PANCREATIC HORMONES DIFFERENTIALLY REGULATE INSULIN-LIKE GROWTH-FACTOR (IGF)-I AND IGF-BINDING PROTEIN-PRODUCTION BY PRIMARY RAT HEPATOCYTES

We investigated the influence of and interactions among pancreatic hor mones on the secretion of insulin-like growth factor-I (IGF-I) and IGF -binding proteins (IGFBPs) by treating primary hepatocytes from young male Long-Evans rats with insulin or glucagon in combination with rat GH (rGH). The concentration of IGF-I secreted into the medium was esti mated by radioimmunoassay after formic acid-acetone cryoextraction, an d secreted IGFBPs were analysed by Western ligand blot and immunoblot; accumulation of IGF-I mRNA was analysed by Northern blot. Both insuli n (0.1-100 nmol/l) and rGH (0.5, 5 and 50 pmol/l) produced a dose-depe ndent stimulation of IGF-I secretion over a 24-h incubation period. In contrast, glucagon (0.1-100 nmol/l) inhibited IGF-I production in a d ose-related manner. Glucagon (10 nmol/l) also inhibited IGF-I secretio n stimulated by rGH (5 pmol/l) and insulin (10 nmol/l). Northern blot analysis of total RNA isolated from rat hepatocytes revealed that rGH (5 pmol/l) elevated IGF-I mRNA levels, glucagon (10 nmol/l) alone had no effect on this parameter, but glucagon significantly reduced IGF-I transcript accumulation in response to rGH. IGFBPs secreted by rat hep atocytes run in two molecular weight ranges on SDS-PAGE: similar to 25 kDa (IGFBP-4) and similar to 29-31 kDa (IGFBP-1 and -2); the predomin ant hormonally regulated IGFBP was identified as IGFBP-1. Insulin prod uced a dose-dependent inhibition of production of IGFBP-1, while gluca gon was stimulatory; when given together at an equivalent concentratio n (1 nmol/l), the effects of insulin were dominant to glucagon on IGFB P-1. These observations provide support for significant opposite roles for the pancreatic hormones, insulin and glucagon, in the regulation of liver IGF-I and IGFBP-1 production. As the production of pancreatic hormones is influenced by nutritional status, these polypeptides may mediate the effects of changing nutritional state on the hormonal cont rol of protein anabolism and glucose homeostasis by directly influenci ng the circulating level of liver-derived IGF-I and its binding protei ns.