IN-VITRO EFFECTS OF INSULIN-LIKE GROWTH FACTOR-I ON GILL NA-ATPASE INCOHO SALMON, ONCORHYNCHUS-KISUTCH(,K+)

The effect of ovine GH (oGH) in vivo and recombinant bovine insulin-li ke growth factor-I (rbIGF-I) in vitro on gill Na+,K+-ATPase activity w as investigated in two seasonal experiments conducted during the parr- smolt transformation period of coho salmon. In 1991, when fish were he ld under a photoperiod of 12 h light : 12 h darkness, the stimulatory effect of oGH (1 mug/g) on gill Na+,K+-ATPase in vivo decreased at the time of expected parr-smolt transformation. Gill Na+,K+-ATPase from c ontrol fish was insensitive to rbIGF-I in vitro from February to June, whereas GH treatment induced sensitivity to rbIGF-I (100-1000 mug/l) in vitro in February and March, but not later in development. In 1992, when fish were held under natural conditions, oGH (4 mug/g) stimulate d gill Na+,K+-ATPase in vivo from February to July. There was, however , a pronounced developmental change in sensitivity of gill Na+,K+-ATPa se to rbIGF-I in vitro. In February, gills from control fish were inse nsitive, but oGH treatment in vivo induced sensitivity to rbIGF-I in v itro (100-1000 mug/l). In April and May, control fish were sensitive t o rbIGF-I in vitro. This sensitivity was not further potentiated by oG H treatment in vivo. In June, gills from control or oGH-treated fish w ere not sensitive to rbIGF-I in vitro, but in July exogenous oGH again induced gill tissue sensitivity to rbIGF-I at 1000 mug/l. Both studie s showed that rbIGF-I stimulates gill Na+,K+-ATPase directly; an abili ty that may depend on priming by endogenous or exogenous GH. This supp orts the role of IGF-I as an endocrine mediator for GH action during p arr-smolt transformation.