The antagonism or genetic deletion of adenosine A(2A) receptors has been shown to exacerbate tissue damage in acute lung injury. Caffeine, a widely consumed behavioral drug, acts as a non-selective antagonist of A(2A) receptor and also has additional pharmacological effects. Thus, the protective vs. deleterious effects of caffeine in acute lung injury should be evaluated. In a murine oleic acid-induced model of acute lung injury, we found that chronic caffeine treatment by drinking water (0.1g/l or 0.25g/l for 2 weeks before acute lung injury) or acute caffeine treatment at high dose (i.p. 50mg/kg, injection, 30min before acute lung injury) significantly attenuated the lung edema, hemorrhage, neutrophil recruitment as well as the inflammatory cytokine tumor necrosis factor-α (TNF-α) and interleukin-1 (IL-1) expressions in both of the wild type (WT) and A(2A) receptor knockout (KO) mice. This profile was accompanied by increased cAMP levels and up-regulation of A2B receptor mRNAs in the lungs. In contrast, acute caffeine treatment at low dose (i.p. 5mg/kg or 15mg/kg, injection, 30min before acute lung injury) enhanced the inflammation and lung damage in WT mice with decreasing cAMP but not in A(2A) receptor KO mice. These results indicate that caffeine either enhances lung damage by antagonizing A(2A) receptor or exerts protection against lung damage via A(2A) receptor-independent mechanisms, depending on the timing of exposure (chronic vs. acute) and dose of administration (low vs. high). These findings provide new insight of caffeine in acute lung injury and highlight the potential benefit and strategy of caffeine intake or administration for preventing acute lung injury.
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