Glucose modulates γ-aminobutyric acid release from the pancreatic βTC6 cell line

To determine if endogenous γ-aminobutyric acid (GABA) is secreted by a pancreatic β-cell-derived cell line and to determine the effects of glucose on GABA release, βTC6 cultures were incubated in the presence of 1 or 10 mmol/l glucose for 12 h and then subjected to a 2-h secretion test in Krebs- Ringer buffer containing 1 or 10 mmol/l glucose. βTC6-conditioned medium was collected at 15, 30, 60, and 120 min after glucose stimulation for GABA analysis by high pressure liquid chromatography-electrochemical detection. After 30 min, medium GABA concentrations were significantly higher (p < 0.05) in cultures that were exposed to high glucose during both the 12-h incubation period and the 2-h secretion test than in the remaining three glucose combinations. To address possible roles of β-cell-derived GABA, the effect of GABA on glucagon secretion from pancreatic αTC6 cells was tested at concentrations released from βTC6 cells. Inhibition of glucagon secretion by αTC6 cells was observed in the presence of GABA at concentrations equivalent to concentrations secreted by βTC6 cells. The inhibitory effects of GABA on glucagon secretion by αTC6 cells were blocked by the GABA(A) receptor antagonist bicuculline and were dissociated from the inhibitory effects of glucose. Together, these results provide the first documentation that endogenous GABA is released from a highly differentiated β-cell line and that glucose and GABA independently attenuate glucagon secretion by a pancreatic α-cell line.