Alan W. Spannagel

Bioactivity of intraduodenally and intravenously infused fragments of luminal cholecystokinin releasing factor (LCRF)

A luminal cholecystokinin releasing factor (LCRF), has been purified from intestinal secretion and found to have a mass of 8136 daltons. The amino-terminal 41 residues have been sequenced. Previous studies showed that intraduodenal infusion of the synthetic amino-terminal 35 amino acid peptide, LCRF1-35 significantly stimulated pancreatic protein and fluid secretion in conscious rats, but the peptide did not stimulate amylase release from isolated, dispersed pancreatic acini. In the present study, several fragments of LCRF were synthesized and tested for CCK-releasing activity (pancreatic protein secretion) to determine whether shorter fragments of LCRF exhibit the characteristic biological activity of native LCRF and synthetic LCRF1-35. Compounds tested were LCRF1-41, LCRF1-35, LCRF1-65 and LCRF11-25. Of the fragments shorter than LCRF1-35, only LCRF11-25 but not LCRF1-6 had significant CCK releasing activity. LCRF1-41 was equivalent to LCRF1-35 in potency and efficacy. Intravenous and intraduodenal infusion of LCRF1-35 elicited nearly identical dose-response curves.

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.

Atropine enhances food-stimulated CCK secretion in the rat.

The effect of atropine on plasma cholecystokinin (CCK) and pancreatic secretion during intraintestinal infusion of a conventional defined formula liquid diet (Ensure HN, Ross Laboratories, 1.06 kcal/ml) was studied in conscious rats. Rats were prepared with cannulae draining bile and pancreatic juice, which were returned to the duodenum at all times. Pancreatic secretion was monitored during intraduodenal infusion of 0.15 M NaCl for 2 h followed by Ensure HN, both infused at 4.62 d h Rats were infused i.p. with atropine (500 μg/kg/h) or vehicle throughout the experiment, beginning 1 h before monitoring of basal pancreatic secretion. Basal and 15 min postprandial plasma CCK concentrations were determined by bioassay. Atropine inhibited basal pancreatic protein secretion by ∼60%. However, protein secretion during infusion of the diet was not decreased by atropine, due to a larger incremental pancreatic protein secretory response in atropine-treated rats. Plasma CCK 15 min after beginning the diet infusion was significantly increased by atropine (8.09 ± 1.77 pM in atropine-treated rats versus 3.14 ± 0.64 pM in controls). The results indicate that rats compensate for loss of cholinergic input to the pancreas by increasing CCK release in response to a meal. This is hypothesized to occur by virtue of reduced feedback inhibition of CCK release due to anticholinergic reduction of basal levels of intestinal protease activity.

Role of Intraluminal Nutrients in Feedback Regulation of Pancreatic Enzyme Secretion

The importance of intraluminal nutrients in stimulating cholecystokinin (CCK) secretion and pancreatic enzyme secretion when pancreatic proteases are not present in the lumen is unclear. In the rat, previous reports showed either a requirement for mixed nutrients, a need for peptone only, or no requirement for nutrients.*-3 To clarify this issue, we studied the effect of different nutrients on the pancreatic protein secretory response (an indicator of CCK release) to bilepancreatic juice diversion in a new conscious rat model, the jejunal bypass rat.4

Intraluminal Regulatory Peptides and Intestinal Cholecystokinin Secretion

The gastrointestinal hormone cholecystokinin (CCK) was purified from porcine intestinal mucosa and sequenced almost 30 yr ago (1), yet the lack of progress in understanding how CCK secretion is regulated is apparent when one compares what is known about the regulation of gastrin secretion with that established for CCK (2,3). This is more remarkable because gastrin, which is phylogenetically CCK’s younger sibling (4), was purified and sequenced from porcine antral mucosa a few years before CCK (5). Thus, whereas neural and paracrine pathways controlling gastrin secretion have been well-established (6), the mechanisms that mediate CCK secretion from the small intestine are relatively unclear.

Pancreatic Hypersecretion in the Jejunal-Bypass Rat

We diverted bile and pancreatic juice from jejunal blind loops of different lengths in conscious rats to see if the pancreatic secretory response was dependent on the length of the jejunal blind loop. Short-term bile and pancreatic juice diversion from a short jejunal blind loop, representing only 8-10% of the total length of the small intestine, stimulated a significant increase in pancreatic exocrine secretion. Short-term bile and pancreatic juice diversion from jejunal blind loops of increasing length resulted in a progressive decrease in the pancreatic secretory response to diversion (i.e., there appears to be a length-dependent inhibition of the pancreatic secretory response to short-term bile and pancreatic juice diversion from the jejunum). Cholinergic receptor blockade with atropine eliminated this length-dependent inhibition of pancreatic exocrine secretion during short-term bile and pancreatic juice diversion. In contrast to what was observed with short-term bile and pancreatic juice diversion, there was a length-dependent increase in pancreatic secretion during long-term bile and pancreatic juice diversion. The jejunal-bypass rat model can facilitate the investigation of the intestinal mechanisms mediating feedback regulation of pancreatic exocrine secretion.