Earlier studies have shown different effects on cell proliferation and weight characteristics by sulfated cholecystokinin-8 (CCK-8S) in the rat pancreas when the
peptide has been administered continuously rather than intermittently. 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 proliferation,
weight gain, and induction of apoptosis and (ii) to examine the effect 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 microg/kg/h or 1%
bovine serum albumin (BSA) (vehicle) by implanted osmotic minipumps. The rats were killed after 4 days. Other rats were either injected subcutaneously only once or injected twice daily for 3 days with either 6 microg of CCK dissolved in 0.5 mL BSA or 0.5 mL BSA alone. The rats were killed 1, 3, 6, and 12 hours after the last injection. One hour before death they received 5-bromo-2-deoxyuridine (
BrdU) intraperitoneally to localize and quantitate the cell proliferation. Plasma was collected for analysis of CCK. The pancreas was dissected and immunohistochemistry was performed for analysis of the expression of the apoptosis promoting
protein bax and the
apoptosis inhibiting protein bcl-2, and for
BrdU and
CCK-A receptor localization. In situ hybridization (ISH) was used for examination and semiquantification of
CCK-A receptor mRNA expression. Continuous infusion of
CCK-8S led to a sixfold increase in plasma CCK and a 40% increase in pancreatic weight without any effect on
BrdU labeling. Immunohistochemistry revealed decreased tissue expression of bax but unaffected expression of bcl-2. Intermittent
injections of
CCK-8S led to hyper-CCK-emia with increased incorporation of
BrdU, indicating increased cell proliferation but no increase in pancreatic weight. Immunohistochemistry showed increased expression of bax, whereas bcl-2 remained unchanged. Immunofluorescence and ISH for the
CCK-A receptor and its gene expression, respectively, showed a lowest intensity at 3 hours after
CCK-8S injections. The results indicate that decreased apoptosis could explain the increased pancreatic weight during continuous infusion of
CCK-8S. An increased apoptosis could explain the lack of pancreatic
weight gain upon intermittent
injections of
CCK-8S despite the stimulation of cell proliferation.
Injections of
CCK-8S only transiently decreased the tissue levels of its receptor.