An
energetic and conceptual model of the physiological role of dietary
carbohydrates and salinity on Litopenaeus vannamei juveniles
C.
Rosas, G. Cuzon, G. Gaxiola, C. Pascual, G. Taboada, L. Arena, A. van
Wormhoudt-2002
Journal of Experimental Marine
Biology and Ecology, 268(1): 47-67
Abstract:
We
are reporting results directed to explain the relation between carbohydrates
(CHO), protein metabolism, and the energetic balance of Litopenaeus
vannamei juveniles. The interaction of dietary CHO and salinity was
measured to try to understand the relation between osmotic control and
metabolism, both from a biochemical and energetic point of view. Two
experiments were done. In the first experiment, shrimp were fed with 0%, 5%,
33%, and 61% CHO and maintained at 15 ppt and 40 ppt salinity. Glucose,
lactate protein, hemocyanin, ammonia concentration, and osmotic pressure
were measured in blood. Digestive gland glycogen (DGG) was measured also. In
the second experiment, shrimp were fed with 0% and 38% dietary CHO and
maintained at 15 ppt and 40 ppt salinity. From that shrimp, absorbed energy
(Abs) was calculated as: Abs=respiration (R)+ammonia excretion (U)
and production (P); assimilated energy (As) was calculated as the
product of R×P. Osmotic pressure, hemocyanin, protein,
lactate, and blood ammonia increased with the reduction in dietary CHO. In
contrast, an increase in blood glucose was observed with an increase in
dietary CHO. Digestive gland glycogen (DGG) increased following a saturation
curve with a DGG maximum at 33% dietary CHO. Blood metabolites of fasting
and feeding shrimp showed the same behavior. Energy balance results showed
that shrimp maintained in low salinity and fed without CHO waste more energy
in U production than for shrimp maintained in high salinity and fed with
high CHO levels. Notwithstanding, the production efficiency was higher in
shrimp fed without CHO than that observed in shrimp fed with high CHO
independent of salinity. A scheme trying to integrate the relation between
CHO and protein metabolism and the way in which both are modulated by
salinity is presented. From published and present results, there are two
factors that apparently control the use of high dietary CHO levels; α-amylase
enzyme-dietary CHO level capacity and glycogen saturation in DG. Production
of glucose is limited in shrimp because of saturation of α-amylase when
shrimp are fed with diets above 33% CHO. This is the first control point of
starch metabolism. The digestive gland is saturated with glycogen in shrimp
fed with dietary CHO levels >33%. This is apparently the second control
point of CHO metabolism that limits growth rate in such conditions. The high
metabolic cost related to high CHO diets could explain why shrimp are well
adapted to use protein as a source of energy.