Effects of insulin-like growth factor-I and cortisol on Na+,K+-ATPase expression in osmoregulatory tissues of brown trout (Salmo trutta)

The effect of recombinant bovine IGF-I (rbIGF-I) on hypo-osmoregulatory abi lity and the effect of rbIGF-I and cortisol (F) alone and in combination on Na+,K+-ATPase expression in fresh water (FW) acclimated brown trout (Salmo trutta) were examined in two experiments. In Experiment 1, fish were given three injections of saline or 0.01 or 0.1 mu g rbIGF-I/g, respectively, an d subjected to a 24-h 25 ppt seawater (SW) challenge test 24 h after the la st injection. Fish treated with 0.01 and 0.1 mu g rbIGF-I/g had better hypo -osmoregulatory ability than control fish as judged by their higher level o f muscle water content and lower plasma osmolality after 24 h exposure to 2 5 ppt SW. Compared with control fish, gill Na+,K+-ATPase activity was uncha nged 24 h after the first injection at either dose but significantly stimul ated after three injections of either dose of rbIGF-I. In Experiment 2, fis h were given three injections of saline, 0.1 mu g rbIGF-I/g, 4 mu g F/g, or 0.1 mu g rbIGF-I + 4 mu g F/g and sampled in FW 24 h after the last inject ion. IGF-I and F had additive stimulatory effects on Na+,K+-ATPase activity and alpha-subunit Na+,K+-ATPase mRNA levels in the gill. Injections of IGF -I and F alone and in combination increased Na+,K+-ATPase-immunoreactive (N KIR) cell number in the primary gill filament but had no effect on secondar y lamellar NKIR cell number. NKIR cells were abundant in kidney tubules, py loric ceca, and posterior intestine, but Na+,Kf-ATPase enzyme activity war unaffected by treatment with F and/or IGF-I in these tissues. F but not rbI GF-I increased in vitro fluid transport capacity in the posterior intestine . In addition to confirming an overall SW-adaptive effect of rbIGF-I and F in FW-acclimated S. trutta, the study suggests the effect to be associated with stimulation of chloride cell development and Na+,K+-ATPase expression in the gill. The study indicates that the stimulatory effects of the two ho rmones on Na+,K+-ATPase expression are additive, highly organ specific, and restricted to the primary filament epithelium of the gill. (C) 1999 Academ ic Press.