DAMAGED RAT BETA-CELLS DISCHARGE GLUTAMATE-DECARBOXYLASE IN THE EXTRACELLULAR MEDIUM

In type I (insulin-dependent) diabetes, destruction of pancreatic beta cells has been associated with the presence of circulating antibodies against glutamate decarboxylase (GAD), a GABA (gamma-aminobutyric aci d) synthesizing enzyme which is located in the beta cells. We examined whether destruction of islet beta cells can lead to discharge of GAD in the extracellular medium, making it a potential autoantigen. Rat is let beta cells were first exposed for 1 hour to streptozotocin and the n cultured for 4 to 24 hours before cellular and medium GAD activities were measured, After 24 hours culture, 70 percent of streptozotocin-t reated beta cells were disintegrated whereas the number of control cel ls remained unchanged. Control cells exhibited a stable cellular GAD a ctivity over the 24 hour period with no enzyme activity detectable in their culture medium. The cells recovered 24 hours after streptozotoci n treatment exhibited 10-fold lower levels of CAD-activity and of GABA ; their culture medium contained GAD, its enzymatic activity reaching peak values after 10 hours. The beta-cell enzymes glutamate dehydrogen ase and glyceraldehyde-3-phosphate dehydrogenase were not detectable i n the medium of control or streptozotocin-treated cells. Similar obser vations were made when beta cells had been exposed to cytotoxic concen trations of alloxan. It is concluded that damage to rat islet beta cel ls results in transient discharge of GAD in the extracellular medium m aking this enzyme a candidate extracellular marker for beta cell toxic processes and a potential autoantigen for immune reactivity. (C) 1996 Academic Press, Inc.