Previously,it was reported that A ₁ adenosine receptor antagonists prevent endotoxin-inducedacute lung injury and pulmonary arterial endothelial cell damage. In competition radioligand binding experiments in membranes prepared from human pulmonary artery endothelial cells (PAECs), lipopolysaccharides (LPSs) of Escherichia coli, Salmonella typhimurium, Klebsiella pneumoniae, and Pseudomonas aeruginosa displaced the binding of a selective A adenosine receptor antagonist [¹²⁵I]-BWA844U (IC₅₀ values: 195 ng/ml, 290 ng/ml, 602 ng/ml, and 693¹ ng/ml, respectively)in a dose-dependent, competitive manner. There was no displacement of this radioligand by enterotoxin (≤ 10 μg/ml), diphosphoryl lipid A (≤ 10μg/ml), and glycolipids, monosialoganglioside(≤ 1μg/ml), lactocerebroside (≤ 100μg/ml), or NBD galactocerebroside (≤ 100 μg/ml). Based on calculated IC values, LPS (E. coli, IC₅₀ 111 ng/ml) _{50 6} displaced the selective A₁ adenosine receptor agonist, [³H]-2-chloro, N -cyclopentyladenosine (CCPA) in human PAECs with a potency profile, CCPA > LPS > 2-phenylaminoadenosine (CV 1808), a selective A₂ adenosine receptor agonist. The potency profile for displacement of the selective A μ ^2a adenosine receptor agonist [³H]-CGS 21680 was CV 1808 > CCPA. LPS (E. coli 0.1 pg/ml—10 g/ml) did not displace [³H]-CGS 21680 binding. In human PAECs, IL-6 and TXA₂ release induced by LPS (0—1 μg/ml) or CCPA (0—1 μM) at high doses was significantly reduced by the selective A₁ adenosine receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine(DPCPX; 1 μM). These data suggest that LPS binds to and activates A₁ adenosine receptors on human PAECs to induce the release of IL-6 and TXA₂. Activation of A₁ adenosine receptors on human PAECs by LPS, may contribute to the pathophysiology of acute lung injury associated with Gram-negative septicemia and endotoxemia.