Fig. 4.

The effect of lactisole and cyclamate on the human T1R1/T1R3 umami taste receptor. (A) Response of the human T1R1/T1R3 stable cell line tol-glutamate (5 mM) and l-glutamate/IMP (1/0.2 mM) in the absence and presence of lactisole (5 mM). (B) The lactisole dose-dependent inhibition curves were determined for l-glutamate (Glu) and l-glutamate with 0.2 mM IMP (Glu/IMP), each at two different concentrations. The IC₅₀s are 0.19 ± 0.02 and 0.21 ± 0.01 mM for l-glutamate at 8 and 80 mM, respectively, and 0.35 ± 0.03 and 0.82 ± 0.06 mM for l-glutamate with IMP at 0.8 and 8 mM, respectively. (B) The dose–responses for l-glutamate, with or without 0.2 mM IMP, were determined in the presence of different concentrations of lactisole. In the presence of 0, 25, or 50 μM lactisole, the EC₅₀s are 9.9 ± 1.5, 7.9 ± 0.5, and 7.0 ± 0.3 mM, respectively, for l-glutamate. In the presence of 0, 100, or 200 μM lactisole, the EC₅₀s are 0.53 ± 0.04, 0.71 ± 0.10, and 0.84 ± 0.10 mM, respectively, for l-glutamate with IMP. Values represent the mean ± SE for four independent responses. (D) The detection thresholds for sweet, umami, and salty taste stimuli were determined in the presence or absence of lactisole. The inhibition effect of lactisole is shown as fold increases in detection thresholds. The detection threshold values were averaged over four trials for three subjects. (E) The responses of the human T1R1/T1R3 stable cell line to threshold level of l-glutamate (4 mM) and endogenous M2 receptor agonist carbachol were assayed on FLIPR in the absence and presence of various concentrations of cyclamate. (F) Dose–responses of the human T1R1/T1R3 stable cell line were determined on FLIPR for l-glutamate with or without 0.2 mM IMP in the absence and presence of cyclamate (8 mM). The activities in B, C, E, and F represent the mean ± SE of fold increases in fluorescence intensities for four recorded wells. The dose–responses in B, C, E, and F were reproduced at least six times independently.