INSULIN BINDING TO ISOLATED HEPATOCYTES OF ATLANTIC SALMON AND RAINBOW-TROUT

The questions addressed in this study were: 1) whether insulin added t o the incubation medium can down-regulate I-125 insulin binding to iso lated hepatocytes of Atlantic salmon (Salmo salar) and rainbow trout ( Oncorhynchus mykiss); 2) whether quantitative assessment of insulin pr ocessing can be made on isolated fish liver cells; 3) how ambient temp eratures can affect insulin binding, and down-regulation of insulin re ceptors. After isolation and a short (up to 4h) ''metabolic recovery p eriod'', liver cells were used either directly in I-125 insulin bindin g assay or first preincubated for 18h at 4-degrees-C or for 3h at 15-d egrees-C, with or without mammalian or salmon insulin in concentration s ranging from 1 to 1000 nM. Preincubation at 15-degrees-C, decreased binding capacity (number of binding sites per liver cell) in all five independent hepatocyte preparations treated with 1000 nM insulin and i n four out of five preparations treated with 100 nM insulin. At 4-degr ees-C insulin binding sites were down-regulated in less than 50% of al l hepatocyte preparations and only in the presence of 1000 nM insulin. Differential quantitive assessment was made of a) intact free insulin ; b) insulin degraded; c) intact insulin bound to the cell membrane; d ) internalized but degraded insulin, and e) intact insulin internalize d by liver cells. Hepatocytes preincubated with 100 - 1000 nM insulin at 15-degrees-C bound and internalized less I-125 insulin. We hypothes ize that in vivo, at water temperatures of 15-degrees-C and higher, ex treme physiological levels of plasma insulin may regulate the numbers of insulin receptors in the salmonid liver. In contrast, in fish inhab iting cold waters the regulation of insulin receptors by circulating p lasma insulin seems to be of little physiological importance.