Search

Cholecystokinin (CCK) is an important regulator of pancreatic enzyme secretion in adult mammals and teleosteans. Although some studies have focused on the interaction between CCK and trypsin in marine fish larvae, little is known about the circadian patterns of the regulatory mechanism involving these two digestive components. In this study, we took advantage of the characteristic change from a diurnal to a nocturnal feeding habit that occurs in Senegalese sole (Solea senegalensis) post-larvae, to conduct an experiment where larvae and postlarvae were submitted to three different feeding regimes from mouth opening: continuous feeding, diurnal feeding and nocturnal feeding. The aim was to establish different daily feeding scenarios to uncover the operating mechanisms of CCK and tryptic enzyme activity over the 24-hour cycle to better understand the regulation of digestion in developing fish larvae. Results show a prevalence of simultaneous and opposing trends of CCK level and tryptic activity as a function of the postprandial time. This finding supports the existence of a regulatory loop between these two digestive components in pre- and post-metamorphic Senegal sole larvae. In addition, CCK level was also modulated by the gut content, tending to be lower when the gut is full and higher when is being emptied. Furthermore, larvae were able to synchronize digestive functions to very different feeding regimes, although it seems to be important having a diurnal feeding phase during pre-metamorphic stages for a proper development., This research was funded by the Spanish Ministry of Economic Affairs and Competitiveness (MINECO) by project RIDIGEST (AGL2011-23722) with FEDER/ERDF contribution granted to M. Yúfera. C. Navarro-Guillén was supported by a doctoral fellowship (BES-2012-051956) from MINECO (Spain).

The present study was aimed to establish the relationship between the functionality of the digestive gland and physiological rates including SFG (scope for growth) in wild mussels, Mytilus galloprovincilis. The experimental set-up consisted in the evaluation of changes in the morphology of the gland, as well as in the activity of some key digestive enzymes (amylase, laminarinase, cellulase and protease) within a broad range of SFG obtained through manipulation of food ration. The higher SFG values were correlated to an increase in both the size of the digestive gland and the activities of enzymes when expressed in relation to individual. In contrast, no clear relations were observed when the activity of enzymes was expressed in relation to soluble protein, with the exception to amylase. The higher protease activities measured in mussels showing lower SFG may reflect an initial stage of catabolic processes intended to compensate the energy deficit produced by food restriction. The potential use of parameters measured in digestive glands in studies of marine pollution was discussed.

Hydrolysis of three different proteins by either crude fish digestive extracts or purified mammal proteases was assayed using two different in vitro systems. The closed system was a modification of the pH-stat method including a previous acid digestion. The open system used a digestion cell containing a semi-permeable membrane which allowed continuous separation of the final products of hydrolysis with a molecular cut-off of 1000 Da. Assays in both systems resulted a similar arrangement of the tested proteins in relation to their ability to be hydrolyzed, with casein>fish meal> or =soybean meal. With the exception of casein, no significant differences were found between results produced by any of the enzyme sources using the closed system. In constrast, significantly higher hydrolysis of all proteins was produced by mammal enzymes under conditions operating in the open system. Differences in the rate of release of amino acids measured in this latter system were related both to the type of protein and the origin of the enzymes. When using purified mammal enzymes, release of lysine or phenylalanine from casein and soybean was high, but low from fishmeal. Isoleucine and valine present in fishmeal were preferentially hydrolyzed by commercial enzymes, but glycine and proline by fish enzymes.