T. Agustsson et al., Growth hormone endocrinology of Atlantic salmon (Salmo salar): pituitary gene expression, hormone storage, secretion and plasma levels during parr-smolt transformation, J ENDOCR, 170(1), 2001, pp. 227-234
A number of studies on the Atlantic salmon (Salmo salar), have reported cha
nges in plasma GH during parr-smelt transformation, but there is a lack of
information about the endocrinology of the GH system during this process. I
n order to elucidate the mechanisms underlying these changes in plasma GH l
evels during the parr-smelt transformation of Atlantic salmon, GH mRNA expr
ession in the pituitary was studied together with total pituitary GH conten
t, in vitro GH secretion rate and plasma GH and IGF-I levels. Atlantic salm
on were kept in outside tanks, under natural condition from early February
until late June. Approximately three times a month fish were killed and pit
uitaries and blood were sampled for investigation. Further, pituitaries wer
e moved to the laboratory for in vitro GH secretion studies. The results sh
ow that the GH system is first activated by all increase in GH secretion ra
te, which leads to an increase in plasma GH levels and causes a drop in the
total GH content of the pituitary. This drop in pituitary GH content is la
ter reversed by an increased GH synthesis seen as an increase in GH mRNA ex
pression. Maximal activation of the GH system is seen to occur in early May
, when plasma IGF-I levels reach highest levels, after which a certain deac
tivation of the GH system takes place. The data show that plasma levels of
GH are to a large extent regulated by the secretion rate from the pituitary
, although changes in the GH clearance rate are also likely to take place a
nd influence the plasma GH levels. The study further underlines the signifi
cant role that the GH-IGF-I axis plays in the parr-smelt transformation of
the Atlantic salmon.