Feedback regulation of pancreatic enzyme secretion occurs in rats. Whether such a system exists in man remains unsettled and the responsible mechanism is unknown. To investigate this question gastrointestinal intubation and perfusion were performed in 12 healthy subjects. Intraduodenal perfusion of trypsin-inhibited phenylalanine-, oleic acid-, and meal-stimulated chymotrypsin and lipase outputs in a dose-related manner. The minimal concentration of bovine trypsin needed to inhibit pancreatic enzyme secretion was 0.5 g/liter. 1 g/liter caused a maximal suppression of 35±4% of the phenylalanine-stimulated chymotrypsin release. This inhibitory effect was protease-specific. Intraduodenal perfusion of phenylalanine and oleic acid increased plasma cholecystokinin (CCK) from a basal level of 0.9±0.06 to 5.3±0.9 pM and 7.2±1.3 pM, respectively. Addition of bovine trypsin to the perfusates significantly reduced the plasma CCK level to basal values. This inhibitory effect of trypsin on CCK release was dose dependent and specific to proteases. Therefore, the present studies indicate that feedback regulation of pancreatic enzyme secretion is operative in man and it is mediated by release of CCK.

Previous studies have shown that trypsin and chymotrypsin in the duodenum exert a negative-feedback regulation on pancreatic enzyme secretion in the rat. The mechanism responsible for this physiological phenomenon is unknown. By use of a specific and sensitive bioassay based on amylase release from isolated pancreatic acini, the role of cholecystokinin in the negative-feedback regulation of exocrine pancreatic secretion was examined. Rats were prepared with duodenal cannulas and pancreaticobiliary cannulas. Diversion of pancreaticobiliary juice resulted in a threefold increase in pancreatic protein output and an increase of plasma cholecystokinin from a basal level of 0.5 +/- 0.08 pM cholecystokinin octapeptide (CCK-8) to 16 +/- 4 pM CCK-8. Perfusion of trypsin (2 mg/h) or pancreaticobiliary juice returned pancreatic protein output to basal levels and plasma cholecystokinin to 2.1 +/- 1.2 and 0.33 +/- 0.1 pM, respectively. The inhibitory effect of trypsin on cholecystokinin release was enzyme and site specific, since inhibition of cholecystokinin release was not observed with perfusion of amylase into the duodenum or with trypsin into the ileum. Intravenous infusion of proglumide abolished the increase in pancreatic secretion following diversion of pancreaticobiliary juice. Intraduodenal perfusion of lidocaine, infusion of tetrodotoxin into the superior mesenteric artery, or intravenous infusion of atropine inhibited the rise in plasma cholecystokinin seen with diversion of pancreaticobiliary juice. These studies suggest that feedback regulation of pancreatic enzyme secretion in the rat is mediated by release of cholecystokinin. Furthermore, the feedback mechanism is neurally mediated, involving a cholinergic pathway.

The biological activity of bovine pancreatic polypeptide (BPP) on rat exocrine pancreatic secretion was compared in vivo and in vitro. In anesthetized rats prepared with a bile-pancreatic duct cannula, BPP inhibited cholecystokinin (CCK)-stimulated (10 IDU . kg-1 X h-1) protein secretion in a dose-related manner (P less than 0.001). CCK, from 5-20 IDU . kg-1 X h-1, did not alter the degree of inhibition by BPP at 40 micrograms . kg-1 X h-1, suggesting a nonsurmountable inhibition. Analogues of BPP, including rat pancreatic polypeptide, neuropeptide Y, peptide YY, and the C-terminal hexapeptide of PP, also inhibited CCK-stimulated protein secretion. To determine whether BPP acts directly on acinar cells to suppress enzyme secretion, in vitro studies were performed. BPP and its analogues did not suppress octapeptide of CCK (CCK-8)-stimulated amylase release from either isolated rat pancreatic acini or preparations of pancreatic lobules. Specific binding of 125I-BPP to pancreatic acini was also not observed. From our data we conclude that BPP acts to inhibit pancreatic enzyme secretion in the rat in a noncompetitive manner. Absence of an effect by BPP or its analogues in vitro coupled with an absence of 125I-BPP binding to acini suggest that the inhibitory action of PP on exocrine pancreatic function is mediated by indirect mechanisms.

Diversion of bile pancreatic juice from the duodenum in rats stimulates cholecystokinin (CCK) release and pancreatic enzyme secretion. Intraduodenal perfusion of trypsin inhibits the release of CCK and pancreatic enzyme secretion. We hypothesized that the increased pancreatic enzyme secretion after pancreatic juice diversion is mediated by a trypsin-sensitive peptide secreted by the small intestine that stimulates release of CCK. To test this hypothesis, rats were surgically prepared with bile-pancreatic cannula and intestinal fistulas. Diversion of bile-pancreatic juice stimulated amylase output fivefold above basal and increased plasma CCK from a basal of 0.5 +/- 0.05 pM to 14 +/- 5 pM. Rapid perfusion (3 ml/min) of the duodenum with phosphate-buffered saline reversed the increase in amylase output and lowered the plasma CCK to 1.2 +/- 0.2. Administration of intestinal perfusate (3 ml/min) collected from a donor rat into the duodenum of a recipient rat with diversion of bile pancreatic juice increased amylase output threefold above basal and increased plasma CCK. The stimulatory activity of the intestinal perfusate was inactivated by treatment with trypsin but not by amylase or lipase. In addition, boiling did not alter the stimulatory activity of the intestinal perfusate. Perfusion of intestinal perfusate from donor rats pretreated with atropine did not stimulate amylase output and CCK release in recipient rats. By use of molecular membrane exclusion filters, stimulatory activity was retained (between 1,000 and 5,000). These results indicate that feedback regulation of pancreatic enzyme secretion is mediated by a CCK releasing peptide whose secretion from the duodenum is cholinergically mediated. This peptide is trypsin sensitive and has a molecular weight between 1,000 and 5,000.

The influence of applied auxin on abscission in explants (excised cotyledonary nodes) of cotton was investigated.Proximal applications to the stem stump accelerated abscission with all concentrations applied. Distal applications to the petiole stumps retarded abscission with all concentrations applied.Simultaneous applications to petiole and stem stumps accelerated, retarded, or were without effect on abscission depending on the relative amounts applied. Relatively high concentrations proximal on stem stumps accelerated abscission whereas relatively high concentrations distal on petiole stumps retarded abscission. The abscission rates from various combinations resulted in a multiphasic abscission response curve as the amounts applied were increased.Consideration of these and related data in the literature indicates that many factors can have profound effects on the nature of abscission responses to auxin.Interest in auxin as a controlling factor in abscission developed rapidly after Laibach (18) found that abscission can be retarded by the application of auxin to debladed petioles. His discovery led to a number of important agricultural practices (1,30,31) and stimulated basic researches on the influence of auxin in the control of abscission. All but the most recent literature has been described in reviews (1,5,16). A comprehensive article describing the physiological factors in abscission, including other hormones as well as auxin, has recently appeared (2).The early investigations established that auxin commonly functions to retard abscission. Further investigations disclosed other influences of auxin. The onset of abscission was found to be correlated with the gradient, or balance, of auxin across the abscission zone, auxin distal to the abscission zone tending to retard abscission, and auxin proximal to the zone tending to accelerate abscission. This general concept is supported by the results of a number of workers with a variety of plant materials from experiments involving extraction, diffusion, applications, and metabolism of auxin (6,14,15,19,21,(26)(27)(28). However, a few experiments have been reported in which proximal applications of auxin have retarded abscission (24), and in some cases distal applications have accelerated abscission (10,12,32 MATERIALS AND METHODSExcised abscission zones (explants) were used in this investigation following a method similar to that described by Addicott et al. (4). Explants were cut from the cotyledonary nodes of cotton seedlings (Gossypium hirsutum L. cultivar Acala 4-42) grown in perlite in galvanized flats in the greenhouse. Seedlings were used when their first internodes had attained a length of 2 cm (21-25 days after planting). Cotyledons were debladed 24 hr before the nodes were excised as this led to more rapid abscission, reducing the time required for an experiment. After excision each explant consisted of two 5-mm petiole stumps, a 5-mm stem stump (first internode), and a 10-mm hypocotyl stump, and thus each contained two abscission zones. The ...