Some forms of acute renal failure (ARF) are associated with upregulation of renal adenosine A₁ receptors (Smith et al. 2000) and this may account for the enhanced renal vasoconstrictor response to adenosine noted in ARF induced by myohaemoglobinuria (Gould et al. 1995). Since hypoxia/ ischaemia are initiating factors for ARF, we have investigated whether chronic hypoxia alters the expression of adenosine A₁ receptor in DDT₁ MF-2 cells, a hamster smooth muscle cell line.

Binding characteristics of [³H]1,3-dipropyl-8-cyclopentyl-xanthine ([³H]DPCPX), a selective adenosine A₁ receptor antagonist, were determined at 37 °C with membranes and whole cells. Binding studies were conducted following 1-16 h culture in either normoxic (P_O2 ~142 mmHg) or hypoxic (P_O2 ~18 mmHg) conditions. Data are given as means ± S.E.M. (n = 3).

Hypoxia of 4-16 h resulted in significant (P < 0.05, Student’s unpaired t test) increases of up to 60 % in the percentage of [³H]DPCPX bound to intact DDT₁ MF-2 cells. Analysis of binding isotherms with membranes isolated from cells exposed to 16 h of hypoxia (Fig. 1) showed a significant increase (P < 0.05) in B_max (1.7 ± 0.5 vs. 0.48 ± 0.02 pmol (mg protein)^-1) with no detectable change in A₁ receptor affinity as indicated by similar K_d values (1.2 ± 0.3 vs. 0.84 ± 0.2 nM). The findings with isolated membranes suggest that the increase in binding noted with intact cells is a result of increased receptor number. Incubation of cells for 4 h with either actinomycin D (5-20 mg ml^-1) or cycloheximide (1-10 mM) markedly attenuated the hypoxia-induced increase in percentage binding of [³H]DPCPX. For example, 4 h of hypoxia resulted in an increase in [³H]DPCPX binding of 67 ± 14 %, whereas in the presence of 1 mM cycloheximide the increase in binding was only 17 ± 9 % (P < 0.01).

In summary, this study has shown that chronic hypoxia upregulates adenosine A₁ receptors in smooth muscle cells. This response to hypoxia appears to be a consequence of increased gene transcription and subsequent synthesis of receptor protein. These findings suggest that hypoxia associated with ARF may trigger the upregulation of adenosine A₁ receptors in renal vascular smooth muscle.