Release of secretin immunoreactivity from the ileum

Prolonged stimulation of pancreatic serous secretions by bile and sodium taurocholate in anaesthetized rats

There have been numerous reports that infusion of either natural bile or bile salts into the duodenum evokes a rapid increase in pancreatic secretion through the release of the hormone secretin from the duodenal mucosa. We have extended this observation by the demonstration of an additional late increase in secretion which persisted for many hours and have sought to identify the processes underlying this increase. In anaesthetised rats, infusion of 20 mM taurocholate into the duodenum caused a staircase-like increase in the weight of pancreatic secretion which extended over many hours during which, the HCO[Formula: see text] and protein output of the secretion showed only minimal changes. This effect was also reproduced with intra-duodenal infusion of natural bile which was inferred to act though its taurocholate content. Since the stimulatory action was also obtained with superfusion of taurocholate or natural bile onto the small intestine and by intravenous injection of taurocholate, it was concluded that taurocholate acted by being absorbed into the bloodstream and then by exerting a stimulatory action on the exocrine pancreas. This action was inhibited by puromycin (a protein synthesis inhibitor), by furosemide (a Na( + )/K( + )/2Cl(-) cotransporter inhibitor), though not by SITS (an inhibitor of Cl(-)/HCO[Formula: see text] exchange). The long lasting increase in pancreatic serous secretion would be consistent with the possible activation of gene transcription by taurocholate leading to increased activity of the Na( + )/K( + )/2Cl(-) cotransporter through which the acinar cells increased their secretions.

Absorption of the cholic acid-conjugated peptide hormone cholylsecretin from the rat ileum in vivo

AIMS

Previously, we demonstrated that gastrin peptides as long as 34 amino acids were absorbed from the ileum of rat after conjugation to the C24 position of cholic acid and that these peptides retained full biological activity. As absorption was specific to the ileum, it was inferred that the conjugated hormone was taken up by the bile salt transporters. We have now extended these experiments to a member of a different family of hormones, viz. secretin, a 27-amino acid hormone that stimulates serous secretions from the exocrine pancreas.

METHODS

After conjugation to cholic acid, the degree of cholylsecretin absorption from the ileum of anaesthetized rats was assessed from the increase in pancreatic secretions.

RESULTS

A complication to the study was that intra-ileal infusion of native secretin caused a transient increase in the levels of pancreatic secretions. This was in contrast to the effects of intra-ileal infusion of cholylsecretin which did not cause this transient increase but, instead, gave rise to a delayed increase in pancreatic secretions which was sustained over several hours during which cholylsecretin was detected in plasma in high concentration by mass spectrometry. The pancreatic response to cholylsecretin was abolished by co-infusion of 50 mm taurocholate, employed to compete with the bile salt transporters, although a transient increase in pancreatic secretions similar to that caused by secretin was now generated. This was shown to arise from an action of taurocholate per se causing the release of endogenous secretin which is present in rat ileum.

CONCLUSIONS

We, therefore, concluded that cholylsecretin had been absorbed from the rat ileum by uptake by bile salt transporters.

Pancreatic dose dependent effect of intraduodenal HCl in the anesthetized rabbit

1. A dose-response relationship between intraduodenal perfusion of different HCl loads (1.8, 4.5, 9, 18 and 45 mmol/hr) and volume, bicarbonate and protein outputs, from the exocrine pancreas of anaesthetized rabbits was observed. 2. This study also shows a dose-response relationship between the different HCl loads and the porta levels of secretin and VIP. 3. The secretin response showed a marked decrease during the stimulation period and this may suggest the involvement of a neural mechanism. 4. The plasma VIP responses were of a magnitude that might suggest a hormone role for VIP in this effect.

Effect of chronically-administered secretin on the nude mouse

Secretin plays an important role in the growth regulation of certain cancers in vitro. The nude mouse is a suitable model for evaluation of the effects of this hormone on tumor xenografts in vivo, but little is known about long-term actions of secretin in this species. We investigated the impact of chronically administered synthetic porcine secretin in the nude mouse. Six groups of mice (eight animals each) received twice-daily intraperitoneal injections of saline or secretin at 0.5, 5, 50, 500, or 5,000 micrograms/kg for 14 days. Body weight and general health were unaffected by exogenous secretin, and no apparent behavioral effects were observed. Seven abdominal organs were examined at necropsy and all were histologically normal. The only organ that showed a weight change was the pancreas (13% decrease at the highest secretin dose). This was accompanied by decreases in DNA and RNA content, indicating pancreatic hypoplasia. Secretin administration caused changes in DNA and/or RNA content (but not protein content or weight) in liver, small bowel, cecum, and large bowel. No effect of secretin on stomach or kidney was observed. Our work demonstrates the safety of frequent injections of pharmacologic doses of secretin in this frail animal and suggests that the nude mouse is an appropriate model for the in vivo study of tumor growth regulation by secretin.