Muscarine and somatostatin enhance an inward rectifier K+ conductance in the AtT-20 pituitary cell line. Both effects are abolished by pertussis toxin (PTX). To determine which PTX-sensitive G protein mediates these agonist effects, we made cDNAs encoding mutant PTX-insensitive Gi alpha subtypes, in which the cysteine residue fourth from the C terminus was replaced with serine. The mutated cDNA was transfected into AtT-20 cells, resulting in stable cell lines overexpressing a Gi alpha subtype. As controls, wild-type Gi alpha cDNA was transfected into AtT-20 cells. The agonist-induced increase of the inward rectifier K+ conductance in the transfectants was examined with the whole-cell clamp method. Only in the cell lines into which the mutated (PTX-insensitive) Gi2 alpha cDNA was transfected, did the muscarine response become PTX-insensitive, suggesting that Gi2 couples to the muscarinic receptor and enhances the activity of the inward rectifier K+ channel. However, PTX-insensitive somatostatin responses were not obtained in any of the cell lines transfected with a mutated Gi alpha cDNA, suggesting either that none of the Gi subtypes is a transducer for the somatostatin effect or that the mutation prevents the coupling of the Gi alpha to the somatostatin receptor.