Mechanisms by which glucose controls glucagon secretion in mouse alpha-cells : role of KATP channels and paracrine factors
(2012)
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- Authors
- Supervisors
- Gilon, Patrick
- Abstract
- (en) The cellular mechanisms by which glucose (G) controls glucagon secretion are poor-understood. In particular, we ignore if G exerts a directly or an indirectly effect on α-cells. The hypothesis of an indirect effect suggests that G stimulates the release of one or several paracrine factors by non-α-cells within islets which would inhibit glucagon secretion. Some factors have been suggested: insulin, GABA released by β-cell, zinc co-released with insulin, or somatostatin released by δ-cells. One major reason for our poor understanding of the physiology of α-cells is that identification of living α-cells among other islet cells is not straightforward. To circumvent this difficulty, we generated two new mouse models, referred to as GYY and RIPYY, allowing easy α-cell and β-cell identification, respectively, because of cell specific expression of EYFP (Enhanced Yellow fluorescence protein). In a first study, I contributed to the validation of the GYY model. 76%α-cells were fluorescent for EYFP. GYY mice displayed normal glycemic control during a fasting/refeeding test or intraperitoneal insulin injection. Glucagon secretion by isolated islets was normally inhibited by G and stimulated by adrenaline. [Ca2+]c responses to arginine, adrenaline, diazoxide and tolbutamide, were similar in GYY and control mice (NMRI or C57BL/6J). In a second study, we compared the effect of G and modulators of KATP channels on various physiological parameters (NAD(P)H fluorescence, IKATP, and [Ca2+]c) of isolated α- and β-cells (from GYY and RIPYY mice, respectively). We showed that α-cells display similarities with β-cells: KATP channels control Ca2+ influx mainly through L-type Ca2+ channels. However, α-cells have distinct features from β-cells: most KATP channels are already closed at low G, G does not affect cell metabolism and IKATP, and it slightly decreased [Ca2+]c. Hence, G and KATP channel modulators exert distinct effects on α-cell [Ca2+]c. The lowering of [Ca2+]c resulting from a direct action of G on α-cells is modest and probably insufficient to account for the robust inhibition of glucagon secretion produced by G in these islets. It is therefore likely that the glucagonostatic effect of G in the intact islets results from a combination of both a small direct effect and a more important indirect effect by islet factors. Three inhibitory paracrine candidates, GABA, zinc and insulin, have been tested on α-cell [Ca2+]c, but we found that none of them accounts for the inhibitory effect of G. In order to study the mechanisms of control of glucagon secretion by G under more physiological conditions, we performed experiments on intact islets and evaluated how G, pharmacological agents and certain paracrine factors affect glucagon secretion. G inhibited glucagon secretion in conditions where KATP channels were pharmacologically bypassed (high concentrations of diazoxide or tolbutamide) or genetically knocked out (islets from Sur1-/- mice), suggesting that it can inhibit glucagon secretion independently from KATP channels. Tolbutamide decreased glucagon secretion at 1 mM G but stimulated it at 7 mM. Diazoxide did not reverse the glucagonostatic effect of G. Removal of the somatostatin paracrine influence (islets from Sst-/- mice or pretreated with pertussis toxin) strongly increased glucagon release, did not prevent the glucagonostatic effect of 7mM G and unmasked a marked glucagonotropic effect of tolbutamide. Knockout of the Zn2+ transporter ZnT8 (ZnT8-/-mice) did not prevent the glucagonostatic effect of G. This suggests that glucose can inhibit glucagon release independently of Zn2+, KATP channels and somatostatin. Closure of KATP channels controls glucagon secretion by two mechanisms: a direct stimulation of α-cells and an indirect inhibition via somatostatin released from d-cell. The net effect on glucagon release results from a balance between both effects. The direct stimulatory effect of tolbutamide on α-cells should be considered during treatment of type 2 diabetic patients by sulfonylureas because stimulation of glucagon secretion by the drugs could contribute to the unwanted hyperglucagonemia found in diabetes.
- Affiliations
Citations
Cheng, R. (2012). Mechanisms by which glucose controls glucagon secretion in mouse alpha-cells : role of KATP channels and paracrine factors. https://hdl.handle.net/2078.5/75500