Ontogenesis of insulin processing in fetal rat hepatocytes

We studied insulin processing and hepatic glycogenesis in cultured hepatocytes isolated from rat fetuses of 17, 19, and 21 days of gestation. Steady-state insulin binding increased by 250% between days 17 and 19, from 145 +/- 8 to 361 +/- 52 fmol/mg protein, and by an additional 40% (405 +/- 69 fmol/mg protein) by 21 days of gestation. At 37 degrees C, 125I-insulin was rapidly (t 1/2 less than 5 min) internalized by hepatocytes at all three ages, reaching maximal levels (63-76% of the total cell-associated radioactivity) by 15 min. 125I-labelled degradation products appeared rapidly (t 1/2 less than 15 min) within the cells. Yet, the majority (68-77%) of the intracellular radioactivity consisted of intact 125I-insulin, even after 4 h at 37 degrees C. Hepatocytes pre-loaded with 125I-insulin and then acid-stripped of surface-bound radioactivity, rapidly released both intact 125I-insulin (retroendocytosis) and its radiolabelled degradation products. While intact insulin was initially released more rapidly (t 1/2 less than 6 min), and reached a plateau after 15-30 min, the degradation products continued to accumulate in the medium for at least 4 h. Methylamine inhibited intracellular 125I-insulin degradation at all three gestational ages and also blocked insulin-stimulated glycogenesis in 19- and 21-day hepatocytes, without altering basal glycogen synthesis. Insulin-stimulated glycogenesis was not induced in 17-day fetal rat hepatocytes in control or methylamine-treated cultures. We conclude that both degradative and retroendocytotic pathways for processing insulin are present in fetal rat hepatocytes by 17 days of gestation.(ABSTRACT TRUNCATED AT 250 WORDS)