Increased plasma insulin-like growth factor-I (IGF-I) following oral and intraperitoneal administration of growth hormone to rainbow trout, Oncorhynchus mykiss.

Changes in plasma insulin-like growth factor-I (IGF-I) and growth hormone (GH) levels in rainbow trout, Oncorhynchus mykiss, were investigated after either intraperitoneal or oral (intragastric) administration of salmon GH. Plasma IGF-I and GH levels were measured by homologous radioimmunoassays for salmon IGF-I and GH. After intraperitoneal injection of salmon GH at doses of 0.1 or 1.0 micrograms/g body weight to rainbow trout, plasma GH levels increased to a maximum after 6 h and declined rapidly thereafter. Plasma IGF-I levels were elevated after 12 h, reaching maximum at 24 h and (after the higher dose of GH) remaining high at 78 h after GH injection. In contrast, following intragastric administration of salmon GH at the same doses, plasma GH levels were significantly elevated 12 h after administration, reaching a maximum at 15 h and declining to basal levels at 24 h. Plasma IGF-I levels increased significantly after 48 h and remained elevated (after the higher dose of GH) for more than 96 h. Effects on plasma GH and IGF-I were dose-dependent notwithstanding the route of administration of GH. These results indicate that plasma IGF-I in salmonid fish is under GH control and that oral administration of the hormone is followed by longer-lasting effects than those achieved by intraperitoneal injection.