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Vol. 273, Issue 3, 1647-1653, January 16, 1998

A High Affinity Glutamate/Aspartate Transport System in Pancreatic Islets of Langerhans Modulates Glucose-stimulated Insulin Secretion

C. David Weaver, Vidar Gundersen^¶, and Todd A. Verdoorn

From the  CNS Drug Discovery, Bristol-Meyers Squibb, Wallingford, Connecticut 06492 and the ^¶ Anatomical Institute, University of Oslo, N-0317 Oslo, Norway

To examine the role of glutamatergic signaling in the function of pancreatic islets, we have characterized a high affinity glutamate/aspartate uptake system in this tissue. The islet [³H]glutamate uptake activity was Na⁺-dependent, and it was blocked by L-trans-pyrrolidine-2,4-dicarboxylic acid, a blocker of neuronal and glial glutamate transporters. Islet glutamate transport activity exhibited a V_max of 8.48 ± 1.47 fmol/min/islet (n = 4), which corresponds to 102.2 ± 17.7 pmol/min/mg islet protein. The apparent K_m of islet glutamate transport activity depended on the glucose concentration used in the assay. In the presence of glucose concentrations that do not stimulate insulin secretion (2.8 mM), the apparent K_m was 34.7 ± 7.8 µM (n = 3). However, in high glucose (16.7 mM) the apparent K_m increased to 112.7 ± 16.5 µM (n = 3) with little or no change in V_max. Like most known plasma membrane glutamate transporters, islet glutamate transporters also transported D-aspartate. Anti-D-aspartate immunoreactivity showed that the islet glutamate/aspartate transport activity was localized to the non- cell islet mantle. In perifusion experiments with isolated islets in the absence of exogenous amino acids, L-trans-pyrrolidine-2,4-dicarboxylic acid in the presence of 8.3 mM glucose potentiated insulin secretion 23.3 ± 2.3% (n = 3) compared with 8.3 mM glucose alone. This effect was abolished in the presence of the -amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione. Furthermore, 6-cyano-7-nitroquinoxaline-2,3-dione alone inhibited glucose-stimulated insulin secretion in isolated islets by 15.9 ± 5.9% (n = 3). Taken together these data suggest that a high affinity glutamate transport system exists in pancreatic islets and that this system contributes to a glutamatergic signaling pathway that can modulate glucose-inducible insulin secretion.

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