title: Ontogenesis of insulin processing in fetal rat hepatocytes.
abstract: 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)
authors.affiliation: Department of Pediatrics, SUNY Health Science Center, Syracuse.
authors.forename: M R
authors.initials: MR
authors.lastname: Benedict
authors.forename: R A
authors.initials: RA
authors.lastname: Richman
mesh_terms: D000818:Animals; D002478:Cells, Cultured; D005333:Fetus; D005865:Gestational Age; D007328:Insulin / Q000378:metabolism* / Q000494:pharmacology; D007457:Iodine Radioisotopes; D007700:Kinetics; D008099:Liver / Q000187:drug effects / Q000196:embryology / Q000378:metabolism*; D008112:Liver Glycogen / Q000378:metabolism; D008744:Methylamines / Q000494:pharmacology; D051381:Rats; D011919:Rats, Inbred Strains; D011972:Receptor, Insulin / Q000378:metabolism; D013696:Temperature
chemical_list: D007328:Insulin; D007457:Iodine Radioisotopes; D008112:Liver Glycogen; D008744:Methylamines; C027451:methylamine; D011972:Receptor, Insulin