Nonparallel pancreatic secretion: its meaning and implications

The endocrine secretion of human insulin and growth hormone by exocrine glands of the gastrointestinal tract

The exocrine pancreas, liver, and submandibular glands of the rat were used to express and secrete two exogenous, human protein hormones (growth hormone and insulin) into blood at physiological concentrations. Transfection, expression, and secretion were achieved by the in vivo retrograde injection of plasmid DNA into the secretory ducts of these glands. Pancreatic acinar cells secreted physiological concentrations of growth hormone into the circulation, and its secretion was enhanced by cholinergic stimulation. A human insulin gene was engineered to allow normal processing of insulin in non-beta cells. With this gene, the secretion of human insulin by the exocrine pancreas normalized elevated blood glucose levels in diabetic rats. These in vivo observations demonstrate the utility of retrograde ductal administration of naked DNA into exocrine organs as a novel method for the regulated systemic delivery of protein-based pharmaceuticals.

Pancreatic digestive enzyme secretion in the rabbit: rapid cyclic variations in enzyme composition

The role and mechanism of nonparallel pancreatic secretion of digestive enzymes, in which enzyme proportions change in rapidly regulated fashion, remain controversial. Secretion was collected from male 2.2-kg New Zealand rabbits in 5-min intervals for 3 h under basal conditions or constant stimulation with cholecystokinin (CCK; 0.1 microgram per kg per h i.v.) or methacholine chloride (MCh; 40 micrograms per kg per h i.v.). Both CCK and MCh produced an 8-fold stimulation of protein output. Enzymes were separated by SDS/PAGE and quantitated by densitometry of Coomassie blue-stained gels. Under both basal conditions and constant MCh infusion, rapid neurosecretory-like 12-min cyclic changes occurred in the proportions of amylase, lipase I, chymotrypsinogen, and trypsinogen. During constant infusion their percentages changed as much as 10-fold, and their ratios cycled by as much as 30-fold. The mean percentage for the entire infusion period for lipase I declined > 25% with CCK or MCh, for amylase it rose approximately 30%, and for chymotrypsinogen and trypsinogen it doubled (for all, P < 0.05). CCK and MCh elicited subtly but significantly different mean enzyme percentages and enzyme ratios (P < 0.05) for amylase, chymotrypsinogen, and trypsinogen; these differences were also confirmed by regression and correlation analyses. The changes in enzyme percentages and ratios were explicitly consistent with secretagogue-caused shifts in the intrapancreatic enzyme secretory sources. Nonparallel secretion of digestive enzymes occurs routinely, even during constant stimulation, and is due to cyclic neurosecretory-like secretion from heterogeneous intrapancreatic sources.

The effects of histamine, pyrilamine, cimetidine, and ranitidine on secretion of lingual lipase and amylase from rat von Ebner's glands

Minced von Ebner's glands of rat tongue were incubated in vitro with histamine and histamine receptor antagonists. At various time intervals, media and homogenates of the tissue were assayed for lingual lipase and amylase activity and percentage secretion calculated. Histamine elicited moderate secretion (approximately 10%) of lingual lipase and amylase. In contrast, pyrilamine, an H1 receptor antagonist, elicited > 60% secretion. There were statistically significant differences between the percentage secretion of lingual lipase and amylase for basal secretion, as well as for histamine- and pyrilamine-evoked secretion above basal. The H2 receptor inhibitors, cimetidine and ranitidine, stimulated secretion of only amylase, but not lingual lipase. When combined with histamine, these antagonists partially inhibited only the secretion of histamine-evoked lingual lipase, but not amylase. The differences in percentage secretion between the two enzymes indicate that exocytosis may not be the only process involved in protein secretion. The anomalous effects of the H1 and H2 receptor antagonists necessitate a more detailed characterization of the receptors of von Ebner's glands.

Nonparallel transport and mechanisms of secretion

After many years of controversy, it is now clear that at least some cells and tissues that secrete more than one product can vary the composition of the secreted mixture as the result of the differential transport of various substances out of the cells that secrete them. In this article we discuss this phenomenon, non-parallel transport or secretion, and how it has and continues to inform us about how cells release the products they manufacture. We focus on expression of the phenomenon in the secretion of digestive enzymes by the exocrine pancreas, where it has been studied most extensively.

Influence of in situ neural isolation of jejunoileum on postprandial pancreatobiliary secretion and gastric emptying

Our aims were to examine the influence of neural isolation of the jejunoileum on postprandial pancreatobiliary secretion. In four dogs, duodenal perfusion and aspiration catheters were implanted, and serosal electrodes were placed along the proximal small bowel. Control studies of gastric emptying, output of bile acids and amylase, and plasma concentrations of peptide YY and neurotensin were performed on three occasions following ingestion of a 340-kcal mixed-nutrient liquid meal. The dogs then underwent our model of in situ jejunoileal neural isolation, and the meal studies were repeated. Neural isolation, when compared to control, did not affect either postprandial conversion of intestinal myoelectric activity to the "fed" pattern, gastric emptying (T1/2, X +/- SE of the liquid meal (74 +/- 6 vs 79 +/- 7 min; P greater than 0.05), or cumulative amylase output (373 +/- 59 vs 305 +/- 66 kU; P greater than 0.05). Neural isolation decreased cumulative postprandial bile acid output from 6.6 +/- 0.9 mM to 3.4 +/- 1.1 mM (P less than 0.05) and increased postprandial plasma concentrations of peptide YY and neurotensin. Our findings suggest that the jejunoileal denervation that accompanies the in situ neural isolation of the jejunoileum is not associated with changes in postprandial motility patterns, gastric emptying, or pancreatic amylase secretion. Loss of this innervation, however, may decrease postprandial output of bile acids and lead to a compensatory increase in the postprandial release of neurotensin and peptide YY.

Hormones as regulators of pancreatic secretion in man

At the beginning of the century, Pavlov suggested that the pancreas was exclusively controlled by the nervous reflex mechanisms. In 1902, Bayliss & Starling published their experiments on secretin and claimed that the nervous regulation is 'superfluous and improbable'. In the following decades, especially after the discovery of CCK, it was generally held that exocrine pancreatic secretion is regulated mainly by hormones. The present summary clearly demonstrates the importance of the cholinergic system in regulating exocrine pancreatic secretion and the complexity of neurohormonal interactions. The question is no longer hormones or nerves, but rather a very complicated coordination of neural, hormonal and possible paracrine effects, resulting in the control of exocrine pancreatic activity. In this complex regulatory system, the cholinergic control is central with hormones such as CCK or secretion modulating the response.

Effects of CCK-8 in combination with natural or synthetic secretin on amylase, lipase, trypsin, and chymotrypsin secretion in rats

In the present study, we examined the interaction between CCK-8 and natural or synthetic secretin on pancreatic enzyme secretion. On anaesthetized rats, a proximal duodenal segment was continuously perfused with saline and amylase, lipase, trypsin, and chymotrypsin were determined in the perfusate. Neither during iv saline nor during iv secretin (natural or synthetic) at doses of 0.01, 0.05, or 0.1 CU/kg/h, any of the four enzymes changed significantly from basal values over a period of 100 min. Iv CCK-8 at stepwise increasing doses of 5, 10, and 20 pmol/kg/h elicited a significant increase of all four enzymes at the medium dose, with a further increase of amylase and trypsin, but not lipase and chymotrypsin at 20 pmol/kg/h. The addition of secretin at all 3 doses potentiated CCK-induced trypsin output. The effects of natural secretin were more pronounced than those of the synthetic peptide. Secretin significantly increased amylase secretion over basal at the lowest dose of CCK-8 (5 pmol/kg/h) that by itself had no effect on amylase release. Only the higher doses of natural but not synthetic secretin augmented lipase secretion during the lowest dose of CCK-8. Both forms of secretin had no further stimulatory effect on CCK-induced chymotrypsin secretion. The present data demonstrate that, first, in rats a potentiation of CCK-8-induced enzyme secretion by low doses of secretin is different for the four enzymes, which suggests a differential regulatory action of these two intestinal hormones on pancreatic enzyme release. Second, the different effects of natural secretin may represent those of contaminants suggesting that only synthetic secretin should be employed in future studies of this peptide on pancreatic enzyme secretion.