Responses to di-sodium guanosine 5'-monophosphate and monosodium L-glutamate in taste receptor cells of rat fungiform papillae.

The 5'-ribonucleotide guanosine 5'-monophosphate (GMP) is used widely as an umami taste stimulus and a potent flavor enhancer as it synergistically increases the umami taste elicited by monosodium glutamate. Transduction mechanisms for GMP and its synergy with glutamate are largely unknown. Using whole-cell patch-clamp and Ca(2+) imaging, we examined responses to GMP, glutamate, and a mixture of GMP and glutamate in taste-receptor cells of rat fungiform papillae. Our electrophysiological results showed that GMP induces responses that are similar to those of glutamate, e.g., an outward current, an inward current, or a biphasic response. Our Ca(2+) imaging results showed that applications of GMP, glutamate, and the mixture increased intracellular Ca(2+) levels. Interestingly, both patch-clamp and Ca(2+) imaging showed that some taste cells can respond to GMP and glutamate independently, indicating that glutamate and GMP likely activate different receptors. Simultaneous application of GMP and glutamate resulted in synergistic responses in a subset of cells; both response intensity and number of responding cells were increased. Most responses to GMP, as well as the synergy between GMP and glutamate, were suppressed by 8-bromo-adenosine 3',5'-cyclic monophosphate (8-bromo-cAMP) in patch-clamp recordings. Together, our results suggest that intracellular cAMP- and Ca(2+)-mediated pathways are involved in umami taste transduction for GMP and its synergistic responses with glutamate.