Insulin kinetics and distribution in rainbow trout ( Salmo gairdneri)

Abstract

The plasma disappearance of intravenously-injected 125I-cod insulin and a mixture of 131I-beef insulin and 125I-beef monoiodoinsulin, studied in rainbow trout ( Salmo gairdneri), was found to be multiexponential, exhibiting both a rapid and a slow component. Biological half-disappearance times ( T 1 4 ) and metabolic clearance rates (MCR) varied significantly with three different assay methods for 125I-cod insulin, while the “apparent” distribution spaces (DS) were similar for all methods. The immunoassay technique with anti-fish insulin serum provided a significantly greater discrimination between total radioactivity and 125I-cod insulin radioactivity than either precipitation with 20% trichloracetic acid (TCA) or charcoal adsorption. The labelled beef insulins were assayed by double-antibody immunoprecipitation using anti-beef insulin serum. The MCR for 125I-beef monoiodoinsulin (1.78 ± 0.20 ml/kg/min) was significantly greater than for either 131I-beef insulin (0.58 ± 0.06 ml/kg/min) or immunoassayable 125I-cod insulin (0.74 ± 0.06 ml/kg/min). The results support the current view that monoiodoinsulin is a more valid tracer for insulin kinetic studies than randomly iodinated forms. A tissue distribution study of 125I-cod insulin showed the presence of TCA-precipitable radioactivity in all tissues and fluids studied, but the kidney was found to be the major accumulation site.