Secretin-Enhanced Magnetic Resonance Cholangiopancreatography for Assessing Pancreatic Secretory Function in Children.

OBJECTIVE: To assess the accuracy and interrater reproducibility of measurements of pancreatic secretory function by magnetic resonance cholangiopancreatography in response to secretin administration and to describe our experience using the technique to noninvasively assess pancreatic secretory function in a pediatric population. STUDY DESIGN: In the accuracy study, phantoms with varying fluid volume (47-206 mL) were imaged using the clinical quantification sequence. Fluid volume was measured by image segmentation (ImageJ). Measurement accuracy was expressed in terms of error (absolute and percent) relative to known fluid volume. In the reproducibility study and clinical experience, 31 patients with suspected pancreatic disease underwent 33 secretin-enhanced magnetic resonance cholangiopancreatography exams. Two-dimensional T2-weighted, fat-saturated single shot fast spin echo sequences were acquired before and after secretin injection (0.2 µg/kg, max 16 µg). Secreted fluid volume (postsecretin minus presecretin) was independently measured by 2 blinded reviewers. Between reviewer measurement reproducibility was assessed based on correlation (Spearman) and bias (Bland-Altman analysis). RESULTS: For the accuracy study, fluid volumes were measured with mean volume errors of -0.3 to +12.5 mL (percent error -0.03% to +9.0%). For the reproducibility study, the mean secreted fluid volumes measured by reviewer 1 and reviewer 2 were 79.1 ± 54.3 mL (range 5.5-215.4) and 77.2 ± 47.1 mL (range 6.7-198.1 mL), respectively. Measured secreted fluid volumes were very strongly correlated (r = 0.922) between reviewers with a bias of only 1.9 mL (95% limits of agreement -40.5 to 44.2). CONCLUSIONS: Measurement of fluid volume by magnetic resonance imaging is highly accurate with <10% (<13 mL) error in measured volume. Measurements of pancreatic secreted fluid volume in response to secretin by magnetic resonance cholangiopancreatography are highly reproducible with a bias of <2 mL between reviewers.

Assessment of exocrine pancreatic function by secretin-stimulated magnetic resonance cholangiopancreaticography and diffusion-weighted imaging in healthy controls.

PURPOSE: To characterize and quantify exocrine pancreatic function by secretin-stimulated magnetic resonance cholangiopancreaticography (s-MRCP) and diffusion-weighted imaging (DWI) in healthy subjects and compare these findings to morphological features, ie, pancreatic volume and secretin-stimulated peak bicarbonate concentration measured in pancreatic juice. MATERIALS AND METHODS: Pancreatic magnetic resonance imaging (MRI) (1.5 T) was performed in 20 healthy volunteers among which 10 underwent gastroscopy with duodenal intubation. MRI included T2-weighted imaging and DWI acquired before and 1, 5, 9, and 13 minutes after secretin administration. Secreted pancreatic juice volumes were calculated based on the sequential T2-weighted images and pancreatic volumes and apparent diffusion coefficient (ADC) values were estimated. RESULTS: The mean pancreatic secretion rate declined from 9.5 mL/min at 1-5 minutes (postsecretin) to 2.9 mL/min at 9-13 minutes. Pancreatic head ADC values significantly increased from baseline (1.29 × 10(-3) mm(2) /s) to 1 minute postsecretin (1.48 × 10(-3) mm(2) /s) (P = 0.003). Secreted pancreatic juice volume at 1 minute after secretin correlated positively with peak bicarbonate concentration (n = 10, P = 0.05). CONCLUSION: Secretin-stimulated MRCP and DWI can characterize and quantify exocrine pancreatic function in healthy subjects. These imaging methods may prove relevant for patients with exocrine pancreatic dysfunction.

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.

Differential transport of a secretin analog across the blood-brain and blood-cerebrospinal fluid barriers of the mouse.

Secretin is a gastrointestinal peptide belonging to the vasoactive intestinal peptide (VIP)/glucagon/pituitary adenylate cyclase-activating polypeptide (PACAP) family recently suggested to have therapeutic effects in autism. A direct effect on brain would require secretin to cross the blood-brain barrier (BBB), an ability other members of the VIP/PACAP family have. Herein, we examined whether a secretin analog (SA) radioactively labeled with (131)I (I-SA) could cross the BBB of 4-week-old mice. We found I-SA was rapidly cleared from serum with fragments not precipitating with acid appearing in brain and serum. Levels of radioactivity were corrected to reflect only intact I-SA as estimated by acid precipitation. After i.v. injection, I-SA was taken up by brain at a modest rate of 0.9 to 1.5 microl/g-mm. Capillary depletion, brain perfusion, and high-performance liquid chromatography were used to confirm the passage of intact I-SA across the BBB. I-SA entered every brain region, with the highest uptake into the hypothalamus and cerebrospinal fluid (CSF). Unlabeled SA (10 microg/mouse) did not inhibit uptake by brain but did inhibit clearance from blood and uptake by the CSF, colon, kidney, and liver. The decreased clearance of I-SA from blood increased the percentage of the i.v. injected dose taken up per brain (%Inj/g) from about 0.118 to 0.295%Inj/g. In conclusion, SA crosses the vascular barrier by a nonsaturable process and the choroid plexus by a saturable process in amounts that for other members of its family produce central nervous system (CNS) effects. This passage provides a pathway through which peripherally administered SA could affect the CNS.

Dibutyltin dichloride modifies amylase release from isolated rat pancreatic acini.

INTRODUCTION: Dibutyltin dichloride (DBTC) is widely used as a stabilizer for polyvinylchloride plastics and is of particular toxicologic interest. AIM: To examine the effects of DBTC on pancreatic exocrine function in isolated rat pancreatic acini. METHODOLOGY: Isolated rat pancreatic acini were incubated with various secretagogues in the presence or absence of DBTC. We investigated the effects of DBTC on amylase release, receptor binding, and protein kinase C (PKC) enzyme activity. RESULTS: DBTC reduced cholecystokinin octapeptide (CCK-8)-stimulated and carbamylcholine-stimulated amylase release and the binding of [(125)I]CCK-8 to isolated rat pancreatic acini. Conversely, DBTC potentiated secretin-stimulated amylase release, although it slightly inhibited [(125)I]secretin binding to its receptors. In addition, DBTC potentiated amylase release stimulated by vasoactive intestinal peptide, 8-bromoadenosine 3', 5'-monophosphate (8Br-cAMP) or calcium ionophore A23187, whereas it had no influence on amylase release stimulated by 12-O-tetradecanoylphorbol 13-acetate. The protein kinase C (PKC) inhibitor calphostin C abolished the DBTC-induced potentiation of amylase release stimulated by 8Br-cAMP or A23187. Moreover, DBTC caused a significant translocation of PKC enzyme activity from cytosol to membrane fraction. CONCLUSIONS: These results indicate that DBTC reduces CCK-8- and carbamylcholine-stimulated amylase release by inhibiting their receptor bindings to pancreatic acini, whereas it potentiates cAMP-mediated amylase release by activating PKC in isolated rat pancreatic acini.

Proliferative response of different exocrine pancreatic cell types to hormonal stimuli. II. Effects of long-term secretin administration.

The effect of graded doses of secretin on exocrine pancreatic growth and cell proliferation was studied in a long-term experiment. After 16.5 and 150 micrograms/kg/24 h of secretin, each administered as two subcutaneous injections daily for 14 days, the pancreatic wet weight decreased, whereas the protein and DNA content of the gland was uninfluenced. When administered as described above for 14 days, 16.5, 50, 150, and 450 micrograms/kg/24 h of secretin did not affect the proliferation rate of ductal, acinar, or centroacinar cells as measured by a labeling index after 7 days of continuous 3H-thymidine administration. In the control groups a higher labeling index was found for ductal cells (19.9%) than for acinar cells (11.0%). During the 7 days of 3H-thymidine administration 80-90% of ductal and acinar cells remained in the G0 phase. There was a significantly higher labeling index in interlobular than in intralobular duct cells.

Pancreatic secretory responses to long-term infusions of secretin and cerulein in humans.

We measured pancreatic enzyme and bicarbonate responses to graded doses of intravenous secretin or cerulein alone or together in healthy human subjects. Bicarbonate responses were steady and well maintained during the last 3.5 h of the 4 h of infusions of secretagogues, giving evidence for a constant pancreatic flow rate. Potentiation (more-than-additive response) was observed between secretin and cerulein for bicarbonate secretion, but not for enzyme secretion. Secretin stimulated pancreatic enzyme secretion. The effect was most pronounced with amylase secretion and less prominent with lipase, trypsin, and chymotrypsin secretion. Changes in the proportion of enzymes were seen over time, with trypsin and chymotrypsin output declining towards the end of cerulein infusion. We conclude that in humans the effects of secretin on pancreatic enzyme secretion are complex and include time-dependent changes in the enzyme mixture, but potentiation between secretin and cerulein does not occur for enzyme output.

Mechanism and kinetics of secretion degradation in aqueous solutions.

In order to clarify the mechanism of secretin degradation in aqueous solutions, the formation of degradation products from secretin, aspartoyl3 secretin and beta-aspartyl3 secretin was investigated; the stabilities of these three peptides were investigated as well. Aspartoyl3 secretion and beta-aspartyl3 secretin, degradation peptides produced during the storage of secretin in aqueous solutions, were isolated by preparative reversed-phase HPLC (RP-HPLC). The amounts of secretin and its two degradation peptides resulting from storage of secretin in various buffer solutions (pH 2.3 to 10.0, mu = 0.5 M, 60 degrees C) were determined by analytical RP-HPLC. Secretin and the isolated degradation peptides were stored separately in various aqueous buffer solutions resulting in the degradation of each peptide. A mixture of secretin and its degradation or cleavage peptides was formed in each solution. The observed degradation rates for each peptide approximately followed first-order kinetics. The pH-rate profiles for conversion of secretin and beta-aspartyl3 secretin were similar, while that for aspartoyl3 secretin was different from these two. Aspartoyl3 secretin was more stable than the others at pH 2.3 to 4.0, but it was easily degraded between pH 5.0 and 10.0. Investigation of aspartoyl3 secretin degradation showed that its degradation was related to the pH value of the solution, and that hydroxide ion catalyzes the ring opening of the aspartoyl peptide. Secretin was most stable in pH 7.0 buffer solution and more stable in acidic solutions than in alkaline solutions. Secretion was mainly degraded through the following pathways: cleavage peptides reversible secretin in equilibrium aspartoyl peptide in equilibrium beta-aspartyl peptide vector cleavage peptides.

Effect of dose, pH, and osmolarity on nasal absorption of secretin in rats. III. In vitro membrane permeation test and determination of apparent partition coefficient of secretion.

Nasal absorption of secretin in rats was enhanced in an acid solution and the maximum absorption was observed at a sodium chloride solution molarity of 0.462. In order to predict how changes in the secretin molecule would affect its absorption through the nasal mucosa independently of structural changes in the epithelial membrane, an artificial membrane permeation test was conducted, and the apparent partition coefficient between octanol and a test solution was determined. The concentration of secretin was measured using high performance liquid chromatography. The amount of secretin that permeated through an artificial membrane was hardly affected by changes in pH, which suggest that the size of the secretin molecule was not changed. The apparent partition coefficient, however, increased as the pH of the test solution rose from 3.81 to 7.0, which suggested that the hydrophobicity of secretin was enhanced. In relation to the osmolarity of the test solution, the amount of permeation was hardly affected by the concentration of sodium chloride, but the partition coefficient increased with the concentration of the sodium chloride solution. It was supposed that the size of the secretin molecule was not changed in spite of the increasing hydrophobicity, and the nasal absorption of secretin at a sodium chloride molarity of 0.462 was dependent on a change in the epithelial cells. When sorbitol was used as an osmoregulatory agent, the apparent partition coefficient hardly varied as the osmolarity of the solution was increased, whereas the amount of permeation decreased, and these findings reflected the nasal absorption in rats.

Human secretin. Biologic effects and plasma kinetics in humans.

The action of synthetic human secretin, which differs in two amino acid residues from porcine secretin, was compared with synthetic porcine secretin in 6 healthy volunteers. Pancreatic secretion was assessed by a marker perfusion technique and plasma secretin concentrations were assessed by a specific radioimmunoassay. Increasing doses of either human or porcine secretin produced increasing bicarbonate output (p less than 0.01), whereas trypsin and lipase were not stimulated over basal. The highest doses of secretin induced a significant increase in pancreatic amylase secretion. The two secretin preparations were found to be equipotent with respect to pancreatic secretion and plasma kinetics. Significant increases of plasma secretin were observed after a steak meal in 15 volunteers (p less than 0.001). When human secretin was infused at postprandial concentrations, significant increases in pancreatic bicarbonate output were observed (p less than 0.05). We conclude (a) that the substitution of two amino acids in human secretin does not affect biologic activity and plasma metabolism of the compound; (b) secretin does not stimulate pancreatic enzyme secretion at physiologic concentrations; and (c) the stimulatory effects of secretin on pancreatic amylase remain to be elucidated. The study suggests that human secretin is a true hormone.

Effects of dose, pH, and osmolarity on nasal absorption of secretin in rats. II: Histological aspects of the nasal mucosa in relation to the absorption variation due to the effects of pH and osmolarity.

Nasal absorption of secretin in rats was enhanced in an acid solution, and the maximum absorption was observed at a sodium chloride solution molarity of 0.462. In order to examine reasons for the variation of absorbability caused by the change of pH and osmolarity in secretin preparations, both a pretreatment study, in which the nasal mucosa was treated with placebo prior to the administration of a secretin preparation, and a histological study were conducted in rats. The nasal absorption of secretin was determined by measuring the increased secretin of pancreatic juice. Similar profiles of nasal absorption, both after intranasal administration of secretin preparations and as a result of pretreatment effects, were obtained in studies of the effects of pH and osmolarity. However, in the pH-effect study, the absorption with the use of active preparations was observed to be significantly larger than that with the pretreatment effect below a pH of 4.79, and significantly smaller than that with the pretreatment effect at a pH of 7 to 8. The results of histological studies revealed structural changes of the epithelial cells of the nasal mucosa at pH 2.94, and shrinkage of epithelial cells was observed at a sodium chloride solution molarity of 0.462.