Characterization of the G protein coupling of a glucagon receptor to the KATP channel in insulin-secreting cells.
The G-protein-mediated coupling of a glucagon receptor to ATP-dependent K channels--KATP--has been studied in insulin-secreting cells using the patch clamp technique. In excised outside-out patches, KATP channel activity was inhibited by low concentrations of glucagon (IC50 = 2.4 nM); the inhibitory effect vanished at concentrations greater than 50 nM. In cell-attached patches, inhibition by bath-applied glucagon was seen most often, although stimulation was observed in a few cases. A dual action of the hormone is proposed to resolve these apparently divergent results. In excised inside-out patches, KATP channel activity was inhibited by addition of beta gamma subunits purified from either erythrocyte or retina (IC50 = 50 pM and 1 nM, respectively). Subsequent exposure of the patch to alpha i or alpha o reversed this effect. In excised inside-out patches, increasing Mg2+ in the bath stimulated the channel activity between 0 and 0.5 nM, but blocked it at higher concentrations (IC50 = 2.55 mM). In most cases (70%), GTP had a stimulatory effect at concentrations up to 100 microns. However, in three cases, similar GTP levels had clear inhibitory effects. In excised inside-out patches, cholera toxin (CTX) caused channel inhibition. Although the effect could not be reversed by removal of the toxin, the activity was restored by subsequent addition of purified alpha i or alpha o. These results are compatible with a model whereby channel inhibition by activated Gs-coupled receptors occurs, at least in part, via association of the beta gamma subunits of Gs with alpha i/alpha o subunits and deactivation of the alpha i/alpha o-dependent stimulatory pathway. On the basis of this hypothesis, a model is developed to describe the effects of G proteins on the KATP channel, as well as to account for the concentration-dependent stimulation and inhibition of KATP channel by Mg2+. An interpretation of the ability of glucagon to potentiate, but not initiate, insulin release is also given in terms of this model and the effects of ATP on KATP channels.
Ribalet
B
B
Department of Physiology, Ahmanson Laboratory of Neurobiology, University of California, Los Angeles 90024.
Ciani
S
S
D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D002772:Cholera Toxin / Q000494:pharmacology; D006224:Cricetinae; D004594:Electrophysiology; D019204:GTP-Binding Proteins / Q000378:metabolism*; D005934:Glucagon / Q000494:pharmacology; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology; D006160:Guanosine Triphosphate / Q000494:pharmacology; D007328:Insulin / Q000378:metabolism; D000078790:Insulin Secretion; D007515:Islets of Langerhans / Q000378:metabolism*; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D008564:Membrane Potentials; D018408:Patch-Clamp Techniques; D015221:Potassium Channels / Q000378:metabolism*; D051381:Rats; D018027:Receptors, Glucagon / Q000378:metabolism*; D014407:Tumor Cells, Cultured
D007328:Insulin; D015221:Potassium Channels; D018027:Receptors, Glucagon; D016244:Guanosine 5'-O-(3-Thiotriphosphate); D006160:Guanosine Triphosphate; D000255:Adenosine Triphosphate; D005934:Glucagon; D002772:Cholera Toxin; D019204:GTP-Binding Proteins; D008274:Magnesium