Figure 2.

Proton concentration-responses at GPR4, GPR65, and GPR68 in the absence and presence of Ca²⁺/Mg²⁺ (a) and EDTA (b). Proton-mediated G_s-cAMP production was measured in transiently transfected HEK293T cells using the GloSensor cAMP assays. Results represented means ± SEM from a minimum of 3 independent assays, each in triplicate or quadruplicate, and were normalized to the proton activity in the absence of Ca²⁺/Mg²⁺ or EDTA for each receptor. Curves were analyzed using Prism 8.0 with the built-in four-parameter logistic function. Parameters (basal, E_max, potency, and Hill slope) were reported in Table 1.