Acta Pharmacologica Sinica 2005 February; 26 (2): 160-165; doi: 10.1111/j.1745-7254.2005.00024.x

Aim: To study the modulating effect of adenosine deaminase (ADA) on yhe adenosine A₁ receptor (A₁R) in HEK293 cells stably expressing the human A₁R.

Methods: cDNA was amplified by RT-PCR using total RNA from human embryo brain tissue as the template. The PCR products were subcloned into the plasmid pcDNA3 and cloned into the plasmid pcDNA3.1. The cloned A₁R cDNA was sequenced and stably expressed in HEK293 cells. The modulating effect of adenosine deaminase on A₁R was studied by using [³H]DPCPX binding assay and an intracellular calcium assay.

Results: HEK293 cells stably expressing human A₁R were obtained. Saturation studies showed that the K_D value and B_max value of [³H]DPCPX were 1.6±0.2 nmol/L and 1.819±0.215 nmol/g of protein respectively, in the absence of ecto-ADA respectively, and 1.3±0.2 nmol/L and 1.992±0.130 nmol/g of protein in the presence of ecto-ADA respectively, suggesting that the K_D value and B_max value of [³H]DPCPX were unaffected by ecto-ADA. In the case of [³H]DPCPX competition curves obtained from intact cells or membranes, A₁R agonist CCPA/[³H]DPCPX competition curve could be fitted well to a one-site model in the absence of ecto-ADA and a two-site model in the presence of ecto-ADA with a K_H value of 0.74 (0.11-4.8) nmol/L (intact cells) or 1.8 (0.25-10) nmol/L (membrane) and a K_L value of 0.94 (0.62-1.41) µmol/L (intact cells) or 0.77 (0.29- 0.99) µmol/L (membrane). The K_L value is not significantly different from the IC₅₀ value of 0.84(0.57-1.23) µmol/L (intact cells) or 0.84 (0.63-1.12) µmol/L (membrane) obtained in the absence of ecto-ADA. Similar results were obtained from the CPA/[³H]DPCPX competition curve in the absence or presence of ecto-ADA on intact cells or membranes. Intracellular calcium assay demonstrated that the EC₅₀ value of CPA were 10 (5-29) nmol/L and 94 (38-229) nmol/L in the presence or absence of ecto-ADA, respectively.

Conclusion: A₁R stably expressed in the HEK293 cells display a low affinity for agonists in the absence of ADA and high and low affinities for agonists in the presence of ADA. The presence of ADA may promote the signaling through the adenosine A₁ receptor in HEK293 cells.

Keywords: adenosine deaminase; adenosine A₁ receptor; radioligand assay; calcium; fluorescence; diagnosis