The effect of temperature on the vitellogenic response in Atlantic salmon post-smolts (Salmo salar).

Effects of low (3 degrees) and high (10 degrees) temperatures on the vitellogenic response to estradiol treatment were analyzed in female and male salmon post-smolts. Vitellogenic response as indicated by levels of circulating vitellogenin, hepatic protein, RNA, and nuclear count depends on the ambient temperature during hormone treatment but not on the previous 1 month acclimation temperature. Thus, the level of vitellogenin in the serum or hepatic RNA did not increase above the levels of the untreated controls in previously warm-acclimated smolts if the fish were exposed to low temperature water during treatment with estradiol. Conversely, cold-acclimated smolts showed increased levels of hepatic RNA and circulating vitellogenin only when transferred to 10 degrees water during treatment. No such response could be elicited if salmon were kept at 3 degrees during estradiol treatment. The negative influence of low ambient temperature on the vitellogenic response could not be eliminated by using higher doses of estradiol. Salmon post-smolts, long-term acclimated to 3 degrees revealed a significant vitellogenic response to estradiol only if the temperature during treatment was increased to 10 degrees. Estradiol treatment at 10 degrees stimulated the growth of liver as determined by the increase in liver somatic index and amounts of total hepatic protein, RNA, and nuclear count. On a per unit liver wet weight basis, however, only RNA increased above the level of the untreated control fish. As the nuclear count per gram of liver is similar in estradiol-treated and untreated groups, estradiol-induced liver growth in the salmon post-smolts may be accounted for by an increase in cell number (hyperplasia) rather than an increase in cell size (hypertrophy).