Expression and activities of pancreatic enzymes in developing sea bass larvae (Dicentrarchus labrax) in relation to intact and hydrolyzed dietary protein; involvement of cholecystokinin
In order to assess the influence of dietary protein on digestive enzyme expression and cholecystokinin (CCK) content in sea bass larvae, four groups of larvae were fed experimental diets from mouth opening until day 42: three isonitrogenous diets with increasing protein hydrolysate levels (0%, 14% and 46% of crude matter) and one diet incorporating starch. The groups fed high starch or high protein hydrolysate level exhibited the lowest growth. The final weight in these groups was 9.5 and 5.6 mg, respectively, whereas it reached approximately 20.0 mg in the groups fed 0% or 14% protein hydrolysate level. The highest levels of trypsin secretion were observed in larvae fed the lowest protein hydrolysate level. Paradoxically, the groups fed diet containing starch also exhibited a high trypsin secretion level. There was a clear allometric relationship between larval CCK content and body mass, but there were also differences between dietary groups. On day 42, the CCK level in the group fed diet incorporating starch (40 fmol/mg dry weight of larvae) was more than twice as high as that found in the other groups. The lowest CCK level (13 fmol/mg) was found in the group fed the highest protein hydrolysate level. Our data suggested that dietary protein level and chain length combined with protein intraluminal proteolytic activity regulate the CCK level in fish larvae as in other vertebrates. The CCK concentration assayed in larvae fed diets with a low protein content or different protein hydrolysate levels is compatible with the existence of an indirect mechanism controlling CCK release and mediating pancreatic enzyme secretion.
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