Insulinotropic glucagon-like peptide-1-mediated activation of non-selective cation currents in insulinoma cells is mimicked by maitotoxin.

Colin A Leech,Joel F Habener

doi:10.1074/jbc.272.29.17987

Abstract

Maitotoxin (MTX) activates a Ca2+-dependent non-selective cation current (ICa-NS) in insulinoma cells whose time course is identical to non-selective cation currents activated by incretin hormones such as glucagon-like peptide-1 (GLP-1), which stimulate glucose-dependent insulin secretion by activating cAMP signaling pathways. We investigated the mechanism of activation of ICa-NS in insulinoma cells using specific pharmacological reagents, and these studies further support an identity between MTX- and GLP-1-activated currents. ICa-NS is inhibited by extracellular application of genistein, econazole, and SKF 96365. This inhibition by genistein suggests that tyrosine phophorylation may play a role in the activation of ICa-NS. ICa-NS is not inhibited by incubation of cells in glucose-free solution, by extracellular tetrodotoxin, nimodipine, or tetraethylammonium, or by intracellular dialysis with 4-aminopyridine, ATP, ryanodine, or heparin. ICa-NS is also not significantly inhibited by staurosporine, which does, however, partially inhibit the MTX-induced rise of intracellular Ca2+ concentration. These effects of staurosporine suggest that protein kinase C may not be involved in the activation of ICa-NS but that it may regulate intracellular Ca2+ release. Alternatively, ICa-NS may have a small component that is carried through separate divalent cation-selective channels that are inhibited by staurosporine. ICa-NS is neither activated nor inhibited by dialysis with KF, KF + AlF3 or GTPgammaS (guanosine 5'-O-(3-thiotriphosphate)), suggesting that GTP-binding proteins do not play a major role in the activation of this current.

Highlights

The consensus model of glucose-stimulated insulin secretion is that closure of ATP-sensitive K1 channels (K1ATP)1 permits membrane depolarization, activation of voltage-dependent Ca21 channels (VDCCs) and the influx of Ca21 [1]
Increased cAMP levels stimulate Ca21 release from secretory granules and reduce mitochondrial Ca21 uptake in b-cells [13, 14]. These observations raise the possibility that the activation of non-selective cation currents by pituitary adenylate cyclase-activating polypeptide (PACAP) and glucagon-like peptide-1 (GLP-1) may be a secondary consequence of glucose- and cAMP-dependent intracellular Ca21 release
The aim of this study is to examine the mechanism of activation of the MTX-sensitive current and to compare the properties of ICa-NS with the current activated by GLP-1, PACAP, and cAMP to determine whether these currents are likely to be carried through the same channels

Summary

Introduction

The consensus model of glucose-stimulated insulin secretion is that closure of ATP-sensitive K1 channels (K1ATP) permits membrane depolarization, activation of voltage-dependent Ca21 channels (VDCCs) and the influx of Ca21 [1]. Individual b-cells, are often unresponsive to glucose alone, but can become responsive by combined stimulation with glucose and hormones, such as insulinotropic hormone glucagon-like peptide-1 (GLP-1; Ref. 2), that elevate intracellular cAMP levels [3,4,5] One mechanism underlying this increased responsiveness is the enhanced closure of K1ATP channels [2]. The physiological role of Ca21 releaseactivated currents in b-cells remains controversial, but such currents have been suggested to play a role in the cholinergic modulation of electrical bursting activity [15], and may control the membrane potential and intracellular Ca21 ([Ca21]i) oscillations in response to nutrient stimulation [10] Both PACAP [16] and GLP-1 [17] are potent insulin secretagogues in the presence of slightly elevated glucose levels. The aim of this study is to examine the mechanism of activation of the MTX-sensitive current and to compare the properties of ICa-NS with the current activated by GLP-1, PACAP, and cAMP to determine whether these currents are likely to be carried through the same channels

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