Feedback regulation of pancreatic exocrine secretion in animal and man

Novel porous starch/alginate hydrogels for controlled insulin release with dual response to pH and amylase

An important principle in the development of oral insulin is to protect insulin from the harsh conditions of the stomach and release it in a controlled manner in the intestine. In the present study, novel insulin-loaded porous starch-alginate hydrogel systems (In-S-Alg) including In-MS-Alg (prepared with porous maize starch), In-WS-Alg (porous waxy maize starch), and In-RS-Alg (porous rice starch) were successfully developed. As a representative, In-MS-Alg was further coated with gelatinized-retrograded high amylose maize starch (HA) films with different thicknesses to prepare In-MS-HA/Alg hydrogel beads for improving the functionality of controlled release of insulin under the action of α-amylase. The In-S-Alg and In-MS-HA/Alg hydrogel beads were evaluated in terms of structural and morphological properties, encapsulation effect on insulin as well as its release behavior. The results show that insulin was distributed in the pores and cavities of porous starch granules. In In-MS-HA/Alg hydrogel beads, insulin was increasingly blocked inside porous starch with the increased thickness of the HA film. Encapsulation efficiency of insulin in all In-S-Alg and In-MS-HA/Alg hydrogel beads was >80%. Amazingly, both the hydrogel beads successfully achieved the goal of triggered release upon pH changes and α-amylase addition. Most of the insulin (about 90%) was retained in the simulated gastric fluid; while the release rate of insulin in the simulated intestinal fluid increased gradually, and was further accelerated in the presence of α-amylase. Furthermore, for the In-MS-HA/Alg hydrogel beads, the insulin release rate can be gradually reduced by increasing the thickness of the HA film, which provided the possibility to match the rate of increase of the blood glucose level after the intake of food with different glycemic indices. Therefore, the novel hydrogel prepared in this study may be a promising and safe delivery carrier for oral insulin.

Inhibitor-stimulated non-parallel pancreatic secretion in man: hormonal and neural regulation?

BACKGROUND

We wanted to study whether a total inhibition of tryptic activity in the duodenum would induce a cholecystokinin (CCK)-dependent increase in pancreatic exocrine proteinase secretion.

METHODS

Concentrations of CCK and activities and concentrations of pancreatic enzymes were measured in human plasma and duodenal juices, respectively, collected during continuous intraduodenal instillations of proteinase inhibitors, with and without intravenous atropine administration.

RESULTS

Inhibitor instillation totally abolished tryptic activity and reduced the chymotryptic and elastase (1 and 2) activities by 95-100%. The inhibitors caused a rapid increase in the concentrations of trypsin, chymotrypsin, and pancreatic secretory trypsin inhibitor (PSTI) but had only a slight or no effect on amylase and elastase 1 secretion. An enhanced secretion of PSTI lends support to a possible connection between PSTI (resembling the monitor peptide causing CCK release in rats) and the enzyme secretion in man. CCK increased from 7 to 12-13 pmol/l. Intravenous atropine almost completely blocked the inhibitor-stimulated enzyme and PSTI secretion and reduced amylase activity by 50%. A further significant (P = 0.002) increase in the inhibitor-induced CCK output was found during atropine administration, as compared with the test situation without atropine.

CONCLUSION

The inhibitor-induced pancreatic secretion during total inhibition of tryptic activity shows a non-parallel secretion requiring different signals for different enzymes. The increase in plasma CCK levels indicates that CCK is feedback-regulated by both an inhibitor-mediated decrease in duodenal enzyme activity and a further decrease in pancreatic enzyme secretion by atropine.

The mediators of bile action on the exocrine pancreas

Under basal conditions, bile and bile salts applied intraduodenally influence plasma levels of several gastroenteropancreatic peptides. Besides those with stimulatory effects on exocrine pancreatic secretion, others with inhibitory or no effects are released as well. Furthermore, cholinergic and peptidergic neural mechanisms may also be activated. Secretin seems to be the most important mediator of bile- or bile salt-induced water and bicarbonate secretion. In addition, VIP released from peptidergic nerve endings in the pancreas may also be involved in the mediation of the hydrokinetic effect. With regard to water and bicarbonate secretion, cholinergic mechanisms probably are of minor importance. Cholinergic mechanisms, however, seem to be the most important mediator of bile- or bile salt-induced pancreatic enzyme secretion. CCK may act as an additional mediator of the ecbolic effect. This statement, however, is based on few results only and has to be confirmed by further studies. Gastroenteropancreatic peptides with an inhibitory action on the exocrine pancreas were also released by intraduodenal bile or bile salts. Somatostatin is released in physiologically relevant amounts to bring about a counter-regulation. Plasma PP levels are also enhanced by bile and bile salts. The amounts of PP released, however, are below those observed postprandially. In contrast to their stimulatory action on basal pancreatic secretion, bile and bile salts have no or even an inhibitory effect on pancreatic secretion stimulated by intraluminal nutrients. Accordingly, the release of gastroenteropancreatic peptides is not influenced (for example, secretin) or even reduced (for example, CCK) when bile or bile salts are added to intraluminal nutrients.(ABSTRACT TRUNCATED AT 250 WORDS)