Inhibition of connexin 36 hemichannels by glucose contributes to the stimulation of insulin secretion

• Published in: American journal of physiology: endocrinology and metabolism 2014-06, Vol.306 (12), p.E1354-E1366
• Main Authors: Pizarro-Delgado, Javier, Fasciani, Ilaria, Temperan, Ana, Romero, María, González-Nieto, Daniel, Alonso-Magdalena, Paloma, Nualart-Marti, Anna, Estil'les, Elisabet, Paul, David L, Martín-del-Río, Rafael, Montanya, Eduard, Solsona, Carles, Nadal, Angel, Barrio, Luis Carlos, Tamarit-Rodríguez, J
• Format: Article
• Language: English
• Subjects: Adenosine triphosphatase; Adenosine Triphosphate - metabolism; Animals; Blood Glucose - analysis; Blood Glucose - metabolism; Cells; Connexins - antagonists & inhibitors; Connexins - genetics; Connexins - metabolism; Gap Junction delta-2 Protein; Gap Junctions - drug effects; Gap Junctions - metabolism; Glucose; Glucose Intolerance - blood; Glucose Intolerance - metabolism; Heterozygote; Hyperglycemia - etiology; Hyperglycemia - metabolism; Insulin; Insulin - metabolism; Insulin Secretion; Insulin-Secreting Cells - drug effects; Insulin-Secreting Cells - metabolism; Male; Membrane Potentials - drug effects; Membrane Transport Modulators - pharmacology; Metabolism; Mice; Mice, 129 Strain; Mice, Inbred C57BL; Mice, Knockout; Rats; Rats, Wistar; Recombinant Proteins - chemistry; Recombinant Proteins - metabolism; Tissue Culture Techniques; Up-Regulation - drug effects
• ISSN: 0193-1849; 1522-1555
• DOI: 10.1152/ajpendo.00358.2013

The existence of functional connexin36 (Cx36) hemichannels in β-cells was investigated in pancreatic islets of rat and wild-type (Cx36(+/+)), monoallelic (Cx36(+/-)), and biallelic (Cx36(-/-)) knockout mice. Hemichannel opening by KCl depolarization was studied by measuring ATP release and changes of intracellular ATP (ADP). Cx36(+/+) islets lost ATP after depolarization with 70 mM KCl at 5 mM glucose; ATP loss was prevented by 8 and 20 mM glucose or 50 μM mefloquine (connexin inhibitor). ATP content was higher in Cx36(-/-) than Cx36(+/+) islets and was not decreased by KCl depolarization; Cx36(+/-) islets showed values between that of control and homozygous islets. Five minimolar extracellular ATP increased ATP content and ATP/ADP ratio and induced a biphasic insulin secretion in depolarized Cx36(+/+) and Cx36(+/-) but not Cx36(-/-) islets. Cx36 hemichannels expressed in oocytes opened upon depolarization of membrane potential, and their activation was inhibited by mefloquine and glucose (IC₅₀ ∼8 mM). It is postulated that glucose-induced inhibition of Cx36 hemichannels in islet β-cells might avoid depolarization-induced ATP loss, allowing an optimum increase of the ATP/ADP ratio by sugar metabolism and a biphasic stimulation of insulin secretion. Gradual suppression of glucose-induced insulin release in Cx36(+/-) and Cx36(-/-) islets confirms that Cx36 gap junction channels are necessary for a full secretory stimulation and might account for the glucose intolerance observed in mice with defective Cx36 expression. Mefloquine targeting of Cx36 on both gap junctions and hemichannels also suppresses glucose-stimulated secretion. By contrast, glucose stimulation of insulin secretion requires Cx36 hemichannels' closure but keeping gap junction channels opened.