Immunohistochemical localization of somatostatin, insulin and glucagon in the principal islets of the anglerfish (Lophius americanus) and the channel catfish (Ictalurus punctata) (1) (2)

Unique localization of disseminated pancreas in the oesophagus of catfish (clarias gariepinus) with reference to sexual dimorphism

BACKGROUND

The fish pancreas has been reported to be composed of two portions: compact and disseminated. However, little has been elucidated in catfish. The present study describes a unique localization of the disseminated pancreas in African catfish.

METHODS

The sections were obtained and used for either routine histological examination following staining with haematoxylin and eosin (H & E), periodic acid-Schiff's, or were subjected to immunohistochemical staining for detection of both insulin-producing β cells and glucagon-producing alpha cells.

RESULTS

Our investigation showed that the pancreas of catfish consisted of both compact and disseminated portions. The compact pancreas was embedded in the mesenteric adipose tissue between the spleen, stomach and liver. However, the disseminated one showed unique localization in the tunica adventitia of the middle portion of the oesophagus. The pancreas consisted of two portions, exocrine and endocrine. Furthermore, in both types of pancreas, the female showed a significantly higher ratio for the endocrine islet area/pancreatic tissue area than that of the male and also a significantly higher ratio for both insulin- and glucagon-positive area/islet area in the female pancreas (compact and disseminated) than that of the male.

IN CONCLUSION

The present study provides evidence on a unique localization of the disseminated pancreas in the oesophagus of catfish. Furthermore, we revealed sex-related difference in the endocrine portion in both pancreatic tissues with more development in the female. The study suggests that sex hormones could be contributed to such sexual dimorphism. However, further investigation is required to compare the degree of development during the spawning and resting seasons.

Immunohistochemical distribution of insulin-, glucagon- and somatostatin-containing cells in the pancreas of Lake Van fish (Alburnus tarichi Güldenstädt, 1814) (Cyprinidae)

The Lake Van fish (Alburnus tarichi) is a species that is endemic to Turkey’s Lake Van basin. In this study, the regional distribution, volume density, and relative frequency of some pancreatic endocrine cells in Lake Van fish were investigated via immunohistochemistry using specific mammalian antibodies. The pancreatic tissue was observed to be surrounded by adipose tissue, which was adjacent to the gall bladder or extrahepatic bile duct, or dispersed in the adipose tissue ranked among coils of post-esophageal swelling and intestine. The pancreatic endocrine cells were examined, including the islets, exocrine pancreas, and pancreatic ducts. According to the modified aldehyde fuchsin staining and immunohistochemistry, insulin-secreting beta cells were observed to localize throughout the islets. Glucagon immune-reactive (IR) cells were observed to be situated moderately on the islet periphery, and were rarely determined in the islet central region. A small number of somatostatin- IR cells were observed in the islet centers and peripheries. Similar distributions of those three endocrine cells were also determined in the secondary islets. Additionally, the endocrine cell percentages did not differ between the primary and secondary islets; insulin-, glucagon- and somatostatin-IR cells comprised approximately 54%, 29%, and 11% of the endocrine cells in the principal islets, whereas they comprised 52%, 27%, and 14% in the secondary islets, respectively. Insulin-, glucagon- and somatostatin-IR cells were also determined among the epithelium and subepithelial connective tissue in the pancreatic ducts or exocrine areas of the pancreas. With this study, the existence, regional distribution, and relative frequency of the insulin-, glucagon- and somatostatin-IR cells were first investigated in the pancreatic tissue of Lake Van fish and the results were discussed.

Immunohistochemical study of the principal pancreatic islet of the toadfish, Halobatrachus didactylus (Pisces: Batrachoididae)

The endocrine pancreas of the toadfish, Halobatrachus didactylus, consists of one large circular principal islet (Brockman body) located in the dorsal side or neck region of the gallbladder, along with various accessory islets of variable sizes and shapes, embedded in the exocrine tissue located within the digestive organs connecting mesenteries. Islet cells showed variable shapes, angular or fusiform, with long cytoplasmic processes, granular cytoplasm, and a large eccentric nucleus. Cells were found scattered or as aggregates or cords. Four primary endocrine cell types immunoreactive for glucagon (α cells), insulin (β cells), somatostatin (δ cells), and pancreatic polypeptide (F cells) were identified within the toadfish principal islet. The α, δ, and F cells were located both at the periphery and in the central regions, while β cells, which were the predominant type, were present only in the central core. α and δ cells were found in moderate frequencies, while F cells were the least abundant. Macroscopically, the Brockman body of H. didactylus is visible as a milky white nodule separated from the exocrine tissue. Its size, location, and ease of extraction suggest that H. didactylus is suitable as experimental subject for biochemical, immunological, and physiological studies of the endocrine pancreas including in vitro investigations of hormone production, storage, and release.

Isolation and characterization of insulin from the Brockmann body of Dissostichus mawsoni, an Antarctic teleost fish

The Brockmann body of fish synthesizes and secretes insulin. The Brockmann body of Antarctic fish has been described anatomically and shown to contain insulin immunoreactive sites, however, the primary structure of an Antarctic fish insulin has yet to be reported. Insulin was isolated from the Brockmann bodies of the Antarctic perciform teleost, Dissostichus mawsoni. The peptide was purified to homogeneity by gel filtration and reversed-phase HPLC. Insulin-containing fractions were identified by radioimmunoassay using antisera raised against porcine insulin. Electrospray ionization-mass spectrometry determined the mass of the isolated product to be 5725.27 a.m.u. The amino acid composition and primary structure were determined for the pyridylethylated A- and B-chains. The amino acid sequences of the A chain and B chain were H-Gly-lle-Val-Glu-Gln-Cys-Cys-His-Gln-Pro10-Cys-Asn-Ile-Phe- Asp-Leu-Gln-Asn-Tyr-Cys20-Asn-OH and H-Ala-Pro-Gly-Pro-GIn-His-Leu-Cys-Gly-Ser10-His-Leu-Val-Asp-Ala-Le u-Tyr-Leu-Val-Cys20-Gly-Glu-Arg-Gly-Phe-Phe-Tyr-Asn-Pro-Lys30++ +-OH, respectively. The primary structure of insulin from Antarctic fish is compared with known structures of insulin from other vertebrates.

Ontogenetic and phylogenetic development of the endocrine pancreas (islet organ) in fish

The morphology of the gastroenteropancreatic (GEP) system of fish was reviewed with the objective of providing the phylogenetic and ontogenetic development of the system in this vertebrate group, which includes agnathans and gnathostome cartilaginous, actinoptyerygian, and sarcopterygian fish. Particular emphasis is placed on the fish homolog of the endocrine pancreas of other vertebrates, which is referred to as the islet organ. The one-hormone islet organ (B cells) of larval lampreys is the most basic pattern seen among a free-living vertebrate, with the two-hormone islet organ (B and D cells) of hagfish and the three-hormone islet organ (B, D, and F cells) of adult lampreys implying a phylogenetic trend toward the classic four-hormone islet tissue (B, D, F, and A cells) in most other fish. An earlier stage in the development of this phylogenetic sequence in vertebrates may have been the restriction of islet-type hormones to the alimentary canal, like that seen in protochordates. The relationship of the islet organ to exocrine pancreatic tissue, or its equivalent, is variable among bony, cartilaginous, and agnathan fishes and is likely a manifestation of the early divergence of these piscine groups. Variations in pancreatic morphology between individuals of subgroups within both the lamprey and chondrichthyan taxa are consistent with their evolutionary distance. A comparison of the distribution and degree of concentration of the components of the islet organ among teleosts indicates a diffuse distribution of relatively small islets in the generalized euteleosts and the tendency for the concentration into Brockmann bodies of large (principal) islets (with or without secondary islets) in the more derived forms. The holostean actinopterygians (Amiiformes and Semiontiformes) share with the basal teleosts (osteoglossomorphs, elopomorphs) the diffuse arrangement of the components of the islet organ that is seen in generalized euteleosts. Since principal islets are also present in adult lampreys the question arises whether principal islets are a derived or a generalized feature among teleosts. There is a paucity of studies on the ontogeny of the GEP system in fish but it has been noted that the timing of the appearance of the islet cell types parallels the time that they appear during phylogeny; the theory of recapitulation has been revisited. It is stressed that the lamprey life cycle provides a good opportunity for studying the development of the GEP system. There are now several markers of cell differentiation in the mammalian endocrine pancreas which would be useful for investigating the development of the islet organ and cells of the remaining GEP system in fish.

Immunocytochemical characterization of the pancreatic islet cells of the Nile Tilapia (Oreochromis niloticus)

The cellular composition and topography of the pancreatic islet of Oreochromis niloticus, now known to be a donor source for islet xenotransplantation studies, were characterized. Whole tilapia islets were harvested using an enzymatic method and then further digested into single-cell preparations. Cell cytospin preparations of islet cells and paraffin sections of whole islets were stained using antisera against tilapia insulin, human glucagon, salmon somatostatin-25 (SST-25), human somatostatin-14 (SST-14), and salmon peptide tyrosine-tyrosine (PYY) using the immunoperoxidase method. Cell counts, performed on cytospin preparations using a Quantimet 570 computerized image analysis system, revealed that O. niloticus islets contained 78% endocrine cells and 22% immunonegative cells (i. e., mainly nucleated erythrocytes and rare tissue eosinophils). The proportions of immunopositive endocrine cell types were: 42.3% insulin immunopositive cells, 11.5% glucagon immunopositive cells, 23.1% SST-25 immunopositive cells, 21.8% SST-14 immunopositive cells, and 1.3% PYY immunopositive cells. Islet cell topography was evaluated using histologic sections of whole endocrine pancreata including large, medium, and small islets. Round to polygonal insulin immunopositive cells with round central nuclei were distributed in clusters throughout both the principal and the smaller islets. Elongate SST-14 immunopositive cells were closely associated with the clusters of insulin immunopositive cells; both were surrounded by SST-25 immunopositive cells, which were similar in shape to the insulin immunopositive cells. There were elongate glucagon immunopositive cells throughout the islets, whereas the PYY immunopositive cells were restricted to the periphery and to channels of fibrovascular connective tissue penetrating the islets.

An immunohistochemical study of the endocrine cells within the pancreas, intestine, and stomach of the gar (Lepisosteus osseus L.)

The distribution and identity of the various endocrine cell types were examined in the pancreas, stomach, and anterior intestine of the phylogenetically ancient actinopterygian, the gar (Lepisosteus osseus L.), using immunohistochemistry. Antisera used were directed against several insulins (INSs) and somatostatins (SSTs), and members of the pancreatic polypeptide (PP, aPY, NPY) and glucagon (GLUC, GLP) families. In the gar pancreas the most pronounced aggregation of islet tissue is among the exocrine acini near the union of extrahepatic common bile duct with the gastrointestinal junction. Four immunoreactive cell types were identified within well-defined islets (A, B, D, and F cells) but immunoreactive cell types were also seen isolated among the exocrine acini. Centrally located B cells were immunoreactive with mammalian and lamprey INS antisera whereas the widely dispersed D cells immunostained with anti-SST-14, -25, and -34. SST was also localized in the epithelium of the pancreatic ducts. There was a colocalization of immunoreactivity for each member of the PP and GLU families at the periphery of each islet to identify F and A cells, respectively. However, colocalization of peptides from both families is suspected for at least some cells. Although the gastric and intestinal mucosae showed a similar pattern of immunoreactivity to GLP and not GLU, they had contrasting immunoreactivity with the two INS antisera. SST immunoreactivity was restricted to the stomach, whereas three of the four PP-family peptides were only immunoreactive in the intestine. Immunoreactivity to the various antisera used in the study imply that there may be an organ-specific processing of preproinsulin, that the gar SST profile may be more similar to agnathan and bowfin rather than either elasmobranch or teleost SSTs, and that only the GLP portion of the preproglucagon gene is expressed in the gastrointestinal mucosa. Our results are consistent with other recent endocrine studies showing that the gar is a widely distinct actinopterygian.

Distribution of peptidyl-glycine alpha-amidating monooxygenase immunoreactivity in the brain, pituitary and islet organ of the anglerfish (Lophius americanus)

Peptidyl-glycine alpha-amidating monooxygenase (PAM; EC 1.14.17.3) is an enzyme that catalyzes conversion of glycine-extended peptides to alpha-amidated bioactive peptides. Two peptides that are processed at their carboxyl-termini by this enzyme are neuropeptide Y and anglerfish peptide Y, both of which possess a C-terminal glycine that is used as a substrate for amidation. Results from previous reports have demonstrated that neuropeptide Y-like and anglerfish peptide Y-like immunoreactivities are present in the brain of anglerfish (Lophius americanus). Furthermore, neuropeptide Y-like peptides, namely anglerfish peptide Y and anglerfish peptide YG (the homologues of pancreatic polypeptide) are present in the islet organ of this species. Neuropeptide Y has also been localized in the anterior, intermediate and posterior lobes of the pituitary gland in a variety of species. In order to learn more about the distribution of the enzyme responsible for alpha amidation of these peptides in the brain and pituitary and to specifically investigate the relationship of this enzyme to peptide synthesizing endocrine cells of the anglerfish islet, we performed an immunohistochemical study using several antisera generated against different peptide sequences of the enzyme. PAM antisera labeled cells in the islet organ, pituitary and brain, and fibers in the brain and pituitary gland. The PAM staining pattern in the brain was remarkably similar to the distribution of neuropeptide Y immunoreactivity reported previously. Clusters of cells adjacent to vessels in the anterior pituitary displayed punctate PAM immunoreactivity while varicose fibers were observed in the pituitary stalk and neurohypophysis. Endocrine cells of the islet organ were differentially labeled with different PAM antisera. Comparison of the staining patterns of insulin, glucagon, and anglerfish peptide Y in the islet organ to PAM immunoreactivity suggests a distribution of forms of PAM enzyme in insulin and anglerfish peptide Y-containing cells, but no overlap with glucagon-producing cells. The results also indicate that PAM immunoreactivity is widely distributed in the brain, pituitary and islet organ of anglerfish in cells, that contain peptides that require presence of a C-terminal glycine for amidation.

Neuronal influence on hormone release from anglerfish islet cells

Pancreatic islets in anglerfish (AF) are macroscopic collections of nearly pure endocrine cells that are densely innervated. Immunohistochemical staining for neurotransmitter biosynthetic enzymes revealed noradrenergic and cholinergic innervation of AF islets. An in vitro preparation of perifused dispersed AF islet cells was developed to study nutrient and neural control of islet hormone secretion. Glucose stimulated insulin and somatostatin-14 (SS-14) secretion in a dose-dependent manner, and 16.7 mM glucose inhibited glucagon secretion. In 2 mM glucose, norepinephrine and isoproterenol stimulated glucagon and SS-14 release. Isoproterenol stimulated insulin secretion, and norepinephrine stimulated or inhibited insulin release, depending on the concentration. Clonidine potently inhibited glucose-stimulated insulin secretion but stimulated glucagon release. Methacholine, a muscarinic cholinergic agonist, stimulated insulin, glucagon, and SS-14 release. The control of AF hormone release by neurotransmitter agonists in vitro was similar to that in higher vertebrate species. Therefore we used this tissue preparation to study postsynaptic interactions between glucose and neurotransmitters in islets.

Pancreatic endocrine cells in sea bass (Dicentrarchus labrax L.) II. Immunocytochemical study of insulin and somatostatin peptides

Insulin (INS)- and somatostatin (SST)-immunoreactive cells were demonstrated by light immunocytochemistry in the endocrine pancreas of sea bass (Dicentrarchus labrax). INS-immunoreactive cells were identified using bovine/porcine, bonito, and salmon (s) INS antisera; the immunostaining was abolished when each antiserum was preabsorbed with its respective peptide but not with unrelated peptides. These cells also reacted with mammal (m) SST-28 (4-14) antiserum. The immunoreaction did not change when this antiserum was preabsorbed by bovine INS. INS-immunoreactive cells were located in the central part of the endocrine areas of the principal, intermediate, and small islets. Two SST-immunoreactive cell types (D1 and D2) were revealed. D1 cells, immunoreactive to SST 14 (562) and sSST-25 antisera, were located next to the glucagon-immunoreactive cells in the peripheral part of the endocrine areas. D2 cells, immunoreactive to SST-14 (562), SST-14 (566), and mSST-28 (4-14) antisera, were found in apposition to the INS-immunoreactive cells. The specificity controls showed that D1 cells expressed sSST-25-like peptides, while D2 cells might contain SST-14 and/or mSST-28-like peptides. The close topographic association between the different SST-immunoreactive cells and both glucagon- and insulin-immunoreactive cells might indicate the existence of a specific paracrine regulation of each endocrine cell type in the sea bass endocrine pancreas.

Maturation of the endocrine pancreas in the sea bass, Dicentrarchus labrax L. (Teleostei): an immunocytochemical and ultrastructural study. I. Glucagon-producing cells

The structure of the endocrine pancreas in the sea bass (Dicentrarchus labrax L.) was studied with special reference to glucagon-immunoreactive cells. As described in most of the teleosts, the sea bass was found to have a diffuse pancreas. In the adult, endocrine cells were clustered in a principal islet and numerous accessory islets where the glucagon A cells were localized peripherally. Under electron microscopy, the A cells displayed a clear hyaloplasm with granules having typical spherical or polyhedral cores, as in other vertebrates. The maturation of the endocrine pancreas was monitored under rearing conditions. The endocrine pancreas appeared during the prelarval stage, 3 days after hatching, and consisted of a single cluster of morphologically similar cells, containing very small cytoplasmic granules. During the larval stage, cytodifferentiation resulted in modifications of cell shape and increased granule size. Typical granules appeared in 8-mm-long larvae. Cells immunoreactive with mammalian glucagon antibodies appeared only at the beginning of the juvenile stage (3 months/20 mm). Electron microscope observations revealed that the storage of hormone in numerous cytoplasmic granules began at this stage.

Isolation and structures of alligator gar (Lepisosteus spatula) insulin and pancreatic polypeptide

Insulin and a 36-residue peptide with homology to pancreatic polypeptide (PP) were isolated from the endocrine pancreas of the alligator gar (Lepisosteus spatula), a ganoid fish, by gel filtration and HPLC. Heterologous radioimmunoassays were used to detect insulin-like and PP-like immunoreactivities during purification of the two peptides. The sequence of the 36-amino acid peptide containing a C-terminal tyrosinamide was identical at 31 of 36 positions to porcine neuropeptide Y (NPY). The amino acid sequence of this peptide is YPPKPENPGEDAPPEELAKYYSALRHYINLITRQRY-NH2. The second peptide, gar insulin, contains 52 amino acid residues and is composed of a 21-residue A chain and a 31-residue B chain. The sequence of the A chain is GIVEQCCHKPCTIYELENYCN. The sequence of the B chain is AANQHLCGSHLVEALYLVCGEKGFFYNPNKV.

Oxytocin-like immunoreactive nerves are associated with insulin-containing cells in pancreatic islets of anglerfish (Lophius americanus)

Recent reports indicate that oxytocin exerts direct effects on the release of insulin and glucagon from the endocrine pancreas of the rat. The purpose of this study was to determine whether oxytocin-like immunoreactivity is present in the anglerfish islet, and if it is associated with subsets of hormone-producing cells. Antisera against oxytocin, insulin, glucagon, somatostatin, neuropeptide Y, and the 200-kd neurofilament polypeptide were applied to serial 5 micrometers sections of pancreatic islets. The antiserum to the 200-kd neurofilament polypeptide labeled nerve bundles and axons, some of which were also stained with the oxytocin antiserum. Oxytocin immunoreactivity was observed in large nerves that branched into varicose fibers. These fibers were consistently associated only with clusters of insulin-producing cells. Successive application of oxytocin and insulin antisera to the same section provided additional verification of this relationship. Oxytocin-labeled nerves were not associated with cells immunoreactive to glucagon, somatostatin, or neuropeptide Y (anglerfish peptide Yg). The results demonstrate that oxytocin or an oxytocin-like peptide is located in fibers that surround only insulin-producing cells in the anglerfish islet. Although the functional significance of this observation remains to be determined, the results imply that oxytocin, or an oxytocin-like peptide, may affect the synthesis or release of insulin from anglerfish islets.

Primary structure of insulin and glucagon from the flounder (Platichthys flesus)

Insulin and glucagon have been isolated from the Brockmann bodies of the flounder, a teleostean fish, and their primary structures established by automated Edman degradation. The A-chain of flounder insulin shows strong homology to the A-chains from the coho salmon (Oncorhynchus kisutch; 100%) and the anglerfish (Lophius americanus; 95%) but homologies in the B-chain region are weaker (salmon 79%, anglerfish 83%). Flounder insulin B-chain contains the novel sequence Val-Val-Pro-Pro at the NH2 terminus and the highly conserved seryl residue at position 10 (B 9 in mammals) is replaced by an alanyl residue. Flounder glucagon is identical to anglerfish glucagon II but shows four amino acid substitutions compared with salmon glucagon.

Secretagogues for pancreatic hormone release in the channel catfish (Ictalurus punctatus)

We investigated the effect of several potential carbohydrate secretagogues, amino acids, a ketoacid, and potassium chloride on insulin, glucagon, and somatostatin release from the in vitro perfused Brockmann body of channel catfish (Ictalurus punctatus). Mannose (15 mM) stimulated the release of insulin and somatostatin. Fructose (30 mM) induced only a small and transient release of somatostatin. Galactose (15 mM) was not a secretagogue. Likewise, glyceraldehyde failed to stimulate hormone release. Among the amino acids newly tested, alanine and leucine, and also alpha-ketoisocaproic acid were without effect. A high concentration of potassium (25 mEq/liter) induced a pronounced release of insulin and glucagon and a moderate release of somatostatin. In conclusion, a striking similarity exists between catfish and higher vertebrates in their pancreatic endocrine response to hexoses; on the other hand, the catfish Brockmann body appears to respond only to a few of the common stimuli of pancreatic hormone release in mammals.

An immunocytochemical and ultrastructural study of the endocrine pancreas of Sparus auratus L. (Teleostei)

The pancreatic endocrine cells of Sparus auratus (gilthead sea bream) are concentrated in two or three principal islets, or Brockmann bodies, and numerous smaller islets embedded in the exocrine tissue. Insulin-, glucagon-, somatostatin-, and pancreatic polypeptide (PP)-immunoreactive cells were identified in all pancreatic islets of S. auratus using an indirect immunocytochemical (PAP) method. Insulin-immunoreactive cells were found in the central region of the islets. Glucagon-immunoreactive cells could be seen at the periphery of the islets and isolated in the exocrine tissue surrounding the large principal islet. Somatostatin-immunoreactive cells were distributed throughout the islets. PP-immunoreactive cells were clustered, in a limited shallow section, being found in no other part of the large principal islet whereas, in the smaller islets, these cells were more numerous and found in the whole peripheral area. Four cell types were identified in the pancreatic islets of S. auratus by electron microscopy. A,B,D and PP cells were characterized by the shape, size, and electron density of their respective secretory granules.

Immunocytochemical identification and localization of peptide hormones in the gastro-entero-pancreatic (GEP) endocrine system of the mouse and a stomachless fish, Barbus conchonius

A large number of antisera mainly raised against mammalian hormones are tested immunocytochemically on the GEP-endocrine system of mouse and fish (Barbus conchonius). The endocrine pancreas of mouse and fish appeared to contain the same four endocrine cell types; insulin-, glucagon-, PP- and somatostatin-immunoreactive cells. In mouse about 13 GEP endocrine cell types are distinguished: 1. insulin-, 2. somatostatin-, 3. glucagon-, 4. PP-, 5. (entero)glucagon-/PP-like, 6. CCK-like, 7. substance P-, 8. neurotensin-, 9. VIP-, 10. gastrin-, 11. secretin-, 12. beta-endorphin-, 13. serotonin-immunoreactive cells. Based on this and a previous study at least 13 GEP endocrine cell types seems to be present in stomachless fish: 1-9 as described for mouse, 10. (entero)glucagon-like, 11. met-enkephalin, 12. VIP-like, 13. unspecific immunoreactive endocrine cells. Coexistence of glucagon and PP-like peptides is found in the gut and pancreas of mice and in the gut of B. conchonius. In mouse pancreas and fish gut, endocrine cells showing only PP- or glucagon-like immunoreactivity are found too. In mouse stomach some endocrine cells showing only PP-immunoreactivity are demonstrated. In the same region coexistence of C-t-gastrin- and FMRF-amide-immunoreactivity is found in endocrine cells. The importance of these phenomena are discussed. Enteric nerves immunoreactive with antisera raised against substance P and GRP are found in mouse, against somatostatin and met-enkephalin in both mouse and fish and against VIP in fish.

Identification of hormone-producing cells of the endocrine pancreas of the sea bass, Dicentrarchus labrax, by ultrastructural immunocytochemistry

The hormone-producing cells of the endocrine pancreas of the sea bass Dicentrarchus labrax have been identified by ultrastructural immunocytochemistry. The glucagon cells have "clear" cytoplasm and contain characteristic electron-dense polygonal granules surrounded by a "halo" of electron-lucent material. The insulin cells have numerous, tightly packed, electron-dense granules that are almost twice as large as the peripherally located granules of the somatostatin cells. The pancreatic polypeptide cells have granules with variable electron density. When specific antisera are applied in the peroxidase-anti-per-oxidase immunocytochemical method at the electron microscope level, each of the four types of granule is identified by the resultant overlying immunoreaction deposit. Especially in older fish, a fifth, nonclassified type of cell has been identified within the endocrine pancreatic tissue. These cells have many ramifying processes and contain a mixture of the granules of the four endocrine cell types as well as granules from the exocrine tissue. It is suggested that these cells may be undertaking macrophage activities. A distinct patterned arrangement of the endocrine cell types in both small pancreatic islets and Brockmann bodies is observed. There is a central core of insulin and somatostatin cells surrounded by an outer peripheral layer of glucagon and pancreatic polypeptide cells. A definite functional interrelationship is suggested by this arrangement.

Distribution, histochemistry and ultrastructure of somatostatin-like immunoreactive cells in the gastroenteric tract of the cartilaginous fish Scyliorhinus stellaris (L.)

Somatostatin-like immunoreactive cells of an open type have been identified in the digestive tract of the cartilaginous fish Scyliorhinus stellaris (L.) by the use of immunocytochemical techniques. In the stomach these cells are numerous both in the corpus (neck zone and tubular glands) and in the pyloric portion (crypts). In the spiral valve, somatostatin-like cells are rare, situated in the intestinal epithelium and without any particular localization. Using semithin serial sections, somatostatin-like cells are found to be Davenport-negative and weakly positive towards the Grimelius silver reaction, and using the semithin and ultrathin technique have been identified at the ultrastructural level; their secretory granules appear electron dense, round or slightly polygonal, and with a limiting membrane tightly adherent to the core. The mean diameter varies from 250-300 nm.

Detection of endocrine cells by immunofluorescence method in the gastroenteropancreatic system of the adult eel, glass-eel, and leptocephalic larva (Anguilla anguilla L.)

Five antisera against insulin (Ins), glucagon (Glu), somatostatin (SRIF), met-enkephalin (met-enk), and serotonin (5-HT) were used for immunofluorescence detection of endocrine cells in pancreas and gastrointestinal tract (GIT) of the European eel (Anguilla anguilla L.) at three stages of development (leptocephalic larva, glass-eel, and adult eel). Comparable distribution of endocrine cells was observed for adults and glass-eels. In their pancreatic islets, positive immunoreactions were obtained only for Ins, SRIF, and Glu; this later was also present in the pancreatic ducts. 5-HT cells were present throughout the GIT. SRIF cells were situated mostly in the stomach and less in the intestine. Met-enk cells were abundant in the pyloric cecum, but less frequent in the intestinal mucosa. Glu cells were present only in the intestine. No insulin-immunoreactive cells could be detected in the GIT. The pancreatic islets of leptocephalic larvae exhibited a strong reaction for SRIF, a weak reaction for Glu, and none at all for Ins, met-Enk, or 5-HT. The GIT of these larvae contained numerous met-enk cells, mainly in the foregut. In the fore- and midgut, cells exhibited a weak fluorescence after treatment with Glu antiserum. No positive immunoreactive cells were observed with 5-HT, SRIF, or Ins antisera.

Regulation of hormone biosynthesis in cultured islet cells from anglerfish

The effects of glucose and arginine on islet hormone biosynthesis were investigated using primary cell cultures prepared from islets of the anglerfish (Lophius americanus). After dispersion under sterile conditions, islet cells were maintained at 23 degrees C in medium containing RPMI 1640 with Hanks' buffer, pH 7.5, modified by the adjustment of glucose (to 0.56 or 5.6 mM) and arginine (to 0.1, 1.15, or 10 mM) with the addition of 10% fetal bovine serum (dialyzed, heat inactivated) and penicillin/streptomycin. After 48 h, media were replaced by incorporation media containing [14C]isoleucine and [3H]tryptophan and incubated for an additional 8 h under otherwise identical conditions. Culture samples (cells plus media) were extracted, desalted, and gel filtered to identify and quantitate [14C]insulin, [3H]glucagon(s) plus [3H]somatostatin-28, and [3H]somatostatin-14. In some experiments, [14C]insulin, [3H]glucagon(s), [3H]somatostatin-28, and [3H]somatostatin-14 were separated by high performance liquid chromatography. Raising the medium glucose from 0.56 (control) to 5.6 mM resulted in an augmentation in incorporation of [14C]isoleucine into insulin and an augmentation of [3H]tryptophan into glucagon(s) and somatostatin-14, but no change in incorporation of [3H]tryptophan into somatostatin-28. Raising the concentration of arginine from 0.1 to 1.15 or 10 mM resulted in a dose-dependent inhibition of labeled amino acid incorporation into all hormones except somatostatin-28. The results demonstrate the usefulness of the culture system for studying the modulation of hormone biosynthesis in anglerfish islet cells.

Processing of an anglerfish somatostatin precursor to a hydroxylysine-containing somatostatin 28

A novel 28-residue somatostatin (SS) has been isolated from anglerfish pancreatic islets and characterized by complete Edman degradation, peptide mapping, and amino acid analysis. The primary structure of this anglerfish SS-28 (aSS-28) containing hydroxylysine (Hyl) was established to be H-Ser-Val-Asp-Ser-Thr-Asn-Asn-Leu-Pro-Pro-Arg-Glu-Arg-Lys-Ala-Gly-Cys- Lys-Asn-Phe-Tyr-Trp-Hyl-Gly-Phe-Thr-Ser-Cys-OH. This sequence (with the exception of hydroxylysine-23, which is replaced by lysine) is identical to the sequence of the COOH-terminal 28 residues of prepro-SS II predicted on the basis of cDNA analysis [Hobart, P., Crawford, R., Shen, L., Pictet, R. & Rutter, W. J. (1980) Nature (London) 288, 137-141]. This is the first instance in which hydroxylysine (to date characteristically observed in collagen or collagen-like structures) has been found in a potential regulatory peptide. Chromatographic characterization of peptides, radiolabeled in islet culture, revealed that aSS-28 contained 10-12% of the radioactivity incorporated into the 8000- to 1000-dalton SS-like polypeptides, whereas 88-90% of this radioactivity was detected in anglerfish SS-14. It appears probable that aSS-28 represents the predominant primary cleavage product derived from prepro-SS II by cleavage at the COOH-terminal side of a single arginine. Based on knowledge of the collagen biosynthesis, it is speculated that hydroxylation may take place as an early post-translational event.

An immunocytochemical study of the pancreatic islet system of the channel catfish

The endocrine pancreas of the channel catfish is segregated into a large primary islet and numerous smaller secondary islets. In view of cell distribution differences in mammalian islets of ventral and dorsal primordia, we have determined the percentage volumes of insulin-, glucagon-, and somatostatin-containing cells in primary and secondary catfish islets to ascertain if these islets correlated with those derived from ventral and dorsal primordia in mammals. Islets were immunocytochemically stained using antisera to anglerfish insulin, porcine glucagon, and synthetic somatostatin and volume densities were quantified on light micrographs by point-counting procedures. In both primary and secondary islets the insulin-, glucagon-, and somatostatin-containing cells comprised approximately 32%, 23%, and 38% of the endocrine cell volumes, respectively. Therefore, the cell populations did not reflect any embryological differences between the two groups of islets. In this study, the volume densities of insulin-reactive cells in the primary islet were less than previously reported, and the overall insulin staining was about one-half of that seen in mammals. The volume density of somatostatin-reactive cells in primary islets was greater than previously reported. Based on these data, primary and secondary islets of the catfish do not appear to have a similar development to the ventral and dorsal islets of the mammalian system.

In vitro study of frog (Rana ridibunda Pallas) interrenal function by use of a simplified perifusion system. VII. Lack of effect of somatostatin on angiotensin-induced corticosteroid production

Somatostatin (SRIF), the somatotropin release inhibiting factor of the hypothalamus, has been reported to inhibit the production of angiotensin II (AII)-stimulated aldosterone in the rat adrenal glomerulosa cells. Since the interrenal of the frog is the homolog of mammalian adrenal zona glomerulosa, the effect of synthetic SRIF on perifused dice of Rana ridibunda was tested. Graded doses of SRIF did not modify the spontaneous production of corticosterone and aldosterone. The highest concentration of SRIF (10(-5) M) did not alter the stimulatory effect of the AII agonist [Sar1-Val5] AII upon corticosteroidogenesis. Thus, in apparent contradiction to recent findings in mammals, SRIF did not alter the effect of AII in the frog interrenal cell.

Subcellular localization and preliminary characterization of islet hormone-degrading activities in anglerfish islet tissue

The degradation of [125I]iodoinsulin in anglerfish islet tissue was studied in a trichloroacetic acid solubilization assay system. The pH optima for insulin breakdown by acidic and neutral enzymes were determined in fish islets and compared with mammalian tissues (rat liver, pancreas, and islets of Langerhans). Two major insulinolytic activities of anglerfish islet tissue were partially characterized: 1) An acidic (pH 3.5) activity showing marked sensitivity to pepstatin, some sensitivity to antipain, leupeptin, phenylmethanesulfonyl fluoride (PMSF), and thiol proteinase inhibitors, but no inhibition by EDTA; and 2) a neutral (pH 7.3) activity showing marked sensitivity to thiol proteinase inhibitors, sensitivity to antipain and leupeptin, but no sensitivity to PMSF, EDTA, or pepstatin. Glucagonolytic activities were also observed at the same acidic and neutral pH optima. Following cell fractionation of anglerfish islet homogenates, acidic (pH 3.5) insulinolytic activities were distributed with the lysosome-rich, microsome, and cytosol fractions, whereas neutral (pH 7.3) activities were found chiefly in cytosol and microsomes. Little or no insulinolysis was observed in secretory granule fractions. The data suggest that insulin is degraded in islet tissue by at least two enzyme systems. Lysosomal insulinolysis was due principally to cathepsin D-like activity. Neutral insulinolysis, partially characterized in the cytosol fraction, was due to thiol proteinase activity. The activity profile indicates that islet tissue resembles other insulin-responsive tissues in its subcellular distribution of insulin-degrading activities. The cellular heterogeneity of islet tissue, and the presence of high concentrations of endogenous islet hormones require further efforts at purification before insulinolytic enzymes are fully characterized in this tissue.

Somatostatin-like immunoreactivity in pancreatic extracts of a sahelian lizard (Varanus exanthematicus) during starvation

In the Varanus exanthematicus, the pancreatic complex comprises the true pancreas as well as an intrasplenic islet comparable to a Brockmann's body. Somatostatin content and concentration were estimated by radioimmunoassay in acetic acid extracts of both organs. Relatively large amounts of somatostatin (SLI) are present in the pancreas (2.17 +/- 0.07 micrograms) without any difference in distribution between the cranial (CP) and mediocaudal (MCP) regions. The intrasplenic islet contains as much SLI material as the whole pancreas (3.38 +/- 0.85 microgram); thus, this primitive organ presents a very high hormonal concentration (109.34 +/- 40.30 ng/mg wt). Serial dilutions of the extracts gave parallel immunoassay displacement curves and gel-filtration revealed two immunoreactive peaks: the most important one was found in the synthetic tetradecapeptide fraction, the other one near the void volume fraction. These results show an immunological similarity between the SLI substance of the pancreatic complex and the synthetic somatostatin; however, a molecular heterogeneity must not be excluded. The results are discussed from a phylogenetic point of view.

Isolated perfused Brockmann body as a model for studying pancreatic endocrine secretion

The release of insulin, glucagon, and somatostatin was investigated in a newly developed in vitro perfusion preparation of the splenic Brockmann body (principal pancreatic islet) of the channel catfish (Ictalurus punctatus). The preparation was viable for up to 7 h by functional and morphological criteria. Glucose evoked a biphasic release of insulin and somatostatin, but had no effect on the release of glucagon. Arginine at basal glucose concentration caused a marked release of insulin and glucagon, but only a minor release of somatostatin.

An immunocytochemical and electron-microscopical study of endocrine cells in the gut and pancreas of a stomachless teleost fish, Barbus conchonius (Cyprinidae)

Four immunoreactive endocrine cell types can be distinguished in the pancreatic islets of B. conchonius: insulin-producing B cells, somatostatin-producing A1 (= D) cells, glucagon-producing A2 cells and pancreatic polypeptide-producing PP cells. The principal islet of this species contains only a few PP cells, while many PP cells are present in the smaller islets. Except for the B cell all pancreatic endocrine cell types are also present in the pancreatic duct. At least six enteroendocrine cell types are present in the gut of B. conchonius: 1. a cell type (I) with small secretory granules, present throughout the intestine, and possibly involved in the regulation of gut motility; 2. a C-terminal gastrin immunoreactive cell, probably producing a caerulein-like peptide; these cells are located at the upper parts of the folds, especially in the proximal part of the intestinal bulb; 3. a met-enkephalin-immunoreactive cell, present throughout the first segment; 4. a glucagon-immunoreactive cell, which is rare in the first segment; 5. a PP-immunoreactive cell, mainly present in the first half of the first segment; 6. an immunoreactive cell, which cannot at present be specified, located in the intestinal bulb. The latter four cell types are mostly located in the basal parts of the folds, although some PP-immunoreactive cells can also be found in the upper parts. Most if not all enteroendocrine cells are of the open type. The possible functions of all enteroendocrine cell types are discussed.

Immunocytochemical localization of hormone-producing cells within the pancreatic islets of the rainbow trout (Salmo gairdneri)

A histological study of the pancreatic islets in rainbow trout, Salmo gairdneri, was undertaken in which polypeptide hormones-producing cells were localized, using immunocytochemical staining techniques. Four different cell-types were identified in this manner. These were the insulin, somatostatin, pancreatic polypeptide and glucagon/gastric inhibitory polypeptide (GIP) cells. The glucagon/GIP cell was designated thus as antisera to both hormones cross-reacted with a common population of cells. A fifth cell-type, commonly referred to as a clear cell, was also identified although its secretory product is as yet undetermined. These functional cell types were compared to the standard tinctorial properties of pancreatic endocrine cells. The relationships of the various cell types with each other was also observed.

Cell types of the endocrine pancreas in the shark Scyliorhinus stellaris as revealed by correlative light and electron microscopy

In the pancreas of Scyliorhinus stellaris large islets are usually found around small ducts, the inner surface of which is covered by elongated epithelial cells; thus the endocrine cells are never exposed directly to the lumen of the duct. Sometimes, single islet cells or small groups of endocrine elements are also incorporated into acini. Using correlative light and electron microscopy, eight islet cell types were identified: Only B-cells (type I) display a positive reaction with pseudoisocyanin and aldehyde-fuchsin staining. This cell type contains numerous small secretory granules (diameter 280 nm). Type II- and III-cells possess large granules stainable with orange G and azocarmine and show strong luminescence with dark-field microscopy. Type II-cells have spherical (diameter 700 nm), type III-cells spherical to elongated granules (diameter 450 x 750 nm). Type II-cells are possibly analogous to A-cells, while type III-cells resemble mammalian enterochromaffin cells. Type IV-cells contain granules (diameter 540 nm) of high electron density showing a positive reaction to the Hellman-Hellerström silver impregnation and a negative reaction to Grimelius' silver impregnation; they are most probably analogous to D-cells of other species. Type V-cells exhibit smaller granules (diameter 250 x 500 nm), oval to elongated in shape. Type VI-cells contain small spherical granules (diameter 310 nm). Type VII-cells possess two kinds of large granules interspersed in the cytoplasm; one type is spherical and electron dense (diameter 650 nm), the other spherical and less electron dense (diameter 900 nm). Type VIII-cells have small granules curved in shape and show moderate electron density (diameter 100 nm). Grimelius-positive secretory granules were not only found in cell types II and III, but also in types V, VI, and VII. B-cells (type I) and the cell types II to IV were the most frequent cells; types V to VII occurred occasionally, whereas type VIII-cells were very rare.

Cloning and sequence analysis of cDNAs encoding two distinct somatostatin precursors found in the endocrine pancreas of anglerfish

Complementary DNAs for two distinct anglerfish somatostatin peptides (termed I and II) have been cloned in bacterial plasmids and sequenced. The nucleotide sequence for somatostatin I encodes a large percursor peptide (molecular weight 13,300) in which the somatostatin hormones is at the carboxyl terminus. The predicted 14-amino acid sequence for anglerfish somatostatin I is the same as mammalian somatostatin. Somatostatin II is also synthesized as part of a larger precursor (molecular weight 14,100) with the presumptive somatostatin hormone also at the carboxyl terminus. The 14-amino acid sequence of somatostatin II differs from somatostatin I at two internal residues (Tyr in place of Phe 7 and Gly in place of Thr 10). The two different somatostatins may have distinct biological activities. Homologies in the amino acid sequences of the two peptides outside the somatostatin moiety suggest other regions of the molecules have biological functions.

Vasoactive intestinal polypeptide-like immunoreactivity in nerves of the pancreatic islet of the teleost fish, Gillichthys mirabilis

In the teleost fish, Gillichthys mirabilis, vasoactive intestinal polypeptide (VIP)-like immunoreactivity is present in some nerve fibres of the principal pancreatic islet and surrounding tissues, the vagus and splanchnic nerves, the coeliac ganglion and the wall of the intestine. The nerves of the pancreatic islet that contain VIP-like immunoreactivity probably correspond to one of the two types of non-cholinergic, non-adrenergic (peptidergic) nerve previously described in this fish. Similarities in the localisation of hormonal peptides in fish and mammals suggest that the regulation of gastroenteropancreatic physiology in fish may resemble that of mammals.

Isolation and characterization of immunoreactive somatostatin from fish pancreatic islets

Using a radioimmunoassay with labeled synthetic tetradecapeptide somatostatin, a large amount of immunoreactive somatostatin was found in the principal pancreatic islet of the channel catfish (Ictalurus punctata). The purpose of these experiments was to isolate and characterize the somatostatin-like material. Extracts of islets were chromatographed on a Bio-Gel P-30 column, and over 90% of the immunoreactive somatostatin migrated with proteins at least twice the size of synthetic tetradecapeptide somatostatin. This fraction was further purified by ion-exchange chromatography on carboxymethyl-cellulose and DEAE-cellulose columns. Two peptides were obtained with identical immunoreactivity, which was approximately 25% that of the synthetic somatostatin. Each peptide was judged to be >95% pure by thin-layer electrophoresis, polyacrylamide gel electrophoresis at pH 8.9, and highpressure liquid chromatography. Further criteria of purity included amino-terminal analysis of fraction IV yielding only aspartic acid. A total of 1.3 mg of fraction II, and 3.8 mg of fraction IV somatostatin-like peptides were obtained from 10 g of fresh frozen islets. Characterization of the two peptides revealed both peptides slightly more acidic than synthetic tetradecapeptide somatostatin. Fraction II had an isoelectric point of 8.0-8.3, and fraction IV 8.3-9.0. Molecular weight estimation by sodium dodecyl sulfate-urea polyacrylamide gel electrophoresis revealed similar mobility of both peptides, between pancreatic polypeptide (mol wt 4,500) and glucagon (mol wt 3,500). The mobility was not altered by reduction, and was approximately twice the size of synthetic tetradecapeptide somatostatin (mol wt 1,800). This confirmed that the peptides were single polypeptide chains and not aggregates, or somatostatin bound to larger proteins. Molecular weight determination by gel filtration chromatography on Bio-Gel P-6 in 8 M urea gave an estimated mol wt of 3,700. Amino acid analysis of the two immunoreactive somatostatins indicated that they were very similar in composition. Both pancreatic somatostatins (1 muM) had full biological activity relative to synthetic somatostatin measured as inhibition of growth hormone release from rat anterior pituitary cells.

Distribution and morphometric quantitation of pancreatic endocrine cell types in the frog, Rana pipiens

Cells reactive to anti-anglerfish insulin, anti-porcine glucagon, anti-synthetic somatostatin, and anti-bovine pancreatic polypeptide were identified in adult Rana pipiens male pancreases using peroxidase anti-peroxidase immunohistochemistry. Insulin positive cells are columnar shaped and arranged in cords. Glucagon positive and somatostatin positive cells are located around the core of insulin positive cells. Isolated cells and clusters of cells of only one cell type are also found. Adjacent sections stained with anti-glucagon and anti-bovine pancreatic polypeptide showed that glucagon positivity and pancreatic polypeptide positivity are found in the same cells. Comparison of double stained adjacent sections confirmed the presence of these two antigens in the same cells, and further showed the occasional presence of cells which are positive to only glucagon or pancreatic polypeptide. Staining of rat pancreas with these two antisera showed that glucagon and pancreatic polypeptide are present in two distinct cell populations. Morphometric quantitation of immunohistochemically stained sections of Rana pipiens pancreases showed that about 2% of the pancreas is endocrine tissue. Of this, 43% is comprised of insulin positive cells, and 43% is occupied by glucagon-pancreatic polypeptide positive cells. Somatostatin positive cell occupy about 14% of the total islet volume. The presence of glucagon and pancreatic polypeptide in the same cell population in the frog, but in different cell populations in mammals, may reflect special functional adaptation in this species, or a close relation of these two hormones and their cells of production during evolution.

Pancreatic endocrine cells of Barbus conchonius (Teleostei, Cyprinidae), and their relation to the enteroendocrine cells

The pancreatic endocrine cells of Barbus conchonius are concentrated in a large (principal) islet, located near the gall bladder, and in a number of smaller islets. Five types of endocrine cells can be distinguished in there pancreatic islets: B cells, A1 (or D cells), 2 types of A2 cells (A2r cells with round granules; and A2fl cells with flocculent granules) and a scarce 5th cell type. The hormones produced by B and A2fl cells are probably insulin and glucagon respectively. The A2r cell contains granules with the same diameter as the granules of the enteroendocrine type III cell of the gut. Both cell types may resemble the enteroglucagon-producing EG cell of mammals. The function of the A1 cells, which are frequently found without secretory granules, and of the 5th cell type, will be discussed. The pancreastic islets of B. conchonius are strongly innervated, which suggests thatpresence of a direct nervous control system. Some intermediate or mixed cells containing exocrine and endocrine A2r granules are found continguous with the principal islet. The origin of pancreatic endocrine cells is also the subject of discussion.

Use of immunocytochemical staining of somatostatin for correlative light and electron microscopic investigation of D cells in the pancreatic islet of Xiphophorus helleri H. (Teleostei)

Somatostatin-containing cells have been demonstrated by immunocytochemistry in semithin sections of the pancreatic islet of the teleost fish, Xiphophorus helleri. These cells were shown by correlative light and electron microscopy to be identical with D cells previously defined in this species by the silver impregnation method of Hellman and Hellerström.

Localization of four polypeptide hormones in the saurian pancreas

Glucagon, insulin, somatostatin, and pancreatic polypeptide have been localized in the anolian pancreas using peroxidase-antiperoxidase immunocytochemistry. The most abundant endocrine cell type contains glucagon. Insulin-containing cells are the next most numerous. Somatostatin-immunoreactive cells tend to be localized at the periphery of the islet cords. Pancreatic polypeptide-containing cells are a minor endocrine component scattered throughout the exocrine pancreas and occasionally within the islet areas. No staining was observed after application of antigastrin serum.