A luminescent calcium biosensor enabling endpoint measurement of GPCR-mediated calcium signaling

G_q/11-coupled GPCRs regulate critical physiological processes through calcium mobilization, making intracellular Ca²⁺ dynamics a key readout for drug discovery and biomarker detection. However, the transient nature of calcium signals necessitates real-time monitoring with specialized equipment, creating barriers for high-throughput screening and limiting accessibility. Here we present CalLuc-2.1, a luminescent calcium biosensor that converts brief Ca²⁺ spikes into persistent luminescence changes readable tens of minutes after stimulation. By eliminating the need for precisely-timed detection, CalLuc-2.1 enables endpoint measurement of GPCR activation using standard plate readers. We demonstrate robust performance (Z' > 0.88) across multiple G_q/11-coupled GPCRs in both agonist and antagonist screening formats. Furthermore, CalLuc-2.1 successfully detects endogenous GPCR ligands directly in human serum, offering a simpler alternative to immunoassays and mass spectrometry for biomarker quantification. This approach makes calcium-based GPCR assays accessible to any laboratory with basic luminescence detection capabilities, potentially accelerating both drug discovery and clinical research applications.