A nonhydrolyzable G protein activator (guanosine 5'-O-(3-thiotriphosphate); GTPγS) and a G protein inhibitor (guanosine 5'-O-(2-thiodiphosphate); GDPβS) were introduced into the labellar taste receptor cells of the fleshfly by treatment of their receptive membranes beneath the tip opening of the chemosensory hair with each reagent in 0.03% deoxycholate solution for 4 min. After treatment with GTPγS, the responses of the sugar receptor cell to D-glucose, D-fructose, L-phenylalanine and L-valine and that of the salt receptor cell to cyclic AMP were markedly enhanced, compared with those after treatment with deoxycholate alone. Treatment with GDPβS depressed these responses. These results strongly suggest that the responses are mediated by G protein. However, the response of the salt receptor cell to NaCl was not affected by treatment with either GTPγS or GDPβS, and thus the response to NaCl clearly is not elicited through a G protein-regulated mechanism.
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