Appearance of insulin-like growth factor mRNA in the liver and pyloric ceca of a teleost in response to exogenous growth hormone

Augmentation of vertebrate growth by growth hormone (GH) is primarily due to its regulation of insulin-like growth factor I (IGF I) and IGF II levels. To characterize the effect of GH on the levels of IGF I and IGF II mRNA in a teleost, 10 μg of bovine GH (bGH) per g of body weight was administered to juvenile rainbow trout (Oncorhynchus mykiss) through i.p. injection. The levels of IGF I and IGF II mRNA were determined simultaneously, by using RNase protection assays, in the liver, pyloric ceca, kidney, and gill at 0, 1, 3, 6, 12, 24, 48, and 72 hr after injection. In the liver, IGF I mRNA levels were significantly elevated at 6 and 12 hr (≃2-to 3-fold, P ≤ 0.01), while IGF II mRNA levels were significantly elevated at 3 and 6 hr (≃3- fold, P ≤ 0.01). In the pyloric ceca, IGF II mRNA levels were significantly elevated at 12, 24, and 48 hr (≃3-fold, P ≤ 0.01), while IGF I mRNA was below the limits of assay accuracy. GH-dependent IGF mRNA appearance was not detected in the gill and kidney. Serum bGH levels, determined by using a radioimmunoassay, were significantly elevated at 3 and 6 hr (P < 0.005). In primary hepatocyte culture, IGF I and IGF II mRNA levels increased in a bGH dose-dependent fashion, with ED₅₀ values of ≃45 and ≃6 ng of bGH per ml, respectively. The GH-dependent appearance of IGF II mRNA in the liver and pyloric ceca suggests important roles for this peptide hormone exclusive of IGF I.