Heterogeneity in glutamic acid decarboxylase expression among single rat pancreatic beta cells

Aims/hypothesis. An isoform of glutamic acid decarboxylase, (GAD)65 has bee n identified as a pancreatic beta-cell autoantigen in Type I (insulin depen dent) diabetes mellitus. We investigated the expression of GAD isoforms amo ng single rat beta cells in culture, under different conditions and the cor relation between GAD65 expression and insulin secretion-rate. Results. Independent of culture conditions, 100% of fresh and cultured beta cells express GAD67. In contrast, considerable heterogeneity in GAD65 expr ession among single beta cells was observed. After 2 days in culture in 2.6 mmol/l glucose, only 24% of the beta cells express GAD65. This percentage increases to 39% in 5.6 mmol/l glucose and to 54% and 56% in 11.6 mmol/l an d 20.6 mmol/l glucose, respectively. Moreover, reducing glucose concentrati on from 11.6 to 2.5 mmol/l for 2 days, reduces GAD65 expression by nearly 3 0%. After 11 days in culture with 11.6 mmol/l glucose, 50% of beta cells co ntinue expressing GAD65, this percentage is further increased to nearly 75% by including either nerve growth factor or dibutyryl cyclic AMP or both in the culture medium. When beta cells are challenged for 1 h with 20.6 mmol/ l glucose, 67% respond forming insulin-immunoplaques. More than two-thirds of insulin-secretors are GAD65-positive, in contrast to only 11% of the non -secreting cells. Moreover, 87% of beta cells that have a high insulin secr etory rate express CAD65. Conclusion/interpretation. These results show that the most active beta cel ls, which secrete more insulin, also express GAD65 and that manipulating ex tracellular glucose may modify the expression of the enzyme and possibly th e autoimmune attack in Type I diabetes.