Stimulation of A2 adenosine receptors inhibits the ingestion of photoreceptor outer segments by retinal pigment epithelium.

PURPOSE Recent studies have shown that A2 adenosine receptors are present in retinal pigment epithelium (RPE). In this study, the effect of adenosine and adenosine analogues on photoreceptor outer segment (ROS) phagocytosis by RPE was investigated. METHODS Primary cultures of RPE cells were incubated with isolated outer segments in the presence of various adenosine derivatives. Changes in adenylyl cyclase activity was measured by cyclic adenosine monophosphate (cAMP) production using a radioimmunoassay detection system. RESULTS Adenosine inhibited the ingestion phase of phagocytosis (IC50 = 50 microM), and this effect was potentiated 80-fold in the presence of dipyridamole (IC50 = 0.6 microM). In the presence of 10 microM 8-phenyltheophylline, the inhibitory effect of 100 microM adenosine was reduced from 80% inhibition of ROS ingestion to 33% inhibition. The rank order of potency of adenosine analogues to inhibit ROS ingestion by RPE was N6-cyclohexyladenosine/5'-[N-ethylcarboxamido]-adenosine (NECA) = NECA > adenosine >> [R]-N6-[2-phenylisopropyl]-adenosine. The greatest stimulation of cAMP production was observed with 33.3 microM NECA: The production of cAMP reached its maximum level after 2 minutes of incubation, and after 10 minutes the levels of cAMP were back to basal. CONCLUSIONS These results suggest that adenosine and adenosine analogues modulate ROS ingestion by RPE via activation of adenosine A2b receptors, possibly through the cAMP intracellular signaling pathway.