Glutamate is a positive autocrine signal for glucagon release

Abstract

An important feature of glucose homeostasis is the effective release of glucagon from the pancreatic alpha cell. The molecular mechanisms regulating glucagon secretion are still poorly understood. We now demonstrate that human alpha cells express ionotropic glutamate receptors (iGluRs) that are essential for glucagon release. A lowering in glucose concentration results in the release of glutamate from the alpha cell. Glutamate then acts on iGluRs of the AMPA/kainate type, resulting in membrane depolarization, opening of voltage-gated Ca(2+) channels, increase in cytoplasmic free Ca(2+) concentration, and enhanced glucagon release. In vivo blockade of iGluRs reduces glucagon secretion and exacerbates insulin-induced hypoglycemia in mice. Hence, the glutamate autocrine feedback loop endows the alpha cell with the ability to effectively potentiate its own secretory activity. This is a prerequisite to guarantee adequate glucagon release despite relatively modest changes in blood glucose concentration under physiological conditions.

Figure 1. Activation of Ionotropic Glutamate Receptors in Human Islets Induces Glucagon Secretion

(A) Glutamate induced glucagon responses that could be blocked by CNQX (10 μM). Kainate and AMPA (both 100 μM) also elicited strong glucagon secretion. (B) Quantification of results in (A) (n = 5 islet preparations). (C) The metabotropic glutamate receptor antagonist CPPG (100 μM) did not affect the glutamate-induced glucagon response. The metabotropic glutamate receptor agonists trans-ACPD (tACPD; 100 μM) and ACPT-1 (100 μM) did not elicit changes in glucagon secretion. (D) Quantification of results in (C) (n = 3 islet preparations). (E) Insulin release was induced by high glucose (11 mM, 11G) but not by kainate (representative of six islet preparations). (F) Glutamate elicited concentration-dependent [Ca²⁺]_i responses in human islets (n = 4 islet preparations). Data were curve fitted using the Hill equation. Results are shown as mean ± SEM.

Figure 2. α Cells in Human Pancreatic Islets Express Functional Ionotropic Glutamate Receptors of the AMPA/Kainate Type

(A) Top row: three sequential images showing [Ca²⁺]_i responses to 3 mM glucose (3G), 11 mM glucose (11G), and 100 μM glutamate in dispersed human islet cells (pseudocolor scale). Bottom left: glucagon (green) and insulin (red) immunostaining. Glucagon-immunoreactive cells 2 and 3 responded to glutamate but not to 11G. Insulin-immunoreactive cells 1 and 4 responded to 11G but not to glutamate. Bottom right: traces of the [Ca²⁺]_i responses of these cells. Arrows indicate the time points at which the images were taken. Bars under traces indicate stimulus application. Scale bar = 50 μm. 1G = 1 mM glucose. Results are representative of four human islet preparations. (B) Gene expression profiling of individual α cells (left) and β cells (right) from human (red circles) and monkey (black circles) islets. Each row represents a single cell; each column represents a different glutamate receptor gene. Black or red circles denote that RT-PCR products were detected.

Figure 3. α Cell Responses to Kainate Require Ca²⁺ Influx through Voltage-Dependent Ca²⁺ Channels

(A) Activation of ionotropic glutamate receptors (iGluRs) elicits inward currents in human islet cells. Top left: a representative whole-cell current evoked by kainate (100 μM). Bottom left: the kainate-evoked current could be blocked with the AMPA/kainate receptor antagonist NBQX (10 μM). Right: the amplitudes of these currents averaged for four cells. Holding potential = −70 mV. Bars over current traces indicate kainate application. (B) Perifusion assays show that kainate (100 μM) stimulated large increases in glucagon secretion (left) that were abolished in the absence of extra-cellular Ca²⁺ (0 Ca²⁺ + 1 mM EGTA) and strongly diminished in the presence of the Ca²⁺ channel blocker La³⁺ (30 μM) or a combination of the specific Ca²⁺ channel inhibitors nimodipine (10 μM), conotoxin GVIA (1 μM), agatoxin IVA (0.1 μM), and mibefradil (1 μM). Average traces are shown (n = 3 human islet preparations). Arrow indicates switch to new solution. (C) An averaged trace (n = 9 cells; three monkey islet preparations) showing that [Ca²⁺]_i responses to kainate were abolished at nominal 0 Ca²⁺. (D) [Ca²⁺]_i responses to kainate were inhibited by CNQX (10 μM, n = 7 cells) and the Ca²⁺ channel blockers La³⁺ (30 μM, n = 14 cells) and nifedipine (10 μM, n = 18 cells). Shown are the means of the peak amplitudes of the [Ca²⁺]_i responses (changes in the 340/380 fluorescence emission ratio) to kainate. Data are from three separate monkey islet preparations. *p < 0.05 by Student’s t test. Error bars represent ±SEM.

Figure 4. Stimulated Primate α Cells Release Glutamate

(A) Confocal images of a monkey pancreatic section containing an islet. Immunoreactivity for the vesicular glutamate transporter 1 (vGluT1) colocalized with glucagon but not insulin immunostaining. Results are representative of three human pancreata. Scale bar = 20 μm. (B) Representative images of islets in experiments using a fluorescent enzymatic assay to detect glutamate release. In this assay, released glutamate is a substrate in an enzymatic chain reaction that generates the fluorescent product resorufin. Resorufin fluorescence is color coded; an increase from low (blue, rest) to high (yellow, kainate) indicates increased glutamate release in response to kainate. There was no fluorescence increase in the absence of the enzyme glutamate oxidase (−GO, bottom panel). Scale bar = 50 μm. (C and D) In the absence of the enzyme glutamate oxidase (−GO), application of kainate and KCl did not increase resorufin fluorescence. Low glucose (1 mM, 1G; p = 0.005), kainate (100 μM, kain; p < 0.001), and KCl (30 mM; p < 0.001) depolarization, but not high glucose (11 mM, 11G; p = 0.289), induced significant glutamate release from islets as compared to kainate without GO (−GO; n = 3 monkey islet preparations; one-way ANOVA followed by multiple-comparisons procedure by Student-Newman-Keuls method). a.u. = arbitrary units. Results in (D) are presented as mean ± SEM.

Figure 5. A Stimulatory Autocrine Glutamate Feedback Loop Is Needed for Effective Glucagon Release

(A) Illustration of the experimental approach used to measure islet glucagon secretion in real time using glucagon-sensitive biosensor cells. (B) Exogenous glutamate elicited glucagon secretion from human islets as measured by [Ca²⁺]_i responses in individual glucagon biosensor cells (traces at left). No responses were seen in the glucagon biosensor cells in the absence of human islets (traces at right). (C) Lowering the glucose concentration from 6 mM to 1 mM (arrow) induced glucagon secretion as measured by [Ca²⁺]_i responses in biosensor cells (n = 6 regions of interest). Rinsing caused an abrupt decrease in [Ca²⁺]_i in biosensor cells. Results in (C) and (D) are presented as mean ± SEM. (D) Quantification of the data in (C) shows that the AMPA/kainate iGluR antagonist CNQX (10 μM) significantly inhibited the effect of glucose lowering on glucagon release by 54% (measured 8 min after reducing the glucose concentration; n = 3 islet preparations; *p = 0.042 by Student’s t test).

Figure 6. Glutamate Signaling in Mouse Islets Is Similar to that in Primate Islets

(A) Left: perifusion assays of mouse islets showed that glutamate (100 μM) stimulated glucagon release that was blocked by DNQX (10 μM) (n = 6 perifusions). Right: kainate and AMPA (both 100 μM) also stimulated glucagon secretion (n = 3 perifusions). (B) Left: the metabotropic glutamate receptor agonists tACPD (100 μM) and ACPT-1 (100 μM) did not elicit changes in glucagon secretion. Right: the metabotropic glutamate receptor antagonist CPPG (100 μM) did not affect the glutamate-induced glucagon response (n = 3 perifusions). (C) Glucagon responses to glutamate (100 μM) in islets from mice lacking the metabotropic glutamate receptor mGluR4 were not different from those of islets from control mice (n = 4 islet preparations per group). (D) Insulin release was induced by high glucose (11 mM, 11G) but not by kainate (left) or by AMPA or glutamate (right) (n = 3 islet preparations). Results are presented as mean ± SEM.

Figure 7. In Vivo Activation of iGluRs Stimulates Glucagon Secretion

(A) Left: mice treated systemically with glutamate (30 mg/kg i.p.; n = 7 mice) or AMPA (15 mg/kg i.p.; n = 8 mice) showed increased plasma glucagon concentrations (*p < 0.05 by ANOVA). Middle: plasma insulin concentrations did not change. Right: 30 min after AMPA injection, AMPA-treated mice showed increased plasma glucose concentrations (●, n = 8 mice; *p < 0.05 by Student’s t test) as compared to PBS-injected mice (○, n = 4 mice). Results in (A)–(C) are presented as mean ± SEM. (B) Hyperinsulinemic-hypoglycemic clamp to provide a constant hypoglycemic stimulus at a blood glucose concentration of ~3 mM (left) was induced with insulin infusion (middle). Glucagon secretion in response to hypoglycemia was significantly diminished in mice after NBQX infusion (right) (10 mg/kg; red circles, n = 7) compared with saline-infused mice (black circles, n = 5; *p < 0.05 by repeated-measures ANOVA). Horizontal bar indicates drug infusion. (C) The glucose infusion rate needed to maintain glycemia after drug infusion was significantly larger in NBQX-treated mice (red bars, n = 7) than in saline-treated mice (black bars, n = 5). *p < 0.05 by Student’s t test. (D) Proposed model for the regulation of glucagon secretion. Activation of α cells depends on an initial stimulus as well as on positive feedback. When glucose levels fall, there is less suppression from β cell-derived GABA, Zn²⁺, or insulin. Positive feedback by glutamate strongly amplifies glucagon secretion. Once glucose levels increase, glucagon secretion is inhibited by insulin, Zn²⁺, GABA, or a combination of the three. Without glutamate feedback, α cells are not fully activated and glucagon secretion is deficient.