CCK-releasing activity of rat intestinal secretion: effect of atropine and comparison with monitor peptide

A bioassay for studying the cholecystokinin (CCK)-releasing activity of intraluminal protease-sensitive bioactive peptides was developed. In conscious rats, bile and pancreatic juice were chronically diverted from the proximal intestine to the ileum to cause chronic stimulation of CCK release and pancreatic protein secretion. CCK-releasing activity of test substances was assayed during transient inhibition of CCK release by intraduodenal sodium taurocholate (78 mumols/h). Intestinal secretion as a source of the putative trypsin-sensitive intestinal CCK-releasing peptide was obtained by rapid intestinal perfusion of isolated Thiry-Vella fistulae of jejunum in conscious rats, collected with or without atropine pretreatment. Partially purified rat pancreatic secretory trypsin inhibitor (PSTI, or "monitor peptide") was compared with ovomucoid trypsin inhibitor (OMTI) and with concentrated jejunal secretions for CCK-releasing activity and trypsin inhibitor activity. Concentrated, heat-treated jejunal secretions were the strongest stimulants of CCK release and pancreatic protein secretion in this model. OMTI had no CCK-releasing activity in this model, whereas a larger amount (approximately 5x, based on trypsin inhibitor activity) of PSTI weakly but significantly stimulated CCK release. CCK-releasing activity manifested by pancreatic protein secretion was equivalent in intestinal washes from atropine-treated and control Thiry-Vella fistula donor rats. Concentrated jejunal secretions had no trypsin inhibitory activity, indicating that the putative intestinal CCK-releasing peptide and "monitor peptide" are different substances.

Regulation of plasma cholecystokinin levels by bile and bile acids in the rat

To determine whether intraduodenal bile acids inhibit pancreatic secretion and cholecystokinin (CCK) release independent of pancreatic proteases, experiments were conducted in rats with bile and pancreatic juice chronically diverted to the ileum. Diversion of bile and pancreatic juice increased plasma CCK concentration to 19.1 +/- 4.0 pmol/L. Intraduodenal sodium taurocholate (78 mumol/h) reduced plasma CCK concentration to 6.6 +/- 1.5 pmol/L after 1 hour, but values increased to 17.3 +/- 2.3 pmol/L after 13.5 hours despite continued taurocholate infusion. Pancreatic protein secretion was also significantly but transiently inhibited by taurocholate. However, neither acute nor chronic intraduodenal bile infusion significantly reduced plasma CCK concentration compared with sodium bicarbonate infusion (13.4 +/- 1.9 pmol/L vs. 15.0 +/- 1.7 pmol/L, respectively). Chronic (13.5 hours) intraduodenal infusion of taurocholate plus pancreatic juice caused a sustained reduction of plasma CCK level to 3.1 +/- 0.5 pmol/L, which significantly increased to 9.4 +/- 1.1 pmol/L after cessation of taurocholate but with continued infusion of pancreatic juice. The results indicate that bile does not inhibit CCK release and that bile acids do not physiologically inhibit pancreatic secretion or CCK release independent of the presence of pancreatic proteases.

Effects of SMS 201-995 on basal and stimulated pancreatic secretion in rats

Somatostatin (SRIF) is a potent inhibitor of most gastrointestinal and pancreatic functions. Recently, we showed that SRIF given either iv or intraduodenally (id) strongly inhibited stimulated pancreatic secretion induced by pancreatic juice diversion (PJD) from the duodenum. In this study we evaluate the effects of iv and id infusion of a long acting analog of SRIF, SMS 201-995 (SMS), on pancreatic secretion during basal conditions (pancreatic juice returned) and PJD. Conscious rats prepared with bile, pancreatic, duodenal, and jugular cannulae were studied 3-8 days postoperatively. Protein and fluid outputs were evaluated, and plasma cholecystokinin (CCK) was measured by bioassay. iv SMS infusion (5 micrograms kg-1 h-1) inhibited basal pancreatic protein and fluid secretion by 84 and 64%, respectively. Addition of atropine (500 micrograms kg-1 h-1 ip) did not cause further inhibition. During PJD, SMS iv from 0.005-1.28 micrograms kg-1 h-1 for 3 h caused a dose-dependent inhibition with maximal 90% and 75% reductions of protein and fluid, respectively, at 1.28 micrograms SMS. Plasma CCK was also reduced by 83% from 3.01 +/- 1.15 to 0.51 +/- 0.22 pM. SMS, id at 1.7 micrograms kg-1 h-1 for 1.5 h before and 2 h after PJD, caused inhibition of basal secretion by 25% and that induced by PJD by 60%. Plasma CCK, measured 1.5 h after diversion, increased from 1.55 +/- 0.06 to 5.9 +/- 1.14 pM in the presence of SMS. Intravenous SMS was 20 times more potent than SRIF in inhibiting pancreatic protein and volume secretion stimulated by PJD. Iv SMS inhibited basal and stimulated fluid and protein pancreatic secretion as well as plasma CCK levels. SMS was also effective when given id in inhibiting fluid and protein pancreatic secretion, but id SMS increased plasma CCK levels. This effect on plasma CCK may be due to the inhibition of hormonal inhibitors of CCK release.

Lack of cholinergic control in feedback regulation of pancreatic secretion in the rat

The effect of atropine (100 micrograms/kg/h, i.v.) on plasma cholecystokinin and pancreatic secretion during diversion of bile and pancreatic juice from the intestine was studied in 8 conscious rats equipped with jugular vein, pancreatic, biliary, and duodenal cannulas, and with pyloric ligation and gastric drainage. Diversion of bile and pancreatic juice to the exterior for 4 h significantly increased pancreatic protein and fluid secretion. Atropine delayed the pancreatic response to diversion, but during 4 h of diversion, neither total nor incremental pancreatic protein or fluid secretion was inhibited by atropine. Plasma cholecystokinin levels were elevated after diverting bile and pancreatic juice and were not significantly reduced by atropine (23.0 +/- 6.6 pM vs. 16.0 +/- 3.9 pM at 1.5 h and 17.3 +/- 5.4 pM vs. 13.1 +/- 2.9 pM at 4 h after bile and pancreatic juice diversion; atropine-treated vs. controls, respectively). These results indicate that cholinergic nerves play no important role in feedback regulation of cholecystokinin release and that the previously reported suppressive effect of atropine on the pancreatic response to diversion of bile and pancreatic juice from the intestine was secondary to inhibition of gastric secretion.

Protein digestion in human and rat small intestine: role of new neutral endopeptidases

Two new phosphoramidon-insensitive neutral endopeptidases were identified and partially characterized in the brush-border membrane of rat and human intestine using N-CBZ-L-Ala-L-Arg-L-Arg-4-methoxy-beta-naphthylamide (Z-Ala-Arg-Arg-MNA) and azocasein or alpha-casein as substrates. Activities in the brush-border membrane of both rat and human intestine were maximum at neutral to alkaline pH, were inhibited by metal chelating and thiol reagents, and were insensitive to phosphoramidon. The results also indicate that these endopeptidases are distinct from pancreatic proteases. The biochemical properties of the enzyme hydrolyzing Z-Ala-Arg-Arg-MNA were shown to be different from that hydrolyzing azocasein or alpha-casein. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis and gel filtration revealed that several native intact protein substrates were rapidly degraded to small molecular weight peptides and amino acids when incubated with rat or human brush-border membrane preparations. During in vivo intestinal perfusion in rats, 11% of the total administered alpha-casein was hydrolyzed and absorbed by the intestine. The results suggest that phosphoramidon-insensitive endopeptidases in the intestinal brush-border membrane may be of nutritional and physiological importance in protein digestion.

Role of rat intestinal brush-border membrane angiotensin-converting enzyme in dietary protein digestion

The role of rat intestinal angiotensin-converting enzyme (ACE; E.C 3.4.15.1) in the digestion and absorption of dietary protein was investigated. Enzyme activity was associated with the brush-border membrane fraction, with the highest activity in the proximal to midregion of the small intestine. Preliminary enzyme characterization studies were carried out using purified brush-border membrane preparations. When a variety of N-blocked synthetic peptides were used as potential substrates for ACE, activity was highest with those containing proline at the carboxy terminal position. The hydrolytic rates observed with these prolyl peptides were comparable to those observed when major digestive peptidases of the brush-border membrane such as aminopeptidase N and dipeptidyl aminopeptidase IV were assayed. When isolated rat jejunum was perfused in vivo with solutions of Bz-Gly-Ala-Pro, the dipeptide Ala-Pro was the main hydrolytic product detected in the perfusates. Absorption rates of the constituent amino acids, alanine and proline, depended on the concentration of peptide perfused. Captopril, an active site specific ACE inhibitor, significantly inhibited hydrolysis and absorption of constituent amino acids from Bz-Gly-Ala-Pro. These results show that intestinal brush-border membrane ACE functions as a digestive peptidase in addition to its role as a regulator of biologically active peptides in other tissues.

Identification and characterization of brush-border membrane-bound neutral metalloendopeptidases from rat small intestine

Neutral metalloendopeptidase enzymes were identified and partially characterized in the brush-border membranes of rat small intestinal mucosal cells using insulin B chain and glutaryl-trialanine-4-methoxy-beta-naphthylamide as substrates. Three different molecular species of endopeptidase were identified by disc gel electrophoresis. These enzymes were shown to be distinct from pancreatic endopeptidases on the basis of the following: enrichment in the brush-border membrane fraction, site of hydrolysis of peptide substrates, sensitivity to specific proteinase inhibitors, and the presence of brush-border membrane-associated endopeptidase activity in mucosal cells of Thirty-Vella loops. Hydrolysis of the substrates was shown to be a two-step process involving initial cleavage by endopeptidase with secondary hydrolysis of the peptide products by brush-border membrane aminopeptidase N. Hydrolysis of both substrates was maximum at a neutral pH and was strongly inhibited by metal chelating agents, phosphoramidone, and amastatin. Intestinal perfusion studies using glutaryl-trialanine-4-methoxy-beta-naphthylamide suggest that these enzymes play a physiologic role in protein digestion. It was concluded that neutral endopeptidases are integral components of the intestinal brush-border membrane and work in concert with aminopeptidase N to hydrolyze dietary protein. This process may be of nutritional importance in normal subjects and those with diminished exocrine pancreatic function.