. Immunohistochemistry showed increased exp ression of bar, whereas bcl-2 remained unchanged, Immunofluorescence and IS H for the CCK-A receptor and its gene expression, respectively, showed a lo west intensity at 3 hours after CCK-8S injections. The results indicate tha t decreased apoptosis could explain the increased pancreatic weight during continuous infusion of CCK-8S. An increased apoptosis could explain the lac k of pancreatic weight gain upon intermittent injections of CCK-8S despite the stimulation of cell proliferation, injections of CCK-8S only transientl y decreased the tissue levels of its receptor.