Fig. 4.

Amino acids synergistically enhance the hummingbird T1R1–T1R3 response to carbohydrates. (a) Dose-dependent responses of Anna’s hummingbird T1R1–T1R3 to mixes containing increasing concentrations of alanine or glycine combined with 100 mM sucrose. The observed response is higher than the combined (additive) response (gray circles, the sum of the response of 100 mM sucrose+the response of the amino acid, see Materials and Methods); notably, synergistic responses were observed at concentrations of amino acids too low to activate the receptor alone (10 and 25 mM) (*P < 0.05, two-tailed t-test; mean±SE, n = 6). The average response to 100 mM sucrose (n = 6) is indicated by the dashed orange line. (b) Synergistic responses of a panel of amino acids were assessed by comparing the response of 50 mM sucrose to combined presentations of amino acids and sucrose (50 mM each); no ligand (buf=buffer) control is also shown. The strongest synergism is observed between sucrose (50 mM) paired with alanine and glycine; responses of sucrose presented with serine, lysine, and methionine are also slightly higher than the combined response (blue, amino acid response; striped bar, amino acid+sucrose; gray, combined [additive] response; *P < 0.01, Welch’s two-tailed t-test; mean±SE, n = 4). (c) Synergistic responses are not observed in chimney swift T1R1–T1R3 (each concentration, n = 4).