Pancreatic weight, [3H]-thymidine incorporation into DNA, labeling indices, and total DNA and RNA content were measured in rats treated with vehicle or 1 microgram/kg caerulein, 100 micrograms/kg secretin, or a combination of these peptides injected every 8 h for 1-5 days. Incorporation of [3H]thymidine into DNA increased 12-fold after 2 days of treatment with the combination of peptides. DNA content increased after 3 days and reached a level 1.8 times control after 5 days. Autoradiography showed that two cell types, acinar and an unidentified type, were the sites of increased DNA synthesis. Different patterns of labeling were seen in the two populations: acinar cell labeling indices were increased at 1 and 2 days (20-fold) and then fell; nonacinar cells showed an increase only after 2 days and maintained this increase after 5 days. Potentiation (greater than additive effects) was found when caerulein and secretin were injected together for all measurements except RNA content. These data indicate that DNA synthesis in two cell populations is affected by secretin and caerulein and support the occurrence of potentiation between secretin and caerulein for trophic effects on the exocrine pancreas.

No abstract

The circadian variations of exocrine pancreatic secretion were studied in conscious rats provided with pancreatic, biliary, duodenal, and peritoneal cannulas and kept in restraint cages under controlled conditions, with a regular 12-h light cycle. Rats were divided into fed and fasted groups, and experiments were performed separately. During a 4-day post-surgical recovery period, rats were fed ad libitum. During the experiment, fed rats had free access to food and water. Food, but not water, was denied fasted rats 10 h before the experiment and for its 48-h duration. During the experiment, pancreatic juice was continuously collected for 4 and 2 days from fed and fasted rats, respectively. Every 30 min, a 20-microliters aliquot of sampled pancreatic juice was removed for total protein, amylase, and chymotrypsinogen assays. The remainder was mixed with bile collected simultaneously, and the mixture was recirculated into the duodenum. Over the 4- and 2-day periods there was a clear circadian rhythm of 24-h duration; for all measured parameters, secretory rates increased in the dark period and decreased during the light period. This major circadian rhythm was unexpectedly found to be superimposed on by a remarkably constant neurosecretory-like minor cycle of 2-h duration present in both fed and fasted states. The amplitude of the minor cycle was diminished by fasting. The outputs of fluid, total protein, and amylase were found to be only modestly correlated with each other, whereas chymotrypsinogen output was virtually completely independent of the others. The results suggest that the spontaneous major increase of exocrine pancreatic secretion in the dark was at least partially independent of food intake.(ABSTRACT TRUNCATED AT 250 WORDS)

A B S T R A C T These experiments were designed to determine whether fasting and feeding were associated with differing rates of protein synthesis in the rat pancreas. It has been established that feeding, acetylcholine, or cholecystokinin-pancreozymin administration was associated with enhanced rates of digestive enzyme secretion; however, the literature is unclear as to effects of such stimulation on enzyme synthesis. Rats fed ad lib. or fasted 24, 48, or 72 hr were used for this study. Pancreases were removed and incubated in tissue culture medium with L-phenylalanine-"C, and incorporation into TCA-insoluble material as well as purified amylase was measured. Compared with fed controls, fasting 24, 48, and 72 hr was associated with 29%, 39%, and 35% decreases in incorporation of L-phenylalanine-14C into protein. Decreases of similar magnitudes were apparent whether the data were expressed in terms of protein or DNA. Pancreatic amylase isolated from rats fasted 48 hr contained 57% fewer counts of L-phenylalanine-'C than amylase isolated from fed rats. Moreover, rats fasted for 24 hr and given bethanechol chloride incorporated greater amounts of L-phenylalanine-1C into protein than fasted controls. Studies were performed to exclude changes in pool size of precursor (L-phenylalanine-"C) or product (amylase) in accounting for decreases associated with fasting. These studies demonstrate that fasting was associated with decreased rates of pancreatic amylase and protein synthesis in rats.

Phosphatidylcholine (PC) metabolism stimulated by caerulein (Cae), a cholecystokinin analogue, was investigated in rat pancreatic acini prelabeled with [3H]choline or [3H]-myristic acid. Both labels were incorporated mostly into PC. An inhibition of choline incorporation into PC was first observed in response to Cae (100 and 500 pM) stimulation, as indicated by reduced [3H]choline incorporation into trichloroacetic acid-precipitable material. Whereas choline incorporation was reduced in PC, Cae (500 pM) significantly increased [3H]choline metabolites release in the incubation medium. Separation of these metabolites by thin-layer chromatography showed that approximately 90% of the labeled products released into the medium was phosphocholine; however, Cae caused significant increases of [3H]choline release after 5, 15, and 30 min. In response to Cae, manoalide, a phospholipase C (PLC) inhibitor, totally prevented phosphocholine release into the medium but did not affect choline release. Staurosporine, a protein kinase C inhibitor, did not influence basal and Cae-induced choline release. In cells prelabeled with [3H]myristic acid, Cae stimulated within 5 min a rapid increase in intracellular [3H]phosphatidic acid (PA) levels in the presence of the PA phosphohydrolase inhibitor, propranolol; this PA production was further increased after 15 and 30 min of stimulation. The time course of [3H]PA formation in the presence of propranolol was similar to that of choline release in the medium. Staurosporine partially blocked PA accumulation stimulated by Cae after 30 min. In contrast, manoalide significantly reduced basal PA accumulation but did not prevent its production in response to Cae. In the presence of ethanol, Cae also significantly stimulated above control values the formation of [3H]phosphatidylethanol. These data indicate that Cae-induced PC hydrolysis in rat pancreatic acini is mediated mostly by phospholipase D (PLD) to produce PA and choline; they suggest a direct action of Cae on PLD activation, an effect independent of PLC activation.

This study was undertaken to determine the effects of hydrocortisone on pancreatic size and composition in suckling, recently weaned, growing, and adult rats. The data demonstrate that hydrocortisone 1) is associated with hyperplasia and hypertrophy of the pancreas in suckling rats, 2) suppressed DNA synthesis in recently weaned rats, but maintained the hypertrphic effect, 3) increased protein and pancreatic hydrolase concentrations in growing and adult rats, and 4) preferentially promoted synthesis of amylase over chymotrypsin only during the suckling period. These experiments suggest that growth of the pancreas may be partly under control of glucocorticoids only during the suckling period, whereas hypertrophy of the pancreatic tissue can be obtained at all ages by these hormones.

Dispersed pancreatic acini were first exposed to carbamylcholine (10(-7)-10(-4) M) for 60 min, washed, and reexposed to this same agonist (10(-8)-10(-3) M) for 15 min. During this second incubation, the functional secretory capacity of these acini was evaluated by measuring amylase release. Acini preexposed to concentrations of carbamylcholine of 10(-6) M or greater showed shifts to the right in the subsequent carbamylcholine dose-response curves of amylase release. A 3-h recovery period (without carbamylcholine) did not restore the altered carbamylcholine dose-response curve. Ca2+ concentrations of 10(-7) M or 2.5 X 10(-3) M instead of 0.5 X 10(-3) M during the 60-min preincubation did not affect the desensitization process. With use of N-[3H]methylscopolamine to evaluate muscarinic receptors, the only changes observed after desensitization were a significant decrease in the high-affinity and an equivalent increase in that of the low-affinity receptors. After cholinergic exposure amylase release stimulated by caerulein was only slightly modified, whereas amylase release in response to a phorbol ester 12-O-tetradecanoylphorbol-13-acetate and to the ionophore A23187 was not altered. These data indicate that short-term desensitization with a cholinergic agent is relatively specific to muscarinic agonists, causes changes in the muscarinic receptor high- and low-affinity concentration but does not alter intracellular steps after calcium mobilization or protein kinase C activation known to be involved in the secretion process.

We investigated the effects of somatostatin (SMS)-201-995, atropine, and MK-329 on the role of cholinergic- and cholecystokinin-related systems and on the secretory relationship between five pancreatic digestive enzymes in rats. Animals kept in restraint cages and provided with pancreatic, biliary, duodenal, and jugular vein cannulas were treated as follows: 1) 0.25 micrograms.kg-1.h-1 caerulein alone, 2) both 0.25 micrograms.kg-1.h-1 caerulein and 100 micrograms.kg-1.h-1 atropine, 3) both caerulein and 5 micrograms.kg-1.h-1 SMS, 4) 91.3 micrograms.kg-1.h-1 carbachol alone, 5) both carbachol and 0.5 mg.kg-1.h-1 MK-329, and 6) both carbachol and 5 micrograms.kg-1.h-1 SMS, respectively. Food, but not water, was denied rats starting 10 h before the experiment and throughout the 6-h experimental period. The secretory patterns over the 6-h experimental period showed noticeably independent regulation of pancreatic secretion of individual digestive enzymes. The relationship between paired enzymes significantly varied according to the treatment. The correlation between chymotrypsinogen and the other enzymes was markedly modulated by MK-329. Our results suggest that SMS is a major "gate-keeper" in the regulation of exocrine pancreatic secretion and that the secretion of each digestive enzyme is individually regulated. Furthermore, they suggest that cholecystokinin and acetylcholine and their respective agonists are essentially initiators of secretory processes of the pancreas. Therefore, the paradigms of the regulation of pancreatic secretion heretofore accepted should be reexamined.