Pancreatic Secretory Response Research Articles

Effect of somatostatin immunoneutralization on gastric acid and pancreatic enzyme secretion in anesthetized rats

The involvement of somatostatin in urethane-anesthesia-evoked suppression of gastric acid secretion has been described. The present study has examined the role of endogenous somatostatin in diminished pancreatic enzyme secretion during anesthesia, while monitoring acid secretion concurrently. Rats were anesthetized with either urethane or sodium pentobarbital. An indwelling catheter was placed into the right jugular vein. The esophagus and the pylorus were ligated, and the stomach was perfused with saline. The common bile duct was ligated at the hepatic hilum, and cannulated at the duodenal end of the duct for collecting pure pancreatic juice. Purified somatostatin monoclonal antibody (CURE.S6) or control antibody (keyhole limpet hemacyanin, KLH) was injected iv in increasing doses (0.05; 0.15; 0.5; and 1.5 mg) every 30 min ( n = 6). Gastric acid and pancreatic amylase secretions were measured. The effect of the antibodies on CCK-8-stimulated (0.25–2.50 nmol/kg/h) pancreatic amylase secretion was also tested. During urethane anesthesia somatostatin antibody induced a dose-dependent increase in acid output, while control antibody did not change it. Basal pancreatic amylase secretion was not affected by either somatostatin or by control antibody. Pancreatic secretory responses to high but not to low doses CCK-8 were found to be significantly increased following immunoneutralization of somatostatin. In sodium pentobarbital-anesthetized rats somatostatin antibody stimulated basal acid secretion but did not affect basal pancreatic amylase secretion. Our data indicate that in anesthetized rats endogenous somatostatin mediates suppression of basal gastric acid secretion but not that of basal pancreatic amylase secretion, and this action does not depend on the type of anesthesia. Furthermore, endogenous somatostatin may play a physiological role in modulating stimulated pancreatic enzyme secretion in this species.

Pancreatic Secretory Response to Intrajejunal Tryptophan

In two sets of dogs with gastric, duodenal, and jejunal fistulas, we studied the effect of atropine (14 nmol/ kg/h) on the pancreatic secretory response to intrajejunal tryptophan (0.12-10.0 mmol/h; given against a secretin background) before (n = 7) and after extrinsic denervation of the jejunoileum (orthotopical autotransplantation; n = 6). Plasma levels of cholecystokinin were determined by radioimmunoassay. The incremental bicarbonate response to tryptophan was not significantly different between the two sets of dogs. Atropine had no effect on the incremental bicarbonate response to tryptophan. In both sets of dogs, intrajejunal tryptophan caused a dose-dependent increase in pancreatic protein output, which was reduced by atropine. The tryptophan-stimulated levels of plasma cholecystokinin were not significantly altered by denervation and or atropine. We conclude that in dogs (1) intrajejunal tryptophan stimulates pancreatic bicarbonate and protein secretion via release of hormones, (2) extrinsic denervation of the jejunoileum does not significantly alter the incremental bicarbonate and protein responses to intrajejunal tryptophan, (3) the cholinergic intrinsic nerves of the jejunoileum and the hormone cholecystokinin are probably involved in control of the pancreatic protein response to tryptophan, and (4) the release of cholecystokinin by intrajejunal tryptophan does not depend on the extrinsic and intrinsic cholinergic nerves of the jejunoileum.

Duodenal and ileal nutrient deliveries regulate human intestinal motor and pancreatic responses to a meal.

It is assumed that in humans pancreatic and gastrointestinal motor responses to a meal are coordinated and regulated mainly by duodenal nutrient exposure. On the other hand, there is evidence that the distal intestine may participate in the regulation of gastrointestinal functions. The aim of this study was to compare human pancreatic and intestinal motility responses to a meal and to correlate them with nutrient exposure of the proximal and distal intestine. After intubation with an oroileal multilumen tube for marker perfusion, duodenal and ileal aspiration, and intestinal manometry, 14 healthy subjects received a mixed test meal (1,257 kJ). Intraluminal nutrient concentrations, enzyme activities, and small intestinal motility were analyzed for 6 h postprandially. Duration of duodenal nutrient exposure was 3.4 +/- 0.2 h, and duration of pancreatic enzyme response and fed motor pattern was 2.5 +/- 0.2 and 3.5 +/- 0.3 h, respectively. Durations of pancreatic secretory and motor responses were correlated (P < 0.05), but neither duration of digestive secretory nor of motor activity correlated with that of prandial duodenal nutrient concentrations. By contrast, they were associated with the relative increase in ileal nutrient delivery late postprandially (P < 0.05). Physiological late postprandial delivery of nutrients to distal intestinal sites is correlated with the termination of digestive pancreatic and motor responses and may participate in their control.

Inhibition of canine exocrine pancreatic secretion by peptide YY is mediated by PYY-preferring Y2 receptors.

It is still unclear, which receptor subtype, Y1 and/or Y2, mediates the inhibitory action of PYY on exocrine pancreatic secretion. The present study was undertaken to characterize functionally the Y receptor subtype that mediates the inhibition of exocrine pancreatic secretion by peptide YY (PYY). In eight conscious dogs with chronic gastric and pancreatic fistulas, we compared the action of intravenous infusion of 200 and 400 pmol/kg/h of the Y receptor agonists PYY 1-36, PYY 3-36, PYY 13-36, Pro34PYY 1-36, and NPY 1-36 on the pancreatic secretory response to secretin (20.5 pmol/kg/h) and cerulein (29.6 pmol/kg/h). PYY 13-36, Pro34PYY 1-36, and NPY 1-36 were also studied by giving a fivefold dose (1,000 and 2,000 pmol/kg/h). PYY 1-36 and the Y2 receptor agonist PYY 3-36 significantly inhibited pancreatic secretory responses to secretin and cerulein, whereas inhibition by NPY 1-36 and the Y2 receptor agonist PYY 13-36 was attainable only at doses of 1,000 and 2,000 pmol/kg/h. The Y1 receptor agonist Pro34PYY 1-36 was without effect on pancreatic secretion. We conclude that in dogs the inhibition of exocrine pancreatic secretion by PYY is mediated via Y2 receptors of a PYY-preferring subtype.

Insulin Resistance, Its Consequences, and Coronary Heart Disease

Approximately 25 years ago, our research group1 2 used quantitative methods to show that resistance to insulin-mediated glucose disposal was present in patients with either impaired glucose tolerance or NIDDM. These initial observations have been confirmed on multiple occasions since then, and there is now a large body of evidence that shows that this defect is present in the vast majority of glucose-intolerant individuals.3 4 5 6 7 8 In addition, several reports9 10 11 demonstrated that first-degree relatives of patients with NIDDM are also relatively insulin resistant compared with well-matched volunteers without a family history of NIDDM. Finally, prospective studies12 13 14 15 16 have identified insulin resistance and/or compensatory hyperinsulinemia as the initial defect in the development of NIDDM. Thus, the central role of insulin resistance in the pathogenesis and clinical course of patients with NIDDM has been well established. Of greater relevance to the IRAS study of “Insulin Sensitivity and Atherosclerosis” published in this issue of Circulation 17 is recent awareness that the ability of insulin to mediate glucose disposal can differ by as much as 10-fold in subjects with normal glucose tolerance.18 19 Because the degree of insulin resistance in a substantial portion of these persons approaches that seen in patients with impaired glucose tolerance or NIDDM, it is assumed that they are able to secrete enough insulin to overcome the insulin resistance and maintain normal glucose tolerance. Unfortunately, the philanthropic response on the part of the pancreatic β-cell is not without cost, and in 1988,20 it was emphasized that these individuals were at increased risk to have higher plasma TG and lower HDL cholesterol concentrations and higher blood pressure. More importantly, it was suggested that the cluster of abnormalities associated with insulin resistance, designated as Syndrome X, significantly increased …

Inhibition of Unstimulated Exocrine Pancreatic Secretion by Peptide YY in the Rat

Peptide YY is an ileocolonic peptide known to inhibit postprandial and cholecystokinin-induced pancreatic exocrine secretion. It has also been shown to increase intestinal water and electrolyte absorption. These findings implicate PYY as being potentially useful for controlling watery diarrhea. Although its inhibitory effect on stimulated pancreatic secretion has been well established, PYY effects on interdigestive, unstimulated pancreatic secretion is not known. This study was designed to evaluate the effect of PYY on basal pancreatic exocrine secretion in a conscious rat. Male Sprague-Dawley rats were prepared with catheters for internal biliary bypass, pancreatic juice collection, and intraduodenal reinfusion of pancreatic juice. Jugular and carotid catheters were inserted for drug infusions and blood sampling. After overnight recovery, fasting rats were infused over 6 hours with saline or PYY (400 or 800 pmol/kg/hr). Pancreatic juice was measured and sampled at 60-minute intervals for volume and its protein and bicarbonate content. The remainder was reinfused into the duodenum. Before and after the experiment, pancreatic juice was automatically reinfused by a photocell-controlled peristaltic pump system. Intraductal pancreatic secretion was not affected by PYY at a dose of 400 pmol/kg/hr. PYY at a dose of 800 pmol/kg/hr significantly reduced the volume of pancreatic secretion and its protein and bicarbonate content. Pancreatic secretory response normalized within 24 hours. In conclusion, unstimulated pancreatic exocrine secretion can be inhibited by exogenous PYY in the rat.

Adaptation to fat markedly increases pancreatic secretory response to intraduodenal fat in rats.

Exposure to higher levels of fat in the diet increases the secretion of fat-digesting enzymes in pancreatic juice. This study examines the functional consequences of this phenomenon and demonstrates that adapting rats to high fat (triglyceride) loads increases the release of cholecystokinin (CCK) and the pancreatic secretory response to intraduodenal fat. Lipolytic activity in the small intestine was also higher in adapted rats. Exchanging pancreatic juice from unadapted rats with pancreatic juice from adapted rats decreased the response to fat in adapted rats and increased the response to fat in unadapted rats. Infusing oleic acid into unadapted rats stimulated CCK secretion and pancreatic exocrine secretion to levels observed with triglycerides in adapted rats. Pancreatic exocrine secretion in response to intraduodenal fat in rats adapted to a high-fat (20%) diet were significantly higher than the responses seen in rats fed a low-fat (5%) diet. Adaptation to fat increases the pancreatic secretory and plasma CCK responses to fat, apparently by increasing the efficiency of triglyceride digestion and thereby increasing CCK release.

Insulin stimulates cationic amino acid transport activity in the isolated perfused rat pancreas.

The effects of exogenous insulin, glucagon and streptozotocin-diabetes on influx (15 s) of L-lysine via a cationic amino acid transporter resembling system y+ were investigated in the isolated perfused rat pancreas. In non-diabetic pancreata, transport of L-lysine was saturable with an apparent Km of 2.11 +/- 0.29 mM and Vmax of 2.21 +/- 0.20 mumol min-1 g-1 (n = 6). Bovine insulin (100 mu u ml-1) increased the maximal transport rate (Vmax = 3.49 +/- 0.30 mumol min-1 g-1, n = 4, P < 0.05) for L-lysine 1.6-fold without altering the Km. L-Lysine transport was not elevated significantly in diabetic pancreata, although insulin (100 mu u ml-1) enhanced transport to values measured in non-diabetic preparations. Human glucagon (1.5 x 10(-9) M) had no stimulatory effect on L-lysine transport. These findings provide the first evidence that exogenous insulin stimulates cationic amino acid transport activity in the exocrine pancreatic epithelium. Activation of the cationic pancreatic amino acid transporter may provide a mechanism to enhance the supply of L-arginine and thus sustain nitric oxide-mediated pancreatic secretion in response to islet hormones and secretagogues.

The involvement of endogenous nitric oxide in vagal-cholinergic stimulation of exocrine and endocrine pancreas in dogs.

Previous studies showed that nitric oxide (NO), synthesized from L-arginine (L-arg) by NO synthase (NOS) in vascular epithelium and nerve terminals, affects exocrine pancreatic secretion, but its role in control of endocrine pancreas has not been studied. In this study, the role of NO in the control of pancreatic secretion in response to vagal-cholinergic stimulation and duodenal infusion of nutrients was determined in conscious dogs with chronic pancreatic fistulas. Sham feeding (SF), urecholine iv infusion, and duodenal perfusion with nutrients were used to stimulate the pancreatic protein secretion, and insulin and glucagon release in tests without and with iv infusion of NG-nitro-L-arginine (L-NNA), an inhibitor of NO synthase, L-arg, a substrate of NOS, or their combination was used. SF, urecholine, and duodenal nutrient resulted in the stimulation of pancreatic protein secretion reaching, respectively, 50, 20, and 42% of cerulein maximum. Infusion of L-arg almost doubled the basal protein secretion and tended to increase the secretory response to SF and duodenal nutrient. After infusion of L-NNA, the pancreatic secretory responses to SF, urecholine, and duodenal nutrient were inhibited by about 70, 30, and 75%, respectively. When L-arg was combined with L-NNA, the reduction in pancreatic secretion by L-NNA was significantly attenuated. SF resulted in a significant rise in plasma insulin and glucagon, and this response was completely abolished by L-NNA infusion. Urecholine and duodenal nutrient also resulted in a marked increment in plasma insulin and glucagon, the insulin (but not glucagon) increment being abolished by the pretreatment with L-NNA and reversed by the addition of L-arg.(ABSTRACT TRUNCATED AT 250 WORDS)

Neuronal regulation of the release and action of secretin-releasing peptide and secretin

The acid-stimulated release of secretin is mediated by a secretin-releasing peptide (SRP) in rats. In the present study we investigated to determine whether a neural mechanism(s) is involved in the regulation of release and action of SRP in anesthetized rats. A concentrated acid perfusate (CAP) containing SRP was obtained from donor rats. CAP administered to recipient rats significantly increased pancreatic flow volume (81.6 +/- 18.3%), bicarbonate output (188.7 +/- 15.6%), and plasma secretin level (from 0.9 +/- 0.2 to 4.4 +/- 0.5 pM). However, this effect was attenuated by CAP from donor rats pretreated with tetrodotoxin (TTX), propranolol, bilateral subdiaphragmatic vagotomy (BSV), or systemic and topical administration of capsaicin. In contrast, CAP from donor rats pretreated with phentolamine, atropine, or hexamethonium did not alter the increase in plasma secretin concentration and pancreatic secretion. Moreover, the action of CAP on secretin release was significantly inhibited in the recipient rats pretreated with TTX, BSV, and topical applications of capsaicin but was not suppressed in the recipient rats pretreated with atropine, hexamethonium, or propranolol. Furthermore, perivagal and duodenojejunal mucosal application of capsaicin abolished the pancreatic secretory response to secretin at 5 pmol.kg-1.h-1. In conclusion, the release and action of both SRP and secretin are mediated by a vagal afferent pathway. beta-Adrenergic receptors also play a significant role in the release of SRP.

Effect of endogenous insulin on pancreatic exocrine secretion in perfused dog pancreas.

Canine pancreas was perfused with an intraarterial infusion of Krebs-Ringer bicarbonate solution containing 5% dog red blood cells, 0.1% bovine serum albumin, and 3% dextran at 15 ml/min, while portal effluent was continuously collected. Pancreatic juice was obtained in 15-min samples via main pancreatic duct cannulation. After a 1-h basal period, secretin and cholecystokinin-8 (CCK), at doses of 2.5 ng.min-1 each, were simultaneously infused for 10 min, with background infusion of a normal rabbit serum (NRS) or an antiinsulin serum (Anti-I) in 5 ml each via a sidearm of the intraarterial catheter. The infusion of secretin and CCK resulted in a significant increase in pancreatic bicarbonate and protein secretion during the infusion of NRS, whereas the pancreatic secretory response of bicarbonate and protein was profoundly suppressed by the infusion of Anti-I in six pancreata so studied. This suppression by Anti-I coincided with significant increases in somatostatin and pancreatic polypeptide levels in portal venous effluent. In three additional pancreata, simultaneous infusions of Anti-I with antisomatostatin (5 ml) and antipancreatic polypeptide (5 ml) serum failed to inhibit the pancreatic exocrine secretion. These results indicate that secretin- and CCK-stimulated pancreatic secretion of bicarbonate and protein depends heavily on local action of insulin. The suppression by Anti-I of pancreatic secretion is mediated, in part, by local releases of somatostatin and pancreatic polypeptide. Thus, the insuloacinar axis plays an important regulatory role in pancreatic exocrine secretion in the dog, and it involves at least three islet hormones including insulin, somatostatin, and pancreatic polypeptide.

On the importance of cholecystokinin in neonatal pancreatic growth and secretory development in guinea pigs.

The role of cholecystokinin (CCK) in pancreatic growth and secretory development in fetal and neonatal guinea pigs was investigated by CCK receptor blockade. For the last 20 days of gestation, sows received the CCKA receptor antagonist, MK329 (25 nmol/kg/h) by continuous subcutaneous infusion. Alternatively, neonates from untreated females received an MK329 infusion for the first 4 or 15 days following birth. Pancreatic weight, DNA, RNA, protein, and amylase content per 100 g body weight and secretory responses to CCK, carbamylcholine, and phorbol ester were determined at birth and 4 days in animals receiving MK329 in utero and were measured at 4 and 15 days in neonatally infused animals. No significant changes in pancreatic growth parameters were seen in MK329-treated animals compared to controls, except for a small (14%; p < 0.02) decrease in weight after 15 days of neonatal exposure. Enhanced amylase secretion in response to CCK and carbamylcholine was seen in all groups receiving MK329 (all p values < 0.00001). Pancreatic growth and secretion were not inhibited by CCKA receptor blockade, which suggests that the effects of CCK mediated by the CCKA receptor are not essential for growth or development of the pancreatic amylase secretory response in the neonatal guinea pig.

Cholinergic dependence of pancreatic response to cholecystokinin in rats and guinea pigs

1. 1. Lorglumide and atropine were used to examine the role of cholinergic mechanisms in the pancreatic secretory response to cholecystokinin in two animal species. 2. 2. Anaesthetized rats and guinea pigs with jugular vein and pancreatic cannulae were used and the bile juice was recirculated. In the rat, the treatment with lorglumide (3 μmol/kg) as well as atropine (100 μg/kg) did not have effects on basal interdigestive secretion, whereas in guinea pigs only atropine decreased the protein output (41%) and the juice flow (47%) of the basal pancreatic secretion. 3. 3. Infusion of cholecystokinin (150 pmol/kg/hr in the rat and 50 pmol/kg/hr in the guinea pig) induced a marked increase in pancreatic juice flow and protein output compared to saline controls. Pretreatment of both rat and guinea pig with lorglumide resulted in a marked attenuation of the cholecystokinin-evoked secretory response. 4. 4. In the rats, atropine decreased the response to infusion of cholecystokinin octapeptide while this antimuscarinic agent had no effect in the response to cholecystokinin in the guinea pigs. 5. 5. This study supports the concept that the influence of cholinergic system in pancreatic response to cholecystokinin shows interspecific differences.

Phosphoinositide synthesis in desensitized rat pancreatic acinar cells

To help understand the possible role of phosphoinositide turnover in the desensitization process, the availability of phosphatidylinositol 4,5-bisphosphate was investigated in normal and desensitized pancreatic acinar cells treated with carbamylcholine (Cch), caerulein (Cae), and the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA). In control acini, incorporation of [myo-3H]inositol into total phosphoinositides was maximal at 120 min, was Cch and Cae dose dependent, and was insensitive to TPA. Cch stimulation increased the proportion of [myo-3H]inositol incorporated into phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2], whereas Cae specifically channeled [myo-3H]inositol incorporation into phosphatidylinositol 3,4-bisphosphate and phosphatidylinositol 3,4,5-trisphosphate. In the desensitized cells, preexposure to Cch and Cae, but not to TPA, increased the subsequent basal rate of [myo-3H]inositol incorporation into total phosphoinositol (PI) by 66 and 50% above control values. There were no subsequent responses to increasing concentrations of Cch, Cae, and TPA during a second incubation. Desensitization of the pancreatic secretory responses to Cch, Cae, and TPA does not seem to result from a decrease either in total PI or in specific PtdIns(4,5)P2 synthesis, which is needed for inositol trisphosphate and diacylglycerol production.

Factors influencing the intestinal phase of pancreatic exocrine secretion in the turkey.

The present study was done to investigate the factors regulating the intestinal phase of exocrine pancreatic secretion in the turkey. The intestine of turkeys equipped with pancreatic fistulas was perfused with peptone solution, fat emulsion and hydrochloric acid (HCl), and pancreatic flow and protein output were measured. Neither peptone solution nor fat emulsion had any effects on pancreatic secretion. HCl enhanced the flow rate of pancreatic juice but not protein output. To clarify the neural mechanism of this phenomenon, the vagal postganglionic blocker atropine was continuously infused and pancreatic secretion in response to intestinal HCl was measured. Atropine completely suppressed both pancreatic flow and protein output. It is suggested that the avian intestinal phase of pancreatic secretion is mainly controlled by cholinergic action though HCl stimulation.

Pituitary adenylate cyclase-activating polypeptide (PACAP) stimulates exocrine pancreas in conscious preruminating calves

The effects of new hypothalamic peptides, PACAP-27 and PACAP-38, and secretin and VIP on the interdigestive pancreatic secretion and duodenal myoelectric activity during the asecretory phase of the pancreatic interdigestive cycle, compared with the milk ingestion phase, were examined in five calves. Peptides were infused for 5 min into the external jugular vein (0, 3, 10, 30 and 100 pmol/kg body wt during the asecretory phase of the pancreatic interdigestive cycle, and the pancreatic secretory response was compared with that obtained during milk ingestion. Intravenous infusion of PACAP-27 caused dose-related stimulation of pancreatic juice flow and bicarbonate and protein output; this effect was identical to infusion of secretin. The effect of PACAP-38 was less pronounced, and that of VIP was the weakest. Pancreatic juice volume and bicarbonate responses during milk ingestion were similar to responses obtained with the highest doses of hypothalamic peptides and secretin, whereas postprandial protein secretion was much greater than the secretion stimulated with peptides. It was concluded that PACAP from the VIP/secretin family may stimulate pancreatic exocrine secretion in conscious calves and a part of the pancreatic response to food intake can be mediated by PACAP.

Pituitary adenylate cyclase-activating polypeptide (PACAP) stimulates exocrine pancreas in conscious preruminating calves

The effects of new hypothalamic peptides, PACAP-27 and PACAP-38, and secretin and VIP on the interdigestive pancreatic secretion and duodenal myoelectric activity during the asecretory phase of the pancreatic interdigestive cycle, compared with the milk ingestion phase, were examined in five calves. Peptides were infused for 5 min into the external jugular vein (0, 3, 10, 30 and 100 pmol/kg body wt during the asecretory phase of the pancreatic interdigestive cycle, and the pancreatic secretory response was compared with that obtained during milk ingestion. Intravenous infusion of PACAP-27 caused dose-related stimulation of pancreatic juice flow and bicarbonate and protein output; this effect was identical to infusion of secretin. The effect of PACAP-38 was less pronounced, and that of VIP was the weakest. Pancreatic juice volume and bicarbonate responses during milk ingestion were similar to responses obtained with the highest doses of hypothalamic peptides and secretin, whereas postprandial protein secretion was much greater than the secretion stimulated with peptides. It was concluded that PACAP from the VIP/secretin family may stimulate pancreatic exocrine secretion in conscious calves and a part of the pancreatic response to food intake can be mediated by PACAP.

Direct, concentration-dependent inhibition by taurocholate of pancreatic exocrine secretion and CCK release in conscious rats

In conscious rats, bile inhibits pancreatic secretion. The role of luminal taurocholate (TC), a major component of rat bile, in the regulation of pancreatic secretion was studied in conscious rats with external bile and pancreatic fistulae. On the fourth postoperative day, after the basal collection of bile and pancreatic juice (PJ) returned to the duodenum, graded doses of TC (0, 0.4, 4, 40 mM) containing 10 mM CaCl2 were infused into the duodenum instead of bile and PJ for 2 hr (1 ml/hr), with or without 1 mg/ml of porcine trypsin. Luminal trypsin activities were not affected by any dose of TC. The increases in pancreatic secretion in response to diversion of bile and PJ were progressively inhibited with increasing doses of infused TC from 0 mM to 4 mM both with and without trypsin infusion. The effects with 4 and 40 mM TC were not significantly different. Changes in plasma cholecystokinin concentrations roughly correlated with changes in protein output in rats without trypsin infusion. We concluded that TC directly inhibited pancreatic secretion independent of the luminal trypsin activity and that its inhibitory action was concentration dependent.

Role of the GP2/THP family of GPI-anchored proteins in membrane trafficking during regulated exocrine secretion.

Identification and characterization of the GP2/THP family of GPI-anchored membrane proteins associated with apical secretory membranes suggest that this new class of GPI-linked proteins plays a critical role in regulated protein secretion and ion transport in polarized epithelial cells in pancreas, liver, lung, kidney, and gastrointestinal tract. Based on recent information obtained from the world literature and from our own investigations we present the following two hypotheses capable of unifying previously diverse observations. Hypothesis 1 is that formation of GP2 tetramers in the acidic milieu of the trans-Golgi network (TGN) organizes a GP2/proteoglycan (PG) matrix tightly associated with the luminal surface of zymogen granule (ZG) membranes, and proposes that this matrix functions in (a) membrane sorting during granule assembly in the TGN, (b) inactivation of ZG membranes during the storage phase of secretion, and (c) regulated trafficking of ZG membranes from the apical plasma membrane (APM) after exocytosis. Hypothesis 2 is that the acinar lumen constitutes a distinct physiologic compartment for coupled biochemical reactions between acinar and duct cells. Because the acidic pH of the TGN plays a critical role in condensation of secretory proteins, alkalinization of the acinar lumen is required for (a) neutralization of the acidic pH of exocytic contents and (b) solubilization of aggregated (pro)enzymes. Further alkalinization appears to be required for pH-dependent release of the GP2/PG matrix from the APM, a process that may regulate internalization of ZG membranes for reuse during secretion. Taken together, the two hypotheses suggest that luminal factors including acid-base interactions and matrix assembly and disassembly processes perform critical functions during regulated storage and release of pancreatic (pro)enzymes. The requirement that coupling reactions be coordinated through the actions of separate hormones [cholecystokinin (CCK) and secretin] on divergent epithelial cells (acinar and duct cells, respectively) provides a new appreciation for the importance of combined CCK and secretin stimulation during pancreatic secretion in response to food intake.

Effect of L-364,718 (CCK receptor antagonist) on exocrine pancreatic secretion of hydrocortisone-treated rats.

Exocrine pancreatic function was studied in control rats and animals subjected to treatment with hydrocortisone (10 mg/kg/day) over 7 days. In both cases, the cholecystokinin (CCK) receptor antagonist L-364,718 (0.1 mg/kg/day) had been administered. The administration of this nonpeptide CCK antagonist significantly reduced the basal pancreatic flow of secretion and enzymes in the control rats but did not cause a decrease in pancreatic weight. From this it may be inferred that factors other than CCK are able to exert the trophic effects of this hormone. Also, the processes of enzyme synthesis and storage, which contribute to maintaining the weight of the organ, continued to occur under conditions of deprivation of the action of endogenous CCK because pancreatic secretion in response to the infusion of CCK was similar in the animals treated and not treated with L-364,718. In contrast, no significant changes were observed either regarding pancreatic weight or basal pancreatic secretion in the animals treated with hydrocortisone and simultaneously with L-364,718, as compared with the rats treated only with hydrocortisone. This points to a predominant effect of glucocorticoids on the action of CCK during the phases of enzyme synthesis and storage, accompanied by a blockade of exocytosis due to hydrocortisone treatment. The secretagogue effect of CCK alone becomes possible with high levels of this hormone of exogenous origin, whose interaction with its specific receptors is in turn positively modulated by an excess of glucocorticoids.