SummarySince glucagon-like peptide-1 (7-36) amide (7-37) (GLP-1) has been found to be a potent insulinotropic hormone, it has been postulated that glucagon stimulates insulin secretion from islet beta cells through the GLP-1 receptor. We therefore examined the effects of a GLP-1 receptor antagonist, exendin (9-39) amide, on glucagon-or GLP-l-stimulated insulin release from isolated perfused rat pancreas. When infusion of 100 nmol/1 exendin (9-39) amide was started 5 min before that of 1 nmol/1 glucagon, the stimulation of insulin release by glucagon was similar to that found in the control situation (preinfusion with vehicle alone). By contrast, when 0.3 nmol/1 GLP-1 was used in the same experimental setting, exendin (9-39) amide clearly inhibited insulin release. These results indicate that glucagon stimulates insulin release mainly through glucagon receptors but not GLP-1 receptors on islet beta cells. [Diabetologia (1995) 38: 274-276] Key words Glucagon, insulin secretion, exendin (9-39), GLP-1, pancreas perfusion.Glucagon stimulates insulin secretion from islet beta cells although its physiologic significance is not yet clear [1]. As a mechanism of this action, it had been generally accepted that insulin secretion is stimulated by glucagon directly through its own receptor on the beta cell, because specific binding sites of ~25I-glucagon have been demonstrated in hamster beta-cell tumours [2] and purified beta cells [3]. Since then however, a proglucagon gene product, glucagon-like peptide-1 (7-36) amide/(7-37) (GLP-1), was shown to stimulate insulin secretion more potently [4,5], and has been accepted as a physiologic insulin secretagogue; a candidate for incretin [6]. Specific receptors for GLP-1 have been detected on rat, mouse, and hamster insulinoma cell lines [7][8][9], and it is bound by glucagon with an affinity 100 to 1000 times lower than GLP-1 [7,8]. The insulinotropic activity of glucagon is less potent than that of GLP-1 in the same order [10], leading to the hypothesis that glucagon exhibits its insulinotropic activity through GLP-1 receptors but not glucagon receptors on islet beta cells [11]. Recently, it has been shown that exendin (9-39) amide is a potent receptor antagonist of GLP-1 [12,13]. In this study, we examined the effects of exendin (9-39) amide on GLP-1 or glucagonstimulated insulin release from isolated perfused rat pancreas in order to clarify whether or not glucagon stimulates the insulin release through its own receptor. Materials and methodsChemicals. Glucagon, GLP-1 (7-36) amide (GLP-1) and exendin (9-39) amide (exendin (9-39)) were synthesized by the stepwise solid-phase method using an automatic synthesizer (model 430A, Applied Biosystem, Foster City, Calif., USA), and then purified by high-performance liquid chromatography (HPLC). The purity of peptides was monitored by analytical reverse-phase HPLC on a column of Nucleosil 5C-18 (4.6 x 150 mm; GL Sciences, Tokyo, Japan) under the isocratic conditions of 0.1% trifluoroacetic acid and 39 % acetonitril, and proved to be ...

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