Assessment of Growth and Its Regulation through Insulin-like Growth Factor-I in Southern flounder, Paralichthys lethostigma

Abstract

Insulin-like growth factor-I (IGF-I) is considered a critical regulator of growth in fish and other vertebrates. Little is known about its regulation under different states of growth in southern flounder (Paralichthys lethostigma). In this study, we cloned IGF-I cDNA in southern flounder and investigated whether alterations in circulating IGF-I and hepatic IGF-I mRNA might accompany changes in growth under different nutritional planes. Southern flounder IGF-I coding sequence has 99 % homology compared to a closely related species, Japanese flounder (P. olivaceus). The mature IGF-I amino acid sequence of southern flounder is identical to that of Japanese flounder and composed of 68 amino acid residues compared with 70 residues in other teleost, amphibian, avian, and mammalian IGF-Is. In the 5'-untranslated region, four different cDNA sequences were detected by 5'RACE, possibly reflecting alternate transcription initiation sites and tissue-specific usage.

To establish whether IGF-I correlates with or mediates growth in southern flounder, we conducted an 8-week trial in which growth rates were manipulated through changes in feeding regimen. Fish were subjected to either four weeks of starvation (treatment group) or four weeks of ad libitum feeding (control group). After the initial four weeks, all fish were fed ad libitum for an additional four weeks. Liver and blood were collected at 4-week intervals throughout the study for measures of IGF-I mRNA expression and plasma IGF-I levels, respectively. For the first four weeks, food-deprived fish showed little growth compared with fed fish. Upon refeeding, the treatment group showed a substantial increase in body weight (132 %) compared to control group (107 %) over the last four week period. However, the growth rate of the treatment group following realimentation was no different than that of the size-matched control group fed over the first four weeks, suggesting southern flounder were unable to compensate for the lost growth. By contrast, the hepatosomatic index (HSI) was completely restored in the treatment group following realimentation and exceeded that of the control group by the end of eight weeks. Overall, it appears flounder do not undergo significant growth compensation, despite a complete restoration of hepatic energy stores, at least under this experimental paradigm. Circulating IGF-I levels of the control group increased over the first four weeks, and then subsided slightly by the end of eight weeks where concentrations remained above that observed in animals at the start of the experiment. The food-deprived fish showed a significant decline in circulating IGF-I levels during the starvation period, and levels were completely restored to that of control fish by the end of the refeeding period. Circulating IGF-I concentrations were positively correlated to changes in specific growth rate in southern flounder (r2 = 0.67, P = 0.0012). By contrast, hepatic IGF-I mRNA levels did not differ between control and the starved treatment group, although a rise was observed in fish upon refeeding (P = 0.08). There was no significant correlation between hepatic IGF-I mRNA and circulating IGF-I levels (r2 = 0.01; P = 0.75) or specific growth rate (r2 = 0.16; P = 0.2).

Although the growth of teleosts is regulated by various environmental parameters, little is known of the effects of ambient calcium concentration on this process. Therefore, we also examined a range of ambient calcium concentrations on growth of southern flounder raised in fresh water. Growth did not significantly differ in animals exposed to 50, 100, and 150 ppm calcium (as CaCO3) over a 10-week period. We conclude that ambient calcium concentrations as low as 50 ppm do not adversely affect southern flounder growth. However, prolonged exposures to waters with lower than 50 ppm calcium may adversely affect disease resistance and survival.