High concentration of GABA and high glutamate decarboxylase activity in rat pancreatic islets and human insulinoma.

The concentration of gamma-aminobutyric acid (GABA) and the activity of glutamate decarboxylase (GAD) in rat and human pancreas were measured by sensitive assay methods. The GABA concentration in rat pancreas was 2.51 millimoles per kilogram (dry weight) and GAD activity was 2.58 mmoles per kilogram per hour. The GABA concentration and GAD activity in rat Langerhans' islets were 18.9 mmole kg-1 and 66.7 mmole kg-1 hour-1, whereas those in the exocrine acini were 1.97 mmole kg-1 and 4.67 mmole kg-1 hour-1, respectively. In an insulinoma region of human pancreas the GABA concentration was 25.5 mmole kg-1 and the GAD activity was 138.2 mmole kg-1 hour-1, but in the surrounding nontumor region these values were only 2.81 mmole kg-1 and 2.01 mmole kg-1 hour-1, respectively, similar to the values in normal rat pancreas.