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 (EC(50) = 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 G(i)- or G(o)-type G proteins stimulated the K-ATP(+) channel (EC(50) = 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 mas unaffected by the presence of pertussis toxin, but stimulation by exogenous alpha-subunits of the G protein G(i) or G(o) was prevented inv 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.