Effects of secretin and cholecystokinin octapeptide (CCK8) on exocrine pancreas during cold vagal blockade in calves

Arterial gastroduodenal infusion of cholecystokinin-33 stimulates the exocrine pancreatic enzyme release via an enteropancreatic reflex, without affecting the endocrine insulin secretion in pigs

OBJECTIVES

Cholecystokinin (CCK)-dependent exocrine pancreatic regulation seems to involve different pathways in different species. The aims were to explore the enteropancreatic reflex in the CCK-mediated regulation of the exocrine pancreas and to evaluate a possible involvement of this reflex in the endocrine insulin release.

METHODS

In anesthetized pigs, CCK-33 in increasing doses (4-130 pmol kg 10 min) was infused locally to the gastroduodenal artery, or systemically via the jugular vein. Also, a low CCK-33 dose (13 pmol kg) was injected to the duodenum/antrum area before and after a bilateral truncal vagotomy.

RESULTS

Cholecystokinin-33 in the physiological dose range 4 to 32 pmol kg 10 min increased protein and trypsin outputs after local infusion to the antral-duodenal area, whereas it had no effect after systemic infusion. Cholecystokinin-33 in the pharmacological dose range 64 to 130 pmol kg 10 min further increased the secretion after both local and systemic infusions. Only CCK-33 infusions in the pharmacological dose range were able to elevate the plasma insulin levels. Vagotomy had no effect on CCK-33-mediated stimulation of the enzyme release, whereas it had a significant effect on the plasma insulin level.

CONCLUSIONS

Cholecystokinin-33 in the physiological dose range 4 to 32 pmol kg 10 min stimulates the enzyme secretion but had no effect on the insulin release via a short enteropancreatic pathway in pigs.

Differential activation of adenylate cyclase by secretin and VIP receptors in the calf pancreas

OBJECTIVE

Secretin is a key regulator of pancreatic secretion, but the molecular basis of its action is not well understood, especially in the calf pancreas. Our study investigated the expression and functional competence of secretin receptors (SEC-R) in calf pancreatic membranes.

METHODS

We used reverse transcriptase-polymerase chain reaction, sequencing, and Northern blot to assess the expression of the SEC-R gene. The functional characterization of SEC-R was accomplished using adenylate cyclase (AC) assay.

RESULTS

We successfully amplified, by reverse transcriptase-polymerase chain reaction, a fragment of the SEC-R gene from 119-day-old calf pancreas. This sequence shows higher homology with SEC-R than with vasoactive intestinal polypeptide (VIP)-1 and VIP-2 receptors from other species. Northern blot analysis detected a 1.8-kb transcript. Accordingly, secretin stimulates AC activity in calf pancreatic membranes isolated from 28- and 119-day-old animals with a potency (Ka) of 1.9 to 2.7 nmol/L. Maximal AC stimulation induced by secretin represented a 3- to 4-fold increase of basal activity. AC activation by secretin was inhibited by the 2 SEC-R antagonists, [psi4,5] secretin (l micromol/L) and [5-27] secretin (10 micromol/L). Interestingly, [psi4,5] secretin was ineffective against VIP-induced AC stimulation.

CONCLUSION

Our data indicate that secretin exerts a direct action on pancreatic secretion through specific SEC-R coupled to the AC system. Calf pancreatic SEC-Rs are coexpressed with VIP-2 receptors that we previously identified by ligand binding and cross-linking experiments.

Kinetics of pancreatic juice secretion in relation to duodenal migrating myoelectric complex in preruminant and ruminant calves fed twice daily

Daily secretion of pancreatic juice, including postprandial responses to food, was investigated in two groups of calves: preruminant (fed with liquid food) and ruminant (fed with solid food). Male Friesian calves (1 week old and 6 weeks old) were surgically fitted with a pancreatic duct catheter, duodenal cannula and two duodenal electrodes. Continuous 24 h collections of pancreatic juice and myoelectrical recordings were performed with minimal restraint and disturbance of animals. In both groups of calves clear periodic fluctuations in pancreatic juice secretion (volume, protein output and trypsin activity) coinciding with duodenal migrating myoelectric complexes (MMC) were recorded. Secretion of juice per cycle and per day was greater in ruminant calves, but the frequency and amplitude of cycles were lower in this group. There were no differences between day and night-time preprandial pancreatic cycles and duodenal MMC in preruminant calves, whilst in ruminant calves, evening MMC were longer than morning MMC. The pancreatic cephalic phase (increase of volume flow, protein output and trypsin activity during and just after food intake) was significant only in preruminant calves following morning feeding. Postprandial pancreatic cycles did not differ from preprandial cycles, except the pancreatic cycle (juice volume and trypsin activity) in which food was offered in preruminant calves. No gastric or intestinal phase was observed in either group of calves. In conclusion, biological cycles of the gastrointestinal tract are present in both preruminant and ruminant calves, and these cycles evolve along with the change from liquid to solid food.

Kinetics of pancreatic exocrine secretion and plasma gut regulatory peptide release in response to feeding in preruminant and ruminant calves

Pancreatic exocrine secretion and plasma cholecystokinin, gastrin, secretin, and somatostatin concentrations were examined in relation to feeding in 70- to 120-day-old preruminant and ruminant calves. The apparatus used was designed to immediately re-infuse the animal's own pancreatic juice and to carry out accurate measurements of the juice flow in real time and to take samples. In the preruminants, pancreatic juice, protein, and trypsin flows increased from 45 min before and until 15 min after the meal and decreased sharply thereafter over a period of 30 min. while protein and trypsin concentrations peaked after feeding. A significant increase in plasma gastrin and cholecystokinin (CCK), a fall in secretin and no change in somatostatin were observed after milk ingestion. By contrast, in the ruminants, feeding had no effect on the pancreatic secretion and on the plasma concentrations of these peptides. Similar and simultaneous patterns of juice flow and secretin, as well as of protein and trypsin concentrations, CCK and gastrin, could support the hypothesis that these gut regulatory peptides play a significant role in the regulation of the pancreatic function. In preruminant calves, the existence of cephalic, gastric and intestinal phases is discussed. In the ruminants, that of the ruminal phase is questionable.

Gut regulatory peptides in young cattle and sheep

This review summarizes recent advances on blood plasma patterns, receptors, secondary messengers and metabolism of gut regulatory peptides (i.e. gastrin, secretin, somatostatin, cholecystokinin, vasoactive intestinal polypeptide, pancreatic polypeptide, gastric inhibitory polypeptide, motilin), in young ruminants during fetal and perinatal stages, during the preruminant stage and at weaning. It also deals with their role in the digestive tract function and postabsorptive changes. Some pathological situations in young ruminants are considered. Recent molecular aspects of digestive enzyme synthesis are also reported.

Plasma secretin fluctuates in phase with periodic pancreatic secretion and the duodenal migrating myoelectric complex in calves

Plasma secretin and cholecystokinin (CCK) levels and the periodic secretions of the exocrine pancreas were studied simultaneously with the duodenal migrating myoelectric complexes (MMC) in six milk-fed calves which had been starved overnight. The experiments were performed first when the calves were 10 to 16 days old and subsequently when they were 36 to 45 days old. Plasma secretin and the secretion of pancreatic juice fluctuated periodically in phase with the duodenal MMC: plasma secretin, and the pancreatic secretion of water and protein were significantly higher during the phase of irregular spiking activity than during the phase of no spiking activity in both investigations. Plasma CCK did not change throughout the MMC. The intravenous infusion of secretin at 120 pmol kg-1 bodyweight for one hour markedly stimulated pancreatic secretion and prolonged the duodenal MMC cycle, but it did not abolish pancreatic and duodenal periodic activity.