Plasma kinetics of injected recombinant chicken somatotropin in juvenile coho salmon (Oncorhynchus kisutch) using a homologous radioimmunoassay.

Although somatotropins are potent growth promoters in salmonids, there is little information on how these proteins are metabolized by poikilotherms. In the present study, the plasma uptake and clearance rates of recombinant chicken somatotropin (rcGH) were investigated in juvenile coho salmon (Oncorhynchus kisutch). Two doses of rcGH were administered by intraperitoneal (ip) or intramuscular (im) injection and blood samples were collected over a period of 32 days. A specific radioimmunoassay was validated and used to discriminate rcGH from endogenous somatotropin. Plasma rcGH concentration was proportional to the dose delivered, but uptake and clearance rates were found to be independent of dose (between 0.5 and 5.0 μg/g). Absorption of rcGH into the plasma was faster from the im site, but the peak levels attained were similar after im or ip treatment (using the same dose) as was area under the curve. Plasma half-life was calculated from the declining phase of the uptake/clearance profile but the results were biased by the concurrent uptake of rcGH from the ip or im reservoir of material, resulting in an over-estimation of the true half-life value. Effective treatment doses and intervals are discussed.