Cholecystokinin (CCK), CCK octapeptide (CCK-8), and caerulein (Cae) are the most potent trophic factors in the pancreas when given exogenously or released from the intestine. Recent studies have suggested that this growth-promoting effect of CCK was initiated through the occupation of the CCKA receptor. This study was then undertaken to determine whether occupation of the high- or low-affinity CCKA receptor is involved in the growth process and to establish which transduction signals have been selectively activated. As an answer to the first question, rats were infused with CCK JMV-180, a CCKA high-affinity agonist, at doses of 50, 100, 150, and 300 micrograms.kg-1.h-1, or Cae (0.25 micrograms.kg-1.h-1) for 4 days. After rats were killed, their pancreatic weight and contents of protein, DNA, RNA, amylase, and chymotrypsinogen were estimated. To investigate the transduction signals, rats were infused for 30 min to 4 h with 300 micrograms.kg-1.h-1 JMV-180, or pancreatic acini were exposed in vitro to 1 microM JMV-180 for 5 or 30 min. After rats were killed, pancreases were used to monitor tyrosine kinase, phosphatidylinositol (PtdIns) 3-kinase, and phospholipase D (PLD) activities. The first set of experiments indicates that JMV-180 caused a dose-dependent effect on pancreas growth, with the 300 micrograms.kg-1 x h-1 dose giving the maximal effect comparable to that of Cae. Furthermore, JMV-180 induced concomitant early increases in tyrosine kinase and PLD activities both in vivo and in vitro and PtdIns 3-kinase in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

Cholecystokinin (CCK) is the major pancreatic secretagogue and acinar cell mitogen. This study was performed to determine by which effector systems CCK regulates tyrosine kinases, phosphatidylinositol (PtdIns) 3-kinase, and phospholipase D (PLD) activities. Pancreatic acini loaded with [3H]myristic acid or [3H]inositol were used to assay PLD and PtdIns 3-kinase. G protein activation with NaF increased particulate and crude cytosolic tyrosine kinase and PLD activities. PLD activation was pertussis toxin sensitive. Inhibition of phospholipase C (PLC) slightly reduced caerulein-stimulated particulate tyrosine kinase and blocked crude cytosolic tyrosine kinase activity without affecting caerulein-induced PLD activity. Ca2+ is an important factor in caerulein stimulation of tyrosine kinase and PLD activities. Protein kinase C and tyrosine kinase inhibition abolished caerulein-activated particulate and crude cytosolic tyrosine kinase and PtdIns 3-kinase activities without any effect on PLD. Wortmannin inhibited PLD and PtdIns 3-kinase activation. Caerulein-induced amylase secretion was partially reduced by tyrosine kinase inhibition, with no effect from wortmannin. Caerulein can stimulate a pertussis toxin-insensitive G protein, leading to particulate tyrosine kinase activation and a Ca(2+)-sensitive cytosolic tyrosine kinase through PLC activation. However, PLD activation by caerulein is pertussis toxin sensitive, cytosolic Ca2+ sensitive, and independent of previous PLC and tyrosine kinase activation.

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To help understand the possible role of phosphoinositide turnover in the desensitization process, the availability of phosphatidylinositol 4,5-bisphosphate was investigated in normal and desensitized pancreatic acinar cells treated with carbamylcholine (Cch), caerulein (Cae), and the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). In control acini, incorporation of [myo-3H]inositol into total phosphoinositides was maximal at 120 min, was Cch and Cae dose dependent, and was insensitive to TPA. Cch stimulation increased the proportion of [myo-3H]inositol incorporated into phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2], whereas Cae specifically channeled [myo-3H]inositol incorporation into phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate. In the desensitized cells, preexposure to Cch and Cae, but not to TPA, increased the subsequent basal rate of [myo-3H]inositol incorporation into total phosphoinositol (PI) by 66 and 50% above control values. There were no subsequent responses to increasing concentrations of Cch, Cae, and TPA during a second incubation. Desensitization of the pancreatic secretory responses to Cch, Cae, and TPA does not seem to result from a decrease either in total PI or in specific PtdIns(4,5)P2 synthesis, which is needed for inositol trisphosphate and diacylglycerol production.