The role of G proteins in cholinergic suppression of Ca2+-activated K current was studied in isolated canine colonic myocytes with the whole cell voltage-clamp technique. Acetylcholine (ACh; 10.0 microM) caused a 64 +/- 2.4% depression in the Ca2+-dependent component of the outward current evoked at potentials between -45 and -15 mV when GTP (0.1 microM) was included in the pipette-filling solution. This effect was reversed within 2-4 min on washout of ACh. Without GTP in the filling solution, ACh caused a 15 +/- 2.5% depression in outward current in 60% of the cells tested. When the non-hydrolyzable GTP analogues, GTP gamma S (0.1 mM) or 5'-guanylylimidodiphosphate (GppNHp; 0.1 mM) were used, the decrease in outward current was greater (85 +/- 4.2 and 78 +/- 6.5%, respectively), and it was not reversed on withdrawal of ACh. Dialysis of the cell interior with pipette solution containing pertussis toxin (1 ng/ml) for 30 min had no effect on the whole cell currents evoked on depolarization, but it abolished the effect of ACh on Ca2+-dependent outward current. These data suggest that coupling of muscarinic receptors to the inhibition of Ca2+-activated K channels is mediated by pertussis toxin-sensitive G proteins in colonic smooth muscle cells. G protein-mediated inhibition is distinctly different from the opening of muscarinic-regulated K channels in other cell types.