Stimulation of intestinal lipase secretion by porcine cholecystokinin in the hagfish, Eptatretus stouti

The good, the bad and the slimy: experimental studies of hagfish digestive and nutritional physiology

The hagfishes provide valuable insight into the physiology of feeding, digestion and nutrient absorption by virtue of unusual and unique features of their biology. For example, members of this group undergo long periods of fasting, and are the only vertebrates known to absorb organic nutrients across their epidermal surface. Such properties engender significant attention from researchers interested in feeding and feeding-related processes; however, the practical realities of employing the hagfish as an experimental organism can be challenging. Many of the key tools of the experimental biologist are compromised by a species that does not readily feed in captivity, is difficult to instrument and which produces copious quantities of slime. This Commentary provides critical insight into the key aspects of hagfish feeding and digestive processes, and highlights the pitfalls of this group as experimental organisms. We also suggest key research gaps that, if filled, will lead to better understanding of hagfishes, and we consider how this group may advance our knowledge of feeding, digestion and nutrient absorption processes.

Activity and post-prandial regulation of digestive enzyme activity along the Pacific hagfish (Eptatretus stoutii) alimentary canal

Hagfishes are living representatives of the earliest-diverging vertebrates and are thus useful for the study of early vertebrate physiology. It has been previously postulated that digestive enzymes account for the majority of digestion because hagfish are agastric with notable zymogen granules in specialized cells of the hindgut. While the presence of some digestive enzymes (amylase, lipase and leucinaminopeptidase) have been confirmed with histochemistry, quantification of enzymatic activity is limited. This study sought to biochemically quantify the tissue activity of six digestive enzymes (α-amylase, maltase, lipase, trypsin, aminopeptidase and alkaline phosphatase) along the length of the Pacific hagfish (Eptatretus stoutii) alimentary canal. In addition, the effect of feeding on the rate of enzyme activity was examined. Overall, maltase and trypsin activities were unchanging with respect to location or feeding status, while the activities of α-amylase and alkaline phosphatase decreased substantially following feeding, but were consistent along the length. Lipase and aminopeptidase activities were elevated in the anterior region of the alimentary canal in comparison to the more posterior regions, but were not altered with feeding. This study indicates hagfish have an assortment of digestive enzymes that likely are the result of a varied diet. The differential expression of these enzymes along the tract and in regards to feeding may be indications of early compartmentalization of digestive function.

Hormonal regulation of the fish gastrointestinal tract

The gastrointestinal tracts (GIT) of fish and other vertebrates are challenged with a diversity of functional demands caused by changes and differences in dietary inputs and environmental conditions. This contribution reviews how hormonal regulation plays an essential role in modulating the GIT functions of fish to match changes in functional demands. Exemplary is how hormones produced by the GIT, the associated organs (e.g., pancreas), and other sources (e.g., hypothalamus, adrenal cortex, thyroid, gonads) modulate the digestive processes (motility, secretion, and nutrient absorption) in response to dietary inputs. Hormones regulate the other GIT functions of osmoregulation (secretion and absorption of electrolytes and water), immunity, endocrine secretions, metabolism, and the elimination of toxic metabolites and environmental contaminants to match changes in environmental conditions and physiological states. Although the regulatory molecules and associated signaling pathways have been conserved during evolution of the vertebrate GIT, the specific responses often vary among fish with different feeding habits and from different environments, and can differ from those described for mammals.

Gut hormones in cyclostomes

The current state of knowledge about regulatory peptides in endocrine cells and nerves of the alimentary canal of lampreys and hagfishes is reviewed. Cyclostomes have a wide range of peptides similar immunochemically to those of higher vertebrates. They include, in the endocrine cells of the intestine, peptides resembling glucagon, gastrin/cholecystokinin, peptide YY (pancreatic polypeptide/neuropeptide Y), substance P, vasoactive intestinal peptide, somatostatin and, in the larval stages at least, insulin. The enteric nerves of some lamprey species contain peptides resembling bombesin (gastrin-releasing peptide) and calcitonin gene-related peptide, as well as serotonin. The occurrence of other peptides is less well documented.Little is known of the molecular structure or the biological roles of the enteric peptides in cyclostomes. Extraction, purification, sequencing and physiological experiments are greatly needed.

Gastrin/CCK-like immunoreactivity in Hatschek's groove of Branchiostoma lanceolatum (Cephalochordata)

Gastrin/CCK-like immunoreactivity was demonstrated in a specific group of epithelial cells in Hatschek's groove. Positive reactions were obtained with antisera directed against the C-terminal amino acid sequence common to mammalian CCK and gastrin, but not with antisera against the midportions of these hormones. No immunoreactivity was obtained with antisera against peptides from hypophysis, neuroendocrine peptides in the gastrointestinal tract, islet hormones, calcitonin, NSE, and S-100. The function of the gastrin/CCK-like peptide found in Hatschek's groove is discussed.

Effects of cholecystokinin peptides on digestive enzymes in killifish in vivo

1. The abilities of cholecystokinin-like peptides to elicit lipase secretion were examined in the stomachless killifish, Fundulus heteroclitus. 2. Cholecystokinin octapeptide, caerulein, and nonsulfated caerulein stimulated lipase secretion in vivo, with caerulein and nonsulfated caerulein being the most potent peptides tested. 3. Lipase secretion was not induced by carbachol, and peptide stimulated lipase secretion was not inhibited by atropine. 4. These data suggest digestive enzyme secretion constitutes an evolutionary primitive role for CCK.

Comparative studies on the effects of cholecystokinins, caerulein, bombesin 6-14 nonapeptide, and physalaemin on gastric secretion in the ascidian Styela clava

The effects of cholecystokinins (CCK), caerulein, bombesin 6-14 nonapeptide (bombesin), and physalaemin on gastric secretion in Styela clava were measured using a perfusion technique. Varying concentrations of both CCK33 and CCK8 produced a significant dose-dependent response. Dose for dose, CCK33 was more potent than CCK8, while the assay was unable to show any discrimination between sulphated and nonsulphated forms of CCK8. The specific CCK inhibitor Bt2cGMP significantly reduced the response to both CCK33 and CCK8. Both caerulein and physalaemin were effective although with a considerably reduced response compared with CCK. Bombesin was the most potent of all secretagogues tested while glucagon was without effect on gastric secretion. It is suggested that the primitive prepancreatic zymogen cells in Styela possess a receptor or receptors with an ability to recognize those peptides which are also active on vertebrate pancreatic acinar cells. It is further suggested that while the results indicate a receptor system less sophisticated than that found in vertebrates, they also imply the presence of an endogenous polypeptide hormone or hormones with a sequence which might be expected to show similarities to more than one vertebrate gastrointestinal peptide.

Inhibition of gastric acid secretion inthe Atlantic cod, Gadus morhua, by sulphated and desulphated gastrin, caerulein, and CCK-octapeptide

Gastric acid secretory effects of gastrin/CCK-like peptides have been assayed in cods rendered "spontaneously" secreting by a continuous intestinal perfusion with diluted (33%) seawater. A high dose of pentagastrin induced a weak stimulation (31%) of acid output, while gastrin 17-II, caerulein and CCK8 were inhibitory. Caerulein was the most potent peptide, with an estimated D50 for inhibition of 0.013 nmol/kg.h. Although displaying lower potencies, also the desulphated forms of gastrin-17, caerulein and CCK8 were inhibitory. The results may be explained by release of an endogenous inhibitor, or by interaction with endogenous "codfish gastrin". In the latter case two alternatives are considered: Either gastrin 17, CCK8, and caerulein possess lower efficacies than "codfish gastrin" and therefore act as partial agonists. Alternatively, "codfish gastrin" is itself an inhibitory principle (gastron), the effect of which is mimicked by gastrin 17, caerulein and CCK8. The actions of gastrin and the gastrin-like peptides in the cod indicate a structure-activity relationship different from previously described systems, both mammalian and submammalian.