NUCLEOTIDE RECEPTOR-SITE ON THE LABELLAR SUGAR RECEPTOR CELL OF THE BLOWFLY PHORMIA-REGINA

Previously we reported that, among nucleotides applied extracellularly to the chemosensillum of the blowfly Phormia regina, a membrane-permeable cyclic GMP analogue, dibutyryl cyclic GMP, was particularly effective, inducing much impulse-discharge. From the results obtained, we discussed the possibility that a nucleotide site exists on the sugar receptor cell. Here we report results of our electrophysiological experiments to investigate this putative nucleotide receptor-site. As previously reported, when dibutyryl cyclic GMP was applied with sucrose, the impulse-discharge was depressed. Cyclic GMP or another membrane-permeable cyclic GMP analogue, 8-bromo cyclic GMP, enhanced the impulse frequency when applied to a chemosensillum with sucrose. However, 8-bromo cyclic GMP partly depressed the impulse-discharge evoked by dibutyryl cyclic GMP. If we hypothesize that the sugar receptor-site has a nucleotide receptor-site separated from the receptor site for sucrose (P-site) on the membrane and that the cell uses cyclic GMP as an intrinsic intracellular messenger mediating the taste transduction, then, these results can be explained as follows. Dibutyryl cyclic GMP inhibits the P-site with its butyryl groups. On the other hand, it binds the nucleotide receptor-site and stimulates the cell like other nucleotides. However, 8-bromo cyclic GMP, which does not affect the P-site, binds the nucleotide receptor-site like a competitive inhibitor. Thus, the impulses induced by 8-bromo cyclic GMP are due to the 8-bromo cyclic GMP acting in place of cyclic GMP as the intracellular messenger. As for the responses induced by dibutyryl cyclic GMP, a part which can be reduced by 8-bromo cyclic GMP is due to the dibutyryl cyclic GMP binding the nucleotide receptor-site, and other part is due to the dibutyryl cyclic GMP acting as the intracellular messenger.