Physiological, nutritional, and immunological role of dietary beta 1-3 glucan and ascorbic acid 2-monophosphate in Litopenaeus vannamei juveniles

摘要

The present study was designed to determine the effect of dietary beta 1-3 glucan (BG) and a megadose of vitamin C on the nutrition (growth and survival, blood glucose, lactate, acylglycerides, cholesterol, hemocyanin [He], digestive gland glycogen [DGG]) and the immunological system (blood cells, prophenoloxidase [ProPO]) in Litopenaeus vannamei juveniles. Each treatment was replicated five times. Nutritional condition and immunological response of shrimp were recorded after 40 days of feeding with specific diets and during 48 h after a salinity shock (35-0parts per thousand). A significantly greater growth rate was observed in shrimp fed with BG or vitamin. C diets than in the control group. Higher blood protein, total blood cells, granular cells, and ProPO activity were recorded in shrimp fed with vitamin C as compared to the remaining treatments. That means that BG was degraded in the digestive gland by beta-glucanases to produce energy, permitting the use of more proteins for growth, whereas vitamin C was used to improve animal health, enhancing general metabolism in shrimp. The salinity stress induced a rapid use of reserves, triacylglycerols (TAGs), and cholesterol were reduced in blood and probably used as a source of energy or synthesis in the digestive gland. The increase in digestive gland glycogen 24 h after the salinity shock evidenced the use of these reserves by shrimp fed with all diets. In general, after the salinity, shock blood cells increased in shrimp fed with glucans and decreased in shrimp fed with vitamin C, whereas ProPO decreased in all shrimp after the salinity shock. This could mean that after the salinity, shock shrimp fed with glucans could synthesize cells and ProPO, whereas in shrimp fed with vitamin C, blood cells were just used to respond to the stress. After the salinity shock, an increment in the ProPO/granular cell ratio was observed in shrimp fed with vitamin C, indicating that these shrimp could rapidly increase the components of their immune system. In contrast, a continuous reduction in ProPO/granular cell ratio was observed in shrimp fed with glucans, revealing that with this type of immunostimulant, shrimp drive their immunological equipment to respond continuously to the stress. These results could explain why shrimp fed with immunostimulants presents immunological fatigue in contrast to shrimp fed with additives that improve their nutritional status, like vitamin C.