Endocrine control of Na+,K+-ATPase and chloride cell development in brown trout (Salmo trutta): interaction of insulin-like growth factor-I with prolactin and growth hormone

A 2-factorial (3 x 3) injection experiment was used to investigate the effe ct and interaction between different hormones on the initial phase of seawa ter (SW) acclimation in brown trout (Salmo trutta). Each fish was given 4 i njections on alternate days in freshwater (FW). Factor 1 was either saline, 2 mu g ovine prolactin (oPRL)/g, or 2 mu g ovine growth hormone (oGH)/g. F actor 2 was either 0, 0.01, or 0.1 mu g recombinant human insulin-like grow th factor-I (rhIGF-I)/g. In each of the 9 treatment groups, half of the fis h were subjected to a 48-h SW-challenge test, and the remaining fish were s ham-transferred to FW one day after the last injection. Hypo-osmoregulatory performance was increased by GH and impaired by PRL treatment as judged by changes in plasma osmolality, [Na+], [Cl-], total [Mg] and muscle water co ntent (MWC) after SW transfer. IGF-I reduced plasma osmolality after transf er to SW but had no effect on plasma total [Mg] or MWC. The effects of the two factors on plasma osmolality, [Na+], [Cl-], and MWC were additive. In s ham-transferred fish, GH and IGF-I, alone and in combination, stimulated Na +,K+-ATPase alpha-subunit mRNA (alpha-mRNA) content in the gill. This was p aralleled by an overall increase in gill Na+,K+-ATPase activity in fish tre ated with 0.01 mu g IGF-I/g. Simultaneous administration of PRL completely inhibited the increase in gill alpha-mRNA observed in the IGF-I-injected gr oups. Combination of GH and IGF-I did not further affect the alpha-mRNA lev el relative to the single hormone-injected groups. There was an overall dec rease in Na+,K+-ATPase activity in pyloric caeca and middle intestine by th e low dose and both doses of IGF-I respectively. No effect was observed in the posterior intestine. PRL and GH treatments did not affect enzyme activi ty in any intestinal segment. Both doses of IGF-I increased Na+,K+-ATPase-i mmunoreactive (NKIR) cell density in gill primary filaments. PRL and GH had no effect on primary filament NKIR cell density. GH and both doses of IGF- I reduced secondary lamellar NKIR cell density, whereas PRL had no effect. The main conclusion is that IGF-I and GH induce an overall redistribution o f NKIR cells away from the secondary lamella onto the primary filament of F W-acclimated trout. This is associated with an overall increased alpha-mRNA level in the gill, which may reflect an increased expression within indivi dual NKIR cells in the primary filament. PRL completely abolished the IGF-I stimulation of alpha-mRNA levels, suggesting a desensitisation of the gill tissue to IGF-I, which may explain the overall anti-SW adaptive effect of PRL.