Proinsulin mRNA and peptide are present in β-cells of diabetic BB rats

Abstract

Previous studies have demonstrated that islets isolated from newly diabetic BB rat pancreata retain the ability to release insulin in culture, although in vivo the insulin response to stimulation is absent. The purpose of this study was to determine whether the β-cells in these newly diabetic animals were releasing stored insulin or whether they were still capable of insulin biosynthesis, since secretory defects may reflect abnormalities in insulin synthetic capacity. Insulin gene transcription was examined using in situ hybridization to detect preproinsulin mRNA (ppImRNA) at the level of the single cell since this technique provides a valid semiquantitative index of insulin biosynthesis. In situ hybridization with digoxigenin-labeled rat insulin probes resulted in strong labeling of β-cells in normal Wistar rat pancreata; other islet and acinar cells were negative. Double labeling of sections with an antibody to insulin confirmed that the labeled cells were β-cells only. The intensity of the staining was variable between different islets within the same section, and sometimes within an islet. Nondiabetic and diabetic BB islets were also positive for ppImRNA not only in normal islets but also in islets affected by insulitis. Islets that contained very few β-cells also contained ppImRNA. A consistent finding was that the intensity of the hybridization signal in many islets from the diabetic BB rats was stronger than in controls, suggesting that there is more ppImRNA in these islets. β-Cells that were positive for ppImRNA but negative for insulin peptide were also observed; these were in islets that were affected by insulitis. These results indicate that insulin biosynthesis was still present in islets of diabetic animals; therefore, a lack of insulin gene expression was not involved in the loss of glucose-stimulated insulin secretion.Key words: BB rat, in situ hybridization, preproinsulin mRNA.