Polyamines in pancreatic islets of obese-hyperglycemic (ob/ob) mice of different ages

Abstract

To further evaluate the role of polyamines in insulin production and cell replication in diabetic pancreatic islets, we have studied hyperplastic islets of obese-hyperglycemic mice of different ages and normal islets of the same strain. The aims of the study were to investigate the impact of the diabetic state and aging on polyamine contents and requirements in these islets. Cultured islets from lean and obese animals contained significantly less polyamines than freshly isolated islets. Spermine-to-spermidine ratio was elevated in freshly isolated islets from young obese mice compared with those from lean mice. In islets from old obese animals, spermidine content was decreased, whereas the content of spermine was not different from that of young obese mice. The physiological significance of polyamines was investigated by exposing islets in tissue culture to inhibitors of polyamine synthesis. This treatment caused a partial polyamine depletion in whole islets but failed to affect polyamine content of cell nuclei. Insulin content was not affected in polyamine-deficient islets of obese mice, irrespective of age, in contrast to decreased islet insulin content in polyamine-depleted young lean animals. Polyamine depletion depressed DNA synthesis rate in obese mouse islets; in lean mice it actually stimulated DNA synthesis. We concluded that important qualitative and quantitative differences exist between islets from obese-hyperglycemic and normal mice with respect to polyamine content and requirements of polyamines for regulation of insulin content and cell proliferation. The results suggest that spermine may be involved in mediating the rapid islet cell proliferation noted early in obese-hyperglycemic syndrome, but changes in spermine concentration do not seem to account for the decline in islet cell DNA synthesis in aged normoglycemic animals.