The neuropharmacological study of endogenous glutamate release from the stomach

为了研究谷氨酸在胃中可能的生理作用，我们研究了血管灌流的大鼠离体胃制剂对内源性谷氨酸的释放。用生物发光法测定谷氨酸，高浓度KCI(30-75 mM)诱导谷氨酸释放呈剂量依赖关系。这种KCI诱导的谷氨酸释放在含有EGTA的无钙介质中被取消。电刺激迷走神经也诱导谷氨酸的释放。这种迷走神经刺激诱导的谷氨酸释放可被钙去除和河豚毒素(TTX)所消除。高K+和迷走神经刺激对培养液中其他13种氨基酸的含量没有显著影响。在接下来的系列中，我们研究了介导内源性谷氨酸从胃释放的钙通道亚型的特性。P/Q型钙通道拮抗剂omega-agatoxin IVA和L型钙通道拮抗剂伊拉地平均显著抑制50 mM KCI引起的谷氨酸释放。N型钙通道拮抗剂欧米芋毒素GVIA和非选择性T型钙通道拮抗剂氟桂利嗪均无作用。与谷氨酸的情况相反，欧米茄椰子毒素GVIA对胃去甲肾上腺素的释放有明显的抑制作用。Omega-agatoxin IVA联合伊拉地平对谷氨酸释放有明显的协同抑制作用。然而，这种抑制作用比镉的抑制要小得多。这些结果进一步证明谷氨酸可能是胃内的一种神经递质，提示P/Q和L型钙通道并存，调节胃谷氨酸的释放。此外，胃内谷氨酸的释放可能还涉及P/Q和L通道以外的未知钙通道。

To investigate a possible physiological role of glutamate in the stomach, release of endogenous glutamate from an isolated vascularly perfused rat stomach preparation was studied. Glutamate was measured by bioluminescence assay method.High concentration of KCI (30-75mM) induced a dose dependent release of glutamate. This KCI-induced release of glutamate was abolished in calcium-free medium containing EGTA.Electrical stimulation of the vagus nerves also induced a release of glutamate. This vagal stimulation-induced release of glutamate was abolished by both calcium removal and tetrodotoxin (TTX). Amounts of other 13 amino acids in the medium, detectable by the automatic amino acid analyzer, were not significantly affected by both high-K+and the vagal stimulation.In the next series, we examined properties of calcium channel subtypes mediating.endogenous glutamate release from the stomach. The 50 mM KCI elicited release of glutamate was significantly inhibited by both omega-agatoxin IVA,a P/Q-type calcium channel antagonist, and isradipine, an L type calcium channel antagonist. Omega-Conotoxin GVIA,an N type calcium channel antagonist and flunarizine, a nonselective T-type calcium channel antagonist were without effect. In contrast to this case of glutamate, omega-conotoxin GVIA induced a marked inhibition in the release of gastric noradrenaline. The combined treatment with omega-agatoxin IVA plus isradipine produced a marked synergistic inhibition of the glutamate release. This inhibition was, however, much less than that by cadmium. These results provide an additional evidence that glutamate probably serves as a neurotransmitter in the stomach, and suggest that P/Q and L type calcium channels coexist to regulate the release of gastric glutamate. Furthermore, it is possible that unidentified calcium channels other than P/Q and L type channels are also involved in the release of glutamate in the stomach.