Short-term insulin-induced glycogen formation in primary hepatocytes as a screening bioassay for insulin action

We describe a novel bioassay to measure specific insulin-like activity in primary cultures of rat hepatocytes by determination of [H-3]glycogen from D-[6-H-3]glucose. The dose-response curve of insulin in this assay exhibited an EC50 of 0.42 (+/- 0.04) nM, which is comparable to the dissociation constant of insulin from its receptor in hepatocytes. We used this assay to examine possible residual insulin-like activity of the four major fragments formed upon insulin degradation by insulin protease. Fragments A(1-13)B(1-9), A(1-14)B(1-9), and A(14-21)B(14-30) showed no measurable activity. Although preparations of fragment A(14-21)B(10-30) displayed dose-dependent agonist activity with an EC50 of 380 (+/-40) nM, we conclude that this was due to an insulin-like impurity since the chemically synthesized fragment showed no such activity. In summary, this bioassay demonstrates the action of insulin on glycogen formation in hepatocytes and provides a rapid and sensitive measurement of insulin-like activity which could facilitate screening studies. (C) 1998 Academic Press.