Characterization of the G protein coupling of a somatostatin receptor to the K+ATP channel in insulin-secreting mammalian HIT and RIN cell lines.
1. The G protein-mediated coupling of a somatostatin (somatotropin-releasing inhibitory factor; SRIF) receptor to the ATP-dependent K+ channel (K+ATP channel) has been studied in insulin-secreting cells using the patch clamp technique. 2. In excised outside-out patches, the concentration-dependent stimulation of the K+ATP channel by SRIF was biphasic. Stimulation reached a maximum at 15 nM (EC50 = 5.5 nM), then decayed to a minimum at 50 nM and returned to maximum stimulation at 500 nM. 3. In cell-attached patches, bath-applied SRIF caused K+ATP channel stimulation in most experiments. In a few cases, however, SRIF suppressed channel activity, a response that was reversed by addition of dibutyryl cyclic AMP (DBcAMP). Channel stimulation by SRIF or by DBcAMP did not occur in the presence of glucose. 4. In excised inside-out patches, the alpha-subunits of Gi or G(o)-type G proteins stimulated the K+ATP channel (EC50 = 29 and 42 pM, respectively). The K+ATP channel stimulation by alpha i- or alpha o-subunits had no effect on the concentration-dependent inhibition by ATP. 5. In excised inside-out patches, K+ATP channel activity was reduced by inhibitors of protein kinase C (PKC) and stimulated by a PKC activator. The stimulatory effect of PKC was unaffected by the presence of pertussis toxin, but stimulation by exogenous alpha-subunits of the G protein Gi or G(o) was prevented by PKC inhibitors. 6. From these data we deduce that SRIF can affect K+ATP channel activity directly via a membrane-delimited pathway or indirectly via a pathway requiring diffusible messengers. In the former case, alpha i/alpha o may either enhance PLC activity, stimulating PKC and thus inducing K+ATP channel phosphorylation with consequent increase of activity, or channel phosphorylation by PKC may facilitate a direct stimulation of the channel by alpha i/alpha o. In the latter case, an alpha i/alpha o-induced fall in cAMP contributes to reduced PKA-mediated phosphorylation and suppression of channel activity.
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Department of Physiology, University of California, Los Angeles 90024, USA.
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D000255:Adenosine Triphosphate / Q000378:metabolism*; D000818:Animals; D002460:Cell Line; D002478:Cells, Cultured; D006224:Cricetinae; D019204:GTP-Binding Proteins / Q000378:metabolism*; D005947:Glucose / Q000378:metabolism; D006801:Humans; D007328:Insulin / Q000378:metabolism*; D000078790:Insulin Secretion; D008647:Mesocricetus; D018408:Patch-Clamp Techniques; D037342:Pertussis Toxin; D015221:Potassium Channels / Q000187:drug effects / Q000378:metabolism*; D011493:Protein Kinase C / Q000378:metabolism; D051381:Rats; D017481:Receptors, Somatostatin / Q000378:metabolism*; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
D007328:Insulin; D015221:Potassium Channels; D017481:Receptors, Somatostatin; D010566:Virulence Factors, Bordetella; D000255:Adenosine Triphosphate; D037342:Pertussis Toxin; D011493:Protein Kinase C; D019204:GTP-Binding Proteins; D005947:Glucose