Effects of exogenous cholecystokinin and gastrin on the secretion of trypsin and chymotrypsin from yellowtail (Seriola quinqueradiata) isolated pyloric caeca

First evidence for the presence of amino acid sensing mechanisms in the fish gastrointestinal tract

This study aimed to characterize amino acid sensing systems in the gastrointestinal tract (GIT) of the carnivorous fish model species rainbow trout. We observed that the trout GIT expresses mRNAs encoding some amino acid receptors described in mammals [calcium-sensing receptor (CaSR), G protein-coupled receptor family C group 6 member A (GPRC6A), and taste receptors type 1 members 1 and 2 (T1r1, T1r2)], while others [taste receptor type 1 member 3 (T1r3) and metabotropic glutamate receptors 1 and 4 (mGlur1, mGlur4)] could not be found. Then, we characterized the response of such receptors, as well as that of intracellular signaling mechanisms, to the intragastric administration of l-leucine, l-valine, l-proline or l-glutamate. Results demonstrated that casr, gprc6a, tas1r1 and tas1r2 mRNAs are modulated by amino acids in the stomach and proximal intestine, with important differences with respect to mammals. Likewise, gut amino acid receptors triggered signaling pathways likely mediated, at least partly, by phospholipase C β3 and β4. Finally, the luminal presence of amino acids led to important changes in ghrelin, cholecystokinin, peptide YY and proglucagon mRNAs and/or protein levels. Present results offer the first set of evidence in favor of the existence of amino acid sensing mechanisms within the fish GIT.

Daily rhythms of intestinal cholecystokinin and pancreatic proteases activity in Senegalese sole juveniles with diurnal and nocturnal feeding

The influence of diurnal and nocturnal feeding on daily rhythms of gut levels of cholecystokinin (CCK) and the activity of two key pancreatic proteases, trypsin and chymotrypsin, were examined in juveniles of Senegalese sole (Solea senegalensis), a species with nocturnal habits. Four feeding protocols were performed: P1) One morning meal; P2) Six meals during the light period; P3) Six meals during the dark period; and P4) 12 meals during 24 h. Daily activity patterns of both proteases were remarkably similar and showed a high correlation in all the experimental protocols. In P1, daily patterns of CCK and digestive enzymes showed a single maximum. In P2, CCK levels exhibited two peaks. Digestive enzymes activities showed slightly delayed peaks compared to CCK, although their daily fluctuations were not significant. In P3, intestinal CCK concentration exhibited two peaks at the end of light and dark periods, but only the second one was significant. The first maximum level of chymotrypsin activity occurred 4 h after the first CCK peak, while the second one coincided with the second CCK peak. Fluctuations of trypsin activity were not significant. In P4, CCK concentration showed three small peaks. Digestive enzymes daily fluctuations were not significant, although they showed an inverted trend with respect to CCK. The daily pattern of the gut CCK content in our study is in agreement with the anorexigenic function of this hormone. Our results support the existence of a negative feedback regulatory loop between CCK and pancreatic proteolytic enzymes in Senegalese sole juveniles.

Characterization and ontogenetic development of digestive enzymes in Pacific bluefin tuna Thunnus orientalis larvae

The major digestive enzymes in Pacific bluefin tuna Thunnus orientalis larvae were characterized, and the physiological characteristics of the enzymes during early ontogeny were clarified using biochemical and molecular approaches. The maximum activity of trypsin (Try), chymotrypsin (Ct) and amylase (Amy) was observed at pH 6-11, 8-11 and 6-9, respectively. Maximum activity of Try, Ct and Amy occurred at 50 °C, that of lipase (Lip) was at 60 °C and that of pepsin (Pep) was at 40-50 °C. These pH and thermal profiles were similar to those for other fish species but differed from those previously reported for adult bluefin tuna. Enzyme activity for all enzymes assayed was found to decrease at high temperatures (Try, Ct, Amy and Pep: 50 °C; Lip: 40 °C), which is similar to findings for other fish species with one marked exception-increased Try activity was observed at 40 °C. Lip activity appeared to be dependent on bile salts under our assay conditions, resulting in a significant increase in activity in the presence of bile salts. Ontogenetic changes in pancreatic digestive enzymes showed similar gene expression patterns to those of other fish species, whereas marked temporal increases in enzyme activities were observed at 10-12 days post hatching (dph), coinciding with previously reported timing of the development of the pyloric caeca in bluefin tuna larvae. However, complete development of digestive function was indicated by the high pep gene expression from 19 dph, which contradicts the profile of Pep activity and previously reported development timing of the gastric gland. These findings contribute to the general knowledge of bluefin tuna larval digestive system development.

Trypsin isozymes in the lobster Panulirus argus (Latreille, 1804): from molecules to physiology

Trypsin enzymes have been studied in a wide variety of animal taxa due to their central role in protein digestion as well as in other important physiological and biotechnological processes. Crustacean trypsins exhibit a high number of isoforms. However, while differences in properties of isoenzymes are known to play important roles in regulating different physiological processes, there is little information on this aspect for decapod trypsins. The aim of this review is to integrate recent findings at the molecular level on trypsin enzymes of the spiny lobster Panulirus argus, into higher levels of organization (biochemical, organism) and to interpret those findings in relation to the feeding ecology of these crustaceans. Trypsin in lobster is a polymorphic enzyme, showing isoforms that differ in their biochemical features and catalytic efficiencies. Molecular studies suggest that polymorphism in lobster trypsins may be non-neutral. Trypsin isoenzymes are differentially regulated by dietary proteins, and it seems that some isoenzymes have undergone adaptive evolution coupled with a divergence in expression rate to increase fitness. This review highlights important but poorly studied issues in crustaceans in general, such as the relation among trypsin polymorphism, phenotypic (digestive) flexibility, digestion efficiency, and feeding ecology.

Molecular cloning and tissue distribution of cholecystokinin-1 receptor (CCK-1R) in yellowtail Seriola quinqueradiata and its response to feeding and in vitro CCK treatment

In vertebrates, the peptide cholecystokinin (CCK) is one of the most important neuroregulatory digestive hormones. CCK acts via CCK receptors that are classified into two subtypes, CCK-1 receptor (CCK-1R; formally CCK-A) and CCK-2 receptor (formally CCK-B). In particular, the CCK-1R is involved in digestion and is regulated by CCK. However, very little information is known about CCK-1R in fish. Therefore, we performed molecular cloning of CCK-1R cDNA from the digestive tract of yellowtail Seriola quinqueradiata. Phylogenetic tree analysis showed a high sequence identity between the cloned yellowtail CCK receptor cDNA and CCK-1R, which belongs to the CCK-1R cluster. Furthermore, the expression of yellowtail CCK receptor mRNA was observed in gallbladder, pyloric caeca, and intestines, similarly to CCK-1R mRNA expression in mammals, suggesting that the cloned cDNA is of CCK-1R from yellowtail. In in vivo experiments, the CCK-1R mRNA levels increased in the gallbladder and pyloric caeca after feeding, whereas in vitro, mRNA levels of CCK-1R and digestive enzymes in cultured pyloric caeca increased by the addition of CCK. These results suggest that CCK-1R plays an important role in digestion stimulated by CCK in yellowtail.

Cholecystokinin: molecular cloning and immunohistochemical localization in the gastrointestinal tract of larval red drum, Sciaenops ocellatus (L.)

The current study sought to clarify the role of cholecystokinin (CCK) in the digestion of larval red drum (Sciaenops ocellatus) in order to better characterize the processes limiting the utilization of microparticulate diets at first feeding. The red drum CCK cDNA, isolated from adult anterior intestine and pyloric caeca, contains a 414 base pair (bp) open reading frame encoding a deduced amino acid sequence of 138 residues which is highly similar to preprocholecystokinin from other vertebrates. The mature CCK octapeptide has the same amino acid sequence as that found in mammals and in Atlantic herring (Clupea harengus). Tissue distribution analysis of adult and juvenile red drum using primers specific for red drum CCK mRNA revealed bright bands in samples from the brain, pyloric caeca, anterior intestine, and gonad with fainter bands seen in all other tissues. Immunohistochemical analysis of larval red drum showed that CCK-immunoreactive (CCK-IR) cells were present as early as 3 days post hatch (DPH) in some fish and were present in all fish by 6 DPH. CCK-IR cells were found in the anterior midgut in early larvae and had spread to the first bend of the gut by day 6. In older larvae (18+ DPH), CCK-IR cells were found in large numbers in the anterior intestine and in the developing pyloric caeca. The sequence and distribution of CCK mRNA along with the presence of CCK-IR cells in early red drum larvae suggest that CCK is present and may be capable of regulating pancreatic secretion in early red drum larvae.

Changes in cholecystokinin and peptide Y gene expression with feeding in yellowtail (Seriola quinqueradiata): relation to pancreatic exocrine regulation

In fish, the regulation of digestive enzyme secretion by hormonal control such as cholecystokinin (CCK) and neuropeptide Y (NPY)-related peptide is not well understood. To investigate the roles of fish CCK and peptide Y (PY) in digestive enzyme secretion, mRNA levels of CCK and PY, pyloric caeca enzyme activities and mRNA levels of pancreatic digestive enzymes (lipase, trypsin and amylase) were measured at pre- and post-prandial stages in yellowtail. Pyloric caeca were sampled at 0, 0.5, 1.5, 3, 6, 12, 24 and 48 h after feeding. The mRNA levels of trypsin and amylase increased after feeding, suggesting that transcription was induced by feed ingestion. Digestive enzyme activities decreased in exocrine pancreas after feeding, suggesting the stored enzyme was secreted from pancreas post-prandially. mRNA levels for CCK displayed a time-dependent increase, peaking between 1.5 and 3 h after-feeding followed by a rapid decrease 3 to 6 h after feeding. The mRNA expression pattern of PY was inverse to the pattern of CCK, decreasing until 1.5 h after feeding and then rising to initial levels by 12 h after feeding. These results suggest that CCK and PY work antagonistically in the exocrine pancreas of yellowtail.