Long Term Inhibitory Effects of Pancreastatin and Diazepam Binding Inhibitor on Pancreatic β-Cell Deoxyribonucleic Acid Replication, Polyamine Content, and Insulin Secretion*

Abstract

The two peptides pancreastatin and diazepam binding inhibitor (DBI) were recently demonstrated in pancreatic islets and were shown to inhibit insulin secretion in short term experiments. In the present study we investigated long term effects of pancreastatin and DBI on the DNA synthesis, polyamine content, and insulin secretion of pancreatic beta-cells in tissue culture. For this purpose fetal rat pancreatic islets enriched in beta-cells were isolated and cultured for 3 days at different concentrations of rat pancreastatin and porcine DBI. It was found that pancreastatin dose-dependently decreased beta-cell DNA synthesis, reaching maximal inhibition at 100 nM. In parallel with this, pancreastatin also decreased insulin secretion and the islet contents of insulin and the polyamines spermidine and spermine. These effects were abolished by a high glucose concentration or addition of GH. Also, DBI evoked a dose-dependent inhibition of beta-cell DNA synthesis but affected neither the islet contents of insulin or polyamines nor insulin secretion. Like pancreastatin, DBI was ineffective in preventing the increased beta-cell DNA synthesis, insulin content, or secretion in response to high glucose or GH. It is concluded that pancreastatin and DBI inhibit beta-cell DNA synthesis and function in vitro. In the case of pancreastatin these inhibitory effects may be mediated by a decrease in islet polyamine content. It is suggested that pancreastatin and DBI may influence beta-cell replication and function in vivo in an autocrine or paracrine fashion.