Chronic hypoxia up-regulates expression of adenosine A1 receptors in DDT1-MF2 cells

As the first step to understand how chronic hypoxia might regulate smooth muscle function in health and disease, we have employed an established immortalised cell model of smooth muscle, DDT1-MF2 cells, to address the hypothesis that adenosine A1 receptor density is modulated by O2 availability. Maximal specific binding (Bmax) of the selective adenosine A1 receptor antagonist, [3H]-DPCPX, to cell membranes increased 3.5-fold from 0.48 +/- 0.02 pmol/mg to 1.7 +/- 0.5 pmol/mg protein after 16 hr of hypoxia and this effect was not accompanied by any statistically significant changes in either binding affinity (0.84 +/- 0.2 nM vs. 1.2 +/- 0.3 nM) or Hill coefficient (1.1 +/- 0.1 vs. 0.99 +/- 0.03). Hypoxia-evoked increases in membrane receptor density were paralleled in intact DDT1-MF2 cells. In addition, the increase in [3H]-DPCPX binding to intact cells was inhibited by co-incubation during hypoxia with the translational inhibitor cycloheximide, the transcriptional blocker actinomycin D and the NFkappaB inhibitor sulphasalazine. Together, these data show that adenosine A1 receptor density is modulated, at least in part, by O2-dependent activation of the transcription factor NFkappaB and adds to the list of processes dynamically regulated by ambient oxygen availability. Since hypoxia is an initiating factor in acute renal failure, similar changes in transcription may account for up-regulation of adenosine A1 receptors noted previously in the renal vasculature of rats with acute renal failure.