Plasma Glucose Level and Insulin-Like Growth Factor-I (IGF-I) mRNA Expression in Chronically Stressed Nile Tilapia (Oreochromis Niloticus L.) Reared Under Sub-optimal Stocking Densities

Lito  S. Guzman,; Emmanuel  M. Vera Cruz,

Plasma Glucose Level and Insulin-Like Growth Factor-I (IGF-I) mRNA Expression in Chronically Stressed Nile Tilapia (Oreochromis Niloticus L.) Reared Under Sub-optimal Stocking Densities

Abstract:

The study evaluated the growth and stress response of Nile tilapia (Oreochromis niloticus L.) to suboptimal rearing densities. A stock of juvenile all male tilapia weighing 65 grams was used in the study. The experimental fish were conditioned for one week at stocking density of 3·m-3 in 300·m2 net enclosure; then reared in 1m x 1m cages installed in pond for three months at stocking densities of 75·m-3 and 150·m-3. The results revealed that 75.m-3 was significantly higher (P<0.05) than 150.m-3 with respect to the specific growth rate (SGR). Also, blood plasma glucose (GLU) was significantly higher (P<0.05) in the stocking density of 150.m-3 than that of 75.m-3. On the contrary, Insulin-like growth factor (IGF-I) mRNA expression was comparable between the two density classes. With respect to their temporal trend, the growth and GLU increased consistently with time while IGF-I increased only from month 1 to month 2; and partly declined on the third month. A significantly positive relationship also exists between the two variables in which GLU increased logarithmically with IGF-I. Their functional relationship is written as Y=27.21ln(x) + 66.134. In like manner, a positive linear relationship; that is, Y= 0.0169(x) + 0.54490, was also observed between SGR and IGF-I. Conclusively, the stocking density of 150.m-3 has been shown to be more chronically stressful than 75.m-3 as evident by the relatively lower SGR and higher GLU observed in the former. Although the expression of IGF-I mRNA was not directly influenced by the stocking density, its declining expression with increasing GLU at the end of the culture period is attributed more on GH mediation and its positive linear relationship with SGR proved its utility as indicator of growth in this species. The result demonstrated in the present study also proved the potential utility of GLU as stress biomarker and that its mechanism of secretion is directly influenced by the expression of the IGF-I mRNA via paracrine route and indirectly via negative feedback mechanisms of GH/IGF-I axis.