*2.2. Compounds*

THC (Dronabinol) was obtained from THC Pharm, Frankfurt, Germany. A967079, COR170, HC030031, ruthenium red (RR) and rimonabant were obtained from Tocris/Biotechne, Wiesbaden, Germany. 1,1-Diethyl-2,2-cyanine iodide (Decynium-22; D22) was obtained from Sigma Aldrich, Taufkirchen, Germany. THC is a partial agonist at CB1 and CB2 receptors, but also at several TRP channels. It is the major psychoactive constituent of the plant *cannabis sativa*. A967079 and HC030031 are lipophilic antagonists at the TRPA1 ion channel, whereas RR unselectively blocks TRP channels at the plasma membrane. Rimonabant and COR170 are inverse agonists at CB1 and CB2, respectively. D22 is an inhibitor of organic cation transporters and monoamine transport.

#### *2.3. Synovial Fibroblast and Tissue Preparation*

Samples from RA synovial tissue were collected immediately after opening the knee joint capsule, and tissue was prepared for cell isolation thereafter [41]. Synovial tissue was cut into small fragments and treated with liberase (Roche Diagnostics, Mannheim, Germany) at 37 ◦C overnight. The cell suspension was filtered (70 μM) and centrifuged at 300 g for 10 min. After that, the pellet was treated with erythrocyte lysis buffer (20.7 g NH4Cl, 1.97 g NH4HCO3, 0.09 g EDTA ad 1 L H2O) for 5 min, recentrifuged for 10 min, and then resuspended in RPMI-1640 (Sigma Aldrich, Taufkirchen, Germany) with 10% FCS. After overnight incubation, RPMI medium was replaced with fresh medium to wash off dead cells and debris.

#### *2.4. Intracellular Calcium and PoPo3 Uptake*

In black 96-well plates, RASF were incubated with 4 μM of calcium dye Cal-520 (ab171868, Abcam, Cambridge, UK) in PBS with 0.02% Pluoronic F127 (Thermo Fisher scientific, Waltham, MA, USA, # P6866) for 60 min at 37 ◦C, followed by 30 min at room temperature. After washing, HBSS or PBS containing 1 μM PoPo3 iodide (Thermo Fisher scientific, # P3584) and respective antagonists/ligands/inhibitors were added for 30 min at room temperature. After that, THC was added and the intracellular Ca2+ concentration as well as PoPo3 uptake were evaluated with a TECAN multimode reader over 90 min.
