ATP-Sensitive K+ Channel–Dependent Regulation of Glucagon Release and Electrical Activity by Glucose in Wild-Type and SUR1−/− Mouse α-Cells

  1. Jesper Gromada1,
  2. Xiaosong Ma2,
  3. Marianne Høy3,
  4. Krister Bokvist1,
  5. Albert Salehi2,
  6. Per-Olof Berggren4 and
  7. Patrik Rorsman5
  1. 1From the Lilly Research Laboratories, Hamburg, Germany
  2. 2Department of Physiological Sciences, Lund, Sweden
  3. 3Department of Medical Physiology, The Panum Institute, University of Copenhagen, Copenhagen, Denmark
  4. 4Department of Molecular Medicine, The Rolf Luft Center for Diabetes Research, Karolinska Institutet, Stockholm, Sweden
  5. 5Diabetes Research Laboratories, Oxford Centre for Diabetes, Endocrinology and Metabolism, The Churchill Hospital, Oxford, England
  1. Address correspondence and reprint requests to Jesper Gromada, Lilly Research Laboratories, Essener Strasse 93, D-22419 Hamburg, Germany. E-mail: gromada{at}lilly.com

Abstract

Patch-clamp recordings and glucagon release measurements were combined to determine the role of plasma membrane ATP-sensitive K+ channels (KATP channels) in the control of glucagon secretion from mouse pancreatic α-cells. In wild-type mouse islets, glucose produced a concentration-dependent (half-maximal inhibitory concentration [IC50] = 2.5 mmol/l) reduction of glucagon release. Maximum inhibition (∼50%) was attained at glucose concentrations >5 mmol/l. The sulfonylureas tolbutamide (100 μmol/l) and glibenclamide (100 nmol/l) inhibited glucagon secretion to the same extent as a maximally inhibitory concentration of glucose. In mice lacking functional KATP channels (SUR1−/−), glucagon secretion in the absence of glucose was lower than that observed in wild-type islets and both glucose (0–20 mmol/l) and the sulfonylureas failed to inhibit glucagon secretion. Membrane potential recordings revealed that α-cells generate action potentials in the absence of glucose. Addition of glucose depolarized the α-cell by ∼7 mV and reduced spike height by 30% Application of tolbutamide likewise depolarized the α-cell (∼17 mV) and reduced action potential amplitude (43%). Whereas insulin secretion increased monotonically with increasing external K+ concentrations (threshold 25 mmol/l), glucagon secretion was paradoxically suppressed at intermediate concentrations (5.6–15 mmol/l), and stimulation was first detectable at >25 mmol/l K+. In α-cells isolated from SUR1−/− mice, both tolbutamide and glucose failed to produce membrane depolarization. These effects correlated with the presence of a small (0.13 nS) sulfonylurea-sensitive conductance in wild-type but not in SUR1−/− α-cells. Recordings of the free cytoplasmic Ca2+ concentration ([Ca2+]i) revealed that, whereas glucose lowered [Ca2+]i to the same extent as application of tolbutamide, the Na+ channel blocker tetrodotoxin, or the Ca2+ channel blocker Co2+ in wild-type α-cells, the sugar was far less effective on [Ca2+]i in SUR1−/− α-cells. We conclude that the KATP channel is involved in the control of glucagon secretion by regulating the membrane potential in the α-cell in a way reminiscent of that previously documented in insulin-releasing β-cells. However, because α-cells possess a different complement of voltage-gated ion channels involved in action potential generation than the β-cell, moderate membrane depolarization in α-cells is associated with reduced rather than increased electrical activity and secretion.

Footnotes

  • This article is based on a presentation at a symposium. The symposium and the publication of this article were made possible by an unrestricted educational grant from Servier.

    • Accepted May 18, 2004.
    • Received March 12, 2004.
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