Wendt Anna, Birnir Bryndis, Buschard Karsten, Gromada Jesper, Salehi Albert, Sewing Sabine, Rorsman Patrik, Braun Matthias
Department of Physiological Sciences, Lund University, Lund, Sweden.
Diabetes. 2004 Apr;53(4):1038-45. doi: 10.2337/diabetes.53.4.1038.
gamma-Aminobutyric acid (GABA) has been proposed to function as a paracrine signaling molecule in islets of Langerhans. We have shown that rat beta-cells release GABA by Ca(2+)-dependent exocytosis of synaptic-like microvesicles. Here we demonstrate that GABA thus released can diffuse over sufficient distances within the islet interstitium to activate GABA(A) receptors in neighboring cells. Confocal immunocytochemistry revealed the presence of GABA(A) receptors in glucagon-secreting alpha-cells but not in beta- and delta-cells. RT-PCR analysis detected transcripts of alpha(1) and alpha(4) as well as beta(1-3) GABA(A) receptor subunits in purified alpha-cells but not in beta-cells. In whole-cell voltage-clamp recordings, exogenous application of GABA activated Cl(-) currents in alpha-cells. The GABA(A) receptor antagonist SR95531 was used to investigate the effects of endogenous GABA (released from beta-cells) on pancreatic islet hormone secretion. The antagonist increased glucagon secretion at 1 mmol/l glucose twofold and completely abolished the inhibitory action of 20 mmol/l glucose on glucagon release. Basal and glucose-stimulated secretion of insulin and somatostatin were unaffected by SR95531. The L-type Ca(2+) channel blocker isradipine evoked a paradoxical stimulation of glucagon secretion. This effect was not observed in the presence of SR95531, and we therefore conclude that isradipine stimulates glucagon secretion by inhibition of GABA release.
γ-氨基丁酸（GABA）被认为在胰岛中作为旁分泌信号分子发挥作用。我们已经表明，大鼠β细胞通过突触样微囊泡的Ca（2+）依赖性胞吐作用释放GABA。在此我们证明，如此释放的GABA可以在胰岛间质内扩散足够的距离，以激活邻近细胞中的GABA（A）受体。共聚焦免疫细胞化学显示，在分泌胰高血糖素的α细胞中存在GABA（A）受体，而在β细胞和δ细胞中则不存在。RT-PCR分析在纯化的α细胞中检测到α（1）和α（4）以及β（1-3）GABA（A）受体亚基的转录本，但在β细胞中未检测到。在全细胞电压钳记录中，外源性应用GABA可激活α细胞中的Cl（-）电流。GABA（A）受体拮抗剂SR95531被用于研究内源性GABA（从β细胞释放）对胰岛激素分泌的影响。该拮抗剂使1 mmol/l葡萄糖时的胰高血糖素分泌增加了两倍，并完全消除了20 mmol/l葡萄糖对胰高血糖素释放的抑制作用。SR95531对胰岛素和生长抑素的基础分泌和葡萄糖刺激分泌没有影响。L型Ca（2+）通道阻滞剂伊拉地平引起了胰高血糖素分泌的反常刺激。在存在SR95531的情况下未观察到这种效应，因此我们得出结论，伊拉地平通过抑制GABA释放来刺激胰高血糖素分泌。
