The role of protein kinase C activation in a coupling of Ca2+-mobilizing receptors/GTP-binding protein/phospholipase C was examined using Xenopus oocytes before and after microinjection of mRNA purified from rat brains. Under voltage-clamp conditions, although the phorbol ester TPA per se never elicited any changes in ionic conductance, chloride current responses of mRNA-injected cells to 5-hydroxytryptamine and acetylcholine (ACh) were suppressed by an 8-min pretreatment of 12-O-tetradecanoyl-4 beta-phorbol-13-acetate (TPA), at nanomolar concentrations. Native ACh response in intact follicular oocytes was also inhibited by the TPA treatment. However, similar current responses triggered by the direct activation of their intracellular signalling pathway with guanosine-5'-O-(3-thio)triphosphate or Ca2+ were not affected by TPA. Biochemical analyses indicated that phosphorylation of 33,000- and 45,000-dalton proteins was markedly enhanced by TPA in vivo, and that stimulation of receptors with agonists as well as TPA treatment increased phosphoproteins in the membrane fraction of mRNA-injected oocytes. These observations suggest that protein kinase C may switch off the signal transduction from receptors to GTP-binding proteins and may participate in the negative feedback modulation of receptor-operated ion channel responses.