SEASONAL-CHANGES IN CIRCULATING GROWTH-HORMONE (GH), HEPATIC GH-BINDING AND PLASMA INSULIN-LIKE GROWTH-FACTOR-I IMMUNOREACTIVITY IN A MARINE FISH, GILTHEAD SEA BREAM, SPARUS-AURATA

We have studied the seasonal relationship between growth and circulati ng growth hormone (GH), hepatic GH-binding and plasma insulin-like gro wth factor-I immunoreactivity in gilthead sea bream, Sparus aurata. Th e seasonal increase in plasma GH levels preceded by several weeks the summer increase in growth rates. In contrast, a marked increase in hep atic GH-binding with a high degree of endogenous GH occupancy was foun d during the period of maximum growth which suggests an enhanced sensi tivity of liver to GH action. Thus, circulating levels of immunoreacti ve IGF-I, probably derived from the liver in response to GH action, we re positively correlated with growth throughout the experimental perio d although a consistent relationship between growth and circulating GH was not found. In spite of this, we consider that, in gilthead sea br eam, as in several other teleosts, the availability of endogenous GH c an limit growth. Thus, under environmental conditions of suboptimal gr owth, a single intraperitoneal injection of recombinant rainbow trout GH (rtGH) induced over the dose range tested (0.75, 1.5, 3 mug g BW-1) an increase in plasma IGF-I-like immunoreactivity comparable to that seen during the period of maximum growth.