Possible role of a GTP-binding protein in the activation of phospholipase C by carbamylcholine in tumoral insulin-producing cells.

Carbamylcholine, but not GTP gamma S, stimulated the production of [3H]inositol mono-, bis- and trisphosphate in intact tumoral insulin-producing cells (RINm5F line) prelabelled with myo-[2-3H(N)]inositol. On the contrary, GTP gamma S, but not carbamylcholine, stimulated the production of [3H]inositol mono-, bis- and trisphosphates in a subcellular particulate fraction derived from the prelabelled RINm5F cells. When the latter cells were first treated by digitonine, carbamylcholine or GTP caused a modest stimulation of [3H]inositol 1-phosphate production. However, in the presence of both GTP and carbamylcholine, the production of [3H]inositol 1-phosphate was much higher: it exceeded the value computed by summing the individual responses to each of these two agents, and reached the same level as that recorded in the presence of GTP gamma S. It is speculated that a GTP-binding regulatory protein, probably distinct from either Ns or Ni, couples the occupancy of muscarinic receptor to the activation of phospholipase C in pancreatic islet cells.