Cholecystokinin-stimulated enzyme secretion from dispersed rabbit pancreatic acinar cells: phosphorylation-dependent changes in potency and efficacy

How does cholecystokinin stimulate exocrine pancreatic secretion? From birds, rodents, to humans

The field of cholecystokinin (CCK) stimulation of exocrine pancreatic secretion has experienced major changes in the recent past. This review attempts to summarize the present status of the field. CCK production in the intestinal I cells, the molecular forms of CCK produced and subsequently circulated in the blood, the presence or absence of CCK receptors on the isolated pancreatic acinar cells and the associated signaling for acinar cell secretion, and the actual circuits and sites of action for CCK regulation of exocrine pancreatic secretion in vivo are reviewed in different animal species with an emphasis on birds, rodents, and humans. Clear differences in the relative importance of neural and direct modes of CCK action on pancreatic acinar cells were identified. Rodents seem to be endowed with both modes of action, whereas in humans the neural mode may predominate. In birds, such as duck, the direct mode needs further assistance from pituitary adenylate cyclase-activating peptide/VIP receptors. However, much further work needs to be directed to the neural mode to map out all sites of CCK action and details of the full circuits, and we foresee a major revival for this field of research in the near future.

Intestinal metabolism and absorption of cholecystokinin analogs in rats

Intestinal metabolism and poor permeability were known to be major barriers for oral absorption of large peptide drugs. Dimensionless wall permeability values of C-terminal octa- and tetra-peptides cholecystokinin analogs (CCK8 and CCK4) were estimated and found out to be greater than 1, suggesting no permeability-limited absorption for CCK analogs. Thus, a strategy employing enzyme inhibitors and a specific delivery site to improve the absorption was developed and tested with CCK8, followed by identification of metabolites of the analogs and their participating enzymes in rabbit brush-border membrane vesicles. Thiorphan and amastatin, a specific enzyme inhibitor for enkephalinase and aminopeptidase, respectively, in pH 4 buffer solution were coadministered with CCK8 to the ileum in fistulated rats. The absolute bioavailability (F) of CCK8 was 5.4% and increased to 19% in the presence of the enzyme inhibitors, while the F values following oral administration were close to zero. These results indicate that peptide oral delivery is possible.

Recovery from TPA inhibition of receptor-mediated Ca2+ mobilization is paralleled by down-regulation of protein kinase C-alpha in CHO cells expressing the CCK-A receptor

Digital-imaging microscopy of Fura-2-loaded Chinese hamster ovary cells, stably expressing the cholecystokinin-A receptor, revealed that both the C-terminal octapeptide of cholecystokinin (CCKB) and its analogue JMV-180, which acts as an agonist at the high-affinity CCK-A receptor, recruited CHO-CCK-A cells dose-dependently in terms of receptor-mediated Ca2+ mobilization. Agonist-evoked cell recruitment was inhibited by short-term (10 min) pretreatment with 0.1 microM 12-O-tetradecanoylphorbol 13-acetate (TPA). In the case of CCKB, inhibition was overcome with increasing of the hormone concentration. In contrast, increasing of the JMV-180 concentration did not reverse the inhibitory action of TPA. CHO-CCK-A cells gradually regained their responsiveness to JMV-180 during prolonged TPA pretreatment. Complete recovery was observed within 1 h following addition of TPA. Western blot analysis using antibodies directed against the various PKC isotypes revealed that recovery was paralleled by the disappearance of PKC-alpha. Surprisingly, short-term (10 min) TPA pretreatment virtually completely inhibited the formation of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] in response to CCKB concentrations at which the effect on cell recruitment was not affected by short term phorbol ester pretreatment. Together with the finding that JMV-180 does not detectably increase the cellular Ins(1,4,5)P3 content, this suggests a large overproduction of this second messenger by CCKB concentrations supramaximal in terms of cell recruitment. Again, full responsiveness was observed after long term TPA pretreatment. The present observations are in agreement with the idea that in CHO-CCK-A cells activation of PKC-alpha leads to inhibition of agonist-evoked Ca2+ mobilization through inhibition of receptor-stimulated Ins(1,4,5)P3 formation.