The present study was designed to understand how carbohydrate (CBH) and protein metabolism
are related in the penaeid shrimp Litopenaeus vannamei. With this information, we
obtained a comprehensive schedule of the protein-carbohydrate metabolism including
enzymatic, energetic, and functional aspects. We used salinity to determine its role
as a modulator of the protein-carbohydrate metabolism in shrimp. Two experiments were
designed. The first experiment evaluated the effect of CBH-salinity combinations in
growth and survival, and hemolymph glucose, protein, and ammonia levels, digestive
gland glycogen, osmotic pressure, and glutamate dehydrogenase (GDH) of L. vannamei
juveniles acclimated during 18 days at a salinity of 15 per thousand and 40 per thousand.
The second experiment was done to evaluate the effect of dietary CBH level on pre-
and postprandial oxygen consumption, ammonia excretion, and the oxygen-nitrogen ratio
(O/N) of juvenile L. vannamei in shrimps acclimated at 40 per thousand salinity. We
also evaluated the ability of shrimp to carbohydrate adaptation. We made phosphoenolpyruvate
carboxykinase (PECPK) and hexokinase activity measurements after a change in dietary
carbohydrate levels at different times during 10 days. The growth rate depended on
the combination salinity-dietary CBH-protein level. The maximum growth rate was obtained
in shrimps maintained at 15 per thousand salinity and with a diet containing low CBH
and high protein. The protein in hemolymph is related to the dietary protein levels;
high dietary protein levels produced a high protein concentration in hemolymph. This
suggests hemolymph is able to store proteins after a salinity acclimation. Depending
on the salinity, the hemolymph proteins could be used as a source of osmotic effectors
or as metabolic energy. The O/N values obtained show that shrimp used proteins as
a source of energy, mainly when shrimps were fed with low CBH. The role played by
postprandial nitrogen excretion (PPNE) in apparent heat increase (AHI) (PPNE/AHI ratio)
is lower in shrimps fed diets containing high CBH in comparison with shrimps fed diets
containing low CBH levels. These results confirm that the metabolism of L. vannamei
juveniles is controlled by dietary protein levels, affecting the processes involved
in the mechanical and biochemical transformations of ingested food. A growth depression
effect was observed in shrimps fed with low-CBH protein diets and maintained in 40
per thousand salinity. In these shrimps, the hemolymph ammonia concentration (HAC)
was significantly higher than that observed in shrimps fed with low CBH and maintained
in 15 per thousand salinity. That high HAC level coincided with lower growth rate,
which suggests that this level might be toxic for juveniles of L. vannamei. Results
obtained for GDH activity showed this enzyme regulated both HAC and hemolymph protein
levels, with high values in shrimps fed with low CBH levels and maintained in 40 per
thousand salinity, and lower in shrimps fed with high CBH and maintained in 15 per
thousand salinity. These differences mean that shrimp with a high-gill GDH activity
might waste more energy in oxidation of the excess proteins and amino acids, reducing
the energy for growth. It was evident that L. vannamei can convert protein to glycogen
by a gluconeogenic pathway, which permitted shrimp to maintain a minimum circulating
glucose of 0.34 mg/ml in hemolymph. A high PECPK activity was observed in shrimps
fed a diet containing low CBH level indicating that the gluconeogenic pathway is activated,
as in vertebrates by low dietary CBH levels. After a change in diet, we observed a
change in PEPCK; however, it was lower and seems to depend on the way of adaptation,
because it occurred after 6 days when adapting to a high-CBH diet and with little
change for the low-CBH diet.