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J Meldolesi

Publications and source records attributed to J Meldolesi.

At least 163 records · Page 9Linked to original sources
PubMed1987

Intracellular transport of pancreatic zymogens during caerulein supramaximal stimulation.

Rats infused with a dose of the secretagogue caerulein that is in excess of that which stimulates a maximal rate of pancreatic digestive enzyme secretion develop acute edematous pancreatitis. We have previously noted that infusion of this dose of caerulein (5 micrograms . kg-1 . h-1) induces the appearance of large heterogeneous vacuoles in acinar cell, blockade of exocytosis, and intracellular accumulation of digestive zymogens [O. Watanabe et al. Am. J. Physiol. 246 (Gastrointest. Liver Physiol. 9): G457-G467, 1984 and A. Saluja et al. Am. J. Physiol. 249 (Gastrointest. Liver Physiol. 12): G702-G710, 1985]. The current studies were performed to further elucidate these phenomena at the electron microscopic level of resolution and employed the techniques of pulse labeling, radioautography, and immunolocalization. Rats were infused with caerulein (5 micrograms . kg-1 . h-1) for 1 h, given a pulse of [3H]phenylalanine, and killed at selected times during the subsequent 5- to 180-min postpulse period during which caerulein infusion was continued. Transport from the endoplasmic reticulum to the Golgi cisternae was not altered by supramaximal stimulation, but transport through post-Golgi elements was altered. In particular, the maturation of condensing vacuoles into zymogen granules was found to be impaired. This led to the accumulation of partially condensed vacuoles and to the development of the large vacuoles containing newly synthesized digestive zymogens as well as the lysosomal hydrolase cathepsin D. The source of the latter could be impaired sorting of lysosomal and digestive enzymes and/or fusion of vacuoles with lysosomes. At the later times after pulse labeling, mature zymogen granules were also found to fuse with these large cathepsin D-containing vacuoles by a process analogous to crinophagy. Thus these studies indicate that the large heterogeneous vacuoles that appear during supramaximal secretagogue stimulation and that contain admixed digestive zymogens and lysosomal hydrolases arise by at least two mechanisms, impaired condensing vacuole maturation and crinophagy.

Animals↗
PubMed1986

Early rise of cytosolic Ca2+ induced by NGF in PC12 and chromaffin cells.

A rise of cytosolic Ca2+ is induced by NGF in rat pheochromocytoma PC12 and bovine chromaffin cells investigated (both in suspension and while attached to polyornithine-coated glass slides) by fluorescence techniques (with quin-2 and fura-2). The effect of NGF on [Ca2+]i is delayed (30-40 s of lag phase), slow (t1/2 = 40 s), relatively small (+50-75%) and persistent (over 10 min). It is due to Ca2+ influx (requires extracellular Ca2+ greater than 10 microM) through a pathway different from the voltage-gated Ca2+ channel, possibly accompanied by intracellular Ca2+ redistribution, and might play a messenger role in NGF action.

Adrenal Gland Neoplasms↗
PubMed1986

PDGF-induced receptor phosphorylation and phosphoinositide hydrolysis are unaffected by protein kinase C activation in mouse Swiss 3T3 and human skin fibroblasts.

Short (1-10 min) pretreatment of intact cells with activators of protein kinase C (e.g. phorbol-12 myristate, 13-acetate, PMA) affects the activity of a variety of surface receptors (for growth factors, hormones and neurotransmitters), with inhibition of transmembrane signal generation. In two types of fibroblasts we demonstrate that the PDGF receptor is unaffected by PMA. Exposure to PMA at concentrations up to 100 nM for 10 min failed to inhibit either one of the agonist-induced, receptor-coupled responses of PDGF: the autophosphorylation of receptor molecules at tyrosine residues, and the hydrolysis of membrane polyphosphoinositides. In contrast, the EGF receptor autophosphorylation (in A 431 cells) and the bombesin-induced phosphoinositide hydrolysis were readily inhibited by PMA. Feed-back inhibition of surface receptors by protein kinase C-mediated phosphorylation is therefore not general, and cannot be the only process responsible for the attenuation of receptor-mediated responses in eukaryotic cells.

Animals↗
PubMed1986

Rat adrenal chromaffin cells become sensitive to alpha-latrotoxin when cultured in vitro: the effect of nerve growth factor.

Previous studies showed that in the intact rat chromaffin cells are insensitive to the presynaptic stimulatory toxin, alpha-latrotoxin (alpha-LTx). In contrast, primary cultures of rat adrenal chromaffin cells respond to alpha-LTx by releasing stored catecholamines. The toxin-induced response was greater in cultures from young (8-10-day-old) than adult (40-50-day-old) animals, and increased when the time of culturing was prolonged from 4 to 20 days. Even greater responses were observed in cells cultured in the presence of nerve growth factor, a treatment which induces a neuronal-like phenotype. Acquisition of sensitivity to alpha-LTx appears to be an early marker of chromaffin cell commitment to differentiate towards a neuron.

Adrenal Medulla↗
PubMed1986

Activation of muscarinic receptors in PC12 cells. Stimulation of Ca2+ influx and redistribution.

Ca2+ homoeostasis was investigated in pheochromocytoma neurosecretory (PC12) cells both before and after treatment with nerve growth factor, which induces a neuronal-like differentiation accompanied by a large increase in the number of muscarinic receptors. The resting concentration of free cytosolic Ca2+, [Ca2+]i, measured by the quin2 technique, was found to be higher and more variable in differentiated cells. Moreover, the [Ca2+]i rises induced by the Ca2+ ionophore ionomycin and by depolarizing concentrations of KC1 were greater and more transient. Exposure to carbachol induced modest, but long-lasting, [Ca2+]i rises, which were faster and greater in differentiated than in non-differentiated cells. These effects were due to the activation of the muscarinic receptor, because they were unaffected by nicotinic blockers (hexamethonium and D-tubocurarine) and completely eliminated by low concentrations of the muscarinic antagonists atropine and pirenzepine [IC50 (concn. causing 50% inhibition) = 2 and 60 nM respectively]. The muscarinic-receptor-dependent [Ca2+]i rises were the result of two concomitant processes: (1) redistribution of Ca2+ from cytoplasmic stores to the cytosol, possibly mediated by generation of inositol 1,4,5-trisphosphate as a consequence of the muscarinic-receptor-coupled hydrolysis of polyphosphoinositides, and (2) increased Ca2+ influx through a pathway of the plasmalemma insensitive to verapamil and thus different from the voltage-dependent Ca2+ channel. The existence of this second process was documented: (a) by the difference of the [Ca2+]i responses brought about by carbachol in Ca2+-containing and Ca2+-free media; (b) by the occurrence of [Ca2+]i rise and increased 45Ca accumulation in cells exposed to 1 mM-CaCl2 after having been treated for 2 min with carbachol in Ca2+-free medium; (c) by typical differences in the quin2 signal kinetics observed in parallel samples of PC12 cells loaded with different concentrations of the dye.

Adrenal Gland Neoplasms↗
PubMed1986

Activation of muscarinic receptors in PC12 cells. Correlation between cytosolic Ca2+ rise and phosphoinositide hydrolysis.

The intracellular signals generated by carbachol activation of the muscarinic receptor [release of inositol phosphates as a consequence of phosphoinositide hydrolysis and rise of the cytosolic Ca2+ concentration ([Ca2+]i, measured by quin2)] were studied in intact PC12 pheochromocytoma cells that had been differentiated by treatment with nerve growth factor. When measured in parallel samples of the same cell preparation 30 s after receptor activation, the release of inositol trisphosphate and of its possible metabolites, inositol bis- and mono-phosphate, and the [Ca2+]i rise were found to occur with almost superimposable carbachol concentration curves. At the same time carbachol caused a decrease in the radioactivity of preloaded phosphatidylinositol 4,5-bisphosphate, the precursor of inositol trisphosphate. Neither the inositol phosphate nor the [Ca2+]i signal was modified by preincubation of the cells with either purified Bordetella pertussis toxin or forskolin, the direct activator of adenylate cyclase. Both signals were partially inhibited by dibutyryl cyclic AMP, especially when the nucleotide analogue was applied in combination with the phosphodiesterase inhibitors RO 201724 and theophylline. The latter drug alone profoundly inhibited the carbachol-induced [Ca2+]i rise, with only minimal effect on phosphoinositide hydrolysis. Because of the diverging results obtained with forskolin on the one hand, dibutyryl cyclic AMP and phosphodiesterase inhibitors on the other, the effects of the latter drugs are considered to be pharmacological, independent of the intracellular cyclic AMP concentration. Two further drugs tested, mepacrine and MY5445, inhibited phosphoinositide hydrolysis at the same time as the 45Ca2+ influx stimulated by carbachol. Taken together, our results concur with previous evidence obtained with permeabilized cells and cell fractions to indicate phosphatidylinositol 4,5-bisphosphate hydrolysis and [Ca2+]i rise as two successive events in the intracellular transduction cascade initiated by receptor activation. The strict correlation between the carbachol concentration curves for inositol trisphosphate generation and [Ca2+]i rise, and the inhibition by theophylline of the Ca2$ signal without major effects on inositol phosphate generation, satisfy important requirements of the abovementioned interpretation.

Adrenal Gland Neoplasms↗
PubMed1986

alpha Latrotoxin of the black widow spider venom opens a small, non-closing cation channel.

alpha Latrotoxin, a presynaptically acting polypeptide neurotoxin, induces massive neurotransmitter release from both synapses of vertebrates and the neurosecretory cells of the line PC12, derived from a rat pheochromocytoma. Single PC12 cells, differentiated by treatment with nerve growth factor, were used to investigate by the patch-clamp technique i) the alterations of the resting cell conditions (membrane potential and resistance) and ii) the microscopic mechanism of the permeability changes that underly the response to alpha LTx. The toxin was found to open a channel, 15 pS in conductance, that is permeable to various cations (Na+, K+ and probably Ca2+) and has little tendency to close. This channel is different from the classical voltage- and receptor-operated channels present in PC12 cells, as well as from the large conductances induced by the toxin in artificial lipid membranes.

Animals↗
PubMed1986

Tumor promoter phorbol myristate acetate inhibits Ca2+ influx through voltage-gated Ca2+ channels in two secretory cell lines, PC12 and RINm5F.

Protein kinase C is known to be involved both in initiation and termination of cellular responses due to phosphoinositide breakdown. Here we report that in PC12 cells (a line of neurosecretory cells derived from a rat pheochromocytoma), pretreatment with nanomolar concentrations of phorbol myristate acetate, PMA, which is believed to specifically activate protein kinase C, inhibits the cytosolic-free Ca2+ concentration rise induced by depolarizing agents. In contrast, plasma membrane potential and 45Ca efflux from preloaded cells were unaffected by PMA pretreatment. Inhibition by PMA and diacylglycerol of the cytosolic-free Ca2+ concentration rise induced by depolarization was observed also in another cell line, the insulin secreting line RINm5F. These results raise the possibility that the voltage-gated Ca2+ channel is under inhibitory control by protein kinase C.

Adrenal Gland Neoplasms↗
PubMed1986

Reconstitution of the purified alpha-latrotoxin receptor in liposomes and planar lipid membranes. Clues to the mechanism of toxin action.

The receptor of alpha-latrotoxin (the major toxin of the black widow spider venom), purified from bovine synaptosomal membranes, was reconstituted into small unilamellar liposomes. These (but not control) liposomes exhibited high-affinity, specific binding of [125I]alpha-latrotoxin. In the receptor-bearing liposomes alpha-latrotoxin induced depolarization and stimulated 45Ca efflux. These responses to alpha-latrotoxin, that were observed only in the presence of external divalent cations, resembled those previously demonstrated in mammalian brain synaptosomes. The alpha-latrotoxin-activated ion fluxes are therefore, at least in part, the result of the direct interaction of the toxin with its receptor. When control and receptor-bearing liposomes were pre-incubated with alpha-latrotoxin and then added to a solution bathing a planar lipid bilayer membrane, single channel cationic conductances were observed. In the presence of the receptor, the conductances induced by alpha-latrotoxin were markedly different from those observed without the receptor, but not identical to those observed without the receptor, but not identical to those recently characterized by patch clamping in the cells of a line (PC12) sensitive to alpha-latrotoxin. These results demonstrate that the reconstituted receptor is functional, and suggest that the cationic channel activated by the toxin-receptor interaction is modulated by additional component(s) in the membrane of synapses and cells.

Animals↗
PubMed1986

Black widow spider venom-induced release of neurotransmitters: mammalian synaptosomes are stimulated by a unique venom component (alpha-latrotoxin), insect synaptosomes by multiple components.

Synaptosomes isolated from the rat brain corpus striatum and locust head and thoracic ganglia were loaded with radioactive neurotransmitter ([3H]dopamine and [3H]acetylcholine, respectively) and then treated with alpha-latrotoxin and other fractions (fractions C, D and E of Frontali et al.8) obtained by Sephadex G200 column chromatography from black widow spider venom gland homogenates. As shown by sodium dodecyl sulphate-polyacrylamide gel electrophoresis, alpha-latrotoxin is a high Mr protein, whereas fractions C-E are mixtures of several proteins, that include small amounts of contaminating alpha-latrotoxin (especially in fraction C). In rat synaptosomes alpha-latrotoxin induced massive neurotransmitter release, and some release was induced also by high concentrations of fractions C and D. These responses were blocked almost completely by a monospecific anti-alpha-latrotoxin serum, indicating that they were all due to alpha-latrotoxin. Release of [3H]acetylcholine from locust synaptosomes was induced by the various preparations investigated. alpha-Latrotoxin was about 10-fold less potent in locust than in rat synaptosomes. The effects of fractions C-E tended to disappear with storage. The most active batches of fractions C and E were even more potent than alpha-latrotoxin, while the D fraction was approximately 5-fold less potent. The anti-alpha-latrotoxin antiserum inhibited part of the responses elicited by fractions C and E, but left fraction D almost unaffected. Release by D and E fractions was maintained even when Ca2+ was removed from the incubation medium.(ABSTRACT TRUNCATED AT 250 WORDS)

Acetylcholine↗
PubMed1985

Tumor promoter phorbol 12-myristate, 13-acetate inhibits phosphoinositide hydrolysis and cytosolic Ca2+ rise induced by the activation of muscarinic receptors in PC12 cells.

Preincubation of PC12 cells (used both before and after differentiation by NGF) with phorbol myristate acetate (PMA) was without effect on the basal concentration of inositol phosphates (metabolites of phosphoinositide hydrolysis) and of free cytosolic Ca2+, but inhibited considerably the increases induced by the cholinergic agonist carbachol via the activation of the muscarinic receptor. Inasmuch as binding was unaffected, this inhibition might occur at the level of receptor coupling to its transduction mechanism(s). Inhibition appeared within 1 min and was maximal after 3 min. The concentrations of PMA needed (10(-9)-10(-8)M) were in the range believed to cause specifically the activation of protein kinase C. The muscarinic receptor, via the hydrolysis of phosphoinositides and the generation of diacylglycerol, participates in the regulation of the latter enzyme. Our data suggest therefore that the receptor operates under stringent feedback control by the metabolites generated as a consequence of its activation.

Animals↗
PubMed1985

Purification of the putative alpha-latrotoxin receptor from bovine synaptosomal membranes in an active binding form.

alpha-Latrotoxin (alpha-LTx, apparent mol. wt. 130 000) is a presynaptically active neurotoxin purified from the venom of the black widow spider that causes massive exocytotic release of neurotransmitters, presumably via binding to presynaptic membrane protein(s). Solubilization and purification experiments were undertaken to identify and characterize this membrane component. An immunoaffinity matrix was prepared by sequentially binding anti-alpha-LTx antibodies and alpha-LTx to Protein A-Sepharose CL-4B. Beads were irreversibly cross-linked with dimethyl pimelimidate. These beads were capable of extracting alpha-LTx binding activity from Triton X-100 solubilized bovine synaptosomal membranes. Following extensive washing, bound material was eluted with 6 M urea. Analysis of silver stained and radiolabel-containing gels revealed one major band (apparent mol. wt. 200 000) under non-reducing conditions and two major bands (apparent mol. wts. 66 000 and 54 000) under reducing conditions. The purified material was still capable of specifically binding alpha-LTx as determined by solid phase assays on microtiter plates. The affinity for alpha-LTx of the purified preparation was similar to that of the native membrane (KA approximately 10(10) M). It is concluded that a putative alpha-LTx receptor protein can be purified from synaptosomal membranes using an immunoaffinity matrix in a form that retains its defined biological property (alpha-LTx binding).

Animals↗
PubMed1985

The effect of alpha-latrotoxin on the neurosecretory PC12 cells differentiated by treatment with nerve growth factor.

Neurosecretory PC12 cells differentiated in vitro by prolonged (at least 2 weeks) treatment with nerve growth factor were exposed to alpha-latrotoxin and studied by morphological and biochemical techniques. Cell monolayers or suspensions responded to the toxin with a prompt and massive release of neurotransmitter. The dose dependency (Km approximately 5 X 10(-10)M) and the maximum release effect (approximately 60% of the stored [3H]dopamine released within 8 min) were not appreciably different from the values found in non-differentiated PC12 cells. Moreover, the concentration dependency of the release was found to correspond closely to that of the [125I]alpha-latrotoxin binding to its specific sites (the alpha-latrotoxin receptors). The number of these receptors was over two-fold higher in differentiated than in undifferentiated cells. Since, however, differentiation implies a large increase in cell size and surface area, the receptor density (number/unit area) remained virtually unchanged. By radioautography the alpha-latrotoxin receptors were found to remain diffusely distributed at the entire surface of differentiated cells even when these were allowed to form synapses with myotubes. This situation is at variance with that demonstrated recently at the frog neuromuscular junction, where alpha-latrotoxin receptors are exclusively localized at the nerve terminal plasmalemma. Scanning and transmission electron microscopy revealed that the enlargements of neurites where dense granules are preferentially accumulated--the varicosities and terminals--underwent swelling and extensive disorganization within a few minutes after the application of alpha-latrotoxin, whereas the cell bodies and the tracts of neurites occupied primarily by microtubules were less severely affected. The greater sensitivity of varicosities and terminals with respect to the other parts of the differentiated cells, rather than the consequence of a specific addressing of the toxin to these structures, might be due to their vulnerability by toxin-induced events, such as the uncontrolled activation of ion fluxes.

Animals↗
PubMed1985

Muscarinic receptor-induced phosphoinositide hydrolysis at resting cytosolic Ca2+ concentration in PC12 cells.

In PC12 cells, cultured in the presence of nerve growth factor to increase their complement of muscarinic receptors, treatment with carbachol induces muscarinic receptor-dependent rises in free cytosolic Ca2+ as well as hydrolysis of membrane phosphoinositides. Experiments were carried out to clarify the relationship between these two receptor-triggered events. In particular, since inositol-1,4,5-trisphosphate (the hydrophilic metabolite produced by the hydrolysis of phosphatidylinositol-4,5-bisphosphate) is believed to mediate intracellularly the release of Ca2+ from nonmitochondrial store(s), it was important to establish whether it can be generated at resting cytoplasmic concentration of Ca2+ (approximately 0.1 microM). Cells incubated in Ca2+-free medium were depleted of their cytoplasmic Ca2+ stores by pretreatment with ionomycin. When these cells were then treated with carbachol, their cytosolic concentration of Ca2+ remained at the resting level, whereas inositol-1,4,5-trisphosphate generation was still markedly stimulated. Our results demonstrate that an increase in the concentration of cytosolic Ca2+ is not a necessary intermediate between receptor activation and phosphoinositide hydrolysis, and therefore support the second-messenger role of inositol-1,4,5-trisphosphate.

Animals↗
PubMed1985

Leptinotoxin-h action in synaptosomes, neurosecretory cells, and artificial membranes: stimulation of ion fluxes.

Leptinotoxin-h (LPTx), a neurotoxin (otherwise designated beta-leptinotarsin-h) known to stimulate the release of neurotransmitters from synapses, was purified from the hemolymph of the potato beetle, Leptinotarsa haldemani, by a simplification of the procedure originally developed by Crosland et al. [Biochemistry 23, 734-741, (1984)]. Highly and partially purified preparations of the toxin were applied to guinea pig synaptosomes and neurosecretory (PC12) cells. When applied in a Ca2+-containing Ringer medium, at concentrations in the 10(-11) - 10(-10) M range, the toxin induced: (a) rapid depolarization of the plasma membrane, which was not inhibited by organic blockers of voltage-dependent Na+ and Ca2+ channels (tetrodotoxin or verapamil); (b) large 45Ca influx; and (c) increased free cytosolic Ca2+ concentration. These latter two effects were unaffected by verapamil. In Ca2+-free media the effects of the toxin were different in the two systems investigated. In synaptosomes, depolarization was still observed, even if the toxin concentrations needed were higher (approximately 10X) than those effective in the complete medium. In contrast, in PC12 cells no effect of the toxin on membrane potential was observed. Binding of LPTx to its cellular targets could not be investigated directly because the toxin was inactivated by the procedures used for its labeling. Indirect evidence suggested however that Ca2+ is necessary for toxin binding to PC12 cells. Interaction of LPTx with air/water interfaces, as well as with cholesterol/phospholipid mono- and bilayer membranes was investigated. The results indicate that the toxin has affinity for hydrophobic surfaces, but lacks the capacity to insert across membranes unless transpositive voltage is applied. Our results are inconsistent with the previous conclusion of Crosland et al. (1984), who suggested opening of the Ca2+ channel as the mechanism of action of LPTx. The effects of the toxin resemble those of alpha-latrotoxin (alpha-LTx) of the black widow spider venom, and therefore the two toxins might act by similar mechanisms. However, the sites recognized by the two toxins might be different, because LPTx does not inhibit alpha-LTx binding.

Adrenal Gland Neoplasms↗
PubMed1985

Leptinotoxin-h action in synaptosomes and neurosecretory cells: stimulation of neurotransmitter release.

Guinea pig brain cortex synaptosomes and neurosecretory PC12 cells were loaded with [3H]3,4-dihydroxyphenylethylamine ([3H]DA, [3H]dopamine) and then exposed to leptinotoxin-h (LPTx) (purified and partially purified preparations, obtained from the hemolymph of Leptinotarsa haldemani). In a Ca2+-containing Ringer medium the toxin induced prompt and massive release of the neurotransmitter. Half-maximal effects were obtained at concentrations estimated of approximately 3 X 10(-11) M for synaptosomes, and 1.5 X 10(-10) M for PC12 cells. Release responses in the two experimental systems investigated were dependent to different extents on the Ca2+ concentration in the medium. In synaptosomes clear, although slow, release of [3H]DA was elicited by the toxin even in Ca2+-free, EGTA-containing medium, provided that high (in the 10(-10) M range) concentrations were used; near-maximal responses were observed at 10(-5)M Ca2+. In contrast, the toxin-induced release from PC12 cells was appreciable only at 3 X 10(-5) M Ca2+, and was maximal at 2 X 10(-4) M and above. In both synaptosomes and PC12 cells Sr2+ and Ba2+ could substitute for Ca2+; Co2+ was inhibitory, whereas Mn2+ failed to modify the release induced by the toxin in Ca2+-containing medium. Organic blockers of the voltage-dependent Ca2+ channel (verapamil and nitrendipine) and calmodulin blocking drugs (trifluoperazine and calmidazolium) failed to inhibit the toxin-induced release of [3H]DA. LPTx induced profound morphological effects. Synaptosomes treated in the Ca2+-containing medium exhibited fusion of synaptic vesicles, formation of numerous infoldings and large cisternae, and alterations of mitochondria. In the Ca2+-free medium the effects were similar, except that their appearance was delayed, and mitochondria were well preserved. Swelling was observed in PC12 cells, accompanied by enlargement of the Golgi area, accumulation of multivesicular bodies, mitochondrial alterations, and decreased number of secretion granules (Ca2+-containing medium). Morphometric analyses revealed a good correlation between the decrease of both synaptic vesicles (synaptosomes) and neurosecretory granules (PC12 cells), and the release of [3H]DA measured biochemically. This is a good indication that the release effect of the toxin is due to stimulation of exocytosis. Taken as a whole, these results confirm the similarity of the effects of LPTx with alpha-latrotoxin of the black widow spider venom, mentioned in the companion article. However, differences in effect and target specificity suggest that the two toxins are specific to separate binding sites.(ABSTRACT TRUNCATED AT 400 WORDS)

Adrenal Gland Neoplasms↗
PubMed1985

In vivo rat pancreatic acinar cell function during supramaximal stimulation with caerulein.

Infusion of a supramaximal dose of caerulein results in acute interstitial pancreatitis in rats. We report studies of in vivo pancreatic acinar cell function during the initial 3.5 h of supramaximal stimulation with caerulein (5 micrograms X kg-1 X h-1). Amino acid [( 3H]phenylalanine) uptake was not altered, and there was no change in the rate or extent of protein synthesis or in intracellular transport of in vivo pulse-labeled proteins from microsome to zymogen granule-enriched fractions. However, the discharge of labeled protein was markedly inhibited. Radioautographic studies indicated that the pulse-labeled proteins retained in the gland were not located extracellularly but had accumulated within acinar cells, with a preferential distribution at the cell apex (presumably in zymogen granules) and in large vacuoles that form within the cell during hyperstimulation. Supramaximal stimulation with caerulein also caused increasing amounts of amylase and labeled proteins to be recovered in the postmicrosomal fraction. These findings suggest that supramaximal stimulation causes digestive enzymes to become localized in organelles that are fragile and subject to disruption during tissue homogenization. These organelles may be the vacuoles noted in morphological studies and believed to represent immature condensing vacuoles and/or crinophagic vacuoles.

Amino Acids↗