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Biomedical subjects

M L Steer

Publications and source records attributed to M L Steer.

At least 19 recordsLinked to original sources
PubMed1992

CCK-JMV-180, an analog of cholecystokinin, releases intracellular calcium from an inositol trisphosphate-independent pool in rat pancreatic acini.

In pancreatic acinar cells cholecystokinin and its analogs, caerulein and CCK-JMV-180, stimulate an increase in intracellular free [Ca2+] by releasing Ca2+ from non-mitochondrial intracellular pools. It is generally believed that the caerulein-induced release of Ca2+ is mediated by phospholipase C-catalyzed production of 1,4,5-inositol triphosphate (IP3). In this study we have investigated the source and mechanism of Ca2+ release induced by CCK-JMV-180 using streptolysin O-permeabilized pancreatic acinar cells. Caerulein-stimulated release of Ca2+ was completely blocked by either neomycin, an inhibitor of phospholipase C, or by heparin, an IP3 receptor antagonist. These observations are compatible with the conclusion that caerulein releases Ca2+ from an IP3-sensitive pool. In contrast to caerulein, however, CCK-JMV-180-stimulated release of Ca2+ was not blocked by either neomycin or by heparin, indicating that CCK-JMV-180 releases Ca2+ by mechanisms which do not involve the generation or action of IP3. CCK-JMV-180 stimulated the release of Ca2+ even after the IP3-sensitive pool had been completely emptied by prior exposure to a supramaximally stimulating concentration of IP3 (40 microM). Prestimulation of permeabilized acini with 20 mM caffeine did not abolish the CCK-JMV-180-induced Ca2+ release. These results indicate that CCK-JMV-180 stimulates release of Ca2+ from a hitherto uncharacterized intracellular storage pool which is insensitive to either IP3 or caffeine.

Amylases↗
PubMed1992

How and where does acute pancreatitis begin?

Circumstantial evidence suggests that gallstone-induced pancreatitis is triggered by obstruction of the pancreatic duct. In this report I will review the results of studies conducted during the last decade that have employed the diet-induced, secretagogue-induced, and duct obstruction-induced models of experimental pancreatitis to investigate the early events that lead to the development of acute pancreatitis. In each of these models, digestive enzyme zymogens and the lysosomal hydrolase cathepsin B were found to become colocalized. These observations have led to the hypothesis that intra-acinar cell activation of digestive enzyme zymogens by lysosomal hydrolases may be an important critical event in the development of acute pancreatitis. Recent morphologic studies evaluating the initial 24 hours after ligation of the opossum pancreatic duct indicate that the earliest lesions in this model of hemorrhagic pancreatitis occur in acinar cells.

Acute Disease↗
PubMed1992

Acute necrotizing pancreatitis in the opossum: earliest morphological changes involve acinar cells.

Acute pancreatitis was induced by ligating the opossum common biliopancreatic duct immediately proximal to its entry into the duodenum, and macroscopic as well as microscopic changes were evaluated during the subsequent 24 hours. Transient pancreatic edema and progressive hyperamylasemia were noted within 6 hours of pancreatic and bile duct ligation. Light microscopic evidence of pancreatic injury including acinar cell necrosis, hemorrhage, fat necrosis, and inflammatory cell infiltration was noted within 12 hours of duct obstruction. Electron microscopic changes included massive dilatation of the rough endoplasmic reticulum and disruption of the apical plasmalemma of acinar cells during the initial 3 hours. These observations indicate that pancreatic and bile duct ligation in the opossum results in the rapid (less than 24 hours) appearance of changes consistent with acute hemorrhagic and necrotizing pancreatitis and that the initial lesion in this model of experimental pancreatitis involves acinar cells.

Acute Disease↗
PubMed1992

Pancreatic secretion of lysosomal enzymes stimulated by intraduodenal instillation of a liquid meal in rabbits.

1. Studies have been performed to determine the effect of intraduodenal food on pancreatic secretion of lysosomal enzymes. 2. Intraduodenal instillation of a liquid meal (3 g/kg body weight; 15.3% protein, 19.7% fat, 59.7% carbohydrate) caused significant increases in pancreatic juice volume and pancreatic secretion of amylase and protein compared with basal values for 2h after instillation in anaesthetized rabbits. 3. Intraduodenal instillation of a liquid meal also caused significant increases in pancreatic secretion of three lysosomal enzymes (cathepsin B, N-beta-acetyl-galactosaminidase and N-acetyl-beta-glucosaminidase) compared with basal values for 2h after instillation. 4. In addition, there were significant correlations between cathepsin B secretion and amylase secretion (r = 0.7764, P < 0.001) and between cathepsin B secretion and protein secretion (r = 0.6216, P < 0.001), both in basal conditions and in response to the liquid meal. 5. These results are evidence for the localization of lysosomal enzymes in the secretory granules-zymogen granules in normal acinar cells, and also indicate that the pancreatic secretion of lysosomal enzymes is gut-hormone-regulated.

Acetylglucosaminidase↗
PubMed1992

Effects of chloroquine and methylamine on lysosomal enzyme secretion by rat pancreas.

In vivo pancreatic secretion of the lysosomal hydrolase cathepsin B was found to be increased by infusion of the secretagogue caerulein. The basal as well as caerulein-stimulated in vivo rate of cathepsin B was further increased by infusion of either chloroquine or methylamine while neither the basal nor the secretagogue-stimulated rates of amylase secretion were altered by the lysosomotropic agents. These observations indicate that neutralization of the acidic prelysosomal compartment by administration of lysosomotropic agents results in lysosomal enzyme entry, by default, into the regulated secretory pathway. In vitro stimulation of pancreatic acini with caerulein was also found to stimulate cathepsin B secretion. That in vitro rate of cathepsin B secretion stimulated by caerulein was not increased in acini prepared from animals infused with caerulein, chloroquine, or methylamine, but the in vitro rate of cathepsin B secretion stimulated by caerulein was increased in acini prepared from animals infused with caerulein plus either chloroquine or methylamine. Under these conditions, redistribution of cathepsin B from the lysosome-enriched to the zymogen granule-enriched subcellular fraction was noted, and lysosomal enzyme-containing organelles became increasingly fragile. These observations indicate that in vivo secretagogue stimulation increases the degree of diversion of lysosomal hydrolases into the regulated secretory compartment when the prelysosomal compartment has been neutralized with lysosomotropic agents.

Amylases↗
PubMed1992

In vivo and in vitro effects of the azidothymidine analog dideoxyinosine on the exocrine pancreas of the rat.

Phase I clinical trials of the purine analog 2',3'-dideoxyinosine (ddl) revealed that 10% of the patients developed pancreatitis, yet there was no clear relationship between increasing doses of ddl and the development of pancreatitis. To test the effects of chronic ddl administration on the structure and function of the rat pancreas, male Wistar rats were given ddl at 100 mg/kg/day i.p. for 35 days or 1400 mg/kg/day for 30 days, in two divided doses. Serum amylase levels, pancreatic tissue water content (edema) and pancreatic morphology by light and electron microscopic examination of pancreata from ddl-treated rats were similar to those of rats receiving saline injections only (controls). 2',3'-Dideoxyinosine administration did not alter the subcellular distribution of the lysosomal enzyme cathepsin B, whose redistribution to a more dense zymogen granule-enriched subcellular fraction is an early indicator of acute pancreatitis. Dispersed pancreatic acini from rats receiving ddl (100 mg/kg/day for 30 days) were incubated in vitro for 15 min with either caerulein or carbamylcholine as secretory stimuli. There was no detectable difference in the stimulatable amylase secretion from ddl-treated animals compared to the control group. Based on these findings, we conclude that ddl has no direct toxic effect on the rat pancreas. 2',3'-Dideoxyinosine may be contributing to pancreatitis in acquired immunodeficiency syndrome patients by potentiating other pancreatotoxic agents or by its action on a pancreas that is already altered by the human immunodeficiency virus infection.

Amylases↗
PubMed1991

The role of oxygen-derived free radicals in two models of experimental acute pancreatitis: effects of catalase, superoxide dismutase, dimethylsulfoxide, and allopurinol.

The role of oxygen-derived free radicals was evaluated in two models of experimental acute pancreatitis by testing the effects of agents which either reduce oxygen-derived free radical generation or scavenge those free radicals. Those agents (catalase, superoxide dismutase, polyethylene glycol-superoxide dismutase, dimethylsulfoxide, and allopurinol) were evaluated using the choline-deficient ethionine-supplemented diet-induced model of acute hemorrhagic pancreatic necrosis and the supramaximal caerulein stimulation model of acute interstitial edematous pancreatitis. In both models, the only effect associated with administration of the test agents was a reduction in the degree of pancreatic edema. These results suggest that oxygen-derived free radicals may play an important role in the development of pancreatic edema during pancreatitis but that those free radicals do not play an important role in the development of acinar cell injury.

Acute Disease↗
PubMed1991

Effects of short-term pancreatic duct obstruction in rats.

The short-term effects of rat pancreatic duct obstruction were evaluated and compared with those recently reported to follow obstruction of the rabbit pancreatic duct. In both species pancreatic edema and hyperamylasemia are noted, and the lysosomal hydrolase cathepsin B is redistributed from the lysosome-enriched to the zymogen granule-enriched subcellular fraction. Theoretically, this redistribution phenomenon might lead to digestive enzyme activation because cathepsin B is known to be capable of activating trypsinogen. The hyperamylasemia and pancreatic edema (but not the cathepsin B redistribution) that follow rat pancreatic duct obstruction were increased by infusion of a submaximally stimulating dose of the cholecystokinin analogue cerulein. Administration of the cholecystokinin-receptor antagonist L-364,718 reduced the hyperamylasemia but did not alter the pancreatic edema or cathepsin B redistribution. These observations indicate that cholecystokinin may modulate some but not all of the effects of duct obstruction. Secretin administration increased the degree of pancreatic edema and had no demonstrable protective effect. The rat duct-obstruction model described in this report may prove particularly useful in future studies designed to clarify the early events underlying the development of acute pancreatitis.

Amylases↗
PubMed1991

Apical secretion of lysosomal enzymes in rabbit pancreas occurs via a secretagogue regulated pathway and is increased after pancreatic duct obstruction.

Lysosomal hydrolases such as cathepsin B are apically secreted from rabbit pancreatic acinar cells via a regulated as opposed to a constitutive pathway. Intravenous infusion of the cholecystokinin analogue caerulein results in highly correlated apical secretion of digestive and lysosomal enzymes, suggesting that they are discharged from the same presecretory compartment (zymogen granules). Lysosomal enzymes appear to enter that compartment as a result of missorting. After 7 h of duct obstruction is relieved, caerulein-stimulated apical secretion of cathepsin B and amylase is increased, but the ratio of cathepsin B to amylase secretion is not different than that following caerulein stimulation of animals never obstructed. These findings indicate that duct obstruction causes an increased amount of both lysosomal and digestive enzymes to accumulate within the secretagogue releasable compartment but that duct obstruction does not increase the degree of lysosomal enzyme missorting into that compartment. Pancreatic duct obstruction causes lysosomal hydrolases to become colocalized with digestive enzymes in organelles that, in size and distribution, resemble zymogen granules but that are not subject to secretion in response to secretagogue stimulation. These organelles may be of importance in the development of pancreatitis.

Amylases↗
PubMed1991

A plasma protease which is expressed during supramaximal stimulation causes in vitro subcellular redistribution of lysosomal enzymes in rat exocrine pancreas.

The complex events by which digestive enzyme zymogens and lysosomal hydrolases are segregated from each other and differentially transported to their respective membrane-bound intracellular organelles in the pancreas have been noted to be disturbed during the early stages of several models of experimental pancreatitis. As a result, lysosomal hydrolases such as cathepsin B are redistributed to the subcellular zymogen granule-rich fraction and lysosomal hydrolases as well as digestive enzyme zymogens are colocalized within large cytoplasmic vacuoles. The current study was designed to create an in vitro system that would reproduce this redistribution phenomenon. Our results indicate that cathepsin B redistribution occurs when rat pancreatic fragments are incubated with a supramaximally stimulating concentration of the cholecystokinin analogue caerulein along with plasma from an animal subjected to in vivo supramaximal caerulein stimulation. Neither the plasma nor a supramaximally stimulating concentration of caerulein, alone, is sufficient to induce in vitro cathepsin B redistribution. The ability of the plasma to induce in vitro cathepsin redistribution is dependent upon its content of a 10,000-30,000-D protein and is lost by exposure to protease inhibitors. In vitro cathepsin B redistribution also occurs when rat pancreatic fragments are incubated with plasma obtained from opossums with hemorrhagic necrotizing pancreatitis caused by bile/pancreatic duct ligation.

Animals↗
PubMed1979

Regulation of human platelet adenylate cyclase by epinephrine, prostaglandin E1, and guanine nucleotides. Evidence for separate guanine nucleotide sites mediating stimulation and inhibition.

A method for preparing human platelet membranes with high adenylate cyclase activity is described. Using these membranes, epinephrine and GTP individually are noted to inhibit adenylate cyclase slightly. When present together, epinephrine and GTP act synergistically to cause a 50% inhibition of basal activity. The epinephrine effect is an alpha-adrenergic process as it is reversed by phentolamine but not propranolol. The quasi-irreversible activation of adenylate cyclase by Gpp(NH)p is time, concentration, and Mg2+-dependent but is not altered by the presence of epinephrine. Adenylate cyclase activated by Gpp(NH)p, and extensively washed to remove unbound Gpp(NH)p, is inhibited by the subsequent addition of Gpp(NH)p, GTP, and epinephrine. This effect of epinephrine is also an alpha-adrenergic phenomenon. In contrast to epinephrine which inhibits the cyclase, PGE1 addition results in enzyme stimulation. PGE1 stimulation does not require GTP addition. PGE1 accelerates the rate of Gpp(NH)p-induced activation. Low GTP concentrations (less than 1 x 10(-6) M) enhance PGE1 stimulation while higher GTP concentrations cause inhibition. These observations suggest that human platelet adenylate cyclase possesses at least two guanine nucleotide sites, one which interacts with the alpha-receptor to result in enzyme inhibition and a second guanine nucleotide site which interacts with the PGE1 receptor and causes enzyme stimulation.

Adenylyl Cyclases↗
PubMed1979

Cholecystokinin/pancreozymin induces the parallel discharge of digestive enzymes from the in vitro rabbit pancreas.

Considerable controversy has surrounded the question of whether the exocrine pancreas discharges digestive enzymes in a parallel or nonparallel fashion. A recent report (Rothman, S.S., and Wilking, H. (1978) J. Biol. Chem. 253, 3543-3549) claimed that the in vitro rabbit pancreas demonstrated nonparallel enzyme discharge after stimulation with cholecystokinin/pancreozymin, but that parallel discharge followed stimulation with the COOH-terminal octapeptide of cholecystokinin/pancreozymin. It was suggested that the full hormone acted to inhibit chymotrypsinogen secretion while stimulating trypsinogen secretion. Because of the fundamental importance of this question to our understanding of the exocrine secretion of exportable proteins, we have repeated these experiments using the same preparation and stimulant but have observed only parallel enzyme discharge. We conclude that it is unlikely that cholecystokinin/pancreozymin causes the selective inhibition of chymotrypsinogen secretion.

Amylases↗
PubMed1979

Protease inhibitors and experimental acute hemorrhagic pancreatitis.

Proteolytic enzyme inhibitors have been reported to decrease morbidity and mortality from certain types of experimental pancreatitis, although recent randomized trials have been unable to demonstrate that they are of benefit in the treatment of clinical acute pancreatitis. We have evaluated the effect of two proteolytic enzyme inhibitors (trasylol and chlorophyll-a, on experimental acute pancreatitis induced in mice by the feeding of a choline-deficient ethionine-enriched diet. The mortality rate and the biochemical and morphological severity of pancreatitis were not altered by either trasylol or chlorophyll-a administration. Thus, in this respect, diet-induced pancreatitis appears to resemble clinical acute pancreatitis. The reasons for the lack of effectiveness of proteolytic enzyme inhibitors in the treatment of both forms of pancreatitis are discussed.

Acute Disease↗
PubMed1979

Localization of bleeding small intestinal lesions using scanning techniques.

Gastrointestinal bleeding remains a major diagnostic problem, and the commonly employed techniques of contrast radiography, endoscopy, and arteriography may not successfully localize the site and/or define the cause of gastrointestinal hemorrhage. Technetium-99m pertechnetate scanning has been useful in identifying Meckel's diverticula, and modifications of this technique may successfully identify highly vascular lesions responsible for gastrointestinal bleeding. A patient with recurrent hemorrhage from a leiomyosarcoma of the ileum is described. The lesion was identified with flow studies and immediate static imaging after injection of technetium-99m pertechnetate. In addition, the lesion was demonstrated by scanning using in vivo technetium-99m pertechnetate labelled autologous erythrocytes. The potential value of these scanning techniques as noninvasive tools for the localization and identification of lesions responsible for gastrointestinal bleeding is discussed.

Aged↗