Increased expression of GAD65 and GABA in pancreatic beta-cells impairs first-phase insulin secretion

The functional role of glutamate decarboxylase (GAD) and its product GABA i n pancreatic islets has remained elusive. Mouse beta-cells express the larg er isoform GAD67, whereas human islets express only the smaller isoform GAD 65. We have generated two lines of transgenic mice expressing human GAD65 i n pancreatic beta-cells (RIP7-hGAD65, Lines 1 and 2) to study the effect th at GABA generated by this isoform has on islet cell function. The ascending order of hGAD65 expression and/or activity in beta-cells was Line 1 hetero zygotes, Line 2 heterozygotes, Line 1 homozygotes. Line 1 heterozygotes hav e normal glucose tolerance, whereas Line 1 homozygotes and Line 2 heterozyg otes exhibit impaired glucose tolerance and inhibition of insulin secretion in vivo in response to glucose. In addition, fasting levels of blood gluco se are elevated and insulin is decreased in Line 1 homozygotes. Pancreas pe rfusion experiments suggest that GABA generated by GAD65 may function as a negative regulator of first-phase insulin secretion in response to glucose by affecting a step proximal to or at the K-ATP(+) channel.